NBSIR 73-244

The Role of Passive Film Growth

Kinetics and Properties in Stress

Corrosion and Crevice Corrosion

Susceptibility

J. Kruger and J. R. Ambrose

Corrosion and Electrodeposition Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

July 1973

Technical Summary Report Number 4

DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED

Prepared for Office of Naval Research

Department of the IMavy

Arlington, Va. 22217

NBSIR 73-244

THE ROLE OF PASSIVE FILM GROWTH

KINETICS AND PROPERTIES IN STRESS

CORROSION AND CREVICE CORROSION

SUSCEPTIBILITY

J. Kruger and J. R. Ambrose

Corrosion and Electrodeposition Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

July 1973

Technical Summary Report Number 4

Prepared for Office of Naval Research

Department of the Navy

Arlington, Va. 22217

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Part I. To be presented at the National Meeting,

Electrochemical Society, Boston, October 1973

REPASSIVATION KINETICS OF 304 STAINLESS STEEL

IN CHLORIDE SOLUTIONS AT ROOM TEMPERATURE

J. R. Ambrose and J. Kruger

Institute for Materials Research

National Bureau of Standards

Washington, D. C.

ABSTRACT

Repassivation kinetics of an AISI 304 stainless steel have been determined

in l.ON NaCl solutions using the triboell ipsometry technique which permits

measurement of film growth and total reaction rates following removal of the

surface film by abrasion. Although deoxygenation of the solution resulted in

little change in either film growth kinetics or the ratio of total change to

film thickness (Rp) , changing the solution pH affected both the mechanism and

rate of film growth which resulted in increased rates of metal dissolution in

acidic (pH3) and basic (pHll) solutions.

In neutral 1,0N NaCl solutions, where film growth increased with more

positive applied potentials, the rate of metal dissolution during the

repassivation transient was initially highest at the corrosion potential, but

decreased with time as the surface became passivated.

1

Part I. To be presented at the National Meeting,

Electrochemical Society, Boston, October 1973

REPASSIVATION KINETICS OF 304 STAINLESS STEEL

IN CHLORIDE SOLUTIONS AT ROOM TEMPERATURE

J. R. Ambrose and J. Kruger

Institute for Materials Research

National Bureau of Standards

Washington, D.C.

A number of workers ^^"^^ in recent years, most notably Scully^^^

(2) and Staehle,^ ' have pointed out the important role that repassi vation

kinetics plays in the complex processes governing stress corrosion cracking

(sec). Scully was among the first to emphasize that the process of repassivation that occurs when protective films on metal surfaces are

broken by slip step emergence upon stressing the metal is probably the

step in the chain of events leading to cracking that is most sensitive

to environmental factors (pH, potential, oxygen concentration, anion and

cation nature and concentrations, and others). Moreover, because most

mechanisms of SCC require that an unprotected surface be available for

either dissolution, hydrogen entry, or adsorption of bond breaking species

to take place, the rate at which that surface becomes reprotected can

be crucial

Attempts to measure the repassivation kinetics of austenitic

stainless steels have been made by Hoar and Hines,^^^ Shibata and Staehle,^^^

(5) and Lees and Hoar.^ ' All of these studies used the straining electrode

or scratching techniques, measuring current transients. A problem with

these techniques is that one cannot separate out the current involved

in dissolution and other processes from that involved in film formation

leading to repassivation. Since the crucial process to study is the

effectiveness of the repassivation on inhibiting dissolution or other

2

processes such as hydrogen entry, we have been developing a technique

that can measure the film regrowth process alone and separate it from

the total current involved in repassivation. This technique called

(6) tri bo-el 1 ipsometry is described elsewhere/ ' In addition to measuring

the rate of film regrowth it is able to measure for a repassivation

transient the repassivation ratio, R , which is defined as follows:

Rp = — =

1^ (1 + Q—)

, where is the total charge consumed in the reX

passivation process, x is the thickness of the film, is the charge

involved in dissolution and other non-film forming processes, and Q

is the charge involved in film formation. The k is a constant that converts X, the film thickness, to charge. Rp is thus a measure of the

effectiveness of the repassivation process.

In this study the first attempts to study the repassivation kinetics

of 304 stainless steel in chloride solutions at room temperature by

tri bo-ell ipsometry are described. Besides its practical importance, this

alloy cracks transgranularly , thereby offering an ideal system to study

tribo-ellipsometrically, since film thickness data are obtained for that

surface where crack initiation occurs. These studies provide direct

n

)

measurements of the rate of repassivation that Scully^ ^ pointed out were

needed to assess environmental factors in SCC. Because such factors as

pH, oxygen concentration, and potential are altered in the environment

of a growing crack, their effect on repassivation was studied. Finally,

because tri bo-ell ipsometry is able to measure the time of repassivation

t and distance of crack advance, L, during this time (assuming crack

advance is due to dissolution) our measurements are compared to predictions

of a new film rupture theory by Vermilyea^^ which calculates L from

3

assumed stress and strain conditions.

EXPERIMENTAL

Measurement Technique

The tri bo-ell ipsometric studies were carried out using the techniques

described in earlier work.^^'^^ However, certain modification of the

technique was required for the 304 stainless steel/sodium chloride solution

system since, under the solution agitation conditions produced by the

abrasion device, film thicknesses and growth rates were of a magnitude to

be beyond the sensitivity of the ell ipsometric detection apparatus

(x < 0.1 nm; dx/dt - 0, tp < 10 msec). This would not be unusual for a

system in which film growth kinetics are determined by mass transport

in the solution. Therefore, surface films were removed by abrasion at

potentials where film regrowth would not be expected to occur (i.e.,

-1000 mV SHE). Following measurement of ellipsometric optical parameters,

the potential was stepped to those values of interest in this study using

a fast rise two-channel potentiostat. Since cessation of the cathodic

hydrogen evolution which occurs at those starting potentials would result

in an ellipsometric light intensity increase upon potential step, modification of the off -null ellipsometry procedure^ ' was employed such that

film growth would give light intensity decreases . Growth and current

transients were displayed on an oscilloscope and the recordings analyzed

as in previous studies.^ '

'

Materials

The material used in this study was a commercially available AISI

304 stainless steel fabricated into cylindrical specimens 1.905 cm in

4

diameter and 0.635 cm thick. Specimens were mechanically polished through

successive grits of silicon carbide metallographic papers and followed

by rotary polishing on nylon cloth with 6 micron diamond paste.

The principal solution used was a KON NaCl solution prepared from

ACS reagent grade chemical and distilled water (7.0 x

10'^ ohm"^ cm"^).

pH adjustment was made with aliquots of l.ON NaOH or HCl solution. Solution deoxygenation was accomplished by dispersing helium gas (99%)

through the solution.

RESULTS

Because one of the important variables in the crack environment is

the pH of the solution, the major emphasis was placed on studying the

effect of pH on repassivation kinetics. A few experiments examined the

effect of lowering the oxygen concentration and changing the potential

to values other than that measured at open circuit which could also

change in the crack environment.

A compilation of the repassivation transient data measured under

all of the conditions studied is given in Table 1. The distance of each

advance, L, during the time of repassivation, tp, which is listed in

the table was calculated on the following basis: the linear log-log plot

of current vs time was extrapolated to a time of 1 msec.

This particular time was selected in order to avoid complications

arising from double layer charging effects on the rather large surface

2 area of the specimen used (2.85 cm ). The value of current at this time,

designated i^^^s was assumed to flow uniformly to all areas of exposed

metal not yet covered by the passivating film. At t , then, one would

5

expect maximum penetration by metal dissolution at these areas of bare

metal last to be protected, and that this distance of penetration would

correspond approximately to the distance of crack advance, L.

L was then calculated from

where M is the average molecular weight of the film formed, assuming it

to be 18% Cr203, 8% NiO, and 78% Fe203; A is the area of the surface

exposed by abrasion; p is the average density of the oxides used to calculate M, F the Faraday and z the valance of the moving species and is

assumed to be +2.

Effect of pH

These experiments were carried out at room temperature at pH 3, 7,

and 11. The reasons for the choice of these values were as follows:

a) The pH 3 solution is at the approximate value^ ' found near crack

tips for a large number of different steels; b) The pH 7 solution is the

neutral value that exists in many natural environments and is important

in crack initiation processes; c) The pH 11 solution attempts to simulate

f 10) conditions that a recent theoretical study^ ' indicates may exist right

at the crack tip surface. This study suggests that the metal ions produced by dissolution may have time to diffuse away from the surface if

the hydrolysis reaction which produces hydrogen ions is sluggish. Thus,

the pH is lowered away from the surface while at the surface the discharge

of diffusion limited hydrogen ions raises the pH.

