PUBUCATipNS

-2083

Development of In-Situ

Techniques for the Detection and

Measurements of Corrosion of

Copper Concentric Neutrals in

Underground Environments

J. Kruger, U. Bertocci, E. Escalante, and J. L. Mullen

Corrosion and Electrodeposition Group

Chemical Stability and Corrosion Division

Center for Materials Science

National Bureau of Standards

U.S. Department of Commerce

Washington, D.C. 20234

AlllDS 1S7EDQ NBSIR 8<

January 1980

Final Report

Issued June 1 980

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80-2083

1980

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NBSIR 80-2083

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DEVELOPMENT OF IN-SITU

TECHNIQUES FOR THE DETECTION AND

MEASUREMENTS OF CORROSION OF

COPPER CONCENTRIC NEUTRALS IN

UNDERGROUND ENVIRONMENTS

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J. Kruger, U. Bertocci, E. Escalante, and J. L. Mullen

Corrosion and Electrodeposition Group

Chemical Stability and Corrosion Division

Center for Materials Science

National Bureau of Standards

U S. Department of Commerce

Washington, D.C. 20234

January 1980

Final Report

Issued June 1 980

Prepared for Department of Energy

Washington, D.C. 20545

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity. Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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Development of IN-SITU Techniques for the Detection and Measurement

of Corrosion of Copper Concentric Neutrals in Underground Environments

Final Technical Report for Period January 15, 1976 to January 15, 1979

J. Kruger, U. Bertocci, E. Escalante, and J. L. Mullen

1. Introduction

This is the final report of a three-year project, whose objective was

to develop techniques and instrumentation for detecting and measuring the

extent of corrosion of the copper concentric neutral (CCN) wires of buried

electric cables without removal of the cables from the soil. By being able

to locate the sites where the rate of corrosion is high, preventive measures

such as altering the environment (e.g., by backfilling with different soils),

developing new CCN materials, application of cathodic protection, or other

yet to be determined ways, can be applied.

The approach to this problem consisted in finding first, by means of

laboratory tests, the conditions that promote a high corrosion rate, and

subsequently to create those conditions on buried cables in order to compare

laboratory and field results. Also necessary was the development of background

knowledge, such as the role of a.c. in promoting corrosion, in order to provide

a basis for various measurement methods.

Measurements methods such as the recording of polarization curves and

electrode potential were evaluated with respect to their ability to detect

and measure corrosion of buried cables. Also, at the same time, other proposed

methods, such as a.c. impedance and electrochemical noise measurements, were

developed and evaluated in the laboratory. The measurement techniques were then

applied in the field to corroding and non-corroding cables, and their potential

for detecting corrosion assessed.

Specifically, our accomplishments were as follows:

1) The development of methods for the determination of corrosion rates

by polarization measurements;

2) The assessment of potential measurements as a means for detecting

corrosion;

3) The demonstration that polarization measurements can be used to

detect and measure in-situ corrosion of CCN wires;

4) The identification of the environmental factors that affect corrosion

of CCN wires;

5) The development of a theory and the experimental validation of that

theory of the effect of a.c. on CCN corrosion;

6) The development of equipment and methodology to make electrochemical

noise and a.c. impedance measurements in the laboratory.
-2 -

This final report describes the details of the above accomplishments

by collecting together reprints of published papers that describe the work

carried out on this project. It also includes preprints of papers that are accepted or have been submitted for publication.

The work described in this report has been successful in identifying

and overcoming the most difficult aspects of the problem of in-situ detection

and measurement of CCN corrosion. For example, as mentioned above, it has

demonstrated that it is possible to make in-situ polarization corrision rate

measurements on underground cables. There still remain, however, a number

of tasks to be accomplished before enough information is available to provide

the basis for the development of simple easy-to-operate field instruments for use by utility personnel. To accomplish this remaining need, the work initiated

by the DOE project is to be continued under the sponsorship of the Electric

Power Research Institute (EPRI). The new EPRI project, which has been set up

for a period of two years, was initiated in October 1979. The new EPRI project

seeks to accomplish the following:

1) Adaptation and evaluation of the corrosion noise technique in the

field.

2) Interpretation of the corrosion noise technique and its correlation

with corrosion rates measured by other means.

3) Completion of the development of polarization corrosion rate measurement techniques by carrying out more extensive current distribution and

corrosion rate measurements.

4) Evaluation of polarization and noise measurements on operating cables.
COBBOSION/77

The International Corrosion Forum Devoted Exclusively to the Protection and Performance of Materials/March 14 18, 1977, San Francisco Hilton Hotel, San Francisco, California

ELECTROCHEMICAL AND CORROSION STUDIES ON COPPER

CONCENTRIC NEUTRAL WIRES

By

Ugo Bertocci and Jasper L. Mullen

Corrosion and Electrodeposition Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

In February 1976 the NBS started work for ERDA to develop methods for

the in situ detection of corrosion occurring on copper concentric neutral

wires buried underground.

The purpose of these studies is to find an easily applicable test that

can detect the locations where corrosion is taking place and can measure the

rate of corrosion. By being able to detect such sites, preventive measures

such as altering the environment (e.g. by backfilling with different soils),

application of cathodic protection, or other yet to be determined ways, can

be applied.

Approach to the Problem

For the detection of underground corrosion, it is necessary to assume

that the corroding wires send out electrical signals different from those

emitted by non-corroding wires or that they respond differently to electrical

stimuli. Therefore, the approach to the problem consists in trying a number

of measurement methods on buried cables under corroding and non-corroding

conditions, and to compare the results.

Four methods have been proposed; two of them are passive, and do not

require the application of an external signal, and two are active techniques.

They are:

1) Potential Measurements. These measurements are the most simple to

carry out and have been already used in the examination of power cable

systems,^ but their reliability is open to question.

2) Noise Measurements. This method as the previous one is a passive

method and consists in analyzing the fluctuations in potential in terms

•Publication Policy

Th« National Asaociation of Corrosion Engineers reserves the right of publication of all papers presented at its conferences and meetings;

however, release of a paper for publication elsewhere may be obtained. Requests for permission to publish a paper presented at an NACE

meeting should be made in writing to NACE, Publications Department, P. O. Box 1499, Houston, Texas 77001. When preprints are distributed

at a conference, other journals may publish reviews, condensation, or abstracts, provided these do not exceed 500 words or one-third ol the

ofiginal paper, and provided acknowledgment of presentation before an NACE sponsored meeting is given. This manuscript has not yet been

reviewed by NACE, and accordingly, the material presented and the views expressed are solely those of the authorls) and are not necessarily endorsed by the Association.

Printed in the USA
of amplitude and frequency „ Although noise measurements are well known

in many areas, very little has been done so far to apply them to electrochemical or corrosion systems, ^ This method is therefore a novel one,

and although very interesting, it is impossible to predict its usefulness,

3) Polarization Techniques. These techniques, consisting in recording

the current-potential response of an electrode-electrolyte half-cell have

been extensively and successfully applied by NBS to corrosion rate measurements on metal in the soil,^ These techniques, under various slightly

different forms, galvanostatic, potentiostatic, or potentiodynamic, are

the most likely to be successful in discriminating between corroding

and non-corroding buried wires.

4) A.C. Impedance Methods. These methods may be considered a special case of the previous one, where the stimulus applied is a sinusoidal

signal of various frequencies. These methods are widely used in electrochemistry,^ but have not been used in underground corrosion studies and

their usefulness is unknown.

Before starting with the field tests, however, a considerable amount

of work at the laboratory scale is necessary. The objectives of this laboratory work are to provide the necessary knowledge for creating the desired

corroding conditions at the field test sites, in other words, to know which

environments are conducive to corrosion, and to collect information on what

responses and values are to be expected from the measurements in the field.

The purpose of this paper is to present the results of the laboratory

tests carried out so far, and to discuss their implications as to the causes

of the concentric neutral wire corrosion and as to the prospective value of

the field test methods proposed.

Experimental Methods

On the basis of an examination of the situation and of previous published

studies^ on the corrosion of concentric neutrals, it was decided to investigate the effect of the following parameters:

1) presence or absence of oxygen,

2) presence or absence of chloride ions in solution,

3) presence or absence of convective motion in the electrolyte,

4) presence or absence of an a.c. signal,

5) coupling of the metal wire to conducting polyethylene (CPE).

As basic electrolyte solution for the corrosion tests and the current-potential measurements, it was decided to choose a neutral 2% solution of Na 2 S04

adding, in a number of cases, 1% NaCl . Later on, following indication by

others^ that sulfur in some forms could be a significant factor in causing

corrosion, tests were carried out also in 0.42 m NaC 104 .

As material to submit to the corrosion tests, tinned copper wire coming

from the same cable to be used for the field tests was chosen. For the

current-potential measurements, experiments were also carried out on pure

copper and on a massive piece of an alloy containing 90% Pb and 10% Sn, equal

to the composition of the tinning bath.

The effect of oxygen was studied by running tests both under an O2 and

an N 2 atmosphere. To study the influence of convective motion, some corrosion tests were carried out in solution gels obtained by adding 1% agar, as done by Schick^.

131/2
Since in the actual buried cables it is likely that alternatinq current

is leaking from the neutral wire into the soil, its effect upon corrosion was examined by applying a voltage signal between the wire under study and on counterelectrode. The circuit employed is shown in Fig. 1. By using a very

small resistance (10i2) for the current measurements, a very large capacitance

(110 yF), and a large counterelectrode to working electrode surface area, a condition approaching a potentiostatic system was achieved.

Cells and circuits used for the electrical measurements are of conventional design, and will not be described here. For the noise measurements,

which are still at a preliminary stage, two circuit schemes are being tested.

The first, shown in Fig. 2, is based essentially on the use of a spectrum

analyzer particularly built for low frequencies. The instrument employed

has 200 channels and at its lowest frequency range is 5 Hz with a resolution

of 0.025 Hz, this being also the lowest frequency measured. Problems arising

from d.c. coupling of the amplifier tend to restrict the lowest measurable

frequency to 0.1 Hz. The second method has the capability of detecting signals whose amplitude is below that of the noise of the electronic equipment

employed, and operates by introducing the noise signal to be measured in

two parallel amplifying and filtering channels, as shown in Fig. 3. The

output signals of the two channels go to a multiplying amplifier with

averaging capability. Each channel signal is the sum of the source noise

and of the instrument noise, but since the instrument noise if uncorrelated,

the average of its product is zero and the output of the multiplying amplifier should be only the square of the source noise.

Experimental Results

A) Current-Potential Measurements.

Potentiodynamic scans have been carried out on wire samples as well as

on copper and Pb-Sn alloy in various solutions. Figs. 4 and 5 give the

results for tinned wire. Figs. 6 and 7 give the scans for pure copper in Na2S0

Fig. 8 in NaClOy, and Figs. 9, 10, and 11 in Na 2 S04 and NaCl . Finally, Figs.

12, 13, and 14 report the results for the Pb-Sn alloy. All data were taken

at room temperature, 23°C.

B) Corrosion Tests.

Wire samples were immersed in the various solutions chosen for testing

and left there for times up to and exceeding 2000 h, in the presence or absence of combinations of the parameters under examination, i.e., oxygen,

chloride ions, CPE coupling, and a.c. signal. In a number of cases, agar

was added so as to prevent convection in order to simulate more closely the

conditions in the soil. If the samples were electrically connected to the

CPE, the short circuit current was also monitored. In the cases where an a.c. signal was superimposed, a.c. cell voltage and a.c. current were

recorded.

At the end of the tests the wires were examined optically, and in some cases with a scanning electron microscope. Analysis of the surface material

was carried out in some instances by means of x-ray energy dispersion.

Some of the results are presented here. Figs. 15, 16, 17, and 18 are photos of the electrode potential versus time obtained in a number of corrosion tests. Other results are summarized in Table I, dealing with the observed short circuit currents between tinned Cu wire and CPE and Table II,

relative to a batch of tests with superimposed a.c. The rest potential versus time of CPE and of a massive piece of Pb-Sn alloy have also been monitored under various conditions. The results are shown in Fig. 19.

131/3
C) Noise Measurements.

The work done so far on the detection of electrochemical noise for

corrosion studies has been concentrated on the testing and calibration of

the instrumentation, and on exploratory work to establish the units of sen- sitivity of the systems. The work is at too early a stage to allow much

discussion. As an example of the results obtained with the method described

in Fig. 2, two typical spectra are shown in Fig. 20.

Discussion of the Results

The extrapolation from laboratory tests to the actual behavior underground is always risky, and requires comparison of the results of measurements in both environments to be sufficiently reliable. For the time being,

therefore, we will discuss the results drawing a minimum of conclusions.

The potentiodynamic scans taken on the tinned wire are rather complex

in shape, particularly in the presence of Cl" ions, but comparison with the

results on pure copper and on Pb-Sn alloy enables us to separate the different

contributions. Essentially all the peaks observed at potentials more negative

than approximately -200 mV vs. S.C.Eo are due to the tinning coat and are absent on copper, and tend to disappear rather rapidly on the tinned wire

itself, if dissolution due to extensive potential cycling is allowed to occur.

When the results on copper are observed, it is clear that no adherent

passive film tends to form in neutral solutions (pH is always close to 7).

Although the copper ions produced anodically eventually precipitate as hydroxide, no protective layer is formed and the anodic branch of the scan increases monotomically in sulfate and in perchlorate. The situation is

somewhat different in the presence of Cl", since CuCl precipitates immediately on the surface, so that at times peaks and inflexion points are observed on the anodic sides. The layer, however, affords no protection,

and anodic current densities are as large or larger than in other solutions.

The precipitate is, however, on the electrode surface so that part of it can be reduced during the cathodic sweep, causing a characteristic peak

between -200 and -400 mV.

The results of the corrosion tests indicate that presence of oxygen

and of Cl" ions tends to increase the rate of attack. In a number of

instances pit formation was observed under large salt encrustations. The

data summarized in Table II show that more rapid attack can be caused by

an a.c. signal, but it is not evident that serious corrosion cannot occur without it. Observation of the wires after the tests showed that the

specimens under N 2 were covered with a CU 2 O deposit, and that the tinning

had disappeared. Surface analysis has confirmed that the tinning coat

might be lost eventually, possibly because of failure to totally prevent

oxygen leakage.

Galvanic coupling with CPE has also been suspected as a possible contributing cause to corrosion of the wires. Some results, reported in Table I,

show that in some instances the short circuit current is cathodic on the

wire. The anodic reaction on the CPE electrode might be the oxidation of

intermediate corrosion products such as Cu'*’ migrating from the metal wire.

By observing the data in Figs. 16, 17, 18, and 19 some general trends

can be found. More positive potentials occur in the presence of oxygen and

in the absence of chlorides. Coupling with CPE generally raises the potential, which is consistent with the data of Fig. 17, showing that the rest

potential of CPE is close to 200 mV. Exceptions, however, are observed,

notably in oxygenated chloride solutions where coupling lowers the potential.

131/4
compared with the corresponding test without coupling. Interestingly, in

these cases the wire is cathodic with respect to the CPE, except for the

solution containing agar, where transport of the corrosion products to the

CPE is much more difficult.

The results obtained so far give a fair idea of the parameters that

enhance wire corrosion. As far as their implications on predicting the

feasibility of detecting corrosion of buried cables by electrical measurements, comparison with field experiments is necessary. Some general trends

can probably be spotted by potential measurements, but current-potential

measurements hold more promise of giving useful indications, if the response

of underground wires is analogous to that of the specimens in our laboratory beakers.

REFERENCES

1) D.H. Kroon, Mat. Perf. 1_5, N.8, 19 (1976).

2) G. Okamoto, T. Sugita, S. Nishiyama, K. Tachibana, BSshokugi jutsu,

23 , 439 (1974); S. Nishiyama, K. Tachibana, G. Okamoto, T. Sugita, ibid.

n, 445 (1974).

3) See, for instance, W.J. Schwerdtfeger, J. Res. NBS, ^C, 271 (1961).

4) See, for instance, M. SI uyters-Rehbach, J.H. Sluyters, in "Electroanalytical Chemistry", Vol. 4, A.J. Bard, ed., p. 1 (1970).

5) G. Schick, Paper N. 128, Intern. Corr. Forum, Toronto (1975).

6) R. Latanision, Private communication.

Table 1

Corrosion current (c) uA/cm^, and electrode potential (E),

mV vs. SCE for tinned copper coupled to CPE

conditions of the test Time, h

chloride atm. ac Agar 0 48 164 212 308 329

No O2 No Yes C +0.13

E +21

+0.23

+21

+0.04

+21

+0.03

+20

' \ -0.01

+19

^ -0.03

+20

Yes O2 No Yes C +7.8

E -83

+9.4 +6.8

-88

+6.8

-90

+6.8

-89

+6.8

-91

Yes "2 No No C +0.65

E -148

+0.73

-150

+0.57

-153

+0 . 59

-158

+0.57

-156

+0.51 1 -156 1 Yes Nj No . No C +0.17

E -174

+0.14

-166

+0.15

-167

+0.36

-164

+0. 50

-163

+0.40 1 -169 1 No N

2

No No C -0.01

E -315

-0.03 -0.02

-310

-0.02

-305

-0.01

-281

-0.01

-275

Yes °2 No No C -0.21

E -31

-0.22 -0.13

-18

-0.11

-19

-0.14

-10

-0.18

-10

TABLE 1 continued....

131/5
TABLE 1 continued.

conditions of the test Time, h

chloride atm. ac Agar 0 48 164 212 308 329

' i No no No C +0.09

E +16

+0.06

+8

+0.10

+7

+0.12

+7

i +0.14

+5

« . 1 +0.17

+7

No °2 No No C +0.19

E +9

+0.23

+9

+0.16

+9

+0.16

+9

+0.21

-3

+0.21 1 -11 1 Yes N'2 Yes No C +0.5

E -119

+3.5

-122

+0.7

-126

+ 1.1

-129

+ 1.4

-130

+2.5

-129

No N2 Yes No C +1.6

E -24

+0.94 + 1.1

-13

+0.93

-28

+0.79

-15

+0.81

-7

Yes °2 Yes No C -1.0

E -55

-0.46

-55

'-0.14

-53

-0.14

-55

-0.14

-55

1 -0.36

-51

No °2 Yes No C +4.0

E +28

+ 1.2 +0.68

+41

+0.57

+31

+0.71

+25

+0.93

+27

Tiil»ln H

Data concerninq 12 corrosion tests, lastinn 20nn hours, a.c. super iniposod

Do. Atm. Cl- CPE coup 11 nq Agar d. c. mV

a.c.

niA/cni'^ short circ. uA/cm2

Optical Examination

1 N, * -240 8 )

1 Rather uniform Cu,0 layer

2 "2

* -220 8 J

^

3 II, -50 4 to 2 CU2O layer

4 "2

* 0 2 + 1/+2 r.u^O crystals

5 Il2 * * -140 8 +2/+5 Cii^O, r.u deposit. Less attack

6 °2

* * -50 1 -0.2 Small green crystals, CU2O

7 °2

* +30 0.8 +1/-0.2 Thin layer of green crystals

8 «2

* -20/+5 2 Oroen scale. Red blisters

9 °2 « it -150/ -60 6 to 2 Similar to 8

10 +20 2.5 7 Sheets and mounds of green

C

1 salt. Pits full of Cu,0

11 02 +20 2 J 4

12 °2

* -50 2 to 1 Large green mounds

131/6
c

Fig. 2 Schematic circuit for noise measurements.

F1g. 3 Noise measuring circuit.

Fig. 5 Potentlodynamic scans for tinned Cu in 2* Na^SO. + 1% NaCI. Scan rate

4 mV/sec. Dashed curve is expansion of sol 10 curve and refers to c.d. scale on left.

Fig. 6 Potentlodynamic scans for Cu in 2 % Na,S0^. Scaa rate 4 mV/sec.

Fig. 4 Potentlodynamic scans for tinned Cu in 2% Na-SO.. Scan rate 4 mV/sec. E^shed curve Is expansion of solid curve and^rerers to c.d. scale on left.

Fig. 7 Potentlodynamic scan for Cu in 2 % Na-SO- up to 800 mV. Scan rate

4 mV/sec.

131/7

,UID/yUJ

‘
10

100

-ii«o -fOO -400 0 >00 4M m l3o

E. aV n. S.C.E.

Fig. 8 Potentlodynonlc scon for Cu In 0.4m No'dO.. Inert otmoiphere. Scin

rate 4 mV/sec.

Fig. 9 Pottntlodynamlc scans for Cu In 21 Na^^SO^ 1X NaC1 . Scan rate 4 mV/sec.

•800 200 400 600

C , mV vs . S.C.E.

Fig. 1l Potentiodynamlc scan for Cu In 21 Na^SO^'-*- It NaCI. Inert atmosphere. Scan rate 40 mV/sec.

Fig. 10 Potent lodynanic scans for Cu ip 2X ha!^S0)| It HaCl. Inert atmosphere.

Fig. 12 Potentiodynamlc scans for Pb-Sn alloy In 2t Na,S0,. Scan rate 4 mV/sec.

Fig. 13 Potentiodynamlc scans for Pb-Sn alloy In 2t Na^SO^ It NaCl. Inert atmosphere. Scan rate 4 mV/sec.

131/8
Fig. 14 Potentlodyniwlc suns for Sn-Pb alloy In 21 Na2S0. * 1% NaC1 . Scan rate

4 mV/sec. Oxygen atsaosphere.

Fig. 15 Electrode potential vs. tirae for tinned Cu wire In Z% Na^SO.. 0^ atnosphere. ^

Fig. 17 Electrode potential vs. time for tinned Cu wire In 21 Na~S0* U

NaCl. 0., atmosphere. ^

Fig. 18 Electrodfe potential vs. time for tinned Cu wire in Zl Na2S0^ 1? NaCI . N., atmsophere.

Fig. 16 Electrode potential vs. time for tinned Cu wire in 255 Na^SO.. atmosphere. ^ ^ ^

131/9
Fig. 19 «#st potlntUl vs. tint for CPE <nd Pb-So «1loy in 21 N»,S0. + II mci. 2 4

0 10 20 U 40 SO

Ii

t

m

k

«• I

Friqutncy, Hi

Fig. 20 Noitt sptctrM It opon circuit. 1) Cu In CuSO^. 2) Pt In boriU.

131/10
PAPER NUMBER 244 r

rn

The International Corrosion Forum Devoted Exclusively to the Protection and Performance of Materials / March 12-16,

1979, Atlanta Hilton, Adanta, Georgia.

A Field Study on the Corrosion of

Concentric Neutral Cable

E. Escalante, U. Bertocci, J. Mullen

Corrosion and Electrodeposition Section

National Bureau of Standards

Washington, D.C. 20234

INTRODUCTION

The corrosion of the outer metallic members of electric cable in underground distribution networks is a threat to the safety and longevity of these

systems. Though the corrosion problem is apparently not widespread, it can be

a serious threat in the local areas where it is found. In fact, realizing

that a corrosion situation exists is a problem in itself. Often times the

corrosion of the cable is found by accident when the cable is excavated for reasons not related to corrosion. Thus, an important part of, this study is to

be able to detect the existence of corrosion on the cable and measure the

extent of deterioration.

MATERIALS AND ENVIRONMENT

The cable used for this study was chosen on the basis of the cable

design most widely used throughout the United States as determined from the

sales of three major cable manufacturers. The specifications were as follows:

//2 AWG stranded conductor in cross-linked, thermosetting polyethylene

insulation with ten strands of #14 AWG coated copper concentric neutral

wires manufactured within AEIC #5 and IPCEA S-61-402 specifications.

Figure 1 illustrates the cross sectional configuration of the cable.

For our purposes the inner stranded conductor and its polyethelene

insulation are of little consequence since they do not play an active role in

the corrosion process. The carbon filled polyethelene jacket around the

insulator is of interest because of its relatively low resistivity and the

chemically noble nature of carbon in the electromotive series. The average

film thickness of this jacket is 0.8 mm, and its resistivity, which was measured to be 50 ohm cm, changed when mechanically flexed during handling and

increased by as much as a factor of two in an irreversible way.

