WPCF Manual of Practice No. 17
          and Protective  Coatings
           for Wastewater
        Treatment Facilities
              Prepared Under Direction

                    of the

             Technical Practice Committee

                    by the

        Subcommittee on Paints and Protective Coatings
                   1969

          Water Pollution Control Federation
3900 Wisconsin Avenue    Washington, D. C. 20016 U. S. A.

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    MANUALS  OF WATER POLLUTION CONTROL
                          PRACTICE

   The  Committee on  Sewage and Industrial Wastes Practice, now
the Technical Practice  Committee, was created by the Board of Con-
trol of the Water Pollution Control Federation (formerly Federation
of Sewage and Industrial Wastes Associations)  on October 11, 1941.
   A primary function of the committee is to originate and produce,
through competent subcommittees,  special reports  dealing  with im-
portant technical aspects of the broad interests of the Federation (see
inside back co^                                     to review tech-
nical practice                                         are indicated
by rese                                               anuals do not
prescri                                               iscourage the
rapid tq                                              the art  and
science                                               They should
instead                                              rith judgment
and due
         D
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Paints and Protective  Coatings
           for Wastewater
        Treatment Facilities

         MANUAL OF PRACTICE NO. 17
              Prepared Under Direction


                    of the

           TECHNICAL PRACTICE COMMITTEE
                    By the

     SUBCOMMITTEE ON PAINTS AND PROTECTIVE COATINGS

               O. H. HERT, Chairman

   A. J. ALTER                    R. M. POWELL
   N. B. HTJME                    J. L. ROBINSON
   0. R. LINDEMAN                  K. SCHILLER
   A.M. MOCK                    C. N. STUTZ
                  T. J. TILLETT

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Copyright © 1969, by the Water Pollution Control Federation, Washington, D. C. 20016 U. 8. A.
                      Library  of Congress  Catalog Card No. 69-17999
                              PRINTED JN UNITED STATES OF AMERICA BY
                               LANCASTER PRESS, INC., LANCASTER, PA.

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                              Preface
  In 1946 the Board of Control of the
Water Pollution  Control Federation,
then the  Federation of Sewage Works
Associations,  approved  the establish-
ment of a Subcommittee on Paints and
Protective  Coatings  of  the Technical
Practice  Committee,  then the Sewage
Works Practice Committee.  The Sub-
committee was charged with, the pro-
duction of a  Manual of  Practice in-
tended to provide designers, operators,
and maintenance  personnel of waste-
water collection and treatment facili-
ties with the fundamental theory and
practical  aspects  of  the  need  for,
choosing, application, and maintenance
of paints and protective  coatings.
  During the  period of preparing the
draft  of the manual the Subcommittee
was  chaired  successively by  Kerwin
L. Mick,  Maurice  L.  Robins, and Oral
H. Hert.   Rapidly changing  technol-
ogy contributed to the problem of con-
solidating the latest information  in  a
manual.   Undoubtedly  changes  will
continue but the manual is  intended
to provide a base to which improved
techniques can be added.
  The  manual was serialized in three
installments in the  September,  Octo-
ber,  and November 1967  issues  of
JOURNAL  WATER  POLLUTION  CONTROL
FEDERATION.   Reader  comment  was
solicited for  a period  following  com-
pletion  of the serialization.  The  Sub-
committee was able to take advantage
of reader suggestions before the man-
ual was printed in  final form,
  To the Subcommittee and those who
have  contributed  to its  efforts  goes
appreciation  for  this contribution  to
the Federation's  manual of practice
series.

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                    Table of Contents
1.  INTRODUCTION AND EXPOSURE CONDITIONS 	    1
   1.1  Exposure Conditions 	    2
       1.11   Submerged Exposures 	    2
       1,12   Moist Atmosphere Exposures	    3
       1.13   Inside Dry Atmosphere Exposures	    6
       1.14   Outside Weather Exposures	    6
       1.15   Miscellaneous Exposures	    7
   1.2  Summary	    8
2.  THE NATURE OP CORROSIVE ACTION 	    9
   2.1  Direct Chemical Corrosion	   10
       2.11   Oxidation	   10
       2.12   Hydrogenation 	   11
       2.13   Chlorination and Other Direct Chemical Reactions	   11
   2.2  Bacteriological Corrosion	   12
   2.3  Fatigue Corrosion	   13
   2.4  Stress Corrosion	   13
   2.5  Fretting Corrosion	   14
   2.6  Cavitation Erosion	   14
   2.7  Filiform Corrosion	   14
   2.8  Electrochemical Corrosion	   IS
       2.81   Bimetallic or Galvanic Corrosion. 	   15
       2.82   Parting	   16
       2.83  Electrolysis  or Stray Current Corrosion	   17
3.  FACTORS AFFECTING THE CHOICE OF CORROSION PROTECTION  19
   3.1  General  	   1°
   3.2  The Absolute Cost	   19
   3.3  Degree of Protection Required	   19
   3.4  Appearance	   20
   3.5  The Ease of Repainting	   20
   3.6   Design 	   20
4.  PREVENTION OF CORROSION  	  21
   4.1   Choice of Materials	   21
        4.101  Cast Iron	   21
        4.102  Malleable Iron  	  21
        4.103  Wrought Iron and Low Alloy Steels	  21
        4.104  Copper  and Copper  Alloys  	  22
        4.105  Stainless Steel	  24
        4.106  Nickel and High Nickel Alloys	  25
        4.107  Silicon Cast Iron	  26
        4.108  Aluminum  	  27
        4.109  Elastomers	  27
        4.110  Plastics	  28

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Vi                 PAINTS AND PEOTECTIVE COATINGS

        4.111  Ceramics, Glass, and Vitrified Clay Products	   28
        4.112  Concrete	   29
   4.2   Control of the Environment	   30
        4.21  Ventilation and Heat 	   30
        4.22  Cathodic Protection 	   30
        4,23  Galvanic or Bimetallic Corrosions	   31
        4.24  Use of Coating to Prevent Corrosion	   32
        4.25  Treatment of Water Systems to Prevent Corrosion	   35
        4.26  Preventing the Corrosion of Portland Cement Concrete by
             Hydrogen Sulfide	   36
   4.3   Summary 	   48
        4.31  Designing and Building to Prevent Corrosion	   49
5.  ACTION OF DESTRUCTIVE AGENTS ON PAINT FILMS	   51
   5.1   General   	   51
   5.2   Destructive Agents	   51
        5.21  Water 	   51
        5.22  Air and Gases	   51
        5.23  Chemicals	   52
        5.24  Sunlight and Heat	   52
        5.25  Oils and Greases	   53
        5.26  Paint Cleaners	   53
        5.27  Abrasion	   54
   5.3   Methods  of Paint Testing	   55
        5.31  General	   55
        5.32  Laboratory Tests	   55
        5.33  Field  Tests 	   56
        5.34  Test Standards	   56
6.  PREPARATION  OF SURFACE FOR PAINTING 	   57
   6.1   Tools for Surface Preparation 	".	   57
        6.11  Hand Tools	   57
        6.12  Power Tools	   58
   6.2   Preparation of Steel Surfaces	   58
        6.21  Justification for  Cleaning 	   58
        6.22  Mechanical Cleaning Methods 	   59
        6.23  Chemical Cleaning Methods	   60
   6.3   Preparation of Concrete Surfaces	   61
        6.31  Concrete Walls	   61
        6.32  Concrete Floors 	   61
   6.4   Preparing Galvanized Iron Surfaces 	   63
        6.41  Types of Galvanized Iron Surfaces	   63
        6.42  Method of Surface Preparation	   63
   6.5   Preparing Wood Surfaces 	   64
        6.51  New Wood 	   64
        6.52  Painted Wood 	   64
   6.6   Preparation of Masonry Surfaces	   64
        6.61  New Masonry	   64
        6.62  Old Masonry	   64

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                   PAINTS AND PBOTECTIVE COATINGS                vii

   6.7  Preparation of Brick Walls	  64
   6.8  Preparation of Miscellaneous Surfaces	  65
   6.9  Conclusion  	  65
7.  PAINTS AND  COATINGS  	  66
   7.1  Metal Surfaces	  66
       7.11   Primers	  66
       7.12   Top Coats  	  68
       7.13   Pigments for Decorative Paints	  70
       7.14   Machine  Enamels  	  71
   7.2  Non-Metallic Surfaces	  73
       7.21   General	  73
       7.22   Walls and Ceilings	  73
   7.3  Concrete  Floors  	  75
   7.4  Wood Floors 	  76
8.  APPLYING THE PAINT 	  77
   8.1  General  	  77
   8.2  Brush Application	  77
   8.3  Spray-Gun Application	  78
   8.4  Thinners  	  80
   8.5  Atmospheric Conditions and Temperatures	  80
   8.6  Drying Time 	  80
   8.7  Number of Coats	  81
   8.8  Safety Precautions	  82
   8.9  Summary  	  83
9.  MISCELLANEOUS FACTORS IN GOOD PAINTING PRACTICE	  84
   9.1  Surface Preparation	  84
   9.2  Painting Problems  	  84
   9.3  Use of Paint for Identification and Safety	  85

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  1. INTRODUCTION AND EXPOSURE  CONDITIONS
  The  installation of treatment facil-
ities for  wastewater in  the United
States has reached large numbers.  As
of January 1962, there were reported
6,898 municipal facilities of the me-
chanical  type.   A  1964  survey indi-
cated the need for 4,749 new treatment
works.   The  number  of  municipal
plants increased by 62 percent during
the period 1945-1962. If this rate con-
tinues, there  may  well be 10,000 me-
chanical-type  treatment  facilities by
1980.
  The  present  installed  replacement
value (in 1963  construction cost)  of
wastewater  utilities  is   estimated  at
$40/cap  for  about  100  million  per-
sons, or  a total replacement value  of
approximately  $4  billion.   The  ex-
penditures from 1963 to 1980 are esti-
mated  at $18 billion to  meet backlog
and future  requirements.   Thus,  the
investment in treatment works by 1980
will approximate $22 billion.
  The above figures apply only to mu-
nicipal facilities.  Industrial treatment
works are probably as numerous as mu-
nicipal works.  A tabulation of indus-
trial plants  with treatment works  in
26  states lists the number at 6,675.
The replacement value of these facil-
ities is probably not known, but annual
expenditures  are estimated at $600 mil-
lion for the next 10 yr to meet the new
and backlog needs.
  If the replacement value of indus-
trial works  was estimated conserva-
tively  at one-half  the value  of mu-
nicipal works, the total  replacement
value of  all existing wastewater treat-
ment works in the United States would
be at least $6 billion.  By 1980, the in-
vestment could  be  expected to  reach
$30 billion.
  Based  on the figures given above, it
is  obvious that plant superintendents
are charged with a tremendous invest-
ment of public and private funds.  It,
therefore, is advisable to save and pro-
tect this investment from deterioration
by  a  thorough and  effective  mainte-
nance program.  An important part of
such a  program  involves the  use of
paints and  protective  coatings to safe-
guard equipment and materials against
the corrosive and otherwise deteriorat-
ing environment common to all waste-
water treatment  works.   Paints and
coatings  not only prevent  deteriora-
tion, but they also preserve plant effi-
ciency  and, in addition, enhance  the
appearance of the facility.
  The subject of corrosion and  protec-
tive coatings is  very  broad and rela-
tively complicated.  The wide variety
of products  and  the voluminous litera-
ture and reference  material which is
available seriously taxes the time of a
busy  plant  superintendent  to keep
abreast of the field.  It is the purpose
of this manual,  therefore,  to provide
information to enable operators to be-
come familiar with the many phases in-
volved.   These phases include the ex-
posure conditions, the types of corro-
sive action, the prevention of corrosion,
the action  of destructive agents on
metals and  paint films, factors affect-
ing the choice of metals and protective
coatings, preparation of surfaces  for
painting, selection of paints, types of
use and conditions, method of applica-
tion, and miscellaneous factors of  im-
portance such  as  painting  records,
color  dynamics,  and paint  for  pipe
identification.
  This  manual  is  not  intended to
obviate seeking counsel from qualified
consultants  and  from  the manufac-
turers of the products.

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                    PAINTS AND PROTECTIVE COATINGS
                    1.1 EXPOSURE CONDITIONS
  Equipment and materials in a treat-
ment works are  exposed to different
kinds of  deteriorating conditions de-
pending on the particular function in-
volved and the nature of the climate.
These exposures  may  be classified as
follows.   There are sub-classifications
of most of these general classifications
as will be  indicated  in the discussion.

1.11 Submerged Exposures

  Submerged exposures are character-
ized by the following conditions which
deteriorate protective coatings:

  (a) "Water is normally present.
  (6) Oxygen is  present in solution.
  (c) Water  line  exposure  is  most
      severe.
  (d) Oils, greases,  and  soaps  are
      present.
  (e) Hydrogen  sulfide is  present in
      certain places.
  (/) Carbon dioxide  usually is pres-
      ent.
  (gf) Floating   material  usually  is
      present.

1.111 Water-line  Conditions:—These
conditions involve most  of  the agents
mentioned above  and  are found in
structures, chambers, and  flumes  con-
taining  or  transporting  wastewater.
The concentration  of  these agents in
various treatment units depends on the
stage of the treatment.
  Obviously, water is  present  in all
submerged and  waterline  conditions.
This agent is  destructive  because  it
acts as an electrolyte, in the  presence
of certain salts, to corrode metal when-
ever it penetrates  a  protective  film.
Water  also  hydrolyzes  many  paint
vehicles so that they lose their strength,
their bond to the metal, and  their re-
sistance to the passage of  oxygen and
acid-forming gases  which may be pres-
ent in solution.  These  gases are also
prime agents of corrosion.
   A feature peculiar to most waterline
exposures is  the   presence  of  oils,
greases, and  soaps in the wastewater.
While these substances tend to coat the
wetted surface below the waterline and
to an  extent protect this  surface  by
preventing the easy passage of oxygen
and acids, their most obvious  charac-
teristic  is, nevertheless, to congeal  on
tank and  sewer walls at the  waterline
in a heavy, black, cheesy crust.  Since
the constituents of this crust are sol-
vents  of many paints, the crust tends
to soften  the paint wherever there  is
contact.  The paint thus becomes more
susceptible to abrasive damage by float-
ing debris and cleaning operations.
  Another characteristic of submerged
exposures  largely confined to the wa-
terline, is the  physical  stress of the
paint  film caused by  wetting-and-dry-
ing, the heating-and-cooling effect in
warm weather, and the freezing-and-
thawing of moisture in  and  on the
paint  film in winter.   The  action of
these  reversing  forces  is  highly de-
structive.
  Ice   may form  on the  surface  of
trickling  filters  in  cold climates, but
rarely is it formed elsewhere in water-
line conditions.  An exception  may be
found where  extreme low temperatures
are sustained. In this case, proper de-
sign  through insulation and auxiliary
heat will  eliminate a large percentage
of the freezing locations and  associated
problems.   Ice, when formed, will grip
paint on side walls and  appurtenances.
When the ice falls away, the paint may
pull with  it,  especially if the paint or
bond has been weakened by the actions
described  above.
   Sunlight also may be  a deteriorating
factor in  waterline attack.  Sunlight
tends to age organic films causing them
to lose their effective life.

1.112  Submergence  in Raw  Waste-
water:—In this exposure the  paint is
submerged in  raw  wastewater or  in
wastewater receiving only preliminary
treatment.

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                    PAINTS AND PEOTECTIVE COATINGS
  Here,  the  water often is  devoid  of
oxygen or nearly so.   There may  be
dissolved salts present but  these, for
the most part, are harmless.  They may
even  be  of   benefit by  neutralizing
strong mineral acids.  Carbon dioxide
and hydrogen sulfide are nearly always
present,  the amount depending largely
on the freshness of the wastewater.   If
there is any agitation  of the  waste-
water so that it takes up oxygen, a part
of  the hydrogen sulfide  will be con-
verted to sulfurous and sulfuric acids,
but these acids will be neutralized
promptly by the  carbonates  in the
wastewater.  The  effect  of the hydro-
gen sulfide on paints will  be discussed
later.
  Ammonia may be a minor constituent
of wastewater at this point, but it, too,
is likely to be neutralized by the min-
eral acids present.  Greases, oils, and
soaps are usually in  abundance  and
sometimes gasoline is present.  The ef-
fect of these  solvents already has been
discussed.   Grit and floating  debris
vary in amount according to the inci-
dence of  storms and the time  elapsed
since  the heavy flow in the  sewer be-
gan.  The amount also varies with the
time of year,  the type of contributing
industries, and with  the amount  of
screening and settling  provided.  Ice
may be a problem in this exposure  in
cold climates.
  In industrial communities, raw waste-
water may contain  strong alkalies  or
strong mineral acids. The alkalies are
particularly  damaging  to oil  paints
while the acids attack  exposed steel
and concrete wherever  they are not
neutralized.

1.113  Submergence in  Aerated   or
Chlorinated  Wastewater:—This type
of  exposure  occurs  in  aeration tanks
and in the settling and chlorine contact
tanks  which  follow these  oxidizing
processes.  An additional exposure  is
found  where an  aerated effluent  is
chlorinated and stored in a supply tank
for use about the treatment facility.
  A large amount of carbon dioxide is
in  solution  which  characterizes  this
exposure.  Greases, oils, and soaps are
present.  Ice may more likely be pres-
ent in the settling and contact tanks
since heat  in the wastewater has been
lost by the earlier processes.
  While  this exposure  is moderately
severe on paints, the condition in area-
tion tanks is less severe on steel as long
as the steel  remains  completely  sub-
merged.  Steel takes on a glassy  iron
oxide film which  is  tight  and fairly
protective so long as it is not exposed
to the atmosphere.   When the tank is
emptied, however, and exposed to  the
weather,   the oxide  coating  quickly
comes loose  and corrosion  then may
proceed at a  rapid rate.
  The exposure in  aeration tanks is
destructive to  metallic  zinc coatings.
This is due apparently to a high  con-
tent of carbon dioxide in solution re-
sulting from the biologic digestion of
the carbonaceous matter.  Another fac-
tor in this destruction may be the high
oxygen content in the liquid.

1.12 Moist Atmosphere Exposures

  Moist atmosphere  exposures  contain
the  following  undesirable  agents  or
conditions:

  (a)  Moisture  and oxygen.
  (&)  Hydrogen sulfide.
  (c)  Carbon dioxide.
  (d)  Sulphur dioxide (occasionally).
  (e)  Carbonic  acid.
  (/)  Sulphur acids.
  (g) Wetting-and-dry ing,  heating-
      and-cooling,  freezing-and-thaw-
      ing.

  Moist atmosphere exposures occur in-
side buildings, manholes,  screen cham-
bers,  wet  wells, grit chambers,  and
closed water  tanks or  wherever waste-
water surfaces are exposed  in an  en-
closed area.   Under such  conditions,
moisture tends to condense  in  a  film
on  cold  surfaces such  as  windows,
doors,  handrails, structural members,
blowers,  pumps, electrical equipment^

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                     PAINTS AND PROTECTIVE COATINGS
           Moisture Vapor Transmission


           Humid Air
           tft
            Water or Salt Water (dilute)
            Direction of  water by osmosis
          (passage of water through a semi-
          permeable membrane  from the di-
          lute  solution in the direction of the
          more concentrated solution).
         O
               Moisture absorbed
               by soluble salt


                Soluble soli or
                salt crystal
                Soluble salt deposit
                or crystals


                Salt Solution
                (concentrated)
                formed by
                moisture vapor
                on soluble salt.
        FIGURE 1.—Diagram of mechanism by which blisters are formed due to
          moisture vapor transmission and osmosis.  (Courtesy Ametcoat Corp.)
pipes,  ducts, conduits, etc., as well as
concrete, brick, and plaster.  This film
of moisture takes up oxygen and other
gases such as  carbon dioxide and  hy-
drogen sulfide  if they are present.
  Experience has shown that hydrogen
sulfide  passes  through  many  paint
films.  When it reaches steel, it attacks
the metal  to  form  black iron sulfide.
This reaction not only destroys the sur-
face to which the paint is bonded,  but
it also frees hydrogen gas  which  col-
lects in blisters beneath the film.  The
loss of the bond and the formation of
these  blisters  make  the paint  more
susceptible to abrasion damage.
  Moisture sometimes also  penetrates
the paint film  along with the hydrogen
sulfide in which case the steel becomes
coated with a black slime instead of the
black iron sulfide.  The presence of this
black slime is evidence that the paint
is not well suited to the surface.
  Some of the hydrogen sulfide in the
moisture film on painted surfaces, in-
stead of penetrating the paint directly,
is oxidized on the surface to sulfurous
and sulfuric acids.  These acids are ac-
tively corrosive  of both steel and con-
crete.   Along with carbonic acid and
oxygen, which  are also in solution in
the moisture, these  aggressive  agents
spread  out over the  painted surface,
pass  through  it  wherever pinholes,
skips, or  abraided  spots  occur, and
vigorously attack the metal  or  cement

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                   PAINTS AND PROTECTIVE COATINGS

         FIGURE 2.—Corrosion by oxidized H2S.  (Courtesy Amereoaf Corp.)
beneath.   The attack tends to spread
laterally  underneath the paint film so
that the damage is extended widely.
  An important  factor in these ex-
posures  is  the physical effect brought
about by frequent changes in dimen-
sion in the paint  film induced by re-
versing stresses.   Such changes in di-
mension  are brought about by wetting
and drying of the paint film,  by heat-
ing and cooling of the paint and metal
on  which  it is  placed, and  in cold
climates, by  freezing and  thawing of
the moisture  in and on the paint film.
  This movement  tends to  thin the
paint over rivets, bolt  heads, and nuts
and over the  sharp edges of plates and
shapes until  tension breaks  the film.
It also tends to pull the film away from
the metal at  these points until the un-
supported film breaks.   Movement also
tends to crystallize the vehicle so that
the paint  becomes increasingly brittle
and  more subject  to  the  cracking.
When the film is  no longer  intact,  it
ceases to protect the surface.
  Moist atmosphere exposures where
sewage gas also is present is perhaps
the most destructive to paint films and
structures of  all  exposures  generally
encountered in a treatment works.

1.121  Exposure  Above Eaw Waste-
water:—This type of exposure occurs
in wet  wells,  in  enclosed  screen  and
grit  chambers, in manholes, and wher-
ever wastewater is allowed to come  in
direct contact with air  confined in an
enclosed space.
  A particularly  severe exposure oc-
curs where  a tall screen  house  with
ventilators in  the  roof is built over a

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6
PAINTS AND PROTECTIVE COATINGS
deep screen pit into which sewers are
running  partly full.   Here  the  tall
building  acts  as a chimney  to draw
warm, wet air heavily laden with sewer
gas from the sewer. When metal parts
and the walls of the housing structure
and its  equipment  become cold, mois-
ture condenses on them.  This moisture
takes up oxygen and sewer gas to do
damage as  described  previously.   In
addition, cold outside air tends to  mix
with warm inside air so that the screen
chamber or wet well may be filled with
a corrosive fog.  The result is that at
certain seasons of the year the paint
remains saturated over long periods of
time and the  damage  is thereby ex-
tended.   Windows  and doors in these
structures suffer most because the wet-
ting-and-drying,   heating-and-cooling,
and freezing-and-thawing processes are
much more frequent on these surfaces
than elsewhere.
  This  particular  exposure,  besides
damaging painted surfaces, makes re-
painting  difficult because the excessive
moisture prevents effective  drying of
the surfaces prior to painting.

1.122  Exposure Above Aerated Plant
Effluent:—This  type  of exposure oc-
curs most  often  where  the  aerated
plant effluent is chlorinated and stored
in a tank for various uses about the
wastewater  facility.  The condition  is
similar to other moist  atmosphere  con-
ditions except  that  traces of free chlo-
rine gas may  be present which  com-
bines  with  the  moisture to  become
highly  aggressive on metal.

1.13 Inside Dry  Atmosphere  Ex-
     posures

  Inside dry atmosphere exposures are
characterized by the  following condi-
tions :

   (a) Little moisture  present.
   (o) Oxygen  is present.
   (c) Hydrogen sulfide  in  sufficient
      concentrations  to discolor  cer-
      tain paints.
                          Sulphur  dioxide  only slightly
                          present.

                     This exposure occurs in offices,  lab-
                   oratories,  pump and blower  rooms,
                   workshops, store rooms,  and the like.
                   Conditions are not as severe as in other
                   exposures about a  plant.  Metal  and
                   other deteriorating  surfaces should be
                   protected  against the  effects of hydro-
                   gen  sulflde, however.  Eegardless of
                   corrosive  conditions,  interiors will no
                   doubt be painted for appearances sake
                   if for no other reason. A well-painted
                   interior is the best assurance of a tidy
                   plant from the housekeeping point of
                   view.

                   1.14 Outside Weather Exposures

                     These  exposures  are  probably  the
                   most variable  of all exposures around
                   a treatment plant.  They  include the
                   following  deteriorating agents or con-
                   ditions :

                     (a)  Actinic light and radiant heat
                          (sunlight).
                     (i)  Hydrogen sulfide.
                     (e) Sulphur dioxide.
                     (d]  Carbon dioxide.
                     (e)  Salt air.
                     (/) Abrasion  by windblown  sand,
                          etc.
                     (flO  Wetting-and-drying,   heating-
                         and-cooling,  freezing-and-thaw-
                          ing.

                     This type of exposure  occurs on the
                   exteriors of treatment plant structures
                   and buildings, fences, guard rails, un-
                   loading docks, etc.
                     The  exposure  is not radically differ-
                   ent from  that experienced outside on
                   any other building in the same region,
                   except that the  presence of sewer gas
                   complicates the problem.  As the sewer
                   gas  usually is  small  in  amount,  its
                   effect on the durability may be of little
                   importance.   Its effect on  the surface
                   appearance, however,  may  be consider-
                   able since it  discolors many pigments
                   which may be present in the paint.

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                    PAINTS AND PBOTECTIVE COATINGS
  Ordinary outside exposures about a
treatment plant are characterized by
th0 effect of sunlight, humidity, tem-
perature  and  temperature variation,
dust and  sand  blowing, and discolora-
tion by sewer gas.
  Rays from the sun greatly stimulate
oxidation of oil paints so that they age
rapidly.  The aging is evidenced by a
chalking of the surface and sometimes
by  a  checking and  cracking  of the
paint film.  Paints that chalk appear
to fade due to a change in the diffusion
of light brought about by the presence
of  the  oxidation powder on the sur-
face.
  Checking and cracking are evidence
that chemical combinations are taking
place which reduce the paint volume
so that the paint fails by tension.
   Coal tar paints  "alligator" due  to
oxidation  and  polymerization of the
top surface and  elimination  of  the
more volatile parts of the tar which
causes a reduction  of the paint volume
and a drawing together of the remain-
ing constituents.   Sunlight increases
the rate of this action.
   Moisture and sunlight  together may
cause certain  soluble  compounds like
acetic  acid to  be  formed.  For that
reason, the  humidity  of the climate
often governs  the type of paint which
is best suited to a given location.
   The physical effects  of wetting-and-
 drying, heating-and-cooling, and freez-
 ing-and-thawing are imporant in out-
 side  exposure.   For   example,  the
 formation of dew at night and its dry-
 ing out in daytime is  one of  the rea-
 sons for  the destructive  nature  of the
 Florida  climate.
   Another factor  affecting paint life
 in an outside exposure is the wear sus-
 tained from  blowing  dust, dirt,  sand,
 and rain.   This  wear accelerates the
 damage  done  by  sunlight  and  other
 agents because it cleans the surface of
 accumulations of  decay so that new
 surfaces  are  presented  for  active
 agents to work on.
    In addition to  damage done to  the
paint, sunlight also affects the color of
certain  pigments.   For  instance,  it
fades prussian blue and causes certain
grades of lithopone to darken.
  Certain pigments are much affected
by  the presence  of sulfur gases from
an  industrial region  or from  sewers.
These  sulfur  gases darken paints in
which  lead  compounds such as white
lead,  lead chromate, or chrome green
are used.  In fact, most all lead pig-
ments are unsuited to decorative coats
where these gases are  strong.  Yellow
ochre and ferrite yellow  (which are
iron hydrates)  also are darkened by
these  gases.    Cadmium   yellow  is
turned white  by the carbonic acid gas
of sewers.  There is probably no yellow
pigment available which  is entirely
satisfactory for use about  a treatment
plant.
  Because of the discoloring effect  of
sewer gas and  sunlight,  careful con-
sideration  always should  be  given  to
the final aesthetic result to be obtained
from top  coats  and the  coloring se-
lected,

1.15 Miscellaneous Exposures

   In the  treatment  plant,  there are
many different types of apparatus and
appurtenances,   and  therefore  many
different  problems  of  maintenance.
 Often  these  problems are  associated
 with the type of surface rather than
 environmental conditions as previously
 discussed.
   Although  these exposures  are dis-
 cussed in more  detail in later chapters,
 it  may be pertinent to  list  some of
 them on which protective coatings are
 indicated:

    (a)  Pumps,  blowers, turbines, and
       motors.
    (B)  Heat  insulation.
    (c)  Plaster,  brick,  and concrete.
    (d}  Floors.
    (e)  Radiators and  bare steam pipe.
    (/)  Boilers,  piping, and controls.
    (g}  Bearing  and  rubbing  surfaces.

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8
PAINTS AND PBOTECTIVE COATINGS
  (h]  Heating coils  and other metal
       surfaces in separate  sludge di-
       gestion tanks.
  (i)  Laboratory facilities.
  In extreme sustained low tempera-
ture areas,  the treatment plant  may
be enclosed  entirely except  for  sludge
storage or  drying.   This method  of
design will  cause additional problems
of maintenance.  The air will be satu-
rated,  in most  cases, and condensation
                   will take  place  making  the  use  of
                   specially designed  coating and meth-
                   ods of application  necessary.  Proper
                   design and selection  of materials is
                   essential in this type  of construction,
                   as well as special means of ventilation
                   or  control of  humid  air.   In  Fair-
                   banks, Alaska,  the aeration chamber is
                   housed in a glass enclosure within the
                   main  structure which  encloses the en-
                   tire plant.
                             1.2 SUMMARY
  Superintendents  and  operators  of
wastewater  treatment plants  in  the
United  States are  charged with  re-
sponsibility  for  maintaining  equip-
ment, materials, and structures valued
in the billions of dollars.  Part of this
maintenance  involves the preservation
of surfaces subject  to deterioration by
the corrosive and otherwise hostile na-
ture of  the environment.
                     Three general classifications  of ex-
                   posures are  of  most serious concern,
                   namely,  submergence  or partial  sub-
                   mergence,  moist  inside  atmosphere,
                   and outside atmosphere.
                     Water,  oxygen, and hydrogen sul-
                   fide are the most common elements re-
                   sponsible for deterioration of surfaces
                   with  steel  surfaces being most  sus-
                   ceptible.

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     2.  THE  NATURE  OF CORROSIVE  ACTION
  Corrosion is the unmaking of metals   they  were  derived.   These  processes
or the process by which they tend to   can be either direct chemical reactions,
revert back  to  the more  chemically   electrochemical reactions, or  a combi-
stable forms of  the ores from  which   nation of both.  Corrosion in all of its
HYDROGEN  GAS
                                         HYDROGEN  GAS
           .H* OH
                    IRON GOING  INTO  SOLUTION
                          AS IRON  IONS
                  Fe (OH)g
                          t" *"  r  •""

                     IRON OR STEEL STRUCTURE
                                         ELECTRON  FLOW AWAY
                                   FROM AREA  OF DISSOLVING IRON
                                            OH"  OH"   H,  H0  Oh' OH
                   ACIDIC AREA
                   EXCESS LOCAL H*IONS
                                     ALK ALINED ARE A
                                     EXCESS LOCAL OH" IONS
Ft — ^FE+++ 2i-
HB0*-H+  4 OH
                                                          •H.
 FIGURE 3. — Top: Conventional diagram of the corrosion process.  Bottom: Corro-
   sion process showing formation of acidic anode  and alkaline cathodic areas.
                       (Courtesy Amercoat Corp.")
                                  9

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10
PAINTS AND PROTECTIVE COATINGS
many manifestations falls within these
three categories  and is not necessarily
a steady-state condition  either as to
rate or type of reaction. In many cases
the exact nature  of the type of reaction
is debatable and distinction is largely
theoretical.
  The term "corrosion"  applies only
to metals  and not to  the  deterioration
                   of concrete, wood, plastic, or other ma-
                   terials of construction found in a treat-
                   ment  plant.  Likewise,  the  term does
                   not apply to  deterioration  by  such
                   physical causes as wear, erosion, vibra-
                   tion,  or stress  but corrosion  may  ac-
                   accompany these physical phenomenon
                   in the form of  a chemical change.
               2.1 DIRECT  CHEMICAL CORROSION
  The most easily understood general
type of corrosion is the direct chemical
union of a metal with one or more of
the components  in its environment.

2.11 Oxidation
  The most familiar form of corrosion
is the oxidation of  ferrous  metals in
the presence of "free"  oxygen which
forms rust  (Pe2O3).  The  rusting of
iron  takes  place in the atmosphere,
when  buried  in the earth, or when
submerged in water or most any com-
mon environment so long as moisture
and "free" oxygen are present and in
direct contact with the metal.   Under
high  temperatures such  as  in welding
or heat treating iron or steel,  a black
oxide (FesCM is formed which is com-
monly known as "mill  scale."  Heat
and the lack of free  oxygen account
for the difference in the type of oxide
formed.
   Oxidation is  not just a union of
metal lie atoms with  oxygen atoms,  but
rather  an  exchange of electrons.   An
iron  oxide  crystal  (FeaOa)  is   not
just a group of iron and oxygen atoms
arranged on a lattice but actually two
iron  ions (iron atoms with three  elec-
trons  missing  on each)  connected to
three oxygen  ions (oxygen  atoms  with
two extra electrons each).   Since the
six  extra oxygen electrons  have  been
given up  to  replace  the  six  missing
electrons, the oxygen is now electrically
neutral and takes on something of the
structure and stability of an inert gas.
Since the  oxide molecule thus formed
also has a neutral charge or potential,
                   it is more stable and reluctant to re-
                   act than  the metal.  The buildup of
                   these neutral ions on the surface  of a
                   metal reduces the  reaction rate  as it
                   gets thicker by  acting  as insulation
                   between the metal  and the active ele-
                   ments in  its environment.   However,
                   if these, or other,  active  elements in
                   the environment are reactive enough to
                   react with the oxides to  form sulfates,
                   chlorides, or some other  chemical com-
                   pound capable of conducting an electri-
                   cal current, or if they are capable of
                   dissolving the oxide, then  the reaction
                   rate may  become accelerated.  Those
                   metals  whose  oxides  have  the  most
                   compact  structure  provide  the  most
                   effective barriers.  The oxides of chro-
                   mium, aluminum,  and nickel, for ex-
                   ample,  will form an effective barrier
                   to further corrosion under normal con-
                   ditions while still of microscopic thick-
                   ness.
                      Since  the oxide coating is  less re-
                   active than the  metal underneath, it
                   becomes the cathode (passive  element)
                   with a negative potential and the metal
                   becomes the anode  (reactive  element)
                   with a positive potential.  In the event
                   that  this  oxide  coating is porous  or
                   becomes scratched  or eroded away, in-
                   dividual  galvanic  cells  are set up be-
                   tween the coated areas and the exposed
                   areas and  the  reaction  rate actually
                   is accelerated until the  exposed areas
                   are "self-healed"  or the  availability
                   of oxygen is  reduced.   If the rate of
                   erosion or oxide reduction equals or
                    exceeds  the rate  of  oxide  formation
                    then the corrosion rate remains  more
                    or less  constant unless some  of the

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                    PAINTS AND PEOTECTIVE COATINGS
                                   11
other factors affecting  the corrosion
rate are altered.
  In  the  case of anhydrous oxidation
(wet  corrosion)  the presence of water
is required  and hydroxides  may form
instead  of anhydrous oxides. The hy-
droxides  generally react later  to be-
come  oxides, sulfates  or,  if acidified,
may  revert back  to the metallic  ion
and water.
  Another  condition  that  frequently
occurs  to pipes or metal  structures
buried  in the earth is the  formation
of "oxygen  cells" caused by variations
in the amount of available oxygen ions
in the  soil  at different  points.   This
results  in lower potentials where  the
oxides are formed  readily and higher
potentials where restricted thus creat-
ing1 galvanic cells and the transfer of
metallic ions from  the area  where the
oxide coating is deficient.