Fig. 1 shows the film growth kinetics for the three pH solutions

at a potential of +90 mV (SHE), the open circuit potential for the pH 7

6

solution. For cracking in a conductive neutral environment this would

probably be the controlling potential. It can be seen that the rate of

rep^ssivation is greatest for the neutral solution and least for the

acidic. Moreover, because the plot of film thickness vs log time is

linear for the films grown in the pH 7 and 11 solutions, the rate law

governing the film growth process is directly logarithmic. This suggests that the film forms by a field assisted growth mechanism^^ ^

^ and,

if this is so, that the film is probably non-porous. The film growth

law in the pH 3 solution appears to follow a more complex mechanism. It

is, as would be expected, thinner than those formed in the pH 7 or 11

solutions for a given time of growth. Therefore, based on the film

dx growth rate data, the repassivat^on rate, , would have the following

order with respect to pH:

(dx^ /dxx (6x^

Mt^pH3 " WpH^^^ ^cFt^pH^

In addition to film growth kinetics, the tri bo-el lipsome trie technique also permits determination of the kinetics of current decay. Fig.

2 is a log-log plot of current vs time for the 3 different pH solutions

studied. The rate of current decay is essentially the same for the pH 7

and 11 solutions but is lower for the pH 3. Moreover, while the magnitude

of the anodic current appears lower for the pH 3 at the beginning of the

passivation process, it eventually exceeds the values found for the other

two solutions. It is quite likely that the anodic current measured for

the pH 3 solution probably is in error due to a cathodt component from

the hydrogen reduction reaction in the more acidic solution. Thus, the

values m.easured for the pH 3 solution are undoubtedly low.

7

When we now combine the current measurements with those of film

formation we can determine the repassivation ratio, Rp, a measure of the

effectiveness of repassivation. Fig. 3 shows^the variation of Rp with

time. The order is the reverse of that for film growth rate,

RpCpHy) < Rp(pHn) < Rp(pH3)

Also, becuase as just pointed out, the current values for pH 3 are

probably low, the R values for that solution must be even higher than

those shown on Fig. 3.

Effect of Oxygen Concentration

A few experiments comparing the repassivation rates of 304 stainless

steel in air saturated and deaerated pH 7 IN NaCl solution were carried

out.

As Figs. 4, 5, and 6 show, lowering the oxygen concentration does

not affect the rate of film growth, current decay, or Rp. However,

the magnitude of these parameters were in all cases slightly lowered.

Thus, there was no mechanistic change, only a slight lowering in the

amount of film formed and the amount of current involved in the most

prevalent anodic processes (film growth and metal dissolution). Except

for the effect of oxygen on the open circuit potential which was purposely

circumvented in these experiments, the effect of oxygen on repassivation

kinetics appears to be minimal. The effect of potential and oxygen concentration will be examined in the next section.

Effect of Potential

Potential of the repassivating surface plays an important role

(12)

in determining stress corrosion susceptibility.^ ' These experiments

8

therefore inviestigate the effect of potential on repassivation kinetics.

Because the potential value at the tip of a crack is probably less than

that measured at the surface of a specimen, we have looked at more negative

potentials than the open circuit value measured for an air saturated

IN NaCl solution £+90 mV (SHE)].

The film formation kinetics in a pH 7 deoxygenated solution at

+90, -134, and -263 mv SHE are shown in Fig. 7. The -134 value is the

open circuit potential for a deoxygenated pH 7 solution, and -263 mV is

the value for the same solution after a number of abrasion cycles, a

condition that produces a high concentration of reduction products and

metal ions that may simulate more closely the conditions after crack

propagation has progressed. It ran be seen from Fig. 7 that at -134 mV

the mechanism of formation is initially the same as at the higher potential

but that it changes at around 40 msec and growth virtually stops. This

is even more pronounced at -263 mV. The current decay behavior (Fig, 8)

is somewhat similar to the higher potentials while the variation in Rp

(Fig. 9) is quite irregular.

DISCUSSION

It is evident from the repassivation data in Table 1 that the

repassivation process is least effective for surfaces exposed to the pH

3 solutions. Besides the fact that the time of repassivation and Rp are

2-3 times as large as they are for the pH 7 and 11 solutions, the ultimate films formed are thinner. Moreover, because the currents measured

during repassivation in the pH 3 solution are probably not the true values

since there exists in the acidic solution a considerable cathodic component,

the effectiveness of repassivation, R is even less than Table 1 would

9

indicate. These results therefore indicate, not surprisingly, that the

lowering of pH in an advancing crack does inhibit repassivation in

chloride environments. These results do not, however, settle the question

of whether the mechanism of SCC for austenitic stainless steel is a

slip-dissolution one or one involving the entry of hydrogen into the

lattice which promotes the formation of martensite or some other brittle

(12) fl4) phase as proposed, for example, by Troiano^ ' and others.^ ' They

do, however, indicate that regardless of whether the mode of crack growth

is due to metal dissolution or embrittlement , the conditions are present

for promoting poor repassivation rates.

Susceptible conditions are also present for the pH 11 solution

where the repassivation kinetics are slower than for the pH 7 solutions.

There are instances of failure of austenitic steel in alkaline environ-

(15) ments^ ' albeit at higher concentrations and temperatures. As mentioned

earlier, recent theoretical studi ndicate that if the hydrolysis

reaction is slow, the pH at the immediate surface may not be too dissimilar for a pH 3 and pH 11 solution. For example, on the basis of a

simple Pick's law diffusion calculation we find that the pH at the surface

can be as high as 10 for a pH 3 solution, assuming a diffusion limited

-4 2

hydrogen reduction current of 3 x 10 amp/cm . This could be the situation

within the confines of a crack where mass transport processes are rate

limiting. These considerations do not explain the order of the repassivation kinetics observed for the solutions studied. They simply point out

that evaluation of environmental conditions at the surface where repassivation takes place in the confinement of a crack is not straightforward.

Thus, it may be that the repassivation parameters measured at pH 11 in our

10

experimental set-up which does not have the confining conditions of a

crack may be more indicative of susceptibility. The repassivation rates

for pH 3 may be too low to promote cracking and instead would result in

tip blunting.

Lowering the oxygen content of the pH 7 solution does not appear

to affect appreciably the repassivation process. Such a decrease in oxygen

concentration would be reasonable in a propagating crack. Apparently, the

main role that oxygen concentration plays is its effect on the potential.

As the potential ejcperiments indicate, the lowering of potential towards

the active region does indeed affect repassivation by altering the rate

and mechanism of film growth. This would agree with the ideas of

Staehle^ ' who pointed out that regions of susceptibility are located

at potentials on the borderline between the active and passive regions

and the passive and pitting, (he labels it transpassive) regions. We

have not yet made studies at potentials near the pitting region because

of surface roughening problems, but we hope to be able to make an approximate measure of tp and Rp for those potentials in future experiments.

None of the measurements of repassivation rates are capable of

predicting susceptibility without also knowing something about the rate

of bare metal production during stressing. As a first estimate, however,

it is useful to use a new theory by Vermilyea^ ' who has developed a

film rupture model for stress corrosion crack propagation. This theory

relates the creep transient in the metal at the crack tip following film

rupture and the critical strain required to rupture the protective film

to the crack propagation rate as determined by the kinetics of repassivation and corrosion. Two parameters needed by the theory are L and t .

11

Using the values listed in Table 1 to calculate L by Eq [IJ we can compare

our measurements to values predicted by Vermilyea's theory. The value

he calculates for the minimum value of L, L . , which will allow SCC that ' min

comes closest to our values calculated from the tri bo-el lipsometrie values

are based on his equation derived for a plane strain condition with a

strain gradient of 1/r where r is the distance in the plane of the crack

from the center of curvature of the crack tip. This equation is

^min = 2(e -2eJ

p c

where is the plastic strain at the crack tip, is the critical

strain for film rupture, and = distance between ductile rupture void

centers. Assuming e„ » 2e- and = 5.08 x lOciJ, we get a value for

p C 0

s J

L^,-.. of 7.62 X

10"^ cm. Since the values listed in Table 1 are all of min

the order of magnitude or somewhat greater, on the basis of repassivation

kinetics only, we would predict that under conditions of plane strain

with a strain gradient of 1/r 304 stainless steel would be susceptible

to SCC. Obviously, both Vermilyea's theory and our calculations of L

have a number of assumptions and simplifications so that, in practice,

this prediction is suspect. Also in practice austenitic stainless steel

in chloride solutions of our concentrations at room temperature are not

very susceptible to SCC. What is needed is more and better measurements

of the mechanical and chemical parameters. The constant strain technique

described by Parkins^^''^ offers some hope for gaining some insight into

the mechanical aspects that are most applicable to tri bo-el lipsometric

measurements of repassivation kinetics. This will be the next direction

our work will take.

12

ACKNOWLEDGEMENT

We are most grateful to the Office of Naval Research who supported

this work under contract NAONR 18-89 NRG 36-082.

13

References

1. J. C. Scully, Corr . Sci . 8, 513 (1968).

2. R. W. Staehle, "The Theory of Stress Corrosion Cracking in Alloys,"

J. C. Scully, ed., N.A.T.O. Sci. Affairs Div., Brussels (1971) p. 223.

3. T. P. Hoar and S. G. Hines, J. Iron Steel Inst. 184, 166 (1956).

4. T. Shibata and R. W. Staehle, Proc. Fifth Int. Cong, on Metallic Corr.,

Tokyo, 1972.