Publication Right

Copyright by the author(s) where copyright is applicable. Reproduced by the National Association of Corrosion Engineers with permission of the author(s). NACE has been given first rights of publication of this manuscript. Requests for permission to publish this manuscript in any

form, in part or in whole, must be made in writing to NACE, Publications Dept., P. 0. Box 986, Katy, Texas 77450. The manuscript has not

yet been reviewed by NACE, and accordingly, the material presented and the views expressed are solely those of the author(s) and are not

necessarily endorsed by the Association.

Pi'inted in I l.'^A
The ten outer strands of coated copper concentric neutral wire are spirally wound around the jacket making one complete turn every 22.5 cm of

cable. Ideally these wires do not touch one another and generally they

remained separated in the 330 meters of cable we received. The diameter of

the wire including the coating is 1.63 mm. During its manufacture, the wire

is coated by, passing it through a 90% Pb, 10% Sn bath and then drawing it

through a die to remove the excess molten material. The resulting film is a Cu Sn intermetallic with some Pb.

In addition to the cable, bare copper wire and an ingot of the tinning

alloy was obtained from the cable supplier. The wire was identical to that

used on the cable, but without the coating. Chemical analysis of the ingot

indicated that antimony was the single major impurity (0.3%) and the sum of

all other impunities was less than 0.7%.

The underground test site is located on the NBS grounds in an area that was found to have the lowest soil resistivity. The ground is slightly inclined

with good drainage and an average soil resistivity of 10,000 ohm cm. Soil ph

has been found to be between 5.5 to 6.0. The first 0.5 m is a brown loamy,

well aerated soil which becomes more clay like with increasing depth. Below

1 m the soil is distinctly clay like and changes to a grey color.

EXPERIMENTAL PROCEDURE

Four cable specimens were cut to a length of 15 m. The ends of each

cable were wrapped with 5 turns of 0.5 mm copper' wire which was then soldered

to each one of the ten concentric neutral wires. The entire end was then

coated with a silicon adhesive, thus, reducing the development of local

galvanic cells. Copper wire was also wrapped and soldered around the cable at

a point midway between the ends of the cable. To this was attached an insulated 14 AWG wire which extended out of the ground after burial and served

as an electrical contact to the concentric neutral wires. Silicon adhesive

was also applied to this joint.

In order to evaluate the electrochemical interaction of the cable components,

sections of cable were cut to a length of 1 m. The concentric neutral wires were removed from the body of the cable and reassembled as illustrated in

Figure 2. The ends of the wires were soldered to a copper conductor and

coated with silicon adhesive. An insulated electrical conductor which extended

above ground was attached to one corner of the specimen. Similar specimens were made from the uncoated copper wire. The remaining body of the cable

which was jacketed with the carbon loaded polyethylene was coated on the ends

with adhesive. At a midpoint between the ends, copper wire was tightly wrapped

around the jacketed cable, coated with adhesive, and connected to an insulated

conductor leading above ground. The 90% Pb, 10% Sn ingot was rolled down to

strips 0.5 cm thick. Specimens were cut from this such that their total

surface area was equal to the total surface area of the concentric neutral

wires on a 1 m section of cable (512 cm^) . The 90% Pb, 10% Sn sheet specimens were then attached to insulated conductors leading above ground.

The specimens were buried to a depth of 1 m in trenches approximately

0.6 m wide and of sufficient length to hold the 15 m cables and cable components.

Figure 3 is a top view of the burial plan. Two parallel cables were located

in each trench and positioned against the side walls so that they were approximately 0.6 m apart along the length of the trench. A wooden post located

between the cables at their midpoint, extended above ground and supported the

electrical conductors attached to the cables. After burial a 60jHz, 45 acV

signal was applied between two of the cables in one trench (cables 1 and 2)

.

This developed an alternating current of 0.7 mA/cm^ on the concentric neutral

wires. Current control was accomplished by using a variable transformer.

244/2
Isolation was found necessary to remove undesired direct current (dc) coming

through the ac line. The second set of cables (cables 3 and 4) had no imposed

ac, however, the soil was locally modified as will be described. In an effort

to increase localized corrosion which could subsequently be detected and

monitored by our measurements on cables 3 and 4, a cubic meter of soil was removed from a section of these cables which were visually inspected at this

time. Laboratory studies indicated that the corrosion rate on the concentric

neutral cable could be raised by increasing aeration in the presence of chlorides

After visual inspection, the excavation was backfilled with a mixture of equal

volumes of soil and sand. As the layers of soil-sand mixture were added, a solution of sodium chloride was poured over each layer. A total of four

kilograms of sodium chloride were added to the backfill material. Three

months after the modification, the soil was qualitatively tested for chloride

content and the soil resistivity measured at three locations at the site and

at three depths at each location. The chloride evaluation was made by taking

100 g of soil and mixing it with 200 ml of distilled water. After standing

for 30 minutes the mixture was filtered so that a clear solution resulted. 1 ml

of a saturated solution of silver nitrate was added to the filtrate causing

any chloride to precipitate as silver chloride.

MEASUREMENTS

Two types of measurements were made on the 15 m cables. These consisted

of open circuit potential survey measurements and polarization measurements.

Open circuit potential surveys consisted of soil surface potential measurements made at the intersections of an 18 m x 2 m grid at 1 m intervals along

the length of each buried cable using a CU-CUSO4 reference electrode. Every

point on the grid is marked al; the field site with a permanent aluminum marker

imbedded in the ground. With these markers it is possible to make repeated

measurements over several months at the same point. The survey allows one to

detect potential changes that may occur along the length of the cable.

The circuit diagram being used for polarization measurements is illustrated

in Figure 4. This diagram can be thought of as two separate circuits with the

section to the left of the specimen providing and measuring the current for

polarization while the circuit to the right measures the resultant potential

change of the specimen. Ac filtering circuits are included so that polarization

measurements can be made in the presence of the ac signal imposed on cables 1 and 2 .

The section on the left has a dc power supply whose current output to the

specimen is controlled by a voltage divider circuit not shown in the illustration

In addition to this voltage divider there is a variable resistor which also

assists in current control. Two switches in parallel, as illustrated, can completely isolate the specimen from the current circuit. The normally open

(N.O) switch is a push button microswitch used for intermittent current control.

The on-off switch bypasses the N.O. switch when needed for longer term current

applications. The output of the dc power supply is measured by a zero resistance

ammeter. Notice that the ammeter is in parallel with a capacitor which serves

as an ac filter for the ammeter, and allows most of the ac to travel to ground

through the counter electrode without affecting the dc current measurement.

The section of the circuit sho\>m to the right of the specimen in Figure 4 is used to measure the potential of the specimen with respect to the Cu-CuCO^

reference electrode. The potential is measured across a Wheatstone bridge

which is used to measure and -compensate for any IR error.

244/3
The high impedance voltmeter is isolated from the background ac by means

of a two-pole low-pass filter circuit which reduces the 60 Hz signal by as much as a factor of 5000.

Measuring the corrosion rate of a finite length of a metallic member in an electrolyte such as soil is, in general, not difficult using techniques

developed here and elsewhere (1,2). However, measuring the corrosion rate of

what amounts to be an infinite length of metal poses a special problem. The

problem is that of not knowing how much of the metallic specimen is being

polarized. If the surface area being polarized is not known, then corrosion

current densities cannot be calculated.

To get around this problem polarization measurements were made at four

points along each cable. The positions of the electrodes were changed for

each measurement as illustrated in Figure 5. The currents necessary to polarize each section of cable to an overpotential of 6 mv are then compared.

The amount of current required to polarize each section of cable 6 mv is

directly related to the corrosion rate of the section.

In order to determine current distribution, current was applied to a cable from a counter-electrode driven into the ground at a given distance from

the cable, and potential measurements made over the length of the cable. The

change in potential with and without polarizing current applied was then

plotted for each point along the cable.

Of secondary interest to the study was the electrical interaction of the

cable components. Though single specimens were included in the study, most

data was obtained from the galvanic couples whose galvanic current was measured

with a zero impedance ammeter. This data gives the magnitude of the current

and the direction of flow.

RESULTS AND DISCUSSIONS

The primary objectives of this study were twofold. The first objective

was to detect the presence of localized corrosion on a buried cable and,

second, to evaluate the intensity of the deterioration. The three conditions

of exposure for the 15 m cables were as follows. One set of cable (1 and 2)

had an imposed ac signal of 0.7 mA/cm“. The second set of cables (3 and 4) was buried without any imposed ac. And finally, the corrosion of a small

section of the cables without ac was increased by a local modification of the

soil

.

Initially a considerable amount of time was spent in improving our ability to make measurements in the presence of the imposed ac and other

interfering signals. By using the filtering circuits described earlier, these

interferences were essentially removed from the measuring circuit allowing us

to observe the dc signal of interest. The effect of the imposed ac on the

open circuit potential of cables 1 and 2 was observed over several months. The

first measurements were made before the ac signal was applied as is shown for

cable 1 in Figure 6. This figure illustrates the potential along the cable at

intervals (positions) of 1 m. This potential was unstable for the first few

weeks after the ac was applied but later stabilized as the three month data

show. Throughout the next several months the potential fluctuated slightly

and gave little indication of what was happening to the cable. Similar open

circuit potential measurements were made on cables 3 and 4 that had no imposed

ac but did have the local soil modification. Again the potentials remained

very stable even in the area of the soil modification as the data for cable 3 in Figure 7 illustrates. The only change observed was that the overall potential

of the cable became more positive (noble) with time.

2kk/k
r

I Polarization measurements were made on the cables 1 and 2 before the ac

I was turned on with the results shown at the bottom of Figure 8. Without ac the

|i polarizing current was less than 500 yA After applying the ac signal,

! however, the polarizing current began to increase with time until one section

of the cable reached a polarizing current of 12,500 yA after 3 months. Ten

|j months later the polarizing current still averaged over 5,000 yA with a high

! of 8,000 yA. At this time the ac was turned off and the measurements repeated

immediately. The polarizing current remained essentially unchanged. Two

months later with the ac still off the polarizing current had dropped to an

f average of less than 2,000 yA.

i The same polarization measurements were mads on cables 3 and 4 which were

ii free of ac. The data for cable 3 over a period of more than 12 months is

shown in Figure 9. In every case the polarizing current was below 600 yA or a factor of ten times less than that measured on the cables exposed to ac. The

suggestion is that the imposed ac has dramatically increased the corrosion of

the concentric neutral cables. These cables were excavated and inspected

after the last measurements were made. Figure 10 is a photograph of a concentric

neutral cable that had imposed ac. The surface is corroded and pitted and is

representative of how the entire cable appears. Figure 11 is also a representative section of the cable without ac. Some attack is visible but clearly

not of the magnitude observed with ac. The measurement in Figure 9 also

1 indicates that the modified soil increased the corrosion of that section by

1 20 to 40% as compared to the sections around it. Figure 12 is a photograph of

a section of the concentric neutral cable that was in the modified soil.

j In order to evaluate the distribution of current over the cable during

polarization, the change in potential over the length of the cable was measured

as current was applied from a vertical steel rod in the ground. The distance

of the current source was varied from 0.5 m to 1 m. The resulting data,

illustrated in Figure 13, indicate that from a distance of 1 m the current

is distributed over the 15 m cable with little concentration of current in the

vicinity of the source located at position 5. As the current source (counterelectrode) is moved to within 0.5 m of the trench, the current distribution begins

to show a distinct concentration around the source with an applied current of

1.8a. When a current of 0.9A is applied at a distance of 0.5 m, 85% of the

current is concentrated along 9 m of the 15 m cable. As the current is increased

the distribution becomes more uniform. This type of information which describes

the current distribution during polarization is necessary for corrosion rate

determinations. However, more data is needed before current densities can be

calculated with confidence.

The galvanic current measurements made on the cable components are listed

in Table 1. This table shows the average galvanic current measured over the

period of burial and the polarity of the components. As expected, the data

clearly indicates that the carbon filled polyethylene is cathodic to the

coated copper wire, the bare copper wire, and to the 90% Pb, 10% Sn alloy, but

the galvanic current generated is of such low magnitude that it caused only

minor damage to the metals. Examination of these components after removal

verified that they had suffered minor corrosion attack. However, the coating

alloy sheet developed severe attack when connected to the bare copper wire, a condition that was expected judging from the current measurements.

SUMMARY AND CONCLUSIONS

It has been found that differences in degree of corrosion along a 15 m

cable can be detected by polarization techniques. This method was used to

determine that the cables with an imposed ac were deteriorating ten times

faster than similar cables without ac, and that the section of cable in the

modified soil was corroding only slightly more than other sections of the same 244/5
cable. These measurements were substantiated by visual examination of the

cable after excavation. The distribution of polarizing current along a cable

can be determined so that current densities and corrosion rates may be calculated, but more work is needed on this phase of the study. The measurements

also revealed that potential readings give no indication of the condition of

the cable.

The galvanic couples of the cable components indicate that the carbon

filled polyethylene is cathodic to the coated copper wire, the bare copper

wire, and to the 90% Pb, 10% Sn alloy, but the galvanic current generated is of

such low magnitude that it caused only minor damage to the metals.

References

1) Schwedtf eger , W. J. , A Study by Polarization Techniques of the Corrosion

Rates of Aluminum and Steel Underground for 16 months, J. Res. Nat.

Bur. Stand. (U.S.), 65C, No. 4, 271-276 (Oct. Dec. 1961).

2) Jones, D. A. and Lower, T. A., ASTM Journal of Materials; "Polarization

Methods for Measuring the Corrosion of Metals Buried Underground, p. 600,

Sept. 1969.

Table 1

Galvanic Currents of Cable Components

Specimens Average

Current

yA

Polarity

Bare copper wire connected

to carbon filled polyethylene

2.8 Bare copper wire anodic

Bare copper wire connected

to coating alloy sheet

682 Coating alloy sheet anodic

Coated copper wire connected

to carbon filled polyethylene

8.8 Coating copper wire anodic

Coating alloy sheet connected

to carbon filled polyethylene

49 Coating alloy sheet anodic

244/6
COATED COPPER WIRE

(CONCENTRIC NEUTRAL)

CARBON FILLED

POLYETHYLENE

POLYETHYLENE

INSULATION

STRANDED CONDUCTOR FIGURE h - Electrical diagram of polarization

ci rcui t.

FIGURE 1 - Cross-section of concentric

neutral cable.

TOP VIB/V

CABLES

/ \

FIGURE 2 - Concentric neutral wires as assembled for burial.

CAIK 4

FIGURE 3

- Top view of NBS test site.

o - +

o

I

I

I

^ Irn ^ 1m

I

I

I

I

1

FIGURE 5

- The positions of the electrodes

during polarization.

244/7
CuCut04

CASLE 1 CABLE 1

AC ON

I MO

0

AC ON

MO

AC ON

AC Off

• INCC JUNI

1

2 4 • 10 12 14 1* It fOSmON. m

AC Off

2 MO

0

FIGURE 6 - The potential along cable 1 at different burial times.

AC Off

tlWCI JUNC

• 2t 77

• 10 12 14 It It fOtITtON. m

FIGURE 8 - The current required to cable 1 6 mV at four positions and

different burial times.

pol ar i ze

a t

CABLE a

600

200

NO AC

••7- 7I

SOIL MODIFIED

I 1

Hi ‘ " (\

NO AC

4« 14>76

ft n

I .

t-1t-77

NO AC

200

too

200

NO AC

t-1t-77

NO AC

t-2t-77

2 4 t t 10 12 14 «t It fOSITION m

FIGURE 7 - The potential

different burial times.

along cable 3 at FIGURE 9 * The current required to polarize

cable 3 6 mV at four positions at different

burial t i mes

.

244/8
FIGURE 10 - Section of concentric neutral

wire with imposed ac. FIGURE 12 - Section of concentric neutral

wire without ac in modified soil.

FIGURE 11 - Section of concentric neutral

wire wi thout ac. FIGURE 13 current a change i

- The distribution of polarizing

ong the cable as indicated by the

overpotential

244/9
Laboratory Corrosion Studies on Tinned

Copper Concentric Neutral Wires*

U. 8ERTOCCI and J. L. MULLEN

Corrosion and Electrodeposition Section, National Bureau of Standards, Washington, DC

Results are given of corrosion tests up to 2500 hours of

tinned Cu neutrals such as are used for grounding residential underground distribution systems. Data were compared for couples between tinned Cu neutrals and conductive poly- ethylene also. Environmental variables included compositions

of electrolytes (Na sulfate and perchlorate. Na chloride) and

superimposed AC. Data ware taken when oxygen was present and absent and when there was convective motion of the

solution. Pitting was found on specimens carrying super- imposed AC, more in nonoxygenated than in oxygenated

environments. Chlorides intensified attack and bottom

pitting was extensive when they were present. Potentio- dynamic scans were made also to determine the current- potential behavior of the electrodes. A specimen of 90 Pb-10 Sn alloy (used for tinning the wires) was tested also with

results indicating irreversible reduction of a Pb sulfate film occurred. The quantity of tin removed during the tests was

measured. AC attack was impeded when corrosion products were present and no significant difference was found between

the effects of perchlorate and sulfate environments. Differential aeration may contribute to effects noted. Field tests are contemplated.

This paper reports the results of laboratorv experiments

carried out on "tinned" copper concentric neutral (CCN) wires at the National Bureau of Standards (NBS) under contract with the Department of Energy (formerly ERDA).

The aim of the research was to develop methods for the detection of corrosion on CCN wire buried underground, and for

this purpose a number of techniques are being investigated, both in the laboratory and in the field. However, before starting the field

tests, a considerable amount of laboratory work was necessary to provide the knowledge for creating the desired corrosion conditions

at the field test sites, and to collect information on what results can

be expected in the field tests. Moreover, the results obtained m

these studies can shed some light on the possible causes of corrosion

of buried CCN cables.

Experimental Methods

Materials

Materials used for these studies were mainly "tinned" U.e., Pb/Sn) copper wires with a diameter of 1.5 mm, taken from a CCN

cable obtained from a major manufacturer of such cables. When

immersed in an electrolyte, the wire ends were masked with a protective paint. Some experiments involved the use of specimens of conductive polyethylene (CPE) wnich covers the insulating sheath

of a CCN cable. These were also cut from a piece of the same cable, and electrical contact was made by electrodepositing copper on the CPE and then winding copper wire tightly over it. The contact was masked with insulating paint. Some measurements were carried out on bare cooper wires. In this case, oxygen free, high conductivity

(OFHC) copper was used.

•Voluntary manuscript submitted for publication April, 1973.

TIME, h FIGURE 1 — Rest potentials vs time of CPE and Pb-Sn alloy

in 2% Na^SOa * 1% NeCI.

The cable manufacturer also provided a sample of the alloy used

in the "tinning" process for the Cu wire. The alloy was found to be 90% Pb and 10% Sn with small amounts of impurities. This alloy was employed in some experiments. The open circuit potential of CPE and of the tinning alloy was monitored for an extended period in various environments. The

results are presented in Figure 1

.

1~~4 On the basis of previously published studies. several environ- mental variables were tested for their influence on corrosion

susceptibility. These were: Composition of the Elactroiyte. Four laboratory test solutions were employed: 2% Na2 S04 , 2% Na 2 S04 + 1% NaCI, 0.42M

NaCI 04 , and 0.42M NaCI 04 + 1% NaCI. The pH of all solutions was

approximately neutral. These solution compositions were chosen to determine the influence of a sulfate ion (thought not to be

particularly aggressive), both in the presence and absence of chloride

ions (which are considered to be corrosive in soil). Solutions containing another nonaggressive salt—perchlorate, instead of sulfate were also tested, because of reports that sulfur in some form could

be a significant factor in causing corrosion.^ Composition of the Gaseous Atmosphere in Contact with the Solution. Both oxygen and nitrogen atmospheres were tested, the

latter to simulate poor soil aeration. Convective Motion in Solution. Since convective transport in the soil is negligible compared to that in a solution, some corrosion

tests were carried out in 1% agar gel to simulate soil conditions. In this case, the solution had the same composition given above. Superimposed AC. Since with actual buried cables, it is likely that alternating current is leaking from the neutral wire into the soil,

its effect upon corrosion was examined by applying an AC voltage between the wire under study and a counter electrode. The circuit employed had a small resistance in series (10^2) for the current measurements and a very large capacitance (110 /iF) to stop direct

Reprinted from MATERIALS PERFORMANCE, Vol. 18, No. 6, pp. 24-31 (1979) June

Copyright 1979 by the National .Association of Corrosion Engineers, P. O. Box 218340, Houston, Texas 77218
TABLE 1 — Solution: 2% Na2 S04

Exper. No. CPE AC Agar Gee ocmV

AC R, n-cm^

Short Circuit Current Density

uA/cm^ Visual Obcarvation

3

• N2 -50 ± 20 180 CU 2 O layer, tin coat excellent underneatn.

4

• • N2 -20 ±30 185 1.0 CU 2 O crystals, tin coat underneath. No visible attack.

101 N2 -50 ± 20 Tiny red spots. Otherwise excellent condition.

103 N2 -30 ± 10 Excellent condition.

104 • N2 -300/-30 +0.04 Almost unchanged.

7

* # 02 +30 ±10 590 +1/0.2 Thin layer of green crystals. Some

small pits. Large part of tin coat

still in good condition.

8

* • 02 +5 ±20 330 Green scale. Rad blisters. Some

pitting under scale.

10 * 02 +20 ± 10 240 Sheets and mounds of green salt. Extensive pitting with CU 2 O in pits.

11 # 02 +20 ±10 250 Same as 1 0.

106 0±5 Good condition.

107 02 0±5 Good condition.

109 * 02 +35 ±5 A few pits (2 or 3) under green mounds.

Otherwise good.

115 02 +10±5 +0.14 Green crystals, white patina. Tinning

in excellent condition.

117 • 02 + 15 ±5 +0.2 Green crystals. Tinning excellent.

118 » • 02 +18±5 -0.14 White-green patina. Excellent.

currents. With a large counter electrode-to-working electrode surface

ratio, a condition approaching a potentiostatic system was achieved. With such a circuit, the call voltage and current density could not be maintained constant. Variations in voltage and currant between 150

and 450 mV and 1 and 8 mA/cm^, respectively, were observed. In the tables that summarize the results, an AC resistance is reported, calculated as the ratio between the average ceil voltage and the electrode current density. This number indicates the ease of AC

flow between the wire and the solution. A smaller resistance leeds to

a larger current density, and vice versa. Coupling with Conductive Polyethylene (CPE). The outer part

of the insulating sheath of the cable Is covered with CPE (formed by

carbon powder embedded in polyethylene), which is in electrical contact with the neutral wire. Since the open circuit potential of CPE tends to be fairly positive (Figure 1), the effect of coupling the CCN wire with CPE was also examined. In these experiments, the short circuit current flowing between CCN wire and CPE was

periodically measured by inserting a current-to-voltaga converter between the two. Since the internal resistance of the instrument was

effectively zero, it was possible to measure the short circuit without

affecting its magnitude. Both open circuit and current-potential measurements were

carried out using a saturated calomel as reference electrode. Conventional Instrumentation and cells were employed for the measurements.

