NBSIR 75-916

The Role of Passive Film Growth

Kinetics and Properties in

Stress Corrosion and Crevice

Corrosion Susceptibility

J. Kruger, J. R. Ambrose, and T. Kodama

Corrosion and Electrodeposition Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

October 1975

Technical Summary Report Number 6

Prepared for Office of Naval Research

Department of the Navy

Arlington, Va. 22217

NBSIR 75-916

THE ROLE OF PASSIVE FILM GROWTH

KINETICS AND PROPERTIES IN

STRESS CORROSION AND CREVICE

CORROSION SUSCEPTIBILITY

J. Kruger, J. R. Ambrose, and T. Kodama

Corrosion and Electrodeposition Section

Metallurgy Division

institute for Materials Research

National Bureau of Standards

October 1975

Technical Summary Report Number 6

DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED

Prepared for Office of Naval Research

Department of the Navy

Arlington, Va. 22217

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton, Secretary

James A. Baker. III. Under Secretary

Dr. Betsy Ancker-Johnson. Assistartt Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Acting Director

s

(To be submitted to J. Electrochemical Society)

Effect of flolybdate Ion on the Repassi vation Kinetics

of Iron in Solutions Containing Chloride Ions

T. Kodama*and J. R. Ambrose

' Institute for Materials Research

National Bureau of Standards

Washington, 13. C. 20234

It has been well established that molybdenum additions improve the

resistance of stainless steels to localized corrosion (especially pitting) in

chloride environments (1-7). Several investigations, however, have given

seemingly contradictory results concerning the role of molybdenum on this

enhanced corrosion resistance.

Rhodin, using microchemical analysis, found that molybdenum content was

increased in the passive film on 316 stainless steel immersed in a 5% HNO- - 0.5% li^Cr^Oj solution (8).

An Auger electron spectroscopic (AES) study by Barnes et al shov/ed

surface segregation of molybdenum in 316 and 304 stainless steels (9). They

used mi 1 1 -f ini shed materials without passivation pretreatment.

Another AES study by Lumsden et al showed that molybdenum was depleted

in the passive film of 316 stainless steel exposed potentiostatical ly to a pH

7 solution at 442 mV NHE (10). They found relative chromium enrichment in the

passive film of the molybdenum-bearing steels. The depletion in molybdenum

seems to be reasonable vyhen we compare Lumsden 's anodization conditions with'

the polarization data of pure molybdenum (11-14); molybdenum is readily

soluble in its hexavalent ion state at the potential and pH that they used for

the anodization.

In all likelihood, then, the presence of molybdenum oxide in the passive

film should not be a prerequisite for the enhanced resistance to pitting. In

fact, it may be that the beneficial effect of alloyed molybdenum on localized i corrosion resistance may be related to the fact that soluble and insoluble

molybdates are known to act as corrosion inhibitors for steels (15-19).

The work described here is aimed at examining whether molybdenum exerts

its beneficial effect on diminishing susceptibility to localized corrosion via

its dissolved, hexavalent state (MoO. ~). Moreover, since recent v/ork from

this laboratory found that molybdenum in ferrous alloys enhanced the rate of

repassivation (20) and rate of repassivation has been proposed as having an important bearing on localized corrosion, v;e have studied the effect of

dissolved molybdate ion on repassivation kinetics of iron. Anodic current

changes following oxide removal by abrasion v^ere measured using a device

developed for tri bo-el 1 ipsometry.

*Guest worker at NBS Nov. 1974 to Oct. 1975 from National Research Institute

for [Petals, Tokyo, Japan.
EXPERIMENTAL

To measure the anodic current transient following oxide film removal by

abrasion, a device which was extensively applied in tribo-ell ipsometric

measurement was used. The surface of a potentiostatical ly controlled iron

specimen was abraded witli SiC paper mounted on a rotating wheel. An abrupt

current increase caused by the retraction of tfie abrasion wheel from the metal

surface triggered an oscilloscope. Details of the device have been described

in previous papers (21-22). Anodic currents during longer time intervals (>500

ms) were measured with a strip chart recorder.

Specimens used were prepared from a Ferrovac iron rod and mounted in

lucite. Specimens were mechanically polished with SiC papers, followed by

rotary pol ishing- wi th 6m diamond paste.

A borate buffer solution (pH 8.4) was used as a supporting electrolyte.

The solution containing 14.30 g NaoB.O-. • 10 H2O and 9.26 g HoBOq per liter

was prepared from ACS grade chemiccilt and distilled water (7 x 10-/ ohm"l

cm- 1 )•

For measuring the effect of chloride or molybdate ion, small amounts of

NaCl or Na^MoO^ were added to the supporting electrolyte. No attempt at

deaeration'was 'made in these experiments.

.