5. D. J. Lees and I. P. Hoar, In preparation, reported in Proc. Int. Conf. on Stress Corr. Cracking and Hydrogen Embri ttlement of Iron Base Alloys,

Firminy France 1973, to be published by N.A.C.E., Houston.

6. J. R. Ambrose and J. Kruger, Corrosion 28, 30 (1972).

7. J. R. Ambrose and J. Kruger, Proc. Fifth Int. Cong, on Met. Corr., Tokyo (1972).

8. J. J. Carroll and A. J. Melmed, Surf . Sci . 16, 251 (1969).

9. G. Sandoz, C. T. Fujii and B. F. Brown, Con . Sci . 10, 839 (1970).

10. J. R. Ambrose, U. Bectocci and S. R. Coriell, In preparation.

11. L. Young, "Anodic Oxide Films," Academic Press, New York (1961) p. 14.

12. H. H. Uhlig and F. Cook, Jr., J. Electrochem. Soc. 116, 173 (1969).

13. A. R. Troiano, Trans. A.S.M . 52, 54 (1960).

14. See page 233 of Ref. (2).

15. D. V. Subrahmanayam and R. W. Staehle, See page 228, Ref. (2).

16. D. A. Vermilyea, "A Film Rupture Model for Stress Corrosion Crack Propagation,"

General Electric Co. Corporate Research and Development Report No. 73CRD063,

Feb. 1973.

14

Table 1

mV(SHE) msec

pH Potential

*P %

*

L, cm

3 yes +090 120 5.96 5.66 1.26x10"^

7 yes +090 65 1 .69 1 .60 2.30x10

n yes +090 35 3.33 3.16 1.10x10"^

7 no -134 70 1.45 1.53 1.47x10'^

7 no -263 60 1.25 1.35 9.50x10"'^

15



10 100

msec

Figure 2 Anodic current decay rates (log]Q mA vs log-iQ msec) for 304

stainless steel in air saturated l.ON NaCl (pH 3, 7 and 11)

at +90 mV (SHE) following removal of surface film by abrasion

at -1000 mV (SHE).

17

10

1.0 I 1 » 0 10 100

msec

Figure 3 Changes in Repassivation Ratio, Rp (login R^ vs logig msec) for

304 stainless steel in air saturated 1.6n N&Cl (pH 3, 7 and 11)

at +90 mV (SHE) following removal of surface film by abrasion at

-1000 mV (SHE).

18

!00

msec

Figure 4 Film growth rates (nm vs logio iTisec) for 304 stainless steel in

air saturated and deaerated l.ON NaCl solution (pH 7) at -i-90 mV

(SHE) following removal of surface film by abrasion at -1000 mV

(SHE).

19

100

\\air sot.

no 02\\^

1 10 KX)

msec

Figure 5 Anodic current decay rates (log]o niA vs. logiQ msec) for 304

stainless steel in air saturated and deaerated l.ON NaCl solution

(pH7) at +90 mV (SHE) following removal of surface film by abrasion

at -1000 mV (SHE).

20

Figure 6 Changes in Repassivation Ratio Rp (logio Rp vs log]o msec) for 304

stainless steel in air saturated and deaerated l.ON NaCl solution

(pH7) at 90 mV (SHE) following removal of surface film by abrasion

at -1000 mV (SHE).

21

Figure 7 Effect of applied potential (+S0, -134 and -263 mV SHE) on film

growth rates (nm vs login msec) for 304 stainless steel in air

saturated l.ON NaCl (pH/j following removal of surface film by

abrasion at -1000 mV (SHE)

22

0 10 !00

msec

Figure 8 Effect of applied potential (+90, -134 and -263 mV SHE) on the

Repassivation Ratio, Rq (logig Rp vs logio msec) for 304 stainless

steel in l.ON NaCl (pH/) following removal of surface film by

abrasion at -1000 mV (SHE).

23
£
Part II. To be presented at National Meeting,

Electrochemical Society, Boston, Massachusetts, October 1973

A New Technique for Studying Crevice Corrosion by Ellipsometry

J. R. Ambrose and J. Kruger

National Bureau of Standards

Institute for Materials Research

Washington, D.C.

ABSTRACT

The early stages of crevice corrosion of AISI 304 stainless

steel in l.ON NaCl solution have been detected using the ellipsometer

to measure changes in optical properties occurring within the crevice

between a polished metal surface and a glass plate. Changes in the

ellipsometric parameters A and ii begin almost immediately upon creation

of the crevice and can be interpreted as resulting from a build-up of

soluble species within the crevice solution, followed by an overall thinning of the protective film and general corrosion attack.

Such optical changes could not be reproduced by deoxygenation

of the bulk solution without the presence of a crevice nor were they

observed during experiments using a Ti-8Al-lMo-lV alloy, which is not

susceptible to crevice corrosion in the l.ON NaCl at room temperature.

24

Part II. To be presented at National Meeting,

Electrochemical Society, Boston, Massachusetts, October 1973

A New Technique for Studying Crevice Corrosion by Ellipsometry

J. R. Ambrose and J. Kruger

National Bureau of Standards

Institute of Materials Research

Washington, D.C.

There has been an increasing interest In the problem of crevice

corrosion in recent years because it is now recognized that it is difficult to design systems that avoid crevices. Thus, crevice corrosion can

(1) (2) be the cause of many equipment failures due to corrosion.^ ' Coburn^ '

has also pointed out that concentration cell crevice corrosion is the

most important example, costing industry more tr.an any other type of

corrosion. Mechanisms based almost exclusively on electrochemical techniques have been proposed which combine a version of the differential

oxygen concentration cell modified to reflect the separation of active13) passive cells by restricted mass transport,^ ^ Recent reviews on the

subject have discussed experimental techniques and proposed mechanisms

(4 5)

to account for the results obtained from such techniques,^ ' ' and have

pointed out the problems arising in interpreting electrochemical data.

In order to complement the available electrochemical techniques,

a method has been devised using a glass plate to create the crevice,

which permits optical detection of changes occurring during the corrosion

process by means of ellipsometry. Use of such transparent windows for

(6 7) visual observation of crevice is not new,^ ' ' but the results from such

studies have been limited to detection of crevice corrosion in its advanced stages. The optical technique of ellipsometry, however, not only

permits measurement of small changes in thickness and optical properties

of surface films on metal substrates,^ ' but can perhaps be used to detect

25

changes in composition of aqueous media as well. By use of this technique

it is hoped that it will be possible to interpret the electrochemical

results obtained in previous studies by being able to detect small changes

occurring in the early stages of the chain of events leading to crevice

corrosion. Since this technique mainly focuses on what happens to the

protective film on the metal in the crevice, it may be useful in developing ways to prevent film breakdown leading to crevice attack.

To investigate the practicality of such a technique in measuring

optical changes which occur during the early stages of crevice corrosion,

we have examined by means of this technique the effect of e crevice on

the behavior of AISI 304 stainless steel in a l.ON NaC^ solution, m

environment in which this material is known to be susceptible to crevice

corrosion attack. We have also compared the behavior observed for the

susceptible stainless steel to that of a titanium alloy which is not-

(4) susceptible to crevice attack in chloride at room temperature.^ ' Since

solution deoxygenation is thought to be a prime factor in establishing

the specific location for initiation of crevice corrosion, a complementary

study into the effect of solution deaeration on system electrochemistry

and protective film properties was also carried out.

EXPERIMENTAL

Apparatus

The device used to simulate a crevice in these stydies is shown

in Fig. 1 and consists of a Teflon adapter which serves to position a

circular pyrex glass plate (3.175 cm diameter x 6.35 cm thick) against

the surface of a polished metal specimen. The thickness of the pyrex

26

disc was such that interference between the beams reflected from the metal

surface and those reflected from the top of the plate did not occur

(Fig. 2). The sides of the Teflon adapter were cut away in order to allow

access of a bulk solution to the crevice. The lateral surfaces of the

specimen were isolated from the environment by a tightly wound layer of

Teflon tape in order to prevent crevice attack where it could not be observed optically. The rear surface of the specimen was similarly protected

during those experiments where potentiostatic control was applied. The

ellipsometer and cells were similar to those described elsewhere.^ '

Materials

The 0.635 cm thick cylindrical specimens used in this study were

machined from 3/4-inch diameter AISI 304 stainless steel rod stock and

Ti-8Al-lMo-lV alloy, then drilled and tapped for mounting in an

electrode assembly. The specimens were hand polished using a series of

silicon carbide metal 1 ©graphic papers followed by rotary polishing on

nylon cloth with 6 and 1 micron diamond paste. Specimens were rinsed

with alcohol and dried in a hot air stream. Solutions were prepared

using ACS reagent grade sodium chloride and distilled water (7.0 x lO"''

ohm"^ cm'^). Deaeration was accomplished by dispersing 99% purity

helium gas through the solution.