Experimental Results

Long Term Corrosion Tests A number of specimens of CCN wire were subjected to long term corrosion tests for times often in excess of 2500 hours. During

the tests, the open circuit potential was measured at short intervals. The alternating current and voltage were also measured for the specimens with superimposed AC, as well as the short circuit current

for the specimens coupled with CPE. Some of these corrosion tests were in duplicate. At the end of the tests, the wires vMre examined

under a low power optical microscope. Some were also examined

with a scanning electron microscope. The results obtained are summarized in Tables 1-4. Examples of the potential-time records

are given in Figures 2 and 3. The results show that under a nitrogen atmosphere, the rate of corrosion was slower than in the presence of oxygen. The electrode

potentials, as a consequence, tended to be more negative; if the tin coating was continuous, the potential could remain at low voltages,

-4(X) to -500 mV vs SCE, for fairly long periods. The tin coating on

the CCN wire used in this work always had many small flaws, and

very low potentials were not maintained for long times. The steady

state values were higher in the absence of Cl~ ions (about 0 mV in NaCI 04 and -50 mV in NajSOa) than in their presence (from -130

to -220 mV in Na2 S04 and from -100 to -200 mV in NaCI 04 ). Potentials were shifted towards more positive values by oxygen, and

25
)

I

J TABLE 2 - Solution: 0.42M NaCI 04 '

Exper. No. CPE AC Agar Gas DC

mV

AC R, n-cm^

Short Circuit Current Density ;2A/cm‘

1

1 Visual Observation

3 N 2 -40 ± 20 67 Almost black. Some indications of attack | on extrusion lines, no pits. Perhaps ' some tin coat left. '

4

* N2 -40 ± 10 186 + 1.5 Dark green with protrusions. Extensive 1 attack underneath. Small, shallow pits I uniformly distributed.

|

101 N2 -200/+1C Excellent condition. Thin white patina. '

103 N2 + 10 ± 10 Almost unchanged.

112 • N2 +20 ± 20 +0.14 Most tin coat can be removed by brushing. ; Otherwise little attack. !

1 7

• O2 +85 ±10 180 2.3 Green and black scale. Attack is not

localized. Copper exposed in large

| areas, but some tinning still present.

^ 8 O2 +40 ± 20 420 Only one pit, with CU 2 O inside. 1 Fairly extensive attack, but some

tinning still on.

10 • O2 +55 ± 15 260 Green-black scale. Pits under blisters.

1

1

* O2 +55 ± 20 220 Same as 10. Tinning is intact on large i areas.

|

106 O2 +55 ± 10

i White-green patina in places. Excellent

condition.

107 O2 +55 ± 10

i More stains and black blisters than 106, but little attack underneath. |

109 * O2 +70 ± 10 Green patina in places. Tin coat can be \ removed by scratching, showing CU 2 O j formation, in one spot. 1

115 O 2 +65 ± 1

0

0.35 Tin coat in good condition. Most red | spots can be removed by scratching.

117 • O2 +70 ± 15 0.22 Tin coat in excellent condition.

118 • O 2 +70 ± 10 0.35 Good condition. Some loss of tin coat. i

they were higher in the absence of chlorides, where NaCI 04

solutrons showed, as under a Nj atmosphere, higher values (+40 to 80 mV) than Na 2 S 04 solutions (from 0 to +30 mV). When

chlorides were present, the differences were also much less pronounced because the spread was larger. In sulfate solutions, the

potential ranged from +30 to -60 mV, and in perchlorate from +50

to -80 mV, with the agar gels showing the lowest values, due to the lower availability of oxygen. The AC resistance showed a definite tendency towards lower

values in nitrogen than in oxygen. Chlorides in solution had the

effect of decreasing the resistance of the electrode even more. When

these findings were compared with the results of visual inspection of the wires after the test, it was clear that a high AC resistance was

associated with, and probably caused by, extensive scale formation. Low resistance specimens in general were only slightly corroded.

Chlorides, on the other hand, promoted corrosion, bur evidently

contributed to the formation of more conductive corrosion products than those formed in their absence. Comparison of the

results in sulfate and perchlorate solutions showed that, all other conditions being equal, the extent of corrosion was greater in perchlorate. Preliminary experiments had shown that corrosion in agar gel under an N2 atmosphere was very small, and, therefore, it was

decided to examine the effect of preventing convective motion only under oxygen atmosphere. Corrosion was found to be rather small

in the absence of chlorides although some pitting was detected. In solutions containing chlorides, however, extensive pitting was found

towards the bottom of the wire, with a pattern suggesting

differential aeration. This was true both in sulfate and perchlorate. The superposition of AC did not have a clear effect on the electrode potential but definitely increased the amount of corrosion. In particular, pitting (as opposed to a more uniform attack)

26
TABLE 3 - Solution: 2% NaiS04 + 1% NaCI

Exper. No. CPE AC Agar Gat ocmVAC R,

Short Circuit Currant Density

/iA/cm^ Visual Observation

1 * N2 -200 ±40 16 Uniform CU 2 O layer, pits underneath, but

also some tin coat left, some attack under the coat.

2

• N2 -220 ±40 30 Similar to 1.

5

• » Nj -130 ±20 14 +3.2 CU 2 O, Cu deposit. Some small pits.

102 Ni -220 ±10 Very little attack.

104 N2 -220 ±10 Little attack. Some loss of tin coat.

111 N2 -150 ± 10 +1.4 Light green patina. Part of the tin coat can be removed by scratching. Underneath

is CU 2 O, and probably pit initiation.

113 ¥ N2 -170 ± 10 +1.4 Similar to 111, but little can'be removed

by scratching.

6

• O2 -55 ±5 420 -0.18 Only one pit, distributed attack, considerable tin coat left. Small green

crystals.

9

* » O 2

-1 50/-60 173 Green scale. Red blisters. Large and deep pits under salt mounds, but only at one level. (Differential aeration?)

12 * O 2 -40 up slowly 390 Large green mounds. Pits under salt mounds not very deep. Probably no

tinning left. Similar to NaCI 04 12.

105 O2 +20

up slowly Some loss of tinning, but little attack on copper.

108 » O 2-1 50/+65 Green layer. Part of tin coat can be removed by scratching, but no extensive

attack of copper. no O 2 -10 ± 10 Tin layer has been attacked considerably. Some Cu attack. No obvious pit initiation.

114 O 2 +30 ± 20 Little attack. One spot could be pit initiation.

115 O 2 -10 ± 10 -0.14 Some attack of tin coat. Otherwise

excellent.

119 • • O2 -90 ±5 +6.4 Brittle surface layer that can be broken

off easily. Red crystals underneath,

but no pits.

was detected almost exclusively on specimens tested with AC. Even

for the tests in the milder N 2 atmosphere, corrosion and pitting could be found. It must be kept in mind that AC in Nj atmosphere was often much larger than under oxygen, and this may have enhanced its effect. On the other hand, in Oj, a smaller current did not prevent enhancement of the attack and pit formation.

Since .the potential of CPE was more positive than that of the CCN wire, as shown In Figure 1, it was expected that coupling between CPE and wire would shift the potential toward more

positive values and that an anodic short circuit current would flow

in the electrolyte from the wire to the CPE. This expectation was

largely fulfilled. In nitrogen, the electrode potentials of the wires

coupled to CPE were almost without exception 20 to 1 00 mV more

positive than the corresponding wires tested without coupling. In oxygen, the situation was less clear, but wires coupled to CPE

tended to be more positive. As far as the direction of the short circuit was concerned, a few

exceptions were found where a cathodic current (always small) was

measured. It is possible that the reversal was caused by transport of some of the corrosion products to the CPE, providing an anodic

reaction there. This explanation is suggested by the fact that some

Cu corrosion products were found on the CPE, and also since m the corresponding tests in agar gel, where such transport could not

occur, the short circuit current was anodic on the wire.

27
TABLE 4 — Solution: NaCI04 + NaCI

Short Circuit Exper. No. CPE AC Agar Gas DC

mV

AC

rj-cm^

Currant Density

Visual Observation

1 • N2 230 ±10 40 CU 2 O layer. Some pits underneath.

2 N2 -190 ± 10 30 Some fairly deep pits under the CujO

layer.

5 N, -100 ± 10 14 +3.2 Brushing removes salt scale and tin coat, showing grooved and attacked Cu wire.

102 -170/-120 Some tin coat can be removed by

scratching. No pits.

104 Nt -100 ±20 Most tin coat can be removed by

scratching, but no pits.

111 * Ni -120 ±20 + 1.4 Tin coat can be brushed away easily.

113 * N2 -120 ±20 + 1.4 The tin coat crumbles away revealing attack underneath.

6 O 2 0 ± 10 460 +0.2 Black-green layer with protrusions. Etching reveals some attack, but no

pitting.

9 * Oj -50 ± 20 120 Heavy attack at one level. Red and

green crystals. Pits underneath. Good

condition at top.

12 • 02 -20/-^60 420 Green mounds, red inside. Pits.

105 02 -M0±10 Soma green salt. Good condition.

108 02 -1 50/+70 Top part good. Bottom part can be removed

by scratching. Cu^O and attack under tin coat but uniform. Pit initiation uncertain.

110 02 -t-10 ± 10 Green layer. Tin coat can be removed by

scratching, but little attack of copper.

116 • 02 t50 ± 10 0.05 Green layer. Tin coat can be removed

by scratching. No pits.

119 * 02 -80 ±10 +4.4 Brittle layer. Copper corroded

underneath.

t

• -

FIGURE 2 — Eloctrod* poUntial vt time for tinned CCN

wire in 2% Na^S04 + 1% NaCI. 0;> atmosphere.

Coupling to CPE increased somewhat the rate of attack on the

wires: in the absence of oxygen, attack occurred under the "tin" coating so that often the coating could be removed by gentle scratching or even by brushing with a soft brush. Cases where there was attack under the tin coating were associated with relatively high

short circuit current densities, that is larger than 1 jLlA/cm^ and up

to 6 /lA/cm^ in one instance. In general, however, this attack was

not associated with pit formation. The short circuit current density was influenced by both the

availability of oxygen, which shifted the potential in a positive direction, and by the formation of a resistive layer on the wire as indicated by the AC resistance. The two factors work against each

other, but the second seems to be more innportant. Therefore, short

circuit currents in Nj atmosphere were often larger than in Oj in spite of the fact that the potential wes more than 100 mV higher in the latter condition.

Potentiodynamic Scans

Potentiodynamic scans afford a useful way to examine quickly

the main features of the current-potential behavior of an electrode.

28
TiyE.h FIGURE 3 — Elactrod* potantial vt tiim for tinnod CCN

wir« in 2% N>7S04 1% NaCI. N} atmosphora.

E.mV M S.C.E. FIGURE 6 — Potantiodynamic aeana for Cu in 2% Na^SOa

1% NaO. Sew rata 4 mV/t.

e, mV vt. S.C.E. FIGURE 4 — Potantiodynamic scant for Cu in 2% Na^SOa.

Scan rata 4 mV/t.

FIGURE 5 — Potantiodynamic scan for Cu in 0.42M

NaCIOa. Inert atmoaphara. Scan rata 4 mV/t.

For this reason, scans on CCN wire were taken in Na^SOa, both in the presence and in the absence of NaCI. The results showed a number of peaks, with a very large one in NaCI at about -150 mV vs SCE. However, there was loss of the tin coating during the measurements, and in order to clarify the contribution of copper

FIGURE 7 — Potantiodynamic scans for Cu in 2% Na2 S04

1% NaCI. Inert atmoaphara.

and of tha tin coating to tha elactrochemical behavior, potentiodynamic scans ware taken both on pure copper and on a sample of tha (Pb/Sn) "tinning" alloy used by tha manufacturar of tha cable. Tha results obtained on pure copper are shown in Figures 4

through 7. In the absence of chlorides, no formation of a passive, protecting film was datacted, whathar in NajSOs or in NaCIOs.

Figura 4 shows the scans in sulfata in oxygen and in nitrogan over

the range of greatest interest (from -800 to -*-200 mV vs SCE). Measurements up to >800 mV showed a monotonic increasa U.e., never decreasing) in tha anodic current. Tha curve was vary similar to that shown in Figure 5 for a perchlorate solution. From these

data, and from visual observation of the solution in the cell, it is quite clear that the Cu^*^ ion produced anodically was precipitated

as hydroxide away from tha electrode surface and did not form a protective layer on the metal surface. As a consequence, no cathodic

reduction took place, in tha absence of oxygan, until hydrogen

evolved during the cathodic sweep.

In the presence of chlorides, a precipitate formed during anodic

attack, which was sufficiently adherent to produce a cathodic p>eak during the negative sweep. However, the precipitate was ineffective

in protecting the metal, and although it caused some poorly

reproducible peaks and inflections on the anodic branch, it failed to

limit the anodic current density Wiich reached 60 mA/cm* at +800 mV (Figure 7). A comparison of Figures 6 and 7 showed that the

29
FIGURE 8 — Potantiodynamic ccant for 90% Pb-10% Sn

alloy in 2% Na2 S04 . Scan rate 4 mV/s.

FIGURE 9 — Potentiodynamic scant for 90% Pb-10% Sn

alloy in 2% Na2 S04 -f 1% NaCI. Scan rata 4 mV/t. Oxygen

atmotphore.

position of the cathodic peak was shifted toward more negative

values by extending the anodic range as well as by increasing the scan rate. The anodic branch, on the contrary, was little affected by

changes in scan rate indicating that the behavior was not transient. The electrochemical behavior of the 90 Pb-10 Sn alloy, both in the presence and absence of chlorides, is shown in Figures 8 and 9. Scans were carried out varying both range and rate in chloride containing solutions. The results confirmed that the more negative cathodic peaks corresponded to the anodic peak and was caused by

the irreversible reduction of a PbS04 film. The more positive cathodic peaks were absent if the scan did not exceed -200 mV and

corresponds to anodic oxidation occurring between -200 and +200 mV, which probably results in the formation of lead chloride. The

metal exhibited passive behavior up to 800 mV vs SCE. The results indicate that some of the features seen from the potentiodynamic scans on tinned wire are indeed due to the "tin"

coating. However, the alloy used in these experiments did not correspond to the composition of the tin coating as discussed in the next section.

Chemical Analysis of the Surface

In order to determine the extent of removal of the tin coating

during corrosion tests, some specimens were analyzed in a scanning

electron microscope.

In the course of these analyses, information on the composition

of the original tin coating was obtained. From a comparison of the

spectra obtained on an untouched CCN wire, the 90% Pb-10% Sn

alloy used in the tinning bath, and on a 40% Pb-€0% Sn alloy, it was

concluded that on the tinned wire, the Sn/Pb ratio is larger than in either Pb-Sn alloy tested. The tinning layer consisted mainly of Sn, with probably only 20 to 30% Pb. This is not surprising, since Sn

forms intermetallic compounds with Cu, and it is taken up

preferentially during the tinning process. Spectra taken after the corrosion tests showed total loss of the

tin coating for many specimens exposed to oxygenated sulfate

solutions, and some loss of Pb when the corrosion tests wore carried out in inert atmosphere. In chloride solutions, the formation of PbCl 2 was indicated by the presence of a small Cl peak. On a sample

exposed to oxygenated chloride solution gelled with agar, the Sn

had disappeared but both Pb and Cl were detected. In this case, because of the lack of convective motion in solution, insoluble PbCl 2 was left on the surface in spite of the total disappearance of the tin coating.

Steady State Current Potential Measurements

Potentiostatic measurements on a number of CCN wires in various solutions gelled with 1% agar were taken at long time

intervals, to estimate the corrosion current by intersecting the anodic and cathodic Tafel lines. The open circuit potentials as well

as the calculated corrosion current densities at various time intervals are reported in Table 5.

TABLE 5 — Open Circuit Potentials and Corrosion Current Densities

for Tinned CCN Wire in Agar Gel. Potentials vs SCE

Solution Composition: 2% Na2 S04 2% Na2 S04 + 1% NaCI Atmosphere; O2 N2 O2 N2

Time, Hours AC

Open

Circuit mV

Corrosion Currant Density

/iA/cm^

Open

Circuit mV

Corrosion

Current Density AiA/cm^

Open

Circuit mV

Corrosion Currant Density

/iA/cm^

Open

Circuit mV

Corrosion Current Density

pA/cm*

460 No +5 1 -45 0.1 -155 0.8 175 0.4 1150 No -25 0.5 -95 0.1 -95 1.5 -175 0.6 1900 No + 15 O.S 125 0.08 -135 1.5 185 0.3 4300 No +35 0.5 -145 0.1 -65 5 -175 0.9 5000 No +45 0.7 -145 0.07 -65 7 -175 0.9 6700 No +25 0.5 -155 0.03 -95 0.7 -185 0.5 1600 Yes +25 0.4 -125 0.3 -65 2 -220 0.2 2400 Yes - - •155 0.2 -45 2 -240 0.3 4200 Yes + 15 0.3 -155 0.06 -5 0.6 •200 0.3

30
The results obtained from the steady state potentiostatic data

are in general agreement with the corrosion tests, as far as the open

circuit potentials are concerned. The corrosion rates deduced from

the polarization values are larger in the presence of oxygen,

particularly in chloride containing solutions, as expected. However, no clear effect of superimposed AC was found. Since ’•hese tests are continuing and the wires have not been inspected visually, it is impossible at writing to correlate the current-potential data with the extent of pitting on the wires.

Discussion The main purpose of this work was to obtain information useful

for planning and interpretation of field tests. At the same time, the

results have given a number of interesting indications concerning the

possible causes of corrosion of buried CCN cables and the interplay of the factors that can initiate pitting on the copper wires. The corrosion tests for long exposure times showed that a superimposed AC was a powerful cause of pitting. However, in some

cases extensive corrosion was found on specimens not subjected to AC, indicating that this is not absolutely necessary to cause

corrosion, but that its presence increases considerably the rate of

attack. The presence of a reducible species such as o.vygen was

expected to enhance corrosive attack, and this was verified experimentally to a certain extent. However, the effect of the attack was to form a layer of corrosion products that slowed further corrosion considerably. Which of the two opposing consequences of

the availability of oxygen will predominate in a certain instance may

depend on subtle differences in the environment, and are difficult to predict. The presence or absence of a protective scale influences the amount of AC leakage, and probably explains why extensive pitting was found in tests vtbere oxygen was kept low by a nitrogen

blanket. In these conditions, no protective scale was found, and

extensive pitting occurred in many cases. Analogous conclusions can

be drawn from those tests in agar gel where oxygen could not

readily reach the bottom part of the specimen, and the attack there was quite severe. These tests also point to differential aeration as a possible cause of failure m the field. The presence of chloride Ions

also enhances corrosive attack, since they tend to reduce the

resistance of the protective layer. Since it has been pointed out’ that sulfates, or perhaps other

sulfur containing materials, might play an important role in the corrosion of CCN wire in the field, corrosion tests employing NaClOa instead of Na 2 S04 as the mam electrolyte were carried out. From the results reported here, it appears that no special role is played by sulfates. Although the effect was quite minor, the NaCI 04 solution was a slightly more corrosive environment than Na 2 S04 . This may have been because perchlorates of the heavy

metals (including Pb) are more soluble than the corresponding

sulfates. The crude measurements of electrode impedance obtained by

monitoring the AC voltage and current have given some interesting clues concerning the susceptibility of the CCN wire to attack. This

suggests that AC impedance measurements can be a useful diagnostic tool.

Referancss

1. Zastrow, O. W. Materials Performance, Vol. 13, No. 8, p. 31

(1974).

2. Schick, G. Corrosion of Concentric Neutrals of Buried Power

Distribution Cables, NACE National Conference, 1975, Toronto,

unpublished paper No. 128.

3. Compton, K. G. Materials Performance, Vol. 14, No. 8, p. 14

(1975).

4. Kroon, D. H. Materials Performance, Vol. 15, No. 8, p. 13 (1976).

5. (.atanision, R. Private communication (1977).

31 Printed in U.S.A.
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Oi«
AC Induced Corrosion. The Effect of an Alternating Voltage

on Electrodes Under Charge-Transfer Control^

U. BERTOCCI*

Abstract

The equation relating current to voltage for an electrode under charge-transfer control has been

solved for a sinusoidal modulation of the electrode potential. The rectified Faradaic component has been obtained, so as to derive its value as a function of the Tafel parameter and of the amplitude of the modulating voltage, as well as of average electrode potential. The case where one electrode

reaction is under diffusion control also has been treated. The amplitude and phase characteristics of the harmonic components have been derived, and their use for determining the kinetic properties of the electrode have been discussed. The capacitative current generated by the alternating voltage also has been investigated, and the conditions under which it can be separated from the Faradaic current have been given. The implications of these results on the corrosion due to AC leakage have been examined.

In recent times, there has been renewed interest in the effect of alternating current (AC) on the corrosion of metals. The interest is due to the fact that serious cases of corrosion of underground power

cables have been reported,* and there have been claims that such

corrosion is caused, or at least greatly enhanced, by AC leakage from the cable to the ground.^ For the study of such an effect, a number of workers have

carried out current-potential measurements with a superimposed

sinusoidal modulation, and indications of a high corrosion current

3 4 density under these conditions have been reported. ' The effect of AC on corrosion was examined many years ago.^ The experiments showed only minor effects that progressively decreased with increasing frequency of the AC. The conclusion at the time was that little harm was to be expected from AC of the frequency used for power transmission. As far as the analysis of the effect of a modulating sinusoidal current or potential on electrodes is concerned, an extensive

literature exists, under names such as redoxokinetic effect,*

7 8 Faradaic rectification, and Faradaic distortion. The results of this analysis have been applied, for instance, to the developqjent of AC

*Submitted for publication June, 1978; revised September, 1978.

’National Measurement Laboratory, Center for Materials Research,

National Bureau of Standards, Washington, DC.

9 polarography

The present treatment, therefore, is not a novel one. However,

while previous workers were mainly concerned with the effects of the modulation on transport in solution, starting with the classical work of Warburg*** and Kruger,* * the emphasis here is on systems

such as those frequently encountered in corrosion practice or research, although the results can be applied also to electrodes in equilibrium with the surrounding solution.

Statement and Analysis of the Problem

The model to be treated here is that of an electrode whose

kinetic behavior is determined only by charge transfer so that the

relationship between current (i) and potential (E) can be written in the form:

i = ic + if = ic + «*P(AnE) - Im*‘m exp(-BmE) (1)

The summation should be extended so as to include all anodic and cathodic reactions that might become significantly large in the range of potentials of interest. Here analysis is restricted mainly to the case where only one anodic and one cathodic reaction are

Reprinted from CORROSION, Vol. 35. No. 5, pp. 2U-21S (1979) May

Copyright 1979 by the National Association of Corrosion Engineers, P. O. Box 986, Katy, Texas 77450
important. Then 300

' “ 'c if “ iof«*P(ATJ) - 0xp(-BT7)J + ig (2)

where ig is the capacitative current, given by the derivative of the electrode charge Q = CE

‘c

drj dC C + E

dt dt

(3)

i (3 represents either the exchange current density (if an equilibrium

situation is considered) or the corrosion current density. A and B

are related to the anodic and cathodic Tafel slopes bg and bg by the

usual relationships A = (InlO/bg, B = (In10)/bg. The voltage (T?) is the difference between the actual electrode potential and the

potential at which no DC is going through the electrode in the absence of an alternating modulation. We now want to examine the current output of the electrode if a voltage signal of the form

FIGURE 1 — Effect of a sinusoidal amplitude of 200 mV

applied to an electrode with anodic Tafel slope (bg) 60

mV/decade and cathodic slope (bg) 120 mV/decade.

f? = J7o V sin CJt (4)

is applied to the electrode by means of a potentiostat. Introducing Equation (4) into Equation (2), the Faradaic current becomes;

if « ig [axplAV sin cot) explAT^g) - exp(-BV sin cot) exp(-Bl7g)] (5)

Since one can make the substitution’^

oo exp(z sin x) =• lg(z) + 2 (-I)** l2k + 1 (*) sin

| (2k + 1) x[

00 + 2 Zk (-1)^ l2k (z) cos (2kx) (6)

1

where Ig (z) is the modified Bessel function of order n, whose value can be calculated by means of the expression

z n ”

'nW-(T) 2k

z 2k {jr*"

k!(n k)l

(7)

the DC component of the Faradaic current as well as its harmonic components can be obtained. When time averages are considered, only the DC term is not

zero, and represents a current which can affect the electrode. Combining Equations (5), (6), and (7), and considering only the DC

component.

