^*»' "' "o,

1<> NBS TECHNICAL NOTE 974

''fAU of

U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

Corrosion of Metallic Pipes

Transporting Potable WaterLaboratory Testing Methods
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The

Bureau's overall goal is to strengthen and advance the Nation's science and technology and

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and the Institute for Computer Sciences and Technology.

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The National Bureau of Standards was reorganized, effective April 9, 1978.
Corrosion of Metallic Pipes Transporting

Potable Water—Laboratory Testing Methods

NATIONAL BTJREATI OF STANDASDS

LIBRARY JVN 2 1 ^S78

(^^

Jean N. Andre

Centre Scientifique du Batiment

Paris, France

James R. Clifton

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

%

^f^Aii Of

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

'-^^^ NATIONALBUREAUOFSTANDARDS, Ernest Ambler, Director

Issued June 1978
National Bureau of Standards Technical Note 974

Nat. Bur. Stand. (U.S.), Tech. Note 974, 36 pages (June 1978) CODEN; NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1978

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

(Order by SD Catalog No. C13.46.974). Stock No. 003-003-01945-3 Price $1.40

(Add 25 percent additional for other than U.S. mailing).
TABLE OF CONTENTS

PAGE

1. INTRODUCTION 1

2. LABORATORY SIMULATION OF PIPE CORROSION 3

2.1 Properties of Metallic Pipes 3

2.2 Chemical Composition of Water 6

2.2.1 Salts 6

2.2.2 Dissolved CO^/^s and O^/^n 7 2(g) 2(g)

2.2.3 Test Water 8

2.3 Flow Rate of Water 10

2.4 Water Temperature 12

2.5 Water Pressure , 13

2.6 Prevention of Extraneous Corrosion 14

3. CORROSION RATE MEASUREMENTS 15

3.1 Weight Loss Measurements 19

3.2 Electrochemical Measurements 21

4. SUMMARY AND CONCLUSIONS 26

5. REFERENCES 27

111
LIST OF TABLES

1, Comparison Between Calculated and Measured Weight Losses

LIST OF FIGURES

1. Schematic of Pipeline Used During Corrosion Studies 4

2. Water Reservoir 5

3. Connection System Developed to Prevent Crevice Corrosion 16

4. Pipe Specimen Holder Assemblies 17

5. Different Corrosion Rate Behaviors Based on Actual Measurements 20

6. Schematic of Circuit for Making Polarization Resistance

Measurements 22

7. Schematic Indicating Location of Reference and Counter Electrodes 23

IV
CORROSION OF METALLIC PIPES TRANSPORTING

POTABLE WATER - A LABORATORY TESTING METHOD

by

Jean N. Andre and James R. Clifton

A pipeline was constructed arid experimental methods developed to determine the effects of several Important factors on the corrosion of pipes.

These factors Included the composition, temperature and flow rate of the

water, and type of metal. Components of the pipeline were carefully selected

or designed to prevent the occurrence of extraneous corrosion.

The rates of corrosion of galvanized steel and copper pipes were measured by direct weight loss measurements and by polarization resistance

methods. It appears that the polarization resistance technique Is a useful

tool which may be used In a variety of corrosion studies of pipe In aqueous

media.

Key words: Copper pipe; corrosion; corrosion measurement; galvanized steel

pipe; metal pipes; potable water; resistance polarization.

1. INTRODUCTION

In the first phase of a program being undertaken to investigate the

processes associated with corrosion of pipes in buildings, the factors

affecting the corrosion of pipes transporting potable water are being

studied. Many factors affect both the type of corrosion and the corrosion

rate of pipes [1-11] . Natural waters are complex media and their chemical
composition, as characterized by pH , dissolved salts, and dissolved gases,

is an important factor influencing the corrosion rate of pipes. Other

important factors include the flow rate and temperature of the water and

obviously the type of metal used in producing the pipe. To ascertain the

relative importance of these factors an experimental test design, test

methods and test apparatus have been developed. Their development is

described in this report.

