pitting corrosion due to deposits in cooling water systems

8/18/2019 Pitting Corrosion Due to Deposits in Cooling Water Systems

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By Sang-Hea Shim, Donald A. Johnson and Barbara E. Moriarty, Nalco Chemical Company

SM

Characterization of Localized andUnderdeposit Corrosionin Cooling Water Systems

Presented at the National Association of Corrosion Engineers, Corrosion ‘88 meeting,St. Louis, Missouri, March 21–25, 1988.

1. Modification of the standard pitting scan technique

which allows for the development of inhibitor films

under oxygenated conditions

2. Galvanometric technique which measures the dif-

ferential current between a well-aerated cathode

and synthetic occluded cells of various dimensions

3. Alternating Current (AC) impedance method whichmeasures change in the surface impedance char-

acteristics caused by the presence of pits

The above methods have been used to characterize

the ability of some major classes of corrosion inhibi-

tor programs to prevent and passivate localized

corrosion. Data is presented on three significantly

different types of corrosion inhibitor packages:

• Neutral pH high-chromate program

• Neutral pH stabilized phosphate package

• High alkalinity phosphonate-based all-organic

inhibitor

Significant differences among these programs were

found. A mechanistic rationale to explain these dif-

ferences is proposed.

EXPERIMENTAL METHODS AND RESULTS

MODIFIED PITTING SCAN

There are many different electrochemical techniques

used to determine characteristic pitting potentials.

Generally, these methods can be divided into

potentiostatic or galvanostatic categories.1 Of thesemethods, potentiodynamic cyclic polarization (also

known as the “pitting scan”) is most widely used to

measure pitting nucleation and passivation potentials.

The standard pitting scan is typically1 conducted

by scanning anodically from the open circuit

potential until a specified current density

ABSTRACT

Localized attack is potentially the most damaging form

of corrosion encountered in cooling water systems. This

paper discusses the mechanism of localized corrosion,

presents some novel techniques for evaluating the

tendency toward and extent of localized corrosion on

carbon steel, and compares the ability of various

inhibitor approaches.

INTRODUCTION

The inhibition of general corrosion by cooling water

is a well-developed technology which has been dis-

cussed extensively in the literature. However, there

is a potentially more damaging form of corrosion

which has received much less attention, i.e., local-

ized corrosion. Pitting, underdeposit corrosion, and

crevice corrosion are related phenomena which can

be described by the generalized term of “localized

corrosion.”The microenvironment which gives rise to localized

corrosion is very different from that associated with

general corrosion. As a consequence, the requirements

for inhibition of these two forms of corrosion are very

different. The testing methods which are typically

used to evaluate inhibitors generally do a poor job of

simulating the conditions occurring on a real-world

“technical” surface on which significant deposits gen-

erally exist. Arguments for extensive cleaning proce-

dures or for invoking unrealistic expectations of

antifoulants or dispersants are really avoiding the

issue of underdeposit corrosion control. What is

needed are evaluation methods which address

underdeposit corrosion as a discreet problem, and

inhibitors which can effectively function in the micro-

environment which is the cause and the consequence

of localized corrosion. This paper discusses some

methods which can quantify localized corrosion. These

methods include:


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3

2. If Eo is between Epp and Enp, no new pits will form,

but existing pits will not passivate.

3. If Eo is greater than both Epp and Enp, new pits will

form and will not passivate.

The following examples illustrate some parameters,

which influence Epp and Enp as measured by this

technique.

A phosphonate-based all-organic corrosion inhibitor

program which is designed to operate at elevated pH

and alkalinity was tested using the above procedure.

This inhibitor program has been described in previ-

ous publications2,3. This sequence of experiments

illustrates the effect of alkalinity, phosphonate and

polymer on Epp and Enp and on the anodic passive

current.

The first comparison (Figure 2) shows the effect of

the phosphonate combination on Epp, Enp and the

anodic passive current. While the anodic passive cur-

rent remains relatively unchanged, a 100 mV increase

in Enp is observed.

The second comparison (Figure 3) shows the effect of

adding an anionic acrylic copolymer to the phos-

phonates. This polymer is an antiscalant with activity

as a calcium phosphate and phosphonate inhibitor.4

The addition of the polymer further increases Enp and

Epp and also causes a decrease in the anodic passiva-

tion current.