2.12  Hydrogenation

  When a metal is immersed in  non-
aerated  water or a non-oxidizing acid,
some  of the water  is reduced to sepa-
rate H- and OH"  ions which then are
free to  react  with the metal  as  well
as the H  ions of acid in the environ-
ment. Under conditions  of stress,  high
temperatures,  or high pressures,  hy-
drogen  penetrates the lattice structure
of the  metal  and  reacts with its in-
ternal  structure.    This changes its
physical properties which  results  in  a
loss of  ductility and  the  creation of
internal pressures.  This loss of ductil-
ity is  called   "hydrogen   embrittle-
ment."   In the  case of  cast iron and
high strength steels, the  internal pres-
sures  may  cause splitting  (hydrogen
cracking)   and,  in  more  malleable
metals,  the  results  are surface blister-
ing.
  As  in oxidation most of the reactions
that occur between the hydrogen and
the metal are single or multi-step ionic
exchanges that   rightfully  could be
called electrochemical  reactions  rather
than direct chemical actions. Students
of electro-chemistry refuse to acknowl-
edge the term "direct chemical action"
and the  evidence evolved in the study
of the processes involved in the oxida-
tion  and hydrogenation  of  a  metal
provide strong proof for their theories.
  Increasing  the temperature,  rough-
ening  of the surface,   working  the
metal, or the presence of an internal
stress in a metal tend to separate the
metals  "structural  boundaries"  or
grain, thus  allowing the hydrogen to
penetrate the metal more readily and
attack the exposed faces  in the interior
of the metal.  As the  ions build up
on these interior surfaces, they slowly
join to form molecules of free hydrogen
which then  are unable  to escape and
the internal pressures result.  Proof of
this theory lies largely in the fact that
free hydrogen is found  in the blisters
of the more ductile metals.

2.13 Chlorination   and  Other Di-
     rect Chemical Reactions

  Because of  the diversified nature of
wastewater in different sections of the
country  the types and  concentrations
of chemicals vary considerably.   Nor-
mally the concentration of any particu-
lar corrosive  chemical will not reach
concernable  proportions  at the disposal
plants because of dilution and reaction
with other materials in the wastewater
collection system.  In coastal areas and
in areas where oil  field brine  wastes
are discharged into the sewer, the con-
centration of sodium chloride and other
chlorides may become a serious prob-
lem.
  The dissolution of the  cement in con-
crete lines and structures and the sub-
sequent erosion of the aggregate leaves
the reinforcing steel  exposed to attack
from  these  salts which  react  directly
with the iron  to form ferric or ferrous
chlorides.  These immediately  dissolve
and leave the metal exposed to continu-
ous attack.    Where splashing  occurs
above the water surface, this condition
is further accelerated by concentration
of the salts due to evaporation and
oxidation by atmospheric oxygen.

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12
PAINTS AND PKOTECTIVE COATINGS
  The organic acids  found  in  waste-
water, and especially in sludge  super-
natant  liquor,  are  very   corrosive.
Since they are generally the product
of bacterial decomposition, they will be
dealt with under the heading of  bac-
teriological corrosion.
  The two gases H2S  and S02, both
in their  gaseous  state and  after they
have reacted  with  water and oxygen
to form  H2SO4, arc perhaps  the most
serious corrosive  problem encountered
around the  average wastewater treat-
ment plant.  In those areas where the
water supply contains  sulfates  the
formation of H2S is usually the result
of    the   sulfate-reducing   bacteria
(sporovibrio desulfuricans)  and also is
covered under bacteriological corrosion.
Some water sources,  however, contain
free  S02 which may remain in trace
amounts in the wastewater.  Possibly
the  most serious problem lies  in the
combustion of digester  gas.   The  H2S
in the gas reacts with oxygen to form
lIoSC>4 in the burner or engine and a
direct chemical  action  takes   place
when the hydrogen  ions in  the  acid
replace the metallic  ions in any ex-
posed metal with which they come  in
contact.   That  which  fails  to react
during  combustion  enters  the atmo-
sphere with the other stack gases and
                   attacks  adjacent exposed  surfaces in
                   the form of sulfuric  acid.
                     The C02 in the digester gas may
                   possibly be considered to be beneficial
                   since it usually  leaves the stack un-
                   changed.  It then may react with wa-
                   ter vapor to form HCOs which reacts
                   slowly with metals to form protective
                   coatings  in  the  form  of carbonates.
                   This  may  contribute to the passivity
                   of the metal  as  well  as  serving as a
                   cathodic coating to react with any sul-
                   furic acid vapors that also contact the
                   surface.
                     In coastal areas the sodium chloride
                   in  solution in  atmospheric vapor  is
                   more damaging than the water vapor
                   and may  extend several miles inland.
                   In  industrial areas, dew can be very
                   corrosive  due  to the absorbed  stack
                   gases from  the  atmosphere  and the
                   chemical salt content of the  dust de-
                   posited  on exposed surfaces.  The ad-
                   dition of  moisture  to the  dust also
                   creates an  electrolyte that can support
                   galvanic corrosion.  Dew is more cor-
                   rosive than rain because the dust and
                   corrosion   products   are  not  flushed
                   away.
                     Besides  being  a  fire  hazard,  free
                   methane from  digester  gases  breaks
                   down in the heat of an  electrical arc
                   and  leaves  free  carbon  deposits  on
                   electrical contacts.
               2.2  BACTERIOLOGICAL CORROSION
   Perhaps the most complicated and
unique forms of corrosion are the re-
sult of bacterial  action either  directly
or indirectly. While most of these bac-
teria are anaerobic there are some that
are  aerobic.   Those commonly found
in sewers are generally of both types.
The sulfate-reducing bacteria  are' an-
aerobic and may be found in wastewa-
ter  or in  the soil.  While they  are
tolerant to a fairly wide range of tem-
perature,  they  are most  active  be-
tween  80°  and  100°F   (26.6°  and
37.8°C) and at least one species found
in the soil is able to survive tempera-
tures  in  excess  of  130°F  (54.5°C).
                    These bacteria do not attack the metal
                    itself but reduce the protective sulfate
                    coating  on  the metal  and  leave  it
                    vulnerable to attack from  the HaSC^
                    that results from oxidation  of  the HaS
                    usually produced in  the sulfate-reduc-
                    tion  process.   A  second  type of bac-
                    teria lives in  the  moist  slimes above
                    the  water  surface and  oxidizes  the
                    II2S  in the atmosphere.  The concen-
                    tration of H2S04  in  these slimes  has
                    been measured  as  high as  10  percent.
                      Still other types attack and destroy
                    asphaltie protective  coatings that  are
                    resistant  to normal  chemical  attack.
                    A wide variety of bacteria  is involved

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                    PAINTS AND PROTECTIVE COATINGS
                                  13
in the process of digesting wastewater
sludge and the  enzymes formed are
organic  acids  that  are  corrosive to
metals and most organic and inorganic
protective coatings.  Like the sulfate-
splitting bacteria they are  non-toler-
ant  to  high pH although  they  will
survive a wide temperature range as
long as  an  abundance  of moisture  is
present.
   In addition  to the  direct chemical
action of these bacteria and their en-
zymes, an equally serious problem re-
sults from the formation of galvanic
cells due to the differences in pli and
salt  content  throughout the  liquid
which  serves  as  an electrolyte.   The
liquid or slime in the immediate area
of each colony of  bacteria,  having a
lower pH and lower potential becomes
a cathode and  the  adjacent  metal  be-
comes anodic with corrosion, appearing
at that point.
  As well as the sulfate-reducing and
sulfur-oxidizing  bacteria,  there   are
other specific  types  that reduce  ni-
trates  to  form  ammonia  and hydro-
genate CO2 to  form methane.
  While this study deals only with the
corrosion  of metals, the damage done
to concrete pipe  and structures  is an
equally serious problem  to  treatment
plant maintenance.
                      2.3 FATIGUE CORROSION
  Any  ductile  metal has  a  relative
limit to the number of  times it can
be bent or otherwise stressed in a non-
corrosive  environment.   When similar
stresses are placed on the same  metal
in a corrosive environment the number
of  times  is  reduced greatly  before
failure  occurs.  The process by which
the  work  limit  is  reduced is  called
"corrosion  fatigue"  or  "fatigue cor-
rosion."  The actual corrosion may  be
oxidation, hydrogenation, direct chemi-
cal,  or  a  galvanic action due to the
heat and stresses generated within the
metal.
   In the first  three instances, the ac-
celeration  is brought about by the dis-
tortion  of the grain boundaries which
tends to  separate  them  and  permits
the  penetration  of  the  corrosive ele-
ment to  the interior  of  the  metal.
The slippage of  the grain  boundaries
also exposes more surface on the metal
faces and helps to erode the protective
corrosion products  that  would other-
wise tend  to retard the rate of cor-
rosion.  The  friction of this movement
generates heat within the metal (which
accelerates  most  chemical  reactions)
and produces slight electrical currents
and differential  pressures  within  the
metal   which are   conducive  to  the
formation  of galvanic cells with sub-
sequent corrosion at the anodes.
                       2.4 STRESS  CORROSION
   Stress  corrosion is similar  to  "fa-
 tigue  corrosion" in  the manner  in
 which  the corrosive  action  actually
 takes  place,  but without  the actual
 working of the metal while  the corro-
 sion  is  taking place.   The stress is
 generally pre-applied and may be the
 result of temperature  (in unannealed
 metals) or strains occurred by working.
 In a  non-corrosive element these in-
 ternal stresses may go undetected for
 months  or  years, only to show up in
 a matter of minutes or hours after be-
 ing placed  in a corrosive environment.
 Cracking  or  splitting are  the  usual
 signs of failure.   In general, tempera-
 ture  appears to have relatively little
 affect on  this type  of corrosion,  nor
 does  the  period  of time that lapses
 between the  incurrence  of  the  stress
 and the time when it is emerged in the
 corrosive element.
   The straining of metal  also produces
 electrical  energy  which  polarizes  the
 metal and increases its "attraction" to
 oxygen  and other  corrosive elements.
 The  electrical  energy  remaining  in
 stressed metal  can  alter its  polarity

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14
PAINTS AND PROTECTIVE COATINGS
so that  an otherwise "passive" metal
becomes  active and  remains so until
the energy is discharged.
  Corrosion generally starts at points
                   of structural disarrangement or stress.
                   Unannealed  metal in the vicinity  of
                   welds is  a very  common example  of
                   this phenomenon.
                    2.5  FRETTING CORROSION
  Fretting corrosion is  a type  of cor-
rosion-erosion.  It is a combination, of
wear and the oxidation or other chemi-
cal reduction of the wear products and
the freshly exposed interfaces  of  the
metal.   While vibration is the most
common instigator of this type  of cor-
rosion, the action of  the flights along
the rails in the  bottom of  a clarifier
also typify this type of  action.  In this
case the wearing action of the metal
                   "shoes"  is complemented by the grit
                   included  in the sludge and the chemi-
                   cal  reduction  is accomplished by  the
                   many corrosive agents present in  the
                   wastewater.
                     Heat  is not a necessary factor  in
                   this  type of  corrosion although high
                   temperatures can accelerate the chemi-
                   cal  action  and, in  some  cases, even
                   prevent  the formation or  buildup  o£
                   protective corrosion products.
                     2.6 CAVITATION EROSION
  While this type of corrosion is gen-
erally  found on  pump impellers and
boat propellers  it  also can  occur  on
venturi tubes and jets or nozzles where
high liquid  velocities and sudden vio-
lent reductions of fluid pressures exist.
A severe pitting of  the surface may
develop  in  these areas even  though
the liquid is of an otherwise non-cor-
rosive  nature.
  Several theories have been evolved to
explain  this phenomenon.   Some  of
the most popular are as follows:
  1. The sudden violent  changes  in
fluid pressure cause unit distortion of
the surface  which assists in  the pene-
tration of oxygen or hydrogen into the
metals  lattice  structure  during mo-
ments  of high pressure. Molecules of
                   the gas combine and  literally explode
                   during  moments  of  reduced pressure
                   to blow off sections of the surface of
                   atomic thickness.
                     2. The penetration of the metals lat-
                   tice  structure in  the  above explained
                   manner results in the oxidation or hy-
                   drogenation of the metal and the sub-
                   sequent erosion of the corrosion prod-
                   ucts by the velocity of the liquid.
                     3. The  formation  of galvanic  cells
                   in the metal as a result  of the differ-
                   ential pressures in the liquid with the
                   subsequent  transfer  of  metallic ions
                   from the anodic area.  Once pitting has
                   started, a  further reduction in pressure
                   occurs in  these areas, thus accounting
                   for  the localized  nature of the cor-
                   rosion.
                     2.7 FILIFORM  CORROSION
   Whenever  a  metallic  surface  is
 coated with an organic coating  there
 is a possibility of "filiform corrosion."
 This  is  caused by pin-point  penetra-
 tion of  moisture through  the coating
 at numerous  points.   By  a combined
 chemical and  electrochemical process
 the corrosion  progresses laterally  in
 narrow  lines  resembling filaments be-
                   neath the coating.  The process is per-
                   petuated by the infiltration of oxygen
                   through the coating and  continues as
                   long as any moisture remains in the
                   "head" of the filament.
                      These filaments  never  cross  each
                   other   nor  themselves  because  the
                   polarity in the corrosion  products  re-
                   mains the same as that in the periphery

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                    PAINTS AND PROTECTIVE COATINGS
                                  15
of the "head"  of the filaments  and
like poles repel each other. The actual
reaction in the "head" of the filament
is galvanic and a groove is left in the
metal  where  the  metallic  ions  were
displaced  during   the  interval  that
point was acting as the anode in the
center of the  head.
               2.8 ELECTROCHEMICAL CORROSION
  While it may be  debatable  as to
whether the  previous  types  of  corro-
sion rightfully should be termed elec-
trochemical processes,  there  is  little
argument  regarding   the   following
types.   Here  the reaction is  galvanic
and involves  the formation  of cells
having  different electrical  potentials.
This induces the flow  of ions between
potentials resulting in  the disintegra-
tion of  the anode.  The formation of
these cells may involve two  or  more
metals,  different physical  properties
within  the same metal,  or  different
physical properties  within the electro-
lyte.  These essentials play a vital role
in  the  rate  at  which  the  electrical
current is generated  and passes from
the anode to  the cathode.   It  is not
only the conductivity  of the electrolyte
that affects the rate  of corrosion but
also its  pH, temperature, velocity, and
chemical  composition.   These factors
determine what corrosion products will
be  formed  and whether  or not they
will be dissolved or eroded away.  They
also may remain as a protective coat-
ing  or  increase  the  passivity of the
anode.  Thus, the process can  be either
continuous, accelerated, or  retarded.
In  an aerated electrolyte  the oxygen
ions can react with the anode just as
the hydrogen ions generally do in an
unaerated electrolyte.   The electrolyte
also may serve only  as a catalyst to
trigger  the reaction.

2.81 Bimetallic or   Galvanic  Cor-
     rosion
  This  type of corrosion involves two
or more metals being immersed in an
electrolyte.   Here  again  the electro-
lyte may be an aqueous  or  non-aque-
ous  solution such as water, earth, or
even atmosphere, or  gas as long as
moisture is present.  The type of cells
thus formed  are referred to  as "dis-
similar  electrode cells"  and the wide
range of conditions  under which  this
type of reaction will take place makes
it perhaps the most frequently encoun-
tered and most difficult to predict and
combat.
  There are various  terms used to  de-
scribe the tendency  of metals to enter
into this type of reaction.  The terms
most  commonly used   are  "electro-
motive  force"  (EMP),  "fluid  pres-
sure," "electrical potential," or  just
"potential."   The metals are listed in
the order in which they  tend  to react:
magnesium, aluminum, zinc, chromium,
iron, cadmium, nickel, tin, lead, hydro-
gen, copper, mercury, silver, platinum,
and gold.
  When any  two metals form such a
cell, it  is the  higher one on  the  list
that forms the  anode  and takes on a
negative polarity as a  result of  the
loss of  the   positively-charged  ions.
Conversely, the cathode is given a posi-
tive charge and an increase in density
as  a  result of  its  acquisition  of  the
ions.
  The rate at which this action takes
place is determined by a number of
factors  which may not be uniform  un-
der all conditions.    The  effect of
proximity  of  the two  metals and  the
conductivity of the electrolyte are con-
stant as is the effect of work or erosion.
An  increase  of  temperature  can  ac-
celerate the process  or actually retard
it by driving the oxygen from the elec-
trolyte which  may result in the forma-
tion of more protective corrosion prod-
ucts,   Raising or lowering  the pH of
the electrolyte can have the same effect.
In  aqueous solutions the rate  of corro-
sion is usually the  most rapid at or

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16
PAINTS AND PEOTECTIVE COATINGS
just below the water surface due to the
availability  of oxygen at  this point.
Not only do steel  tanks  corrode  in
this manner  on standing while partly
filled  with water,  but also steel-hulled
boats  are frequently  "cut off" at the
waterline when they  are left anchored
for a  long period  of  time without cor-
rosion protection.
  The extent of  the dissimilarity  of
two metals need not  be great in order
to set up  a "dissimilar electrode cell."
Impurities in the  metal  are a common
cause  and even the  difference  in the
composition  of the  cast iron used  in
the fabrication of pipe fittings by two
different companies  frequently  is ade-
quate to  cause corrosion in buried  or
submerged piping systems.  The use of
brass  or bronze valves with iron  pipe
is particularly conducive to this  type
of  action as is  the  use  of galvanized
and uncoated pipe.   It is  not a case
of one section being protected  and the
other  unprotected but actually  the ac-
celeration of corrosion by the  instiga-
tion of bimetallic corrosion.
  Pipe lines  buried  in the ground are
subject to a  number  of different types
of electrochemical attack as a result of
differences in the composition and tem-
perature of its environment.  Perhaps
the most  commonly  encountered form
is "the concentration cell." This can
be  due  to the difference in the chemi-
cal composition  of  the backfill ma-
terial around the pipe,  the difference
in the amount of  moisture  in the soil,
or  the difference in  salts dissolved  in
the water  in  saturated  conditions.
These  are commonly referred to  as
salt concentration cells and  also can
affect pipes  running through  a series
of  tanks  where evaporation, dilution,
or a bacteriological or chemical  process
has created  a non-uniform condition
in  the  various sections of the  same
tank.
  An equally common type of concen-
tration  cell  is the "differential aera-
tion cell" in which  case  the  amount
of  oxygen or other  gases dissolved in
                    the electrolyte are dissimilar.  This re-
                    sults in different potentials being cre-
                    ated  at various  points  with the  re-
                    sultant  flow of  current  from  those
                    points having the higher "potentials."
                    This   type  of  action  is  generally
                    "local" and results in pitting.
                      A  third  type of concentration cell
                    frequently   encountered  in  pipelines
                    is the "differential temperature  cell."
                    This  usually is caused by  introducing
                    hot liquids into  a  pipe and the subse-
                    quent cooling as it passes through the
                    pipe.  It also can be created in shallow
                    or exposed  pipelines that pass through
                    shaded and sunny  areas.  Different po-
                    tentials are created within the  metal
                    and either or both the liquid inside the
                    pipe  and the environment outside the
                    pipe  can act as  electrolytes.  Which
                    one  is  actually serving  can  be deter-
                    mined by which surface is pitted.
                      Any of these local types of corrosion
                    can  be serious because failure can  oc-
                    cur  at one point while the remainder
                    of the system is relatively unattacked.
                    When most of  a system is protected by
                    cathodic protection, or  especially  by
                    protective  coating, and  a  relatively
                    small area is exposed, the small area
                    generally becomes anodic and the cor-
                    rosion  rate is  accelerated greatly  by
                    the dissipation of the  current created
                    in  the  whole  system   discharging
                    through  the small unprotected  area.
                    When various conditions such as those
                    listed above, are known  in advance  of
                    installation  of  a pipeline, sections  of
                    non-conductive   pipe   are  sometimes
                    used to  separate  these  sections and
                    thus  disrupt the flow  of current.

                    2.82  Parting

                      When any alloy is  immersed  in an
                    electrolyte  a series of dissimilar elec-
                    trode  cells are formed  between  the
                    various metallic  components  of  the
                    alloy.   In many  eases  this action is
                    instigated deliberately  by selection and
                    quantity of the components in  order
                    to cause the  formation of  corrosion
                    products that will give the  metal a

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                    PAINTS AND PEOTECTIVE COATINGS
                                   17
passive or protective coating.  Some-
times,  however,  unexpected results oc-
cur when the corrosion products  are
soluble in the electrolyte or are eroded
by the velocity  of the electrolyte.  In
these   instances  the anode (which is
generally the lesser  percentage)  is re-
moved from  the alloy but leaving  the
cathodic portion intact.   The nature
of the  alloy can result in a number of
different forms.   When the base  metal
is ductile the removal of the second
metal may create the appearance of a
sponge or if the  percentage of  the
metal that was removed was originally
small,  the only  change visible to  the
naked  eye may be discoloration (as in
the case  of bronze) but a reduction of
ductility  and   strength  usually  has
taken place.
  In some cases, the  corrosion  products
tend to  coat the outer surface as  the
corrosion itself  penetrates   into  the
alloy.  "When the dissipation of molecu-
lar oxygen  or  hydrogen  is  impeded,
"layering"  occurs.  This  also may be
due to the granular structure of  the
alloy but in  either  case the  corrosion
progresses in planes  parallel to  the
surface and  the layers of uncorroded
metal  literally  are  lifted  off.  When
these layers are  thick the term "spall-
ing"  is  used to describe  the process
and when they are thin such  as flakes,
it is known as "defoliation."
  Two specific  types of parting have
been identified  separately  because of
the frequency  with which  they  are
encountered.   The first is  the removal
of zinc from brass leaving the copper
in its original form.  The zinc consti-
tuting  a relatively small percentage of
the alloy, leaves the  copper in  its
original  shape,  however,  the  strength
of the  copper is  reduced greatly by the
removal  of the zinc.  This is known as
'' dezincification.''
  The  other type of  parting, is just
the reverse of  dezincification in that
the metal constituting the major por-
tion  is reduced  and the  trace  metal
and corrosion products remain in  the
original shape.  This is called "graph-
itic corrosion" and occurs  when gray
cast iron is submerged in a  non-oxidiz-
ing1 acid environment.   The iron is re-
duced to the oxide and hydroxide salts
as well as some sulfates and chlorides
which in turn tend to  cement the par-
ticles of graphite remaining from  the
cast iron.  Where pressures are slight
and no movement occurs, a graphitized
pipe  may continue  to  carry  the flow
for months  or  even years after  the
corrosion has been completed.   The
black  coating  of graphitic corrosion
should not be confused with the soft
black coating that frequently  is found
inside steel or iron pipe that has been
carrying wastewater or sludge.   In
this case  the deposit  is usually  iron
sulfide, formed by the  reaction of iron
with  the H2S in the liquid being car-
ried.

2.83  Electrolysis or  Stray Current
      Corrosion

  A  type  of corrosion that  has  re-
ceived a great  deal of publicity and
causes millions  of dollars  in  damage
each year is '' electrolysis or stray cur-
rent  electrolysis."   This type of cor-
rosion, is of little concern to the aver-
age wastewater treatment plant. It is
caused by a  stray or external  current
of  electricity   passing through  the
ground or water in which  a  metal is
submerged.   Since the  metal is usually
a  better conductor  than the  environ-
ment, the current enters the metal at
a point nearest its source and leaves at
a point nearest its destination.  At the
point where  the current  leaves  the
metal (in the form of  metallic ions)
disintegration takes place.   This also
is known as the anode, but since  the
current is from an external source and
is  virtually  "pushed"  through  the
metal the  anodes  have   a   positive
polarity.
   The most  common location for this
type  of electrolysis  is  in pipelines
adjacent to streetcar tracks.   A cer-
tain amount  of  current "leaks" from.

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18
PAINTS AND PROTECTIVE COATINGS
the tracks  and takes the  path of least
resistance  (the pipelines)  on  its re-
turn trip to the powerhouse.  This is
direct  current  electricity  with  high
amperage and the corrosion rate could
be extremely high.   Since electrolysis
rarely  occurs with alternating current
(due  to its reciprocating nature) it
should be  of  little  concern  to waste-
water  treatment plant operation  and
                   maintenance.  About  the only place
                   direct current is still used is in the
                   starting and lighting systems of mobile
                   equipment and in electronics devices.
                   Careless operation of a stationary bat-
                   tery  charger might conceivably  give
                   some trouble but  since the two elec-
                   trodes are so close together it appears
                   unreasonable to suspect an interception
                   of any stray current would be possible.

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      3. FACTORS AFFECTING THE CHOICE  OF
                 CORROSION  PROTECTION
                            3.1 GENERAL
  The basic consideration in selecting
a method  of protecting equipment is
to obtain the most durable  protection
for the  least amount of money.   Re-
sistance  to  corrosion can be obtained
by  either  selecting  materials of con-
struction that resist  chemical attack in
the required service  environment such
as  stainless  steel,   copper  alloys,
aluminum, or plastics, or by applying
a protective coating  to the material in
the form of paint, plating, or galvaniz-
ing.
  There are  several good reasons to
pay more initially for a good level of
protection:
  1. The use of corrosion resistant al-
loys may be advantageous.
  2. If  plating  or  galvanizing  is
called for, the separate parts must be
processed  individually.   This is  not
practical after  the material has been
erected.
  3. Surface cleaning and preparation
for optimum bond between metal and
paint  can be  done more economically
and with better control at the factory.
  4. If  the surface is not  prepared
properly,  subsequent   field  painting
cannot adhere  properly.

  The cost of corrosion protection may
be resolved into capital cost and main-
tenance cost, which are interdependent,
i.e., the higher  the capital cost, the less
the maintenance cost.  It is not a sim-
ple problem to evaluate the relation-
ship between these two costs but sev-
eral factors to  be considered are:

  1. The absolute cost;
  2. The degree of protection required;
  3. Appearance;
  4. Ease of repainting;
  5. Design; and
  6, The cost of painting.
                     3.2 THE  ABSOLUTE COST
  This is simply the actual cost of the
equipment over its life expectancy.  It
is the sum  of the initial cost and the
cost  of the maintenance required  to
keep it in service. The environment in
which the equipment is to serve  must
be evaluated carefully  to  determine
the frequency  of repainting required.
This will vary from the very mild con-
ditions present  in the plant office and
maintenance shops where the air is dry
and  clean to the more  difficult condi-
tions present in the pumping stations
and  screen  wells where there is a  large
amount of moisture and hydrogen sul-
fide present.
  An extreme case of severe environ-
ment is pump impellers. Here  the de-
terioration  is the result of both cor-
rosion  and  erosion.   Pumps  are se-
lected  with great care because  their
power  consumption constitutes  an im-
portant cost item in the operation of a
plant.  To  maintain their original op-
erating efficiency for a reasonable pe-
riod of time, the impeller should be of
the best material  available for this
service.
            3.3 DEGREE  OF PROTECTION REQUIRED
  This factor takes into account the
importance of keeping the  equipment
in service.  A good example of this is
the protection of mounting bolts for-
                                   19

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20
                    PAINTS AND PEOTECTIVE COATINGS
settling tank weir  plates.   Failure  of
these small items would take a major
component  out  of  service  and  drasti-
cally reduce the efficiency  of  the  en-
tire plant.   Obviously, corrosion  re-
sistant alloy bolts are justified.  Other
items in this category are buried  elec-
trical conduit and submerged gas en-
gine cooling piping.
                           3.4 APPEARANCE
  Surfaces that are in full view must
be maintained in near perfect condi-
tion  to present an  attractive  appear-
ance.  This will require  much greater
attention  than cases where the struc-
is important.   The  use  of galvanized
surfaces,  painting,  and  corrosion re-
sistant  alloys may  well be justified.
The surface should be  able to  with-
stand repeated  washing with  various
tural integrity of the metal is all that   detergents without deteriorating.
                  3.5  THE EASE  OF REPAINTING
  In many  instances,  the surface un-
der consideration may be virtually in-
accessable.   Good examples of this are
the annular space between a tank and
the lift of a gas holder and  the space
between the roof and  the bottom  of a
floating digester  cover.  The extreme
difficulty  of  repainting  these areas
greatly increases repainting  cost  and,
therefore, justifies a much higher level
of initial protection than would be re-
quired for readily  accessible surfaces.
If the structure is large  the use of
alloys  or  galvanizing would be  pro-
hibitively expensive.   The best choice
is  to supervise carefully the cleaning,
preparation, and painting of the steel
surfaces at the factory or during erec-
tion so  that the best possible chemical
bond between  the protective  coating
and the metal surface is achieved.   It
also is  important  to  select the best
paint for the service intended, but this
is  of little value if improperly applied.
The cost of  this  supervision  is,   of
course,  part  of the total  cost of  the
equipment.
                               3.6 DESIGN
  Steel with sharp edges requires more
frequent repainting than smooth sur-
faces.  Equipment  can  be designed  to
avoid sharp  edges and inaccessable
areas.  In submerged structures struc-
tural shapes  should be  replaced by
tubular members where possible. These
tubes should be sealed to prevent cor-
rosion of the interior surfaces.
  Conventional protection on gratings
for walkways is quite satisfactory for
building interiors but is not adequate
in exterior applications, particularly
where  subjected  to hydrogen sulfide
and  moisture.  After corrosion starts,
they cannot be protected properly be-
cause of the multiplicity of sharp edges
and  inaccessible   corners.   Gratings
should be  used only where  necessary
and  should be  of non-corrosive alloys,
removable  precast  concrete  slabs, or
combinations  of  non-corrosive alloys
for support and steel floor plates which
can be repainted easily.

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            4.  PREVENTION  OF  CORROSION
  There  are many  ways  to  prevent
corrosion; there are also  ways of re-
tarding or diminishing its damaging
effects.  The choice  of methods to  be
used is determined by the  conditions
encountered and the economic advan-
tage to  be  gained  by  using  such
methods.
  The  choice of materials is very im-
portant.   By substituting  a more re-
sistant  material  for one  previously
used the  service life of  a structure
some  times can be increased many-
fold.  Thus, a thorough knowledge  of
the characteristics of materials of con-
struction is important.
  Changing the environment so it is no
longer  corrosive  is   a  much  used
method.  This  includes removing  the
corrodent or  providing a  protective
coating.  In either  case the corrodent
is  kept from  making  contact with  the
material  to cause corrosion.   By keep-
ing corrosion prevention in mind dur-
ing the designing and construction pe-
riods  many  difficult  corrosion  prob-
lems can be avoided.
                    4.1  CHOICE OF  MATERIALS
4.101 Oast Iron
  Cast iron, corrodes at about the same
rate as steel  under similar  conditions
but, because of its increased thickness,
and sometimes its surface inclusion of
sand from  the mold, it stands up well
in some corrosive environments.  The
rust coating on cast iron is dense, com-
pact, and adherent when  compared to
steel.  Once formed it tends to retard
further corrosion over a long period
of  time.   Numerous examples of this
are available in cast iron water and gas
mains, cast iron  road signs,  cast iron
sprockets  on sludge-collecting mecha-
nisms, etc.  Cast iron also resists corro-
sion at higher temperatures, as are ex-
perienced in furnace grates  and doors
and incinerator parts.  In these loca-
tions the temperature is usually  under
1,000'F (536°C).
  Grey cast iron is subject  to graphi-
tization when  immersed in salt water,
acid mine waters, or buried  under-
ground  in some  soils,  particularly
those containing sulfates.  It occurs
over a period  of time as a result of
the ferrite in  the east iron  dissolving,
leaving the graphite intact.   This con-
dition results in porosity of the struc-
ture and loss  of  density  and  some
mechanical strength, but without out-
ward  appearance  of  any  damage.
White  cast iron is immune.

4.102 Malleable Iron
  Malleable iron has similar corrosion-
resisting properties to  cast iron and
for that reason it  is used for many
things,  including  pipe fittings and
chain links  on  sludge- and grease-col-
lecting mechanisms. It has the added
advantage  of  being less  brittle than
cast iron and able to withstand greater
shock   and  impact   loads  without
failure.

4.103 Wrought Iron and Low Al-
      loy  Steels

  Although controversy once  existed
as  to the relative corrosion resistance
of wrought  iron and low-carbon steel,
it now is recognized that in soils and
natural waters, their  inherent corro-
sion rates are  similar.   The composi-
tion of iron or steel within the usual
commercial  limits of carbon and low-
alloy steels  has no practical effect  on
                                    21

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 22
                    PAINTS AND PROTECTIVE COATINGS
 the corrosion rate in natural water or
 soils.  Only when steel  is alloyed in
 the  proportions of  a  stainless  steel
 (> 12 percent  Cr)  or a high  silicon-
 iron  or  high  nickel-iron  alloy,  for
 which 02  diffusion no longer controls
 the  rate, is corrosion  reduced appre-
 ciably.
   For atmospheric exposures, the situ-
 ation is  changed because the addition
 of certain elements in small  amounts,
 e.g., 0.1-1 percent Cr,  Cu, or Ni, have
 a marked effect on the  protective qual-
 ity  of  naturally-formed rust  films.
 The rust film which forms on the sur-
 face is more  dense and adherent and
 slows down the corrosion attack.  It
 must be  remembered  that different at-
 mospheric exposures  cause a marked
 difference  in  corrosivity, even  of  low
 alloyed steels.