Included in the development of test methods was the identification and

modification of existing corrosion measurement techniques and procedures

which would provide the desired information. A commonly used method is

based on measuring the weight change of test specimens; however, this method

is normally used to obtain results integrated over a long period of time,

usually months. The average corrosion rate over such a long period may not

be the corrosion parameter controlling the service life of the pipe. Frequently, the corrosion rate changes significantly with time and the magnitude

of the corrosion rate at the end of an experiment could be the controlling

parameter. Therefore, methods for measuring instantaneous or quasi-instantaneous corrosion rate were reviewed and the resistance polarization method

was selected. Its applicability for measuring the corrosion of pipes is

discussed in this report.

In addition to investigating the factors affecting the corrosion of

pipes, a preliminary assessment of the efficiency of silicates in reducing

the corrosion rates of metallic pipes has been made. A common method of

controlling corrosion in aqueous media is to use inhibitors, but in potable

water the type and the quantities of inhibitors which can be used are

2
limited because of health considerations. Silicates, phosphates, polyphosphates, zinc salts, carbonates and hydroxides are conunon inhibitors which

are often added to potable water in Europe. Similar inhibitors are also

added to some water systems in the United States. Recently, a literature

review was conducted by J. N. Andre of corrosion in natural waters, which

included the use of polyphosphates (or condensed phosphates) as inhibitors

in potable water; the review was published by the Centre Scientifique et

Technique du Batiment (CSTB) [12].

2. LABORATORY SIMULATION OF PIPE CORROSION

The experimental test design and the test apparatus developed to investigate the effects of factors associated with corrosion of pipes in buildings

are discussed in this section. An important aspect of this work was the fabrication of a functional pipeline, which is shown in Figures 1 and 2. The components and operation of the pipeline are also described herein.

2.1 Properties of Metallic Pipes

One of the most important features affecting the corrosion of commonly

used metals for pipes (black steel, copper, and galvanized steel) is the condition of the interior pipe surfaces, presence of scale or containment films,

and smoothness of interior surfaces. Metallurgical features, such as the

carbon content of steel (in the range of mild steels), appear to be of

secondary importance with pipes of the same metal and their effects are generally uniform from one pipe to another [12] . Therefore, test specimens

were fabricated from commercially available black steel, copper and galvanized

3
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Figure 2. Schematic of Water Reservoir,
steel pipes and used without being subjected to heat treatment. The quality

of the galvanized coating on galvanized steel often varies from pipe to pipe

and may even vary on a specific pipe. Test specimens were made from pipes of

the same origin to minimize metallurgical differences. Only small differences

were found in the corrosion behavior of samples taken from the same source.

Test specimens, 10 cm in length were cut from 1.3 cm i,d pipe using a

lathe. This small diameter made it possible to obtain a high water flow rate

using a low capacity pump (maximum pumping rate of 21 litres/minute). Cutting

with the lathe facilitated the preparation of clean and smooth cross sections

which could be fitted tightly into plastic connectors. Furthermore, cutting,

with the lathe did not damage the galvanized coating.

Immediately prior to use, test specimens were (1) cleaned in ethanol,

(2) immersed in hydrochloric acid (1 minute in concentrated acid for black

steel and copper samples, about 3 seconds in a 1 Normal solution for galvanized test specimens), (3) rinsed in distilled water, then (4) dried with

ethanol, and (5) finally weighed.

2.2 Chemical Composition of Water

2.2.1 Salts

The salts normally present in natural water can be divided into two

main categories, (1) those responsible for the carbonate hardness of water

and (2) soluble salts. The salts responsible for the carbonate hardness

of water are calcium and magnesium carbonates and their bicarbonates.
These salts are only slightly soluble and if supersaturating conditions are

attained the salts will precipitate and slowly form protective scales on the

walls of the pipes [4, 7],

The commonly encountered soluble salts having the greatest effect on

corrosion are the alkali chlorides, sulfates, nitrates, and sodium carbonate

and bicarbonate. One process by which these salts can increase corrosion

rates is by increasing the conductivity of water. Sulfate and chloride ions

have especially great influences on the corrosion rate and type of corrosion.

The effects of other impurities in water such as organic matter, which

is often present in surface water, or naturally occurring silicates (sometimes referred to as the cristalloidic form) are difficult to reproduce.