' /

' ' RESULTS AND DISCUSSION

Anodic current transients, recorded by oscilloscope and strip chart

recorder were plotted on a log-log scale, and are shown in Figs. 1, 3, and 4.

Although in an experiment the oscilloscope was triggered by the abrupt current

increase accompanied by the retraction of the abrasion wheel, there was an uncertainty of time interval between the trigger pulse and the starting of

film growth. Conventionally, we used the time of the current maximum appearing in tne oscilloscope picture as the start of film-growth initiation (t=0).

The current maximum was not reproducible since it was dependent on the

effective metal surface abraded and the roughness of an abrasive paper.

Attention was, tiierefore, placed only on the relative change of anodic

current.

a) Repassi vation in the absence of chloride

Fig. 1 shows the anodic current transients of iron in a plain borate

buffer solution (curve A) and in a borate buffer solution with 0.01 M Na^MoO.

added (curve B), both at 442 mV NHE. Since the buffer solution was used as a supporting electrolyte, the pH shift resulting from NapMoO, addition was very

slight; pH for borate buffer - 0.01 M Na^MoO, solution ms 8.43, while the

borate solution alone showed pH 8.40.

Both curves A and B represent typical current decays accompanying

virtually complete or "perfect" passivation. Assuming the anodic current

efficiency for film formation alone to be unity, we obtain the following

relationship between the film-growth rate law and the slope on the log i-log t

plot (m):
for logarithmic rate law m = -1 for cubic rate lav/ m = -2/3

for parabollic rate lav/ m = -1/2

The derivation of these relations is given in Appendix 1. Slopes

corresponding to m = -1 , -2/3, and -1/2 are also depicted in Fig. 1. It

should be pointed out that all the above rate laws are based upon a uniform

film-growth model.

It can be seen in Fig. 1 that the passive film grows approximately in

accordance with a logarithmic rate law after the lapse of 300 ms. A

logarithmic growth of a passive film on iron has been found from the current

decay that is observed wlien the metal surface is brought to a potential in the

passive region (23,24). The initial stage of current decay (t < 250 ms)

cannot be represented- on a log-log plot by a straight line, since the gradient

is continuously changing from -1/3 to -1. This part was re-plotted in

semi -logarithmic scale and is shown in Fig. 2. The plots of log i vs. t are represented by straight lines with a discrete break in the shape of the decay

curve at t=80 ms; current decay can be expressed to obey the following

equation:

i=i^ exp(-k^t) + i2 exp("k2t) (1)

A siirple exponential current decay,

i=i^ exp(-k^t), (2)

can be derived if we assume the first layer of an oxide film develops in a similar manner to that found for monolayer adsorption: after the passive film

nucleates randomly on the bare surface, uniform film thickening occurs. By

this model, v;e further assumed that oxide-covered sites are inert enough to

prevent active dissolution or film growth, while uncovered sites are subjected

to active dissolution. This model can be regarded as an extreme case of the

generalized nucl eation-and-growth theory. In this case, nucleation rate >> two-dimensional film growth (25). The equation thus derived is expressed as:

o o i=(ij + q^k)exp(-kt), (3)

o o where i, and q^ respectively, denote the active dissolution current density at

uncovered sites and the amount of charge required to form the monolayer-type oxide

film per unit surface area. Details of the derivation of equation (3) are

given in Appendix 2. Equation (1) could be obtained in the same way as that

for equation (3), except that there are two different film-nucleation

probabilities depending upon, e.g., grain orientation or lattice defects of

the bare metal surface.

Perfect passivation kinetics of iron can be represented by the

following two steps: the first layer of an oxide film develops

two-dimensional ly, where oxide nucleation is rate-determining; this first step is

followed by film thickening which obeys a logarithmic rate law.

-3-
Another point to be mentioned in Fig, 1 and 2 is a good parallelism

between curves A and B. Apparently, dissolved molybdate ion does not affect

the growth law for a perfect passive film on iron.

b) Repassi vation in the presence of chloride

Shown in Fig. 3 are anodic-current transients of iron in a borate-buffered 0.01 M NaCl solution at different potentials. These curves demonstrate the onset of pitting corrosion. The growth of pits is indicated

by the current rise following initial passive film formation. During the

stage of passive film formation, neither any noticeable potential dependence

of the current decay slope nor the effect of chloride addition could be

observed.

Even with the technique of abrasion, we cannot determine when pit

initiation occurs. However, this technique does eliminate the so-called

induction period which otherwise is very dependent upon surface pre- treatment

and history, thus it allows a clearer definition of the pitting potential.

Anodic current transients in a buffer solution containing 0.01 M NaCl

and 0.01 M Na^floO^ are shown in Fig. 4.