Experimental Procedure

Following electrode assembly and specimen polishing, the entire

electrode unit was rinsed with alcohol and dried under hot air. Those

specimen surfaces which were to be protected were wrapped in Teflon tape,

and the electrode assembly inserted into the electrochemical cell. The

27

cell was flushed once, then filled with l.ON NaCl and electrical connections made. Cell and specimen positions were adjusted in order to achieve

proper alignment for the ellipsometric measurements. Initial readings

of A and were made within two minutes after exposure of the specimen

to the l.ON NaCl solution. Following measurement of the open circuit

co-rosion potential, the crevice device was attached to the specimen;

the cell was repositioned for maximum optical sensitivity. A and ij;

were again measured within two minutes after creation of the crevice.

A, ijj, and current (or potential, depending on whether potentiostatic or

open circuit mode of control was used) were monitored for periods up to

approximately 2000 minutes or until crevice attack was evident. The

crevice device was then removed, and final a and \i) values were recorded

RESULTS AND DISCUSSION

Effect of Solution Deoxygenation

There appears to be general agreement with the fact that oxygen

depletion within the crevice constitutes the initial phase of crevice

corrosion attack. Therefore, we sought by making ellipsometric

measurements, to determine the effect of oxygen depletion on the protective film on the metal in the absence of a crevice. These measurements

give values of A, the relative phase retardation, and the tangent of

which is the relative amplitude reduction. For a metal with absorbing

films such as stainless steel, A is the parameter most sensitive to

changes in film thickness. ^^^^ Both parameters are affected by film

optical property changes. Results for 304 stainless steel exposed to

l.ON NaCl without a crevice, in both potentiostatic and open circuit modes

28
'

'

.'.1
are shown in Figs. 3 and 4. In the open circuit operation mode, oxygen

depletion results in the expected shift in corrosion potential toward

more negative values. A values are virtually unchanged throughout the

exposure period, increasing by only 0.08 degrees after 1350 minutes.

The value of ^ similarly is constant during the first 100 minutes or so

of exposure, but then increases during a period where the rate of

potential change appears to increase. Overall, ^1^ is increased by 0.260

over the exposure period.

When the specimen potential was fixed at a value corresponding

to the most negative potential attained during deaeration of the solution

(-400 mV SHE), A and ijj were virtually unchanged during the first 50

minutes or so while the cathodic current was decreasing by an order of

magnitude. Both A and ij; then varied simultaneously in a direction that

earlier studies^ ' had shown were characteristic of surface roughening.

Net change in A and were -0.60 and +0.38, respectively.

We can infer from these data on the effect of deoxygenation on

the protective films on 304 stainless steel that when initially exposed

to a deoxygenated solution when the potential is changing slowly

(9.4 mV/minute), the ellipsometer parameters A and ^i) are rather constant

with only a very slight increase in both parameters being evident. This

means very slight thinning or changes in the nature of the film have

occurred. Based on the reaction:

O2 + 2H2O + 4e" i 40H" [1]

the overall potential change of -94 mV would correspond to a 6 order

of magnitude decrease in oxygen concentration. However, later on where

there is a rapid rate of potential change, there is a significant increase

in values of \i) after 100 minutes or so, while A remains essentially constant. This would appear to indicate a greater change in the nature of

the film has occurred, but with still no significant thinning, possibly

as a result of the decrease in supply of available oxygen as evidenced

by the constancy of A. Since micropits were observed at this stage, such

behavior indicates a localized breakdown in passivity.

Placing the specimen under potentiostatic control at a potential

where cathodic reduction of oxgyen would be enhanced also accelerates

those processes which are characterized by changes in the ellipsometer

parameters. The abrupt change in tj< which occurs after about 30 minutes

and is followed 20 minutes later by a decrease in A could be due to surface roughening. As before, the observation of micropits on the surface

of the specimen upon later visual examination confirmed this conclusion.

At this point, our results are in agreement with those recently

published by Bates ^^^^ where he correlated various crevice processes

to breaks in potential -time curves. However, our ellipsometric studies

suggest that, contrary to Bates' proposals depletion of oxygen alone

does not lead to uniform film thinning and loss of passivity over the

whole surface, a behavior characteristic of crevice corrosion attack,

but instead results in a discrete change in the nature of the film

followed by a localized loss of passivity that is more characteristic

of pitting.

Effect of the Crevice

As shown in the past section, oxygen depletion alone does not lead

to the general attack observed under crevices. The next series of experiments were directed at looking at the role of the crevice. Results from

30

these studies in which a crevice was introduced for the open circuit and

potentiostated modes are shown in Figs. 5 and 6. For the open circuit

mode several observations are apparent. Concurrent with a slow shift in

potential to more negative values, there is a significant decrease in

A values (6A = 'V'O.S*'). Although the value of ^ appears to fluctuate

and was not particularly reproducible, there was a general increase in

^ values during the course of exposure {sii = M}.7°). After a period of

20-45 minutes, A began to increase steadily, reaching a maximum value

(6A = 'v2.4°) in 200-300 minutes, after which it decreased markedly. At

this time, several gas bubbles were clearly visible behind the glass

plate and a discoloration was apparent at the periphery of the specimen

surface.

When potentiostated at the initial immersion corrosion potential

(+42 mV), A values again decreased markedly during the first 20 minutes

of exposure (6a = ^-0.12°) before increasing to a maximum value (5A = +.60°),

after about 1450 minutes. The t|i values, however, decreased regularly

during the first 200 minutes (5ij; = =0.65°) then increased for the remainder

of the exposure period. Current values slowly increased from +0.66 yA

to a final value of 300 yA at the end of the run. The major increase

in the current, which indicates an increase in corrosion rate occurred

at about the same time as the A values started to decrease sharply.

It is obvious from both the open circuit and potentiostatic experiments that completely different optical results are obtained when a

crevice is introduced. Although the process of oxygen depletion is

probably accelerated within the low volume crevice, the initial direction

of change of both A and ^ is opposite to that observed in oxygen depletion

31

studies where a crevice was not present, at least for the open circuit

experiments (great fluctuations in values for the potentiostates crevice

system make any concise analysis impossible at this time).

Before attempting to interpret the ell ipsometric observations, it

is necessary to discuss the effect of the glass plate on the ellipsometric

observations. Since the glass plate is a dielectric there will be no

phase change with transmission through the glass plate and the A measured

will not differ from that resulting from optical conditions at the metal

surface. The value of measured, however, will be changed due to the

transmission coefficients at the surfaces of the glass plate. These

transmission coefficients will in turn depend upon the refractive index

of the solution in the crevice. This latter is generally unknown and

changing during the course of our experiment.

We sought to determine empirically the effect of increase of the

solution refractive index on A and i|j outside of a crevice. To do this

we made measurements on a stainless steel surface, with a crevice, in

both IN NaCl and 2.5N NaCl solutions, the latter having a higher refractive index. As Table I shows that the measured value of A with the

crevice in place decreased markedly in value in the more concentrated

solution. The value of ii increased

32

slightly. Thus, based on the changes in A shown in Figs. 5 and 6, the

initial stages of crevice corrosion are consistent with and increase in

crevice solution index of refraction, i.e., £n increase in the concentration of Cl~ and metallic ions. Because \i) is difficult to interpret in the

presence of a crevice for the reasons given above and because of the araater

fluctuations in the values measured, we cannot judge whether the if;

measurements are indicative of crevice solution concentration during the

first part of the chain of events leading to crevice corrosion.

The substantial increase in a and continued decrease in \i)

following the first phase is consistent with an overall decrease in

thickness of the protective surface film. Since oxygen depletion alone

has been shown not to produce such an effect, we can assume that such

thinning results from whatever happened during the first phase of the

process, where a was observed to decrease . Finally, the marked decrease

in A is in the same direction as was previously noted for the noncrevice oxygen depletion studies, and is characteristic of surface

roughening and/or film growth. Notably, this decrease comes at the

same time that the current measurements indicate an increased rate of

corrosion.

The effect of a crevice on the corrosion rate and ellipsometer

parameters of the Ti 8-1-1 alloy is shown in Fig. 7. This alloy, which

is not susceptible to crevice corrosion in this environment at room

temperature, exhibits none of the ellipsometric variations associated

with the onset of crevice corrosion attack on 304 stainless steel when

potentiostated at the open circuit corrosion potential. In fact, A,

and i are relatively constant over the entire period of exposure.