These impurities are generally considered to have beneficial effects [13] in

controlling corrosion and no attempt was made to incorporate them in our

study.

2.2.2 Dissolved CO^/o) ^nd O^y \

The two gases which usually have the most significant affect on the

corrosion of water-transportating pipes are O^f ) and CO,, .. The amount of (dissolved vs/ 2(g)

COo/g-) controls the pH of the water as indicated by the simplified equation:

C0o/'„\ + H^O XT . >, -^ HoCO^ -^H"*" + HCOT -»-2H"'" + C0^~ (1) 2(g) 2 (liq) -^23-^ 3-«-

The CO2 content also affects the extent of scaling by carbonates.
The amount of oxygen dissolved is very important because the electrode

reaction:

1/2 02(g) + H20(iiq) + 2e - 20H-(^q) (2)

is generally the main cathodic reaction involved in the corrosion process.

This reaction is, in most cases, diffusion controlled, which means that its

rate is dependent on the oxygen concentration gradient next to the pipe walls.

If the oxygen at the wall surface is consumed at a rate rapid enough to substantially drop the oxygen concentration, then the diffusion rate of oxygen

will control the cathodic reaction. At a specific temperature, the main

factors affecting the diffusion rate of oxygen are the oxygen concentration

in the bulk solution, and the thickness of the diffusion layer between the

bulk solution and the water-metal interface.

2.2.3 Test Water

To evaluate the effects of dissolved salts and gases on the corrosion

of pipes, experimental waters were made by adding the desired salts and

gases to distilled water. The pH's of the solutions were lowered by

bubbling C02(-2') through them or increased by flushing dissolved C02/o.\

from the solutions with Nr, ^-> . For example, the pH was lowered with

CO2/ \ to permit the complete solution of CaCO^ and MgCO^. The gas content

in the test water was controlled during studies by continuously bubbling

a mixture of

02(e.) ^^'^ ^*^2(r) through the water (Fig. 2).
The corrosion process Itself progressively changes the composition of

the water. Oxygen and eventually CO2/' n are depleted if they are not

replaced and the concentrations of metallic ions coming from the corrosion

of the pipes are increased if they are not precipitated as oxides, hydroxides, or carbonates. The scaling process in a water of positive Langelier

indexii/ will progressively reduce the concentration of calcium and magnesium

carbonates. In a large operational system the water is continuously being

changed which is difficult to do in a laboratory controlled experiment

because of the need to adjust the composition of the test water. The

effect of the corrosion process on water composition may be minimized by

2 / using a very high ratio of corroding surface to volume of test water. _'

However, a more realistic approach would be obtained by using a ratio similar to the one encountered in the field. A value of 140 cm /litre is

given [13] as being a sufficient value for the ratio of corroding surface^

volume of water

if the test water is replaced every five hours. In the present experiments,

a ratio of 65 cm /litre was used and the test water was replaced approximately every 24 hours. In non-scaling water, as used in our first experiments, this replacement rate appeared to be sufficient. In conditions where

scaling is to be expected, the replacement time should be determined by

monitoring changes in the composition of the water. It is felt that concentration changes of less than 20 percent would have little effect on the

corrosion rate.

—T^angelier index is the difference between the actual pH of the water and

the pH this water would have once brought in equilibrium with calcium

carbonate [4]

.

2 / —'Corroding surface indicates the internal surface area of the pipe system

in contact with the water.
2.3 Flow Rate of Water

pDV,

The Reynolds number, RV = (where p is the fluid density, V is

y the average fluid velocity, D is the pipe diameter, and y is the fluid

viscosity), has been developed based on fluid dynamic considerations [14]

and is regarded as being a more significant parameter than the average fluid

velocity when considering corrosion. In the present study, the internal

diameter of the pipe is the same for every test specimen, 1.3 cm, and p and

/x are known for water as a function of temperature. Therefore, measuring the

average water velocity with a flowmeter is sufficient to determine the Reynolds

number at a given temperature. The transition between turbulent and laminar

flow which occurs around RV = 2000 [14] corresponds, for a diameter of 1.3 cm,

to a velocity of approximately 7 cm/s at 60°C.