(if)dc

(AV)2'< oo (BVi^**

( 8)

Equation (8) shows that both the anodic and cathodic DC partial currents have been multiplied by a factor that depends on the

kinetics of the reaction (through A and B) and the amplitude V of

the modulation. In particular, if the average potential (TJg) 'S zero (the open circuit voltage), a DC current may be measured, as

('f^dc, T)o “ o

V2k

2^(kl)2

(A^k . B^k] (9)

which will be zero if A = B, that is if the anodic and cathodic Tafel slopes are the same, or it may be either positive or negative, if the

activation energy of the anodic reaction is, respectively, smaller or

larger than the cathodic one.

This result shows also that If a Tafel plot (potential vs logarithm

of the current) is obtained under sinusoidal modulation of the

voltage, the Tafel slopes will be independent of V, but their locations will be shifted horizontally by amounts given by the terms

in A and B of Equation (9). Figure 1 shows the effect if a sinusoidal amplitude (V = 2(X) mV) is applied to an electrode whose anodic

Tafel slope (bg) is 60 mV/decade, and the cathodic slope (bg) is 120

mV/decade. The figure also illustrates the fact that the intersection

of the two new slopes occurs at a value of 17q

different from zero, and that the value of the corrosion current (or the exchange current

density, if the plot refers to an electrode in equilibrium) obtained from the intersection is larger. This value of ig, obtained under

sinusoidal voltage modulation, will be designated as igorr 'o- The shift in the potential at which no DC is flowing, which has been observed experimentally,^ is a qualitative indication of what would happen if the circuit were constructed so that no DC could

circulate, as for instance, if a capacitor were inserted in the currant

circuit. The correspondence between voltage modulation, as treated

here, and current modulation has to be considered only qualitative, because while in this case the nonlinear characteristics of the electrode reaction causes harmonic distortion in the current, in the

galvanostatic case, the harmonic distortion would appear in the

voltage signal.

In the preceding paragraphs, the DC component of the Faradaic current has been assumed to be also the DC component of the total current, that is, it was assumed that no rectification occurred for the

capiacitative current. The next section will discuss the limits of

validity of this assumption.

Capacitative Current

The value of double layer capacitance is in general a function of

potential and can vary quite significantly when adsorption processes

take place at the electrode. If only voltage modulations of periods

large compared with the relaxation times of adsorption reactions are considered, then the double layer capacitance is only a function of the electrode potential (7)) but not of its derivatives. It is then

C - Cg + Skak’?'* nO)

In this case, when Equation (10) is introduced into Equation (3), and since

dC dC di7

dt dr) dt

The capacitative current is given by

•c ” (Cg + Z|<akr7** + ZhkakT)**"^) ojVcoscJt

which can be rearranged into

ig “ (Cg + ZkakT)**) CJVcoaCJt (11)

In Equation (11), each term of the summation is of the form

212 CORROSION-NACE
TABLE 1 — Values of the Ratio Between the Corrosion Current With and Without Voltage Modulation

and of the Open Circuit Potential Shift Due to the Alternating Voltage for Various Values

of the Tafel Slopes (ba and be) and of the Modulating Voltage Amplitude (V)

be ba mV/dec

V

mV

30 mV/dacVo

•corr/j^ "•V

40 mV/dac

Vo

•corr/j^

60 mV/dec

Vo

•corr/j^

120 mV/dac

Vo

•corr/j^

00

corr/i^ 0 E>

10 1.15 0 1.11 -0.5 1.07 -1.0 1.04 -1.4 1

- 1.9 30 30 2.84 0 2.26 -3.0 1.74 -6.4 1.31 -10.0 1 -13.6 100 . 316 0 115 -13.2 31.2 -30.1 5.84 -52.0 1 -75.0 300 8.4.10® 0 2. 4. 10’ -41.8 4.9.10® -96.9 1477 -173 1 -268

10 1.08 0 1.06 -0.5 1.03 -0.9 1 -1.4 40 30 1.90 0 1.55 -3.5 1.25 -7.3 1 -11.1 100 53.8 0 19.4 -17.7 4.82 -41.9 1 -69.3 300 3.1.10® 0 1.1.10® -57.8 832 -143 1 -259

10 1.04 0 1.02 -0.5 1 -1.0 60 30 1.36 0 1.17 -3.9 1 -8.0 100 9.83 0 3.57 -26.4 1 -59.5 300 11892 0 325 -93.8 1 -245

10 1.01 0 1 -0.5 120 30 1.08 0 1 -4.2 100 2.16 0 1 -40.0 300 53.9 0 1 208

sin^CJt'COSCJt, Mhose integral is proportional to sin*^'*'^ ^CJt, and

therefore, the time average is zero. In other words, no rectification

is produced by the double layer capacitance as long as it is a function that can be expanded in powers of T).

It is also interesting to look at the phase relationships between

the harmonic components of the capacitative current and the modulating voltage assuming that Equation (10) holds. It can be shown that all even haromics (that is components with frequency

2CP, 4CJ, etc.) are sine functions and therefore bear a zero or a ±7T relationship to the modulating voitage as given by Equation (4), whiie all odd harmonics (CO, 3C0, SCO, etc.) are cosine functions, and

therefore out of phase by i7T/2 with the modulating voltage.

Diffusion Limited Electrode Reactions

Although the present treatment does not apply if the modulating voltage causes changes in the concentrations of the reactants

at the electrode, it can be used if one of the electrode reactions is under limiting diffusion conditions. This is often the case in corroding systems when the cathodic reaction is the reduction of oxygen reaching the electrode. When this is the case, the corre- sponding Tafel slope is infinity, and the coefficient A or B is zero. If for instance, tha cathodic reaction is under diffusion control

(B = 0), the second summation in Equation (8) is equal to 1 and

Equation (9) is accordingly simplified, the DC being only a function

of A. The condition at which the DC is zero will be reached by

lowering the potential (l7o), the more so the larger the modulating

amplitude V is, until the anodic partial current, as expressed by the

first term in Equation (8) is equal to ig, the corrosion current that can be sustained under diffusion control. If the electrode is allowed

to remain at this average potential, no enhanced corrosion should be

experienced.

In order to give an idea of the effect of the modulation on

corrosion current (or apparent exchange current density, if this analysis is applied to an electrode in equilibrium) and on the shift in the potential for \Mhich the net DC is zero as a function of A, B, and

V, Table 1 has been prepared. One should note that interchanging A

with B does not change igorr absolute value of I7g. Only its sign is reversed. The values show that even relatively small sinusoidal amplitudes

cause very large increases in iggrr. particularly if the Tafel slopes are

small. This is not surprising, since the electrode potential oscillates

in a range 2V around the average value T]q, bringing it to values

where the current, if only change-transfer limited, would be quite

large.

Frequency Analysis The increasing availability of instrumentation that can perform

signal analysis, allowing one to transform a signal from the time to the frequency domain as well as to obtain information on phase

relationships, makes it interesting to explore what can be obtained from examination of Equation (5) when Equations (6) and (7) have been substituted into it. The result is a representation of the Faradaic current as a Fourier series. The first observation is that all odd harmonics (CJ, 3CJ, 5CP, etc.) are sine functions in phase with the modulating signal, while the even harmonics are ±7T/2 out of phase. Remembering the phase

behavior of the capacitative currant (ig) as discussed in a preceding

section, it appears that, within the limits of validity of the present modal, it is always possible to distinguish between capacitative and

Faradaic components. The amplitude of the odd and of the even harmonics of the Faradaic current, expressed in terms of ig, the corrosion (or exchange) current density in the absence of a modulating voltage,

are the following: odd term of frequency (2k-1)CJ

,2n-1

'(2k-1) k ^ — 1 - I " I*’-’ *

exp(-BT?o)l

( 12)

even term of frequency 2ku)

•2k ^2n-1

,2n

(n + k)l (n - k)l [A^" exp(A77g) -

B^" exp(-Bl7o)] (13)

As Equations (12) and (13) show, the symmetry or lack of it of the electrode reactions as contained in the values of A and B, is reflected in the frequency spectrum of the current. In particular, if

Vol. 35, No. 5, May, 1979 213
TABLE 2 — Amplitude of the Various Harmonic Components for an Electrode With

ba = 40 mV/dec and = 120 mV/dec. All Values are in Decibel Referred to ig = 0.

n. V Frequency

mV mV DC CO 2CJ 3U) 4gj 5oj 60J 7CJ 15o)

20 52.7 52.6 41.4 26.8 9.80 -9.07 -29.5 -51.2 -258 50 62.9 67.0 61.9 54.2 44.5 33.2 20.4 6.32 -138 +100 100 84.6 89.8 87.4 83.4 78.0 71.4 63.6 54.8 -44.1 200 131 137 136 134 131 128 124 119 58.8 300 180 186 185 183 182 179 177 173 131 500 277 283 283 282 281 280 278 276 250

20 50.3 41.8 21.4 -2.60 -29.8 -57.5 -87.6 -118 -450 50 51.9 50.6 37.8 21.7 3.08 -17.5 -39.6 -62.4 -285 -300 100 56.7 59.4 51.8 41.2 28.3 13.6 -2.63 -19.6 -195 200 69.8 74.5 70.7 64.7 56.9 47.5 36.8 25.5 -103 300 34.6 89.8 87.3 83.3 77.8 71.0 63.1 54.8 -48.2 500 116 121 120 117 114 109 104 99:9 49.1

A = B, all even harmonics disappear at TJq = 0. The presence of peaks

for the even harmonics in the spectrum at this potential is an

indication of the differences in the anodic and cathodic Tafel slopes. Since all amplitude components contain the double exponential

expression that is characteristic of charge-transfer kinetics, plots of any component amplitude of the current versus TJ will yield the same Tafel slopes if put in semilogarithmic form. [Examples are given in Figure 1 where the amplitude of the third (30J) and sixth

(6CJ) harmonics are also plotted.) This also indicates that all amplitudes increase when the electrode is polarized, so that high harmonics, which are small when tJq is close to 0, might become

detectable with larger Tjg, even for moderately small values of the modulating amplitude V. For similar reasons, because a larger modulating amplitude pushes the electrode periodically away from

T7q

= 0, more power is proportionally found in the higher harmonics when V is increased. Some idea of the effect of varying the modulating amplitude V

on the various harmonics at constant anodic and cathodic polariza- tion can be obtained from Table 2. All values are given in decibel

referred to Iq in the absence of modulating signal as zero.

Discussion

Limits of Validity of the Model

The very large values of current that can be calculated from moderate modulation voltages, overvoltages, and corrosion (or exchange) current densities, some of which are reported in Tables 1 and 2, immediately point to the limitations in the applicability of

this treatment: certainly other factors in these circumstances would

limit the current, such as transport in solution. The formation of

films of anodic products or other alterations of the electrode surface leading to passivation would also restrict the amount of current to much lower values than those predicted by this model. One can imagine a current-potential plot for the system under investigation, ranging between [(flo^max ^maxl t(i7o)min " '^maxL maximum and minimum values of the DC potential plus or minus

the largest voltage modulation under consideration. If such a plot cannot be described by Equation (2), this treatment is not

appropriate, although sometimes its pertinence can be extended

with the aid of Equation (1), taking into account other reactions such as oxygen evolution at positive potentials and hydrogen

evolution at negative ones. Effects such as slow adsorption or desorption that can cause rectification of the capacitative current can also limit the validity of this treatment.

Effect of Frequency

It may be useful at this point to discuss the influence of the frequency of the modulating voltage upon the current response: as Equations (8), (91, (12), and (13) show, there is no effect on the Faradaic part of the current. This is a consequence of the fact that

charge-transfer processes occur in times much shorter than the

reciprocal of the frequencies that are in common use in electro- chemistry. In experimental work, the upper frequency limit is therefore set by the response of potentiostats and amplifiers and by

the geometry of the cell. On the other hand, high frequency current

leakage cannot be expected to cause less corrosion than low

frequency current leakage if conditions akin to a potentiostatic system exist. In laboratory practice, it might be advantageous to study the effect of changing frequency as a means to detect the influence of slow processes such as transport in solution or chemisorption. The capacitative current, on the contrary, increases linearly with frequency as indicated by Equation (5). This fact should not, however, affect the DC component nor complicate the frequency

analysis of the Faradaic current, as long as phase information is available and within the limits already discussed. The experi- mentalist should, however, remember that at high frequency, the

large capacitative current can very well exceed the maximum output

of the potentiostat.

Derivation of Information Concerning the Electrode Kinetics

In the preceding sections, some of the information that can be obtained from examining the various components of the current has been pointed out. This information concerns the symmetry of the

electrode reaction, and values of the electrode capacitance (and its dependence on potential) as well as Iq and its dependence on the modulating voltage. We would also like to mention some other

points. Away from tJq = 0, in the so-called Tafel region. Equations (12) and (13) contain only the terms in A or in B. Then, from Equation

(7), it can be derived that the ratio between the amplitude of the nth and the (n-l)th harmonic component is approximately equal to A (or B) times V/2n. The Tafel slope can be obtained from a single frequency spectrum. The accuracy increases with the order of the harmonics and with decreasing V, in other words when smaller amplitude peaks are used. A few numerical examples are given here, taking the values from Table 2. On the anodic side, if the ratios between the 4th and the 3rd harmonics for V = 20 mV are used, the slope calculated is 40.7 and for V = 100 mV, the 7th and 6th harmonics yield b3 = 45.3. On the cathodic side, for V = 300 mV

and using the 6th and 5th harmonic, bg is calculated as 141 mV/decade, while for V = 50 mV, the third and fourth harmonics

give a value of 1 21 mV/decade. More elaborate mathematical schemes can be devised to obtain

the kinetic parameters from the frequency and phase spectra, but

only extensive experimental testing can show their usefulness. In fact, one can expect difficulty in the recovery of the information if a significant ohmic drop occurs; then the modulation of the electrode voltage would not be a simple sinusoid, and a phase

214 CORROSION-NACE
difference would exist between control voltage and electrode

voltage. Another source of complications would be the presence of harmonic content in the modulating signal. If the harmonic content

is known, however, attempts can be made to estimate its effect. In

fact, in analogy with Equations (4) and (5), if the signal is in the form

7} = l7o + S|<Cii|< sin k ojt + cos k cut (14)

Equation (5) becomes

i = io (explASkCtk sin k cut) exp(ALid3k cos k cut) exp(AT?o)

-exp(-BZkCi!|( sin k cut) exp(-BZ|^|( cos k cut) exp(-B77o)l (15)

so that the analysis outlined in this treatment can be applied to each term of the summation. Without further elaboration, it must be pointed out that it might be interesting to analyze in detail the consequences of using, instead of a sine function as modulating

voltage, either a square wave signal or a triangular scan, since their Fourier representation involves only odd harmonics and, therefore, should allow the separation of the Faradaic and capacitative components.

Consequences in the Field of Corrosion

According to the treatment described above, if the corrosion

current is derived from the intersection of the Tafel slope or from

the linear polarization method, or by means of some curve fitting procedure,*^ using the values of the DC component of the current under alternating voltage modulation, it will be found that the corrosion current is larger than in the absence of the alternating

signal. The only exception is when either the cathodic or the anodic

reaction is under diffusion control, but even in this case, the same

conclusion could be drawn if the shift in the potential for which 'DC “ 0 's neglected and the current given by one Tafel line at

TJo = 0 is assumed to represent the corrosion current. The conclusion that AC modulation enhances corrosion is therefore justified, provided that two important points are given proper consideration: one is that the extent of the enhancement

depends on the value of the average potential (tJq), since the

potential at which the net DC current is zero will vary, in general, with the amplitude V of the modulating voltage, as illustrated in Figure 1. If conditions are such that TJq can shift to the value for which no net DC flows, the enhanement of corrosion will be minimized. In particular, if all cathodic reactions in the potential range down to (l?o-V) are either diffusion-controlled or negligibly

small, no enhancement of corrosion will take place. This point is illustrated in Figure 2; if T7q

is zero when a modulation of amplitude V = 200 mV is applied, the net DC, marked as iQc on the drawing,

is 8.8 times the ig in the absence of a modulating signal, and the corrosion rate, marked as igorr- 9 3 times ig. If, however, TJq shifts to about -120 mV, where iQC ~ 0- corrosion current is equal to

'o- The second point to be considered is that the effects described

in this work arise from the superposition of an alternating voltage, whose amplitude is independent of frequency. If, on the contrary,

an alternating current of constant amplitude is applied, a large fraction of it will be shunted by the double layer capacitance, the more so the higher the frequency, without causing material transport across the electrode interface. This explains some of the conclusions of earlier work in this area.^ To which extent in a practical situation, the AC leakage can be described as a current or

as a voltage has to be decided after careful examination of the system. The same considerations apply to the manner of carrying out

corrosion tests in the laboratory. Control of the electrode potential,

rather than superposition of an alternating current to the specimen,

simplifies the analysis of the results, but dictates caution in applying

FIGURE 2 — Current-potential curves for an electrode with

anodic Tafel slope bg of 120 mV/decade and cathodic

reaction under diffusion control (be = °“). DC components of the current in the absence and in the presence of an

alternating modulation of 200 mV.

the information obtained to the real cases.

Acknowledgment

This work has been performed as part of contract E(49-1)3800

with the Department of Energy (formerly ERDA) whose financial support is gratefully acknowledged.

References

1. 0. W. Zastrow. Materials Performance, Vol. 13, No. 8, p. 31 (1974); K. G. Compton. Materials Performance, Vol. 14, No. 8,

p. 14 (1975); D. H. Kroon. Materials Performance, Vol. 15, No.

8,p. 13 (1976).

2. G. Schick. Paper No. 128, Intern. Corr. Forum, Toronto

(1975).

3. J. Kruger, U. Bertocci, E. Escalante, and J. L. Mullen. Annual

Report MBSIR 77-1232 (ERDA), April, 1977; D. T. Chin, T. W.

Fu, G. F. Kamrowski, and S. Pookote, 1978 NACE Research

Conference, Houston (1978); U. Bertocci and J. L. Mullen, submitted for publication; D. J. Nesslage, Paper No. 162, Corrosion/78, NACE (1978).

4. K. G. Compton. INCRA Research Report Project 265 (1977).

5. B. McCollum and G. H. Ahiborn. Tech. Pap. BS T72 (1916).

6. K. S. G. Doss and H. P. Agrawal. Proc. Indian Acad. Sci., Vol. 34A,p. 263 (1951); Vol. 35A,p. 45 (1952).

7. K. B. Oldham. Trans. Faraday Soc., Vol. 53, p. 80 (1957); G. Barker, R. Faircloth, and A. Gardner. Nature, Vol. 181, p. 247

(1958); G. Barker. Anal. Chim. Acta, Vol. 18, p. 118 (1958); P. Delahay, M. Senda, and C. H. Weis. J. Phys. Chem., Vol. 64, p. 960 (1960); I. Imai and P. Delahay, ibid., Vol. 66, p. 1108, 1683 (1962).

8. H. H. Bauer. Austral. J. Chem., Vol. 17, p. 591, 715 (1964).

9. B. Breyer and H. H. Bauer. Alternating Current Polarography and Tensammetry. Interscience, New York (1963); P. Delahay. New Instrumental Methods in Electrochemistry, Interscience, New York (1954); D. E. Smith. Electroanalytical Chemistry, A.

J. Bard, ed., Vol. 1, Ch. 1, M. Dekker, New York (1966); T. McCord and D. E Smith. Anal. Chem., Vol. 41 , p. 116 (1969).

10.

E. Warburg. Ann. Phys. U. Chem.. Vol. 67, p. 493 (1899).

11 F. Kruger. 2. Phys. Chem., Vol. 45, p. 1 (1903).

12. Handbook of Mathematical Functions, M. Abramowitz and I. A. Stegun, eds., p. 376, NBS Publication 55 (1965).

13. F. Mansfeld. Advances in Corrosion Science and Technology,

Vol. VI, M. G. Fontana and R. W. Staehle, eds.. Plenum Press, New York (1976).

Vol. 35, No. 5, May, 1979 Printed in U.S.A. 215
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PAPER NUMBER 242 r

V J

Th« international Corrosion Forum Devoted Exclusively to the Protection and Performance of Materials / March 12-16,

1979, Atlanta Hilton, Atlanta, Georgia.

Corrosion Induced by an Alternating Voltage. A Comparison Between

Theoretical Predictions and Experimental Results

U. Bertocci and J. L. Mullen

Chemical Stability & Corrosion Division

Center for Material Science

National Bureau of Standards

Washington, D. C. 20234

1. Purpose of the Work

In a large number of practical situations metallic structures are

subjected to alternating voltages which cause currents to flow into ionic

conductors in contact with them, the most common of which is soil. These

currents might flow because of failure of insulation, but there are also

cases where the metals are exposed without protection. There is, therefore,

a substantial interest in understanding what happens in these situations,

so as to be able to predict when and to which extent corrosion can be

caused by the effect of alternating currents. [1]

As a part of research carried out at NBS concerning the corrosion

of copper concentric neutral cables, a mathematical analysis of the

effect of an alternating voltage on electrodes under charge- transfer

control has been developed [2]. It is the purpose of this paper to

present briefly the principal points of this analysis and to compare

its predictions with experimental results.

Publication Right

Copyright by the author(s) where copyright is applicable. Reproduced by the National Association of Corrosion Engineers with permission of the author(s). NACE has been given first rights of publication of this manuscript. Requests for permission to publish this manuscript in any

form, in part or in whole, must be made in writing to NACE, Publications Dept., P, 0. Box 986, Katy, Texas 77450. The manuscript has not

yet been reviewed by NACE, and accordingly, the material presented and the views expressed are solely those of the author(s) and are not

necessarily endorsed by the Association.

'n I IS A
2 . Mathematical Analysis

a. Faradaic Current

We will present here in a simplified form some of the results of

the analysis which has been given in detail elsewhere [2]. For an electrode

under charge-transfer control, the partial anodic or cathodic current can

be expressed as

i^c ^

where and A are positive for anodic processes and negative for cathodic

ones. E is the electrode potential, taken with respect to some convenient

reference point. As it is well known, (1) can be used to express the

total current in the so-called Tafel region, that is when the partial

current of opposite sign is very small. If the voltage is sinusoidally

modulated, so that

E = E + Vsin u)t (2) o

The expression for the faradaic current can be obtained and by harmonic

analysis the current can be separated into a constant (or d.c. as

customarily called) component and into the various harmonics (of frequency

0), 2w, . . . nuj) of the modulating signal.

It can be shown that each of these components can be expressed

in the form

i = I exp (AE )

• F (A,V) (3)

f,n o on

that is as a product of the constant current i*^ in the absence of a

modulating signal, times a term which depends only on the coefficient

A (which determines the Tafel slope) and on the amplitude V of the

modulating signal. The index n indicates the order of the harmonics,

with zero designating the d.c. term.

•) A second piece of information is derived from the analysis,

concerning phase relationships. It can be shown that the components

of the faradaic current of odd order (co, 3o), 5ai, . . .) bear alternately

242/2
a 0 or ± IT phase relationship with the modulating signal, while even

(2(jq, 4o), 6o), . . .) components are shifted in phase by !

with respect

to the modulating signal.

From equation (3) it can be immediately deduced that current

components depend On the constant component of the voltage in the

same way as (1), giving parallel lines in a semilogarithmic , or Tafel,

plot. This is illustrated in Fig. 1, where both anodic and cathodic

reactions are taken into account. Also, since the factors F do not n

depend on the frequency of the signal, the amplitude of all the current

components should be independent of the frequency. It can also be

shown that F^, the factor for the d.c. component of the faradaic current is always greater than 1, showing that a sinusoidal modulation

of the voltage produces a rectified current that will enhance the rate

of the electrode reaction.

As it is well known, the coefficient A is related to the Tafel

slope by the equation A = - . Therefore, since the factors F^

depend on A, if the anodic and cathodic reactions are associated with

different Tafel slopes, their intersection, which determines the

open circuit potential, that is the potential at which no d.c. current

flows, will be shifted by the application of a sinusoidal modulation.