The third comparison shows the effect of varying al-

kalinity on modified pitting scan results. Figure 4

shows the effect of alkalinity variation on Epp and

Enp. The pitting nucleation potential increases to a

greater extent than the pitting passivation potential.

The passive current also decreases with increasing

alkalinity (Figure 5). The carbon steel specimens were

examined under an optical microscope after each pit-

ting scan measurement. The number of pits was larg-

est on the specimen exposed to 100 ppm alkalinity,

and decreases with increasing alkalinity. These re-

sults show the correlation between the pitting nucle-ation potential and passive current results. When the

film is weaker, the passive current on the pitting scan

is higher and pits can easily nucleate on the surface,

causing the low Enp and large number of pitting sites.

Pitting scan techniques have been widely used as a

predictive tool for characterizing the tendency of a

material to experience pitting corrosion. Although

valuable information can be obtained from these tech-

niques, they have their limitations. Used in a cooling

water environment on carbon steel, they are prima-

rily a measure of the resistance of an inhibitor film

to disruption by anodic corrosion. In conducting anexperiment, a tremendous electrical perturbation is

applied to the specimen, essentially destroying it as

a relevant metal specimen. This limitation makes this

technique a “snapshot” method which cannot follow

the corrosion and inhibition processes over time. This

method, like other potentiodynamic methods, char-

acterizes the tendency of a material in a medium to

experience corrosion under conditions which are dras-

tically different than those typically encountered in

service. Therefore, other techniques are needed.

Figure 3 — Effect of polymer addition in the all-organic inhibitor solution

Figure 2 — Effect of phosphonate on pitting scan


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AC IMPEDANCE MEASUREMENTS

The AC impedance technique has been extensively

used to study corrosion phenomena5,6. By applying a

low-amplitude oscillating potential to the working

electrode, the perturbation of the electrode is mini-

mized and useful information about the electrochemi-

cal characteristics of the inhibitive and corrosion pro-

cesses on the surface is obtained. This technique is a

powerful tool for characterizing inhibitive films on

metal surfaces. In addition, it has been recently sug-

gested7 that AC impedance is applicable to the studyof localized corrosion. Mansfeld proposed that char-

acteristic patterns are observed in the impedance

spectra in the presence of pitting corrosion.

Experiments were done in order to determine the

effect of pitting corrosion on the impedance charac-

teristics of metal surfaces and to determine differences

in the film characteristics of two radically different

inhibitor programs. The programs studied were a

chromate-only inhibitor which was dosed at a marginal

level, and the previously-described phosphonate-

based all-organic program.

The chromate was dosed at a marginal level (200 ppm)

without the presence of zinc or any other inhibitors.

The pH of the chromate experiment was controlled

at 7 using controlled addition of carbon dioxide gas

into the air sparger, while the all-organic program

was tested at high alkalinity levels, equilibrated withair. The resulting pH was typically greater than 9.

Both tests used air-saturated solutions. All measure-

ments were performed at 120°F.

The measurements were performed on mild steel

rotating cylinder electrodes using a Princeton Applied

Research model (PAR) 273 potentiostat and a PAR

model 5301 lock-in amplifier. The potentiostat and

lock-in were controlled by an Apple II™ computer

using the PAR AC impedance software.

Figure 6 shows the Bode plot obtained at 1.5 hours

after immersion of the specimen into the chromate

solution. The figure shows only one peak, which isrelated to the double layer capacitance. In Figure 6,

the Bode plot taken after 24 hours of immersion is

also shown. This plot shows an additional small peak

in the high frequency range. This phenomenon is also

clear when the Nyquist plot (Figure 7) is examined.

The Nyquist plot shows a small semi-circle in the high

frequency range. This phenomenon appears to be

related to the formation of a primarily anodic passive

film on the metal surface. At 72 hours into the ex-

periment, another distortion appears on the Bode plot.

