United States
Environmental Protection
Agency
Water Engineering Research
Laboratory
Cincinnati OH 45268
Research and Development
&ERA Project Summary
EPA/600/S2-85/079 Aug. 1985 ™ *
Vyf »v
Corrosion and Calcium
Carbonate Saturation Index in
Water Distribution Systems
J. Edward Singley, Rodolfo A. Pisigan, Jr., Abdul Ahmadi, Portia 0. Pisigan, and
Ting-Ye Lee
Corrosion in water distribution sys-
tems was studied to gain a better
understanding of the processes needed
in developing control strategies. Equa-
tions and calculation methods for de-
termining the pH, (or the theoretical pH
at which the water is saturated with
CaCO3) were developed using a chemi-
cal model with and without ionic speci-
ation. Several calculation procedures
for determining pH, were analyzed and
compared. Laboratory experiments
using weight loss and electronic tech-
niques for measuring corrosion rates
were conducted in laboratory batch and
continuously circulating systems using
waters with different saturation indices
(SI), pH, alkalinity, chlorides, sulfates,
chlorine residual, dissolved oxygen,
organic color, calcium, buffer capacity,
and other water quality parameters.
Changes in water quality were moni-
tored in experiments with mild steel,
galvanized steel, and copper coupons.
Corrosion products obtained from cou-
pons exposed to continuously circulated
solution were analyzed by wet chemical
techniques and X-ray diffraction. In-
creases in dissolved oxygen, chlorides,
sulfates. chlorine residual, and hydrogen
ion concentration accelerated corrosion
of metal specimens. Higher pH, calcium
content, buffer capacity, and organic
color decreased the corrosion rate. A
model was formulated that expresses
corrosion rate in terms of chlorides,
sulfates. alkalinity, dissolved oxygen,
calcium, buffer capacity, saturation
index, and immersion time. A four-
variable corrosion rate model was de-
veloped from the multivariate analysis
of corrosion data for mild steel in the
batch studies. The model included total
dissolved solids (TDS), dissolved oxy-
gen, saturation index, and exposure
time. Results indicate saturation index
cannot be used alone to indicate the
extent of corrosion or to measure the
corrosivity of a water.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati, OH, to announce
key findings of the research project that
is fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Corrosion of pipes in community water
supply systems has been recognized as a
serious problem since the beginning of
the 20th century. Economic estimates of
corrosion costs in the water industry have
increased from $40 million in 1947 to as
high as $700 million in 1980. In addition
to the economic consequences, as the
metallic structure deteriorates, drinking
water quality is degraded. The resulting
corrosion products also pose potential
health risks. Consequently, the U.S.
Environmental Protection Agency (EPA)
has amended the National Interim Pri-
mary Drinking Water Regulations to
require that public water utilities carry
out a corrosion control program. This
program includes identifying the presence
and sources of corrosion products and
subsequently implementing effective
corrosion control measures.
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CaCOa deposition on the metal surface
has been one of the better control strate-
gies for combatting corrosion. Using the
parameter pH8 (or the pH of CaCO3
saturation) is commonly employed to
ensure that the processed or treated
water has sufficient protective property.
Because most of the pHs calculation
methods are not accurate enough and
provide only an approximate value, more
precise methods are needed for deter-
mining pH8. Likewise, further studies are
still needed to assess the effects of
important water quality parameters on
corrosion of metals used in the distribu-
tion systems.
Research Objectives
This investigation was conducted:
(a) to derive a more accurate theoretical
expression for evaluating pHs;
(b) to make a comparative analysis of
other methods for calculating pHB;
(c) to develop a simplified method for
accurately predicting the exact pH,;
(d) to evaluate the corrosive behaviors of
waters with different saturation
indices;
(e) to study the effects of several water
quality factors on the corrosion rates
of mild steel, black steel, galvanized
steel, and copper; and
(f) to analyze the corrosion data and
develop empirical and statistical cor-
rosion rate models.
Experimental Section
Different synthetic waters with differ-
ent saturation indices were prepared by
mixing appropriate quantities of the re-
quired reagents. A 24-hour equilibration
test in which analytical reagent CaCO3
was added to each water determined the
actual saturation state with respect to
CaCOa. The changes in pH, Ca, and
alkalinity were determined before and
after the equilibration period.
Two sets of corrosion evaluations at
room temperature (20+3°C) were con-
ducted: jar tests and loop studies. In the
jar tests (non-flowing system), several
cleaned metal coupons were suspended
and immersed by nylon cords in the
plastic jars containing water with differ-
ent chemical compositions and saturation
indices. The jars were placed over a
stirring apparatus (Figure 1), which can
be used simultaneously for aerated and
deaerated systems. The weight loss was
used to calculate the corrosion rate:
- Compressed Air -
Figure 1. Stir ing apparatus (D = deionized water; F = glass wool filter; J = jars).
before and after a specified immersion
period, the oven dried weight of each
coupon was determined. In addition to
the weight-loss method, the corrosion
rate in some experiments was electron-
ically monitored at specific time intervals
using a Magna Corrater Model 1120,
which employed three-electrode probes.