33
I-

!

i

I
Although these mechanistic inferences are consistent with the

observed ellipsometric variation, they are by no means the only possible

interpretation. This is so because the ellipsometric n^asurements reflect

average overall changes occurring under the glass plate. In other words,

one process - i.e., solution concentration - could easily mask out

small changes in the film/metal substrate. Much further work is necessary

to make ellipsometric observations of crevices more definitive and quantitative. However, the results just described are important qualitatively

because they indicate the times at which changes occur in the crevice

in the processes leading to crevice attack. Moreover, a reasonable

picture, consistent with the optical measurements of what events are

initiated during these changes can be developed using measurements in

special environments without a crevice. This sequence of

events for the susceptible stainless steel exposed to a crevice in a

chloride solution can be summarized as follows:

1. Depletion of dissolved oxygen within the confines of the

crevice accompanied by a build-up in concentration of dissolved metal

and chloride ion. (Slight film dissolution resulting from the consumption

of hydrogen ion generated by metal ion hydrolysis is probably masked in

these ellipsometer measurements.)

2. Overall film thinning which probably continues to consume

hydrogen ions, accompanied by increased metal dissolution.

3. A sharp decrease in pH when the protective film is gone,

accompanied by surface roughening as metal dissolution proceeds into the

final stages of crevice corrosion attack. C thodic reduction of H"^

could also be expected to occur.

34

In contrast, for the non-susceptible Ti alloy, a constancy of

el 1 ipsometric and current measurements over the entire exposure period

indicate that none of the crevice solution concentration effects which

were detected for the susceptible 304 stainless steel had occurred.

This study has thus shown that it is feasible to study crevice

corrosion by means of el 1 ipsometry. It has demonstrated a difference in

the optical behavior between a metal susceptible to crevice corrosion

(304 stainless steel) and one that is not (Ti 8-1-1). It has been shown

capable of detecting a number of stages leading to crevice attack.

Much further work remains, however, to enable us to develop the technique

into a more quantitative tool, future work will also involve modifying

the technique to enable pH and potential measurements to be made within

the crevice.

ACKNOWLEDGEMENT

We wish to thank Dr. Elio Passaglia for his invaluable aid in

interpreting the ell ipsometry in the presence of crevices.

We are also most grateful to the Office of Naval Research,

especially Dr. Phillip Clarkin, for encouraging us to undertake this

work and for their support under contract NAONR 18-98NR036-082.

35
c
REFERENCES

1. R. J. Landrum, Chemical Engineering 118 , 24 (1969).

2. S. K. Coburn, Materials Protection 6, 33 (1967).

3. D. A. Vermilyea and C. S. Tedman, J*^., J. Electrochem. Soc. 117 ,

437 (1970).

4. W. D. France, Jr., "Localized Corrosion - Cause of Metal Failure,"

ASTM Special Technical Publication 516, M. Henthorne, Chairman,

Atlantic City, 1971 (ASTM, Philadelphia), p. 164.

5. I. L. Rosenfeld and I. K. Marshakov, Corrosion 20, 115t (1964).

6. R. B. Mears and U. R. Evans, Trans. Faraday Soc. 30, 417 (1934).

7. U. R. Evans, J. Inst. Metals 30, 239 (1923).

8. J. Kruger, Corrosion 22^, 88 (1966).

9. J. R. Ambrose and J. Kruger, Corrosion 28, 30 (1972).

10. J. F. Bates, Corrosion 29, 28 (1973).

36
; A'
TABLE I

NaCl Solution Refractive A ij^ SA, « « c St/;-. « « r Concentration Index

i.i^^.o

l.ON 1.3432 113.86 29.38

2.5 1.3567 m.70 29.41 -2.16° +.03*

37

Specimen Electrode Assembly

Figure 1 Schematic of the Teflon adapter and glass disc used to simulate

a crevice where ellipsometer measurements could be made.

38

39

Time,mln.

Figure 3 Effect of solution deoxygenation on ellipsometer parameters

A and 4^, and the corrosion current, as a function of logio

time for 304 stainless steel with no crevice in l.ON NaCl

Specimen is potent iostated at -400 mV SHE.

40

-E.

mV.SHE

500

400 _

1.0 - 300

(^ 0.5' - 200

>— 100

6

0« ——o— —o o A

-0.5 0

1 10 100 1000

Time , min.

Figure 4 Effect of solution deoxygenation on ellipsometer parameters

A and and the corrosion potential, as a function of logio

time for 304 stainless steel with no crevice in l.ON NaCl

41

Figure 5 Effect of a crevice on ellipsometer parameters A and ijj, and the

corrosion potential, as a function of log-in time for 304 stainless

steel in l.ON NaCl

42

10 100 1000

Time , min.

Figure 6 Effect of a crevice on ellipsometer parameters A and t, and the

corrosion current as a function of log-jo time for 304 stainless

steel in l.ON NaCl ; specimen is potent! ostated at +42 mV SHE.

43

0.5 05

SA 0 - 0 S\//

0.5 -0.5

10 100 Time, min.1000

Figure 7 Effect of a crevice on el 1 i psometer parameters A and as a function

of log-iQ time for Ti 8-1-1 alloy in l.ON NaCl ; specimen is

potentiostated at +169 mV SHE.

44

Part III. To be submitted to the Journal of the Electrochemical Society

TRIBO-ELLIPSOMETRIC STUDY OF THE REPASSIVATION KINETICS

OF A TI 8AL-1M0-1V ALLOY

J. R. Ambrose and J. Kruger

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

ABSTRACT

The tri bo-el lipsometry technique with which measurements of film

growth and total reaction rate are made following film removal by abrasion

was used to determine repassi vation kinetics and stress corrosion cracking (sec) susceptibility for Ti-8A1 -lMo-1 V alloy. Repassivation transient

behavior in a l.ON NaCl solution, where cracks have been found to propagate,

was compared to that in a l.ON NaNO^ solution where SCC susceptibility

has never been detected.

Film growth kinetics in both solutions were consistent with a

Fleischmann-Thirsk mechanism of oxide patch nucleation and two-dimensional

growth, although the film growth rate was significantly slower in the

l.ON NaCl solution. This was in turn responsible for an increase in metal

dissolution in a solution where crack propagation velocities have been

measured, but at an apparent rate slower than necessary to propagate

such cracks by metal dissolution alone.

45

Part III. To be submitted to the Journal of the Electrochemical Society

TRIBO-ELLIPSOMETRIC STUDY OF THE REPASSIVATION KINETICS

OF A TI 8AL-1M0-1V ALLOY

J. R. Ambrose*and J. Kruger

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

Theoretical treatments of the relationship between repassivation

kinetics and susceptibility to stress corrosion cracking have recently

been incorporated into mechanisms for cracking of a number of materials,

noticeably mild steel and titanium alloys. Studies by Beck^^^

relating crack propagation velocities to anodic current densities measured

following fast fracture of titanium alloy wire utilized an empirical relationship to estimate film growth rates during the repassivation transient.

The recently developed technique of tribo-ellipsometry , with which

a determination of both anodic current and film growth rates can be made

following passive film removal by abrasion, makes possible a separation

of the total repassivation process into film growth and metal dissolution

components.

This work describes the results of a study comparing repassivation

behavior of a commercial titanium alloy (Ti 8AI-IM0-IV) exposed in a solution where cracks are known to propagate (l.ON NaCl ) and in l.ON NaNO^,

where no susceptibility to cracking has been detected.

An attempt has been made to correlate measured film growth rates

with tne Fleischmann-Thirsk patch nucleation and growth mechanism and to

estimate crack propagation velocities which would result from metal dissolution during repassivation.

*A portion of this work has been included in the research dissertaiton submitted to the University of Maryland in partial fulfillment of the requirements for the Ph.D. degree.

46

EXPERIMENTAL

The description of the experimental apparatus and technique of

tri bo-ell ipsometry has been presented in prior publications. * '

A Ti 8AI-IM0-IV commercial alloy was used in this study: 1.905

cm diameter x 0.635 cm thick cylindrical specimens were machined from

rod stock and polished with silicon carbide metallographic papers followed by rotary polishing on nylon cloth with 6y diamond paste. Airsaturated solutions of l.ON NaCl and l.ON NaNO^ were prepared from ACS

reagent grade chemicals and distilled water.

Specimen potentials were controlled potentiostatically , with the

repassivating current pulse generated upon cessation of abrasion used

to trigger the sweep of a dual channel oscilloscope which also recorded

the output of the photometer system used to monitor changes in ellipsometer light intensity.

Total coulombs, Qj, passed during the transient were determined by

graphical integration of the total current, i-j-; film thickness, x, was

determined from light intensity changes which were converted using predetermined calibration data.

THE REPASSIVATION RATIO. R

P

Although measurement of the total current, ij, and the film thickness, X, as a function of time during the repassivation transient is

relatively straightforward, several uncertainties exist that make it difficult to render the data completely unambiguous and quantitative in any

treatment of data obtained utilizing this technique. These uncertainties

are as follows:

47

1. The total current involved in repassivation, iy, is the current

supplied by the potentiostat to maintain the specimen potential at some

fixed value with respect to a reference electrode potential. If any

cathodic reduction were also to supply current, a determination of i^,

the anodic current used during repassivation, becomes difficult. Of

course, i^ can be made negligible by operating at potentials 50 mV or

so more positive than the open circuit or corrosion potential

this would defeat the point of attempting to predict susceptibility to sec for materials immersed in specific environments.