4.104  Copper  and  Copper Alloys
   Copper  is a metal  widely  used be-
 cause of  good corrosion resistance com-
bined with mechanical workability, ex-
 cellent electrical and thermal conduc-
tivity,  and   ease  of  soldering  and
brazing.
   Copper  and its  alloys have a low
position  in  the electromotive series.
Therefore, as would be expected, they
are excellent  corrosion-resistant mate-
rials in many environments.   The cor-
rosion resistance of these metals is due
to the formation of a protective coat-
ing  on  their  surface  and  the  very
slight  tendency of the metal to dis-
solve in  most aqueous solutions.
  Copper  exposed  to the atmosphere
slowly develops a green coating called
a patina.  This thin protective coating
consists  of  basic copper  sulphate, ex-
cept at the seashore where it contains
some  copper oxychloride.
  Where there  is no  oxidizing  agent
present,  copper has very good resist-
ance to corrosion by hydrochloric and
cold   dilute  sulfuric   acids;  also,  to
non-oxidizing salt solutions and other
fluids which quickly corrode iron and
steel.  For this reason, it  is a good
material to use inside a vacuum filter
to convey  the filtrate from the sludge
cake to the head valve because of the
hydrochloric acid used in cleaning the
filter  surfaces.
  Copper and  the  brasses (Cu-Zn al-
loys)  do  not resist  hydrogen  sulfide
and will  form a  black  discoloration
rapidly when it is present.
  Copper is resistant to seawater, the
corrosion  rate   being  about  0.001  to
0.002  in./yr  (0.05  cm/yr)   in  quiet
water and  somewhat higher in moving
water.  It is one of  the very few metals
which remains free of fouling organ-
isms,  normal corrosion being  sufficient
to release  Cu  ions in concentrations
which poison marine life.
  Copper is sensitive to  corrosion by
high velocity water and aqueous solu-
tions,  called impingement attack.  The
rate increases with DO content, where-
as in  oxygen-free  high velocity water
up  to at least  25 fps (762.5  cm/sec),
impingement attack is either  small  or
zero.
  In summary,  copper is resistant to:

  1. Seawater.
  2. Fresh water, hot or cold.  Copper
    is suited especially to convey soft
    waters high in  DO, low in car-
    bonic  acid  and manganese salts.
  3. Deaerated, hot or cold, dilute sul-
    furic acid, phosphoric acid, acetic
    acid,  and  other   non-oxidizing
    acids.
  4. Atmospheric corrosion.

  Copper is not resistant to:
  1. Oxidizing  acids, e.g., nitric, hot
    concentrated sulfuric and aerated
    non-oxidizing  acids   (including
    carbonic acid).
  2. Ammonium hydroxide  (plus oxy-
    gen).   Substituted ammonia com-
    pounds  (amines) also  are corro-
    sive.   These compounds are the
    ones  that  cause stress  corrosion
    cracking of susceptible copper al-
    loys.
  3. High velocity aerated waters and
    aqueous solutions.  In  corrosive

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                    PAINTS AND PROTECTIVE  COATINGS
                                                                     23
waters (high in 02 and CO2, low
      +"  and Mgf+) the  velocity
     n
    should  be  kept below 4 fps (122
    cm/sec) ; in less corrosive waters
    greater  than  150 °F  (65°C)  be-
    low 8 fps  (244 cu m/sec).
  4. Oxidizing  heavy metal salts, e.g.,
    FeCl3,  Fe2(S04)3.
  5. H2S sulfur and some sulfur com-
    pounds.
  Copper forms  useful  alloys  with
many metals to increase  its strength,
machineability,  and  corrosion  resist-
ance to  different  media.   In general,
alloys  of copper  and zinc are called
brasses,  and  alloys  with  aluminum,
silicon,  tin,  and some other metals are
called bronzes.
  Commercial  brasses contain zinc in
amounts varying from 5 to 45 percent.
Brasses  are  readily  machineable and
the compositions can be varied to give
a wide  range of physical  properties.
  Although brass is resistant to  many
types of corrosion, a brass which is in
an internally strained condition  has a
tendency to develop cracks along the
grain boundaries  when subject  to  at-
tacks by corrosive agents, even those in
the  atmosphere.   This  condition  is
called season cracking or stress  corro-
sion cracking.  Season cracking seldom
occurs  in brasses  containing less than
15 percent zinc.
  One  of  the major corrosion  proc-
esses  of the  Cu-Zn alloys  (brasses)
is dezincification.   As the  name  im-
plies, zinc is lost  from the alloy, leav-
ing as  a residue,  or by  a process of
redeposition, a porous mass of copper
having  little   mechanical   strength.
Soft  waters especially,  may  lead  to
corrosion failures from  localized  de-
zincification of the brasses containing
much zinc, such  as  Muntz  metal  (60
percent Cu, 40  percent  Zn), non-in-
hibited  aluminum brass  (76 percent
Cu, 22 percent Zn, 2 percent Al), and
yellow brass  (67 percent Cu, 33  per-
cent  Zn) containing no dezincification
inhibitor.   Bed brass (85 percent Cu,
15  percent Zn)  and other alloys  con-
taining less than  15 percent Zn  gen-
erally  resist dezincification, which ex-
plains  their widespread  use  as piping
materials.
  The  addition of tin or arsenic (also
antimony  and  phosphorous)  to  the
brasses containing more  than  15 per-
cent zinc usually  is  quite effective in
slowing up or inhibiting the dezincifi-
cation  action  in  fresh  and seawater.
A few examples are  admiralty  metal
(1 percent tin), naval brass (f percent
tin), arsenical aluminum brass  (0.04
percent arsenic), and arsenical Muntz
metal  (^  percent  arsenic).   These are
appreciably more  resistant than  the
Cu-Zn alloys  free of  the  inhibiting
alloy additions.
  The  bronzes  are more costly  than
brasses, but have  compensating advan-
tages.   Bronzes are not subject  to a
type of corrosion  analogous to dezinci-
fication,  by which  one  constituent  is
removed.  They  also, as a rule, are
stronger than the brasses.
  The  commercial  copper-tin bronzes
contain 12 percent or less tin.  They
have higher strength and hardness, are
more resistant to  impingement attack,
and yet have  the  same high resistance
to corrosion as  copper.
  Aluminum  bronze is an  alloy  of
copper and  aluminum  containing 10
percent or less aluminum.  Mechanical
properties of aluminum bronzes,  par-
ticularly   resistant  to   wear,  exceed
those  of  the  copper-tin bronzes,  and
their  resistance to corrosion is better,
especially at high temperatures.   They
are  the most resistant  of  any bronze
to hydrogen sulfide and acids.
   Silicon  bronzes  usually contain one
to four percent silicon and in addition,
small  amounts of either  iron,  man-
ganese,  tin,  or zinc.  Typical silicon
bronzes are those sold under the  trade
name  of Everdur,  Type A.   Everdur
has  a  composition: Copper, 96 percent,
 silicon,  3 percent, and  manganese, 1
 percent.   Silicon  bronzes  as a  class,
 have  good mechanical  properties, are
 readily  weldable  and are  resistant to

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24
PAINTS AND PROTECTIVE COATINGS
corrosive  compounds, particularly, hy-
drochloric and  sulfuric acid,  alkalies,
and certain organic compounds.  These
alloys   are  used  for   gates,  valves,
screens, wire,  ladders, bolts, and other
structural parts in corrosive  environ-
ments.

4.105  Stainless Steel
  The  stainless  steels are  metal alloys
that resist  corrosion to a remarkable
degree.   This  resistance  to  corrosion
is due  to  a property of passivity being
induced in the steel by the addition of
chromium and nickel as alloys.  Steels
containing less than 11.5 percent chro-
mium  usually  are not  classified  as
stainless steel.
  There  are  three  main classes  of
stainless  steels  designated  in accord
with   their  metallurgical  structure.
Each class  contains many types, each
of which  has somewhat differing alloy
compositions,   but  related  physical,
magnetic, and corrosion properties.

  1. Chromium  (11.5  to  17 percent)
     -iron alloys  with   carefully con-
     trolled  carbon  content.   These
     may be hardened  by proper heat
     treatment  to  a martensite  struc-
     ture which  is magnetic.  They
     are,  therefore, known as marten-
     sitic stainless steels.  Typical ap-
     plications  include  cutlery,  steam
     turbine blades, and  tools.   The
     A.I.S.I. type  numbers are in the
     400 series.
  2. Chromium  (17 to  27  percent)
     -iron alloys with low carbon con-
     tent.  They cannot  be  hardened
     by  heat treatment,  but can  be
     hardened somewhat by cold work-
     ing.   Their  crystal  structure  is
     essentially ferrite, which is  also
     magnetic.   They  are called  fer-
     ritic stainless steels.  Their  at-
     mospheric  corrosion  resistance  is
     superior to that of the martensite
     class.  Uses include trim for auto-
     mobiles and as a  major  material
     of construction for  synthetic  ni-
                        tric  acid  plants.   The  A.I.S.I.
                        type numbers  are  in  the  400
                        series.
                      3. Chromium  (16  to  26  percent)
                        -nickel  (6 to 22 percent) -iron al-
                        loys  with  low  carbon   content.
                        They are not hardenable by heat
                        treatment.   Since  they  have  a
                        crystal  structure of non-magnetic
                        austenite, this class  is called the
                        austenite  stainless  steels.    The
                        basic composition  in this class is
                        the  18  Cr- 8  Ni  alloy  which is
                        the most popular of all the stain-
                        less steels  produced.
                        The nickel content contributes to
                        improved corrosion resistance and
                        is  responsible for the  retention
                        of  the  austenitic  structure.   The
                        austenitic class of  steel is notable
                        for  its  ductility.
                        Some uses  of austenite stainless
                        steels include general purpose ap-
                        plications, architectural and auto-
                        mobile  trim,  and  various  struc-
                        tural units for the  food and chem-
                        ical industries.  The A.I.S.I. type
                        numbers are in the  200 and 300
                        series.
                        The highest  general  corrosion re-
                        sistance  is  obtained   with  the
                        nickel-bearing  austenitic  types,
                        and in  general, the highest nickel
                        composition  alloys in  this  class
                        are  more resistant than the low-
                        est.  All  grades,  however,  have
                        the  same general resistance char-
                        acteristics.   For  example,  they
                        are all resistant to most concen-
                        trations of  nitric acid,  but the
                        austenite grades usually show the
                        least attack.
                        All the stainless steels have good
                        resistance to alkaline solution and
                        most organic acids,  but are not
                        resistant to  halides  (Br,  Cl, F)
                        in  any form,  seawater  (unless
                        cathodic protection is  used), oxi-
                        dizing  chlorides,  and  some or-
                        ganic acids,
                        Some stainless  steels are  subject
                        to  pitting and iutergranular cor-

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                    PAINTS AND PKOTECTIVE COATINGS
                                                                          25
     rosion.   By  the   selection   of
     proper  alloys,  the  correct heat
     treatment,  and the  exclusion  of
     certain chemicals, these faults can
     be overcome.

  Passivity is a state  of  electrochemi-
cal activity  which is  attained under
suitable  conditions  by  certain  base
metals, notably iron, nickel, chromium,
and by some of their alloys. Its nature
has been the subject of lively and con-
tinuing1  disagreement  for which   we
have no  room here.  It is sufficient to
state that  it is  a surface  phenomenon
between  the metal  and  an  oxidizing
agent  that creates  a  barrier to cor-
rosion.
   Maintenance  of passivity  requires
the  continuous replenishment of  the
oxidizing agent.  As an  example,  DO
in  seawater is  sufficient  to  maintain
passivity on clean surfaces; however,
the  metal  becomes active beneath a
barnacle or in a crevice, since the rate
of  oxygen  replenishment is too slow
to  maintain passivity,  and  corrosion
occurs.

4.106  Nickel and  High Nickel Al-
       loys
   Nickel is a very important metal  for
resisting corrosion when used by itself
or as  high nickel alloy  with other
metals.
   Nickel is a  white,  malleable, non-
corrodible metal,  having high strength,
relatively  high heat conductivity, and
good heat resisting properties.  These
characteristics  make  nickel  desirable
for many  uses  where other metals  are
not suitable.
   The  high nickel  alloys,  i.e.,  the
nickel-base  alloys   containing  more
 than 50 percent  nickel, are  in a class
 by themselves since they have physical
 and mechanical  properties  not dupli-
 cated  readily  by  other base  alloys.
 These alloys are tougher,  stronger,  and
 harder than copper and aluminum al-
 loys,  and are as strong  as  alloy steel.
 They  are  highly resistant to corrosion
by most of the normal and special cor-
roding agents found in industries, and
they resist oxidation and scaling at ele-
vated  temperatures.   All  the   high
nickel alloys are characterized by ex-
ceptionally high  corrosion  and  heat
resistance, good strength, toughness,
and high  ratios of strength to ductility
in  all conditions of  mechanical and
thermal treatments.
  The high  nickel  alloys  have  been
classified  into six main groups accord-
ing to their composition.  Most of these
alloys have  proprietary trade names
which are used generally to  identify
them.

   Group  I,  Nickel—93.5  to 99.5 per-
     cent  nickel (and  a maximum of
     4.5  percent  manganese).   There
     are  five specific grades  of  com-
     mercial   nickel;  namely,   "A"
     nickel, "D" nickel,  "E" nickel,
     "L"  nickel, and "Z" nickel.

   "A" nickel is the base material and
 is a  commercially pure, malleable ma-
 terial having  an average  nickel con-
 tent of 99.4 percent.  The other grades
 contain small  amounts of alloying ele-
 ments that alter the properties slightly
 for specific  purposes.
   Nickel  combines  excellent  mechani-
 cal properties with good  corrosion re-
 sistance.   It resists hydrogen  chloride,
 chlorine,  caustic soda,  oxidation, and
 scaling, and retains its strength at both
 high and  low  temperatures.  It is free
 from stress corrosion in atmospheric
 conditions.

   Group  11,  Nickel-Copper—63  per-
      cent to  70 percent nickel, 29 to 30
      percent  copper.   These  are  the
      so-called  monel type alloys,  all of
      which were developed  by the In-
      ternational Nickel  Company.

    Monel is the most important alloy in
 this group.   It is more resistant than
 nickel in reducing conditions  and  is
 more resistant than  copper in  oxidiz-
 ing conditions.  As a net result, it  is

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26
PAINTS AND PROTECTIVE COATINGS
in general more resistant to corrosion
than  either of its principal  constitu-
ents.   Monel  metal often is used  in
the form of wire mesh behind the filter
cloth  on vacuum  filters.  Where  lime
and ferric chloride are used as condi-
tioning chemicals, the mesh  has to  be
cleaned periodically of a deposit which
closes the holes in the mesh.   Muriatic
acid  (hydrochloric), containing an  in-
hibitor, usually is used to clean this
mesh.
  Monel metal finds  its greatest use-
fulness  in seawater  involving  high
velocities as in the case of pump shafts,
impellers,  and piping.  It is not  re-
sistant to nitric  acid, sulfurous  acid,
or ferric chloride, except in dilute solu-
tions.

   Group  III,  Nickel-Silicon—85 per-
     cent  nickel,  10  percent  silicon.
     The trade name of the best known
     of these alloys is Hastelloy D.
   This metal is strong,  tough, and  ex-
tremely hard.  It has properties simi-
lar to a high grade cast  iron and is  not
workable.  Because of  its high hard-
ness, about 360 Brinell, it can be ma-
 chined only with difficulty, and must
 be finished by   grinding.   Its  chief
 characteristic is its exceptional resist-
 ance to corrosion in  hot or cold  sul-
 furic acid, acetic  acid, formic acid,  and
 phosphoric acid.   However, it is  not
 resistant to strong oxidizing acids.
   These  alloys are sometimes  used as
 pump  and  valve parts  where  other
 corrosion  resistant materials are  not
 strong or tough  enough.

   Group IV, Nickel-Chromium-Iron—•
      54 to 78.5 percent nickel, 12 to 18
      percent  chromium, 6 to  28   per-
      cent iron.
    The most common alloy in this group
 is called Inconel. It combines the in-
 herent  corrosion resistance,  strength,
 and toughness of nickel with the extra
 resistance  to atmospheric   and  high
 temperature oxidation that is imparted
                   by chromium.   It resists the attack of
                   many corrosive chemicals, but its chief
                   attribute is  exceptional  corrosion  re-
                   sistance at high temperatures.   Also,
                   the ability to withstand repeated heat-
                   ing and cooling.

                      Group V,  Nickcl-Molybdenum-Iron
                        —55 to 62 percent nickel, 17 to 32
                        percent Molybdenum, 6 to 22 per-
                        cent iron.

                      Two  well known alloys fall into this
                    group,  Hastelloy A  and  Hastelloy B.
                    They are characterized  by  their high
                    resistance to corrosion in hydrochloric
                    acid  and wet  hydrogen  chloride gas.
                    They are expensive and would be used
                    only  in exceptional  cases of  corrosive
                    exposures.

                      Group VI,  Nickel-Chromium-Molyl)-
                         denum-Iron—51  to 62  percent
                         nickel, 15 to 22  percent chromium,
                         5 to 19 percent molybdenum, and
                         6 to 8 percent  iron.

                      Hastelloy C and Illium G are in this
                    group.  They  are especially character-
                    ized  by their  high corrosion resistance
                    to oxidizing acids and mixtures, such
                    as nitric, chromic,  and  sulfuric acids,
                    copper  sulfate, etc.   They  are rather
                    hard  alloys  and   difficult  to  work.
                    They have high resistance  to thermal
                    shock.
                      These alloys are used when a strong
                    alloy is required that will resist strong
                    oxidizing acids and oxidizing agents
                    such as free chlorine, bleaching agents,
                    and the like.   They  are used for pump
                    and valve  parts, spray nozzles,  and
                    piping.

                    4.107  Silicon  Oast Iron
                       Silicon alloyed  with  iron imparts
                    corrosion resistance to  a  variety of
                    chemical media, in particular,  strong
                    non-oxidizing acids.   The  alloys are
                    brittle  and are, therefore,  sensitive to
                    fracture by  thermal  shock or by im-
                    pact.   These  alloys are available only
                     as  castings,  and usually  any subse-

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                    PAINTS AND PROTECTIVE COATINGS
                                  27
quent  forming  must  be   done  by
grinding.
  Optimum  resistance  for  minimum
silicon content is about 14,5 percent,
and  this  is  the  composition  of  the
commercial  alloy.   Duriron,  Corros-
iron, and Tantiron are the trade names
o£ the most prominent  ones available.
Duriron has  been used  successfully in
wastewater treatment plants  for  pip-
ing to convey waste chemicals.
  Ducichlor is a  modified type o£ 14.5
percent silicon cast  iron.  It contains
about three percent molybdenum and
has better resistance to  hydrochloric
acid than  Duriron.  Durimet  20 re-
sists hot  sulfuric acid  up  to  180°P
(82°C),  thus, it is appropriate  for
handling  hot pickling  liquor  that is
being used as a sludge  coagulant.
  Other  uses for this  class of alloys
are centrifugal pumps,  valves,  ejectors
for chlorine mixing, spray nozzles, and
agitators.

4,108  Aluminum

  Aluminum has a number  of valuable
properties. Its lightness, Q the weight
of steel)  and  high  strength-to-weight
ratio  allow significant  weight reduc-
tions in engineered products.  The cor-
rosion resistance imparted  to alumi-
num  by  the  stable oxide  coat  that
forms  in  air or  under special  treat-
ment  (anodizing)  gives  it a  tremen-
dous  advantage  over other  structural
metals.
  The properties of aluminum can be
varied by  alloying,  heat treating, and
cold working.  In  actual  practice,  a
good  knowledge  of the nomenclature
and characteristics of each of the many
aluminum  alloys and tempers is neces-
sary to take advantage  of the metal as
a construction material.
  Hydrogen  sulfide, methane, and car-
bon dioxide have little  or no effect on
aluminum.   Sulfur dioxide as a gas
does not  attack  aluminum, but when
oxidized to sulfuric  acid, a slight  at-
tack is noticeable, which increases with
an increase in  concentration and tem-
perature.   Aluminum  is  satisfactory
for distilled  water or soft water  that
does not contain heavy metal ions such
as copper, iron, etc.  It has excellent
resistance to rural, urban, and indus-
trial atmospheric exposures with lesser
resistance  to  a   marine  atmosphere.
Lime and fresh concrete are corrosive
as well as other  strong alkalies.  The
corrosion  rate  in  concrete is reduced
when the  cement sets  and  continues
only  if the concrete  is kept moist  or
contains deliquescent salts, e.g., CaCla.
  Aluminum  is  used  at  wastewater
treatment  plants  for doors, windows,
sash, floor  plates,  gratings,  ladders,
hatch covers, etc., where lightness and
freedom from  painting are important
considerations.
  Aluminum  is high in  the  galvanic
series of metals so one must be careful
when  coupling it with other metals.
Cadmium,  zinc,   and  magnesium  in
most cases can be coupled without suf-
fering high corrosion rates; but there
are  special cases for zinc and mag-
nesium in alkalies  and seawater, re-
spectively. With steel and  other metals
below aluminum in the galvanic series,
special  care must be taken in  making
couplings  to  avoid  accelerated corro-
sion.

4.109 Elastomers
  Elastomers are  the commonly called
rubbers such as  natural rubber,  neo-
prene,  isoprene,  butyl,  etc.   Natural
rubber has been  used  in  the  past to
protect steel piping from  chlorine so-
lutions,  ferric  chloride,  and  other
chemicals.  Plastic pipe is being sub-
stituted for the  elastomers in many
of these applications.  Where  rubber
linings were previously used to protect
pump impellers  and casings,  as  well
as   fans,   and   appurtenances,   the
plastics are now competing to replace
them.  One   of   the   big   uses  of
elastomers  today  is  as  sealants  or
gaskets in pipe joints.  Neoprene, be-

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28
PAINTS AND PROTECTIVE COATINGS
cause of its resistance  to greases and
petroleum products, as well as oxida-
tion, is good for this purpose.

4.110 Plastics

  The term  plastics  covers  a  largo
group of materials.  In general, there
are two  defined  classes or groups  of
plastics:  thermoplastics—those plastics
which can be heated to a plastic state
and  molded, then heated  again and
remolded.   Examples  of  this group
are  the  vinyl  family  (PVC,  PVA,
vinyl,  vinylidene chloride),  acrylics,
Polyamides    (nylon),   polystyrenes,
polyethylenes,  and  polyterafluorethyl-
enes.
  In the second group, called thermo-
setting,   a  chemical  change   occurs,
usually at the  time of application  of
heat  and  pressure during  forming.
As a result  of this chemical change,
the material  does not soften on subse-
quent  heating;  but chars or is  de-
stroyed  if the heating is  carried  to
excess.   Examples of  this group  arc
phenolics, polyesters,  ainino-formalde-
hyde, and  epoxies.
   Plastic pipe  resists  most   of  the
chemicals foimd  in wastewaters and
used  in  the treatment  process.  These
chemicals  include   ferric   chloride,
ferric and  ferrous  sulfate,  sulfuric
acid, hydrochloric acid, and chlorine.
   Plastic pipe does not corrode and
form tubereulations like steel and cast
iron pipe,  thus, they maintain a good
value  or smoothness  factor.   Plastic
pipe  (PVE)   is used  in  chlorination
systems  and in buried pipe  sytsems,
such as for irrigation pipe and sprink-
lers.
   Reinforced  polyester pipe  is used
for  submerged  aerators  in  aeration
systems.  Reinforced  polyester sheets
are   used  in  construction  to  cover
tanks and as windows.  They are trans-
lucent,  but  do not  break  as easily
as glass and are  lighter in weight.
   Plasticized PVC  is  used as a lining
to protect concrete structures and pipe
                   from  corrosion  in  atmospheres  con
                   taining hydrogen sulflde gas.
                      One of the major disadvantages of
                   plastics  is loss  of  strength  and form
                   at  high temperatures.   The  thermo-
                   plastics  normally are not used  above
                   150°F  (65.5°C), while some  thermo-
                   setting plastics  can handle some  ap-
                   plications as high  as  300°F  (149°C).
                   Other disadvantages are  high thermal
                   coefficient   of   expansion   (thermo-
                   plastics), low   strength  compared to
                   metals,  and  costs.
                      The scope of plastics is ever-widen-
                   ing as  they  are being fabricated  into
                   pump  impellers  and casings,  fans,
                   structural members, and even fasten-
                   ers, such as bolts and nuts as substi-
                   tutes  for metals   and corrosive  ex-
                   posures.
                      The substitution  of plastic for glass
                   in laboratory equipment has been ex-
                   tensive due to their light weight,  cor-
                   rosion resistance, and resiliency.

                   4.111 Ceramics, Glass,  and Vitri-
                          fied  Clay Products

                      Vitrified clay  products, such as  clay
                   pipe, are one of the most corrosion re-
                   sistant materials used in construction.
                    They are  resistant  to  moisture or
                   chemicals in the soi], strong  domestic
                   wastes,  industrial  wastes,  and acids
                    formed  by  oxidation  of hydrogen sul-
                   fide in  the sewer.
                      Vitrified  clay pipe will carry every
                    known  chemical, in any state of  con-
                    centration, without  being harmed.  The
                    only exception  is hydrofluoric acid.
                      Vitrified   clay products  are made
                    from blended  clays mixed  and shaped
                    under  pressure.    After  a period of
                    drying  they  are   burned  at  a  tem-
                    perature of about  2,000"F  (1,090°C).
                    This  burning  fuses  the particles of
                    clay together  into  a  strong, chemical-
                    proof bond.
                       The major  weakness in the  use of
                    clay pipe  in  the  past  has been  the
                    materials used to join them.    This
                    deficiency has  been overcome recently

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                    PAINTS AND PROTECTIVE COATINGS
                                   29
with  the  use  of  pre-formed  plastic
joints.  Also, clay pipe at the present
time is limited in size to about 42 in.
(1.05 m) in diam.
  Vitrified  clay  tile liner  plates cast
in  place have  been used for  many
years   to    protect  large   concrete
sewers  and  structures  from  acid at-
tack.   The major  deficiency  in their
use has been the jointing material used
between  the  plates  and the permeabil-
ity of the plates to corrodents.  The
clay  products   manufacturers   have
made advances in attempting to over-
come  the problems of jointing and of
permeability; however,  both  still re-
main  as problems.   In an acid cor-
rosion environment, the failure  of any
of the components making up the clay
tile system will lead to the loss of the
tile protective cover.  Inasmuch as clay
tile is a brittle  material,  any  chemi-
cal reaction of  the  concrete backing
will result in expansion and the break-
ing of the anchoring lugs thus causing
the tile to be displaced.  Tile also have
been  broken loose  by  the expansive
action  of   reactive  aggregates   with
Portland cement  alkalies.
  Bituminous joints are emulsified and
dissolved by soaps,  oils, and greases
in the wastewater.   Sulfur joints do
not adhere well  to the  clay  and  are
attacked  by sulfur  bacteria.   Acid
proof  cement joints appear  to  offer
good protection, but they are costly.
  Glass-lined steel  tanks are  used for
the handling and storing  of corrosive
chemicals.   Recently, glass-lined  steel
vent stacks have  been used where cor-
rosive vapors were  being  conveyed.

4.112  Concrete

  Portland cement concrete is one  of
the most widely used materials of con-
struction   for  wastewater  collection
systems and  treatment plants.  Its use
in  large  diameter  pipe,  tanks,  and
structures  indicates its superiority  in
corrosion resistance  and  economy  to
most  other  materials.   Its resistance
to  natural  atmospheric  corrosions is
excellent. Coatings generally are used
to  cover it  for  decorative  purposes
only.  In the  flowing  wastewater, in
digesters, aeration, and settling tanks
containing domestic wastewater, it has
an  indefinite  life  with no protection
required.  In climates  where freezing
weather is experienced,  concrete is sub-
ject to freeze and thaw damage.  Gen-
erally, this type  of damage can  be
avoided  with the  use  of admixtures
and other devices as well  as the proper
mixing,  placement, and curing of the
concrete.  Steel incased  in concrete, be-
ing in an alkaline environment, is well
protected against  corrosion.   Cement
mortar is used extensively to coat steel
and cast iron  pipes  both inside  and
outside to protect them from  corro-
sion.
  Asbestos cement is a material used
extensively in pipe, roofing, and siding
for  construction.   It  is  composed of
Portland cement, fine silica sand,  and
asbestos  fiber mixed  with water  and
then formed  under pressure into pipe
and sheets.  It is usually steam cured.
Asbestos cement pipe  and  sheets are
hard, dense, arid resistant to  oxidizing
and weathering  conditions,  the same
as Portland cement concrete.
  Portland cement products  are sub-
ject to  severe  corrosion in  an  acid
environment.  Therefore, in  wastewa-
ter  systems that contain low pH in-
dustrial  wastes  or  generate quantities
of hydrogen  sulfide under conditions
where  it  will  be  converted to sulfurous
and sulfuric  acid, Portland  cement
should not be used without protection.
  The properties  of concrete  may be
varied within wide limits.   They de-
pend on  the quality of the ingredients,
the  relative  proportions  of  the in-
gredients, the method  of mixing and
placing,  and the curing  or treatment
after placing.
  The  Portland  Cement Association is
an excellent source of information for
all  phases of concrete  work.

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30
      PAINTS AND PEOTECTIVE COATINGS

4.2 CONTROL  OF THE  ENVIRONMENT
  Since  corrosion  is  the  destructive
attack on a  material  by chemical or
electrochemical reaction with  its en-
vironment, it follows that a  change
in environment, to reduce or eliminate
these destructive forces, will prevent
corrosion.
  An electrolyte,  such as water  and
its  dissolved  chemicals, is a necessary
element  for  the  corrosion of  metals.
Therefore, any means to prevent  con-
tact between  the metal and an  electro-
lyte will prevent corrosion.
  Sometimes   existing structures  can
be  altered to  eliminate  open waste-
water flow and  thus reduce exposure
to moisture and corrosive gases. These
and other methods are discussed in
this section.

4.21 Ventilation and Heat
  The lowering  of the humidity so
moisture will not condense has a bene-
ficial effect  in  controlling corrosion.
Good ventilation is  also of prime im-
portance. These can be accomplished
with fans and  heaters.   Even  open
windows  and natural  drafts  can be
used to  advantage.
  Detroit reports, '' The clearing up of
an  annoying case of severe  corrosion
in the wet well of a pumping plan was
accomplished by reversing the flow of
air  into  this space,   Thit  wet  well
contained airlines,  trolley  beams, and
an  air hoist.   The metal parts of this
equipment were deteriorating very fast
in the damp atmosphere.  By forcing
fresh air into this  area and  keeping
it under a slight positive pressure, the
corrosion rate dropped  to  ordinary
proportions."
  Massillon,  Ohio,  reports,  "Forced
draft ventilation in all chambers and
pump  rooms  definitely has helped to
reduce paint maintenance. Rooms are
kept dry.   Sweating  is  reduced or
eliminated.  Corrosive gases cannot ac-
cumulate and condense on damp)  cold
surfaces.''
                           Winona, Minnesota,  reports, "We
                         have  reduced  our  painting  mainte-
                         nance by 50 percent by improving the
                         ventilation at  the end  of our  pipe
                         tunnel."
                           Circleville, Ohio, reports, "We have
                         found that forced air ventilation  had
                         reduced   condensation  in  the  non-
                         heated screen and grinder room  and
                         that paint life has been  tripled."
                           Pontiac, Michigan,  reports,  "In the
                         space between the  roof  and  bottom
                         plate of the P.F.T.  floating  digester
                         cover,  rot and  rust were prevented
                         by  installing two roof-type  ventila-
                         tors."
                           Providing  heat in unheated areas,
                         or mots heat where it is inadequate to
                         promote  dryness  and prevent conden-
                         sation, will  help  in reducing  the  cor-
                         rosion rate and painting  frequency.
                           Combustion products from gas heat-
                         ers always  should be  vented to the
                         outside.  Natural and digester gas in
                         burning  produces water and  harmful
                         gases.   The  gases would  be  harmful
                         to personnel  and  the moisture  on  con-
                         densing would accelerate  corrosion.
                           Baltimore  reports, "Installation of
                         heaters  in the coarse screen  building
                         and in  the  fine  screen building  pre-
                         vents condensation  on   metal  truss,
                         window   frames,   etc.,  during   cold
                         weather;  reducing maintenance."

                         4.22 Cathodic Protection
                           Cathodic protection by definition  is
                         the reduction or  prevention of corro-
                         sion of  a metal surface by making  it
                         cathodic,  by the  use of sacrificial an-
                         odes or  impressed  currents,  for ex-
                         ample.
                           Cathodic  protection  is  one of the
                         most important approaches to the  con-
                         trol of  corrosion  of metals in use to-
                         day.  It is used extensively to protect
                         condenser  tubes,  buried  pipe lines,
                         water storage tanks, clarifiers, sludge
                         digesters, aerators, and  other metal
                         equipment which contains, or is ex-

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                     PAINTS AND PEOTECTIVE COATINGS
                                   31
 posed to, water and other electrolytes.
 Needless to say,  cathodic  protection
 has no effect above the water line.
   By the use of an externally applied
 current,  corrosion can  be  reduced
 virtually to zero  and a metal surface
 can be maintained in a corrosive en-
 vironment without deterioration for an
 indefinite  period.
   In the usual application of cathodic
 protection  the metal  to be  protected
 is connected electrically to the negative
 terminal of a source  of current  such
 as a  rectifier, generator, or battery.
 The  positive terminal is connected to
 an anode  in the  corrosive electrolyte.
 Current   from   the    anode  passes
 through the electrolyte to the protected
 metal, making it  cathodie and  revers-
 ing the  current at the anodes of  local
 cells on the protected metal.
   The  applied voltage  needs only to
 be sufficient to supply  an adequate cur-
 rent density to all parts of the pro-
 tected structure.   In  soils  or  waters
 of high resistivity, the applied voltage
 must be higher than  in environments
 of low resistivity.  The source of cur-
 rent  is  usually  a rectifier  supplying
 low voltage d-c of several amperes.
   The voltage required to give protec-
 tion  from  corrosion  is  determined
 through measuring the potential  of
 the protected structure.  This measure-
 ment is of  greatest importance in prac-
 tice,  and  is the  criterion   generally
 accepted and used by corrosion engi-
 neers.  It is based on  the fundamental
 concept that cathodic protection is just
 complete when the protected structure
 is polarized to the open-circuit anode
 potential of local  action  cells.   This
 potential for steel, as determined em-
 pirically, is equal to 0.85v vs. the Cu
 saturated  CuSO4  half cell,  a survey
 instrument used  for this  purpose.
  In  a cathodic  protection system
where sacrificial anodes are used  as
the current source, the anode must be
a  metal more  active  in the galvanic
series than the metal  to be  protected.
   In the protection of iron or steel,
there are three readily available metals,
aluminum, zinc, and magnesium, each
of which forms reasonably strong gal-
vanic  cells when combined  with iron.
Magnesium  forms  the  strongest  cell
(highest voltage) of the three, and is,
therefore,  most  often used.
   Aluminum operates theoretically at
a voltage between magnesium and zinc
but  tends  to become passive in water
or  soils,  with  accompanying  change
of potential, to a value  approaching
or more noble than steel.  Whereupon
it ceases to   function as  a sacrificial
electrode.  Special methods to  combat
this  have been  used but none  are too
dependable.
   Zinc's chemical  action,  with regard
to sulfides  and carbonates in waste-
water as  well  as  its lower  voltage,
make it less  effective when used as a
sacrificial anode than magnesium.
   There have been many  installations
of cathodic protection systems in waste
water treatment plants in recent years.
Some have been successful and some
have not. It  is suggested that a compe-
tent   corrosion  engineer be consulted
before the design  and installation of
a cathodic protection system.