3 / The corrosion rate is affected by the thickness of the boundary layer±'

because the chemical species involved in the corrosion processes must pass

through this layer. An increase in velocity of the water decreases the

thickness of this boundary layer and accelerates mass transfer processes

[15, 16, 17], The smoothness of the pipe walls also affects the thickness

of the boundary layer.

A moderately high water velocity is generally a favorable condition

for preventing localized corrosion because the strong stirring action helps

to prevent the formation of concentration cells. At higher velocities

3/— Thin zone of flowing water in the immediate vicinity of the surface of

the pipe [14]

.

10
corrosion-erosion (impingement attack) may occur. In addition, high water

velocities can influence the deposited film characteristics by causing the

removal of loose or slightly adherent deposits.

In this work, the effects of a range of velocities, low velocity (less

than .07 m/s), medium velocity (.4-. 6 m/s), and high velocity (around 2 m/s),

were investigated. After passing through the pump, (see Fig. 1) the water

was separated into four equal portions during passage through segment A of the

pipe network (medium flow), then the water was recombined and passed through

segment B (high flow), except for a small amount which passed through segment C

(low flow). The flow through segment C was regulated by a needle value. The

needle value was calibrated by collecting and measuring the water flowing

through segment C during a specific time interval. The bulk flow was measured

by the flowmeter F. The low flow was probably laminar and the medium and high

flows were considered to be turbulent.

Two experiments have been carried out to determine the effects of the

flow rate of water on corrosion. The first experiment lasted one month and

consisted of exposing copper pipes at 60°C to a test water with a pH between

8.3 to 8.5 (measured at ambient temperature). No attempt was made to measure

the pH at 60° C and the water was aerated with room air. The test water

initially contained 80 ppm of CaCO^, 225 pm of NaHCO^, 60 ppm of NaCl, and

20 ppm of MgSO, .7H^0. Under these conditions, the corrosion rate was little

affected by changes in the water velocity. For an exposed surface of 43.3

2

cm , the average weight loss was 12 mg for low flow (ca ,07 m/s), in medium

flow (0,37 m/s) the weight loss was 16 mg, and in high flow (1,5 m/s), the

i weight loss was 10 mg. In the second experiment, galvanized steel was

11
tested at 60°C for 60 days. The test water was similar to that used in the

previous experiment except that the pH was lowered to 6 (measured at ambient

temperature) by bubbling C02/- \ through the solution. Under these conditions,

the corrosion rate was significantly dependent on the water velocity. The

average weight losses were 1,4 g at high velocity (2.7 m/s), 0.46 g at medium

velocity (0.46 m/s), and 2 g at low velocity (ca. .07 m/s) for samples with

exposed surfaces of approximately 40 cm . Visual inspection of the galvanized

test specimens indicated that localized corrosion took place at low velocity.

These results clearly indicate that the flow rate of the transported water

can significantly affect the corrosion rate of certain types of metallic pipes.

In obtaining information on the effect of a range of water velocities

on the corrosion rate, constant water velocities were used, that is each

velocity was kept constant. In real systems, however, there are alternate

periods of low, medium and high water velocities. While it is possible to

reproduce those conditions in the laboratory the interpretation of the results

would be more difficult.

2.4 Water Temperature

Temperature is an important variable which affects the kinetics of

chemical reactions. The reaction rates of many activation controlled

processes are roughly doubled for every 10°C Increase. While diffusion

controlled processes are less affected by temperature, they are slightly

accelerated because diffusion rates are temperature dependent. The temperature may also effect the thermodynamic equilibrium controlling the

phase stability of oxides and hydroxides, and the solubilities of salts

12
and gases in water. Considering the materials used in water supply systems

and the range of temperatures encountered (10 to 70°C) an increase in temperature will usually increase the corrosion rate.