2. The charge utilized in the formation of an oxide film is given

by

Q = ^ zFgAx j-^-j ^x 0 X M

where i = current consumed by film formation

-, - w=.i««^« equ ivalents

z = valence, ^ ^^i; - mole

r r- J I J. i coulombs

F = Faraday's constant,

p = density of oxide,

cm « A = surface area of oxide layer, cm

X = film thickness , cm

M = molecular weight ^ of oxide, 5?t-« mole

It is difficult to know the value of area. A, for the surface where film

growth (and metal dissolution) occurs since the efficiency of the abrasion

process is difficult to ascertain.

(5)

To get around these difficulties a recent paper^ ' expressed the

experimentally measured variables of total current, ij, and film thickness,

X, in the form of a ratio, designated Rp and giv^n by:

48

X [2]

t M

Since i-j-dt = Q-j. and x = ^1^®"

R„ = where k

= zFpA

• [3]

If we assume that = + then

1 . -d

[1 -n^]

X

[4]

where = charge consumed by metal dissolution.

Although evaluation of A is difficult, it is sometimes useful to

express R as a dimensionless variable, designated R and equal to

r r

[5]

since results would be independent of the particular experimental procedures and apparatus employed. Although R and R differ in magnitude

only by the constant, k, R values are determined directly from experimental measurements; therefore, discussion and conclusions will be based on

Rp values. Differentiating Rp with respect to time

dR.

dt

2. = I

dQj dQ^

^x at" ^d 3t~

[6]

1 Vd - Qd^x

[7]

1 ^d 1 Vx

F \

0 4

" F

IX J

[8]

49
-1,1.
In this derivation we have neglected the cathodic current, i^. This is

not an unreasonable assumption since it is generally quite low except in

acid solutions, hence negligible compared to i^ when the anodic overvolta age is high.

dR The values of R^ and p can be used to distinguish between metal

P dt

dissolution and film repair and thus provide a parameter for predicting

stress corrosion cracking susceptibility. As an example, consider the

various limiting cases:

Case I ^d \

i. Rp becomes very large and i^ is nearly 0.

dR

dt

^> 0

This would correspond to a situation where general corrosiotj without any

repassivation was occurring.

Case II: i >> ij

X d

1

dR Q.id'x

Q.

from eq. [8]

This corresponds to virtually instant repassivation with no metal dissolution and little susceptibility to SCC.

Case III

Rp=k 1

dR

dt

0

i i . X > 0

Subcase Ilia dx^ > 0; (at large values of t)

50

In this case, the film formed is not protective, but is similar in characteristics to Case I because general corrosion or pitting results.

dx

Subcase Illb:

^ Ut large values of t).

These conditions would be those most consistent with the initiation and

propagation of stress corrosion cracks based on a metal dissolution mechanism which is dependent upon the rate of repassivation. In other words,

AD

a relatively large value of R , with small value of ^ and dx indicate

P Ht clt that metal dissolution accompanies film regrowth, but that repassivation

eventually occurs, thus protecting the walls of the advancing crack and

restricting propagation of the crack to the area directly at its tip.

These conditions are summarized in Table I.

RESULTS

1 . Effect of Anions

Results of the tribo-ellipsometric repassivation kinetics experiments conducted in IN NaNO^ and IN NaCl solutions at respective corrosion

potentials are shown in Figs. 1 and 2. The value of the repassivation

ratio, Rp, is significantly higher in the NaCl solution, approaching a

limiting value of about 2 "^^^"^ when monolayer coverage of 0.3 nm is

obtained as compared to the NaNO-j solution (R„ =0.3 "^^^""^

) . The values

3 p^.^ nm of the ell ipsometrically determined film thickness are also shown. In

NaNO^, an oxide thickness of 0.30 nm is present on the metal surface by

the time the abrasion wheel had retracted to allow ellipsometric measurements

*Although corrosion potentials are extremely difficult to reproduce, depending upon specimen surface treatment and historyv."/, the open circuit

potential established for the particular specimen used in these studies

after several abrasion cycles was the same for Ti 8-1-1 in both solutions

used and was fairly reproducible even when the specimen was repolished

or solutions changed.

51

('V 5 msec), indicating extremely rapid early stage film growth. In

NaCl , on the other hand, film growth is so slow that a film thickness

of 0.30 nm is not reached until 38 msec, following cessation of abrasion.

2. Effect of Potential

The results of repassivation experiments conducted in the IN NaNO^

and IN NaCl solutions at a higher anodic potential (+706 mV SHE) are given

in Figs. 3 and 4. R values are significantly higher than at open cirPlim

cuit in both solutions ('v3.00 for NaCl; '\^1.02 for NaNO^). Initial film

thickness growth is substantially slower at +706 mV than at -222 mV for

the NaNO-j solution. A comparison of oscilloscope tracings for Ti 8-1-1

in IN NaCl and IN NaNO^ at 706 mV SHE is given in Fig. 5.

Peak current densities for the transients in NaCl and NaNO^ were

65.0 and 18.8 mA, respectively.

Film growth kinetics were determined and are not inconsistent with

a parabolic growth law - i.e., a straight line was obtained when film

thickness was plotted against the square root of time. These plots for

Ti 8-1-1 in IN NaCl and IN NaNO^ at both potentials are given in Fig.

6. The values of Rp, Rp , T^ (the time required for obtaining 0.30 nm

1/2 film thickness), dx/d(t ' ), and i^^^ are given in Table II.

DISCUSSION

1 . Repassivation Ratio and Stress Corrosion Susceptibility

From Fig. 1 and Table II it can be seen that for Ti 8-1-1 in

IN NaNO^ at the corrosion potential of -222 mV SHE, a low value of Rp with

dRp/dt - 0 was maintained throughout the repassivation transient. Based

on the size of the oxide ion a monolayer of Ti02 would be approximately

52

0.30 nm in thickness; it formed in <5 msec, in this environment. These

results are characteristic for a Case II repassivation transient in which

most of the current transient is utilized in rapid film growth without

significant metal dissolution. When this is compared to the results obtained in IN NaCl at the same potential, where crack propagation velocities

on the order of 10"^ have been measured '^^ it is seen that is sec * p significantly higher as is dRp/dt indicating substantial metal dissolution

which can be accounted for by low rates of film growth in IN NaCl . Although monolayer coverage is not obtained until 38 msec, following cessation of abrasion, Rp changes slope at about 12 msec, indicating some inhibition of metal dissolution, perhaps by formation of a precipitated

salt layer on the surface resulting from saturation of the diffusion layer

with TiCl^ (Fig. 2). At any rate, repassivation is eventually achieved

(Case Illb). Similarly, at an applied potential of +706 mV SHE, R is

r quite high in the chloride solution with Rp changing slope at about 10

msec, which would substantiate a precipitated salt passivation mechanisn

since a saturated solution of TiCl^ would be obtained more rapidly at these

higher current densities. The time to complete monolayer coverage, t^^,

is about 20 msec. Rp and dR/dt for IN NaNO^, although larger than at

the lower potential, are far less than the NaCl environment.

Based upon analysis of the Rp, dRp/dt and t^^ parameters, it

appears that, at least for titanium alloys, a Case I -> Case Illb transition in dRp/dt with an initial high Rp predicts susceptibility to stress

corrosion cracking under the experimental conditions employed.

We should, nevertheless, not rule out a mechanism based on fracture

of titanium hydride (TiH^) produced by the cathodic reduction reaction:

53

Ti + 2H^ + 2e -> J\H^ [9]

Since the results of our study show lower repassivation rates in

the environment known to be susceptible to SCC, repassivation kinetics that

can control or limit access of H to the Ti surface may still be relevant.

Secondly, the fractography found for titanium alloys that have undergone

fo)

SCC show a great deal of cleavage. . This could rule out metal dissolution as a factor during crack propagation. However, the significantly

higher metal dissolution in CI" solutions suggested by Beck^^^

confirmed

in this study cannot be ignored. The effect of repassivation on metal

dissolution must be important in the initiation stages of a crack when,

because the pH of neutral chloride solutions would be near bulk solution

values, the concentration would be too low for the reaction in Eq. 9

to play an important role. During later stages of propagation, where

metal ion hydrolysis leads to pH and iR drops within the crack, hydride

cleavage mechanisms must be considered in addition to the effects of

environmental variations on film growth and metal dissolution kinetics.

2. Film Growth Kinetics

The first stage of film growth appears to be consistent with the

f 9) Fleischmann and Thirsk analysis of oxide nucleation and growth.^ ' They

2 assume an instantaneous nucleation of n patches/cm which spread in 2

dimensions until formation of an oxide monolayer is complete. The equation

for the density to the oxide patches is given by [9]:

i^= 2m Ct exp [-Ct^] [10]

where m = number of monolayers

coulombs

= charge density of oxide monolayer

54

t = time, seconds

2

C = nn 'V2

-2

n = number of oxide patches, cm

= thickness of a monolayer, cm

2

i2 = current density at periphery of oxide patches, amperes/cm

Since the patch thickness is assumed coiistant until complete monolayer coverage is obtained, would be constant; C would also be independent of time. This allows integration of Eq. 10 [from (2)]:

1 - e = exp [-Ct^] [11]

where e is the fraction of surface coverage.