4.23  Galvanic  or Bimetallic Cor-
     rosion
  In modern terms, galvanic corrosion
may be defined  as the accelerated elec-
trochemical  corrosion produced when
one metal is  in electrical contact with
another more noble metal, both being
immersed in  the same corroding me-
dium (electrolyte).   Corrosion  of this
type results,  usually, in an accelerated
rate  of  solution for  one  member  of
the couple and protection for the other.
The  protected metal, the one that does
not  corrode,  is  called  the noble  or
cathode metal.  Note that as galvanic
corrosion  is  generally understood, it
consists of the total  corrosion which
comprises the normal corrosion  that
would occur  on a metal exposed alone,
plus   the  additional amount  that  is

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32
PAINTS AND PROTECTIVE COATINGS
due  to contact with  the  more noble
metal.
  With a  knowledge  of the galvanic
corrosion   behavior  of  metals   and
alloys, it  is  posible to  arrange them
in a  series which  will  indicate their
general tendency  to   form  galvanic
cells,  and  to predict the probable  di-
rection of the galvanic effects.
  Some of  these metals may be grouped
together.   These group  members have
no strong tendency to produce galvanic
corrosion  on each  other and from a
practical standpoint they are relatively
safe  to use in contact  with each other.
But  the  coupling of two metals from
different groups, and distant from each
other in the list, will result in galvanic
or accelerated  corrosion of the metal
higher on the list.   The farther apart
the metal  stands, the greater will be
the galvanic tendency.
  The relative position of  a  metal
within a group sometimes changes with
external  conditions,  but  it  is  only
rarely that changes occur  from group
to group; however,  the  stainless steels
are in two different places.  They  fre-
quently change places,  depending on
the corrosive media.    The most  im-
portant reasons for this are the  oxi-
dizing power and acidity  of the solu-
tions  and the presence  of active ions,
such   as   halides.   In  environments
where these  alloys  ordinarily  demon-
strate good  resistance to  corrosion,
they will be  in  their passive  condition
and   behave  accordingly  in  the  gal-
vanic  coupling.
  Some of the more important practi-
cal rules  that  have been  derived by
corrosion engineers to  prevent or mini-
mize  galvanic  corrosion are:

  1.  Select combinations of metals as
     close  together  as  possible in the
     galvanic series.
  2.  Avoid   making    combinations
     where the  area of the less noble
     metal is relatively small.  It  is
     good  practice  to  use  the more
     noble metal  for  fastenings,  and
     other  small parts  in  equipment,
                        that are built largely  of  less re-
                        sistant  material.
                      3. Insulate dissimilar  metals wher-
                        ever practical.   If complete in-
                        sulation cannot be achieved,  any-
                        thing such as paint or a plastic
                        coating  at the  joints will  help
                        increase resistances  of the circuit.
                      4. Apply coatings with caution.  For
                        example,  do  not  coat the  less
                        noble material without also coat-
                        ing  the  more   noble, otherwise
                        greatly accelerated  attacks might
                        be concentrated  at  imperfections
                        in the  coatings on  the less noble
                        metal.   Keep such coatings  in
                        good repair.
                      5. If possible, increase the electrical
                        resistance  of  the liquid path.
                      6. If possible, add  suitable chemical
                        inhibitors  to  the corrosive solu-
                        tion.
                      7, If you  must  use dissimilar  ma-
                        terials  well apart  in  the series,
                        avoid  joining them by threaded
                        connections, as  the threads  will
                        probably  deteriorate  excessively.
                        Brazed joints are preferred, using
                        a brazing  alloy  more noble  than
                        at least one of  the metals to  be
                        joined.   Also, don't  use  anodic
                        or less  noble  metals  for  critical
                        structural  material.

                   4.24 Use of  Coating  to  Prevent
                         Corrosion
                      One of the most common and widely
                   used methods of preventing corrosion
                   of a material is to cover  it with an-
                   other  material  which has  greater re-
                   sistance to corrosion.  Such a material
                   is called a protective  coating.
                      Protective coatings fall into  two
                   main  categories: those  that act  as  a
                   physical  barrier against the  environ-
                   ment, and  those that corrode  prefer-
                   entially and  save the base metal from
                   attack. Aside from zinc and cadmium
                   coatings,  which fall  in  the sacrificial
                   category,  most coatings are  of  the
                   barrier type.
                      Protective coatings usually  are  sub-

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                    PAINTS AND PROTECTIVE COATINGS
                                  33
divided further  into  metallic  or in-
organic and organic  coatings.
4.241 Metallic Coatings:—Following is
a summary of the most important types
of metallic  coatings available.   These
coatings can  be  applied  by  electro-
deposition,  flame  spraying, hot dip-
ping, cladding,  and other  techniques.
  Zinc and  cadmium coatings both are
less noble than steel under most condi-
tions.  Thus, they are  used  to cathodi-
cally or galvanically protect iron and
steel.  In the process  the coatings are
consumed preferentially and  the base
metal remains intact.   A  further ad-
vantage of  these coatings is that they
cause little  difficulty  from  the stand-
point of dissimilar metal contact when
they adjoin aluminum or  magnesium.
  The zinc  coating  (commonly  called
galvanizing) is usually applied by dip-
ping in a molten zinc bath.   The re-
sulting  coating  is measured either in
mils  (thousandth of  an  inch)  or in
"ounces per square foot."  An average
coating of 5 mils  (0.005 of an in.) is
approximately 2J oz/sq ft  (0.07 g/sq
cm).
  Past  experience indicates  that  the
effective service  life  of a  galvanized
coating varies directly with its  thick-
ness.
  The  service  life of  a  galvanized
coating varies  greatly  with  the  ex-
posure. In  heavy industrial areas con-
taining smoke, soot,  acid  fumes, etc.,
a 5- to 10-yr life can be expected while
in  a rural  area 20 to 25  yr can be
expected.
  Sherardizing  is another  method of
applying  a  zinc  coating in which the
material to  be coated is packed in zinc
dust in an airtight revolving container
and heated to a temperature  close to
the melting point  of zinc.  This causes
an  alloying of the zinc with the steel.
This method is more suitable for small
pieces and  does not produce as thick
a coating as the hot-dip method.
  For specifications and standards on
hot-dip galvanized  coatings  refer to
A.S.T.M.  Standards which have  been
compiled  by  the American  Hot-Dip
Galvanizers Association into one book.
  Nickel coatings, unlike cadmium and
zinc,  are  more noble than iron and
steel  and  do  not provide  sacrificial
protection.  To protect the base metal
they must provide an impervious, non-
porous barrier.  Electropolated nickel
coatings vary in  thickness from 0.5 to
10  mils,  the  thicker  coating  being
used  in the chemical industry.  For
added adhesion,  they  usually are ap-
plied over a very thin layer of copper.
Nickel also  can be applied by electro-
less  plating and by  cladding.   The
electroless   coatings  are  particularly
useful in areas that cannot be reached
by   electrodeposition  and  where   a
heavy-duty  coating is needed, as for
tank  cars  handling corrosive liquids.
  Chromium  electroplates  are especi-
ally  useful  where  tarnish  resistance
combined   with  hardness,  wear re-
sistance, or  a low coefficient of friction
is needed.   They are used most fre-
quently to preserve the  appearance of
nickel electroplates.
  Silver  electroplates can  be  useful in
some corrosive applications.   They are
immune  to attack by  most  dry and
moist  atmospheres  and  although at-
tacked by ozone,  they resist the effects
of oxygen at high temperatures.  Most
halogen gases  will attack silver  plate
but  the  initial  film  that is formed
inhibits further  attack.   However, as
is well known, the coatings will  tarn-
ish  when  subjected  to  moist  sulfur-
bearing compounds.
  Other metal coatings such  as alumi-
num, tin, lead, monel, stainless  steel,
and various hard facings arc used fre-
quently  to  protect  iron and  steel
against corrosion.  Hot-dipped alumi-
num  coatings are  especially useful
where a  combination of heat and cor-
rosion is  encountered and they  have
high resistance  to  corrosive conden-
sates which  form when  a heated part
cools down.   Tin, of course,  is widely-
known  for  its use on  corrosion re-

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34
                    PAINTS AND PROTECTIVE COATINGS
sistant food containers.  Lead coatings
are noted  for their ability to form a
film of environmental reaction  prod-
ucts,  such as lead sulfate, which are
highely resistant to corrosion.
  A recent newcomer to the coatings
field  is  the  inorganic  zinc coating,
which probably  belongs in the metallic
coatings classification.  It  is composed
of metallic zinc  particles and a vehicle
of sodium  silicate.  A curing agent or
hardener   is  used  to  complete  the
chemical  action  of the formation of
the coating.   Inorganic zinc coatings
perform well because they are bonded
tightly to  the metal surface through
reaction with iron base and are  basi-
cally  conductive,  permitting the zinc
to corrode preferentially to protect un-
derlying steel.  Much of the corrosion-
protective value of the zinc is thought
to be related  directly to formation of
relatively  stable  insoluble  corrosion
products,  such  as  oxides,  hydroxides,
and basic  carbonates.

4.242  Non-Metallic or Inorganic Coat-
ings:—The  inorganic  coatings  are
used  to  form a physical barrier be-
tween the  corrosive  environment  and
the material to be protected. They get
the name organic from the use of or-
ganic vehicles,  thinners,  drying oils,
and resins in their compounding.  The
pigments  for these  coatings  usually
consist of metallic oxides, e.g., titanium
oxide, chromate,  lead  carbonate,  etc.
Synthetic  resins now are  used quite
often as vehicles or components  of ve-
hicles  to  enhance the ability  of the
coating to resist acids and alkalies.
  Vinyl resins have good resistance to
penetration by water.  Silicone  resins
are  used  at elevated  temperatures.
Epoxy resins  show resistance to many
chemicals  as  well as excellent  adhe-
sion.
   It  is sufficient  to  say that organic
coatings cover a tremendous field from
the linseed oil paints through the coal
tars and  asphalts to the newer syn-
thetics  such  as vinyls,  epoxies,  and
urethanes.    This  field  includes  the
primers that are used in conjunction
with many of these coatings.   The de-
tailed uses of  these coatings  are cov-
ered  in  the chapter  on  Paints and
Painting.

4.243 Chemical Conversion Coatings:
—Inorganic films produced through a
chemical reaction are classed as chemi-
cal  conversion  coatings.   Such films
actually  become  an  integral  part  of
the surface of the  base metal  being
processed.  These films vary in physi-
cal  characteristics such as durability,
appearance, and cost, depending  on
the processing compound selected  to
produce  the  desired  end  result.
  Chemical conversion  coatings  may
be employed to produce a decorative
effect on a finished  product,  act as a
conditioner  or  an  adherent  base  for
an organic finish, protect  against cor-
rosion,  provide wear-resistant  prop-
erties,  assure   lubricant adhesion, in-
sulate, reflect heat, or form a dielectric
film.
  Typical  inorganic  chemical conver-
sion  films used today  include  coatings
produced  with phosphate, chromate,
various strongly alkaline-oxidizing so-
lutions, fused  dichromate, and anodic
or electrolytic  immersion.
  Two of the most widely used are the
phosphate  coating and the controlled
oxidation method of applying a black
finish to metal parts.
  A phosphate coating is a crystalline
non-metallic layer formed on  the sur-
face of a metal by the chemical reac-
tion  of phosphoric acid and the metal.
The solutions used most commonly con-
tain zinc and iron or manganese along
with iron phosphates.   Small articles
such as bolts,  nuts, etc., are coated  by
dipping  them  in the  phosphate  solu-
tion.  Large pieces are sprayed.
  The phosphate coating depends  on
the attack of  the base metal  to form
the coating.    Consequently,  anything
which interferes with this attack will
influence  the  coating.   This  fact ex-

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                    PAINTS AND PROTECTIVE COATINGS
                                  35
plains the  great  importance of clean-
ing in the proper deposition of a qual-
ity phosphate coating.
  A phosphate coating is not intended
to prevent corrosion by itself, but finds
its greatest use as  a base on which to
apply   subsequent   coats   of   paint.
Paints adhere much better  to  a  phos-
phate-treated metal surface  than  to an
untreated one.
  Corrosion under a paint film  that
has been scratched is retarded effect-
ively  by the  phosphate coating.   This
characteristic  is particularly valuable
on the edges  of  articles where corro-
sion tends  to  start.
  The adhesion of paint to new zinc
or zinc alloy surfaces is usually  very
poor.  A phosphate  coating not  only
provides the necessary bonding  layer
but also increases the durability of the
subsequent paint coatings.
  Parkerizing  and  Bonderizing  are
trade  names  applied to  commercial
phosphate  coatings.   They are   used
widely for refrigerator cabinets,  office
equipment, and automobile  panels.
  The black oxide finish process was
originated  as an  alternate  or substi-
tute process for  plating during the
war.   At that time,  the scarcity and
high  cost of plating material turned
manufacturers to other sources for an
attractive, durable, and protective fin-
ish.    Blackening,  obtained with the
controlled  oxidation  process, was em-
ployed widely because  of its  ease  of
use and excellent  protective qualities
and continued to be popular even after
chrome again was available.
  Typical applications for such black-
ening  include modern metal furniture,
machine  parts,  guns,  tools,  spark-
plug  bodies, gears, typewriter parts,
hinges, screws,  nuts, bolts, washers,
and similar items.
  The black  chemical conversion  coat-
ings usually  are  obtained by exposing
the metal  parts  to hot, oxidizing so-
lutions or  gases.   These coatings are
very thin, normally ranging from 0.02
to  0.2 mils  and have little,  if  any,
effect on dimensional accuracy.  Basi-
cally such  coatings are  used to  im-
prove  the  appearance of the  finished
item, to provide protection from cor-
rosion,  or  a base for  painting.
  One  of  the  most  commonly  used
methods of blackening ordinary steel
is the immersion of the metal in  a  hot
strongly alkaline solution.   Here,  as
with the phosphate solution, the  metal
must be cleaned before immersion  for
a good protective surface to develop.

4.25 Treatment of Water Systems
     to Prevent Corrosion
  The use of water as a heating and
cooling agent is widespread.   To pre-
vent  corrosion  in the process equip-
ment and  operate at peak  efficiency,
treatment  methods  have been devel-
oped to alter the corrosive character-
istics of the water used.
4.261   Cooling-Water   Systems:—In
general there are two types of  cooling-
water  systems,  the  once-through and
the recirculating system.
  The ones-through system usually is
not treated chemically because of  the
large quantities of inhibitors required
and the problem of water  pollution.
Sometimes additions of about two to
five mg/1 sodium or calcium polyphos-
phate are added to help reduce corro-
sion of  steel equipment.  In such small
concentrations  polyphosphates  are  not
toxic and water disposal is not a prob-
lem.  Otherwise use must be made of
a suitable  protective  coating or  of
metals  more corrosion resistant than
steel.
  Eecirculating  cooling  waters, such
as  engine-cooling  systems,   can   be
treated   with    sodium    chromate,
Na2CrO4, in the amount of 0.04  to 0.1
percent  (or  the equivalent amount of
Na2Cr207-2H20 plus  alkali  to  pH
8).   Chromates inhibit  corrosion  of
steel,  copper,  brass,  aluminum, and
soldered components of such systems.
As chromate is  consumed slowly,  ad-
ditions must be made at long intervals.
in  order to maintain the  concentra-

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                    PAINTS AND PROTECTIVE COATINGS
tion at the right level.  For diesel or
other heavy-duty  engines, 2,000 mg/1
sodium chromate (0.2 percent) is rec-
ommended in order to reduce damage
by  cavitation  erosion  as  well as by
aqueous corrosion.
  Chromates  should not  be  used  in
the presence of anti-freeze solutions be-
cause of their tendency to react with
organic substances.   There  are many
proprietary inhibitor mixtures on  the
market which usually  are   dissolved
beforehand in methanol or in ethylene
glycol in order to  simplify  the pack-
aging   problem.   Borax   (Na2B407-
10H20)  is a common  ingredient,  to
which sometimes is added sulfonated
oils which produce  an oily  protective
coating,   and   mercaptobenzothiazole
which specifically inhibits  corrosion of
copper  and at the  same time  removes
the accelerating influence of  dissolved
copper on  corrosion of portions of  the
system.
  The treatment *  of the  water used
at  the  Hyperion  Sewage  Treatment
Plant  of the  City  of Los Angeles  in
the diesel dual-fuel  engines with vapor-
phase cooling and steam-recovery sys-
tem, is  as follows.
  First the  water  is put  through  a
Zeolite-Nalcite softener  and  sand and
gravel filter.   It  is softened to zero
hardness  as indicated by  tests with
the  Boutron-Boudet   soap   solution.
Then  the  following  chemicals   are
added:
Sodium  Sulflte  (Santosite)     104  mg/1
Sodium Hexametaphosphate
  (Calgon)                    47.5 mg/J
Hagan  Dispersive  (Haganite)     47.5 mg/1
Sodium Hydroxide (Caustic Soda)   9.5 mg/1
Cobaltous Chloride              0.24 mg/1
  Hot water heating systems normally
are closed steel systems in which  the
initial  corrosion  of  the system soon
uses the dissolved oxygen; thereafter,
corrosion is negligible so far as life of
the metal  equipment is concerned.
  * This is a once-through system;  therefore,
this particular treatment might  have to be
modified where steam  was condensed  for
reuse  as feed water to the system.
  Medium or hard waters are relatively
non-corrosive and do not require treat-
ment of any kind for corrosion control
in municipal water systems.  Soft wa-
ters on  the other hand cause  rapid
accumulation  of rust  in iron piping,
are  contaminated  readily  with toxic
quantities  of  lead  salts  on  passing
through  lead  piping,  and  cause blue
staining of bathroom fixtures by copper
salts originating from slight corrosion
of copper and  brass piping.  Vacuum
deaeration of  such waters  would  be
ideal as  a corrosion control  measure.
The expense is high for treating  the
large quantities of water involved and
no  practical  installations  apparently
have been constructed as yet for com-
munity water supplies.
  Chemical  treatment of potable wa-
ters is limited  to  small concentrations
of  inexpensive,  non-toxic  chemicals,
such  as  alkali  or lime.   Some  water
supplies  are treated with  about two
mg/1  sodium   polyphosphate   which
helps reduce the red color  originating
from ferric  salts or suspended rust in
water. This treatment also reduces the
corrosion rate to a modest extent wher-
ever water  moves with  some velocity
and is aerated  fully.   In  stagnant
areas of  the distribution system, how-
ever, there  is  probably  no  practical
benefit.

4.26 Preventing the Corrosion  of
     Portland Cement Concrete by
     Hydrogen Sulfide

4.261  Nature   of Concrete  Corro-
       sion:—
  4.2611 General:—The  concentration
of  dissolved sulfide in wastewater is
indicative of  the corrosion potential.
Dissolved sulfide  itself  is not cor-
rosive to the concrete below  the surface
of  the wastewater  in the  concentra-
tions  normally present.    It  evolves,
however, from the wastewater flow in
the form  of hydrogen sulfide and,  by
bacterial action, is oxidized on  the  in-
terior  surfaces of the sewers and ap-

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                    PAINTS AND PROTECTIVE COATINGS
                                  37
purtenant structures, above the waste-
water  level, to  form  sulfurie  acid.
The resultant sulfurie  acid then will
react with any  non-resistant material
resulting in surface deterioration.  On
a  concrete  surface the sulfurie  acid
will be neutralized by the basic  con-
stituents of  the concrete.   Complete
neutralization usually is not achieved,
however,  because of the  inter f err ing
film of the reaction products.   On  an
acid-resistant surface  free  acid  may
concentrate to as much as 25 percent.
Thus,  when  the concrete  is protected
by tile or  other jointed resistant ma-
terial, the joints must  be sealed  with
acid-proof material.  The porosity of
both  the  protective material and the
joints also must be kept to a minimum.
If this is not done, the acid will pene-
trate and  react  with the  concrete be-
hind the  protective material.  Behind
bonded  ceramic tile  or  similar  ma-
terials, such reaction, accompanied by
the  greater  volume of  the reaction
products,  will  break  the bond  and
cause  the  protective surfaces to fail.
When the  protective  membrane  and
jointing remains  sound,  the  concen-
trated acid  tends  to  collect and  run
down the  walls.   If wastewater does
not reach the level  of the protective
membrane for  most of the time, the
unprotected surfaces below the mem-
brane can  be  corroded rapidly  and
severely.
   4.2612  The  Corrosion  Cycle:—The
corrosion cycle, as it applies to sewers
and  appurtenant  structures, can be
described as follows:
   First, the formation of sulfides in
the wastewater  results from the  bac-
terial reduction of compounds contain-
ing  organic  sulfur,   such  as  homo-
cystine,   cysteine,   methionine,   and
jenkolic acid;  or  from the reduction
of  inorganic  sulfur-containing  com-
pounds  such as sulfates,  sulfltes, and
thiosulfates.  All  of these  reductions
are anaerobic in nature and take place
 only in the absence of free or dissolved
 oxygen.  In the sewer snlfide produc-
 tion may occur in the  anaerobic invert
slimes even  though dissolved oxygen
exists  in  the flowing wastewater.
  Second, the release of hydrogen sul-
fide into the sewer atmosphere occurs.
This  rate of release, and  to  some ex-
tent the formation of hydrogen sulflde,
is dependent on both the characteristics
of the wastewater and  the geometric
design of the sewer and appurtenant
structures. Temperature and hydrogen
ion concentration are the  wastewater
characteristics effecting  the proportion
of the dissolved sulfides  available  as
hydrogen sulflde.  Flow velocity and
turbulence  are  the design  considera-
tions   most  effecting  the  release  of
hydrogen sulfide gas.
  Third, the oxidation of hydrogen sul-
fide  to sulfurie acid takes  place  on
moist, exposed surfaces as a result of
bacteriological  or  non-biological  ac-
tions.  An intermediate oxidation prod-
uct may  appear  in  the form  of  ele-
mental sulfur  accumulation  on  the
surface.   A portion of the  hydrogen
sulfide may  escape  from the sewer as
an  odorous gas.
  Finally, the reaction of  sulfurie acid
with  the  calcium compounds of port-
land  cement concrete  forms calcium
sulfate.   The  calcium, sulfate  reacts
with  the  tricalcium  aluminate in the
cement to form an expansive, complex
salt  (tricalcium  alumino  sulfate  hy-
drate).  This and other expansive cor-
rosion products  cause  the concrete to
soften and spall.
   The bacteria  primarily responsible
for hydrogen sulfide formation are the
sulfate-reducing bacteria,  desulphovib-
rio desulfuricans.   These  bacteria, in
deriving their necessary energy, reduce
sulfates to  sulfides.  While the sulfur-
oxidizing  bacteria  (Thiobacillus thio-
parus, Th.  thiooxidans, and Th. con-
cretivorus}  act as the biological inter-
mediaries in the formation of sulfurie
acid  from hydrogen sulfide, sulfur, and
thiosulfate.   It is not clear  at  this
 time if these are  the only species in-
volved in these reactions.   They have
been isolated by certain workers in the

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38
                    PAINTS  AND PROTECTIVE COATINGS
field and have been  found  either to
reduce  or oxidize sulfur compounds.
Other  species  also may function  just
as efficiently.
  The manifestation  of the  "Corro-
sion Cycle,"  as relating to  concrete
corrosion,  is  schematically  shown as
follows:
In the Sewer Flow-

         Sewage Nutrients + Sulfate-Reducing Bacteria 	>  Dissolved Sulfides

                                      pH and Turbulence  	>
                                                             4-
                                                       Hydrogen Sulfide
Above the Wastewater Flow—

    Hydrogen Sulfide + Non-Biological Agencies and Sulfur-
               \    Oxidizing Bacteria
                 \
    Sulfur          -f- Sulfur-Oxidizing Bacteria

    Sulfuric Acid    + Calcium Compounds in Concrete
                  Sulfur


                  Sulfuric Acid

                  Corrosion Products
4.262  Control Methods:—

  4.2621 Method  of  Presentation:—
  The control  methods, which are ap-
  plicable  to the  sulfur cycle and its
  products as  it  applies  to  sewers,
  have been separated as to  principal
  effect  into bacteriological,  physical,
  and chemical categories for purposes
  of presentation,

  4.2622 Bacteriological  Control:—
1.  General
     In a manner analagous to that of
   all  other forms of life,  the  growth
   and activity of wastewater bacteria
   will be  affected by changes in their
   environment such as sudden changes
   in  temperature,  pH,  presence of
   toxic material,  or the supply  of nu-
   trients.   Bacteria,  too, are  subject
   to destruction  by  other  forms of
   living organisms.
     Bacteria stabilize organic matter
   as a function of their metabolic ac-
   tivity.    This  stabilization,  which
   consists basically  of  hydrogen  re-
   moval  and  its transfer to  an ap-
   propriate hydrogen  acceptor,  will
   proceed so that the  higher energy-
   yielding reaction  takes  place first.
   These hydrogen acceptors, in order
   of  descending  energy  yields, are
   DO,  nitrates,  sulfates,  oxidized  or-
   ganic matter, and carbon  dioxide.
2.  Sterilization of  Sewage
   (a)  Treatment  with  Bacteriostatic
      Agents:—In most systems steri-
      lization of the wastewater can-
      not be achieved  economically.
      However,  certain chemicals pos-
      sessing bactericidal action have
      been used.   These include chlor-
      ine,   trichlorophenol,   phenol,
      pentachlorophenol,  orthodichlo-
      robenzene,   quaternary  ammo-
      nium  compounds, heavy  metal-
      lic  salts,   and  a  number   of
      commercial    materials    with
      claimed bactericidal  properties.
      Except under  special conditions,
      only chlorine  has been  effective
      in quantities  economically fea-
      sible.   Chlorine is a strong oxi-
      dizing agent;  therefore,  it  is
      utilized in many side reactions
      before acting  as a  baeterieide.
      Since  wastewater contains large
      amounts  of ammonia, much  of
      the chlorine is utilized in  form-
      ing chloramines.   These  are
      weaker than  free  chlorine   as
      bactericides.
   (6)  Lime:—Lime  in  sufficient con-
      centrations  is   a    baeterieide

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                   PAINTS  AND PROTECTIVE COATINGS
                                39
      which  can be both inexpensive
      and effective.  If enough lime is
      added  to wastewater to  boost
      the pll to 12, a  dosage  rate of
      6,000 to 8,000 mg/1 for a period
      of at least 45 min, the bacteria
      in the  slimes below the  surface
      of the wastewater are destroyed.
      Sulflde  generation then  will be
      reduced   materially  until the
      bacteria   and slimes   build  up
      again.   A period of two to four
      weeks  is  usually required for
      this  build up.  If lime  dosages
      are not sufficient to hold the re-
      quired  pH  over the required
      time, little  benefit  occurs.
3.  Oxidation  of  Wastewater
   (a) Methods:—One or all of the fol-
      lowing  physical  methods of in-
      creasing the DO content of the
      wastewater  may  be used; com-
      pressed  air or  oxygen  intro-
      duced  into the wastewater flow,
      wastewater diluted with  oxygen-
      ated fresh water, or  turbulence
      in high velocity  of flow utilized
      for surface absorption.
         The demand for oxygen also
      may be met through the use of
      chemical  additives.   Chemicals,
      which have been used with vari-
      ous  degrees of success, are chlo-
      rine nitrates, hypochlorites,  hy-
      drogen  peroxide,  and  hexava-
      lent chromium.
         As an  alternate to  the  physi-
      cal and  chemical means, the ox-
      ygen demand  may be  reduced
      by  partial  purification  of  the
      wastewater at some  intermedi-
      ate  point in  the sewer system.
       This could include the biologi-
       cal  use  of  algae  in treatment
      lagoons.
   (&) Chlorine:—It  has been  shown
       by  investigations that  chlorine
       oxidizes  sulflde  to  sulfate,  not
       to free  sulfur,  and  that  the
       ratio of chlorine demand  to sul-
       fide  is  8.87:1.   Chlorine also
    will  raise  the  oxidation-reduc-
    tion potential of the wastewater
    and  has germicidal  action  on
    the sulfide-producing  bacteria.
    In actual  operational  use be-
    tween 12: 1 and  15 :1 parts of
    chlorine per part of sulflde have
    been found necessary in gravity
    sewers and 15:1 to 20:1 parts
    in force mains for odor and cor-
    rosion control.
(c)  Nitrate:—Nitrate   addition  to
    relatively  fresh  wastewater  is
    sometimes an effective means of
    providing  the  wastewater with
    a  reserve  oxidizing  capacity.
    The  nitrates are depleted only
    after  the  DO  has   been  ex-
    hausted.   If  septicity already
    exists  in the  wastewater,  this
    method is  not  as applicable be-
    cause the required nitrate  con-
    centration   becomes  extremely
    high.  The effective dosing rate
    is 7.3 lb nitrate/lb of expected
    sulflde generation.  As with all
    chemical dosages, if not enough
    nitrate  is  added the  treatment
    is generally  useless.   Nitrate
    appears  to  be  most  effective
    under ponding conditions.
(d)  Compressed  Air:—Compressed
    air is most applicable to force
    mains where  it  is desirable  to
    maintain the  maximum solubil-
    ity of air  under the given con-
    ditions without  adding an ex-
    cess  of  air.   Too much  com-
    pressed air will increase pump-
    ing costs by causing air pockets
    which  increase  friction   head
    and may constitute a potential
    danger.  Small amounts of hy-
    drogen  sulfide can  collect  in
    the air pockets and be oxidized
     biologically  to  sulfuric  acids.
     This in turn  can cause severe
     localized corrosion. The proper
     amount  of air to be added may
     be calculated  as  follows:
     cfm  of  air needed to saturate
     the  wastewater  flow = (cfm of

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40
                    PAINTS AND PROTECTIVE COATINGS
      waste water)  (absolute  pressure
      in main)  (solubility of  air  at
      the  particular temperature) -—
      atmospherie pressure.   In prac-
      tice  a  minimum of 1 cfm/1 in.
      of pipe diam  (0.011 cu m/ruin/
      cm)  is recommended  for trial.
      Efficacy of injected  air  is af-
      fected by the pressure.  11: the
      sewer   main   operates  under
      high pressure, the injected air
      dissolves more readily and more
      effective control is obtained.
  (e] Reducing  Oxygen  Demand:—
      As an alternate to providing a
      method for increasing the DO
      content  of the  wastewatcr,  a
      partial purification of the waste-
                                        water to reduce the oxygen de-
                                        mand   may   be   undertaken.
                                        With the development of small,
                                        compact wastewater  treatment
                                        plants,  the use of these  plants
                                        for an  in-line  system  of  waste-
                                        water treatment at intermediate
                                        points  in  the  sewerage system
                                        appears possible.
                                    (/) Dilution with  Water:—In  the
                                        early years  of  a  sewer system
                                        when small  flows  exist, it  may
                                        be feasible economically to  con-
                                        trol  sulfide production by dilut-
                                        ing  the wastewater  with  oxy-
                                        genated water.  The amount of
                                        water to be  added may be  cal-
                                        culated  by the formula:
     Where
         Q»/Q. =  (EBCD/Marginal BOD)5 - 1

            Qw =  Quantity of water;
            Q, =  Quantity of sewage; and
        EBOD =  Effective BOD, or BOD X  1.07'C-M.

Marginal BOD =  BOD limit a sewer can carry without sulfide buildup.
  4.2623 Physical Control:—
1. Wastewater :-—
   (a)  Factors:—The buildup  of sul-
       fides in gravity  sewers can  be
       related to  the temperature, pH,
       BOD, DO, velocity,  slope, and
       area of wetted surface in any
       section under  consideration.
         Sulfide  generation will  occur
       during long  detention  periods
       in force mains and gravity sew-
       ers.   If there  is no  loss to the
       atmosphere  or  no   oxidation,
       buildup   will   result.    Force
       mains, since they run full, will
       not  be subjected to  corrosion;
       however, when the wastewater is
       discharged into  partially full
       gravity sewers and  wet  wells,
       the   sulfide  generated  in  the
       force main will  be  released  as
       hydrogen  sulfide  and,   where
       conditions are favorable,  cause
       corrosion  and odors.
                                           The addition to fresh waste-
                                         water of  cesspool  and septic
                                         tank cleanings and  industrial
                                         wastes may  bring about a loss
                                         of DO and the establishment of
                                         anaerobic conditions with a rise
                                         in sulfide  production.
                                           The rate of sulfide generation
                                         virtually is independent of sul-
                                         fate concentration as long as the
                                         sulfate concentration  is greater
                                         than about 50 mg/I;  therefore,
                                         under normal wastewater  con-
                                         ditions the concentration of sul-
                                         i'ates is  not considered an  ap-
                                         preciable factor.
                                     (Z>)  Interrelationship  of Factors:—
                                         Attempts have been made to  re-
                                         late  the  various  factors which
                                         affect sulfide generation into ex-
                                         pressions which will give a mar-
                                         ginal value,  or upper limit, for
                                         the   BOD  of the wastewater

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                    PAINTS AND PROTECTIVE COATINGS
                                                                         41
      which can  be  conveyed by a
      particular gravity  sewer with
                                             little   or   110  sulfide   buildup.
                                             Such an expression is as follows:
where
               Marginal BOD X 1.038<|-«s> = 7,5QQQWf(Q/Qt)

                         t = temperature, °F;
                         Q = actual flow, cfs;
                        Qf = flow capacity of full pipe, cfs;
                         S = slope; and
                   f(Q/Qf) = a function of relative flow.

