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
G.
C.
E.
W,
0.
J.
B
:UEST
ION
3ND
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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.
-------
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-
-------
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
-------
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
-------
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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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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