Although initial, non-logarithmic current-decay slopes are somewhat

steeper than those in Fig. 1 and 2, the major effect of molybdate shows up in

a slow return of the anodic current to pre-abrasion levels following a fairly

high, constant current level. On this current plateau, small current

oscillations were also measured, possibly indicating repetitive film rupture

and recovery (Fig. []) . As seen in Fig. 4, the plateau current was lov/er with

decreasing anodic potential, and at 242 mV NHE current decay was similar to

that for perfect passivation. This current level was also decreased with

increasing molybdate concentration. Fig. 6 shows anodic current transients in

borate buffer - 0.01 M NaCl - xM NaMoO. solutions (0 ^ x £ 0.03) at a constant

potential of 200 mV.

The current transient curves in Figs. 4 and 6 can be regarded as the

intermediate case between perfect passivity (Fig. 1) and pitting corrosion

(Fig. 3). Apparently, pit initiation preceded the current plateau, this was confirmed by microscopic examination.

Optical photomicrographs, shown in Figs. 7a and b indicate active pits

formed in a 0.01 M iJaCl solution at 442 mV NHE and inactive pits in a 0.01

M NaCl-0.01 M Na2Mo0(i solution at 442 mV NHE respectively. Iron pits induced

by chloride (Fig. 7aj are of crystal loqraphic nature, which indicate active

dissolution of iron in the form of Fe^^ at the pit wall. The corroded surface

in the solution containing both chloride and molybdate also exhibited

localized attack (Fig. 7b). However, no crystal lographic characteristics were observed in the dissolution morphology. In other words, molybdate seems to

inhibit three-dimensional propagation.

From these results, it is concluded that molybdate is effective in

retarding the propagation stage of pitting corrosion.

-4-
c) Mechanism of the molybdate effect on retardation of pitting

The chemistry of pit propagation has been fairly well established in

comparison to the initiation process. It is characterized by reduced pH and

potential and chloride ion accumulation within the pit. These factors favor

the active dissolution of iron at the pit wall.

A possible mechanism by which molybdate effectively inhibits pit

propagation might be similar to those which have been proposed for chromate or nitrite. Molybdate might oxidize ferrous ion resulting in a copreci pitation

of less soluble ferric oxide and molybdenum suboxide. However, by

superimposing Pourbaix aiagrams (6), it can be seen that molybdate is not an oxidizer of ferrous ion (Fig. 8). Experimentally, this has been proved by the

classic works of Robertson (15) and Pryor (16). The coprecipi tation mechanism

thus can be excluded.

Molybdate shows a tendency to form insoluble compounds with cations

other tharji alkali metals and magnesium. From the aG° value of FeMo04, Fe

and MoO^ listed in the NBS selected values of thermochemical data (27), we

can calculate the solubility product of FeMoO/,; the solubility product thus

obtained is K^o = [Fe^^ ] [Mo04'^"] = lO'^O-S at 298°K. Ihe.estimated valued

by O'Hare et al was somewhat larger than this value (10~ " ) (28).

Using the K^o value, and assuming that molybdate ion is

electrochemical ly inert with respect to dissolved iron species, we can construct an E-pH diagram for Ee in a f1o04 ~ solution. Fig. 9 represents an E-pH diagram for Fe in a [MoO. "] = 10"^ M solution. Iron hydroxides, Fe(OH)p

and Fe(OH)-, were accepted as the solid iron compounds, since hydroxides were

thought more likely to exist during pit propagation than were anhydrous

oxides

.

Another E-pH diagram, in which the redox reaction of molybdenum is

involved, can be drawn by superimposing Fig. 9 and Pourbaix 's diagram for Mo

(26) and is shown in Fig._10. In Fig. 10, total dissolved molybdenum and iron

species are 10~ M and 10" M, respectively.

These diagrams are not full^ consistent, particularly with regard to the

selection of aG° values for MoO. . Pourbaix et a\ in constructing the

diagram referred to Latimer's delta (29) (aG° (Mo04^~) = -205.42 kcal/mole),

while our2calculations of Kso was based on the NBS selected values

(aG°(MoO, ') = -199.9 kcal/mole). Moreover, the aG° for FeMoO. employed here

is that for the thermally synthesized, well crystallized compound (30). It is

not quite certain whether this value would still be valid in aqueous systems,

where molybdate shows a tendency to form colloidal, amorphous precipitates.

Despite the above uncertainties, we can derive some useful qualitative

insights from these diagrams: Fe ion reacts with MoO, ~ forming solid

FeMoO. at neutral to slightly acid pH regions and at a potential where active

dissoTution of metallic iron occurs. The region at which FeMo04 occurs corresponds well to those regions of potential and pH as measured inside

a pit.

-5-
Spectrophotometric studies of solutions containing Fe Fe and MoO^i"

ions have shown a solution complex to be formed which is related to the

initiation process (32).