Figure 8 illustrates that the low frequency phase shift

does not approach 0 degrees. Rather, it decreases and

approached –32 degrees at 0.001 Hz. The shape of the Bode plot did not change after this time for up to

108 hours. The rotating disk electrode was examined

under an optical microscope, and the presence of pits

Figure 4 — Effect of alkalinity on Enp and E pp in theall-organic inhibitor solution

Figure 5 — Effect of alkalinity on passive current in

pitting scan curves in all-organic inhibitor solution

Figure 6 — Bode plot at 1.5 and 24 hours after immersion of the mild steel electrode into thechromate solution


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• Rct is used as a measure of the general corrosion

rate

• Cfilm and Rfilm are the capacitance and resistance of

the inhibitor film

• Rpit is related to the pitting corrosion rate

• Cpit is the double layer capacitance over the pit

• W is a Warburg impedance which is due to diffu-

sional processes of oxygen or corrosion products

through the pit

The measurements taken in the chromate solution

indicate that a detectable film was formed at about

24 hours after the immersion of the specimen. This

is demonstrated by the appearance of the high fre-

quency peak. Figure 10 shows the open circuit

5

was observed. The low frequency phase distortion of

the Bode plot appeared to coincide with the forma-

tion of pits on the metal surface.

In order to analyze these results, the development of an equivalent circuit to model the impedance behav-

ior is useful. The various processes at work in the

metal/liquid interface can be visualized as elements

in the equivalent circuit. A proposed equivalent cir-

cuit model for the chromate results is shown in Fig-

ure 9. The physical and chemical processes analogous

to these elements are:

• Rs is the resistance of the solution

• Cdl represents the double layer capacitance

Figure 7 — Nyquist plot at 24 hours after immersionof the mild steel electrode into the chromate solution

Figure 8 — Bode plot at 72 hours after immersion of the mild steel electrode into the chromate solution

Figure 9 — Equivalent circuit for the impedanceresponse in the chromate solution

Figure 10 — Change in open-circuit potential in thechromate solution


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potential throughout the experiment. The increasing

tendency of Ecorr indicates the formation of an anodic

passive film. At 72 hours, visible pitting was observ-

able and the low frequency phase distortion began to

appear in the Bode plot.

Results of the measurements taken with the all-

organic inhibitor are quite different than those obtained

from the chromate solution. Up to 48 hours into the

experiment, the Bode plot showed a single peak, and

the Nyquist plot only one semicircle (Figures 11 and

12). However, after 72 hours, the shape of the Nyquist

plot changed significantly. The low frequency region

is linear with a slope of about 45 degrees. Figure 13

shows that the 45-degree region spans the frequency

range of 0.02 to 0.005 Hz. No visible pitting was

observed on the specimen throughout the test. The

test was continued for 96 hours with the results shown

in Figure 14. The Bode plot shows that the region

showing Warburg characteristics was extended to

lower frequencies (0.002 Hz), indicating a thickening

of the cathodic film. Figure 15 shows the open circuit

potential measured throughout this experiment. The

decrease with time is indicative of the formation of a

cathodic passive film.

The impedance response observed with the all-organic

program is typical of a diffusion layer of finite thick-

ness. As the all-organic program is predominantly a

cathodic inhibitor8, an electron insulator film is

formed on the metal surface. Therefore, it seems prob-

able that the impedance behavior is caused by oxygen

diffusion through this finite layer.

OCCLUDED CELL MEASUREMENT

There are many similarities between pitting, under-

deposit corrosion and crevice corrosion. Often, local-

Figure 11 — Bode plot at 48 hours after immersion of the mild steel electrode in the all-organic inhibitor solution

Figure 12 — Nyquist plot at 48 hours after immersionof the mild steel electrode in the all-organic inhibitor solution



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ized corrosion contains elements of all three forms.

The test methods, which have been previously de-

scribed, all start with a clean surface and measure

some element of the resistance of passive films to

breakdown by anodic corrosion. In the case of

underdeposit corrosion, the ability of an inhibitor to

act may be limited by its ability to diffuse to a sur-

face through a porous deposit and by its ability to act

in the environment resulting from the localized cor-

rosion process. Although the previously discussed test-

ing methods simulate this process to some extent, thespecific geometry through which diffusion must oc-

cur is somewhat a matter of chance.

An occluded cell apparatus was discussed by Krisher8.

In order to specifically address the issue of inhibitor

mobility and effectiveness, a similar apparatus was

constructed to allow for variation of cell geometries,

simulating various pit depths. This apparatus is a

standard laboratory recirculator apparatus (Figure 16)

which contained an electrode assembly (Figure 17)

consisting of a free electrode and occluded electrodes

in various configurations.