  Expressions also have been proposed for the calculation of sulfide generation on
submerged surfaces as shown below.
Gravity Sewers:

where
                           S = EBOD X A X K
                    S = sulfides, Ib/day;
               EBOD = Effective BOD, mg/1;
                    A = area of submerged surfaces, sq ft; and
                    K = constant, 5 X lO"6.
Force Mains:


where
                        S = A! (EBOD)
(1  + O.Olrf)
     d
                          8 = sulfides, mg/1;
                         K = constant = 0.0026;
                          t = time of passage, min;
                    EBOD = Effective BOD, mg/1; and
                          d = diam of force main, in.
         The above  equation  is based
       on an assumption of no absorp-
       tion of. oxygen at the free waste-
       water  surface   in
                              gravity
      sewer.  When such oxygenation
      exists the  sulfides  buildup  in
      the wastewater will be less than
      calculated.
         Earlier  work  indicated  that
      the  buildup of  sulfldes,  where
      slimes existed, was in the range;
      of 0.3 to 0.6 lb/day/1,000 sq ft
      (1.5  to 2.9  g/day/sq  m)   of
      slime surface per 100  mg/1  of
      BOD.
         A relationship of wide appli-
      cation, taking into consideration
      all variables of the control prob-
      lem,  remains to  be  formulated.
      Velocity: — By  increasing veloc-
      ity in the wastewater stream the
     internal partial  pressure  is de-
     creased thus increasing the rate
     of oxygen  absorption at the sur-
     face  and  limiting the  sulfide
     buildup.   It  has been shown
     that  the rate of  oxygen, absorp-
     tion  by a flowing stream is pro-
     portional to the velocity to the
     1.75  power.  The amount of dis-
     solved  sulfide   existing  in   a
     sewer tends toward  a dynamic
     equilibrium between the snlfides
     oxidized  by  oxygen   diffusing
     into  the stream,  the sulfides lost
     as hydrogen sulfide, and the sul-
     fides being generated from  sul-
     fur compounds in the anaerobic
     slimes.  The loss of sulfldes, as
     hydrogen  sulfide,  to  the  sewer
     atmosphere is dependent on the
     internal pressure,  pil, tempera-

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42
PAINTS AND PROTECTIVE COATINGS
      ture, and  turbulence.   It  lias
      been proposed,  however,  that
      the minimum velocities required
                         to prevent sulfide  buildup  for
                         various values of Effective BOD
                         may be calculated by:
or
where
                         V = 0.137(EBOD)°-we
                       NKe = 5,700 X -- (EBOD)"-*9,
             V = velocity, fps;
        EBOD = Effective BOD;
           NRC = minimum Reynold's modulus for no buildup of H2S;
             A — cross-sectional area of flowing wastewater, sq ft; and
             b = width of surface of flowing wastewater, ft.
      These  equations  are  limited in
      use  to  sewers  operating  under
      "normal"   flow   conditions.
      They  do not  provide  for the
      effect  of  numerous  other  vari-
      ables and can be considered only
      as an approximate guide in cor-
      rosion  calculations.
  (d) Temperature:—A high waste-
      water   temperature  will  influ-
      ence biological action resulting
      in increased sulfide  production.
      Within the temperature  range
      found in sewers  an  increase of
      15 °C  may  double  biological
      metabolic rate.  High tempera-
      tures,  by reducing gas solubil-
      ity, also will cause the un-ionized
      hydrogen sulfide to  escape  into
      the  sewer  atmosphere.    The
      production  of  sulfides is neg-
      ligible below a wastewater tem-
      perature of  about 60°P (15°C).
      At  wastewater  temperatures be-
      tween  60° and 70°F  (15°  and
      21 °C)  sulfide buildup generally
      will be moderate; however, se-
      vere corrosion  can  occur.  By
      adding cool, unpolluted water,
      wastewater  temperature will be
      reduced and sulfide  production
      lessened.  For sewers this is not
      recommended   generally   since
      useful capacity will  be used by
      the unpolluted  water.
                     (e) Alkalinity:—Decreasing   the
                         wastewater's pH  causes hydrol-
                         ysis  of  the hydrogen sulfide.
                         Ilydrated lime or caustic  waste
                         are considered to be  the  most
                         economical  chemicals  for  pH-
                         control  treatment.  Except for
                         the use of certain  industrial
                         wastes,  such pH  control would
                         be  uneconomical.    Chemicals
                         containing  sulfides should  not
                         be added to the sewers.
                     (/) Cleaning:—Mechanical cleaning
                         should  be done periodically in
                         order to remove sludge deposits
                         from  the  bottom  and  slimes
                         from  the  submerged  walls.  It
                         appears  that   cleaning is  the
                         least  expensive and  the   most
                         effective method of sulfide con-
                         trol.  Sewers with heavy sludge
                         and slime deposits can generate
                         more  sulfides  than the theory
                         states.  Regular cleaning should
                         be the  foundation  of any sys-
                         tem-wide  sulfide  control  pro-
                         gram.    Sewer  design should
                         provide for easy cleaning.
                     (gr) Ponding  and   Flooding:—Only
                         that  concrete   exposed to  the
                         sewer atmosphere is  subject to
                         rapid corrosion.   Corrosion  of
                         concrete  in sewers,  therefore,
                         can be eliminated by  restricting
                         the wastewater flow  to  main-

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                 PAINTS AND PROTECTIVE COATINGS
                            43
    tain  a full pipe at  all times.
    This method of corrosion  con-
    trol  has been  referred to  as
    ponding  and  can   be  accom-
    plished  by  using  adjustable
    gates  built into the sewer line
    at   appropriate  locations   to
    maintain  the  required  liquid
    level.   Properly  weighted  flat
    gates  can  be installed in man-
    holes  to accomplish ponding in
    sewer lines where  gates  do  not
    exist and where the installation
    of  sluice   gates and  appurte-
    nances would  be  difficult and
    expensive.   Ponding,  however,
    may increase sulfide buildup in
    the wastewater  since wall slime
    area  is   increased.   Ponding,
    therefore, may  increase severely
    the odor and corrosion problem
    downstream of  the  sewer being
    protected.   In  addition, pond-
    ing  will increase  the cleaning
    job  by allowing sludge deposits
    and heavy  slimes to build up.
      Periodic  washing of  the sewer
    walls  by  flooding  with  waste-
    water or fresh water may lessen
    corrosion  of the  concrete  by
    preventing the buildup of heavy
    sulfuric   acid   concentrations.
    This practice also tends to  re-
    tard the activity of the sulfur
    autotrophes proliferating on the
    sewer walls  above  the flowing
    wastewater.  Such flooding robs
    these  organisms of the required
    nutrients,   such as  oxygen,  hy-
    drogen  sulfide, carbon dioxide,
    nitrogen  compounds,  and  an
    acid environment.   The neces-
    sary frequency and duration of
    the  wetting  for corrosion con-
    trol depends  on the intensity of
    the  corrosive gases  in  the sewer
    atmosphere.
(/i) Turbulence:—Hydrogen sulfide
    is soluble in water to the extent
    of 3,850 mg/1 at ordinary tem-
    perature and dynamic equilib-
    rium with 100-percent hydrogen
sulfide in surface contact.   Its
evolution  from  the  wastewater
is  not visible to  the  eye.   At
points of higher  than  normal
turbulence, rates of release are
far greater than from smoothly
flowing wastewater.  Turbulence
can be caused by high velocities,
obstructions in the line,  or as a
result of improper  design  of
structures  including  junction
manholes  which  permit sewer
lines to intersect at right angles
or at  different elevations.  Tur-
bulence also is found at the out-
let  end of  force  mains where
free fall exists, at sudden grade
changes,  at weirs, and at sharp
bends.  These conditions should
be  avoided  in  locations where
hydrogen sulfide cannot be tol-
erated.  Even where wastewater
contains  but  0.1  mg/1  of  dis-
solved sulfides,  turbulence  can
cause excessive release of hydro-
gen sulfide gas.   By undertak-
ing   structural   alterations   it
may be possible to  reduce sub-
stantially the emission of hydro-
gen sulfide  into the sewer  at-
mosphere.
  The mass  transfer of hydro-
gen  sulfide  from  wastewater
into sewer atmosphere also  de-
pends to  some lesser degree on
the liquid surface tension  and
the vapor pressure.  The sur-
face  tension  of  a liquid  de-
creases as the wastewater tem-
perature  rises resulting in  in-
creased molecular motion.   An
increased  turbulence   in   the
wastewater  also  will  cause a
corresponding  increase  in  the
kinetic energy  of  the liquid.
Any  increase  in  the  average
kinetic energy of  the  molecules
of  the liquid, therefore, results
in a decrease in surface  tension.
  The concentration of  H2S in
an  atmosphere  in  equilibrium
with  a  HzS-containing liquid

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 44
                    PAINTS AND PROTECT!VK COATINGS
       may be  found by  multiplying
       the concentration in the liquid
       by  a  factor which  varies  with
       the  temperature.   At  68 °F
       (20°C), this factor is  286, giv-
       ing the  concentration  at equi-
       librium  in  mg/1  by   volume.
       The pit and  turbulence  have
       more effect than temperature on
       the evolution  of  hydrogen sul-
       fide from the  wastewatcr.

4.263 Sewer Atmosphere:—
  4.2631  Methods of Control:—When
odorous  and  corrosive  gases  already
are  present  in  the sewer  atmosphere,
they  can  be  withdrawn  and certain
components eliminated by  properly de-
signed scrubbing or incineration equip-
ment.
  Other  methods of treating this with-
drawn air are the passing of the air
through   activated   carbon,   earthen
beds, activated sludge  cultures,  and
trickling  niters.
  In order to prevent an offensive  odor
condition, control  should  be consid-
ered when the concentration of hydro-
gen  sulfide in the atmosphere reaches
0.7  mg/1.
  Tile,   plastic  sheet,  stainless   steel
sheet, or  protective coatings will  give
various  degrees  of  surface corrosion
protection to  sewer structures in  con-
tact  with such contaminated atmo-
spheres  prior to treatment.   Sewer
ventilation also may be used to supple-
ment physical protection.
  4.2632  Ventilation:—The oxidation
of hydrogen  sulfide on the sewer walls
depends  on conditions favorable to sul-
fur bacteria, including a moist surface,
A flow of unsaturated air  tends to re-
move the moisture from  the walls and
would   therefore  retard   bacterial
growth  and  resultant  conversion of
hydrogen  sulfide  to  sulfuric  acid.
Ventilation has a secondary  effect in
that it can help prevent the  accumu-
lation  of  toxic or  explosive  gases.
Ventilation may, however, result in a
serious odor problem  since large quan-
tities of  odorous air have  been con-
centrated into a point source.
   Effective  drying of the sewer walls
is related to the quantity  and relative
humidity  of  the  air  being  drawn
through the sewer.  The distance that
a given fan can provide the required
drying  must be determined by  full-
scale field tests under normal operating
conditions.
   If, however, it is necessary only to
maintain negative  pressures at  the
manholes for  odor  control,  then it
would  be possible  to determine  unit
pressure losses in the  sewer  for appli-
cation to the design  problem.   Theo-
retically  this  could  be accomplished
by measuring the  static  pressure  at
two  consecutive  manholes in the line
during  ventilation test periods for dif-
ferent conditions of  flow.   This pres-
sure or head loss then would  be used
with  other  known  or measured data
in Darcy's  equation  in order  to  de-
termine  a factor "/," thus:
where

Hj — head loss between  manholes  in
      in. of HSO;
 L = distance bet. manholes, ft;
 V = air velocity, fpm;
  r = hydraulic radius, ft; and
  / = factor.

4.264 Sewer and  Sewer  Appurte-
      nances:—
  4.2641 Consideration:—The  protec-
tion  of the  sewers  and  appurtenant
structures  from  hydrogen  sulfide by
the  use of  protective  coatings  and
liners needs to be considered where the
presence of the gas is anticipated and
where it is impractical to fully control
its generation.
  4.2642 Coatings:—Protection   of
concrete from  corrosion  may  be ob-
tained by applying  protective coatings
to the sewer  walls.   Three  factors af-
fect  the ability of the coating  to  pro-
tect  the concrete: (a) chemical resist-
ance of the coating,  (&) permeability

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                    PAINTS AND PROTECTIVE COATINGS
                                   45
of the film,  and  (c)  adhesion of the
film  to the concrete.  A successful coat-
ing  must  have these  properties and
retain them over a long period of time.
  Many  coating materials have  suit-
able   chemical  resistance  but  when
brushed, sprayed,  or rolled on seldom
give complete  protection.   Such  coat-
ings  are  likely to  have minute per-
forations or pinholes.  When  this hap-
pens the  imperfections  rapidly  in-
crease in  size until   failure  occurs.
Abrasion  of  the coating from floating
objects  occurs between  high  and low
wastewater levels  leading  to  corrosion
of the exposed concrete.   Few if any
presently   known  coatings, therefore,
have been  effective in preventing con-
crete attack under highly  corrosive  or
abrasive conditions.  The epoxies show
the  most   promise,  primarily due  to
their good adhesive  properties.  Coal-
tar epoxy  coatings over aluminous  or
Portland  cement  concrete  have  been
used with  some degree of success.  A
silica loaded coal-tar epoxy liner ma-
terial has  been developed and can  be
factory  applied to new concrete pipe.
  4.2643 Plastic Liner:—Flexible pol-
yvinylchloride  (PVC) sheeting,  either
cemented  to the  concrete  or cast  in
place using integral  " T" shaped PVC
projections on the back of the  sheet,
has  proved  to  be  a  very successful
lining material. PVC has been shown
to be safe from  degradation under
microbial   attack.    Some   authorities
consider it to be the only proven sul-
fide  barrier.  Data on long-time expo-
sure  of PVC to sewer atmospheres are
limited and in view of its known water
absorption characteristics, slow  hard-
ening, and sensitivity to some  solvents,
length of  service  is not known now.
Except  for a tendency  to  mechanical
damage when subjected to high  veloc-
ity   wastewater flows,  with  a subse-
quent repair problem,  PVC protection
has  an  excellent record.
  4.2644 Stainless   Steel:—In  struc-
tures where  PVC sheet liner protec-
tion   may  be  subject  to  mechanical
damage, the use of stainless steel is rec-
ommended.   Sheets  of the type 316L
with a thickness of -fo to 1 in. (0.5 to
0.6 cm)  are providing  excellent  pro-
tection.
  4.2645 Tile Liner  Plates:—Clay tile
liner plates have been used  for many
years and glass plates have  been  pro-
posed.   The  clay  product  manufac-
turers  have  made advances in attempt-
ing to overcome problems of jointing
between plates and of permeability  of
the plates;  however,  both  remain  as
major  problems.  To  find  a satisfac-
tory jointing material is quite difficult.
Bituminous  joints are  emulsified  and
dissolved by  soaps,  oils, and  greases
in the wastewater  and sulfur joints
do  not adhere  well to  the  clay  and
are attacked by sulfur bacteria. Acid-
proof  cement  joints appear to  offer
good protection, but they are  costly.
  The  failure  of  any  of  the  compo-
nents making up the  clay tile system
will lead to the loss  of the tile protec-
tive cover.   Inasmuch  as clay tile is a
brittle  material, any chemical reaction
of the concrete will result in  expansion
and  the breaking   of  the  anchoring
lugs thus  causing   the  tile  to drop.
Tiles also have been broken loose  by
the expansive reaction of concrete ag-
gregates with the alkalies of Portland
cement.
  4,2646  Impregnation:—Any    mea-
sure that will  reduce  the  quantity  of
free lime and other reactive compounds
present in concrete would greatly im-
prove  its  chemical  resistance.   One
method was the so-called "fluating"
process.  This  involved treating the
surface of  the concrete  with a water
solution  of  a  fluoride,  usually  mag-
nesium  fluoride, which  reacted  with
the free  lime to form a layer of cal-
cium  fluoride  of high  chemical  re-
sistance.   The main disadvantage  of
this  method was the  very  thin  and
easily broken layer  of protection ob-
tained.
  The  same principle  is  used for  a
newer method called " Derating."  The

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46
PAINTS AND PROTECTIVE COATINGS
only difference between the two  proc-
esses is that the free lime in the ocrat-
ing  process  is  converted into  hard,
insoluble calcium fluoride by means of
a fluorine-containing gas,  silicon tetra-
fluoride.  The  chemical action is said
to be as follows:
2Ca(OH)»
               2CaF2 + SiC-2 + 2H2O
  4.2647 Lime Coating : — A thin coat-
ing of dry lime, ^ to ^ in.  (0.2 to 0.3
cm)  thick, can be applied to the walls
and  soffit  through  the  manholes  by
means of large-volume blowers.   One
blower is set to direct the  discharge
of lime  into  the  sewer  at one  man-
hole  and the other  is set to exhaust
the air from the next manhole  down-
stream.  Some of the lime is deposited
from the air flow on the  moist surface
of the sewer and forms  a semi-hard
coating.  This coating  tends  to absorb
moisture  and   neutralize  any  acid
which may be formed.  Depending on
the rate  of acid generation, the lime
may  be  effective for a period  of  up
to three  months  provided it is not
removed  by high flows.
  4.2648  Cement  and  Aggregates:-—
  (ft)  Portland  Cement: — There are
      various kinds of  cement, each
      of which  differ  in their  resist-
      ance  to acid attack.   The most
      economical  is Portland cement.
      Portland  cements, according to
      type, may vary in the degree of
      susceptibility  to corrosion de-
      pending mainly on the propor-
      tion  of  tricalcium  aluminate
      they  contain.
         Portland  cements  are  avail-
      able  in which the amounts of
      this constituent  are kept  below
      the  normal average.  ASTM
      Type  II  and ASTM Type V
      Portland  cements have a  trical-
      cium aluminate  content of ap-
      proximately eight  and five per-
      cent, respectively.  It appears,
      however,  that a high degree of
      resistance to sulfate attack alone
                         will not prevent destruction of
                         the concrete by  sulfuric  acid.
                           Pozzolans  when  added  to
                         Portland cement concrete, with-
                         out  reduction  of cement  con-
                         tent, may increase its resistance
                         to  corrosion.    The  pozzolans
                         combine with  the free  lime in
                         the cement to form a cementi-
                         tious  compound,  monocalcium
                         silicate.  The use  of pozzolanic
                         cements for sewer construction,
                         however, has been very limited.
                     (5) Aluminous and  Supersulfated
                         Cements:—The  basic  constitu-
                         ent  of   aluminous cements is
                         alumina and,  after hydration,
                         free alumina is present rather
                         than  free  lime.   The alumina
                         does not react readily with acids
                         in a pH range  above  three and
                         thus may provide better short-
                         term  protection against corro-
                         sive attack.   High aluminous
                         and Supersulfated cements con-
                         tain approximately 40  and  13
                         percent,  respectively,  of A12O3.
                           Aluminous cement is  being
                         used for sewer  construction in
                         several cities of the world where
                         concrete corrosion exists.   In
                         Southern California its use, un-
                         til recently, has  been limited to
                         repair of corroded concrete sur-
                         faces.  It is found that extreme
                         care must be exercised  in  mix-
                         ing, placing, and  curing to in-
                         sure that the aluminous  cement
                         concrete will  provide the  de-
                         sired dense, adherent, and cor-
                         rosive  resistance surface.
                     (c) Calcareous  Aggregates:—The
                         use of calcareous aggregate pro-
                         vides  additional mass of an al-
                         kaline material  for neutralizing
                         the acid,  thus  requiring more
                         acid to effect a given amount of
                         corrosion.  The  rate  at which
                         corrosion penetrates is directly
                         proportional to  the amount of
                         acid  available   and  inversely
                         proportional to  the exposed sur-

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                    PAINTS AND PROTECTIVE COATINGS
                                  47
      face area  of  reactive material.
      Other  factors which  also  will
      determine  this rate,  however,
      are  compressive strength,  spe-
      cific  gravity,  abrasion  resist-
      ance,  soundness,  and  absorp-
      tion.   The physical character-
      istics  of  limestone  aggregate
      vary  over a  wide range, even
      in the same quarry;  therefore,
      careful  testing  and  control
      methods must be. exercised when
      such   an  aggregate  is used.
      Dolomite containing magnesium
      carbonate  should  not  be used
      due to the reactivity  of mag-
      nesium  salts   with  cement  al-
      kalies.   It is  questionable  that
      calcareous  aggregate  concrete,
      without  extra  thickness,  will
      provide the necessary protection
      where  the  attack by acid is se-
      vere  and  concentrated  in  one
      location.  The cost of sacrificial
      calcareous  aggregate  concrete,
      in any case, must be considered
      against  the cost of other  pro-
      tective  methods.

4.265 Chemical Control:—
  4.2651 General:—The  sulfide gen-
erated in  the sewers is in  the  equilib-
rium H+ + HS^±H2S.  Whether the
H2S or the HS'  ion  predominates de-
pends on the pH of  the solution.  The
equilibrium  proportions are  constant
and unchangeable except  by a change
in pll.  The  total mass of material  in
each of these forms  is affected  by the
loss of H2S from the wastewater phase.
At  points of extreme turbulence, the
diffusion  of  HaS is accelerated  and
the reaction is  driven strongly toward
the  H2S  component.  Normally  the
wastewater is  near  neutral  and  pos-
sesses a strong buffering  capacity  so
that the ratio of  HS~ to H2S is about
three or four to one.  When a soluble
metal salt, which will react with sul-
fide is added, the action  is toward the
removal of all sulfide ions and, thus,
the prevention of H2S release to the
atmosphere.
  4.2652 Zinc : — Zinc salt solution has
been  found to be  very successful  in
the removal of sulfide ions.  Although
the zinc will react with other materials
present in the wastewater, the desired
reaction  is  satisfactory and  the  ZnS
formed is insoluble.  In addition,  rela-
tively  inexpensive zinc salt solutions,
in the form of industrial  waste, are
available as a result of certain indus-
trial processes.
  Zinc ions have been  investigated  in
their  reaction  with  the  four  anions
usually considered most important  in
wastewater  applications.    These are
OH",  S=, C03=  and the NH3  complex.
The  solubility  product  constant  of
ZnS,   Zn   (OH) 2  and  ZnCO3,  are
1 X 10-20, 5 x 10-ir and 2 X 1Q-10, re-
spectively.  Therefore, the ZnS should
precipitate first.  When hydrogen sul-
fide is passed into a  zinc  chloride so-
lution, precipitation takes place as
ZnCl2 + H2S
2CI-
                      -f 2H+

  The precipitation of ZnS soon ceases.
The II*  ions  produced  reach a  high
enough  concentration to  establish an
equilibrium.
           H++ S-
          HS- + H+ — H2S
  If the 11+ ions are  buffered with  a
salt,   such   as    sodium   acetate
(NaC2H3O2),  the   reaction  forming
ZnS can be driven  to  completion

     11+ + CJIjOa- ^ HCsHjOz
  The wastewater  generally contains
buffering materials adequate to main-
tain pll levels near  neutral at  the
normal zinc salt dosage rate.
  In  the presence  of hydrogen  ions
the concentration of  sulfide in  equi-
librium  with  the  hydrogen sulfide is
smaller  than  in a neutral  solution.
As  the pH increases,  the fraction of
the dissolved sulfides existing as sulfide
ions becomes so  small that a very high
concentration  of zinc ions  would be
necessary to exceed the  value of  the
solubility product of zinc sulfide.  Such

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48
                     PATNT8 AND PROTECTIVE COATINGS
a  condition would not be expected in
the outfall sewers unless a heavy acid
spill  was present.    The theoretical
ratio of zinc  metal to sulflde content
for complete  reaction  is 2.04 Ib to 1
Ib, respectively.
   4,2653  Copper  and  Other  Heavy
Metals:—The  salts   of  copper and
most  of the  other  heavy metals, such
as  lead,  trivalent   chromium,  and
cadmium, also  will precipitate sulfides
as insoluble metal sufides.  The use of
these metals is generally  too expensive
for routine sulfide control.  In a large
city, however, certain  amounts of such
heavy metals will reach the sewers as
industrial waste and aid in sulfide con-
trol.  Copper and  other  heavy metals
also can  exhibit strong  bacteriostatic
and bactericidal  action and,  although
presenting a problem during treatment
when such metals are present in high
concentrations, will inhibit the action
of bacteria in the sewers.
   4.2654 Iron:—It would be  expected
that  the  ferric salts  would  be more
effective than the ferrous compounds.
The principal effect of the ferric salt,
when added to waste-water of low sul-
fide content,  seems to be oxidation  of
sulfide  at the  time of mixing;  very
little  precipitation occurs  unless  the
initial sulfide concentration exceeds  10
mg/1.
   A large excess of iron is, therefore,
required when the concentration of the
 dissolved sulfides  is below 1.0 mg/1.
 It has  been found that  a mixture of
 iron containing about two-thirds  Fe++t
 and one-third Fe++  is more  effective
 than either alone.
   In contrast to  iron, zinc has the ad-
 vantage of a practically  complete re-
 action with dissolved sulfides.  Experi-
 mental  results indicate that the  Fe*+*
 has only 12 percent of the effectiveness
 of Zn++.  This difference in  effective-
 ness between the two ions is  due ap-
 parently  to the  hydrogen concentra-
 tion.   The  addition of Fet+l" ions can
 make the mixture  strongly acidic due
 to the  hydrolysis  of the ferric  ion.
 The IIS- +  IIf ;=± II2S equilibrium then
 is shifted to the  right.  For large out-
 fall sewers, however, the buffering ca-
 pacity is so great  that  the acid  shift
 does not become  apparent until 100 to
 200 mg/1 of Fe++f  is added.  For nor-
 mal  dosage rates,  Fef+l' would, there-
 fore, be preferable to Fef+, with zinc
 better than either.
  4,2655  Ammoniation:—It has  been
 theorized  that   ammonia  gas,  when
 added to  the sewer atmosphere, would
 dissolve  in the  moisture on  the ex-
 posed wall surfaces and neutralize the
 sulfuric  acid being formed.  Applica-
 tion  of  this proprietary  method has
 certain   technical  difficulties   arising
 from losses and dilution  of the gas as
 well as the  high cost of  maintaining
 the distribution  sytsem.
                             4.3  SUMMARY
  It generally is  considered that the
dissolved  sulfide level  in  the flowing
body of wastewater represents an equi-
librium between the production within
the subsurface slimes and sludge de-
posits,  the  oxidation  by  the oxygen
being  absorbed continually from  the
surface, and the evolution  of hydrogen
sulfide to the  atmosphere.   It is be-
lieved  that  little  sulfide  production
occurs  in  the flowing wastewater.
  Sulfate   is  considered  the  chief
source of sulfides; however, other or-
ganic sulfur compounds also contribute
to sulfide formation.  Sulfates are re-
duced easily  by  microbes;  moreover,
with the large amount generally found
in the wastewater, the rate of sulfide
production would be little  affected by
any   attempt  to  control  the sulfate
concentration.
  Due to the  rapid conversion of hy-
drogen  sulfide to  sulfurie acid on the
exposed sewer walls, the concentration

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                     PAINTS ANT) PROTECTIVE  COATINGS
                                   49
 of hydrogen sulfide in the sewer atmo-
 sphere generally  is far below the ex-
 pected value  for  equilibrium with the
 wastewater.   Concentrations of  5 to
 10 percent acid generally are found on
 acid resistant surfaces where this con-
 version is taking  place.
   In  the  design  of branch or  local
 sewer  systems, consideration should be
 given  to the proper velocities required
 to  maintain  the  generation-oxidation
 equilibrium  at  a level  to prevent  a
 sulfide buildup.  Turbulence should be
 minimized to  prevent hydrogen sulflde
 release in large interceptors and  land
 outfalls   where  odor  and  corrosion
 problems are  most  common.   Large
 concrete sewers probably  can be  pro-
 tected most positively from corrosion
 by PVC sheet liner  where normal flow
 velocities exist.  Because of the danger
 of mechanical damage to the PVC lin-
 ing high  velocity  lines should be  con-
 structed of clay pipe.
   Bate of production of  sulfide in sew-
 ers may be diminished by  diluting the
 wastewater with unpolluted water, by
 removing industrial wastes  of  high
 temperature  or with high content  of
 organic matter, by restricting the ad-
 dition   of septic  tank  and  cesspool
 pumpings, or by  partial treatment  of
 the  wastewater  to  lower  the  BOD.
 Regular and  thorough  cleaning  also
 will  aid materially in limiting  sulfide
 production.
  Air  or  oxygen may be injected  into
 force mains  in  order to  prevent  the
 sulflde generation.  A controlled with-
 drawal of air, however, should be  con-
 sidered to prevent air pockets  in  the
 top of the  sewer to avoid corrosion
 and increase in pumping  head.
  Chlorine is  considered useful  in the
 control of sulfides.  When an adequate
 amount of chlorine is applied, it leaves
 the wastewater  in an oxidized state,
and existing sulfides may  be oxidized
to sulfate.
  Zinc and other heavy metal salts are
effective in the treatment  of wastewa-
ter containing dissolved sulfides.
 4.31 Designing and  Building  to
      Prevent Corrosion

   Many  instances of corrosion could
 be corrected by better design and con-
 struction methods.   The case for  dis-
 similar  metals  or galvanic  corrosion
 has been mentioned  previously and is
 very  important.  Corrosion   of   an
 aluminum rivet can  be expected when.
 it  is used to fasten steel  sheets  to-
 gether.   Similarly if a  steel  rivet  is
 used to fasten aluminum sheets, then
 undercutting galvanic corrosion of the
 aluminum sheet  will result in loose
 rivets, slipping, and possible structural
 damage.   Corrosion  of  this type  can
 prevented by applying  a non-harden-
 ing insulating joint  compound in the
 area where the sheet and rivet  or bolt
 are in  contact.   Another approach  is
 to apply a zinc chromate primer to all
 contacting surfaces and  then coat the
 primed area with an aluminum paint.
  Another cause of galvanic corrosion
 that should  be  avoided is the  use of
 steel and brass  or copper pipe in the
 same system.
   In the use of structural steel avoid
 sections that are hard  to clean and
 coat  such  as   back-to-back   angles,
 beams,  etc.;  also  flat and dished sec-
 tions that will collect and retain mois-
 ture.
  Whenever   possible sharp  features
 where moisture, liquids, and solids can
 accumulate should be avoided and  all
 corners   and  contours   should   be
 rounded.
  Construction  of  angles,  channels,
 and beams should be arranged so  as
 not  to  leave catchment  areas for liq-
 uids.  If this is not possible the  ap-
 propriate size and number  of  drain-
 age  holes should be  provided.  These
 should  not only be  kept clean from
 blockage but also should be sited care-
 fully and attention  paid to disposal
 of the  drainage.
  The   various  methods  of  joining
 should be considered.  For ease of pro-
tection  butt-weld joints are preferable

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50
                    PAINTS AN].) PROTECTIVE COATINGS
to those that are lap-welded.  If,  how-
ever,  it is essential to use the latter,
all exposed  edges should be treated in
such a way as to prevent the access and
retention  of liquids  and  dirt  in the
crevices.
  Any storage  containers  and  tanks
should be supported  on legs  so  that
free  circulation of  air is possible and
condensation is prevented.  Condensa-
tion  also can be prevented by the use
of insulation.
  Evaporation of condensed  moisture
often is   retarded  on  sheltered  hori-
zontal surfaces such  as those under
the eaves  of buildings.  Such features
should  be  provided  with  breathing
holes  or  given additional protection
such as a  coat of water-resistant finish-
ing1 paint.
  Where  the  coating  to be  used will
not be  harmed by fabrication  or  can
be touched up easily after  such fabri-
cation, surface preparation and coating
applications should be done in the shop
where controlled conditions such  as
temperature  and humidity  exist and
good  inspection  is available.  An ex-
ample of the above is in the shipbuild-
ing   industry.   Many  prefabricated
parts of ships are stored in the open,
subject  to  corrosion  for  as long  as
two  years  without protection.   Some
ship  builders  are  now coating these
sub-assemblies   with   inorganic  zinc
coatings over  a  shot-blasted or sand-
blasted  surface.  This provides  protec-
tion  during the building period and a
good  base for a top coating after com-
pletion.  If welding is used to join the
parts, only  a small area of coating is
destroyed and it can be retouched.

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     5.  ACTION  OF  DESTRUCTIVE AGENTS  ON
                           PAINT  FILMS

                            5.1  GENERAL
  The  slogan  of  the  Paint Manu-
facturers' Association, "Save the Sur-
face and You  Save All,"  is  most
appropriate  advice.  Corrosion begins
on the surface of the metal and works
inwardly.  Destructive  action on the
paint film normally begins on the sur-
face of the paint and works inwardly.
  If it were possible to provide a coat-
ing over the surface of metals which
would  cover them  completely and re-
main that way,  adhere  tenaciously  to
the base material, be impervious to all
liquids and gases, be inert to chemical
union with the metals and its environ-
ment, be a non-conductor of electricity,
and stand up under abrasion and ex-
posure to  sunlight, there would be no
serious corrosion  problem.   However,
since it is not possible or even practical
to isolate  metals  in this  way, it re-
mains to investigate the agents which
destroy or make ineffective the surface
protection afforded  by paint films.
                    5.2 DESTRUCTIVE AGENTS
5.21 Water
  Water, with its property of dissolv-
ing  more  materials than  any  other
single liquid, and its capillary attrac-
tion, provides the close contact between
the paint film and other  destructive
agencies.   Water will find imperfec-
tions in the coating and filter through
which to reach the metal beneath.  The
products of corrosion, or merely the ex-
pansion of  the water at  the metal sur-
face, tend  to  separate the paint film
from the metal.  This action first may
be noticed as a series of isolated  spots.
If left  to  run  its  course,  the  spots
enlarge   until   their   circumferences
meet, thus lifting more and more paint
and exposing progressively larger areas
to metallic  corrosion  (see  Figure 4).
  Water will dissolve or soften  many
paints  making the  film more vulner-
able  to ther destructive agents.  Water
will carry acids and alkalis from other
areas into direct  contact with the paint
surface.
  The painting of a metal surface that
is wet, or painting on a day when the
relative humidity of the  air is high,
is  certain to retain enough  moisture
in the film or beneath it to cause early
paint failure and  incipient  corrosion.
The  inclusion of water in an oil paint
will  have the same effect.
  The presence of an excessive amount
of moisture in wood that  is  painted
will  result in the water vapor forming
blisters  with a subsequent  lifting of
the coating.

5.22 Air and  Gases
  The  air is another agency that  de-
teriorates paint films.  The  oxygen of
the  air unites  with the pigments or
vehicles of the  paint, causing them to
form products which may be granular
and  porous.  It also may produce com-
binations which require less volume
than the original  coating,  thus pro-
ducing many tiny  cracks or checks in
the film. It may dry out the coating
to the point where it is no longer flex-
ible, causing  it ultimately  to  crack
and  peel.   The aging  of paint is a
gradual oxidizing  of the vehicle and
the pigment, resulting in a brittle and
chalky surface.
                                    51

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  52
                      PAINTS AND 1'1,'oTKi'TIVIC COATINGS
          FIGURE 4.—Results of water vapor  penetration of  a  coating thus
        forming a concentrated solution which blisters at the coating-steel inter-
        face.  (Courtesy Ametcoat Corporation.)
   Cerliiin gases in the air which  are
 prevalent in industrial areas are very
 injurious to ordinary paint films.  The
 two most important of these are hydro-
 gen sulflde and  sulfur  dioxide  .  As
 mentioned  under  metallic corrosion,
 they attack meials  readily because of
 the acids they form in their union with
 water  and oxygen.  Many paint pig-
 ments  are metallic  derivatives  and
 their reactions with the two gases,  di-
 rectly  or with the acid forms,  produce
 substances which no longer protect the
 metal  beneath.
  Another result is  the discoloration
 of the  paint film.  A familiar example
 of this is the  darkening of white lead
 paint  by the  action  of hydrogen  sul-
 fide  in the air.  In  this instance the
white lead carbonate is changed slowly
to the  black lead  sulflde.  As little as
 1 to  10 mg/1 of hydrogen sulfide in the
atmosphere will be enough to  make a
noticeable  change in the whiteness of
white lead paint.
 5.23  Chemicals
   The  acids  and  alkaline  substances
 used  in  wastewater  treatment  proc-
 esses  and those which  are  sometimes
 brought in with ihr  wastewater ;i.s in-
 dustrial Avaste arc very destructive to
 ordinary paint  films.   This  usually is
 lirought about by the direct action of
 the  chemical  on  the  paint  coating,
 rausing  it to form another  compound
 which has no protective value or one
 that  loses its  bond  to  the  metal be-
 neath.