Figure 2 shows the loop set-up con-
structed with nonmetallic materials to
study corrosion of metals in contact with
circulating water of different saturation
indices. A Manostat Varistaltic* pump
(advanced model) was connected to the
water reservoir to feed the pipe sections
and these regulate the water flow. Bypass
tubes were provided to prevent disrupting
the entire system operation when remov-
ing metal specimens for examination and
analysis. Each PVC tube section in the
loop system held four metal inserts. The
corrosion rate of the exposed internal
surface of each pipe coupon was deter-
mined by the weight-loss method. After
each corrosion test, the loop set-up was
flushed with acidified water and then
rinsed twice with deionized water to
remove any adhering corrosion products.
An aliquot of the last wash water was
analyzed for the presence of metal ions to
ensure that no metallic substances still
remained in the system.
Dried corrosion products obtained from
loop studies of mild steel, black steel, and
galvanized steel were analyzed by wet
chemical methods and X-ray diffraction
analysis. X-ray diffractograms of standard
compounds were developed simultane-
ously during the sample analysis.
Results
Theoretical A spects
After presenting and reviewing several
CaCOaSaturation concepts and corrosivity
indices, two alternative calculation me-
thods for pH8 were derived. The first
equation was a quadratic pHs formula
(QUAPHS) developed from a chemical
model of CaCO3 solubility equilibrium in
which several ionic association equilibria
were considered. The final form of the
equation for hydrogen activity at CaC03
saturation was a function of the first and
second thermodynamic dissociation con-
stants of H2CO3 (Ki and K2), thermody-
namic CaCOa solubility product constant
(Ks), free calcium ion concentration, total
equilibrium carbonate concentration, and
activity coefficients of Ca+2, COa2, and
HCOa.
A computer program, constructed dur-
ing the study, calculates QUAPHS through
iterative calculation procedure whereby
values of the variables, except equilibrium
constants at a given temperature, initially
are given approximate values. Solving for
pHs continues until both sides of the
equation are almost numerically equal.
Another simplified pHs (SIMPHS) ex-
pression similar to Langelier's equation
was derived from equilibrium relation-
ships in which ion pairing was neglected
and a new salinity correction factor was
added. The resulting pHs equation was
SIMPHS = -log
[Ca+2]
Z.5S/T
+ 3.63 I
'Mention of commercial products or trade names
does not constitute endorsement or recommenda-
tion for use.
1.0+3.30N/T+2.61 I
where: [Ca+2] = total calcium concentra-
tion, moles/L
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'/2-Inch Pipe Section
Plexiglas -.-
Cylinder (D**" M-1*
:? , , *x , , xx
1-x-L-Lx-l l-x-LJ-x-T fx1—lx-f
3A-lnch Pipe Section
/ -/nc/7 P/pe Section
C) Pomp
Water
Reservoir
Figure 2. Set-up of the loop system for evaluating metal pipe corrosion in circulating water.
(Alk) = total alkalinity, equiv/L
I = ionic strength, moles/L
Several statistical predictive pHs equa-
tions were analyzed and compared with
the equations derived in this study. Two
predictive equations, one containing 4
variables (STA4PHS) and the other, 5
variables (STA5PHS), were chosen. A
comparative analysis was done among
the two statistically derived predictive
equations and among other calculation
methods such as the WATSPEC2 com-
puter program (WSP2PHS), Langelier's
formula (LANGPHS), Larson-Buswell
equation (LARBPHS), and graphical ap-
proximation of pHB using Caldwell-
Lawrencediagrams(CDLWPHS).ThepH8
values of the 155 water samples (see
Table 1) were calculated using each
calculation procedure. When the pHa
values obtained via computer calculation
of QUAPHS were compared with those
derived from other methods, pHs values
for QUAPHS, SIMPHS, WSP2PHS,
STA4PHS, and STA5PHS are generally
close to one another. LANGPHS and
LARBPHS are comparatively lower than
the pHs values obtained from QUAPHS
and the first four aforementioned calcu-
lation methods. The highest values are
those derived from the approximations in
Caldwell-Lawrence diagrams.