The free electrode (cathode) was a section of carbonsteel tube. The occluded cells (anodes) were carbon steel

cylinders imbedded in a nonconductive plastic block.

The depth of the anodes was adjusted by using dif-

ferent lengths of plastic tubing inside the cells. Three

different cell depths were used with three replicates

of each cell depth. The depths of the shallow, medium

and deep cells were 4, 11 and 18mm, respectively.

The anodes were polished with #600 grit emery paper

and rinsed with acetone before they were inserted in

the occluded cells. The cell box was filled with test

solution and mounted in the recirculating loop. The

temperature of the recirculating solution and the pH

of the solutions were controlled by automatic control-lers. To maintain the inhibitor concentration, makeup

water was pumped continuously into the basin and

out an overflow. Inhibitor was added proportionately

to account for the overflow by a syringe pump. The

current between the anodes and cathode was moni-

tored by zero resistance ammeters and a computer

data acquisition system.

Three different cooling water inhibitor programs were

tested using this device. They were chosen to represent

a spectrum of inhibitors currently in wide commercial

use and a variety of inhibitor types. They included

the phosphonate all-organic, a chromate program, and

a neutral pH-stabilized phosphate program. The

conditions of the three experiments are shown in Table 1.

Figures 18, 19 and 20 show typical results obtained

from these experiments. The reproducibility of the

currents from the replicate anodes was quite good.

The passivation time of the occluded electrodes in the

all-organic and stabilized phosphate programs varied

depending on the depth of the cells. In the all-organic

experiment, the shallow cells took about 80 hours for

passivation to occur. The medium and deep cells took

Figure 15 — Change in the open circuit potential in

the all-organic inhibitor solution

Figure 16 — Recirculator apparatus for the occludedcell experiments

Figure 17 —Occluded cell assembly

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140 and 200 hours respectively. The stabilized phos-

phate program took less time to achieve steady state

currents. The current from the shallow cells leveled

out at about 15 hours. The medium and deep cells

required 60 and 80 hours respectively. The steady-

state currents observed in the all-organic program

were far lower than those seen in the phosphate pro-gram. The average current from the deep occluded

cells after passivation was approximately 0.7 µ A for

the all-organic program and typically 9 µ A for the

stabilized phosphate program. The passivation times

observed with the chromate program were relatively

insensitive to the depth of the cell. All three depths

gave sudden and effective passivation after 75 hours.

After completion of the experiment, the anodes were

microscopically examined. The exposed area of the

anodes from the stabilized phosphate program

showed significant visible etch with depth. The

visible effects on the anodes from the other two pro-

grams were significantly less, with little visible etch.

DISCUSSION

The typical bulk cooling water corrosion environment

consists of air-saturated water at neutral to slightly

alkaline pH with varying amounts of hardness and

other solutes in the presence of corrosion inhibitors.

The metal surface environment is typically carbon

steel of varying cleanliness with numerous small

niches and crevices. These are obvious starting points

for localized corrosion. However, under these condi-

tions, even initially clean carbon steel equipment canexperience underdeposit corrosion. Defects in passive

films or variations in metal composition can cause

local dissolution of iron. The primary product of this

reaction is Fe+2. Compounds of Fe+2 are quire soluble

under these conditions. However, once the Fe+2

reaches the oxygenated bulk water, it is quickly

oxidized to Fe+3. There are numerous oxides and other

components of this ion which are very insoluble and

quickly precipitate. Thus, what starts out as a pit,

quickly acquires a layer of deposit. In the case of

Table 1 — Operating conditions for occluded cell experiments

Inhibitor program

All-organic stabilized phosphate Chromate

Product concentration (ppm) 200* 100** 300***

pH 9* 7** 7***

Temperature (°F) 120* 100** 120***

Ca (ppm as CaCO3) 360* 360** 360***

Mg (ppm as CaCO3) 200* 200** 200***

Alkalinity (ppm as CaCO3) 350* 50** 50*** Conductivity (microsiemens) 2200* 2200** 2200***

***corresponds to 12 ppm phosphonates as PO 4

* **corresponds to 21 ppm total phosphate as PO

4

***corresponds to 300 ppm chromate as chromate

carbon steel, pitting and underdeposit corrosion are

intimately related.