 5.24  Sunlig-ht  and  Heat
   The ultra-violet light  in  the sun's
 rays causes some paint films to change
 their  chemical composition,  which re-
 sults  in  fading of colors, drying  out,
 and cracking.
   Heat above  the ordinary  range of
temperatures  produces  disintegration
 of paint  films by the breaking down
 or drying out  of  the  vehicular oils.

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                    PAINTS AND  PROTECTIVE COATINCS
                                   53
This  is  particularly  noticeable  011
boiler  fronts,  breechings,  and  stacks
where  a heat-resistant paint  has not
been used.

5.25 Oils  and Greases
  The  mineral  oils and  greases used
around a wastewater  treatment plant
for lubricating  purposes  and  the  fats
and organic greases brought  in with
the  waslewater  have  a  deteriorating
effect on ordinary paint films.
  The  mineral  oils and  greases  dam-
age  paint  coatings  by  softening or
dissolving   them.   These   lubricants
sometimes   contain  traces  of  sulfur
which  may have been in the  oriiriiui!
crude  oil  or  left  from   the  refining
process.  They also may  contain small
portions of the lighter ends of  the frac-
tional   distillation   process,   such  as
kerosene and gasoline.
  Greases are made by adding a lubri-
cating oil  to a  soap  base to  get the
desired consistency.   These soaps are
a mixture  of  fats and an alkali, usu-
ally  lime or  soda.   The  presence of
any  free alkali  in the grease explains
their damaging effect on  paint films.
  The fats and organic greases usu-
ally  found in  wastewater oxidize read-
ily on contact with the air.  As  oxida-
tion   proceeds,  fatty  acids  are pro-
duced.  These acids dissolve and soften
paint, or  destroy  its bond  with the
metal  beneath,  causing  it  to  slough
off in large pieces.   On  metal parts
of sludge and grease-collecting  equip-
ment,   such  as   chains,   sprockets,
troughs, etc., which usually  are not
painted, these  acids  may  attack the
iron.

5.26  Paint Cleaners
  Paint-cleaning compounds can be es-
pecially injurious to the ordinary paint
rniiiin.u-s if  not  used properly.   Many
of these compounds are strongly alka-
line while others contain solvents which
           FIGURE 5.—Abrasion test result.  Circle shows path of abrasive
                 wheels on coating.  (Courtesy Amercoat Corporation.)

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                    PAINTS AND PROTECTIVE COATINGS
  FIGURE 6.—Chipping causes cracks in coatings resulting in more extensive damage.
                         (Courtesy Ametcoat Corporation.)
soften paints.   Each type  cleans  by
removing the thin outer layer of the
paint film which has  become  chalky
and rough.  The rough, uneven surface
provides the tooth to which dirt  can
cling.
  A too-concentrated solution of either
kind of cleaner will remove more than
the top layer and shorten the normal
life of the coating. For average clean-
ing  jobs a  mild  soap-and-clean-water
mixture should be sufficient.  Abrasive
cleaners  should be avoided.

5.27 Abrasion

  The damage to paint films  by fric-
tional  abrasion  (see  Figure 5) is due
in most  cases to ordinary wear.   The
familiar examples are  the worn spots
that show up in areas of concentrated
traffic on  painted floors, stairs,  and
handrails.  When it  is  considered  that
th<> average  paint film is about 1/500
in.  (0.0051  cm)  thick, it  is quite re-
markable  it  stands up  as well as it
does under this type of use.
  Mechanical damage to paints, aside
from abrasive wear, consists of chip-
ping,  scratching,   loss  of luster,  etc.
Chipping  usually occurs to paint films
that  have  become  too  thick  from
repeated coats or have become dry  and
brittle (see Figure 6).  Scratches re-
move a portion of the  paint  film, or
all  of it, depending on the severity of
the scratch (see Figure 7).  This re-
duces the protection  to the  metal if
the scratch is light. If the scratch goes
entirely through the paint coating, the
door is open for corrosive action.
  The loss  of luster  may be  due to
several causes, but  under  the  heading
of mechanical damage it usually is due
to repeated wiping of the paint to keep
it clean.  Even though the wiping is
done with a soft rag, the luster gradu-
ally is worn off.
         FIGURE 7.—Scratches cause further flaking away and further damage.
                         (Couttesy Amercoat Corporation.)

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                    PAINTS AN!) PROTECTIVE COATINGS
                5.3 METHODS OF PAINT TESTING
5.31 General
Laboratory or field  tests are  only ap-
proximations  of  the  conditions  ex-
pected  to  be  encountered in  actual
service.  The test for quality is dura-
bility,  and no  test  or group of  tests
can replace actual exposure.  However,
good  laboratory or  field tests  intelli-
gently applied usually can. be depended
on to give reliable data on the physi-
cal properties of different  coatings.

5.32 Laboratory Tests
   There are  three   general  types of
tests performed  in  the laboratory:

   1. Package  and   fluid  properties
which measure  such properties as set-
tling,  floating, viscosity, specific  grav-
ity,  reducibility,  spraying properties,
and odor.
   2. Chemical  and  physical properties
—includes  chemical analysis on  pig-
ment,   binder,  solvent, determination
of pigment volume,  ash,  suspended
matter, etc.
   3. Panel    performance    tests—in-
 cludes tests which   are  based  on  the
 dry film characteristics of the material
 such as color,  water  resistance  flexi-
 bility,  hardness and adhesion and is
 conducted under accelerated test con-
 ditions.  This  test is  considered  the
 heart  of the  performance test.   To
 be of  value, it must simulate as nearly
 as possible  actual  service  conditions.

   The  performance  of a  paint film
 will vary with the  substrate used.  A
 paint  which  tests   well  on  glass  or
 metal  may be  destroyed quickly  on
 concrete.   The substrate used should
 be the same as  the substrate in  actual
 service.
 - The  finish  used   on the test  panel
 should be  exactly   the  same  as used
 in actual service.  Test panels should
 be treated physically (scraped, sanded,
 buffed, polished,  etc.)  as  in  actual
 service.  The condition of the surface
 will  affect the  mechanical  adhesion.
The  life of  a paint applied to  iron
and  steel is  determined primarily by
the surface condition of the metal.
  Paints are  applied by spraying  Dip-
ping,  brushing,  roller coating,  etc.
The  method  of application to the test
panel should  be the same as in the field.
The  temperature of the substrate and
the  paint,  the use of thiner, etc., all
play a part  in the rate of drying and
in the appearance of the finish.  They
should  correspond  to  field practices.
   Film  thickness  is  very  important
and  must be controlled and measured.
Top coats and primers should be tested
together according to the paint system
to be used.
   The  principal   factors  which  are
evaluated  in panel testing of paints
are   hardness,   adhesion,   blistering,
brittleness, wrinkling, softness,  fading,
darkening,   chalking,  checking, crack-
ing, rusting, and discoloration.  These
are  rated as slight, moderate, and ex-
tensive, or  good  to excellent,  fair to
good, and bad to poor.
   It is important  in  estimating out-
door durability to determine resistance
to moisture.  Moisture can affect  paint
both chemically and physically.   The
 chemical reaction is principally hydra-
 tion.  This is a slow reaction generally
 not measurable in laboratory tests but
 may  become  important  in the field
 where the paint  is immersed for long
 periods of time.   The physical effects
 are colloidal,  electrical,  and mechani-
 cal, and largely are dependent on  the
 nature  of the paint film  and the sub-
 strata.  The colloidal effects are gen-
 erally inhibiting.  The electrical effects
 are caused by potential differences and
 mechanical  effects are caused by  the
 penetration of the water  through  the
 film.   The  effect  of each separate re-
 action generally is not known,  only the
 net effect of all  three is determined.
   For  comparative   purposes,  water
 testing should be carried out within
 plus  or minus 2°C in distilled water.

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56
PAINTS AND PEOTBCTIVE COATINGS
As a  general  rule,  the longer  the
immersion period, the more severe the
test.

5.33 Field Tests
   Outdoor exposures are regarded  by
paint  chemists  as the  ultimate  test.
Yet  outdoor  exposure is in  itself  a
highly variable test.  Florida  exposure
is used widely  by paint laboratories.
There  is  less  month-to-month  weather
fluctuations than in  other  locations,
and in addition  the rate of destruction
is much greater.
   The   angle  of  inclination  of  the
panels is  difficult to select.  Exposures
at 45 deg do not give the same results
as vertical exposures.  The angle needs
to be selected to correspond  with the
field use.
   Panels  that receive dew followed  by
sunshine show more rapid failure than
others  where there is  no dew.  Water
plays an  extremely important part in
the destruction of finishes even exceed-
                   ing  that  of  light.  Fumes  and gases
                   in the air particularly  can be destruc-
                   tive.   These observations  all point to
                   the need  of locating the test panel in
                   the  same  atmosphere  where  the  use
                   will be.
                     Because of  the  variable  conditions
                   encountered  in field testing, it is im-
                   portant to conduct numerous tests so
                   the  results  can be evaluated statisti-
                   cally.

                   5.34  Test Standards
                     Laboratory  testing  of  paints  re-
                   mains an art and  subject to personal
                   interpretation  despite  the  many in-
                   struments and scientific tests that are
                   available. The fundamental properties
                   of hardness, adhesion,  cohesion, flexi-
                   bility, etc., when judged by a trained
                   expert generally is more  useful  than
                   the results  of  mechanical tests.  The
                   fiingernail and the trained  eye  be-
                   come the tools of  the expert to make
                   the evaluation.

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  6. PREPARATION  OF SURFACE FOR  PAINTING
  Too much emphasis cannot be given
to the  manner  of  preparing the sur-
face as the most important phase of
the  operation   of   applying   paint.
Proper surface  preparation easily can
account for the major proportion of
the manhours for the whole paint pob.
In  fact, it may account  for a  major
portion of the  total cost of the  job.
It is most important that the surface
preparation be done  properly  or  all
may be lost.
  The performance of any paint appli-
cation is affected profoundly by  water,
grease,  oil,  mill scale, rust,  alkalies,
hydrogen  sulflde, sunlight, air,  micro-
organisms, DO, and  other items  in-
cluding the physical  qualities of the
cleaned surface. Surface preparation
for painting in wastewater treatment
plants  must take most of these  enemy
agents  into account.   The types of
surfaces needing protection are more
varied than found elsewhere.  Every
type  of  construction  material is in-
volved, submerged both in wastewater
and  cleaner waters as well as those
exposed to the elements.
  The quality  of the prepared surface
must  be  judged from the standpoint
of both its freedom from contaminat-
ing substances as  well as its  ability to
provide firm anchorage for  the paint
applied.  Since the many kinds of  sur-
faces  needing  protection  are common
to both the large  and the small plant
it will at times be difficult for the op-
erator of the  smaller plant to select
and  use  the best methods of surface
preparation due to the restrictions of
manpower, equipment  available,  and
money resources.  Considerable thought
and ingenuity  may be required, there-
fore,  to  balance  resources  and  cost
against satisfactory results.
           6.1  TOOLS FOR SURFACE  PREPARATION
  Tools available for the  preparation
of surfaces for painting are many and
of varied types.   They can be divided
into two basic groups, hand  tools and
power tools.

6.11 Hand Tools
  The steel "wire  brush is available uni-
versally  in a  variety  of sizes  and
shapes to fit the need.
  Scrapers also  are available in many
sizes  and forms.  Many  have blades
that can be resharpened. An excellent
scraper can  be   made  by reforming
large flat files.   The  end is turned,
widened, edged,  and tempered.  They
are effective and long lasting.
  The chipping   hammer  is  available
at hardware and mill supply stores.
  Sand and emery paper are available
almost  everywhere.   The   so-called
aluminum oxide open-coat  production
papers are well suited for  good  abra-
sion and longer life.   A cloth-backed
emery also is available and can be used
wet.
  Steel  wool  is available  in  various
grades.  It is used normally on a smooth
surface  and quite often in conjunction
with  a  cleaner.
  The  blowtorch  or  similar  device
often is used for intense  heat applica-
tion and  for  scale or paint removal.
  There are various  chemicals  avail-
able that  might be classified as  hand
tools. They are washing powders,  de-
tergents,  and trisodium   phosphate
 (TSP).
  Solvent cleaning, although the least
efficient of the chemical removal  meth-
ods, still is  used commonly  to remove
grease,  oil, and films  prior  to a more
                                    57

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58
PAINTS AND PROTECTIVE COATINGS
effective cleaning method.   Some of
the solvents are naptha, Stoddard sol-
vent,   toluene,  trichlorethylene,  and
mineral spirits.
  Steam and water jets are used where
conditions  so indicate  and when the
apparatus  is  available.
  Effort and  patience are  of  prime
importance and  are  required  for all
of the above tools.  The workman must
have  had  proper  instruction  in the
use of the tools  for cleaning and the
work must be  inspected to see  that
it was done properly.

6.12  Power  Tools

  The  ever  increasing  cost of  hand
labor finds the use of power tools es-
sential.  If they are not a part of the
plant  equipment  inventory, they can
be leased or rented in most areas.  It
would be  well to  evaluate their cost
vs. hand  tools and labor.
   The air or electric motor with flex-
ible shaft is used with a disc or wheel-
type  steel wire  brushes.  There is  a
large range  of sizes and shapes avail-
able.   One  company  recommends  a
speed of  450 rpm using  a working
pressure of  150 psi  (10.5  kg/sq cm)
for  thorough,  fast,   and  economical
work. The wire brushes specified were
austentitic  chrome  nickle  steel  wire
bristles.
   Air or  electric motors with rotating
                   heads  using  disc,  wire  brushes,  or
                   rotary impact cleaning tools are used
                   often on steel surfaces for removal of
                   some mill scale and rust.
                     Air  driven  paint scrapers and  spe-
                   cial  chisels are available and used ef-
                   fectively.
                     Motor-driven   power  sanders  are
                   adapted  to use discs, drums,  or cones
                   of  varied  sizes  to   suit  the need.
                   Aluminum oxide open-coat papers are
                   used on  this machine.
                     Sand  arid shot  blasting  equipment
                   are  efficient and effective when prop-
                   erly used.   Blasting of steel surface
                   will  remove  rust, mill scale, and old
                   paints along  with some  of  the  base
                   metal.  There are three types of blast
                   cleaning: (a) abrasive in a  stream of
                   high-pressure air,  (&) abrasive in a
                   stream of high-pressure liquid, such as
                   water, and (c) the  abrasive discharged
                   from the periphery  of a rotating paddle
                   wheel  traveling at  high  peripheral
                   speed.   The  first  two are  known as
                   nozzle blast  cleaning.  There  are sev-
                   eral types of abrasives used  in blast
                   cleaning  such  as  metallic,  siliceous,
                   synthetic non-metallic,  and nut shells.
                     Flame priming and descaling equip-
                   ment also are used.  This is a method
                   for  preparing   ferrous  surfaces  by
                   passing  high  velocity  oxyacetylene
                   flames over  the  surface.   Flame tips
                   are  provided for the particular type of
                   operation in which they are used.
             6.2 PREPARATION OF STEEL SURFACES
 6.21 Justification for Cleaning
   It is fundamental that paint on steel
 surfaces will not adhere permanently
 nor  prevent corrosion  unless  placed
 in intimate  contact with sound, clean
 metal when  that metal is dry.
   Ferrous surfaces present  the  most
 complex problems for paint protection
 of all surfaces.   This is due to the well-
 known tendency of iron surfaces to be
 attacked by  chemical and electrochemi-
 cal reactions in the presence of mois-
 ture, oxygen,  and other corrosion  ac-
                    celerators.   The  two  main  problems
                    to be dealt with are rust and mill scale.
                    6,211  Bust:—Iron and steel products
                    are never  homogeneous in structure.
                    Therefore,  their  surfaces  present  in-
                    numerable points  of differing electrical
                    potential.   Electric  currents, carried
                    by moisture  laden with soluble salts,
                    attack the  metal.  At  the anode, the
                    positive pole, iron goes into solution
                    forming ferrous hydroxide by combin-
                    ing   with    the   moisture   present.
                    Simultaneously, hydrogen is released

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                    PAINTS AND PROTECTIVE COATINGS
                                                                         59
at the  cathode, or  negative  pole,  of
this minute battery.  At the anode, the
ferrous hydroxide formed is  changed
to ferric hydroxide by the oxygen pres-
ent in  the air.  This compound is rust.
The rust is insoluble and deposits onto
the metal surface.  Rust is hygroscopic
and, therefore, tends to retain moisture
to continue the battery action. Hence,
the old saying,  "rust begets  rust."
Rust has bulk and  tends  to  create  a
heaving action on  mill scale and paint
films.
6.212  Mill Scale:—Mill  scale  is es-
sentially  an  oxide  of  iron  (Fe30i).
It is  formed on the surface of  steel
during the process of rolling  the steel.
It is brittle  and  subject  to  cracking
and  scaling.   Since its coefficient of
expansion differs from  that of  the
base metal,  temperature  changes af-
fect  the uniformity of its adherence.
Moisture   then  tends   to  seep  under
seemingly tight  scale  to form  rust.
Mill scale is  cathodic to  steel so that
electrochemical action  causes an ac-
celerated local corrosion in  the pres-
ence of moisture.   Complete removal
of mill scale immediately before  prime
coating is the best  method of protec-
tion from its presence.
   It is obvious that  the  presence of
rust and  mill scale  presents a difficult
mechanical  problem  particularly  on
old equipment and where  angles, rivet
heads, and gussett plates complicate the
surface.   The usual  specification for
the  preparation  of a ferrous  metal
surface for  painting  states  that the
surface shall  be  dry,  free  from  mill
scale,  rust, oil, grease, paint films, and
all other  deposits.

6.22  Mechanical Cleaning Methods
   Effective mechanical methods to ac-
complish cleaning include wire brush-
ing, chipping and  scraping, sanding,
sand or shot blasting, and flame  con-
 ditioning.
   Hand  or  power steel-wire brushing
is the  most  widely  used method.  The
method, however, tends to remove  only
the more  loosely adherent scale, rust,
and paint films.  Power wire brushing
is  by  far the more effective.   Wire
brushing  in  general, is considered to
be a "high-spot" hitting method.   It
should be followed  by scraping.
  Chipping and scraping offers one of
the  least  effective  methods.    Hand
scraping  removes only  loosely adher-
ing scale  and paint.  Power chipping
hammers  are not recommended since
they tend to beat corrosion products
into the metal and leave ridges  and a
roughened surface that cause the sub-
sequently applied paint films  to vary
in thickness.
  Sanding is only useful on small  and
slightly corroded surfaces that are not
too irregular in shape.  Sanding  would
be incapable of removing mill scale.
   Sand or shot  blasting are  methods
generally  used  for  thorough  cleaning
of steel, both in the shop and in the
field. With this method there are three
degrees of cleanliness of steel  which
can  be had.
   (a)  White metal  blast described in
Steel  Structure   Painting   Council
specifications SSPC - SP5 - 52T.   This
classification calls for the complete re-
moval of all  corrosion  products,  all
mill  scale,  all paint,  and all  other
foreign matter. The metal after clean-
ing  has  a light gray uniform surface
with a good anchor pattern for excel-
lent adhesion by the paint coating.
   (i>) Commercial  blast  as   is   de-
scribed  in  Steel Structure Painting
Council    specifications  SSPC - SP6 -
52T. This classification calls for  a good
blast  but not perfect as in  the  case
of white  metal.  Practically  all  mill
scale, paint,  and rust will  have been
removed.  The surface will not neces-
sarily be uniform in appearance.  This
grade of blasting also  will give a good
 anchor pattern.
   (c) Brush-off blast  cleaning  is de-
 scribed  in the Steel  Structure  Paint-
 ing Council specifications SSPC  - SPG -
 52T.  This  classification calls for the
 removal  of  loose rust  and loose  mill

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60
PAINTS AND PROTECTIVE COATINGS
scale.  The blast should clean the sur-
face sufficiently to give an anchor pat-
tern for paint coatings.
  The type  of  protective  coating  to
be used often dictates the type of blast
to be used.  The operator should  be
sure he provides the best blast surface
he can afford economically.  The blast-
ing  should  be  performed on days  of
low humidity, and it is very important
that the compressed air be dry and free
of oil vapor.  Dry sand free of dirt
is a prerequisite  for good work . The
blasted surface should be brushed  or
cleaned  with dry  air just  prior  to
applying the paint. It is essential that
the clean metal be painted that same
day or  as  soon  as possible for  the
clean metal will begin to rust and de-
feat the purpose  of blasting.
     Flame cleaning is  effective for pre-
paring steel  surfaces for painting.  In
addition to  its cleaning  action,  flame
cleaning leaves the surface dry, warm,
and in  good condition  for  receiving
paint.  The flame loosens  the scale and
rust.  As soon as the flame  head has
passed,  the  steel  surface  is   wire
brushed to  remove the loose  material.
It is an advantage then  to apply the
paint  while the steel is still  warm.
In  this  way, painting sometimes  can
be done under cold or  damp conditions
which  otherwise  might  cause  delay.
Multiple flame-in-line  heads  are  tised
ranging in  width  from  1 to  12  in.
 (2.54  to 30.5 cm) and  attached  to
standard welding blow pipes.  With a
6-in. (15.3-cm)  flame  head, relatively
clean  surfaces can be treated  at the
rate  of  1,000 sq ft/hr  (93 sq m/hr)
while  on heavily rusted  riveted  sec-
tions the rate  will drop  to about 200
sq  ft/hr (18.6 sq m/hr).  There may
be  considerable danger connected with
the use  of  flame cleaning at a waste-
water treatment  plant,  especially in
confined spaces where gasoline fumes,
methane, etc., may be  present or where
the  paint itself may contain  explosive
types of volatiles.  This method  is not
recommended for submerged  metals.
                   6.23 Chemical Cleaning Methods
                     An alternate approach to the prepa-
                   ration of ferrous metal  surfaces for
                   painting  involves  chemical methods,
                   such  as  pickling,  weathering,   phos-
                   phatizing  and   chromatizing,   alkali
                   primers, and  the use of solvents for
                   degreasing.
                   6.231 Pickling:—This  method  need
                   not  be considered  for use  in  waste-
                   water  treatment  plants.   This process
                   as well as bonderizing and parkerizing
                   are strictly factory methods.
                   6.232 Phosphatizing-Chromatizing: —
                   If a rusted surface is free  from mill
                   scale, it may not be desirable or prac-
                   tical to remove all the rust.   The alter-
                   nate method of phosphatizing depends
                   on the principal  that the normal elec-
                   trochemical action can be slowed down
                   or arrested by passivating  the  metal
                   by forming an oxide layer  on its sur-
                   face.  The Metropolitan Sanitary Dis-
                   trict of Greater Chicago recommended
                   a  solution for this  purpose carrying
                   15 percent of phosphoric acid (H3PO4)
                   by weight  of  the  total  liquid.  The
                   liquid is also to  contain   a wetting
                   agent  in sufficient  amounts to make
                   the acid miseible with the water.  This
                   acid solution is to  be used  at the rate
                   of 1 gal/1,500 sq  ft  (0.3 1/sq  m)  of
                   surface.   The solution is to be scrubbed
                   thoroughly  into  the  prepared  sur-
                   face  and allowed  to dry over  night.
                   Pools  of   excess  liquid  should  be
                   avoided or removed.   No water should
                   be allowed  to contact the treated sur-
                   face.  When ready to apply the paint,
                   the surface should be dry and present
                   a sprinkling of hard, dry, white phos-
                   phate crystals.  Military specifications
                   have  been  written for  a  conditioner
                   similar to  the above  and  carry the
                   number MIL-M-10578A  Type II.
                      Chromic  acid  is  used in a  similar
                   manner.  Some of the acid inhibiting
                   solutions  offered for sale contain both
                   phosphates  as  well as chromates.
                   6.233  Alkali   Primers:—Another
                   method of checking rust formation  by
                   slowing  down the electrochemical ac-

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                    PAINTS AND PROTECTIVE COATINGS
                                  61
tivity is to create an  alkaline atmo-
sphere at the metal surface.  This pro-
cedure is based on the acid reaction of
the rusting process.  Red lead has an
alkaline reaction which partly accounts
for its  long  and  favored use as  a
metal priming pigment.  The sugges-
tion of applying a preliminary coating
of raw linseed oil  ahead of the primer
coat to drive moisture out of the pores
of the metal appears to have merit.
  One well-known paint manufacturer
offers a metal primer whose  pigment
is Portland cement  suspended in  a
linseed  oil  vehicle.  Here  again,  a
strong alkaline  reaction  is  produced
at the metal surface.  Zinc  chromate
pigments offer similar properties.
6.234 Other  Methods:—When  paint-
ing  metal  surfaces  in damp  places,
such  as wet wells, where it  is quite
impossible to attain  dry surfaces, the
Chicago  Metro  District  directs that
after proper cleaning of the surface
it shall be  washed with  rags soaked
in alcohol,  mineral spirits, or turpen-
tine.   The  surface   shall  be  well
scrubbed so as to get penetration into
all cracks and crevices to drive out the
moisture.   The  surface then  shall  be
wiped with  dry, clean cloths  and the
primer coat immediately applied.
         6.3 PREPARATION OF CONCRETE SURFACES
  In addition to the general and fun-
damental  rule  that  surfaces  shall be
free from all loose dirt, scale, grease,
oil, etc., concrete  surfaces, especially
if new, need  to be "cured."

6.31 Concrete Walls
  When  Portland cement hardens, a
considerable  amount of  calcium  hy-
droxide  (Ca(OH)2)  is  formed.   If
this compound is not neutralized prop-
erly,  the  alkaline calcium  hydroxide
in  contact with   linseed  oil  vehicles
tends to saponify the  oil,  producing
soaps that destroy the values of the
coatings applied.
  The usual  remedy is to wash  the
surface with  a solution  of zinc sulfate
(ZnSO.f),  using a solution of 2 Ib of
the zinc  salt/gal  (0.24  kg/1).   The
zinc sulfate combines with the calcium
hydroxide  to   form   zinc   hydrate
(Zn(OH)2)  and calcium  sulfate (Ca-
804), both of which are  used as  pig-
ments in  paint.   This  process consti-
tutes  the  "curing" of the  wall.  A
two percent zinc chloride-three percent
phosphoric acid  solution  may be  a
better wash than the zinc sulfate solu-
tion.  If  the surfaces are sufficiently
aged  and weathered   by time,  this
treatment will not be necessary.  Con-
crete  should  be two years old before
it can be coated safely with oil paints.
The  zinc sulfate wash will neutralize
the alkali on the surface of new con-
crete, but more  will come out.
  If  concrete walls  have been  aged
sufficiently and have been painted pre-
viously with  water  base  paints  or
wainscoated  with  bituminous paints
and it is desired to apply more perma-
nent coatings such as the modern rub-
ber base paints or enamels, it is neces-
sary  to clean the surface thoroughly
by sand blasting to the original sur-
face.  This procedure at  the same time
"tooths" it.   The  further stipulation
is  that  the first or prime coat should
be brushed on.  Succeeding coats may
be brushed, rolled, or air applied as
desired.

6.32 Concrete  Floors
6.321 Free From Oil and  Grease I—-
Where it is desired to renew the paint
on old painted concrete  floors, one of
three methods may  be  chosen:

  1.  The floor  can be cleaned of old
coatings by the  use of a sanding  ma-
chine.   This  method  is  very effective
but extremely dusty.  Such  a proce-
dure, however, has  the  advantage of
leaving the surface well roughed so
that  if the prime  coats are thinned
properly, adherence will be  excellent.
  2.  The old paint  can be removed by

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62
PAINTS AND PROTECTIVE COATINGS
paint removers.  This procedure is ef-
fective  and free from  the dust  prob-
lem.  It, however, involves the tedious
job of removing the resultant solvent-
film mess and the washing of the floor
with turpentine or  mineral  spirits.
  3. A  third  procedure could be  the
dry  method  of  scraping  and  wire
brushing.  This method does  not  in-
volve the dust or the muss of the other
methods but  it  is  also not quite as
effective.  This  method, while tedious,
might be the better to adopt for  small
areas.

  In general, the method adopted must
be  determined  by the area, room con-
tent,  thickness  of old films, manpower,
and equipment available.
  Before painting concrete floors, they
should be etched with an acid solution
made by diluting  one  part of  full
strength muriatic acid with three parts
of water. The operator should use all
safety precautions while working with
acid.  The solution should be prepared
in a plastic or  wooden bucket.  Apply
the solution  with a  stiff fiber brush.
Scrub well while applying.  A  gallon
should treat  75 to 100 sq ft (7 to 9
sq  m)  of surface  area.  When  the
bubbling has stopped (it takes about
20  min) flush  the floor clean and  let
it dry thoroughly.  Almost every floor
paint requires  a dry floor before  the
paint can be  applied.
6.322 Greasy Floors:—When it is  de-
sired to  paint  a previously unpainted
concrete floor that is impregnated with
grease and oil, one  of three methods
may be selected, namely, a wet scrub-
bing  method, a  dry solvent method,
and a caustic  lye method.
  6.3221 The Wet Scrubbing Method:
•—The wet method consists of  a  thor-
ough scrubbing of  the surface  with
stiff bristle brushes and a warm water
solution  containing 0.5  Ib (0.2 kg) of
trisodium phosphate, or its equal per
1 gal (3.8 1)  of water. To this also
should be added a sufficient quantity
of  a wetting  agent.  The  scrubbing
should be vigorous  and the operator
                   should protect the  hands with rubber
                   gloves.  This procedure should be fol-
                   lowed  with a  thorough rinsing with
                   clean water to remove all alkali.  Then
                   the cleaned surface should be  etched
                   with muriatic  acid using a 5- to 10-per-
                   cent solution  by  volume  in  water.
                   This step  mildly  roughs the  surface
                   and  removes the glaze  resulting from
                   too smooth troweling.  Sufficient etch-
                   ing is indicated if a slight sprinkle of
                   water tends to  sink into the  surface.
                   The  acid  treatment  must  be  rinsed
                   completely away with clean water and
                   the floor  allowed  to dry  thoroughly
                   before  applying paint.
                     6.3222 The  Solvent  Method:—The
                   solvent  method for removing  grease
                   and  oil from  concrete  floors  consists
                   of covering the surface  with  about 3
                   in,  (7.6  cm)  of saw dust.  The saw
                   dust then  is soaked with  a high sol-
                   vent, low  volatile thinner such  as hy-
                   drogenated  petroleum  naptha.   The
                   whole surface then should be covered
                   with a rubberized  cloth  or  similar
                   covering to help retain the  solvent and
                   allowed to stand for 16  to 24 hr.  The
                   solvent  should  be  renewed as  neces-
                   sary.   At the  end  of the  soaking
                   period, the saw dust layer is  removed
                   and  the  floor  thoroughly  scrubbed
                   with stiff  bristle  brushes   and  clean
                   solvent  to  remove  completely all  oil
                   and  grease from cracks and  crevices.
                   This process  should be followed with
                   the etching procedure described above,
                   washed  clear  of acid,  and  allowed to
                   dry.  The  solvent  method  is  not de-
                   sirable where  open  flames or sparking
                   electric  equipment  is present, and is
                   in fact so dangerous as  to preclude its
                   use  except  ~by  experts  with  special
                   equipment.   Even  a  spark  from  a
                   shoe might set it off as an explosion.
                     6.3223 The  Caustic Lye  Method:—
                   A wet method that  involves no danger
                   from flame or spark consists of cover-
                   ing the floor with a thin layer of saw
                   dust and  saturating the layer with a
                   solution, of caustic  lye  at the rate of
                   1 Ib/gal (0.2 kg/3.8 1) of water.  This

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                   PAINTS AND PEOTBCTIVK COATINGS
                                  63
mixture  is allowed  contact  with  the  water  and the etching treatment ap-
floor over night and then scraped off.  plied as described  in  6.3221.   Repeat
The  floor  then is washed  with  hot  the treatment if necessary.

        6.4 PREPARING GALVANIZED IRON SURFACES
  Galvanized iron is used in numerous
structures in  and around wastewater
treatment plants.  Often it is  desir-
able that such structures be protected
further  by   painting.    Ninety-two
plants answered a  Federation  ques-
tionnaire that asked  if galvanized iron
surfaces were painted, whether paint-
ing  was satisfactory,  and what pre-
treatment, if  any, was used.  An anal-
ysis of these replies showed  that  63
plants  painted   galvanized  iron,  26
plants did not   paint,  and  4 plants
gave no reply.   Furthermore,  9 plants
used  a pretreatment of acetic  acid
(vinegar), 8 plants used  muriatic acid,
16 plants allowed time for weathering,
3 plants used a copper  sulfate  wash,
and 8  plants reported no preparation.
Nine plants reported that the  painting
of galvanized iron has proven unsatis-
factory.

6.41 Types of Galvanized Surfaces
  Types of galvanized surfaces are as
follows:

  1. Iron is coated  with zinc  commer-
cially  by an  electroplating  process,
hence  the term  "galvanized."  Elec-
troplated zinc is laid on in fine plate
crystals, leaving the  surface smooth
and bright.
  2. Iron also is coated  by a dipping
process whereby the acid cleaned iron
is  "fluxed"  with a solution of  am-
monium  chloride and dipped one or
more times into  a bath of molten zinc.
Dipped galvanizing  produces a con-
tinuous  non-crystalline  film  that  is
more smooth  and shiny than the plated
method.
  3. A third and more recently  devel-
oped process applies zinc as a molten
spray  using  air pressure.   This  is
called  metallizing.   The process pro-
duces a rough surface that tends to be
porous.  In  a short  time rust spots
seep through.

  Of the above  three,  dipped galvan-
ized metal  offers the better protective
properties since  its film is continuous
and can be  reinforced   by multiple
dippings.  Its surface,  however, is not
adapted to receiving paint by reason
of  the  absence  of  an  etched surface
to which the  paint may  bond.

6.42 Method of Surface Prepara-
     tion
  There are  two  common methods
used.

  1. The most  common   of  all is to
allow   the   galvanized   surface   to
"weather," since the  purpose of the
galvanizing  itself was to  protect the
base  metal.   Weathering  produces  a
roughened  surface  by allowing  time
for a film of zinc oxide to form.  This
process changes  the  surface from  a
shiny  finish  to  a dull gray to which
paint will bond.
  2. Many   galvanized surfaces  are
damaged or need painting at the time
of  their installation.   It  is not desir-
able or practical to wait  for weather-
ing, so one of several primers may be
used.
   (a)  Vinyl  wash coat  which  is  a
phosphoric acid solution  can be  used
and a zinc  dust  paint  is recommended
as the primer.
   (Z>)  Acetic  acid  also is  used  com-
monly.
   (c)  Where an acid wash is not prac-
tical  the telephone  company has used
a  zinc dust—zinc  oxide  primer for
pretreatment. This primer is a modi-
fication of  Federal Specification  TT-
P-641,

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64
PAINTS AND PEOTECTIVE COATINGS
                6.5  PREPARING WOOD  SURFACES
  While wood surfaces are not as prev-
alent in and  around wastewater  treat-
ment plants as  other  surfaces, the
preparation  of  a wood  surface for
painting is  an  important item in its
preservation.  Moisture  is the princi-
pal enemy of a good paint job on wood.
Other  important factors  include the
method of paint application and the
drying time  allowed between coats.