In our apparatus, water is heated in a 5 litre round bottom glass flask

by a jacket heater with controlled power (Fig. 2) . The temperature of the

laboratory is relatively constant and consequently heat losses are fairly

uniform, therefore 60°C is easily obtained within - 2°C. Significant changes

in temperature only occur when a portion of the test water is replaced. At

i this time, the temperature would decrease by as much as 15°C, however in less

than one hour the temperature increased back to its original value. The

temperature difference between the inlet water and the outlet water in the

flask was found to be less than 0.5°C. The temperature in the branch with

the low flow (segment C, Fig. 1) appears to be slightly lower than that in

the main part of the pipeline. In further experiments this problem will be

eliminated by wrapping the low flow branch with a thermal insulation material.

2.5 Water Pressure

The main influence of externally applied pressure on corrosion rates

in water appears to be to increase the solubility of ©2 and CO2 gases.

Similar results can be obtained by bubbling through the water gaseous

mixtures of O2 and CO2 with, if desired, high partial pressures of either

gas.

13
The experimental pipe system was not pressurized, it was connected

to the atmosphere of the room through a water-cooled condenser to avoid

excessive evaporation of the test water.

2.6 Prevention of Extraneous Corrosion

Components of the pipeline were carefully selected or designed to

prevent the occurrence of corrosion extraneous to the scope of this work,

such as deposition, crevice, and galvanic corrosion.

Copper ions in concentrations less than 1 ppm can cause the deposition

corrosion of metallic pipes such as black steel and galvanized steel [7],

Such small concentrations can be readily generated by the corrosion of copper

or copper alloy parts, especially in a recirculating system. As a consequence,

an apparatus for testing black steel and galvanized pipes has to be free of

components containing copper or copper alloys. Separate pipelines were setup

to test copper pipes, black steel, and galvanized pipes individually and there

are no copper components in the pipeline used for the study of black steel and

galvanized pipe. For example, the pump has an epoxy body and epoxy impeller,

the flow meter has a stainless steel body and glass flow tube, and the needle

valve consists of a glass housing and poly(tetraf luorethylene) adjustable

needle.

A method was developed to connect test specimens to the water loop in

such a way that neither differential aeration nor unusual turbulence occurred

at the junctions. This connection system shown in Figure 3 has prevented the

occurrence of any corrosion localized (crevice corrosion) at the edges of the

14
specimens. The plastic used for the connector was Initially poly(tetraf luorethylene) but at 60°C this material creeped so much that leaks developed

at the metal-plastic joints and therefore Is being replaced with chlorinated

poly( vinyl chloride) which has performed satisfactorily to date.

Galvanic corrosion is prevented by constructing the pipe network

(Figure 1) from 1.3 cm l.d. chlorinated poly(vinyl chloride) pipe. All

other components, such as the pump, needle valve, test specimen holders,

were connected to the pipeline with chemically and thermally resistant

synthetic tubing. Three pipe specimens tested in series in branches B and

C (Figure 1) were separated from each other with chlorinated poly( vinyl

chloride) spacers (Figure 4).

3. CORROSION RATE MEASUREMENTS

Classical corrosion studies on metallic pipes usually involved visual

observation of the samples, weight loss measurements and, if pitting took

place, measurements of the depth and density of pits. These techniques are

not entirely satisfactory because visual observation gives only qualitative

information on the extent and rate of corrosion.

Weight loss measurements and the measurement of pits are destructive

in that the actual state of the sample is changed during cleaning of the

sample; also only the amount of corrosion is determined, whereas the rate

of corrosion may be more Important. As the measured weight values are

often scattered, a number of identical samples should be tested to check

the reliability of the results (for example, 3 samples were tested in the

15

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17
present study). Considering the destructive character of the measurements

and the necessity of using several samples for each measurement, the desirability of employing methods for the nondestructive, instantaneous measurements of the corrosion rate is understandable.

Several electrochemical techniques have been developed for nondestructive

corrosion rate measurements. Of these methods, polarization resistance measure

ments [18, 19] appear to be most appropriate for measuring the corrosion of

metallic pipe. It has been shown that under proper conditions the corrosion

current, Ic, of a metal specimen in a solution is proportional to the applied

voltage, AV, usually 10 millivolts. The formula is:

^ K_ ^ KAI (3)

^ RP AV

where RP (polarization resistance) is the ratio AV/ AI and K is a constant

whose value depends on the specific metal.