The local environment in an actively corroding

underdeposit site is significantly different than that

in the bulk water, in that:

• Oxygen is depleted• Fe+2 is the predominant iron form

• The pH is very low

• Ionic concentrations differ depending on electro-

phoretic mobilities

There are a number of issues which determine the

ability of an inhibitor to prevent and passivate

underdeposit corrosion. These are:

• The quality and integrity of the inhibitor film and

its ability to resist attack

• The presence of components in the inhibitor whichcan effectively passivate in the underdeposit

environment

• The ability of these components to diffuse to the

corroding site

The three experiments presented focus on various as-

pects of the problem. The pitting nucleation potential,

Enp, measured by the modified pitting scan, is an in-

dication of the resistance of preformed inhibitor films

to rupture by anodic dissolution of iron. In the two

examples presented, increasing the alkalinity and

including an appropriate polymer had beneficial ef-

fects on film strength. Increasing the alkalinity alsoincreased the pitting passivation potential Epp, indi-

cating an improved ability to passivate existing de-

fects in the film.

The AC impedance method provides a great deal of

information about the characteristics of the film. Low

frequency phase distortion in the Bode plot appears

to correlate well with the appearance of pits. The

appearance of a Warburg-like characteristic consist-

ing of a linear 45 degree region in the all-organic im-


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pedance spectrum is indicative of the formation of a

diffusion layer of finite thickness. This provides evi-

dence of the formation of a well-defined cathodic film

on the mild steel electrode. The extension of the fre-

quency range of the Warburg-like region corresponds

to thickening of this layer. The formation of an an-

odic passive film by the chromate program was indi-

cated by the appearance of a small impedance peak

at high frequency, corresponding to a capacitance-re-sistance type of circuit.

The occluded cell experiment emphasizes the ability

of an inhibitor to diffuse to a remote surface and to

function in the oxygen-depleted, low pH, Fe+2-rich

environment encountered under deposits.

The ability of the chromate program to act as an anodic

inhibitor in the absence of oxygen and to promote the

conversion of Fe+2 to Fe+3 probably accounts for its ef-

fectiveness in this test. The insensitivity to pit depth

in this experiment indicates that this process is not

diffusion-controlled. The potential of the occluded

electrode is determined by the IR drop between the

free electrode and the occluded electrode, where I is

the current between the free electrode and R is mainly

solution resistance. The occluded electrode potential

can then be expressed as follows:

Ecell = Eo – IR

where Ecell is the potential of the occluded electrode

and Eo is the open-circuit potential of the free elec-

trode. Since Eo increases (Figure 10) and I decreases

(Figure 20) with time, Ecell increases as the occluded

cell measurement proceeds. This change in Ecell af-

fects the passivation of the occluded electrode sincethe solution contains chromate ions. When Ecellreaches the critical potential for passivation, the cur-

rent drops because of the passivation of the occluded

electrode. This decrease in current causes a further

increase in Ecell. Figures 21 and 22 show this process.

In Figure 21, Ecell is lower than the critical potential

for passivation; therefore, the corrosion rate is very

high. In Figure 22, Ecell increases to above the critical

potential for passivation and the resultant corrosion

rate is very low.

The passivation of the occluded cells in the two

nonchromate programs was different from that ob-

served with the chromate program. These programs

function primarily as cathodic inhibitors2 in contrast

to the anodic character of the chromate program. The

difference between the passivation currents observed

in the stabilized phosphate and all-organic programs

is probably due to the great difference in the alkalin-

ity at which these programs are operated. At a pH of

7, the alkalinity is typically lower than 50 ppm

(CaCO3). At a pH of 9, the alkalinity will be in the

400-500 ppm range.

Figure 18 — Results of occluded cell measurement inthe all-organic inhibitor solution

Figure 19 — Results of occluded cell measurement inthe stabilized phosphate inhibitor solution

Figure 20 — Results of occluded cell measurement inthe chromate solution


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Alkalinity can influence the underdeposit environ-

ment in two ways. First, the increased buffer capacity

afforded by a high alkalinity will tend to minimize

the pH depression at an anodic occluded site, caused

by hydrolysis by ferrous and ferric ions. Secondly, thepresence of the CO3= ion provides the opportunity for

the precipitation of FeCO3(Siderite). While phosphate

could potentially form Fe3(PO

4)

2(Vivianite), this re-

action appears to be less effective in passivating

occluded cells. The Siderite model of corrosion inhi-

bition in drinking water circuits has been extensively

discussed by Sondheimer et al.9 They propose that it

is an important intermediate phase which undergoes

isomorphous transformation into oxidized iron phases

such as Geothite.