Taking logarithms of both sides gives:

log [1 - e] = -Ct^ [12]

Since e is proportional to the ellipsometer parameter A which

depends on average film thickness, hence also to aP (the change in polarizer

reading), a plot of log aP vs t should be linear if the Fleischmann-Thirsk

model is an appropriate one. Results are given in Fig. 7.

From Table II, we find that a monolayer (approximately 0.30 nm) has

formed on Ti 8-1-1 in the NaNO^ solution at the open circuit potential

in less than 5 msec. A comparison with the NaCl solution at the same

potential shows that monolayer coverage is not obtained until some 30 msec,

later, and the log (1 - e) vs t plot is linear, indicating possible

applicability of the Fleischmann model. Experiments run at the higher

anodic potential give approximately the same effect, that is, film growth

is substantially slower in the chloride solution than in the nitrate. The

fact that first stage growth rates at the higher potential are lower in

the nitrate solution may be due to a localized pH lowering from sol va ted

55

+3

Ti hydrolysis. This could measurably affect film growth kinetics. Such

an effect would not be noticeable in the chloride solution where hydrolysis

of the metal ions probably results in pH decreases in both potentials.

The values of i^^ for both solutions at the two potentials would substantiate this conclusion. In NaNO^ the relative increase in peak current

resulting from film removal at the higher potential is about 6:1 whereas

only about a 3:1 increase is noted for chloride solution (Table II).

Simultaneous determination of both film thickness and total current

allow an estimate of the amount of metal dissolution occurring during the

repassivation transient. Such an estimate would allow prediction of the

crack propagation velocities to be expected if propagation proceeds via

(2) an electrochemical dissolution mechanism as suggested by Beck.^ '

Assuming that metal dissolution occurs by Tafel kinetics on the

bare metal between patches:

\ - ^d.o^'^P ["'^a - Eo.+)/RT] [13]

A where i^j q

= exchange current density, —^ ' cm

= transfer coefficient

= applied potential (potentiostat)

a

^

= equilibrium potential for dissolution reaction.

Total metal dissolution current over the entire surface is proportional

to the area of bare metal exposed. It can be assumed, however, that

the local dissolution current density on bare metal surfaces remains constant over the span of the repassivation transient until the patches coalesce.

This would allow an estimate of the maximLHn penetration of metal by electrochemical dissolution. Since the local dissolution current density is given

imav/A. x„ , the maximum penetration at those areas of surface last

Pmax

56

covered by the monolayer, is given by:

i M

X, =-3^t^ [14]

where M = atomic wt. of metal

,

z = valence^ —

g

- a

3

p = density of metal, gras/cm

2

A = initial bare surface area produced, cm

F = Faraday's constant. equiv

t^ = time for monolayer formation, sec.

Using data from Table I for Ti 8-1-1 in IN NaCl at the open circuit

potential of -222 mV SHE, we obtain a value of x of 2.3 x 10'^ cm or

_6 ""^^ a maximum propagation velocity of 6.0 x 10" cm/sec. Even if the 1^ - max

i^ approximation Is in error due to localized cathodic reactions, as

as overestimating bare metal surface area, propagation velocities based

on electrochemical metal dissolution do not come close to the observed

pro

(2)

_3

stress corrosion crack propagation velocities of 8 x 10 cm/sec

Beck at this potential.

However, penetration velocities are potential dependent, v

being calculated as 16.2 x 10" cm/sec at a potential of +706 mV SHE.

From this we would predict that upon increasing the applied potential by

1 volt the propagation velocity would increase by a factor of three, in

agreement with Beck.^ ^ This would suggest that although metal dissolution does not account for the observed crack propagation rates, it may be

a necessary condition for such propagation.

57
' 'n^v'' ? j 0'' ' a'
The Fleischmann-Thirsk model for patch growth, coupled with the

assumption of constant metal dissolution current density at the metal surface,would, however, predict creation of localized penetration sites.

The geometry of these sites would lead to increased stresses at the point

of maximum penetration as well as effecting environmental composition

variations due to the occluded nature of these sites (Fig. 8).

Whether crack propagation then proceeds via a dissolution assisted

mechanical cleavage process or by fracture of a brittle region produced

by entry into the metal of hydrogen formed by cathodic reduction cannot

be determined on the basis of the results presented here. The specific

function of halide ions (e.g., CI") in restricting lateral oxide growth

is also highly speculative but may result from preferred ion adsorption

inhibiting oxide growth kinetics. Green and Sedricks suggest that on

the basis of their el 1 ipsometric potential step experiments film growth

kinetics play no part in determining SCC.^^^ However, they did not

study film growth kinetics on bare metal surfaces, making their conclusions somewhat irrelevant to stress cracking. By varying the water content of 3.5% LiCl in dimethyl sulfoxide solutions they have been able to

control crack propagation velocities in Ti 8-1-1.^^^^ Their results suggest the possibility of a hydrogen embrittling mechanism since no availabl

hydrogen is present in pure DMSO; however, neglect in measuring their

solution conductivities leaves open the possibility of crack propagation

rates being limited by mass transfer of some rate determining species.

On the other hand, this work indicates that film growth rate and SCC

susceptibility appear to be related for those environments studied.

- 58 -

CONCLUSIONS

T. Ti 8-1-1 alloy repassivates rapidly in NaNO^ solution as indicated by a low R and dR /dt with a monolayer of oxide being formed in

less than 5 msec.

2. Ti 8-1-1 alloy repassivates less rapidly in NaCl solution than

in NaNO^ solution as indicated by a higher R with an initially higher

o p

dRp/dt. Although monolayer coverage is not obtained until 38 msec, following oxide removal, a decrease in dR^/dt is observed at 12 msec, probably

to inhibition of metal dissolution by a precipitated salt layer.

3. An analysis of ellipsometrically determined film growth kinetics

on Ti 8-1-1 shows that the Fleischmann-Thirsk model of oxide patch

nucleation and growth is applicable in both IN NaCl and IN NaNO^ solutions.

4. A calculation of the maximum penetration resulting from T1

dissolution in Ti 8-1-1 in NaCl solution during the 38 msec, time interval for oxide monolayer formation shows that crack propagation velocities

observed by others cannot be accounted for by metal dissolution alone,

ACKNOWLEDGEMENT

We are most grateful to the Office of Naval Research which supported

this work under contract NAONR 18-89 NRO 36-082.

_ 59 -

REFERENCES

1. T. P. Hoar & J. R. Galvele, Corrosion Sci . 10, 211 (1970).

2. T. R. Beck, "The Theory of Stress Corrosion Cracking in Alloys,"

J. C. Scully, ed., NATO Scientific Affairs Division, Brussels (W. S.

Maney, Leeds, 1971 ) , p. 64.

3. J. R. Ambrose and J. Kruger, Corrosion 28_, 30 (1972).

4. J. Kruger et al , National Bureau of Standards Report No. 10-876,

Technical Summary Report No. 3, Contract NAONR 18-69, NR 036-082, 1972.

5. J. R. Ambrose & J. Kruger, Proc. 5th Int. Cong. Met. Corr., Tokyo, 1972, in

pri nt

6. T. R. Beck, J. Electrochem. Soc. VH, 551 (1967).

7. J. A. S. Green and A. J. Sedricks, Metall. Trans. 2^, 1807 (1971 ).

8. J. C. Scully, "The Theory of Stress Corrosion Cracking in Alloys,"

J. C. Scully, ed.,(NATO Scientific Affairs Division, Brussels, 1971),

p. 127.

9. M. Fleischmann and H. R. Thirsk, Advances in Electrochemistry and

Electrochemical Engineering ^, (Interscience , New York, 1963),

p. 123.

10. J. A. S. Green and A. J. Sedricks, Corrosion 28, 220 (1972).

11. H. H. Uhlig, Proc. Int. Cong. Fundamental Aspects of Stress Corrosion

Cracking, Ohio State University, 1967 (NACE, Houston, 1969), p. 93.

60

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62

Fig. 1. Change in thickness, x, and repass ivation ratio, Rp, after

removal of the abrasion lAflieel of the tribo-ellipsometric

apparatus for Ti 8-1-1 in M NaN03 at 25°C. The alloy is at

the open crrcuit (corrosion) potential -222 mV (SHE) .