Table 1. Mean Values and Ranges of the
Analytical Parameters for 155
Samples of Florida Water
Parameter*
Mean
Range
TDS
Ca
Mg
Na
Cl
sot
Fe
F
Har
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time) yielded a satisfactory graphical fit to
the experimental data. Multivariate re-
gression analysis of the correlation be-
tween the corrosion rate of mild steel and
various water quality parameters pre-
sented in Table 2 revealed that the
corrosion rate (CR4) is best predicted.
under the experimental conditions em-
ployed in the present study, by
CR4 =
(TDS)0.253(DO)0.820
(10s')00876 (time, day)0373
The above predictive equation appears
I
I
^
T
1
T
1
T
_L
-
-
0.37
0.18^
"
2
X
J
0.12
0.06
n
Water A Water B Water D Water E Water F
SI = -0.72 SI = -0.48 SI = +0.04 SI = +0.57 SI = +0.94
to work well in waters with medium to
high DO level (5.0-9.0 mg/L). Represen-
tative plots for comparing the observed
and CR4- predicted values in three waters
are shown in Figure 6. The experimental
corrosion rates follow a decreasing trend
with time as do the corrosion-time curves
generated by the predictive equation.
Conclusion
Two equations for determining pH,
were developed. One was a quadratic
expression derived from a CaCOa solubil-
ity equilibrium model, which incorporated
interaction among the major ions. The
other, derived without taking into account
ion pairing, was similar in form to
Langelier's equation but with a new ionic
strength correction factor. A simpler
method of computing pH9 was found in
which pHs is defined by a statistical
predictive equation composed of a linear
combination of the logarithmic values of
total dissolved solids, total alkalinity, and
analytical concentrations of calcium,
magnesium, and sulfate. Although a more
accurate method of determining the
CaCO3 saturation index was found, the
traditional interpretation of the indices
was not consistent with the experimental
corrosion data obtained from the jar and
loop studies. Analysis of X-ray diffracto-
grams of the corrosion products sugges-
ted that CaCOa would still precipitate in
waters with a negative saturation index
while in contact with a corroding metal
surface. Several water quality parameters
influenced the corrosion of metals used
in the study and would have to be taken
into account when assessing corrosive
behaviors of waters. An empirically con-
structed, eight-variable model indicates
that the corrosion rate of mild steel under
the experimental conditions used is af-
fected not only by the water's tendency to
precipitate or dissolve CaCOa, but also by
dissolved oxygen, chloride, sulfate, cal-
cium, alkalinity, buffer capacity and im-
mersion time. A four-variable model
derived from multivariate regression
analysis defines corrosion rate as a
function of four parameters: total dis-
solved solids; dissolved oxygen; saturation
index; and exposure time.
The full report was submitted in fulfill-
ment of Grant No. R805400-01 -1 by the
University of Florida, under the sponsor-
ship of the U.S. Environmental Protection
Agency.
Figure 4. Bar graph showing the corrosion rates of black steel pipes after 21 days in the loop
system (DO = 5.0 + 0.2 mg/L) using five waters of different saturation indices.
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Figure 5. X-ray pattern of corrosion products from black steel pipes in contact with water B
(SI = -0.46).
Table 2. Means and Ranges of Analytical Variables* for the
Batch Corrosion Studies
Parameter Mean
TDS
Ca
Mg
Na
Cl
S04
Alkalinity. mg/L as CaCO3
pfmg/LasCaCOa/pH)
pH, units
pH* units
Saturation Index, units
Ionic Strength. mole/L
Dissolved Oxygen
Corrosion Kate fmpy)
Corrosion Rate (mm/yr)
473
68
16
64
119
79
116
16
7.77
7.79
-O.02
O.O081
5.1
11.9
0.301
1 7 Waters Used in the Mild Steel
Range
116-791
10-12O
2-52
7-1 3O
18-213
16-2O8
21-232
3-42
6.70-8.95
7.60-9.07
-2. 12-+1.90
O.OO01-O.O198
1.4-9.1
2.1-36.6
0.053-0.930
"All values are expressed in mg/L except as indicated.
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40
30
20
10
oc
I
o
O
30
20
10
Water e
S\=-2.12
D0=5.0±0.2
Water e
S\ = -2.12
DO = 9.0±0.2
t \
Water k
SI = +0.07
DO=5.0±0.2
Water k
SI = +0.07
DO = 9.0±0.2
Water q
SI = +1.90
DO=5.0±0.2
Water q
SI = +1.90
DO = 9.0±0.2
02 46 8 10 2468 10 24 6 3 10
Days
Figure 6. Predicted corrosion—time curves and experimental values (*J for corrosion
of mild steel in waters e, k. and q.
.S. GOVERNMENT PRINTING OFFICE: 1985/559-111/20666 19519F
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J. E. Singley, R. A. Pisigan, Jr.. A. Ahmadi. P. 0. Pisigan. and Ting-Ye Lee are with
University of Florida. Gainesville. FL 32611.
Marvin Gar dels is the EPA Project Officer (see below).
The complete report, entitled "Corrosion and Calcium Carbonate Saturation Index
in Water Distribution Systems," (Order No. PB 85-228 112/AS; Cost: $22.00,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use 5300
EPA/600/S2-85/079
QCOC329 PS
U S ENVIR PROTECTION AGENCY
REGION 5 LIBRARY
230 S DEARBORN STREET
CHICAGO IL 60604
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