CONCLUSIONThe conditions under which underdeposit corrosion

occurs are different from those controlling generalized

corrosion. Issues of inhibitor diffusivity and ability to

control corrosion in deoxygenated environments be-

come important. The test methods used to evaluate

inhibitors should take these factors into account. The

experiments described in this study were intended

to provide information about various aspects of local-

ized corrosion such as:

Figure 21 — Occluded cell potential and currentbefore passivation

• The resistance of inhibitor films to pitting attack

• The ability of inhibitors to stop crevice and under-

deposit corrosion once it has begun

• The effect of water chemistry on localized corrosion• The kinetics of pit passivation

The described three localized corrosion measurement

techniques provided important interrelated informa-

tion. The major findings of the study are as follows:

1. In the all-organic program, phosphonates (which

comprise the major corrosion inhibitor in the pro-

gram) did not decrease the passive current in the

modified pitting scan curve confirming that they

are cathodic inhibitors. However, Enp increased

about 100mV, indicating a pitting inhibition prop-

erty of the chemical.

2. The polymer in the all-organic program improved

the protective property of the passive film and in-

creased pitting nucleation potential.

3. The cathodic passive film formed in the all-organic

program acted as a finite thickness diffusion layer

and was detected as a Warburg-type impedance in

the impedance measurements. The thickening of

the film with time was detected by the measurements.

Figure 22 — Occluded cell potential and current after passivation

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4. Higher alkalinity and pH of a mainly cathodic in-

hibitor seemed to be beneficial for the passivation

of the occluded cells.

5. The anodic passive film formed in the chromate

solution was detected by impedance measurements

as a parallel capacitance-resistance type of circuit

at the high frequency range.

6. The pitting formation by the underdosed chromateprogram was detected by impedance measure-

ments at the low frequency range.

7. The passivation of the occluded electrodes in an

anodic inhibitor seemed to be related to the poten-

tial of the electrode determined by the open circuit

potential of the free electrode and the IR drop.

REFERENCES

1. Z. Szklarska-Smialowska, Pitting Corrosion of

Metals, National Association of Corrosion Engi-

neers, Houston, Texas, 1986.

2. G. Bohnsack, K.H. Lee, D.A. Johnson, E. Buss, “In-

vestigation of the Mechanisms of Organic Com-

pounds Used in Cooling Water Corrosion Control,”

Materials Performance, Vol. 25, No. 5, p.32, 1986.

3. K.E. Fulks, A.M. Yeoman, “Performance Evalua-

tion of Non-Metal Cooling Water Treatments,” pre-

sented at Corrosion/83, Anaheim, California, April,

1983.

4. J.E. Hoots, G.A. Crucil, “Role of Polymers in the

Mechanisms and Performance of Alkaline Cooling

Water Programs,” Materials Performance, Vol. 26,

No. 4, p. 17, 1987.

5. D.D. Macdonald, “Theoretical Analysis of Electro-

chemical Impedance,” Corrosion/87, Paper No. 479,

National Association of Corrosion Engineers, Hous-

ton, Texas, 1987.6. I. Epelboin, C. Gabrielli, M. Keddam, H. Takenouti,

“Alternating-Current Impedance Measurements

Applied to Corrosion Studies and Corrosion-Rate

Determination,” Electrochemical Corrosion Test-

ing, STP 727, ASTM, Philadelphia, Pennsylvania,

p. 150, 1981.

7. F. Mansfield, presented at the International Con-

ference on Localized Corrosion, Orlando, Florida,

1987.

8. A.S. Krisher, “A Synthetic Crevice Device to Moni-

tor Corrosiveness of Aqueous Systems,” Corrosion/

81, Paper No. 153, National Association of Corro-

sion Engineers, Houston, Texas, 1981.

9. H. Sontheimer, W. Kolle, A. Kuch, Internal Cor-

rosion of Water Distribution Systems, AWWA

Research Foundation, Denver, Colorado, 1985.

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Registered Trademarks of Nalco Chemical Company Printed in U.S.A. 12-88

pitting corrosion due to deposits in cooling water systems

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