This fact is also illustrated in Fig. 1. Here the anodic Tafel

slope is smaller than the cathodic one, and the open circuit potential

is shifted in a negative direction.

b. Capacitative Current

For the analysis of the effect of an alternating voltage on

an electrode it is necessary to consider the capacitative current i^

as well, since the electrode-solution interface constitutes a circuit

with a capacitance and resistance in parallel. Therefore the total

current is

i = i + i (4)

r c 242/3
The electrode capacitance in general varies with potential and it is

strongly affected by adsorption phenomena, some of which can be rather

slow, so that relaxation times can be large compared with variations

in electrode potential.

However, in many instances the capacitance is only a function

of potential, and as a matter of fact, its variation can be relatively

minor within certain potential ranges. The analysis has shown that if

the electrode capacitance is only a function of potential, a sinusoidal

modulation as in (2) does not cause any d.c. component of i^. Moreover, it can be shown that its odd harmonic components (oj, 3o), 5o), . . .

are shifted by with respect to the modulating signal, while the

1 even components are shifted by 0 or tt.

Comparison of these phase-shifts with those of the faradaic

current shows that when the hypotheses employed in this analysis are

applicable, it is possible to separate the faradaic and capacitative

components of the current, provided that phase information is available.

3 . Experimental Aspects

a. Introduction

The experiments studying the effect of a modulating voltage on

the correct response of an electrode have three main purposes:

1) Verification of the correctness of the theoretical predictions

2) Investigation of the usefulness of different types of

measurements for gathering information of diagnostic character

concerning a particular corrosion system.

3) Examination of the behavior of electrodes beyond the limits

of validity of the present model.

In this paper we report the results of experiments which concern

point 1) and only in part address point 2) . This work is still in progress . We intend in the future to collect information related to point

242/4
3) in nhe hope to widen our understanding of the effect of alternating

voltage to larger classes of electrode systems, and to contribute the

experimental basis for a more encompassing theoretical model.

b. Experimental Methods

With the exception of some preliminary runs, the experiments aimed

at testing the validity of the theoretical model have been carried out

on the hydrogen evolution reaction. The electrode material was OFHC

copper and the solution composition was 0.5 M Na2S0^ + 0.1 M H^SO^. The

reason for the choice was that of employing a system which could be considered under charge- transfer control over a potential range as wide as

possible. By minimizing other cathodic reactions, in particular oxygen

reduction by bubbling Ar in the cell, and favoring transport in solution

by means of vigorous stirring, it was possible to obtain a straight Tafel

plot with a slope of 120 mV/decade over a potential range larger than 300 mV.

As further test of the applicability of the hypothesis used in the

mathematical analysis to the electrode system under study, the electrode

impedance was measured by applying a very small (<2mV) modulating signal,

about in the middle of the potential range where a straight Tafel slope

was obtained. The results are given in Fig. 2, where the imaginary component Z" of the electrode impedance (for unit surface area) is plotted

versus the real component Z'. The plot is approximately a semicircle,

indicating that there is no detectable contribution of transport to

the electrode kinetics. From the plot the ohmic drop error is found

2 2 to be about • cm > while the charge-transfer resistance is 450f2’cm and the

2 electrode capacitance of the order of 30yF/cra .

The .circuit employed for the measurements is given in Fig. 3.

Two voltages, a constant or slowly varying one (Eq of equation (2)) and

a pure sinewave (of amplitude V) , where higher harmonics had been

minimized by filtering, were added and applied to the control voltage

input of the potentiostat . The resulting current was then decomposed

242/5
using a low pass filter to obtain the constant (or d.c.) term. A

phase-sensitive lock-in amplifier gave the amplitude of the current

both at the frequency w of the modulation, and at twice that frequency,

each one divided in the component in phase with the modulating signal

and the component at 90 degrees.

Inputs and outputs of the circuit, with the symbols used here

are given in Table I.

n In a few instances the current signal was fed to a 200 channels

spectrum analyzer. In this way the amplitude of higher order harmonics

could be obtained. However, all phase Information was lost and only

the absolute value of the current density of angular frequency ncu

i '= Ai’ + (i"

no) n(D no)

was obtained.

4. Results and Discussion

Current-potential measurements in the absence and in the presence of a modulating sinusoidal voltage were carried out. Fig. 4

shows the Tafel plot of i, in both conditions, as well as i* , 1"

dc 0) 0) and i'' . In the figure are also drawn the lines calculated from the

2o) mathematical model. They have a slope of 120 mV/dccade. A.s shown,

the d.c. component runs parallel to the value without modulation and

it is increased by a factor which is in good agreement with tljat

given by (3). A similar behavior is shown by i^ and i” which, as

discussed before are both components of the faradaic current. The

major difference is that the potential range where the slope is

close to 120mV/decade tends to decrease with the order of the harmonics. The component 90 degrees out of phase i|^, on the contrary,

is largely independent of potential, indicating that the electrode

capacitance remains nearly constant in the range studied. In agreement with this conclusion the term i' was found to be negliglibly

Z(i) 242/6
small, suggesting that all higher harmonics of the current are almost exclusively faradaic.

As an additional test, the current signal was examined with a

spectrum analyzer for a value of of -506mV vs. N.H.E. and a modulation

at 25 Hz of 35.4 mV r.m.s. The output values recorded, during the

spectrum analysis, as well as those obtained from the spectrum analyzer

are reported for comparison in Table II, which also gives theoretical

and experimental ratio between the various current components and the current

density in the absence of modulation. The spectrum is given in Fig. 5,

together with the spectrum obtained substituting a pure resistor for the

cell, and passing a current of the order of lOOyA rms at 25 Hz. The

spectral purity was excellent: the amplitude of the 50 Hz harmonics

was below the detection limits (about 10 ^A) and could be measured at

— 8 about 4 ’10 A only after filtering out the large 25 Hz signal. As Fig. 5

shows, the only spurious signal detected on the resistor was a 60 Hz

peak of 5*10 ^A. The small peak at about 20 Hz which can be seen on the

side of the 25 Hz peak of the electrode current is caused by the magnetic stirrer and could be eliminated by shutting it off.

Another Tafel plot, for a modulation of 88.3 mV rms. (V = 125 mV)

at 100 Hz, is shown in Fig. 6. The behavior of the d.c. component is

close to the theoretical prediction over a range of about 300 mV, but significant distortions can be seen in the other frequency components.

This is not surprising, since the peak-to-peak amplitude was 250 mV

and, therefore, almost everywhere the oscillations of the electrode

potential exceeded the linear Tafel region. As expected for a frequency higher than that used to take the data of Fig. 4, the capacitative component i^ was larger. The significant deviation of i\ which

is constant above -350 mV, is due to the effect of the ohmic drop error.

When the faradaic resistance becomes large (as shown by the value of i*

dc

and the circuit becomes equivalent to a resistance and capacitance

242/7
in series, causing the observed phase shift in the current. The results

observed for i* and i" above -350 mV can be accounted for by an olimic

Ci) 0) 2 2 drop resistance of 6.5 fJ'cm and an electrode capacitance of 50 pF/cm .

Comparisions between the predictions about the amplitude of the

faradaic components and the experimental values are summarized in Fig. 7.

On it the experimental ratios between i, , i' and i’’ with respect to i* , dc oj 2(u dc

as well as the theoretical lines are given. The agreement is very good

for amplitudes V up to about 200 mV. Beyond that the current density caused

by the modulation was less than predicted, because the electrode potential

was being driven outside the linear Tafel range.

The effect of the frequency of the modulating signal has also

been examined. Fig. 8 shows the ratio between the dc current i.

dc

and i* . The measurements were carried out at the same potential

dc

(E^ = -456 mV vs. N.H.E.) and at the same modulating amplitude (V =

100 mV), at frequencies ranging from 25 Hz to 3kHz. The results

agree satisfactorily with the prediction of independence of the current

response on the frequency of the modulating voltages. The deviations

from the theoretical values at frequencies larger than 500 Hz are due

to a large extent to the ohmic drop error, which tends to become the

largest element of the circuit. Attempts are being made to further

reduce the IR drop by changing the cell geometry and by testing positive

feedback systems. These are however difficult to apply to the present

work, because of the stringent need to avoid introducing spurious phase

shifts

.

The experimental results presented here show conclusively that

the mathematical analysis previously presented [2] predicts accurately

the current response to a sinusoidal voltage of an electrode under

charge-transfer control, and can be used, therefore, as a basis for

the understanding of the effect of a.c. on corrosion. It also shows

that frequency analysis holds promise to be an important tool for the

242/8
study of electrodes undergoing corrosion. There is, however, a need

to extend the experimental work as well as the theoretical analysis

to encompass other kinds of kinetic control.

This work has been performed as part of contract E (49-1) 3800

with the Department of Energy, whose financial support is gratefully

acknowledged.

References

[1] B. McCollum, G. H. Ahlbom, Tech. Pap. BST72 (1916)

K. G. Compton, INCRA Research Report Project 265 (1977)

D. T. Chin, T. W. Fu, G. F. Kamrowski, S. Pookote, 1978 NACE Research

Conference, Houston (1978)

D. J. Nesslage, Paper N. 762, Corrosion 78, NACE (1978)

[2] U. Bertocci, Corrosion, in press.

T^l« I

Input and Output 0\iantiti«s

Input Qumntitiee

V U a 2«V

d.c. component of Amplitude of raodU'- Angular frequency of

electrode potential lating voltage modulating voltage

Output Quantities

constant Id.c.l components

In phase with modulating voltage

i*/2 with respect

to modulating voltage dc

current density for

^dc current density for

V > 0 current density

at frequency u current density

at frequency u

current density

at frequency 2u» current density

at frequency 2u

Table II

Values obtained from circuit in Fig. 2 and from spectrum analyzer.

E = -506 mV vs. N.H.E.„ V = 35.4 mV r.m.s. at 25 Hz. Current

O • 2 density values in pA/cm r.m.s.

^dc ^dc

i

'

0) (jj /i /

CO 2co ^2w / 'S /

2co /3o) Circuit 186 238 152 161 221 0.1 32 32

Spectrum 215 31 7.3

Ratios i / i * dc^ dc

i ' /i;;'

(jj dc

/i5

dc

i' /i5

3tj dc

Theoretical

Found

1.24

1.28

0.76

0.82

0.176

0.17

0.0276

0.039

242/9
Fig. 1. Example of calculated curves. Electrode with anodic Tafel slope

of 60 mV/decade and cathodic slope of 120 mV/decade. Solid line:

d.c. current In the absence of alternating modulation. Dashed

line: d.c. current In the presence of alternating modulation,

amplitude 200 mV. Dotted line: faradalc component at frequency

3;lt. Dot-dashed line: faradalc component at frequency 6u.

Fig. 2. Imaginary Z" vs. real component Z' of the electrode Impedance.

Cu/H^SO^ f Sa^SO^. E^ -506 mV vs. N.K.E. Frequency as

parameter.

Fig. 4. R.m.s. current density vs. electrode potential for copper

In H.SO, Na.SO, with and without voltage modulation (V -

2 4 2 4 SO mV, V » 23 Hz). Solid lines are calculated values.

Fig. 5. Solid line: spectrum of current density for a Cu electrode

in H.,S0, + Na.,S0, . E

• -506 mV vs. N.H.E., V - 50 mV, . *

2 4 2 4 o

25 Hz. Scale is log (r.m.s. A/cm“) vs. Hz. Average over 128

spectra. Dotted line: spectrum of current through resistor.

Scale is log (r.m.s. A) vs. Hz. Average over 128 spectra.

Fig. 3. Slock diagram of the circuit uied for the measurements. 242/10
-100

10“* 10“S 10“* 10~^ 10'*

CURRENT DENSITY, r.m.t. A/cn*

Fig. 6. R.m.s. current density vs. electrode potential for Cu in H.,SO^ +

Na.jSO^, with and without voltage nodulation (V 125 aV, »

100 Hz). Solid lines are calculated values.

Fig. 8. Ratio between d.c. current In the absence and In the presence

of modulation, as a function of frequency. Cu/H^SO^ + Na^SO^.

E -456 mV vs. N.H.E., V 100 mV. Solid line Indicates

theoretical value.

Fig. 7. Ratio between various components of the faradaic current and

d.c. current in the absence of modulation, as a function of

the amplitude of modulation. Cu/H.SO, N'a.,S0, . E ® -506 mV

Z •* - 4 o vs. N.H.E. . » 103.3 Hz.

242/1
[ Published in "Proceedings of the International Congress on Metallic

Corrosion" ABRACO, av. Venezuela 82, Rio de Janeiro, RJ Brazil (1979),]

Detection and Analysis of Electrochemical Noise

foe Corrosion Studies

U. Bertoccl National Bureau of Standards Institute for Materials Research

Washington, D.C. 20234

, ABSTRACT

This paper describes the work done at the U.S. National Bureau of Standards for

the study of the fluctuations In current and potential of electrochemical systems, which

arc commonly referred to as electrochemical noise.

Since control of the electrode potential Is desirable In corrosion studies and

since commercial potentlostats tend to generate considerable noise that can often Interfere with the measurements, a battery-operated potentlostat was developed, with

a special amplifier for the detection of the current fluctuation. The results of the

testing of such an Instrument are reported. It was found that the Instability of the

control voltage generates a signal of the order of 5*10 ^ V//Hz, while the amplifying

system contributes a noise current of 3*10 A//Hz.

Two different experimental arrangements for the analysis and detection of electrochemical noise are described. With one of them, noise lower than that generated by the

Instrumentation can be measured.

The equipment described has been used for Investigating a number of electrochemical

reactions. One of them was the clectrodcposltlon of nickel. In this case, the noise

detected ap*''>rently was t.'lated to the change-transfer process.

Two systems of interest to corrosion science have been Investigated, iron and

aluminum, both In neutral solution. It Is shown that anodic polarization above the

pitting potential Increases the noise generated by a Fe electrode. In the case of Al,

addition of small amounts of chlorides, that are known to cause pitting, was found to

enhance the noise current by two orders of magnitude.

2010
INTKODUCTION

The ncasutement and analysis of the fluctuatlona In current and potential of

electrodes, which are generally called electrochenlcal noise, are receiving Increasing

attention as a means for the study of electrode kinetics [1], but are also being applied

to the field of corrosion, where the method holds promise to be an useful cool for

/ the detection of localized corrosion [2].

In the area of electrode kinetics one of the advantages Is that there is no need

to apply an external signal to the system under study. On the other hand Che development

of mathematical models for the Interpretation of the results Is still at an early stage

and the difficulties arc conslder?blc [1], [3].

In the field of corrosion, early work [4] had shown that a very simple experimental

arrangement was sufficient to show the correlation between the reduction In corrosion

rate caused by an Inhibitor and the decrease of the amplitude of the electrochemical

noise. With a more modem and complex Instrumentation there Is reason to believe that

the method can be quite powerful and susceptible of application outside Che laboratory

for the monitoring of corrosion In underground cables and pipes, metallic structures

and chemical plants.

At the present time there Is a need, however, for accurate laboratory measurements

In order to gather information about Che range of applicability of electrochemical noise

studies, and to collect a data base for Che theoretical understanding of the physicochemical processes Involved.

The purpose of this communication Is to describe the work done so far at KBS

for Che development of Instrumentation, and to present some experimental results

obtained.

INSTRL’MENTATION

Two methods are being used in our laboratory for Che analysis of the noise

generated by an electrode so as to obtain Information about Its frequency characteristics. The first consists In processing Che noise current at constant voltage or Che

noise voltage at constant current by moans of a spectrum analyzer. Since Che signals

to be studied are often very small, suitable amplification Is necessary. The basic

k J

2011
block diagram for chls kind of moasuremcnta In chc case of poccncloscaclc configuration Is shown In Fig. 1. For a galvanostaclc conf Iguratlooi a constant current power

auppl/ Is substituted for the potcntlostat

.

Since for corrosion studies It Is desirable that the potential of the electrode

be controlled, and since the analysis of noise often requires the processing of

signals of very small amplitude. It was concluded that a low-noise potentlostat with

characteristics better than those of commercial Instrument was necessary. Fig. 2, for

example, shows the voltage noise that a commercial potentlostat generates when qon5 netted to a purely resistive load. In the range between 0.25 and 50 Hz. Although for

most purposes the characteristic exhibited by such a potentlostat are excellent, the

noise level seldom exceeding 3uV, this Is still too high In many Instances for noise

studies. Also, since the instrument is a.c. powered, large peaks, about two orders

of magnitude greater than those shown In Fig. 2, can be found at 60 Hz and at its

harmonics

.

The potentlostat developed and built at NBS Is battery-powered, and In Its first

version has a maximum output of 5 mA. By careful selection of Its components, the noise

level generated by the instrument Is considerably lower than that shown previously.

Fig. 3 shows the noise voltage found across a resistive load up to 200 Hz. The .

figure shows that some Increase in noise occurs when the maximum current 5 mA is passing.

The two peaks at 60 and 180 Hz are due to a.c. pick-up, and con be further reduced by additional electromagnetic shielding. In the figure it is also shown the spectrum obtained when the potentlostat is switched off, which corresponds to the noise generated inside

the amplifying circuit. The noise introduced by the potentlostat is less than 10 times

the background.

The potentlostat, as schematically Indicated In Fig. 1, has built-in a special

amplifier for the detection of the f luctu.itlons of the current running through the

t electrochemical cell under study. The frequency range is between 10 mHz and 1 kHz,

which should cover most of the f luctu : .ns of interest In electrochemical systems.

Several spectra of the noise current obtained through this amplifier for resistive

loads of various values are shown in Fig. 4. The results can be summarized and described

< J

2012
^in the following way: the no^a* current potenttoetat is the siaa of

« current caused by the instability of the control sroltes« •“«* ®f • current gencrated Inside the a.c. aaplifier. ^

"

‘CV

I. (1)

As shown in Fig. 4 the noise current, which is ^rgely Independent of frequency

except at the very low frequencies where flicker noise predoninates, is inversely

proportional to the value of the load resistance t, that is

lev - Vev/F O)

where is the noise voltage in the control voltage generator.. This instability is

about S'lC"^ V//Hz, as can be deduced directly iron the spectrum of Fig. 3. When the

resistance R is larger than a few '<n, the noise current given by (2) becomes negligibly

small when compared with the intrinsic noise of Che amplifier, which la of the order

of 3*10‘“ A/.1I2.

The figures shown and the numbers reported here give an idea of Che limits of

detection of electrochemical noise which are sec by Che Instrumentation developed. The

experimental results, which will be discussed in Che next section. Indicate that this

instrumentation is adequate for a number of electrochemical systems. A graphic example

of the capabilities of the instrument is given in Fig. S. Here a SpV signal at 500 Rz

was added to the control voltage so as to generate a current of 10 ^A in a 500 R resistor. The spectrum shows how clearly such a small current signal stands above the background noise.

Tor noise amplitudes smaller than the background generated by tbe amplyflers, it is

necessary to separate the different sources. One such scheme is being eaiploycd in our

laboratory. This scheme, which is indicated in Tig. €, should be able, at least in

principle, to measure noise levels lower than Che noise of the instruments used to detect

it. It consists of two ident4.cal amplifying and filtering channels, which are connected

to two reference electrodes. The output of the two channels is then fed to a multplying

and averaging amplifier. Since the noise of tbe two asgtllfying systems, including that

produced by the reference electrodes, ia uncorrclatcd, its timi—average should be zero,

while the signal produced by the working electrode is present in both channels and will

3013
appear as a poelclve value (since It Is squared) at Che output. The mala drawback of the

system Is Its slowness, since one measurement at each frequency requires Che resetting

of the active filters and adequate time must be allowed for averaging the output. This

time can be quite considerable for low frequency measurements. The system, nevertheless,

has been used In some Instances. An example of Its calibration Is shown In Fig. 7.

Here the noise voltage from a 10 resistor was measured. Below 10 Hz the noise could

be detected although It was smaller, up to about a factor of 10, than Che Instrumental

noise. The large peak shown Is due to 60 Hz pick up and has no significance.

EXPERIMENTAL

Ve will describe here the results obtained on various electrode systems, which

have been examined to gain familarlty with the capabilities of Che InsCrumenCacion. Some

of these systems ace of Interest in Che corrosion field.

An electrochemical reaction examined was the deposition of Ni from Che Watts bath,

whose noise has been already studied [5]. The measurements have been carded out both

potentlostatlcally and galvanostaClcally . In Fig. 8 the spectra under galvanosCaclc

conditions show a slight decrease in voltage noise during cathodic deposition as compared

with open circuit. The potentlostatlc results of Fig. 9 show that the current noise Is Ch(

higher Che higher Is Che deposition current density. These results arc in agreement with

Che conclusion that the cliange-transf er resistance Is the principal source of noise and

that it decreases when the electrode Is cathodlcally polarized. No evidence of clecfrocrystalllzatlon noise could be obtained from these results.

Aiuong Che electrode systems of Interest in corrosion science the Fe electrode

In a neutral borate solution with a small amount of chloride added has been examined.

The voltage noise under galvanostatlc conditions wos much larger when Che electrode was

polarized anodlcally. As shown by the spectra In Fig. 10, the noise levels increased

by more than a factor of ten when the electrode was above the pitting potential. Correspondingly, observation of the volta?.c signal as a function of time revealed sudden

bursts of oscillations which were probably related to the formation of pits. Fig. 11

compares the relatively small voltage Instabilities at open circuit with the much larger

fluctuations above the pitting potential.

k .J

2014
'

effect of tMll eddltlone of NaCl, which la known to cause pitting. Fig. 12 ehowo tho

spectra obtained under potentiostatic conditions, in tho absence of NaCl. Anodic polarization up' to 1.3V vs SCE has only a ninor effect on the results. By cosiparison, the

apectruB obtained under cathodic polarization is also shown. The high noise level in this

case is believed to be related to the evolution of hydrogen bubbles.

Vfhen NaCl was added, although the noise spectrun at the corrosion poteetlal was

practically the same as that without chlorides, a snail anodic polarization was sufficient

to Increase the noise current by nore then two orders of magnitude, as ahown in Fig. 13.

This high noise level coincided «rith the fometion of pits over the surface. The noise

spectrua was found to consist in many cases of fairly well defined peaks, as shown, for

example in Fig. 16. Whether or not these peaks are specific of these conditions and

can be used to infer the onset of pitting cannot ha answered at the moment, and will require a much more extensive study.

CONCLUSIONS

In this paper, instrumentation suitable for the study of electrochemical noise,

particularly for systems of interest in corrosion, has been described. This instrumentation has a low level of intrinsic noise allowing the detection of relatively small

signals.

The results obtained on soae electrodes indicate that noise levels can vary quite

strongly as a function of the corrosion conditions, pointing to noise analysis as a new

and promising tool in corrosion science.

KZFEKENCES UJ V.A. Tyagal, Elektrokhimlya, 3, 1331 (1967).

C. Blanc, C. Cabclelli, M. Keddaa, Electrochlm. Acta, 22.687 (197S).

C. Blanc. I. Epelboln, C. Cabrlelli, M. Keddaa. J. Electroanal. Chem. 39 (1975).

(2] C. Okamoto. T. Suglta, S.. Nlshiyama, K. Tachlbana. Boshoku Cijutsu, 23. 639, 665

(1976).

[3] G. Blanc, C. Cabrlelli, M. Kcddam, C.B. Acad. Sci. Paris, C, 283 . 107 (1976).

C. Blanc, J. Epelboln, C. C.abrlclll, K. Keddaa, J. Electroanal. Cbem. IS, 97 (1977).

(6J W,. P. Iverson, J. Electrochcm. Soc. m, 617 (1968).

|SJ C. Cabrlelli, M. Ksourl, R. Viart, J. Electroanal. Chem. 86, 233 (1978).

C. Blanc, C. Cabrlelli, M. Ksourl, R. Ulart, Elcctrocliim. Acta, 23, 337 (1978).

k J

3015
S^CTIUI.

»M*».ITO0e.

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4 r

Fi-. 1. Schematic diagram of circuit for potentiostatic noise studies.

lig, 2. Noise spectrun of commercial potcntiostat.