6.51 New Wood
  New wood should present a clean,
smooth, dry surface.   Knots  should be
blow-torched  to partially draw out the
resin, which should be scraped of£ and
the surface  then coated with shellac.
Depressions  should be filled  with suf-
ficient putty to  allow for contraction
and later surfaced with  sand paper.

6.52 Painted Wood
  Painted wood,  if  the  existing  coat-
ings are adherent and free from  paint
                   defects, may be brushed clean,  sanded
                   where necessary, and repainted.  Pre-
                   viously painted  wood,  however,  may
                   have such an  accumulation of paint
                   and be otherwise so defective  that it
                   will be necessary to remove it.  This
                   may be done either by thorough scrap-
                   ing, sanding, and smoothing the edges
                   around sound paint areas, or else the
                   old paint will have to be removed com-
                   pletely.  The tedious and time-consum-
                   ing  job  of  completely removing  such
                   coatings  is  best  done  with  a  blow
                   torch  and  scrapers.   This work re-
                   quires a day when there is little wind
                   to avoid the cooling of the surface and
                   slowing of the  work, since there  must
                   be sufficient heat to soften and blister
                   the  paint film.  Paint removers are
                   likely  to be too expensive for large
                   areas.   The use of caustic soda is  ill
                   advised since it tends to penetrate the
                   surface and thus deteriorates the new
                   coatings.
         6.6 PREPARATION OF  MASONRY SURFACES
  Masonry surfaces which are to  re-
ceive  paint  should be  dry and  clean
of all dirt,  grime,  and  foreign par-
ticles  before painting.

6.61 New Masonry
  New masonry  should be aged  prior
to painting with oil-base  paints  for a
period of 30 to 60 days.  This  permits
the removal of  moisture  and in the
case  of lime plaster to  decrease the
alkalinity  of the surface  film.
  If the water-based paints are  to be
                  used, the drying period can be short-
                  ened to two weeks.

                  6.62 Old Masonry
                     Old   masonry  that  is  dirty  and
                  greasy  will have to be  cleaned with
                  a  hydrocarbon solvent to remove  the
                  oil and grease.  Once this  is removed
                  then the surface can be  cleaned with
                  a  trisodium phosphate solution using
                  sponges.  Circular   motions  are  less
                  fatiguing and only  small areas should
                  be washed between  rinsings.
              6.7 PREPARATION

  Old brick walls  should be dry and
swept clean before they are  painted.
If they  have been painted  the scaled
areas should be scraped or  brushed
until there is no loose material.  Blast-
ing of the  surface may be necessary
if all of the material is to be removed.
                 OF BRICK WALLS

                    Brick surfaces that have  effloresced
                  will have  a  calcium sulfate  deposit.
                  This deposit should  be scrubbed with
                  muriatic acid solution (10 percent by
                  volume)  and then washed down.
                    If the walls  have been marred by

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                   PAINTS AND PROTECTIVE COATINGS
                                             65
soot and smoke, they should be washed
with a strong soap,  detergent, or solu-
tion  of trisodium  phosphate.  After
cleaning the walls, they  should  be
             rinsed thoroughly  with clear  water.
             There may be occasions when steam
             cleaning will  be  necessary to remove
             stubborn stains.
     6.8 PREPARATION OF MISCELLANEOUS SURFACES
  Surfaces previously painted  with
bituminous paints  or surfaces  that
have  been coated  with  cork-asphalt
compounds for  insulating  purposes,
such as "No-Drip," should  be sealed
before they are painted.   If  this  is
not  done, the  asphalt  will  bleed
through the paint.
  Beaver  board and cellulose  mate-
rial must  be sealed properly before a
paint can be used.
               Surfaces that have been coated with
             an enamel or gloss paint or that  are
             varnished should be prepared for re-
             coating by either roughing the surface
             with steel wool or medium sand paper.
             There  are some  solvents or chemical
             solutions  available  for  application to
             the surface which will soften or permit
             the  new  paint  to  bond  to the  old
             surface.
  The workmen who prepare the sur-
face should be given proper instruc-
tions in the use  of  the tools  that are
to be used.  They also should be shown
what is  expected as the minimum ac-
ceptable surface preparation.   Inspec-
tion on the part of the  supervisory
staff is as important as any phase of
the  total job.
6.9 CONCLUSION
               Irrespective  of  the  type of  paint
            used  or method of application,  the
            more  thoroughly any type of surface
            has been prepared to receive the paint,
            the greater will be the dividends re-
            turned in  its "life" and its protective
            value.

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                 7.  PAINTS  AND  COATINGS
                        7.1  METAL  SURFACES
7.11 Primers
  The most important paint in a dry-
ing-oil painting system is the first or
prime coat.  Other paints  in  the  sys-
tem are,  of course,  important, but the
efficiency of the entire paint job is de-
termined to a  considerable extent by
the effectiveness of  the  prime  coat.
Its composition, thickness, adhesion to
the metal, and  suitability for the pur-
pose  are, therefore, all important con-
siderations.
  To be  of universal service on metal
in or around  a wastewater treatment
facility, an  ideal priming paint of the
drying-oil  type must  serve two  very
distinct purposes.
  It must be thoroughly gas and water-
proof at the surface, i.e.,  it not only
must  be impervious  to moisture  and
acids, but  it  also  must be gas  tight
against  hydrogen sulflde.   The hydro-
gen sulfide gas has a very  small mole-
cule  and  will  penetrate   most  paint
films.   If  the  gas penetrates  it  will
attack the  steel and form iron sulfide.
This formation, of course, destroys the
paint bond to  the steel and when the
bond is lost the loose paint is damaged
more  easily by abrasion.   The  prime
paint also must not soften  appreciably
when covered by accumulations of oils,
greases,  and soaps, nor be damaged
easily by the abrasion of floating mat-
ter which  usually accompanies  these
oils,  greases, and soaps.
  The paint not only must be  impervi-
ous  to  its  surroundings,  but  it  also
must furnish good bond of  itself to the
steel and itself provide good bond for
the top coats.   These  several  physical
qualities are afforded largely  by the
vehicle, although the pigment does add
considerably to the quality  of the paint
and  its  durability.
  The pigment adds to the protective
value of the paint film by increasing
the paint density.  The pigmentation,
however, must not be so great as to
decrease the imperviousness.
  The size and shape and, to an extent,
the composition  of the  pigment  also
affect the  performance  of  the paint
film.   For  instance,  mica  (and  es-
pecially graphitic mica) when used in
moderate amount adds considerably to
the  life and usefulness  of a  metal
priming paint.   Possibly this  results
because the mica  in effect increases the
film  thickness, which is one  of the
factors governing the paint durability.
It  also is  possible that the graphite
in the graphitic mica spreads itself as
a film on the surface of the mica flakes
and being a  poor wetter by water, its
effect is to waterproof the whole paint
film and thus prolong the  paint  life.
  A second, even  more important func-
tion of  the prime coat is  to "inhibit"
corrosion of the metal whenever the
corroding   liquids   eventually   get
through the  paint film to the steel as
they  inevitably do.
  A number of pigments function well
in  this  respect.   The best known and
most  used inhibitors are zinc chromate
and  basic lead  chromate.   Red  lead
also  has been classed among  the in-
hibitors; its most useful contribution
to  the  paint formulation  is  that  it
furnishes a  tough,  impervious,  and
strongly adherent lead soap.
   Note that  a  priming  paint  which
serves well under one set  of conditions
may  not do  well at all under another
set.
   Steel is in many  locations  in and
around a wastewater treatment facility
where the paint  remains damp practi-
                                    66

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                    PAINTS AND PROTECTIVE COATINGS
                                   67
cally all of the time and where hydro-
gen sulfide gas is abundant,
  The presence of these more severe
conditions in a treatment plant necessi-
tates a somewhat different approach to
the painting problem than where the
conditions are more mild.  The vehicle
in particular must be more highly re-
sistant  to  these  conditions  than  is
necessary for paints used on a  bridge
and on the steel of most buildings.
  7.111 Red-Lead Primers:—When  a
priming paint is mentioned  one natu-
rally thinks of a red lead and oil paint
and for most situations a red lead and
oil paint does make a very satisfactory
metal primer.  Such  paints have been
used with success for years.  Recently,
however, there have been a number of
other paints developed for  the  most
severe conditions of service which have
gained  considerable  popularity.
  7.112  Zinc  Dust-Zinc Oxide  Prim-
ers:—Zinc dust-zinc  oxide  paints are
very useful for painting new galvan-
ized  sheet metal  and for touching up
the threads and damaged spots of new
galvanized pipe.   It  also  has  been
found that zinc dust-zinc oxide paints
serve very well  as a  touchup prime
coat  in  a repaint job to be  finished
with one coat  of  aluminum.   It also
has been used as an all over prime coat
under the one coat of aluminum.  These
paints  are rather  expensive as  com-
pared with  other  primers, however.
  7.113 Iron  Oxide-Zinc  Chromate:
•—In  plant  construction  it  is  always
convenient to standardize on one prim-
ing  paint  for  use  everywhere  irre-
spective of  where the steel  is  to  be
used.  Top coats applied after erection
can be varied according to the location
to combat the peculiarities of the ex-
posure,  but the engineer cannot very
well  tell the fabricator  to  vary his
shop  coat  on  different parts  of the
work  according to where  the steel is
to be  placed.   Such instructions,  if
given,  would result in endless confu-
sion  and countless  errors.    Even  in
maintenance  repainting work it is less
bothersome to stock one type of prim-
ing paint so that the painters can use
it  everywhere.  This practice reduces
the amount of stock necessary for the
storekeeper to have on hand.  Such a
paint,  however, must  be designed for
the worst conditions encountered in the
plant if it is to be applied  generally.
  Some  waste treatment  plants  have
standardized  on iron  oxide-zinc chro-
mate as a single metal primer for gen-
eral use.  This has given very  good
service,  except that it does not  seem
to  be  suited  to  use  in  very damp
atmospheres  such  as are  encountered
in  screen  houses  and grit  chambers
and also on the underside of the roof
of water tanks.
  Neither  is  the  red  metal primer
suited  to painting new galvanized sur-
faces where zinc dust-zinc oxide paints
have  proven  better.  Otherwise, the
red metal  primer  has been found to
outlast most other paints,  even in sub-
merged locations,  if it is  applied cor-
rectly  to properly prepared surfaces
and properly covered by  suitable top
coats.
  The  primer has  a marked  advantage
over red lead as a shop  coater in that
material painted today can be shipped
out tomorrow without a great deal of
damage being done to the paint by the
handling.  It  also appears  to endure
exposure to sunlight and weather with-
out being protected by  top  coats bet-
ter than does  red  lead.   A  red metal
primer paint  formulation is as  fol-
lows:
                           Percent by
                             Weight'
                             (volatile
                            free basis)
Zinc chromate (P44)
lied lead (ASTM D83-41)
Red iron oxide (P42)
Graphitic mica (P43)
Crystalline silica (P33)
Grinding varnish (V37.25),
  nonvolatile
22.80
 1.63
19.54
17.91
 3.25

34.87
Total Nonvolatile               100.00
Pigment to nonvolatile vehicle ratio J to \.

  Note: This  ratio  is varied according to
the  fineness  of  the grind.  Drier (V75) is

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68
PAINTS AND PROTECTIVE COATINGS
  7.114 Vinyl  Primers:—This  is  a
two-part  formulation.   The   primer
base is mixed with the primer  liquid
before  using.    Because  of  chemical
changes which take place when allowed
to stand for long periods of time that
affect  the  adhesive  qualities  of  the
paint,  only  the amount  to be used for
one  day's operation  should be  mixed
at a time. When used properly, vinyl
paints have  a tenacious  adhesion and
toughness  superior to  most  conven-
tional  varnish-type primers.    Vinyl
paints can  be  immersed  immediately
in water  since  they cure in  water as
well as in air.  The primer will toler-
ate  slight  moisture  condensation  on
the metal without harm to its adhesion
qualities.
  Coatings made from these resins ex-
hibit great flexibility and are resistant
to  most  caustic  and  acid  solutions.
They are almost totally unaffected  by
oils, greases,  and aliphatic petroleum
solvents.  Their resistance to salt so-
lutions has  enabled them to be used
for painting ship bottoms.

7.12 Top Coats
  Under this  heading those paints are
discussed  which are suited more par-
ticularly to  use on steel over the pre-
viously described drying-oil  primers,
although some of  these paints may  be
used directly on metal  without  inter-
position of a primer.  This broad sub-
ject, will  not be covered fully; how-
ever,   some   of the  more important
factors which govern their proper se-
lection will  be pointed  out.
  Top coats serve (a)  to protect the
prime  coat,  as  for instance from  the
full  effect of continued immersion  in
water  and  from  the  softening  effect
of the oils,  greases, and soaps, and
added to make the paint dry to touch in not
less than  4 hr and  dry  hard  in not more
than 20 hr.  Thinner  also is added but ia not
to  exceed  50 percent by weight of the total
vehicle.  1-%  Ib  (0.74 kg)  of lecithin is
added  as  a wetting  agent to  each  100 gal
(380  1) of the paint as  made.
                   the abrasion of floating matter; (&) to
                   decorate the surface, as in an office or
                   laboratory; and  (c)  they  may serve
                   both to protect the prime coat and to
                   decorate the  surface, as for  instance
                   outside  in the  sunlight where  the
                   decorative   coats  shield  the  varnish
                   of the prime coat  from the effects of
                   actinic light;  also in a screen chamber
                   where  the  decorative coats protect the
                   prime  coat from  the full effect of the
                   atmospheric moisture  and  hydrogen
                   sulfide.
                     7.121 Bituminous  Coatings:—Bitu-
                   minous coatings of various kinds  have
                   been used for many years on both iron
                   and  steel  but usually without inter-
                   position  of a prime coat.   (Here in
                   mentioning a prime coat we do  not
                   include the application of a  clear coat
                   of bitumen which is sometimes done to
                   make  a  heavier  coat bond  better to
                   the surface.  The prime coat referred
                   to is that used to inhibit corrosion  and
                   also  to waterproof the surface.)
                     Bituminous materials are  supplied
                   in four different forms:

                      (a)  as a hot coat material,
                      (6)  as  a cutback paint,
                      (c)  as an asphaltic varnish, and
                      (d)  as a water emulsion.

                     7.122 Hot  Coats:—Most  engineers
                   are  familiar  with  the hot  tar   dip
                   which  often is prescribed for use di-
                   rectly  on the metal of cast iron pipe.
                   Engineers  know  from experience  that
                   this  coal  tar dip  in most  situations
                   provides excellent   protection to  the
                   metal,   especially   in  underground
                   work.
                     In fact,   it may  be  said  generally
                   that  if a  heavy hot  coat  of either
                   coal  tar  or asphalt could be  applied
                   uniformly  without  pinholes  or  flaws
                   and if that coating material  could be
                   designed so that it would remain intact
                   without alligatoring, cracking,  or flow-
                   ing in  the sunlight and  weather, it
                   would  provide about the best protec-
                   tion for underground and under waste-
                   water  treatment  plants because these

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                    PAINTS AND PROTECTIVE COATINGS
                                  69
hot coat materials are quite waterproof
and, while they may be somewhat soft-
ened, are  not affected seriously by  the
oils,  greases,  and  soaps  present   in
wastewater and resist damage by float-
ing debris very  well.   Unfortunately
these high ideals usually are not  at-
tainable.  Coal  tar tends to alligator
and flow  in hot weather and crack in
cold  weather  when exposed for  one
reason  or another,  and  asphalts  by
nature are not entirely water and  gas
tight, although very thick coatings  ap-
proach  tightness.
  When the article to be coated  can
be dipped into bitumen, as is cast iron
pipe, or  where  the hot material  can
be  spun  on the  interior,  as often is
done  inside  large pipes, applying it to
considerable thickness, the coating can,
with  care,  be made  almost  perfect,
uniform,  even, and free of  pinholes,
skips, and other flaws.  However, when
a hot coat is applied to steel construc-
tion already erected, the hot material
must be daubed onto the surface.  Ex-
perience  has  shown  that it  is very
difficult to daub  on a  hot coat  in  this
way without leaving pinholes and flaws
which can be detected by an. electric
brush drawn over  the  surface  and
which, of  course, detract from the pro-
tective value of the coating.
  Hot bituminous coatings, or  in fact
any other type of bituminous coating,
protect the  steel only by being water
and gas tight.   If they are  not water
and gas tight they furnish  no inhibi-
tion  of corrosion such as  do  prime
coats containing zinc or lead chromate.
  It  is important,  therefore,  that bi-
tuminous  coatings be  quite  thick  and
that  they be  applied  as a  solid, con-
tinuous film over the  surface.
  7.123 Cutbacks:—From the stand-
point of  the protection afforded,  cut-
backs are by far the  poorest  of  the
four  types  of   bituminous  coatings.
While they  often display a  bright  and
pleasing appearance when first applied
they  are  almost  never water and  gas
tight.
  Cutbacks have  two faults.   First,
the solvent used,  particularly in  the
coal-tar cutbacks, is very likely to  lift
the prime coat and thereby  greatly
diminish its usefulness.   Second, cut-
backs  harden by  evaporation of  the
solvents and thinners. As the volatile
leaves  the coating the bitumen which is
left  behind  begins to  stiffen.   The
volatile  then   forms   concentration
centers to  which the remaining vola-
tile drains to escape.  These points of
thinner  concentration   are  the   last
places  in  the  film  to  dry.   As  the
bitumen is freed  of the  volatile it
shrinks because of  a reduction of vol-
ume and as it shrinks it draws  away
from the  centers of thinner  evapora-
tion so that in the end  when the film
is wholly dry these  vortices remain as
little wells which do not close because
the bitumen  by that time is too stiff
to flow back  into the holes.  The film,
as a consequence,  remains pervious to
water  and gas.
  7.1231 Asphaltic   Varnishes:—As-
phaltic varnishes are made by cooking
gilsonite, wurtzilite, or elaterite (which
are ancient solidified forms of asphalt)
with tung  oil or linseed  oil  or combi-
nations of the  two and  then thinning
to working consistency.  They are true
varnishes;  the asphalt reacts as a kind
of resin to combine with the drying
oil.
  The   varnish  films, when  dry,  are
quite waterproof,  but are quite  sensi-
tive  to oils, greases, and soaps.  They
then are not suited to submerged waste-
water  application but  do fairly well
applied in several  coats on surfaces
submerged in clean water. They make
fairly  good black  paints for use  in-
side, even in  rather  damp locations.
They are not so satisfactory for out-
side painting because of the action of
sunlight.
  7.1232 Bituminous Emulsions:—Bi-
tuminous   emulsions  are usually dull
and uninteresting in appearance.  They
are,    however,   quite    water     and
gas  tight  when dried  in thick coat-

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70
PAINTS AND PROTECTIVE COATINGS
ings and they do not alligator.   They
harden by precipitation of the bitumen
when the emulsion '' breaks.''  In hard-
ening they solidify from the bottom up
so that  there  is no tendency to form
pinholes as  is the case  with cutbacks.
They, therefore, present a  solid,  uni-
form, and  continuous  film  when  dry.
Neither  do  water  emulsions lift the
prime coat as  do some of the cutbacks
since the water is not a solvent for
the primer.  The emulsion must dry for
at least two weeks and preferably for
a month before it is  submerged or  it
will re-emulsify.
  7.1233 Chemical  Emulsions:—Soap
emulsions have not proven to  be as
good as the  clay  emulsions in  sub-
merged  locations.   So-called  chemical
emulsions where the emulsifying agent
is made very small in percentage also
are said to  be  suitable  for  submerged
conditions.
  In making  the  clay  emulsion  it
should  be  specified  that  the   clear
emulsion, in addition  to the bitumen
and clay (required for its  emulsifica-
tion) shall  contain  10  percent  by
weight  of  zinc  oxide   (powdered va-
riety).   Zinc  oxide  is  used not  only
to keep the emulsion alkaline so  that
it  does  not  so easily re-emulsify, but
also that zinc may be present to  arrest
any hydrogen  sulfide which  may try
to penetrate the film to the  steel,
  To the clear emulsion add asbestos
fiber  in  amount  equal  to  13 percent
of the total weight to make the asphalt
cling together  better when  the  emul-
sion is being applied in a heavy  coat
and also make it remain better in place
when it is softened by the oils, greases,
and soaps of wastewater. Two grades
of  fiber  are used, 1/3  being what  is
commercially known as 7-M  grade, and
2/3 being what is called asbestos pulp
or float.
  7.124  Grease Coatings:—These  con-
sist  of  bituminous waxy  compounds
made rust  preventive by the addition
of  chemical  rust inhibitors.  Beside
being applied  easily and quickly,  they
                   seem to be the answer to some of the
                   most  annoying  rust  problems.    The
                   surface to be protected need not be
                   thoroughly clean and dry.  Any  heavy
                   rust or loose paint should be chipped
                   off.  The grease coatings are non-dry-
                   ing or semi-drying.  They gradually
                   soak through the rust and coat the
                   underlying metal.  Further corrosion
                   is stopped.  The film remains soft and
                   plastic and can be described as  norm-
                   ally self-healing.  Any damage beyond
                   self-healing is repaired easily without
                   any surface preparation.  Grease coat-
                   ings obviously cannot be used in  places
                   where  a workman would come in con-
                   tact with  them because of  their non-
                   drying nature.  A grease coating  would
                   rub off on clothing.

                   7.13 Pigments for  Decorative
                        Paints
                     Before taking up decorative paints
                   in detail,  it  will be  well to consider
                   first the suitability of various pigments
                   for use in such paints in and around
                   a wastewater treatment facility.   The
                   number of pigments which can be used
                   in top coat paints exposed to waste-
                   water is rather limited because sewage
                   gas and especially hydrogen sulfide re-
                   acts chemically with many of the pig-
                   ments  to change  their color.  Carbon
                   dioxide, another constituent of sewage
                   gas, also reacts with some of the pig-
                   ments  to change their color.   Sulfur
                   dioxide from  industrial  gases  may
                   cause  a similar darkening  of certain
                   pigments.
                     It is desirable,  therefore, to  know
                   something  about  the behavior of the
                   various pigments when they are con-
                   tacted  by  sewage or industrial gas.
                     While it may not be  complete, the
                   following  list of  pigments is believed
                   to include all of the common color pig-
                   ments  which  have been found  satis-
                   factory for use in decorative paints at
                   wastewater treatment facilities.

                     Whites: Zinc oxide, zinc  sulfide, ti-
                       tanium  dioxide.

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                   PAINTS AND PROTECTIVE COATINGS
                                  71
  Blacks: So  far as known none  of
    the blacks are affected by sewage
    gas.
  Oranges and Yellows: Cadmium and
    selenium sulfides,   Hansa  yellow
    is stable but  quite expensive.  In-
    ternational   orange   (dinitroani-
    line) also is  quite stable.
  Greens: Chrome oxide  green  is  an
    excellent color not affected by sew-
    age gas.
  Blues: There are no very good blues.
  Beds: Practically all iron oxide reds
    are stable.
  Browns and Grays: Any  derived
    color which can be made of any
    combination  of  the  above   color
    pigments  almost certainly will  be
    stable.   Of course, dark shades of
    gray and brown show less  dark-
    ening by  hydrogen  sulfide,  even
    though  some  of the pigments may
    be affected.

Metallic  Powders and Pastes
  Aluminum:  Turns slate color due to
    formation of  aluminum hydroxide
    which  is  white  and  presence  of
    impurities  like   copper   which
    darken  it.
  Zinc: Whitens  due to formation of
    zinc oxide and  zinc  sulfate.
  Bronze: Blackens  due to formation
    of brown-black copper sulfide.
  Chrome: Unaffected by sewage gas.

7.14 Machine Enamels
  The engineer often wants larger and
more showy pieces of equipment like
pumps,  motors, blowers, and the like
to have a high  gloss.  This requires
the use of an enamel.  The principal
difference between an ordinary paint
and an enamel is that  a  paint  is de-
signed for  durability and protection,
while an enamel is designed more par-
ticularly for appearance.  Usually the
pigment volume is made somewhat less
in the enamel than  it  is in a  paint,
but the  same  effect  may be  accom-
plished  by  choosing  pigments  which
have low oil absorptions.
  Machine  enamels must be very flex-
ible since the machine temperature is
often  subject to wide variation as be-
tween  the  cold end  of  a blower  or
pump  in the winter and the  hot end
of the blower in the summer.  They
also must be resistant to the lubricat-
ing oils and greases and particularly
to the oil  which a mechanic uses  to
wipe  off the dust and dirt from the
machine.   On  a  steam  turbine  the
enamel must withstand steam leakage
and  perhaps  a  temperature  up  to
350°F (176°C)  on uninsulated trim.
It also must withstand  blows from
wrenches and other tools and abrasion
from  other equipment coming in con-
tact.
  The  pigment nonvolatile vehicle vol-
ume ratio in many enamels is usually
made  1 to  3, but in black enamel, be-
cause  of the flattening effect of carbon
black,  the ratio is 1 to 4.
  A good metal primer first should be
applied evenly  on the surface  to  be
painted.  When this  is dry it should
be sanded lightly to remove the gloss.
A suitable  machine filler then should
be applied all over either with a brush
(if the surface is already quite smooth)
or with  a  knife (if  it  requires con-
siderable filling to bring the surface
level).  This filler should be smoothed
out to an even, uniform  surface.
  When the  filler is  fully dried and
hard,  it should  be sanded thoroughly
to make the surface perfectly smooth,
after  which a thin coat  of clear phe-
nolic  varnish should  be applied  all
over to seal the filler against entrance
of moisture  and oil.  This  seal coat
should be lightly  sanded, after  which
the enamel coats of paint may be ap-
plied, sanding between coats.  Usually
two coats of enamel will be sufficient.
If, however, the enamel coatings as ap-
plied  still  do  not produce the  gloss
which the   operator  desires,  a  clear
coat of varnish may be added  over the
enamel.
  7.141 Vinyl Coating System:—The
best known vinyl  coatings are those

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72
PAINTS AND PEOTECTIVB COATINGS
based on vinyl chloride-vinyl acetate
co-polymer.  These coatings at normal
temperature  are  inert and unaffected
by strong  or weak acids  and alkalies.
They are not affected by water or ani-
mal  and vegetable greases and there-
fore are well suited for use in waste-
water treatment plants.  They are ap-
plied usually as a system  consisting of
primer,  intermediate, and finish coats.
Vinyl paints require more coats than
other types of paint because the film
per coat is thin.   Extensive  and care-
ful surface preparation is  imperative
and  application by skilled painters is
essential. As many as six to nine coats
are required.  They  are  used on  the
underside  of the roof of a steel water
tank and also on the metal equipment
and steel construction in a screen house
or a  grit  chamber and on the metal
parts of a vacuum filter drum.
  The system is made up  of three dif-
ferent paints, each serving a different
purpose and applied  in  a  specified
order: primer, intermediate  coat, and
top coat.
  7.142  SDC No. 232 Wash Primer:
—The primer is  shipped  as  two sepa-
rate  solutions  which must be  mixed
together just before  the  paint is  ap-
plied and  the  application must be to
an absolutely clean steel surface which
means  that  in most cases  the steel
must be sandblasted or  pickled.  The
dry   thickness  of the wash  primer
should  be between 0.3 and 0.5 mils.
  The intermediate coat material must
be interplaced between the primer and
the top  coat because the latter will not
bond to the  former but  will bond to
the intermediate  coat and the inter-
mediate coat will bond satisfactorily to
the primer.  The  dry thickness of the
intermedate  coat should be between 3
and  4 mils.
  The co-polymer of vinyl chloride and
vinyl acetate which  affords the high
resistance  of the  system  to  untoward
conditions is the top coat.  It usually
is applied  in several coats and may be
of various colors.
                     The total dry thickness of  the  top
                   coat should be  between 3 and 5 mils
                   except  in  the  submerged  locations
                   where the thickness should be doubled.
                     7.143 Vinylidene Chloride Paints:—
                   Another group of paints closely re-
                   lated to the vinyl type of paints is the
                   vinylidene chloride type.   The two
                   types are  about equally  satisfactory
                   for use in damp places if each is ap-
                   plied properly to clean surfaces.
                     The paints  are highly  resistant to
                   most chemicals and quite impervious
                   to  moisture.   The  paints  are  made
                   both  as  aleoholic-ketonic solutions  of
                   the resins and as water emulsions,  Both
                   types within certain limits can be pig-
                   mented as desired.   The prime coat
                   contains the zinc chromate for inhibit-
                   ing  the corrosion while the top coats
                   afford the waterproofiing. They, there-
                   fore, afford very good protection to the
                   steel.
                     The water emulsion types are of par-
                   ticular interest because  they not only
                   afford good protection to the steel, but
                   they  also  can,  to  better   advantage
                   than most other types of paints, be used
                   in  confined quarters such  as on  the
                   inside surfaces of a water tank  or on
                   the interiors of vacuum  filter drums.
                     These  paints and  the vinyls have
                   another advantage which is of interest
                   to  a plant operator  where  sludge is
                   being filtered  and  dried.   When  the
                   metal  parts  of the   drums  of  the
                   vacuum  filters are  painted with ordi-
                   nary paints, the sludge adheres to the
                   paint  so  that  a  considerable  cake
                   builds up  on  the surface.   The filter
                   drums then come to have an unkempt,
                   neglected appearance.   When  painted
                   with either the vinyl or the vinylidene
                   chloride  types of paints,  the  painted
                   surface sheds the sludge and the filters
                   look cleaner  and better cared for.
                     The   vinylidene   chloride   paints
                   should  be applied  to  an  absolutely
                   clean metal surface;  the  metal must
                   be free not only of all organic matter,
                   oil,  grease, and soap,  but also of all
                   old  paint  and  mill   scale.   When

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                    PAINTS AND PEOTECTIVE COATINGS
                                  73
cleaned the  surface  then  should be
treated with  phosphoric acid to afford
the  best  bond  before applying the
paint.
  7.144 Oleoresinous Enamels:—Out-
side paints  require  a vehicle  which
will retain its elasticity over long pe-
riods of  time.   Of  the many oleo-
resinous enamels on  the  market, the
type based  on  a phenolic tung oil-
linseed  oil  varnish  of  medium  oil
length  has been used satisfactorily in
wastewater   treatment   plants.   Its
chemical resistance is very good and
its outside life satisfactory.
  7.145 Alkyd  Type  Vehicles:—Al-
kyds are  synthetic resins  based on a
combination  of  certain  alcohols such
as glycerol with certain acids such as
phthalic.  Alkyd types  of  vehicles are
quite satisfactory  for use in  outside
paints  where the  conditions  are not
too damp.   They will keep  cleaner
in industrial atmospheres than will the
linseed oil paints.
                   7.2 NON-METALLIC SURFACES
7.21 General
  Non-metallic surfaces  in  a  waste-
water  treatment  plant when painted
are, as a rule, painted only to improve
their   appearance.   In  a  few  cases
the  painting  may  be  to   brighten
up  some dark corner for operational
reasons,  but  protection of the  under-
lying surface is  rarely  an important
consideration.  In   this  respect   the
painting of non-metallic  surfaces dif-
fers  radically  from  the painting  of
metallic  surfaces where the  preserva-
tion of the metal is the prime  reason
for the  painting.   Non-metallic  sur-
faces  often painted are those of  con-
crete;   plaster;  brick,  stone,  and
cement-block  masonry; heat insulation;
and wood.
  In view of  the fact that the preserva-
tion of the underlying surface  is  not
the primary  purpose  of  the  painting
of most of these non-metallic  surfaces,
the basis of  the  paint selection is re-
duced  to a  consideration of the  one
question: How will the paint react to
its surroundings?  Most decorators are
familiar with  these  problems  in  the
ordinary situation,  but they  may  not
be so familiar  with the  special  condi-
tions which prevail around a wastewa-
ter treatment plant.
  One of the first things to note about
plant exposures is the presence and the
effect of sewage  gas  on  the  color  of
paints.
7.22 Walls and Ceilings
  The walls and ceilings of offices, lab-
oratories,  pumping stations, and other
buildings  where wastewater  is  not  in
direct contact with the atmosphere  of
the rooms to saturate it  are not par-
ticularly difficult to paint. Very often
the effect  of  the  sewage  gas  on the
color  is  the only special matter  to
command  attention.  The surface to  be
painted, however, may  itself  require
special  consideration and treatment.
  Very  little trouble has been experi-
enced in painting the walls and ceilings
in these relatively  dry rooms when the
concrete,  plaster,  and brick are first
primed  with  one  coat  of aluminum
paint consisting of % Ib  (0.34  kg)  of
aluminum paste to 35-gal  (138-1) phe-
nolic varnish.   Over this seal coat one
can  apply a  flat  paint  of  the color
desired.  This  top coat paint  also  is
made usually with a phenolic  varnish
vehicle,  because often these surfaces  do
become  damp and the phenolic varnish
is reasonably resistant.
  When the  surface  to  be painted  is
very porous or  very damp,  or where
the underlying  material  contains an
alkali,   some  special provisions  may
need to be made.   These surface con-
ditions  often require some  kind of seal
coat.