A value of 35 volts was used for K when calculating I^ for zinc, iron

and copper. This value was obtained by assuming a Tafel slope of 60 mv/decade,

two electron oxidation processes, and that the cathodic reaction is diffusion

controlled [4]. As the polarization resistance method involves only small

changes of electrical potential from the open circuit corrosion potential,

the actual corrosion state of the sample should not be seriously altered by

the measurement. Therefore, measurements can be taken on samples in situ

as many times as desired. The polarization resistance method is sensitive

to small changes in the corrosion condition of metals and when coupled with

weight loss measurements to check its validity, it is a very useful tool.

18
Open circuit corrosion potentials and polarization resistance measurements were taken after 1, 2, 3, 4, 6 and 8 weeks of testing. At the end of

the test period, the test specimens were visually Inspected, weighed, then

cleaned and reweighed, and finally the cleaned surfaces were Inspected for

evidence of pitting. The early polarization resistance studies Indicated

the necessity of taking additional measurements at 1, 3, and 10 days because

it was found that corrosion rate changes rapidly during the first two weeks

of the test. Actual measured curves are shown in Figure 5 Illustrating

different corrosion rate behaviors. These curves indicate the importance

of commencing the taking of corrosion measurements at early test periods

and also that the duration of the testing is very important. Our first

results, and those of other laboratories [6], indicate that a minimum time

is required before the corrosion rate is stable. It appears that a reasonable period for short-term experiments would be at least 2 months. Under

some conditions, a sufficiently stable rate may be realized in less than 2

months, but such processes as the formation of a protective scale are usually

slow and may not form within a short test period.

When investigating a specific problem, long-term testing is usually

feasible, but in a systematic testing program a reasonably short period

must be selected based on some understanding of the corrosion processes

taking place,

3.1 Weight Loss Measurements

Test specimens were cleaned and then weighed before being placed in

the pipe network. After the experiment was completed the samples were

19
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TIME, WEEKS

GALVANIZED STEEL IN SYNTHETIC WATER 60°C

LOW FLOW CONDITION (few cm/s]

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2 3 4 5 6

TIME, WEEKS

GALVANIZED STEEL IN SYNTHETIC WATER+StLICATES 60°C

HIGH FLOW CONDITION [2.5 m/s]

8

Figure 5. Different Corrosion Rate Behaviors Based on Actual Measurement.

The Corrosion Rate Shown by Curve A was Initially Low then

Rapidly Increased and did not Completely Stabilize even after

2 Months of Testing. The Initial High Corrosion Rate Shown by

Curve B Rapidly Decreased then Gradually Increased,

20
cleaned again. The cleaning agent for copper was either concentrated hydrochloric acid or hydrofluoric acid. A preliminary test indicated that these

cleaning agents did not produce any significant corrosion of copper (1 mg)

when the immersion time was approximately 15 minutes. The cleaning of galvanized steel was accomplished with a saturated aqueous solution of ammonium

acetate and a soft brush. A galvanized specimen lost up to 20 mg when subjected to the cleaning process for 15 minutes. Therefore, this cleaning

method should be used only when, at least, medium corrosion rates are anticipated. It was suitable for our first experiment with galvanized steel,

in which the measured weight losses were between 0.5 g and 2.5 g. Black

steel test specimens were cleaned with Clarke solution (2 parts of SboOo,

5 parts of SnCl2, and 100 parts of concentrated HCl, by weight). All the

samples were dried with ethanol and weighed to within 0.5 mg.

3.2 Electrochemical Measurements

The circuit used for polarization resistance measurements is shown in

the schematic of Figure 6. A saturated calomel electrode was used as the

reference electrode and a stainless steel rod was the counter electrode.

The geometry of the system used for making preliminary polarization resistance measurements with copper specimens is shown in Figure 7. Different

results were obtained if made with counter electrode 1, counter electrode 2,

or with sample 2 as the counter electrode (Figure 7A). There was more than

a tenfold difference between the weight loss calculated from polarization

I resistance measurements and the actual measured weight losses. An investigation indicates that IR drop was influencing the polarization measurements.