Fig. 3. Noise voltage across

resistor. J

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» lA *

'

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• c g*|ssl $ If * « t • s f'SIf IMf*Af • « 4 t tsvt 2SI iggcitg

L_ _ _

fKOUEICT. Ml

Fig. 4. Effect of load resistance on noise current.

Fig. 5. Saall amplitude current

signal superimposed to noise background.

Fig. 6. Schematic diagram of circuit for measuring low noise signals.

2017
rig. 7. Noise generated by the Fig* Noise spectra for elcctrodcposition

instrumentation and by of nickel. Potentiostatic conditions a 10 k & resistor.

Pig. 9. Noise spectra for electro- deposition of nickel. Potentiostatic conditions

Fig. 10. Noise spectra for the corrosion

of Fe in neutral borate solution

2018
F«/Hj1O3-W»2»4Ot*0 01 "» **«CI r

Al (••I

rig. 11. Oscillographic record of Fig. 12. Koise spectrua of Al in electrode potential of Fe in H3§03-Na2B40y

1:1 H3BO3 - NaaB^OT O.Ol N

NaCl above and below the pitting

potential

rig. 13. Effect of NaCl on noise

spectrum of Al. rig. 14. Peaks in noise voltage of

Al in Boric • Borate NaCl.

301t
r

DISCUSSION

i

(

Q: Hideo Kitagawa ^ Wnai kinds of noises from corrosion neasure in the practical dirty

surfaces, solution with impurities, etc.)

A: U. Bertocci-We hope to be able to use the technique under a variety

of conditions. Tlie noise measured should be characteristic of the corrosion

process in any environment being examined.

Q: Tatsuo Ishikawa - Sapporo, Japan

1) When Al in boric acid borate solution is cathodically polarized at -2.4V, kdiat is occuring on the Al surface that causes a high level of noise?

2) If you have, I would like to know the data for stainless steel in HjSO. Cl~ system. If not, what results could you expect to the noise

spectrum of this system?

A: U. Bertocci

1) Probably hydrogen evolution.

2) Electrochemical noise on stainless steel has been studied by Okamoto

and coworkers [Boshoku Gijutsu, 439, 445 (1974)].

Q: M. Froment - Paris, France

You have found in the case of anodic polarization of aluninua different

behaviors according to the presence or absence of chloride. It is possible to observe a sliarp maximum of the level of noise at 500 Hz. Have you a electrochesiical model to explain the result?

A: U. Bertocci - The apparent peak at about 600 Hz is an artifact of the instrumentation. In the version used for these studies the a.c. noise

in the control voltage of the potent iostat was being filtered out above about 500 Hz. As a consequence, the current response which was increasing

with frequency because of the impedance rolloff due to the double layer

capacitance, started decreasing beyond -veoo Hz, thus creating the ap- pearance of a peak. We have since extended the frequency range wherf

control voltage noise is about constant to about 5 kHz.

V :

2020

**''*^

.
[Accepted by the Journal of the Electrochemical Society]

Applications of a Low-Noise Potentiostat

in Electrochemical Measurements

Ugo Bertocci

Chemical Stability and Corrosion Division

National Bureau of Standards

Washington, D.C. 20234

Abstract

Measurements on two electrochemical systems, copper in copper sulfate

and aluminum in boric acid: sodium tetraborate buffer with and without

chloride added, have been carried out employing a low-noise potentiostat

developed and built at MBS by recording the amplitude spectrum of the

fluctuations in the current density. In the case of copper, the current

spectra resulted to be the deterministic response of the electrode to the

noise voltage generated by the potentiostat. The electrode characteristics

for charge-transfer and for diffusion could be obtained from the impedance

plots derived from the measurements when the level of the applied signal was

of the order of 10*^ V. In the case of aluminum, the deterministic response

observed in the absence of pitting was overcome by random fluctuations in

the current in conditions leading to pitting. It is shown that the onset

of pit formation can be detected from noise measurements. The significance

of the information obtained in electrochemical noise measurements is briefly

discussed.
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Applications of a Low-Noise Potentiostat

in Electrochemical Measurements

Ugo Bertocci

Chemical Stability and Corrosion Division

National Bureau of Standards

Washington, D.C. 20234

INTRODUCTION

The study of random fluctuations in current and potential of

electrodes, usually known as electrochemical noise, is receiving

increasing attention, and interesting applications are being envisaged

in the field of corrosion [1]. In these studies, it is often necessary

to detect and measure very low amplitude signals, and it is therefore

very important to reduce the noise generated by the measuring instruments.

Since in most electrochemicc > studies it is desirable to control the

electrode potential, a low-noise potentiostat is a particularly

useful device [2].

Elsewhere [3], the circuit and performance of a potentiostat

designed and built at NBS have been described. The purpose of this

communication is to report on measurements carried out on some

electrochemical cells, taking advantage of the characteristics of

such a potentiostat. These measurements are not restricted to the

detection of noise, but concerns also the observation of the current

response to very small voltage signals.

EXPERIMENTAL PROCEDURES

The measurements were carried out with the apparatus shown

schematically in Fig. 1. All the instruments inside the dot-dashed

enclosure are battery operated, and the enclosure represents electromagnetic shielding as well as some protection from mechanical vibrations.

The potentiostat provides a low-noise dc control voltage and for the

measurement of the dc current. A built-in ac amplifier is employed

for the detection of the fluctuations in the current in the frequency

range approximately between 0.1 Hz and 2 kHz. The cell is provided

with two reference electrodes, a low impedance one, which is used as

a sensor for the potentiostat and for the measurement of the voltage

noise, and a second one, often a SCE, employed for the monitoring of

the dc electrode potential. The values of the dc voltage and current

are recorded on a two-channel recorder.

An additional voltage signal can be added to the dc control

voltage. In this work, the superimposed voltage consisted of a constant

amplitude, swept frequency signal, varying linearly over a factor of

ten. The repetition rate was slightly less then the time of acquisition

of a spectrum by the spectrum analyzer.

The frequency analysis of the output signal, either the current

or the electrode potential depending on the position of the switch

shown in Fig. 1 was carried out by a spectrum analyzer with a frequency
resolution of 1/200 of the range. In the 50 Hz range, for example,

the spectrum consisted in 200 values at intervals of 0.25 Hz. The

acquisition time for one spectrum is equal to one period at the

lowest frequency, that is 4 seconds for the range mentioned above.

The input waveform to the spectrum analyzer as well as the

instantaneous and average spectra were observed continuously on the

oscilloscope. Average spectra were then recorded on a X-Y recorder,

either on a linear or on a logarithmic frequency scale.

The time involved in acquiring the average spectra in the low

frequency ranges can be considerable. In the 5 Hz range, each spectrum

requires 40 seconds, so that for an average over 64 spectra, the

acquisition time is of the order of 40 minutes.

EXPERIMENTAL RESULTS

The results presented here concern two quite different electrode

systems, one having low and the other high resistance to the faradaic

current.

As a low impedance system, the reaction between Cu metal and a 0.5 M CUSO4 + 0.1 M H2 SO4 solution was examined. Figs. 2 and 3 show

the voltage and current spectra, respectively. The reference electrode

was also copper, and the data were taken in conditions of zero dc

current. The spectra were taken also when a swept frequency signal

was added to the dc control voltage. The amplitude of the signal as shown in Fig. 2 was about 10 to 100 times the background noise but

never larger than about 2 pV. Fig. 3 shows the current response with

and without the superimposed voltage signal.

The impedance of the electrode /Z/ can be obtained as the ratio

of voltage to the current at every frequency, and the results are

shown in Fig. 4. These ratios were calculated after subtracting from

each value the instrumental noise, which had been previously measured

[3]. The points in Fig. 4 obtained with and without the superimposed

voltage signal are very close. This is because the electrode impedance

is rather small, and therefore the instrumental noise in the current

density is small compared with the recorded value of the current.

The increased signal-to-noise ratio obtained by a larger voltage

signal has little effect on the accuracy of the results.

The results have been fitted to an equivalent circut (shown in

the inset in Fig. 4). The line drawn in Fig. 4 shows /Z/ as calculated

from this circuit, using the following values: ohmic drop resistance

Rj2=5 n, charge-transfer resistance R^t = 15 n, equivalent to an exchange current Iq for the rate-determining step [4] Cu'*"*' + e~ ^

Cu'*' of 0.42 mA/cm^; double layer capacitance C = 35 pF/cm^.

t

(

ii

-2-
Since one of the species involved in the rate-determining step,

Cu^, is present only in low concentrations, a significant Warburg

impedance is present at low frequencies, the value of the Warburg

impedance

RT

**F*(Cu'^)''2iJ)cu+

I -5 2 + -4

' was calculated choosing 0^^+ = 1.38 10 cm /sec and Cu = 2.9 . 10 M.

II Most of these values are in excellent agreement with the literature;

|! for instance, the exchange current density [5] and the diffusion

|| coefficient of Cu+ [6]. The Cu'*' concentration that gives the best

I fit is about 80% larger that, that calculated from the reaction

l| , + Cu = 2 Cu"^

! whose equilibrium constant is 7.10“^ [6] [7] and taking the activity

coefficient of 0.5 M CUSO4 as 0.062 [8]. The value of the Cu

concentration affects the magnitude of the Warburg impedance and

therefore that of /Z/ only at low frequencies, where the reliability

' of the experimental points, as shown by the error bars in Fig. 4, is

not as good as those at higher frequencies.

The second system examined was 6061 Aluminum in a 1:1 mixture of

saturated H3BO3 and Na2B407 with and without addition of 0.05 M NaCl

The current and voltage spectra in the solution not containing chlorides

are as given in Fig. 5.

;| In the chloride-containing solution, which is known to cause

pitting if the potential is raised approximately above -700 + 50 mV

vs SCE [9], the current spectra obtained below and at the pitting

potential are reported in Fig. 6 together with the voltage spectrum. «

i During the recording of the spectra, the dc current was about 10 uA/cm'^

i cathodic at -800 mV, while at -650 mV, it became anodic, increasing

I slowly to about 50 yA/cm^.

•')

! The impedance plots for this system .are given in Fig. 7. Here

il again the onset of pitting is marked by a fundamental change in the

I impedance plot up to 1 kHz.

DISCUSSION

The results shown for the Cu/CuS04 electrode indicate that

il random fluctuations in electrode characteristics are so low that they

. do not affect significantly the current response to the broadband

I noise voltage signal. For this reason, the impedance plot shown in

I Fig. 4 can be accounted for satisfactorily by the deterministic

- 3-
response of the circuit drawn, from which reliable values concerning

the kinetics of the electrode can be obtained. Therefore, although

the intrinsic noise of the control voltage is still too large in this

case for the detection of random processes at the electrode, good

measurements can be carried out even for signals below 0.1 yV,

detecting currents in the nA range.

In the case of aluminum in boric:borate solution, when pitting

does not occur, random fluctuations are below the limits of detection,

and the electrode response is largely that of a capacitor of about

0.3 yF/cm2. The data of Fig. 7 seem to indicate a larger capacitance

in chloride containing solution, where thinning of the film can occur

[10]. Above the pitting potential, the noise current increases more

than two orders of magnitude, and the Impedance plot in Fig. 7 cannot

be explained other than caused by random fluctuations of the electrode

current, independently of the voltage signal applied.

The detection of this current noise is a good indication of the

beginning of pitting, and the large fluctuations are probably due to

hydrogen evolution from the pits [11]. The onset of the large current

noise has shown to be reproducible within a range of 10 to 20 mV.

The noise current observed is the sum of several contributions

since the fluctuations of the control voltage interact with the

random variations of the electrode resistance and capacitance causing

intermodulation terms. They are, however, much smaller than the

contributions dueto the fluctuations of the dc current, which are Independent of the control voltage noise. Since the dc current,, upon

pitting, increases to several yA/cm^, the noise current observed can

be accounted for by oscillations of the order of a few percent of the

average dc current.

More difficult it is to estimate the possible contribution of

fluctuations in the double layer capacitance; they can be caused by

changes in surface area as well as changes in the thickness of the

protecting film, a more likely case for A1 in neutral solution. The

current generated is

where Ezc is the potential of zero charge. If the capacitance varies

by a fraction k at angular frequency ai, that is

C=Cq (i+k sinu)t)

The current at that frequency is

i =(E-E,^)wC k coswt

c zc 0

-4-
The potential of zero charge for A1 is at about -0.75 V vs. SCE [12],

therefore lE-E^cl can be estimated at being of the order of 0.1 + 0.2

V. Since Cq = 0.3 yF/cm^, for k=l , the rms value of the capacitative

noise current v/ould be about 1.3 • 10"^A at 1 Hz and 100 times larger

at 100 Hz. Since the experimental value of the current decreases

with frequency and is about 10"°A at 100 Hz, it is safe to conclude

that the higher frequency fluctuations of the capacitance are very

small, but it is not possible to unambiguously assign the noise

observed to variations in the faradaic rather than in the capacitative

current.

The results presented show that reduction of the instrumental

noise is essential for the study of random fluctuations and that it

can be quite useful in all circumstances allowing measurements with

very little perturbation of the systems under investigation. The

fact that deterministic measL 'ements can be done using the almost

white noise in the control voltage [13] has, however, important

implications for a proper understanding of the current spectra when

"noise" measurements are attempted. Only if the data cannot be

explained by assigning constant values to the components of the

equivalent circuit, it is appropriate to describe the signal observed

as electrochemical noise.

- 5 -
V n

^ ?6 CC'" I^ 10 } S^'Tfint? frlil^r TO fVrQ«|»7eq 9H ^ "• 0‘> r.O 4^i94 ^

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oriT ,f*ot3ii^ri|i9Vrt^ ‘ij ^.rs^U od.r?n^2t)*9«\)6t t)|fi«FAFirf9}9b icrf^ JOttV'

aofixioqwr ,tovsv<oH ,e^ fo’tanoa srix ftf oaFon aJhtv# . -nortw xndrwo artX Tseqoi*<J’'8M4^'^difl’;fiairqniF ^ ^

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tAf ijn:V‘ * Vv’v{^i‘^tl 0n of

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4V*«^' 'v - L*.itji.f<T$ u;

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: i>
REFERENCES

[1] W. P. Iverson, J. Electrochem. Soc., 115 , 617 (1968).

6. Okamoto, T. Sugita, S. Nishiyama, K. Tachibana, Boshoku Gijutsu,

23, 439, 445 (1974).

G. Blanc, I. Epelboin, C. Gabrieli i, M. Keddam, J. Electroanal. Chem.,

45, 97 (1977).

U. Bertocci, 7th International Congress on Metallic Corrosion, Rio de

Janeiro, 1978, in press.

[2] I. Epelboin, C. Gabrieli i, M. Keddam, L. Rail Ion, J. Electroanal.

Chem., 9]_, 155 (1978).

[3] U. Bertocci, R. W. Shideler, NBS J. Res., in press.

[4] £. Mattson, J. O'M. Bockris, Trans. Faraday Soc., 55, 1586 (1959).

[5] A. Damjanovic, T.H.V. Setty, J. O'M. Bockris, J. Electrochem. Soc.,

113 , 429 (1966).

U. Bertocci, C. Bertocci, B. C. Larson, J. Cryst. Growth, 13/14 , 427,

(1972).

D. Posti, G. Eichkorn, H. Fischer, Z. Phys. Chem., (NF), 77, 138 (1972).

[6] L. M. Gedansky, E. M. Woolley, L. G. Hepler, J. Chem. Thermod.,

2, 561 (1970).

[7] CODATA Bulletin 28, Internat. Council of Scientific Unions (1977).

U. Bertocci, D. R. Turner, in "Encyclopedia of Electrochemistry of the

Elements", A. T. Bard, ed., Vol . II, p. 387, M. Dekker, New York (1974).

[8] R. A. Robinson, J. H. Jones, J. A.C.S., 959 (1936).

[9] M. Pourbaix, Corrosion, 2^, 431 (1970).

J. R. Galvele, S. M. de DeMicheli, Corr. Sci., 10_, 795 (1970).

K. Nisancioglu, H. Holtan, Corr. Sci., 18, 835 (1978).

[10] M. A. Heine, D. S. Keir, M. J. Pryor, J. Electrochem. Soc., 112 , 24 (1965).

[11] C. B. Bergeron, R. B. Givens, J. Electrochem. Soc., 124 , 1845 (1977):

Corrosion Res. Conf., NACE, Atlanta (1979).

[12] S. Trasatti, J. Electroanal. Chem., 33, 351 (1971).

G. Blanc, I. Epelboin, C. Gabrielli, M. Keddam, Electrochim. Acta,

20, 599 (1975).
Figure Captions

Fig. 1. Experimental circuit for the recording of current and voltage

spectra under potentiostatic conditions.

Fig. 2. Voltage spectra recorded under potentiostatic conditions (Ij =0) on Cu in 0.5 M CuSO^ + 0.1 M H2SO4.

Fig. 3. Current spectra recorded under potentiostatic conditions (I . =0) on Cu in 0.5 M CuSO^ + 0.1 M H^SO^.

Fig. 4. Absolute impedance |Z| vs. frequency for Cu in 0.5 M CuSO^ +

0.1 M HpSO^ obtained from current and voltage spectra. Solid line

calculated^ for circuit shov/n.

Fig. 5. Current and voltage spectra recorded in potentiostatic conditions

(E=-330 mV vs. SCE) on A1 in H2B02:Na2^407 solution.

Fig. 6. Current and voltage spectra recorded under potentiostatic conditions

below and at the pitting potential on A1 in H-BO^iNa^B.O^ +

0.05 M NaCl.

J ^ ^ ^ /

Fig. 7. Absolute impedance |Z| vs. frequency for A1 in H-BO-,:Ma«B.O-,

with and without chloride added.
Fig.
Voltage, V/vlHz o o o

00 6>
Frequency,

Hz

Current Density, A/CM2 ^Hz

oI O

zOt
P -STi

ZH ‘Aouanbajj

cOI- zOI- 01^ I- lOt

Absolute

Impedance

|Z|,

Q

CM^
Frequency,

Hz

Voltage Amplitude, V/viHz

(O 6
Voltage,

V/\/Hz

9 *STi

Current

Density,

A/CM

VHz
105

104

103

102

10

1

1 ^

“1

AM

A

^ "a

A A*<] i J

A.A

— -330 mV vs. SCE No Chloride

i .800 mV .8. SCE-,

„ g|

-650 mV vs. SCE J

°

^ ^1 10-1 1 10 102 103

Frequency, Hz

Fig. 7
r.

.9

'r

l-y

*

<

^,i

*oo a

A

•' A

'•a a

A

*** '^ -^•***>1

" .

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A obholrtO oM A §9,?>av Vf^et^

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w30S .8v Vm 038" o

u v^.

^or

j.

'

:

Q a

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-aor

or

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X

JL

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,

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• I
[Reference: NBS Journal of Research 85, 211-217 (1980).]

A Low-Noise Potentiostat for the Study

of Small Amplitude Signals in Electrochemistry

U. Bertocci

Corrosion and Electrodeposition Group

Center for Materials Science

and

R. W. Shi del er Special Analytic.! Instrumentation Group

Center for Analytical Chemistry

National Bureau of Standards

Washington, D.C. 20234

INTRODUCTION

The ability to detect and measure small amplitude signals in

electrochemistry can be very useful when there is a need to minimize the

input signal in order to perturb as little as possible the electrode/

electrolyte interface as well as to decrease unwanted effects such as the ohmic drop error, and when the aim of the research is to study the

spontaneous fluctuations in current and potential, which are commonly

referred to as electrochemical noise. This area of endeavor is re- ceiving increasing attention in the field of electrochemistry [1], and

it is being applied for the detection of corrosion [2]. One of the

problems encountered in these studies is caused by the need of controlling the potential of the electrode without causing an unacceptably

high level of interference from the noise generated by the potentiostat

[3j. In order to improve its performance with respect to commerically

available instruments, a low-noise potentiostat was designed and built,

whose characteristics will be described in the following sections.

INSTRUMENT CHARACTERISTICS

The low-noise potentiostat was designed and developed by the

Special Analytical Instrumentation Group of the Center for Analytical

Chemistry at the National Bureau of Standards.

The design has exceptional low noise and stable low frequency

performance as well as moderately wide bandwidth.

The characteristics of the potentiostat include:

a. Highly stable operation over a wide range of loads with up

to + 5 niA output.

b. A useful frequency range from DC to greater than 2 kHz.
c. A precision low noise voltage reference source variable

from 5 V, having a stability of better than 10 yV.

d. A fast recovery, low-noise, ac current signal amplifier

with a frequency response of between 5 x lO”^ and 2*10^ Hz.

Additional features include an external input to allow small signal

modulation of the internal voltage reference and the ability to be used

as a constant current source or galvanostat. The circuit scheme is

shown in Fig. 1.

DESIGN AND CONSTRUCTION CRITERIA

'

In order to achieve the characteristics desired, the design was chosen so that the working electrode was grounded, facilitating the

shielding of stray signals. To reduce capacitative currents across the

reference electrode, the lead shield was maintained at the same potential,

effecting a guarded input circuit. The high impedance guarded input

amplifier is followed by the potentiostat loop amplifier and an output

current buffer. (Total noise referred to input is less than 3 yV rms.)

The frequency response of the circuit crosses unity gain at approximately six kHz. Immunity to oscillation for all values of cell R and C

was secured by tailoring the frequency characteristics of the feedback

amplifiers. No provision to compensate for cell parameters at high

frequencies is included. A differential amplifier is used to extract

the dc current signal from the potentiostat current driver. Signal

output is from_an ac coupled amplifier having a very low frequency

cutoff (5 X 10”2 Hz) and exhibits extremely low noise and prompt overload recovery. It has three gain settings up to a maximum gain of 1000.

For ac components within the frequency range, an ultimate resolution as small as ‘3 nA (3 mV output) can be measured. A fast recovery circuit

brings the amplifier into its linear range within a few seconds for any

overload.

Power *,s supplied by a pair of rechargeable 12 volt gelled lead

acid batteries to assure a flat and constant discharge as well as complete freedom from ac pickup. All amplifier circuits make use of

high performance operational amplifiers. The amplifier chosen. Precision Monolithics OP-07, is a device having excellent noise and drift

performance with long-term drift stability approaching that of chopper

stabilized amplifiers.

A double box construction was used to reduce ambient thermal effects and improve immunity to electrical noise. Resistors used were

glass metal-film, and all capacitors used in active circuit locations

were of polystryrenc foil construction. All loose wiring was mechanically secured by using dabs of silicone rubber sealant to reduce vibrations causing microphonics.

-2 -
PERFORMANCE CHARACTERISTICS

The potentiostat is employed as a part of the circuit shown schematically in Fig. 2. For testing purposes, the cell was substituted

with a passive load, which was either a pure resistor or a resistor and

a capacitor in parallel. With the switch in the upper position the

fluctuations in the current were analyzed, while with the switch down,

the noise voltage applied to the load was recorded. If desired, either

a constant or a swept frequency signal of small amplitude was superimposed to the control voltage. All spectra shown in this work were

recorded in four sections, over the 5, 50, 500, and 5,000 Hz range. The

spectrum analyzer has a frequency resolution of 1/200 of the range,

requiring a time of 40, 4, 0.4 and 0.04 s respectively for each spectrum. All recordings are averages over 64 spectra for the lowest range,

and 256 or 512 spectra for th higher frequency ranges. All data given

are rms values.

Fig. 3 shows the noise current for three values of the load resistance at zero current, as well as at 1 niA of dc current. Fig. 4

shows the noise voltage Vf^ for any resistive load at 1=0 and 1=1 mA, as

well as the noise of the amplifier with the input shortcircuited.

Subtracting this noise from the upper curve (at 1=0) gives the dashed

line, which represents the noise content V^y of the control voltage of

the potentiostat in the frequency range of interest

This no.ise is largely independent of frequency above 1 Hz and is

about 2.5 • 10’® V//Hz. When applied to a resistive load this voltage

signal generates the noise current shown in Fig. 3. The noise current observed is the sum of the current I generated by the fluctuations in the control voltage and of a noise^signal produced by the

ac amplifying stage in the potentiostat.