63

c

t ime, msec

Fig. 2. Change in thickness, x, and repassivation ratio, Rp, after

removal of the abrasion wheel of the tribo-ellipsometric

apparatus for Ti 8-1-1 in IN NaCl at 25° C. Ihe alloy is at

the open circuit (corrosion) potential -222 mV (SHE)

64
L
Fig. 3. Change In thickness, x, and repasslvatlon ratio, Rp, after

removal of the abrasion wheel of the tribo-ellipsometrlc

apparatus for Ti 8-1-1 in IN NaCl at 25°C. The alloy is at

+706 mV (SHE)

65

Pig. ^« Change in thiclmess, x, and repassivation ratio, Rp, after

removal of the abrasion wheel of the trib o-ellipsometrie

apparatus for Tl 8-1-1 in IN NaNOo at 25°C. The alloy is at

+706 mV (SHE) .

66

TITANIUM ALLOY (8-1-1)

in chloride (susceptible

and nitrate ( non- susceptible

)

Pig. 5. A conparlson of oscilloscope repassivation transients for

Ti 8-1-1 in W NaCl and YlaNO-^ solutions at +706 mV (SHE)

[Ellipsometric film growth - jagged trace; current - solid

line trace].

67

E

C

0

0

-222

706

''^

.0

706

-222

6

1/r

time, msec

Fig. 6. Changes in film thickness, x, for Ti 8-1-1 in IM NaNOo

( — 0—0) and IN NaCl ( ) at -222 and +706 mV (SHE)

at 25° C.

68

1.0

CL

<

0)

0

-222

706

706

-°

^

+

' 4.'

-222

0.1 0 100 200 300 ..2 2 time .msec

Pig. 7. Changes in lognQAP for Ti 8-1-1 in W NaNOo ( — . — .) and

W NaCl ( ) at -222 and +706 mV (SHE) at 25°C.

69
I-
a. t=to

b. t

= tn*At

Fig. 8. Schematic, of the effect of the Pleis chmann-Thipsk model of

oxide path nucleation and growth on substrate penetration by

metal dissolution, (a) Nucleation of oxide patches at cessation

of abrasion; (b) metal dissolution during lateral patch growth

resulting in formation of localized crevices where patches

coalesce (c)

70

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Si-cunty clas.-ii fication of tiife, body uf nby'ract and indcxiiif^ unnotHfi on n\u^t be entered v.hcn the overall report is cht s si fied) -3'. - iNiG-CT'/'TY^ Corporate author

}

National Bureau of Standards

Washington, D. C. 20234

2a. REPORT SECURITY CLASSIFICATION Unclassified

2b. GROUP

3 REPOR" TITLE The Role of Passive Film Growth Kinetics and Properties in Stress Corrosion and

Crevice Corrosion Susceptibility

CE5C'-:iPT:/E NC tS fType of report and inclusive dates) Technical Summary Report No. 4

A'jT'-iO^ Si fFirst name, middle initial, last name) J. Kruger

J. R. Ambrose

6. REPORT - i -E 7a. TOTAL NO OF PAGES 7b. NO. OF REFS

6a. CONTRACT OR GRAN - NO NAONR 18-69, NR036-082

b. P ROJ EC T NO- 3120448

9a. ORIGINATOR'S REPORT NUMBER(S)

NBSIR 73-244

9b. OTHER REPORT NO(S) (Any other numbers that may be as signed

this report)

1' DISTRIBUTION S-a-evEnDistribution of this document is unlimited

11- S,_,PPLEMENTA^y NCES 12. SPONSORING MILITARY ACTIVITY Office of Naval Research

Repassivation kinetics of an AISI 304 stainless steel have been determined in

l.ON NaCl solutions using the triboel 1 ipsometry technique which permits measurement of

film growth and total reaction rates following removal of the surface film by abrasion.

Although deoxygenation of the solution resulted in little change in other film growth

kinetics or the ratio of total change to film thickness (Rp), changing the solution pH

affected both the mechanism and rate of film growth which resulted in increased rates

of metal dissolution in acidic (pH3) and basic (pHll) solutions.

In neutral l.ON NaCl solutions, where film growth increased with more positive

applied potentials, the rate of metal dissolution during the repassivation transient

was initially highest at the corrosion potential, but decreased with time as the

surface became passivated.

The tri bo-ell ipsometry technique was used to determine repassivation kinetics and

stress corrosion cracking (SCC) susceptibility for Ti-8A1 -IMo-lV alloy. Repassivation

transient behavior in a l.ON NaCl solution, where cracks have been found to propagate,

was compared to that in a l.ON NaNOs solution where SCC susceptibility has never been

detected.

Film growth kinetics in both solutions were consistent with a Fleischmann-Thirsk

mechanism of oxide patch riucleation and two-dimensional growth, although the film

growth rate was significantly slower in the l.ON NaCl solution. This was in turn re- sponsible for an increase in metal dissolution in a solution where crack propagation

velocities have been measured, but at an apparent rate slower than necessary to propagate such cracks by metal dissolution alone

DD 1 NO V 65 147Q I *T / kJ

S/N 0101.307-680 1

(PAGE 1 Unclassified

Security Classification

Unclassified

Security Classification KEY WORDS LINK A LINK B LINK C 1

HOLE W T ROLE W T ROLE W T

Stress corrosion cracking

Repassivation kinetics

Crevice corrosion

Titanium alloys

Stainless steel

El 1 ipsometry

Nitrates

Chloride

DD :.T.A473 (back)

PH

Dissolved oxygen

(PAGE- 2) Security Classification

The early stages of crevice corrosion of AISI 304 stainless steel in

l.ON NaCl solution have been detected using the el 1 ipsometer to measure

changes in optical properties occurring within the crevice between a polished

metal surface and a glass plate. Changes in the ellipsometer parameters

A and 4-' begin almost immediately upon creation of the crevice and can be

interpreted as resulting from a build-up of soluble species within the

crevice solution, followed by an overall thinning of the protective film

and general corrosion attack.

Such optical changes could not be reproduced by deoxygenation of the

bulk solution without the presence of a crevice nor were they observed during

experiments using a Ti-8Al-lMo-lV alloy, which is not susceptible to crevice

corrosion in the l.ON NaCl at room temperature.

;-0;-.:.-. N3S.n4A (5-711 J.o. DEPT. OF COMM. 1. P'u Bl.iCATlON OK RLPOilT NO. 2. Gov i Accession

! SiBLIOGRAPHIC DATA NBSIR-73-244

b t 1

5. Recipicru .-j '-.ccess ion N o.

4. TrTLlI AM) SL.'bTITLE The Role of Passive Film Growth Kinetics and Properties in

Stress Corrosion and Crevice Corrosion Susceptibility

5. Publication Date

6. Performing Organization Code

7. ALiHOR(s/j^^^^^

Kruger and John R. Ambrose

8. Pc-rfornuns Organization

9. PERFORMIMG ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT Or CO'/.MERCE

WASHINGTON, D.C. 2023-^

10. Project/ Task/ '.Vork Unit No, 3120448

11. Contract/Grant No. NAONR 18-69, MR 035-032

12. Sponsoring 0:ganLzation Nair.c anJ Address

Oftice of Naval Research

Arlington, Virginia 22217

13. Type of Report & Period Covered intcriiii

3/1//2 - .</2o//o

14. Sponsoring, A^-,ent.> Cade

15. iUPPLEMENTARV NOTES

16. AT-)STR.ACT A 2C0--.vord or le:;s factual summat) of mosr .sij^rificant inforiiiat ion. If docurr.ci'f inciudes a significant

bibliography or literature survey, mention it here.) Repassi vation kinetics of an AISI 304 stainless steel have been determined in

l.ON NdCl solutions using the triboellipsometry techiiiqi'e which permits meo.surtment of

film growth and total reaction rates following remov-?.! of cho si;rfdCG film oy abrasion.

Although deoxygenation of tne solution resulted in Uttlo change in either film growth

kinetics or the ratio of total change to film thickness (Rn)., changing the solution pH

affected both the mechanis:n and rate of film growth which r.'^sulv.ed in increased rates i of metal dissolution in acidic (piiS) and basic {pHll] solutions.

j The tri bo-el 1 ipsometry technique was also used "co determine repassi vation kinetics \ and stress corrosion cracking (SCC) susceptibility for Ti -SAi - 1 Mo-1 V alloy. Repassi va- I tion transient behavior in a l.ON NaCl solution, wtiere cracks lieve been found to pro-

[ pagate, was compared to that in a l.ON NaN03 solution where SCC susceptibility has never;

been detected. Susceptibility was found to be related to film growth kinetics in the

two solutions.

The early stages of crevice corrosion of AISI 304 stainless steel in l.ON NaCl

solution have been detected using the ellipsometer to measure changes in optical properties occurring within the crevice between a polished metal surface and a glass

plate. Changes in the ellipsometer j^arameters iy and begin almost immediately upon

| creation of the crevice and can be interpreted as resulting from a build-up of soluble i species within the crevice solution, followed by an overall thinning of the protective

film and general corrosion attack.

'7. KEY UOKDS (Alphabetical oracr, separated by s erriicojons/ Chloride; Crevice corrosion; Dissolved oxygen; El 1 ipsornetry ; Nitrates; pH;

Repassivation kinetics; Stainless steel; Stress corrosion cracking; Titanium alloys

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