21
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O QJ •H 4-1 U +J O c d O QJ o 3 2: iH

•H

O

W +J o o

rt s >-i O M-l Q) o o 4-1 hJ •

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4-1 •r-l 0) CO CO 4= O O Pi 4-1 •H CM M

13 ^ C J= 4-1 H • 00 •H • cn •H S PQ a QJ 33 •H 13 XI C

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23
Even with specimens as short as 10 cm, it appears that a homogeneous potential

field was not induced by the applied current. In subsequent experiments, the

system was modified. The counter electrode consisted of a cylindrical stainless steel rod, 3 mm diameter, placed along the axis of the specimen (Figure 7B).

That portion of the counter electrode which extended beyond the specimen was

covered with an epoxy coating. The reference electrode was positioned as near

as possible to the edge of the sample. The calculated weight losses from polarization resistance measurement of galvanized steel and copper test specimens

using this arrangement were within a factor of 2 of the measured weight losses

(Table 1). These results are very encouraging considering the assumptions made

in calculating weight losses based on polarization resistance measurements.

It does appear, however, that a few weight loss measurements need to be made

to calibrate the polarization resistance technique if it is to be used for

quantitative measurements of the corrosion rate.

A few other difficulties were encountered during making the electrochemical measurements. One problem which is inherent in electrochemical

measurements carried out in flowing hot water, is the instability of the

open circuit corrosion potentials. Random fluctuations of sometimes more

than 10 mV were observed. These fluctuations necessitated changing the

electrical potential by more than 10 mV during polarization resistance

measurements. This introduces an additional error in the determination of

the corrosion current. When the open circuit potential was highly unstable,

a potential change higher than the usual value of ±10 mV was imposed; however, the potential change was kept as low as possible so that the conditions

for valid polarization resistance measurements could be met. Measurement of

24
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25
potential changes were made when the potential was sufficiently stable,

generally 10 to 60 seconds after the current was applied.

4. SUMMARY AND CONCLUSIONS

The effects of the composition of water (characterized by pH, dissolved

salts and dissolved gases), the flow rate of the water and a silicate inhibitor

on the corrosion of galvanized and copper pipes have been investigated. The

experimental test design and testing apparatus developed for this investigation

provides a reasonable simulation of field conditions and contains considerable

flexibility. The advantages of such laboratory testing are that selected

variables can be studied in a relatively short test period and at modest cost.

Certainly field use is the ultimate test; however, laboratory data are needed i

to develop performance criteria to form the basis of selecting durable materials for field use.

The polarization resistance technique gave satisfactory results in this

preliminary study of the corrosion of metallic pipes transporting water.

Furthermore, it appears that the polarization resistance technique is a

useful tool which may be used in a variety of corrosion studies on pipe

carried out in aqueous media [20, 21] . However, the results from polarization resistance measurements should be correlated with results from actual

weight loss measurements and with visual inspection of test specimens for

the detection of localized corrosion, especially pitting.

26
I

! 5. REFERENCES

1. G. Butler and H. Ison, Corrosion and Its Prevention in Water (Leonard

Hills Books, London, 1966).

\ 2, U. R. Evans, Corrosion and Oxidation of Metals: Scientific Principles

and Practical Applications, pp. 87-131, 160-164, 205 (Edward Arnold

Publishers Ltd., London, 1969).

I

ri

I 3. H. H, Uhlig, Corrosion Handbook, pp. 496-506, 953-965 (John Wiley

and Sons, Inc., New York, 1948).

4. H. H. Uhlig, Corrosion and Corrosion Control, 2nd ed. (John Wiley

and Sons, Inc., New York, 1971).

5. M. F. Obrecht, M. Pourbaix, Corrosion of Metals in Potable Water

Systems, J.A.W.W.A. 5_9, 977 (1967).

6. T. E. Larson, Corrosion by Domestic Waters, Illinois State Water

Survey Bulletin 49 (Urbana, 1975).