When the resistive load R is sufficiently low (<500 o), I

P

redominates, as shovm by the two upper curves in Fig. 3. When'^R is

arge, however, ^

-v l^, as shown by the lowest curve of Fig. 4. Ia

is also largely independent of frequency above 1 Hz and results to be

about 3*10’i^ A//HZ.

The potentiostat was also tested on a circuit formed by a lOQ resistor

in series with a parallel combination of a 5kll resistor and a 5yF capacitor

-3 -
(as shown In the inset in Fig. 5). In this case the load impedance is frequency

dependent. The voltage and current spectra are given in Fig. 5. Taking the

ratio between these values at the same frequency should give the absolute

value of the impedance |Zl

|Z|((o) .

However, as shown before in the case of a resistive load, impedances

greater than several hundred ohms are too large to give a current significantly

above if one relies only on the control voltage noise. Because of

the low intrinsic noise of the potentiostat, the addition of a very low

amplitude swept frequency signal to the control voltage was sufficient

for obtaining good impedance values. The voltage and current spectra

obtained are shown in Fig. 6, and the plot of /Z/ vs. frequency from

which the value of the circuit components can be obtained, is shown in

Fig. 7. These measurements were obtained applying no more than a few

tenths of a mV (in the lov/est frequency range) and generating currents

less than 100 nA. The possibility of measuring at low amplitudes can be an important advantage when, otherwise, distortions or irreversible

changes in the electrochemical system could be produced.

PERFORMANCE AS A 6ALVAN0STAT

As mentioned before, the instrument can be switched in a configuration where it supplies a constant dc current; up to 5 mA, provided

the total load does not exceed 5 V. The ac content of the potential

difference between working and reference electrode appears at the output

of the ac amplifier.

Recordings taken with a resistor as load are shown in Fig. 8. The

noise voltage, in a manner analogous to the potentiostatic case, is the

sum of the noise generated by the instability of the control voltage

v/hich in this configuration becomes a noise current through the

load, and of the amplifier noise V^.

( 4 )

Rj is the internal resistance (Rj = 1 to) on v/hich V<-y acts to

determine the current, and Z((u) is the load impedance, which in this

case, is a resistance R. In curve A of Fig. 8, the resistance is zero,

and therefore Vm = V/\. The noise in the control voltage can then be

.obtained from (4), and it is close to that obtained in the potentiostat

configuration. Curve 0 shows some increase in noise when a dc current

Is being drawn.

-4 -
REFERENCES

[1] V. A. Tyagai, Elektrokhimiya , 1331 (1967).

G. Blanc, C. Gabrieli i, M. Keddain, Electrochcm. Acta, 687 (1975).

G. Blanc, I. Epelboin, C. Gabrielli, M. Keddam, J. Electroanal. Chem.

,

§

2 , 59 (1975).

G. C. Barker, J. Electroanal. Chem., 8^, 145. (1977).

C. Gabrielli, M. Ksouri, R. Wiart, J. Electroanal. Chem., 8£, 233 (1978).

[2] W. P. Iverson, J. Electrochem. Soc., 115 , 617 (1968).

G. Okamoto, T. Sugita, S. Nishiyama, K. Tachibana, Boshoku Gijutsu,

23, 439, 445 (1974).

G. Blanc, I Epelboin, C. Gabrielli, M. Keddam, J. Electroanal. Chem.,

97 (1977).

U. Bertocci , 7th International Congress on Metallic Corrosion, Rio

de Janeiro, 1978, in press.

[3] I. Epelboin, C. Gabrielli, M. Keddam, L. Raillon, J. Electroanal. Cfiem.

,

91, 155 (1978).
FIGURE CAPTIONS

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6

Fig. 7

Fig. 8

Schematic circuit of the potentiostat.

Experimental arrangement for recording current and voltage

spectra.

Current spectra recorded in potentiostatic conditions on different resistive loads. Solid lines: I. = 0. Dotted

line; I^^ = 1 mA.

Voltage spectra. Upper solid line: potentiostatic conditions,

1=0. Dotted line: same, 1=1 mA. Lower solid line: noise

from amplifier with shortcircuited input. Dashed line:

Difference between solid lines.

Current and voltage spectra recorded under potentiostatic conditions

on circuit shown in inset.

Current and voltage spectra recorded in potentiostatic conditions

on the circuit shovm in the inset in Fig. 5. Swept frequency

signal superimposed to the dc control voltage.

Absolute impedance /Z/ vs. frequency for circuit shown in inset

in Fig. 5. Experimental points obtained from spectra in Fig. 6.

Solid line: calculated values.

Voltage spectra recorded in galvanostatic conditions. Curve A:

R = 0, 1=0, Curve B: R = 500 q, I = 0, Curve C: R = 1 Kn,

1=0, Curve D: R = 500 q, I = 3 mA.

t

Fig.

to
5

kQ

Noise Current, A/VHz

10-8
Frequency,

Hz

Voltage Amplitude, V/\^
Frequency,

Hz

Voltage Amplitude, V/\/Hz

o

00

O

6) O

cn
Frequency,

Hz

Fig.

6

Voltage Amplitude, VIVHz

o• oI ot o• cn ^
i

I
0

Q

o

OO CD<

CO

CM

(O

O

N.

o

zha/A ‘aSejiOA

10-8
[To be published in Proceedings of the ASTM Symposium

•’Progress in Electrochemical Corrosion Testing ]

Corrosion Enhancement Due to Large Voltage Modulations.

Frequency Analysis of the Response of Electrodes Under Charge-Transfer

Control

U. Bertocci and J. L. Mullen

Chemical Stability & Corrosion Division

Center for Material Science

National Bureau of Standards

Washington, D, C. 20234

1 . Introduction

In a large number of practical situations metallic structures are

subjected to alternating voltages which cause currents to flow into

ionic conductors in contact with them, the most common of which is soil.

These currents might flow because of failure of insulation, but there

are also cases where the metals are exposed without protection. There

is, therefor?', a substantial interest in understanding what role an

alternating iignal can play in affecting the kinetics of an electrode,

so as to be ab ? to predict when and to what extent corrosion can be

caused by the ffect of alternating currents. [1]

As a part of research carried out at NBS concerning the corrosion

of copper concentric neutral cables, the conditions under which ac

enhanced corrosion could be expected were examined, and, as a con-

¥ sequence, a mathematical treatment was developed describing the current

response of an electrode under charge- transfer control to a large sinusoidal

modulation of the voltage. The possibility of measuring separately the

faradaic and the capacitative components of the current was also examined,

leading to the identification of the conditions necessary for such a

separation [2].

The theoretical treatment showed the potential usefulness of

frequency analysis for the recovery of information concerning the

mechanism and rate of the electrode reaction, as well as data concerning

the enhancement of the corrosion process.
In this paper the principal points of tho mathematical treatment

are briefly reviewed. The effects of ohmic drop error on current

measurements are also discussed. Finally frequency analysis techniques

are applied to an electrode under charge-transfer control and the

experimental results are compared with the theoretical predictions.

2. Mathematical Analysis

a. Faradaic Current

We will present here in a simplified form some of the results of

the analysis which has been given in detail elsewhere [2]. For an

electrode under charge-transfer control, the partial anodic or cathodic

current can be expressed as

where and A are positive for anodic processes and negative for

cathodic ones. E is the electrode potential, taken with respect to some

convenient reference value. As is well known, (1) can be used to

express the total current in the so-called Tafel region, that is when

the partial current of opposite sign is very small.

If the voltage is sinusoidally modulated, so that

the expression for the faradaic current can be obtained, and by harmonic

analysis the current can be separated into a constant (or dc as customarily called) component and into the various harmonics (of angular

frequency u, 2a>, . . . nw) of the modulating signal.

It can be shown that the amplitude coefficients of each of these

components can be expressed in the form

^dc

'

^0 (1)

I E ( 2 ) = + Vsin tut 0

'f.n

=

'<, (3)

which can be rewritten as

( 4 )

- 2 -
because, in the absence of a modulating signal, (2) becomes ^

therefore

Introduction of (5) into (3) gives (4). The r.m.s. value of each of

these sinusoidal components of the faradaic current is

Eq. (4) shows that every frequency component of the faradaic

current is proportional to the current in the absence of modulation ij^

multiplied by a coefficient whose value depends only on the kinetic

parameter A (which determines che Tafel slope) and on the amplitude V of

the sinusoidal modulation. The index n indicates the order of the

harmonics, with zero designating the dc term. An obvious consequence of

(4) is that all frequency components of the faradaic current, when

plotted against the electrode potential on a semi logarithmic scale,

will exhi'iit the same Tafel slope as ij^.

Additional information can be derived from the analysis of the

phase relationships among the various current components. It can be

shown that the components of the faradaic current of odd order (w, 3uj,

5w, . . .) bear alternately a 0 or _+ tt phase relationship with the

modulating signal, while the phase of the even (2ai, 4oj, 6u), . . .)

components is shifted by with respect to the modulating signal.

Reflecting the fact that the relaxation times for charge- transfer

are extremely short, the factors do not depend on the frequency of

the signal, and therefore, the amplitude of all the current components

is independent of the frequency. It can also be shown that F^, the

factor for the d.c. component of the faradaic current, is always greater

than 1, so that a sinusoidal modulation of the voltage produces a

rectified current that increases the rate of the electrode reaction.

iJc = Iq exp (AE^l (5)

( 6 )

- 3 -
As it is well known, the coefficient A is related to the Tafel

slope b by the equation A = Therefore, since the factors

depend on A, if the anodic and cathodic reactions are associated with

different Tafel slopes, their intersection, which determines the

potential at which no dc current flows, will be shifted by the application of a sinusoidal modulation.

b. Capacitative Current

For the analysis of the effect of an alternating voltage on an

electrode it is necessary to consider the capacitative current i^ as

well, since the electrode-solution interface constitutes a circuit with

a capacitance and resistance in parallel. Therefore, the total current

is

1 = If +

1, (7)

The electrode capacitance in general varies with potential and it is

strongly affected by adsorption phenomena, some of which can be rather

slow, so that relaxation times can be large compared with variations in

electrode potential.

However, in many instances the capacitance is only a function of

% potential, and often its variations can be relatively minor within

certain potential ranges. The analysis has shown that if the electrode

capacitance is a function of potential only, a sinusoidal modulation as

In (2) does not cause any dc component of i . Moreover, it can be shown

that its odd harmonic components (w,' 3o), bw, . . .) are shifted by

with respect to the modulating signal, while the even components are

shifted by 0 or TT.

Comparison of these phase-shifts with those of the faradaic current

shows that when the hypotheses employed in this analysis are applicable,

it is possible to separate the faradaic and capacitative components of

the current, provided that phase information is available.

- 4 -
c. Ohmic drop effects

The theoretical treatment assumes that no ohmic drop error is

included into the circuit to which the voltage given by (2) is applied. •

In practice, however, is never zero and this can affect substantially

the values measured experimentally, causing a frequency dependence when,

according to the theory, none should be found. A careful analysis of

the effects of the ohmic drop is therefore necessary in order to examine

to what extent it can explain some apparent discrepancies between

theoretical predictions and experimental results.

The response of the working electrode to the voltage signal E given

by (2) can be described, neglecting the phase relationships between

different frequency components, by the equivalent circuit of Fig. 1, to

00 which a new voltage signal E' = E^ + Vz^ sin (kwt) is applied. Each

0 ^ K frequency component of the current is then separated by a tuned filter

F before dividing into a faradaic component through a resistor R^^ (with

n indicating the order of the harmonics) and a capacitative component

through a capacitance C^. The constant, or dc, component obviously has

no capacitative counterpart. All current components will flow through

the ohmic drop resistor R. '

The value of the resistances R^ can be obtained from (3), since R^

is the derivative of E^ with respect to i^

^

1 (7)

' “V.n'

^

AIoexp(AEo)Fn(A.V) ‘ ySTVT

where R* is the charge-transfer resistance for the electrode at the

potential E^ in the absence of sinusoidal voltage modulation.

The values of depends on how the double layer capacitance varies

with potential. Assuming that its variation in the range of potentials

investigated is negligible.

- 5-
( 8 ) "'I "'dl while all other C^, for n > 1, are equal to zero.

Experimental observations on the electrode employed for this work

have shown that for n > 1 are small and that equation (8) is a good

approximation.

Once the values to be assigned to the components in the circuit of

Fig. 1 are calculated, it is possible to calculate the in-phase and outof-phase components of the current at angular frequency noj. Calling

2

a = 1 +

6

- Rjj (1 + [nuC^R^]^) + R„

6 = nu)C R

n n

(9)

( 10 )

01 )

we have

('nJrms = ^ a6

^ +

6^

(12)

and (1" ) ' nojVms

VH

a6

2 2 e + 6

(13)

It is easy to show that if the ohmic drop resistance goes to zero

V

(12) and (13) revert to the theoretical values: (i„ ), nojC^V ,no)'rms ^ R.

It is also interesting to see what is the effect of varying the

average electrode potential E^, upon which, as shown by (7), R^ depends

exponentially. At high overvoltages, that is for values of such that

the dc current density i^^ is large and R^^ is small with respect to R^^,

equations (12) and 13) become

V (14)

R„ . 0

- 6-
and

lim (i'' = lim

' no) rms

nu)C„R^V

n n

R.

= 0 (15)

The current becomes Independent of R^ and is limited only by the ohmic

drop resistance. On the other hand, when is shifted to values where

2

the current density is small, R^ becomes larger, so that (nwC^R^) >>1.

Then

V (no)C„)'^R n-\ lim (ii = — o ^

[l+(nyC^Rj,)^]

(16)

and

lim (i"

no) rms

R

n

no)C.

^

[l+{n(i)C^Rjj)^]

(17)

These current values are proportional to the admittance of a RC series

circuit, where the resistance is the ohmic drop resistance

At both limits the in-phase current, instead of varying with on

a Tafel slope, becomes constant, depending only on the ohmic drop R^^.

One of the effects of an ohmic drop error is that of introducing a

phase-shift in the current so that the identity between faradaic current

and in-phase component is no longer true. This ^Iso introduces an

additional dependence of the current components on frequency which can

be calculated from (12) and (13) by varying oi at constant E^.

In the experimental part it will be shown that the effects of the

ohmic drop resistance described here account well for the deviations

from the theoretical behavior observed.

3. Experimental

a. Purpose of the experimental work

- 7-
The distortion of the faradaic current which leads to an increase

of the dc current and can cause enhanced corrosion is not dependent on

special conditions concerning the electrode surface, such as the

presence of a rectifying semiconductor film, but is a consequence

only of the non-linearity of the current-voltage characteristics of the

electrode, and therefore, should be observed in a large class of

electrode-electrolyte systems. The effect is graphically illustrated

by the waveforms shown in Fig. 2, which are experimental recordings.

The lov/er trace shows the sinusoidal voltage signal applied to the

electrode, while the upper trace is the current response, showing the

high degree of distortion which is the cause both of the rectification

and of the appearance of harmonics of the modulating frequency.

The particular quantitative mathematical treatment employed here

was developed for a specific case of kinetics, charge-transfer control,

but should maintain a fair degree of applicability for the prediction of

the behavior of electrodes having a more complex kinetics.

The main purpose of this work was that of verifying the validity of

the mathematical model as quantitatively as possible. For this reason

t it was more convenient to choose an electrochemical system that was

known to be under charge-transfer control over as wide a range of

current densities as possible.

b. Experimental Methods

The experiments aimed at testing the validity of the theoretical

model have been carried out on the hydrogen evolution reaction. The

electrode material was OFHC copper and the solution composition was 0.5

M NagSO^ + 0.1 M H2S0^. By minimizing other cathodic reactions, in

particular oxygen reduction by bubbling Ar in the cell, and favoring

transport in solution by means of vigorous stirring, it was possible to

-8 -
obtain a straight Tafel plot with a slope of 120 mV/decade over a

potential range larger than 300 mV.

As a further test of the applicability to the electrode under study

of the hypothesis used in the mathematical analysis the electrode

impedance was measured by applying a very small (<2mV) modulating

signal, about in the middle of the (dc) potential range where a straight

Tafel slope was obtained. The results are given in Fig. 3, where the

imaginary component Z" of the electrode impedance (for unit surface

area) is plotted versus the real component Z'. The plot is approximately a semicircle, indicating that there is no detectable contribution

of transport to the electrode kinetics. From the plot the ohmic drop

2

error is found to be about 6f2cm , while the charge-transfer resistance

2

is more than 400fi‘cm .

The circuit employed for the measurements is shown schematically in

Fig. 4. Two voltages, a constant one (E^ of equation (2)) and a pure

sinewave (of amplitude V), where higher harmonics had been minimized by

filtering, were added and applied to the control voltage input of the

potentiostat. A phase-sensitive lock-in amplifier measured the amplitude

of the current both at frequency ca of the modulation, and at twice that

frequency, each one divided into the component in phase and the component

in quadrature with the modulating signal. By means of the low pass

filters the dc components of the voltage (E ) and of the current (i. ) 0 ' dc^

could be recorded. With the filters switched off, the total current and

voltage signal could be stored in the digital oscilloscope and suc- cessively reproduced on the X-Y recorder in the form illustrated in Fig.

2. Inputs and outputs of the measuring system, with the symbols used

here are given in Table I.

- 9-
In a few instances the current signal was fed to a 200 channels

spectrum analyzer. In this way the amplitude of higher order harmonics

could be obtained. Hov^ever, all phase information was lost and only the

absolute value of the current density of angular frequency nw

was obatined.

Results and Discussion

Current-potential measurements in the absence and in the presence

of a modulating sinusoidal voltage were carried out. Fig. 5 shows the

Tafel plot of i^^ in both conditions, as well as i^, i^ and i^^. The

solid lines represent the theoretical values for the various components

of the faradaic current calculated from the experimental values

'dc

by means of equation (3). The factors F^(A,V) were calculated according

to the formulae given in Ref. [2].

Linear Tafel behavior is shown by the dc current between -350 and -

650 mV vs. NHE. When the modulating signal was applied, i^^ was larger

than ij^ by a factor in good agreement with the theoretical one. The

capacitative component i^ is largely independent of potential, indicat-

» ing that the electrode capacitance remains nearly constant in the range

studied. In agreement with this conclusion the term ii was found to be

negliglibly small, suggesting that all higher harmonics of the current

are almost exclusively faradaic.

The current signal was also examined with a spectrum analyzer for a

value of of -506mV vs. NHE and a modulation at 25 Hz of 35.4 mV

r.m.s. The output values recorded during the spectrum analysis, as well

as those obtained from the spectrum analyzer are reported for comparison

in Table II, which also gives theoretical and experimental ratios between

the various current components and the current density i* in the

absence of modulation.

- 10-
The values reported in Table II confirm that the capacitative

currents at 2w and 3u) are negligibly small, which is equivalent to say

that the double layer capcitance varies very little in the electrode

potential range (100 mV) covered by the modulation. The ratios are also

in excellent agreement with the theory.

The spectrum is shown in Fig. 6, together with the spectrum obtained

substituting a pure resistor for the cell, and passing a current of the

order of lOOymA rms at 25 Hz. The harmonic distortion is extremely

small: the amplitude of the 50 Hz harmonics is below the detection

-7 -8

limits (about 10 A) and could be measured at about 4*10 A only after

filtering out the large 25Hz signal. As Fig. 6 shows, the only spurious

signal detected was a 60Hz peak at 5*10”^A. The small peak at about 20

Hz which can be seen on the side of the 25 Hz peak of the electrode

current is caused by the magnetic stirrer and could be eliminated by

shutting it off.

Another Tafel plot, for a larger modulation of 88.3 mV r.m.s.

(V = 125 mV) at 100 Hz, is shov/n in Fig. 7. The behavior of the dc

component is close to the theoretical prediction over a range of about

300 mV, but significant distortions can be seen in the other frequency

components.

The main reason for the deviation from the theoretical behavior is

the ohmic drop error, as discussed in a preceding section. The

deviations can be accounted for quantitatively, as shov/n in Fig. 8,

which reproduces in detail part of Fig. 7, where the dc current density

absence of modulation as well as the experimental values of

the ac components of the current at the frequency of modulation w are

shown. The dashed lines represent the theoretical values of i^ (from

2

Eq (3)) and of i^ (assuming a double layer capacitance of 52y/cm ).

- 11 -
In the absence of ohmic drop error. The dot-dashed lines are calculated from (12) and (13) for an ohmic drop resistance of 6n, the value

obtained from the high frequency end of the semicircle in Fig. 3. As

Fig. 8 shows, the experimental values fall quite close to the calculated

lines. It should be pointed out that no empirical, adjustable parameters

enter these calculations, once the Tafel slope for the current density

in the absence of modulation, here shown as a solid line, is drawn

through the experimental points.

Comparisions between the predictions about the amplitude of the

faradaic components and the experimental values are summarized in Fig.

9. On it the experimental ratios of i^^, i^ and i^^ to ij^, as well as

the theoretical lines are given. The agreement is very good for amplitudes V up to about 200 mV. Beyond that the current density caused by

the modulation was less than predicted, because the electrode potential

was being driven outside the linear Tafel range.

The effect of the frequency of the modulating signal has also been

examined. Fig. 10 shows the ratio between the dc current i^^ and i*^.

The measurements were carried out at the same potential (E^ = -456 mV

vs. NHEl and at the same modulating amplitude (V = 100 mV), at frequencies

ranging from 25 Hz to 10 kHz. As expected, the deviations caused by the

ohmic drop error become quite. large when the frequency is increased.

The results shown pertain to two different electrodes having different

ohmic drop resistance. With = 6n, significant deviations from the

theoretical behavior begin at about 300 Hz, while reducing to about

0.8 increases the frequency of the beginning of the deviations by a

factor of 10, to around 3 kHz. The effect of the ohmic drop error is

even more pronounced on the current components at the modulation frequency.

- 12 -
Fig. 11 shows the values of i' and i" as a function of frequency, together with the lines calculated from (12) and (13), assuming a dc

capacitance of 48pF and = 0.7n. The agreement is satisfactory.

Indicating again that the greatest part of the deviations from the

theoretical behavior is due to the ohmic drop error. It should be noted

that, in its absence, i^ would be independent of frequency.

Conclusions

The experimental results presented here show conclusively that the

mathematical analysis previously published [2] predicts accurately the

current response to a sinusoidal voltage of an electrode under chargetransfer control, and can be used, therefore, as a basis for the understanding of the effect of ac on corrosion. It also shows that frequency

analysis holds promise to be an important tool for the study of electrodes

undergoing corrosion. Since the model employed does not involve special

properties of the electrode surface in order to produce the effect which

has been called faradaic rectification [3], it is to be expected that an

increase in the corrosion rate due to ac leakage is a widespread

phenomenon, with considerable practical implications.

This work has been performed as part of contract E(49-l)3800 with

the Department of Energy, whose financial support is gratefully acknowledged.

- 13-
References

[]] B. McCollum, G. H. Ahlborn, Tech. Pap. BST72 (1916)

K. G. Compton, INCRA Research Report Project 265 (1977)

D. T. Chin, T. W. Fu, G. F. Kamrowski , S. Pookote, 1978 NACE Research

Conference, Houston (1978)

D. J. Nesslage, Paper N. 762, Corrosion 78, NACE (1978)

U. Bertocci, J. L. Mullen, Paper No. 242, Corrosion 79, NACE (1979).

[2] 1). Bertocci, Corrosion, in press.