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 74
PAINTS AND PROTECTIVE COATINGS
   1. Where the  surface  is  very  ab-
 sorbent  the porosity  will  cause  what
 is called "suction" whereby the  top
 coats  are  robbed  of  their  vehicle.
 Since  this suction usually  varies over
 the surface, the texture and color will
 become mottled.   This mottling of  the
 texture  and color is  objectionable in
 that it spoils the artistic value of  the
 paints.
   2. Where the   concrete,  plaster, or
 masonry is  still  green and  damp at
 the time or where the wall or ceiling
 remains damp due to seepage, the top
 coats  must  either themselves not be
 subject to damage by the moisture, or
 they must  be protected by an under-
 coat which will waterproof  the surface.
   3. Where the cement, the plaster, the
 aggregate, or the tempering water con-
 tains an alkali and the walls or ceiling
 either  remain damp continually or are
 occasionally wet, the  top  coat  must
 either  itself be of a nature that is not
 subject  to  saponification or  it  must
 be protected by  an undercoat which
 will not be affected by the alkali.
   Sometimes none of  the above three
 conditions  require  a  sealing of the
 surface and sometimes only one or two
 of them cause trouble. Where any of
 them are present the  paints  next to
 the concrete, plaster, or brick  must be
 suitable.
   Sometimes the decorative paint itself
 will seal pores  of the  surface  suf-
 ficiently and resist the moisture  and
 saponification, but very often  a sealer
underneath the color coat is required.
 One of  the  best  of these  sealer  coat
materials (which also may serve as the
 decorative coat if desired) is a poly-
styrene paint made from resins.
   If the  conditions are very bad,  es-
pecially  in  basements, tunnels,  and
storage rooms and also on  brick, con-
crete, and block  walls, serious consid-
eration should be given to the use of a
Portland cement  paint which will not
only seal the  surface,  but also  will
serve  to decorate.
                     A  workable formulation  is  as  fol-
                   lows:

                                               Percent by
                                                Weight
                                               (measured
                                                 dry)
                   Portland Cement                 40.0
                   Sand (Well graded but all passing
                     No. 16 mesh and not more than
                     5 percent finer than the No. 200
                     mesh.)                        59.7
                   Either Ammonium or Calcium
                     stearate                       0.3
                   Total dry materials

                   Water added to make a creamy
                     mixture.
100.0
                     Where the wall  surfaces are rough
                   like those of cinder block, a stiff fiber
                   brush like a fender brush produces the
                   best coatings.  Where the surfaces are
                   smoother  like a brick wall,  a softer
                   fiber brush  like a roofing brush gives
                   the best coatings.  The  coatings must
                   be well rubbed  into the pores of the
                   wall or  ceiling to make them bond well
                   to the surface.
                     The  above discussion  relates to the
                   painting of  walls and ceilings in  rela-
                   tively dry rooms where  the walls and
                   ceilings  themselves  may  need  some
                   treatment before the paints can be ap-
                   plied  properly.    There  are  rooms,
                   however (such  as  those in  a screen
                   house,  an operating gallery, or a grit
                   chamber),  where the atmosphere  of
                   the room is always in  contact  with
                   wastewater  and,  therefore,  always
                   near saturation.   Sometimes  the out-
                   side walls and ceilings of these rooms
                   are very thin and uninsulated so that
                   moisture from the atmosphere will  be
                   condensed  on their  inside  faces es-
                   pecially in the winter when  they are
                   cold.   The  surface  then may remain
                   wet for weeks  or  even  months at a
                   time.  Unless the paints  used on these
                   surfaces are very water  resistant  they
                   will be  damaged greatly by this  con-
                   tinued   saturation.   Moreover,  if  the
                   moisture freezes on  the  surface, some
                   of the paints which might be used will

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                    PAINTS AND PEOTBCTIVE  COATINGS
                                  75
be pried off by the frost action (as for
instance a cement wash paint).
  The polystyrene paints are probably
the most  satisfactory ones to use un-
der these circumstances and a phenolic
varnish type  probably the next  best
coating material.
  The best solution  to the problem,
however, if it can be  done, is to cover
the cold surfaces with  standard  sheet
insulation  to  prevent the  walls  from
becoming  so  cold.   If  that solution
seems impracticable a  somewhat less
effective measure  would be to spray
on  a heavy  coat of  insulmastic  cork
paint or no drip as suggested for use
on  "sweating pipes."  The insulating
value  of  these  materials  depends
greatly  on their dry  thickness so  they
should never be made less than y^-in.
(1.3-cm)  dry thickness for  this  pur-
pose. Since  these are bituminous ma-
terials it  will be necessary  to  paint
their exposed surface  with  at least one
coat of aluminum paint when they are
thoroughly  dry before applying any
color coats  to  prevent the  bitumen
from bleeding through into the top
coats.  The number of coats of alumi-
num required will depend on how dry
the bituminous coat is  at the time of
painting.
  In connection with the painting of
walls and ceilings for decoration  a
word should be added  concerning the
architectural value of  different  kinds
of paints.  Gloss paints  generally are
not  considered  to  be so good archi-
tecturally as are flat paints  when used
over large flat areas because the lights
and  shadows of  the reflection from the
gloss paint  brings  out  all of the un-
evenness and imperfections  of the sur-
face  which is painted.  Since its shows
up all  of  these imperfections in the
workmanship,   the  gloss  seems  to
cheapen the  appearance of the whole
construction.
   On the other  hand, flat paints seem
to level out these imperfections so that
they  do not appear.  The  quality of
workmanship of the entire  job, there-
fore, seems to be  enhanced.  The flat
paints  then  for this reason are  con-
sidered  architecturally   better  than
are the gloss paints.
   Gloss paints,  however, are usually,
but not always easier to keep clean be-
cause the dirt does not  adhere to tho
surface easily.   However,  if  the  ve-
hicle of the  flat  paint  is fashioned
of a hard varnish it,  too, will shed
the  dirt fairly  well.  Most engineers
choose  the flat  paints for their  walls
and  ceilings where they want the rooms
to look well.
   The new silicon water repellants are
very satisfactory for use on masonry,
and  paints can be applied over them if
desired.  The silicon water repellants
should  be  applied  when  the masonry
is new, before effluorescence begins.
                       7.3 CONCRETE FLOORS
  Concrete floors to be painted must be
clean and  free  of  all material which
will detract from the life of the paint.
"When properly made and thoroughly
cured  and dried,  ready for painting,
the pores of the surface should be open,
clean, and  unfilled  and the whole sur-
face free  of dust and moisture.   Oc-
casionally  concrete  floors  need to be
pretreated  with the zinc chloride and
phosphoric acids as discussed above for
walls and ceilings.
  Rubber-base paints are probably su-
perior to water-cement paints for con-
crete in general  as they  are easily
cleaned and washed and are more re-
sistant to corrosive gases and fumes.
Most of  these  coatings  are based on
chlorinated  rubber or  butadiene-sty-
rene co-polymer.  They possess remark-
able resistance to humidity, acids, alka-
lies,  and  other  destructive  agents.
They are unaffected chemically by the
lime  found  in  all  masonry.   This

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76
PAINTS AND PROTECTIVE COATINGS
should be kept in mind when painting
concrete.    Rubber-base  coatings  are
marketed in attractive colors.   Color
selections should be made  from  the
                   wide range of colors which do not con-
                   tain lead pigments.  Colors based on
                   lead pigments are unsuitable for waste-
                   water treatment plants.
                           7.4 WOOD  WORK
  Wooden floors  should be made  of
dry, well-seasoned lumber, and their
surface should be  machine sanded  to
bring to an even,  smooth finish.  Tra-
verse the floors sufficiently to remove
all  warpage and unevenness.  Corners
and inaccessible  areas  along  walls
where the machine cannot enter should
be  hand-scraped  and  sanded  to an
equivalent even surface.
  After  this  primary  leveling the
whole surface should be gone over with
fine sandpaper or steel wool to polish
it.
  The  floor then may be either waxed
or varnished using a phenolic varnish.
  Wooden baseboard, window and door
casings,  and  other wooden construc-
tion around  a wastewater treatment
plant are  subject  to  rot  due  to the
prevalence of  moisture.   For best serv-
ice  all wooden construction should be
treated   with  penta-chlorophenol  or
equal fungicide and  then  all hidden
surfaces  preferably backprimed with
aluminum paint.   The joints, after fit-
ting but  before  fastening together,
also should be coated with this same
aluminum paint if possible to keep out
the moisture.
  The  face  surfaces  may be  either
varnished, waxed, or painted.  Wooden
floors are better when they are painted
on the back face  (bottom) surface.
  Laboratories  are  sometimes  fur-
nished with  wooden-topped chemical
tables.    The  wood for this  purpose
should  be hard and close grained, free
of knots  and other imperfections.  The
surface should be  sanded to a smooth,
even surface before finishing.
                     "Carbonized  black  acid-proof  fin-
                   ish" has proved to be very satisfactory
                   applied in two  solutions  composed as
                   follows:
                             Solution No. 1
                   Chlorate of potash
                   Chloride of copper
                   Water

                             Solution No. 2
                   Anilin hydrochloride
                   Water
300 g
360 g
   41
600 g
  41
                     A full treatment consists of four ap-
                   plications.   Each  application consists
                   of one coat of solution No. 1 applied
                   and dried, after which two  coats of
                   solution No. 2 are applied and dried.
                     Sufficient time  is  allowed  between
                   coats for the wood to dry thoroughly.
                   After  each complete  application of
                   three coats and when the  last  coat
                   has become thoroughly dry, the surface
                   is washed with clean water  and again
                   allowed to  dry before proceeding with
                   the next application  of three coats.
                     After the final coat of  the final ap-
                   plication has dried completely and the
                   surface has been  washed and  dried,
                   the surface is  given a full coat of raw
                   linseed oil  thinner with about 15  per-
                   cent by  volume turpentine  to  which
                   mixture sufficient cobalt drier shall be
                   added to make the oil dry within 8 hr.
                     If necessary, to  fill the  pores  of the
                   wood, additional coats of linseed oil as
                   above  specified  shall be added  and
                   dried,  after which the surface  shall
                   be rubbed to an even color,  dull black
                   finish.

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                   8.  APPLYING THE  PAINT
                             8.1 GENERAL
  Proper application  of paint is  very
important,  sometimes more  important
than  surface  preparation,  depending
on the amount of labor involved.  It
is estimated that the cost of  this phase
of the work is about 65 to 75 percent
for labor,  and 25  to  35  percent for
materials,  regardless of  the  method
used.  Painting  is not as  simple  as
slipping  a  brush  up and  down  or
passing a spray gun in the vicinity of
the surface.   On  the  contrary, there
are many fine points to painting, which
spell  the difference between a  lasting
job and  one that must  be  repainted
prematurely.
  Many different methods of applying
protective  and  colorful  finishes  are
used  today.  However, since the con-
cern  here  is  in  the  application  of
paint in a wastewater treatment plant,
only brush and spray methods will be
considered.
  Painting of plant structures  should
not be for show purposes only,  even
though painting for appearance sake is
often desirable; such programs ought
to be the  exception  rather  than the
rule.  Too frequent painting should be
avoided because  it wastes labor  and
material, adds to fire hazard, and may
cause paint failure by  cracking  from
the added  film thickness.  It may be
found that soiled surfaces which are
subjected to a treatment with a scrub
brush, rather than a paint brush, may
be the wisest move.
                      8.2 BRUSH APPLICATION
  The brushes selected should  be  of
the proper  style and  quality to per-
mit the paint to be applied efficiently
and  with minimum effort.   By  im-
proper  use  high quality  brushes  can
be ruined making them unfit for the
next job.  It is better to use a fully
oversized brush than   an undersized
model.   Pure bristle  brushes are the
best  but their cost may prohibit their
use on  all types of work.  Excellent
results are obtained if the bristles are
animal  bristles, deformed  nylon,  or
other  comparable  material  that  is
capable of holding maximum amounts
of paint on  the brush.  Brushes made
of pure bristles exterior and horeshair
interior are less expensive  and satisfac-
tory for large flat areas.  Synthetic
brushes are  finding favor  in all  appli-
cations  as they are less expensive and
tougher than natural brushes and pro-
vide longer  wearing  life  with  rough
service.
  New brushes must be broken in simi-
larly  to a new pair of  shoes.  In the
absence of the brush manufacturer's
breaking-in instructions, a new brush
may be soaked in raw linseed oil from
48  to 72 hr to  prevent  the  porous
bristles from absorbing  pigment par-
ticles.  This will make the brush more
flexible, easier to clean,  and better to
use. The brush should be wrapped be-
fore suspending  it in  linseed  oil by
folding it in  heavy paper to cover the
bristles from the ferrule  to the  tip.
This will  allow the brush to hold its
shape  when  it is  rested on its end.
The soaking  should  be  followed by
washing in mineral spirits or turpen-
tine until  all excess oil is removed.
The brush now is ready for use.
  To keep the brush in good condition.
clean  the coating material  from it im-
mediately after  every  use,  even  for
an  overnight interruption.

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78
PAINTS AND PBOTECTIVE COATINGS
       If a brush has been used in:

1. Paint, enamel,  or  varnish
2. Shellac or alcohol stain
3. Lacquer

4. Water or casein painta
5. Epoxies
                              To clean the brush:

                   Use turpentine or equivalent synthetic solvent.
                   Use alcohol as a solvent.
                   Use lacquer thinner, preferably by the same
                     manufacturer who made the lacquer.
                   Wash  out  immediately in plain cold water.
                   Use lacquer thinner or preferably  use the
                     thinner  or cleaner as recommended by the
                     manufacturer.
  Be sure to work the solvent well into
the heel.  When all paint has been re-
moved,  the  brush should  be washed
in warm sudsy water, rinsed in clear
warm water, dried,  and wrapped  in
paper to protect the bristles.  Do  not
allow the brush to stand on its unsup-
ported bristles as this will force it  out
of shape.
  It is  important that paint cans  be
opened in the proper way, keeping the
cover flat and unbent so that it can be
used again.  If a skin has  formed on
the  surface  of the  paint,  remove it
carefully and  discard  it.   The  paint
must be thoroughly mixed and thinned
in accordance  with  the directions  of
the  manufacturer  before  using.   If
there are particles or skins dissolved
at this point, remove them by straining
the  paint  through a wire screen  or
cheesecloth.
  At the  completion of the  present
work the  remaining  paint should  be
stored  properly.   Pour the  unused
paint into smaller containers, seal, then
place in a cool, dry place. If the  origi-
nal label has been lost, it is well  to
label each  can  in  front  showing
formula number and date of manu-
facture.  By placing the  oldest cans
in front, they will be used first  when
the next job  is started. Turn the cans
bottom up at least every six months.
  In applying paint  to the surface it
                   is important  to  get the correct grip
                   on the brush.  The  brush is held well
                   up  into the hand with the first three
                   fingers  resting on the metal band in
                   position so that it is at a 45-deg angle
                   to the surface of  the work.  The brush
                   is dipped  into the paint  a distance
                   half the length of the  bristles which is
                   far enough to  load the brush, without
                   dripping.  Pat the brush gently on the
                   inside of the can, not the edge,  to re-
                   move excess paint.  At all times paint
                   should be  kept from  getting into the
                   heel of  the brush.  Its accumulation
                   there can cause a great deal of trouble.
                     Brushing should be done in a man-
                   ner that will provide a smooth coat of
                   uniform thickness.   Brushes should be
                   kept full of paint, and excessive brush-
                   ing  should be avoided.   Apply  the
                   paint with short brush strokes deposit-
                   ing uniform amounts with each stroke;
                   brush paint  thoroughly into all sur-
                   face irregularities;  finally, smooth or
                   level the paint film with longer strokes
                   at about right angles  to the direction
                   of the  first strokes  allowing  only the
                   tip  of  the bristles to drag  so  that  a
                   film without deep brush marks will re-
                   sult.  Always  brush  paint  toward
                   rather  than  away  from  the  freshly
                   painted or wet  edges.   Work  paint
                   well into crevices and corners.  Brush
                   out all  sags and runs in the film.
                   8.3 SPRAY-GUN APPLICATION
  The easiest method of painting large
or irregular  surfaces  is by means  of
a  spray gun.   With this method  a
painter with  sufficient  know-how  in
                   handling this equipment can apply  a
                   coat of paint in either a thick or thin
                   film far more evenly than he can with
                   a brush.

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                    PAINTS AND PROTECTIVE COATINGS
                                  79
FIGURE 8.—Spray painting is fast but must be done thoroughly and properly to assure
                complete coverage.  (.Courtesy Amercoat Corporation.)
  Spray equipment must be capable of
properly atomizing  the paint  and  be
suitably controlled with pressure regu-
lators and  gauges.   Separators  should
be in the lines to provide a means to
drain  the  oil and  condensed  water
periodically from the  compressed air.
Spray guns, air caps,  nozzles, needles,
and  pressures should  be  used as rec-
ommended  by the paint manufacturer
as best suited to handle  his  product.
Directions governing the use and limits
of airless spray  apparatus and paint
atomizing  devices,  as given by  the
equipment  manufacturer, should  be
followed strictly.
  Pressure on material in  the pot and
of air at the  gun should  be adjusted
for optimum spraying  effectiveness and
to suit changes  in elevation   of  the
spray gun  over the pot,  air  pressure
at the gun should be  high enough to
atomize the paint properly,  but  not
so high as to  cause excessive fogging
of paint, excessive evaporation  of sol-
vent, or loss of material by overspray-
ing.  Manufacturers of spray painting
equipment have done a splendid job in
providing illustrations of the  opera-
tion,  care,  and  maintenance  of this
apparatus, so  only a  brief comment
for comparison  with  brush painting
will be made.
  In application, the spray gun is held
at a distance from  6 to 10 in.  (15.2 to
25.4 cm)  away from the work.   The
stroke is  made with a free arm mo-
tion.   Keep the gun perpendicular to
the surface at  all points of the stroke
since  a sweeping  or  arc  stroke will
cause  uneven application.  Release trig-
ger at the end of each  stroke while the
gun is still moving and start gun mov-
ing at beginning of next stroke so gun
is in  motion when trigger is  pulled.
Direction  of spray strokes should  be
toward rather  than away from edges.
The pattern of paint deposited at each
stroke should overlap  the edge of the
pattern last deposited.   When film
thickness  requirements  make multiple
layers necessary  within a  single  coat,
subsequent layers should be applied at
right  angles to  the direction  of  the
one previously  applied.  All runs and
sags in the film should  be brushed out
immediately or the paint should be re-
moved and the surface repainted.

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80
PAINTS AND PHOTECTJVE COATINGS
                            8.4  THINNERS
  Never thin any paint more than is
absolutely  necessary.   The  necessity
for thinning usually  is present only
under  certain  circumstances  as  (&)
in cold weather to get a paint to flow
easily,  (i) for spray painting if  the
paint is not specifically formulated  for
spraying  and if  the  proper adjust-
ments  of  the spray  equipment  and
air pressures  do not  permit a satis-
factory  paint  application,  and   (c)
on porous surfaces where absorption is
                   rapid  the thinner serves  to  carry  a
                   protective coating of paint  into all
                   pores,  cracks, and crevices.
                     For a list of thinners suggested for
                   various types of  paint, and the maxi-
                   mum amount to be used, always check
                   the  manufacturer's  directions on the
                   paint  container as to how much and
                   when to use.  Basically the amount of
                   thinner to be used should never exceed
                   V8 gal/gal.
    8.5 ATMOSPHERIC CONDITIONS  AND TEMPERATURES
  Painting may be done at any  time
during  the year  if certain rules  are
born in mind,  chief of which is  that
the weather should be clear, dry, and
warm.  Paint should not be applied to
exposed surfaces in rain,  snow,  fog,
mist,  frost, dew,  or  other forms  of
moisture.   Relative  humidity of  the
surrounding air should not exceed  85
percent.
  The  air  temperature  should not  be
below  40°F  (4.4°C)  and the  work
should never be done  after a sudden
sharp drop in  temperature, or if the
                   temperature is expected to  drop to
                   32°F (0°C) before the paint has dried.
                   The best results  can be secured for
                   paints if they are applied at tempera-
                   tures above 70°F (21° C) which is con-
                   sidered normal.
                     In applying  heat resistant paints,
                   they should be put on at temperatures
                   between  60°   and  100°F  (16°  and
                   38°C) in a thin, even coat,  and allow-
                   ances for setting of  at least 3 hr must
                   be  made before  the temperature  is
                   returned to the highest point.
                          8.6 DRYING TIME
   The basis of the theory for  drying
is varied according to vehicle constitu-
ent. For instance, linseed oil products
dry by oxidation, tung oil base paints
by polymerization, lacquers and spirit
finishes by  evaporation,  and thermal
setting resins  utilize heat  for drying.
   Many factors  influence  the  speed
with which paint dries: (a) slow dry-
ing often  is  caused  by oil, wax,  or
grease under  the paint film; (6)  the
type of surface often varies the  drying
time as metals or other hard surfaces
absorb none of the paint and tend to
cause slower drying; (c) cold weather
retards drying; (
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                    PAINTS AND PBOTECTIVE COATINGS
                                  81
                       8.7 NUMBER  OF  COATS
  Generally  five mils are  considered
the minimum total dry film thickness
for a paint system applied over steel
surfaces.  It is necessary to know the
spreading and flow properties, mixing
and thinning limitations,  drying char-
acteristics, and safety requirements for
handling  many  types of paint rela-
tively unknown  a decade ago.  There
are a bewildering number  of generic
types and sub-types of paint  formula-
tions available, all of which have their
individual characteristics  and  behavior
patterns.  They range from the alkyds,
through bitumens, chlorinated rubbers,
epoxies, furanes, hydrocarbons, metal-
lies,  neoprenes,  oil-based  materials,
phenolics, styreue polymers, urethanes,
and  vinyls  to the zinc-rich  formula-
tions  often  mistakenly used  alone as
priming coats in paint systems.
  Surfaces which are to  be coated for
the first  time, or  which  are found to
be relatively porous, will soak  up large
amounts  of  paint.   In such instances
the first step should be an application
of  a  size or sealer  which  has the
faculty to bridge the pores or  fill them
at the surface, thus  reducing suction
or absorption of  the paint. As a conse-
quence,  these compositions are very
helpful in saving paint and providing
uniform appearance.
  Before  painting  any wood  surface,
all knots and resin deposits should be
covered first with a thin coat of shellac
before the prime coat is applied. This
would  be followed by the undercoat
on which is  applied  the  finished coat
in one or additional layers.
"Concrete surfaces which are in good
condition  usually must be coated with
some type of approved filler and seal-
ing compound.  The chief danger and
cause of paint  failure is due  to the
fact that the  concrete  may  contain
moisture which will force  the film from
the surface.  A practical test for mois-
ture is  made by fastening a rubber
mat on the  surface to be painted and
allowing it to remain for two or three
days. If moisture collects on its under-
side, it is necessary to wait until the
concrete is thoroughly dried out.  All
cracks first  should be filled with suit-
able  compound  before   any surface
treatment begins.
  In repainting surfaces which have
been coated previously,  it  is well to
observe the  following.  Nearly  all as-
phalt paints  will bleed  through the
surface of any ordinary paint put over
them.   This  may  be retarded  by a
heavy  coat  of aluminum paint  which
will help to seal the  asphalt and give
a good base  for any painting coat.  Do
not  try to  paint  over any calcimine
work as this must be  washed  thor-
oughly  from  the  surface  before an-
other paint  is applied.  Oil  paints and
enamels should not  be  applied over
casein paints on wood surfaces without
priming.  Paint derived from coal tar
products contains volatile  substances
which  constantly  evaporate and will
tend to stain any paint put  over them.
There  are  no definite suggestions to
be made for painting over tarred sur-
faces.  The best solution where surfaces
are coated with cold water  paint is to
remove all  of  the  old  paint  before
applying the new.  Aluminum  paint
usually will resist the powerful bleed-
ing  action   of  creosote  although  it
should  never be  applied  before the
creosote has  been weathered  for at
least 10 weeks,  in order to allow the
volatile oils  to escape.  Lacquer cannot
be used successfully over other  paints
because certain of its ingredients are
very strong solvents,  often  being used
in  paint removers,  which  cause the
paint layers on which it is  applied to
lift.
  The  first  coat  always  should  be
brushed carefully  over  all parts of
the surface so that all cracks, openings,
and holes will receive enough paint to

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82
PAINTS AND PROTECTIVE COATINGS
wet the surface.  Openings and cracks
should be filled properly as soon as the
primary coat  is  dry.   After about a
week, if the atmosphere has been  clear
and  dry,  this  should  be  dry enough
to receive the  second coat.  Sometimes
breaks appear in the surface of a paint
coating which rather resemble the ap-
pearance  of an alligator  hide.   This
is sometimes caused by the application
of hard  finishing  coats  over  a soft
primer  or especially before the primer
has thoroughly dried.   A priming coat
should  be allowed  to  dry thoroughly
and  it  always should be as hard  or
harder than the outer coats.
   Subsequent  coats should be applied
carefully,  the  number dependent  on
the service or  life expected.  For  ordi-
nary work both interior and exterior,
a good prime coat followed by a single
finish coat is satisfactory.  Often times,
after a good  scrubbing a  previously
painted surface can be brought  back
as good as new with  one  coat.  Loca-
tions in  damp areas  or  where  sub-
jected  to  corrosive liquids or gases
will  require special paints and an  in-
creased number  of coats.   Heat  re-
sistant paints are placed in a thin, even
                   coat,  with one  coat being  sufficient
                   on interior surfaces.  For underwater
                   painting,  two prime coats  are recom-
                   mended followed by the last  coat be-
                   fore the water is reintroduced. It has
                   been reported that it takes a minimum
                   thickness of four coats to prevent salt
                   water penetration, which is good advice
                   to the wastewater treatment plant op-
                   erator for many areas.  Since fewer
                   coats  may result in a paint  job  with
                   weak  spots,  it  is well in multi-coat
                   work  to  vary the color of each  suc-
                   cessive coat slightly so as to avoid any
                   skips  or misses.
                     It is a wise operator  who heeds the
                   sign for changing the paint guard from
                   time to time. When the gloss has gone
                   from  the  paint or  the colors begin
                   to look washed  out,  it is  a  warning
                   that  it  is  time  to change  the guard.
                   The usual life of a good exterior paint
                   coating  is from four  to  five years.
                   There are on every structure, however,
                   some  danger spots  such  as edges,
                   corners,  crevices,  rivets,  bolts,   and
                   welds which, when they reveal the need
                   for painting, usually indicate that an
                   entirely  new  job  is needed  for the
                   structure.
                     8.8 SAFETY PRECAUTIONS
  Some painting must be done in con-
fined areas.  Unless provisions are made
to change the supply of air the  paint
fumes will cause dizziness and finally
fainting.  It is well to watch  for the
danger signs.  Headaches or dizziness
are  warnings to get out in the  fresh
air.   Most paint materials  are highly
inflammable and must be handled with
care, avoiding contact  with flame or
heat.  Saturated oily rags in  confined
places can cateh fire through spontane-
                   ous  combustion.    Removal  of paint
                   from the skin with solvents may cause
                   irritation so it is a good precaution to
                   keep the body covered as much as pos-
                   sible. Ropes, ladders,  and safety belts
                   always  should be  inspected  before  a
                   job  is   started.   The paint  bucket
                   should  be  secured  thoroughly when
                   working from heights, and other tools
                   should  be  anchored to prevent  their
                   falling on persons passing  underneath.

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                    PAINTS AND PEOTECTIVE COATINGS
                                  83
    FIGURE 9.—Paints and protective coatings pull back from sharp edges; care
          must be exercised in application.  (Courtesy Amercoat Corporation.)
                             8.9 SUMMARY
  Adherence to a few general rules will
help to insure a satisfactory paint job.
  1. Surface dryness and preparation
to prevent moisture from breaking out
beneath the paint film.
  2. Sufficient number of coats, not to
be too thick  as they are applied.
  3, Removal of  part or all of the old
coating  that  has  become too heavy.
  4. Thorough drying of each coat be-
fore another one is  applied.
  5. Proper use and care of tools and
the correct type of paint for each par-
ticular job.
  6. Consideration of the importance
of  weather and temperature  on  the
outcome  of the  work.

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       9. MISCELLANEOUS FACTORS IN GOOD
                     PAINTING PRACTICE
  The main factors used in good paint-
ing practice  have been discussed  in
previous  chapters  of  this  manual.
Some of these factors will be reviewed
again.
                   9.1 SURFACE PREPARATION
  Surface preparation  of  metals and
concrete for painting are difficult jobs
at times  due to the moisture  and gas
conditions found  in  nearly all waste
treatment plants.
  The most  common  method of clean-
ing  metal  surfaces   involves  wire
brushing,  scraping, then washing the
surface with a phosphoric acid solu-
tion.  The surface also may be washed
with  mineral  spirits,  turpentine,  or
alcohol.    The prime  coat should  be
applied immediately.
  To paint wet or damp pipe surfaces,
wash  with  hot water  along  with  a
good cleaning  compound, such as tri-
sodium phosphate, dry pipe by wiping
with turpentine or alcohol, and paint
immediately.
  A handy tool for cleaning rust and
scale can  be  made from worn rasps
or mill files.  They make fine surface
scrapers when fitted with a handle.
  Woodwork usually is cleaned by wire
brushing,  blow torch,  sandpaper, or
some chemical remover of paint.  In
the priming of woodwork, the paint
should  be  thinned  with turpentine to
permit the first coat to soak into the
pores of the wood.
                     9.2 PAINTING PROBLEMS
  Bleeding,  peeling, and blistering of
the paint coating  has  caused  trouble
on many jobs.  Bleeding often happens
when applying paint over tar or creo-
sote.  Peeling  can  result from a poor
foundation  being  provided  by  the
primer, a poor grade of paint, or too
thick an application.    Blistering is
caused by moisture being trapped be-
neath the paint film or high tempera-
tures before the oil has set.
  Driers should be  used very sparingly
as they tend to shorten the life of the
paint film.  The thinner recommended
by  the  paint manufacturer  always
should be used.
  The primer is the real life  of the
paint job and should be selected and
applied  carefully.   The  ideal  primer
should have  a hard tenacious film, good
waterproof qualities, and rust-inhibit-
ing pigments.  The prime coat should
be harder than the finish coat.  A soft
undercoat may cause the finish coat to
crack.
  No painting should be done except
in dry weather.  Paint should not be
applied  in  foggy,  frosty,  misty,  or
snowy weather, or when temperatures
are below 40 °F (4.4°C).  The best re-
sults will be obtained when tempera-
tures are  around  60°F  (16°C)  or
higher.
  Whether   the  paint  is applied  by
spraying or brushing does not matter
too much.   Good  results can be ob-
tained by either method if the paint
is applied  properly under favorable
conditions.  Spraying is considered the
best  for   cold-weather  conditions.
Again some say that brush application
of prime coats promotes good adhesion,
while spraying  is satisfactory for top
coats.   Brush application is  used  on
                                   84

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                    PAINTS AND PEOTECTIVE COATINGS
                                  85
most  small jobs.    Some  of the  ad-
vantages  claimed  for spray  painting
are:  three  or  four   times  faster,
smoother  and more even  distribution
of paint;  better penetration on  porous
surfaces;  and more economical.  Ad-
jacent  equipment   such  as  motors,
switch gear, speed reducers,  and simi-
lar equipment within reach  of spray
painting should be  protected by  ade-
quate covering.   This  procedure  will
add to the painting cost.
   The painting of  wire  fences  is  a
task that confronts many wastewater
treatment plant personnel.  The prob-
lem is  how  to  do  this  rapidly  and
effectively, without  wasting  paint  or
getting it  all over other areas beside
the fence.   There  are two  methods
which have proven  satisfactory.  One
involves  spraying  the paint on  the
fence, using a movable backboard fitted
with a drain trough across the bottom
to catch the excess paint for reuse.  The
other method involves application  of
the paint by means of a large diameter
roller  with  extra  long  nap  on  the
sheepskin covering.
  How often to repaint is a question
often  asked  at  wastewater  treatment
plants.  It is much more economical to
keep a paint job in good  repair than
to wait until a greater  part of  the
coating has been  destroyed and then
paint.   No two  plants  have the same
conditions to  deal  with on the  paint
problem.  The paint problem seems to
be much more serious in northern lati-
tudes  than it is in warmer climates.
Where  metal is exposed  to moisture
and gases  it may need touching up once
in six  months or possibly once a year,
depending on the exposure.  Most out-
side exposures are painted on the aver-
age of every two or three years.  Mild
exposures  may  be  all right for four
to five years.  Eecords should  be kept
of the  painting  or repainting of all
structures and  equipment,  including
the date,  method of cleaning,  kind of
paint,  number of coats, etc.
    9.3 USE OF PAINT FOR  IDENTIFICATION AND SAFETY
  To many wastewater treatment plant
operators, paint  is  just  a cover and
protection applied to conceal the ear-
marks  of use and time and to protect
against the effects of  wear,  weather,
and corrosion; but paint  is useful  in
other ways  such as  for identification.
When  this  characteristic is  utilized,
series  of pipelines  can  be  identified
readily as to function.  The identifica-
tion code as recommended in the  Great
Lakes-Upper Mississippi  River Board
of State Sanitary Engineers, Recom-
mended  Standards for  Sewage Works
(Ten-State Standards), is as follows:

  Painting:  The use of paints  containing
lead  should be avoided.  In order to facili-
tate identification of piping,  particularly in
the  large plants, it ia  suggested  that the
different lines have contrasting colors. The
following  color scheme  is  recommended for
purposes of  standardization:
  Sludge line—brown.
  Gas line—red.
  Potable water  line—blue.
  Chlorine  line—yellow.
  Sewage line—gray.
  Compressed air line—green.
  Water lines for heating digesters or build-
ings—blue, with a 6-in. red band spaced 30
in. apart.

  Protruding   ledges,   low  over-head
pipes,   beams,  unexpected  steps,  or
curbings  will  draw  attention  when
spotlighted  by  some contrasting color.
Paint  also is useful for beautification
even  though this is a secondary pur-
pose.   The  tasteful  use  of  colorful
paints can contribute  much  to the at-
tractiveness  of  any   plant.   Paint,
teamed with light, can provide a daily
tonic of considerable  value.
  Color can flash danger warnings, lo-
cate  vital equipment, identify machine
parts,  and brighten the rooms of the

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86
PAINTS AND PEOTECTIVE COATINGS
  FIGURE 10.—Paint can be both  functional and attractive as illustrated  at the Mill
Creek Water  Pollution Control Plant, Cincinnati, Ohio.  (.Courtesy Inertol Company of
Koppets Company, Inc.)
plant.  Applied  to machines,  the first
job of  color  dynamics  is  to  separate
the critical from the non-critical parts
of the  machine.   The critical or  op-
erating parts  of the machine should be
given a color  that comes quickly to the
eye,  a color that stands out in  strong
contrast  to   the  stationary   or  non-
critical parts  of  the  machine.   This is
known  as a  focal  color  because  it
focuses the worker's attention exactly
where it  should be—on the  working
parts of the machine.
  There  are  certain receding  colors
which are used to  cause the non-criti-
                   cal parts of the machine to drop back.
                   "Machine gray" has  been used  to  a
                   certain extent for this  purpose.  Green
                   is considered one of the best receding
                   colors as it has a relaxing effect on the
                   human eye.  The wide  spread of green
                   by nature  in  the  forests and field is
                   the proof of this  color.
                      Color applied to  the walls  and ceil-
                   ing of a room will produce a feeling of
                   cheerfulness  and   restfulness  along
                   with  good  visibility.   Color  not  only
                   has a physiological  effect on the work-
                   er's eye and body,  it also has a physio-
                   logical  effect on  his mind.

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               PAINTS AND PHOTECTIVE COATINGS
87
PAINTS AND PROTECTIVE COATINGS  FOR
WASTEWATER TREATMENT FACILITIES
—MOP  17

Technical Practice Committee, Subcommittee on  Paints and
  Protective Coatings

1969

  New Manual of Practice No. 17, title as above, is intended to provide design-
ers, operators, and maintenance  personnel  of wastewater collection and  treat-
ment  facilities with the fundamental theory  and practical aspects of  the need
for, choosing,  application, and maintenance of paints  and protective  coatings,

Keywords: coatings, corrosion,  corrosion  control,  corrosion  effects, corrosion
environments,  corrosion prevention, maintenance, (Manual of  Practice), paint-
ing,  paints, plants,  protective  coatings, sewage  treatment, (Water Pollution
Control Federation),

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   5  Air Diffusion in Sewage Works	      t         t
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      ment  	   $ 0.50   $ 0.75
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