7. NACE Basic Corrosion Course, Chapters 7, 8 (National Association

of Corrosion Engineers, Houston, 1971),

8. W. F. Langelier, The Analytical Control of Anticorrosion Water

Treatment, J.A.W.W.A. 28, 1500 (1936).

27
9. L. Legrand and G. Poirier, Theorie des Eaux Naturelles (Eyrulles,

Paris, 1972).

10. H. Campbell, A Review: Pitting Corrosion of Copper and Its Alloys,

pp. 625-635, in Localized Corrosion (National. Association of Corrosion

Engineers, Houston, 1974).

11. Second Corrosion Study of Pipe Exposed to Domestic Waters, National

Association of Corrosion Engineers Report 7B170 (Houston, 1970).

12. J. N. Andre and M. Rubaud, Utilisation des Phophates Condenses dans

la lutte Contre la Corrosion des Reseaux de Distribution d'eau en

Immeubles, Cahiers du Centre Scientifique du Batiment (Grenoble,

France, November 1976).

13. T. E. Larson, Corrosion Phenomena - Causes and Cures, Chapter 8,

Water Quality Treatment, 3rd ed. , pp. 295-312 (McGraw-Hill Book Co.,

New York, 1971).

14. H. Rouse, Elementary Mechanics of Fluids (John Wiley and Sons, Inc.

,

New York, 1964).

15. G. Butler and H. Ison, The Influence of Movement and Temperature on

Corrosion of Mild Steel, Water Circulation Apparatus and Test in

Teddington Water, J. Appl. Chem. 10, 80 (February, 1960).

28
16. G. Butler and E. G, Stoud, The Influence of Movement and Temperature

on the Corrosion of Mild Steel, Part 1, J. Appl. Chem. 10, p. 80 (1960);

Part II, J. Appl. Chem. 15, p. 325 (1965); and Part III, Brit. J.

Corros. I, p. 110 (1965).

17. A. J, Romeo, R. T. Surinde, and R, Eliassen, Proceedings of the American

Society of Civil Engineers, Vol. 84, #SA4, July 1958, paper 1702.

18. M. Stern and A. L. Geary, Electrochemical Polarization, J. Electrochem.

Soc. 104, 56 (1957).

! 19. M. Stern, A Method for Determining Corrosion Rates from Linear Polarization Data, Corrosion j^, 440t (1958).

pi 20. M. Stern and E. D. Weisert, Experimental Observation on the Relation

between Polarization Resistance and Corrosion Rate, Proc. Am. Soc.

Testing Mater. Proc. _59, 1280 (1959).

21. T. E. Larson and R. V. Skold, Measurement of the Instantaneous

Corrosion Rate by means of Polarization Data, Corrosion 13, 139 (1957).

22. R. W. Lane, C. H. Neff and S. W. Schilsky, Silicate Treatment to Inhibit

Corrosion of Hot, Potable Water Systems, NTIS AD-767902.

29
(4BS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN 9742. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Corrosion of Metallic Pipes Transporting

Potable Water-Laboratory Testing Methods

5. Publication Date June 1978

6. Performing Organization Code

7. AUTHOR(S)

Jean N. Andre and James R. Clifton

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

Centre Scientifique du Batiment National Bureau of Standards

Paris, France Department of Commerce

Washington, DC 20234

10. Proiect/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as 9.

13. Type of Report & Period Covered Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary ol most si^iticant information. If document includes a significant

bibliography or literature survey, mention it here.) Many factors affect both the form of and the corrosion of pipes transporting potable

water including the composition, temperature and flow rate of the water, type of

metal and the physical condition of the pipe. A pipeline was constructed and experimental methods developed to determine the effects of the above factors on the corrosion

of pipes. Components of the pipeline were carefully selected or designed to prevent

the occurrence of extraneous corrosion.

The rate of corrosion of galvanized steel and copper pipes were measured by direct

weight losses measurements and by polarization resistance methods. It appears that the

polarization resistance technique is a useful tool which may be used in a variety of

corrosioniStudies of pipe in aqueous media.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Copper „ pipe; , corrosion; . corrosion . measurement; galvanized ...

steel pipe; metal pipes; potable water; resistance polarization.

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