[3] B. Breyer, H. H. Bauer, "Alternating Current Polarography and

Tensammetry" , Interscience, Mew York (1963)

- 14 -
Table I

Input and Output Quantities

Input Quantities

V a> = 2nv

d.c. component of

electrode potential

Amplitude of modulating voltage

Angular frequency of

modulating voltage

Output Quantities

constant (d.c. ) components

In phase with

modulating voltage

±tt/ 2 with respect

to modulating voltage

dc ^dc

i'u i"

0)

current density

for V = o current density

for V > 0

current density

at frequency u current density

at frequency oj

i

2(1) i'o

2(1)

current density

at frequency 2(u

current density

at frequency 2(d
Table II

Values obtained from circuit in Fig. 2 and from spectrum analyzer.

Eq = -506 mV vs. N.H.E. V = 35.4 mV r.m.s. at 25 Hz. Current

density values in yA/cm^ r.m.s.

ijdc ^dc

i'

0) i"

U) /i /

0) ^2u)

i"

2u /ip /

- ^ /i^ /

3(1) Circuit 186 238 152 161 221 0.1 32 32

Spectrum

Analyzer

215 31 7,3 •

Ratios i /i*

^dc^ dc Cl) dc

/i5

2u) dc

i ' /i*

3u dc

Theoretical

Found

1.24

1.28

0.76

0.82

0.176

0.17

0.0276

0.039

'
Figure Captions *

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.

Equivalent circuit of the electrode/electrolyte system under

large sinusoidal voltage modulation.

Voltage input signal and current response for Cu in 0.5M Na2S0^

+ O.IM H2S0^.

Imaginary impedance Z" vs. real impedance V for Cu in

0.5M Na.SO, + O.IM H«S0.. E^ = -506 mV vs. NHE.

Frequency as parameter.

Schematic diagram of the circuit used for the measurements.

R.m.s. current density vs. electrode potential for copper

in O.IM H2S0^ + 0.5M Na2S0^ with and without voltage modulation (V = 50 mV, V = 25 Hz). Solid lines are calculated values.

Solid line: Frequency spectrum of current density for a Cu

electrode in O.IM HoSOy, + 0.5M NaoSO*. E^ = -506 mV vs.

NHE, V = 50 mV, v = 25 Hz. Scale is log (r.m.s. A/cm^)

vs. Hz. Average over 128 spectra. Dotted line: spectrum

of current through resistor. Scale is log (r.m.s. A) vs.

Hz. Average over 128 spectra.

^ R.m.s. current density vs. electrode potential for Cu in

O.IM H2S0^ + 0.5M Na2S0^, with and without voltage modulation (V = 125 mV, V = 100 Hz). Solid lines are calculated values.

Current density in the absence of modulation ij^, and in-phase

(i^ ) and out of phase (i^) components of the current density

at the modulation frequency (v = 100 Hz). Dashed lines calculated without ohmic drop (Equivalent circuit shown in inset

A). Dash-dot lines calculated with an ohmic drop error of

6n (Equivalent circuit shown in inset B)-.

- 15 -
Fig. 9. Ratio between various components of the faradaic current and

d.c. current in the absence of modulation ij^, as a function

of the amplitude of modulation. Cu/H250^ + Na2S0^. = -506 mV

vs. NHE^ = 103.3 Hz.

Fig. 10 Ratio between dc current in the absence and in the presence

of modulation, as a function of frequency for two values

of the ohmic drop resistance. Cu/H2S0^ + Na2S0^. E^ = -456 mV

vs. NHE, V = 100 mV. Solid line indicates theoretical value.

Fig. 11 In-phase (i^) and out-of-phase (i^j) components of the current

density as a function of the frequency of modulation. Constant amplitude V = 100 mV. Solid lines are calculated for

Rj^ = 0.7fi and C = 48yF/cm^.

%

- 16 -
Fig.

i

i’r

i

1

I

I

39

a.

o
10

ms I
Fig.

3

I

Z",0-cm2 N3O

CO

CD

CD
PHASE-SENSITIVE

LOCK-IN

AIVIPLIFIER

A.C.

SIGNAL

FREQUENCY

-cc h

'

r-»

GENERATOR

—

COUNTER
CURREWT,

r.m.s.

A/cm
9 ‘StJ

ooz

ZH 'A3W3nD3aj

091 001 09 0

Vai03dS 8Zl U3AO 3DVH3AV

J I L
CURRENT

DENSITY,

r.m.s.

A/cm

Eo, mV vs. N.H.E

N»

1

1 1 1 1 CD U1 4^ &0 O C3 CD o o

o O O o

O
CURRENT

DENSITY,

A/cm

Eq, mV vs. N.H.E.

I I I CD m 4^

I N3

-300
Fig. 9
RATIO

Irfc/Id

FREQUENCY, Hz

Fig. 10
CURRENT

DENSITY,

rms

A/cm

Fig. 11
[From Proceedings on International Conference on Non-traditional

Approaches to the Study of the Solid-Electro-ly te Interface. To be published

in Surface Science]

Studies of Passive Film Breakdown by Detection and

Analysis of Electrochemical Noise

U. Bertocci and J. Kruger

National Measurement Laboratory

Chemical Stability and Corrosion Division

National Bureau of Standards

Washington, DC 20234

ABSTRACT

Random fluctuations in the passive current of electrodes undci'

potentiostatic conditions have been measured on aluminum in boric acid:

borate solution and on a Fe-Cr-Ni alloy, both in the amorphous and in

the ci'ystalline state, in sulfuric acid. The onset of pitting can be

detected by the large increase in current noise. The noise level is

different in the amorphous and crystalline Fe-Cr-Ni alloy, indicating

that the breakdown of the passive film differs in the two conditions.

The experimental aspects involved in carrying out meaningful noise

measurements in electrochemical systems are also discussed.
Studies of Passive Film Breakdown by Detection and

Analysis of Electrochemical Noise

U. Bertocci and J. Kruger

National Measurement Laboratory

Chemical Stability and Corrosion Division

National Bureau of Standards

Washington, DC 20234

The processes of passive film breakdown and healing are of fundamental

importance in the understanding of the initiation and development of

localized corrosion. A new approach which shows promise of producing new

insights into these processes, is that of detecting and analyzing the fluctuations from the average values of the current and electrode potential. This

has been recognized by a number of workers who have employed various techniques

for the study of what is currently called ’'electrochemical noise" [1].

Use of this kind of measurement to study, passive film breakdown

acquires additional interest because there have been recent suggestions

[ 2 ] that the event leading to breakdown consists of a dynamic breakdownrepair stochastic process which manifests itself as potential and current

fluctuations.

In the present work, we report on some of the experimental aspects

that are involved in making meaningful measurements, describe the use

of these measurements to study metals covered by passive films and the

changes in the signal that result when the passive film breaks down and

pitting occurs.

-1 -
EXPERIMENTAL APPROACH }

Of the various methods of processing stationary or quasistationary

random signals, the one employed in our laboratory has been that of

transforming the output of the electrochemical cell in the frequency domain

by means of a spectrum analyzer. Details of the experimental circuitry

are given elsewhere [3].

As far as the conditions of the electrochemical cell are concerned,

two main modes can be employed, that is the observation of the voltage

fluctuations under galvanostatic conditions and the observation of the

current fluctuations under potentiostatic conditions. Both methods

have some specific advantages and the detection of the increases in

noise level associated with pitting can be done in either mode. An

example of noise levels under galvanostatic conditions is sho\m in

Fig. 1, which gives the voltage spectra for Fe in borate buffer with

a small concentration of chloride added. The noise level is larger

when the potential of the electrode is above the pitting potential.

The potentiostatic method, however, is preferable in most cases where

passive films are being examined, because it maintains definite energy

conditions at the electrode/electrolyte interface. The main problem,

however, is to carry out the measurements so as to introduce the lowest

possible level of instrumental noise maintaining the largest practicable

bandwith. For this purpose, a battery-operated low-noise potentiostat

has been built at the NBS. Its specific characteristics have been

given elsewliere [4]. For the present purposes, its performance can be

summarized as follows: the noise in the control voltage is about

-2-
•»7 *"8 » 10*” V/Zn^ at 0.3 llz and about constant at 2.5 10 V//TTz from lb Hz to

about 2000 Hz; the useful bandwith extends from 0.1 Hz to 2 kHz; the

instrumental noise introduced by the ac amplifier which is driven by

the current fluctuations at the working electorde is equivalent to about

A/ZITz.

With the characteristics described, it follows that the potentiostatic

generated noise can be used to obtain useful impedance data over a wide

frequency range provided that ^he absolute impedance |Z| of the electrode

does not signific2intly exceed 1000 n. If the random fluctuations of the

electrical characteristics of the electrode are negligibly small compared

with the noise injected by the potentiostat , measurements of a deterministic

nature can be carried out, the input being essentially white noise [5].

NOISE SOU; ES IN POTENTIOSTATIC MEASUREMENTS

For ti interpretation of the experimental results, it is useful to

examine which sources of noise current may be contributing to the observed

signal. The electrode can be represented by an equivalent circuit having

a faradaic admittance Y_ in parallel with a capacitative Y_, across which

r c the potentiostat maintains a constant potential E plus a noise signal,

which at any frequency takes the form.V sin wt. The two admittances may

fluctuate causing the flow of an ac current even in the absence of an

alternating signal in the control voltage. In order to simplify the

argument, let us assume that the dc faradaic current can be described

by a simple expression such as:

‘^F’do = (1)

-3-
where S is the surface area of the electrode, is a quantity proportional

to the rate constant of the electrode reaction and which takes into account

the choice of reference for the measurement of E. When the voltage E is

applied, the faradaic admittance can be written as:

„ F dc o /ttFE V . Yp=-g— = — “P(-RT->- <2>

Fluctuations in can be caused either by changes in surface area (during

cathodic deposition of anodic dissolution) or in i^, for instance,

because of passive film breakdown. If Y fluctuates, it is obvious from

r

(1) and (2) that also the faradaic current will do so. The amplitude of

the fluctuations can be expressed as a fraction x of the average faradaic

admittance (2) or as a fraction x of the dc current.

Ip = EYp (1 + X sin (i)t) = (Ip)jjj, + *^^F^dc

If the voltage applied in turn oscillates with amplitude V, the ac

faradaic current consists of two main components, neglecting intermodulation

terms, which is permissible when the fluctuations are sufficiently small.

The first term, due to the fluctuations of the faradaic admittance at constant

applied voltage is given by the last term in (3), whose amplitude is:

*T1

= *<^F’do

The second term is the product of the voltage fluctuations and the differential

admittance calculated for the average current given in (1)

^2 ~= V« dE (5)

Since the phase difference between the two currents varies in a random

fashion, the resulting amplitude is only little more than the larger of the

two terms.

With similar reasoning it can be shown that there are two main terms

for the capacitntive current. The first is due to the fluctuations of

-4-
I the electrode capacitance caused by the same processes that affect the

j faradaic admittance. Assuming that the capacitance varies by a fraction

y at frequency o)»

C = C (1 + y sin wt) (6)

the capacitative current has an amplitude

*C1

=

“y'^av'

where e = E-E is the difference between the average applied potential

and the potential of zero charge.

The second terra of the capacitative current is due to the fluctuations

of the control voltage acting on the average value of the capacitance.

The amplitude is

*02 =

The phase difference between the second term of the faradaic current and

that of the capacitative current is fixed at tt/ 2, but that between the

terms due to fluctuations in the electrode characteristics can vary. In

many cases a process causing an increase of electrode resistance, such as

an increase xn thickness of a protecting film or a decrease in surface

area can cause a decrease of the capacitance, but this may not be always

true. Moreover, the size of the capacitative term depends on the size

of the difference c between the electrode potential and the pzc.

• Even this brief examination of the sources of noise current allows

one to estimate their relative importance in a typical situation, and

it is useful in interpreting the experimental results. For instance,

—2 ~1

if we take an electrode whose differential faradaic admittance is 10 n —5

and whose capacitance is 10 F, the amplitude of the faradaic and

capacitative terms is shown in Table I. If the fluctuations of the faradaic

_5 current are 0.1% of its average value of 10 A and the variations of the

capacitance are also of 0.1%, while e = 0.2 V, the amplitude of the current

-5-
terms is also shown in Table I. 1

As the numbers show, the capacitative terms predominate at 1 kllz

over the faradaic ones. What is also noteworthy is that fluctuations of

the order of 0.1% in the electrode resistance and capacitance would cause

much larger currents than those due to the potentiostat instability..

APPLICATION OF NOISE MEASUREMENTS TO B STUDY THE BREAKDOWN OF PASSIVITY

a) Aluminum in borate buffer.

The results obtained with 6061 Al in borate buffer solution to which 0.05M

NaCl had been added, have shown that it is possible to determine the

pitting potential by observing the significant increase of noise current

that attends the formation of pits. An interesting example is given in

Fig. 2, where are reported the noise spectra below the pitting potential

(-700 mV vs. SCE) and at the pitting potential (-650 mV vs. SCE) during the

initiation period and after development of pits (-600 mV vs, SCE). In the

latter case, the noise current is more than two orders of magnitude greater

than that below the pitting potential in the frequency range up to 50 Hz.

The corresponding time record of Fig. 3 shows how the increase in. noise

covers the whole frequency range. On the contrary. Fig. 4 shows the time

record taken during the induction period, and must be compared with spectrum

2 of Fig. 2. Here, isolated current bursts whose decay times range between

0.5 and 1.5 seconds, can be observed. Consequently, the increase in noise

is largely limited to the low frequency end. The decay times of the current

bursts correspond to the maximum repassivation times that still do not

result in the formulation of a stable pit.

The noise in the current increases steadily with time: spectrum 3

was taken again at -650 mV about 10 min. after taking spectrum 2. However,

2

the dc current was still cathodic, about 4 yA/cm .

-6 -
The results indicate that while the very large noise levclSj such as

in spectrum 4 of Fig. 2 are associated with active pitting and are connected with hydrogen evolution from the pits [6], a significant increase

in noise with respect to the passive state can be detected well in advance

of pit formation, and is largely limited to the low frequency range,

indicating that the fluctuations affect the resistive characteristics

rather than the electrode capacitance. These results support the suggestions

of Videm [2] that the pitting of Al involves repeated breakdown-repair

events. Moreover, our results ndicate that such events occur during pit

initiation, assuming they are manifested as electrochemical noise. However,

the character of the noise observed during pit initiation differs from

that found during propagation,

b) Amorphous and crystalline Fe-Ni-Cr alloy

An amorphous passive film is generally considered more protective

than a cryst' lline one, as shown by the fact that an increase in Cr in

Fe-Ni alloys increases the resistance to attack as well as the tendency

to form glassy oxide films. The passive film grovm on an amorphous metal

substrate is thought to have less tendency to localized breakdown because

of its particularly uniform structure [7]. To gather more information

on the differences in the properties of the passive films gro\m on amorphous

and on crystalline substrates, an amorphous Fe-Ni-Cr alloy, to which P and

B have been added to increase its tendency to cool in the glassy state,

is being investigated. For comparison, specimens of the same alloy have

been annealed in vacuum at 350°C for 3 months, so that the alloy became

crystalline, as shown by the presence of broad x-ray diffraction peaks

corresponding to the (110), (200) and (220) orientations, but without

-7-
phase separation because of the very low rccrystallization temperature.

The current spectra of the amorphous alloy in 1 M H SO at +550 mV vs.

NHE do not show any features which can be attributed to fluctuations in

the electrode characteristics. Dividing the current density by the voltage

at each frequency gives the absolute impedance, as shown in Fig. 5. The

relatively large value of the capacitance indicates that the passive film

is thin. The spectra taken on the amorphous and on the crystalline alloy are

shown in Fig. 6. Here the constant value of the current with frequency

for the crystalline sample indicates that the oxide film developed on it is

much thicker than on the amorphous crystal, so that the capacitance is

very low.

Even more striking is the difference in behavior upon anodic polarization as shovm in Fig. 7. Spectra 1 and 2 were taken on the amorphous

alloy. Even polarizing at 1.59 V, where transpassive dissolution begins

to occur and the dc current density increases more than 20 times to about

—3 2 3.5*10 A/cm , the fluctuations are small, so that the increase in noise

at low frequency can be attributed to a moderate decrease in reaction resistance. The crystalline alloy, on the contrary, shows the characteristics of

localized attack: an increase in potential of 70 mV from 1.37 V to 1.44 V

causes only a modest increase in the dc current density, from 130 to 200

2

yA/cm , but has a spectacular effect in the noise level as shown by spectra

3 and 4 of Fig. 7. The monotonic decrease in noise current with frequency

in spectrum 4 indicates that the fluctuations of the faradaic impedance

are much larger than those of the capacitative terms. The results can be

accounted for by fluctuations in the dc current density of the order of

-8-
5«10“^ I at 50 Hz to at 500 Hz.

The remarkable difference in the noise spectra of the amorphous and

crystallized alloy clearly shows that the latter has a much greater tendency

to localized attack, no doubt due to structural inhomogenity of the passive

film. Moreover, the noise measurements have revealed that the superior

resistance to breakdown of the passive film in the amorphous alloy is not

due to the siatic properties of this film because the overall current densities were observed not to differ greatly when comparing the

crystallized and amorphous allo^ s. Rather, the greater resistance to breakdown of the amorphous alloy lay in the ability of its more homogeneous film

to inhibit the dynamic processes (probably repeated breakdown-repair events)

that result in electrochemical noise. These results with the amorphous and

recrystallized alloys bolster the suggestions [ 2 ] that the breakdown

mechanism i volves a dynamic fluctuating set of events rather than a steady

deterioratio ) (by, for example, damaging species penetration) of the

properties of the passive film.
i REFERENCES

1. W. P. Iverson, J. Electrochera. Soc., 115 , 617 (1968)

G. Okamoto, T. Sugita, S. Nishiyama, K. Tachibana, Boshoku Gijutsu,

23, 439, 445 (1974).

G. Blanc, I. Epelboin, C. Gabrielli, M. Keddam, J. Electroanal. Chem.

,

45, 97 (1977)

U. Bertocci, 7th International Congress on Metallic Corrosion, Rio

de Janeiro, 1978, in press.

2. K. Videra, Kjeller Report KR-149, Institutt for Atoraenergi , Kjeller,

Norway (1974)

G. Okamoto, K. Tachibana, S. Nishiyama, and T. Sugita. Passivity and

Its Breakdown on Iron and Iron Base Alloys, R. W. Staehle and H. Okada,

Editors, NACE, Houston, Texas, P. 106 (1976).

J. Kruger, Passivity and its Breakdown on Iron and Iron Base Alloys

,

R, W. Staehle, Ed., Nat. Assoc. Con. Eng., Houston, 1976, P. 94.

3. U. Bertocci, J. Electrochem. Soc., to be submitted.

4. U. Bertocci, R. W. Shideler, to be published.

5. G. Blanc, I. Epelboin, C. Gabrielli, M. Keddam, Electrochim. Acta,

599 (1975)

6. C. B. Bergeron, R. B. Givens, J. Electrochem. Soc., 124 , 1845 (1977)

Corrosion Res. Conf. NACE, Atlanta (1979)

7. M. Naka, K. Hashimoto, T. Masumoto, Corrosion, 32, 146 (1976)

-10-
TABLE I

» Amplitude of the faradaic and capacitativc current components at two

frequencies, rms A.

(I_) , = 10~®A; C = 10~®F; d(I„) , /dE =

F dc * av * F dc

e = 0.2V; x = y= lO”^

Component Formula 0.3 Hz . V = 10~^ V

1 kHz

V = 2.5”®V

^2 (5)

to 10”®

^C2 (8)

-12

1.9«10 1.6*10”®

^1 (4) 7.1*10”® 7.1*10”®

^C1 (7) 2.7*10”® 8.9*10”®

11-
I

FIGURE CAPTIONS

1. Noise voltage spectra of Fe in borate buffer + 0.01 M NaCl. Galvanostatic

conditions.

2. Noise current spectra of 6061 Al in borate buffer + 0.01 M NaCl.

Potentiostatic conditions. Reference; SCE. Averages over 64 spectra.

1) -700 mV. 2) -650 mV. 3) -650 mV after 10 min. 4) -600 mV.

3. Current-time signal taken with spectrum N.4. Fig,' 2.

—7 2 Horizontal: 0.4 s/div. Vertical; 4.3 • 10 A/cm • div,

4. Current-time signal taken with spectrum N.2, Fig. 2,

-9 2 Horizontal: 0.4 s/div. Vertical; 8.6 • 10 A/cm • div.

5. Absolute impedance |Z| vs. frequency derived from current and voltage

spectra on ^^

32^^ 3 6 ^^14^12^6 ^ ^ ^ ~ ^ Dashed

line is calculated for R = 4200 fl • cm , C = 5 yF/cm^.

6. Current spectra for amorphous and crystalline Fe Ni Cr P B in

O ^ 00 J. 4 O 1 M H-SO . Dotted line: amorphous. Solid line: crystalline.

^ 4

7 . Current spectra for amorphous and crystalline Fe Ni Cr P B in

oO X4 0

1 M HgSO^. Amorphous: 1) E=1.14 V; i^^ = 1.5 •10”^ A/cm^;

2) E = 1.59; i^ = 3.5 • lO”^ A/cm^. Crystalline: 3) E = 1.37 V, i^^ =

1.3 • 10~'^ A/cm^. 4) E = 1.44 V, i^^ = 2.0 • 10~'^ A/cm^.

Reference: NHE. Averages over 256 spectra.

-12-
SPECTRAL

POWER

AMPLITUDE,

V/\/H^

Fig.. 1
Current

Amplitude,

A/cm^

vl^

1

1 I I r

10

" 7

Frequency, Hz

Fig. 2
Fig. 3

Fig. 4
IZI,n»cm

10^

lO'^ - \\

Na

\

CM \

10- Na\

\“

\

\

\\

10 -

R=4200 n»cm^, C=5|iF/cm^

\

10 100 1000

Frequency, Hz

Fig. 5
Current Amplitude, A/cm

oI CO

oI 00
Current

Amplitude,

A/cm

^

VHz

Frequency, Hz

Fig. 7
NBS-114A (REV. 9-7B)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 80-2083 ydfw'zr /'A'""" -

• . — - 4. TITLE AND SUBTITLE

Development of In-Situ Techniques for the Detection and

Measurements of Corrosion of Copper Concentric Neutrals in

Underground Environments

5. Publication Date June 1980

pisHic^iii 0^^ Cod«

7. AUTHOR(S)

J. Kruger, U. Bertocci, E. Escalante, and J. L. Mullen

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS 18, Pfof^^aHt/TIforH Unit No.

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

K-i ^

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fSfreer. City, state, ziP)

Department of Energy

Washington, DC 20545

13. Type of Report & Period Covered

'14, Spawning Code

15. SUPPLEMENTARY NOTES

r~1 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16* ABSTRACT (A 200^word or less factual summary of most significant information. If document includes a significant bibliography or

literature survey, mention it here.) This is a final report on the Development of In-Situ Techniques for the Detection

and Measurements of Corrosion of Copper Concentric Neutrals in Underground

Environments which collects all of the papers written for the project. The titles of

the papers included are: 1) Final Technical Report on the Development of In-Situ

Techniques ... Underground Environments; 2) Electrochemical and Corrosion Studies on Copper Concentric Neutral Wires; 3) Laboratory Corrosion Studies on Tinned Copper

Concentric Neutral Wires; 4) Corrosion Induced by an Alternating Voltage. A Comparison

Between Theoretical Predictions and Experimental Results; 5) Detection and Analysis of

Electrochemical Noise for Corrosion Studies; 6) Applications of a Low-Noise

Potentiostat in Electrochemical Measurements; 7) A Low-Noise Potentiostat for the Study

of Small Amplitude Signals in Electrochemistry; 8) Corrosion Enhancement Due to Large

Voltage Modulations. Frequency Analysis of the Response of Electrodes Under ChargeTransfer Control; 9) Studies of Passive Film Breakdown by Detection and Analysis of

Electrochemical Noise; 10) A Field Study on the Corrosion of Concentric Neutral Cable;

11) AC Induced Corrosion. The Effect of an Alternating Voltage on Electrodes Under

Charge-Transfer Control.

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)

Corrosion; corrosion noise; electrochemistry; polarization techniques; underground

corrosion

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