The formation of FeMoO. at the pit wall exUins the anodic current transients

shown in Fig. 3. Since molyodate is inert to Fe ion, it affects less the

formation and breakdown of the passive film. On the contrary, molybdate reacts

with ferrous ion dissolving from the pit wall to form a solid compound which

contributes to retardation of propagation. This stage is thought to

correspond to the current plateau in Fig. 4.

In order to verify this hypothesis, an electron probe microanalysis

(EPMA) of materials wittiin pits was carried out. Figs. 11 and 12 show EPMA

images of pits formed at 442 mV NHE in a 0.01 M NaCl -borate solution and in a 0.01 I'l NaCl-0.01 ;i MapMoO^ solution, respectively. In both solutions, CI

accumulation was observed inside pits, but was significantly less in the

molybdate-containing solution. Secondly, Mo accumulation in the pit is quite

noticeable in the molybdate-containing solution. Ma was not detected in

either solution. EPI^iA results strongly suggest that molybdate ions displace

chloride ions within the pit. These results are also in agreement with that

inhibition mechanism in which formation of FeMoO^ contributes to repassi vation

of pits.

It should be emphasized that FeMoO, v;ould occur only at occluded sites

where both potential and pH are lower than those of the bulk solution, i.e.,

under those very conditions expected to be characteristic in active pit

growth. The bulk potential and pH used in those experiments described by

Figs. 6, 7, 11, and 12 (442 mV NHE and pH 8.4) are in a region, however, where

Fe(0H)2 or Fe^O^ is stable. It is thus quite reasonable that Mo could not be

found By EPMA in areas outside pits. For the same reason, Mo could not be

detected in studies by Lumsden and Staehle (10). In this work, the analytical

technique of Auger Electron Spectroscopy would not be expected to detect

concentrations of elements across small areas such as pits.

Studies by Okada at el have shown that Mo is present in the film when CI"

ions are present in the solution using a similar AES technique (33).

However, if analytical techniques capable of looking at specific areas

of surface covered by pits were used, then high concentrations of molybdenum

would be expected to be present. Studies by Yahalom et al (31) did in fact

show the presence of molybdenum in films covering pits on molybdenum bearing

stainless steels.

A variety of molybdenum chlorides and oxychlorides could contribute to

the pitting inhibition. Among those found in literature, even the most

probable one, Ho02Cl2> is soluble in aqueous solution. A reaction, for

instance, MoO^^- + 201" + 4H -> MoO^Cl^ (aq) + 2H2O would favor pit

inhibition since it decreases the activities of both chloride and hydrogen

ions. However, it fails to explain the corrosion inhibition of iron under a system of limited [loO, " and unlimited CI" supply. No further discussion will

be made regarding the formation of the molybdenum oxychlorides since no suitable thermochemical data were available.

-G-
The results of this investigation clearly demonstrated that dissolved

molybdate ion is effective in repassivating iron undergoing pitting corrosion.

The mechanism proposed in the present work can hopefully be extended to

elucidate the mechanism for enhanced corrosion resistance of Ho-bearing

stainless steels in neutral chloride media. However, it should be noted that

the present mechanism gives only one aspect of the complex chemistry of

alloyed Ilo. As has been reported in our previous study (20), the

repassivation rate for times less than 100 ms following initiation of

repassi vation is increased with increasing lAo content in steels, while the

present study has shown that dissolved molybdate does not affect the

repassivation rate of iron. Also Mo-bearing stainless steels are reported to

exhibit excellent corrosion resistance in acid environments (1-7) where FeMoO,

is not stable. As is expected from Figs. 8 to 10, molybdenum trioxide, MoO^,

becomes less soluble with increasing acidity, suggesting direct involvement of

f'io in the passivation or Mo enrichment in the film to occur: this is in

agreement with the microanalytical work of Rhodin (8), who used an acid

solution for passivation treatment. The chemistry of Mo is, however, far more

complex especially in acid regions than is shovin in Pourbaix's diagram because

of the variety of polymolybdates and valence states. Further work using

various environments is needed to elucidate the role of Mo on the increased

corrosion resistance of Mo-bearing alloys.

ACKNOWLEDGEMENT

We are grateful to Dr. J. Kruger for discussions and encouragement

concerning this work, to the Office of Naval Research who supported this

work under Contract No. NAONR 18-69, NR 036-082, and to the Japanese Science

and Technology Agency who sponsored one of the authors' (T. Kodama) visit

to the National Bureau of Standards.

-7-
APPthDIXiiS

Appendix 1

In electrochemical oxidation, the amount of charge for oxide film formation (Qf) is proportional to the weight gain (aW) if the anodic current

efficiency for film formation is unity.

aW Qf

Parabolic, cubic, and logarithmic rate laws for film formation are respectively

expressed as follows as a function of time:

• aW - Qf = At V2 (4)

Qf = At (5)

Qf = Alnt + B (6)

Wfiere A and B are constants.

Since the derivative of Q^ gives anodic current, eqs. (4) to (6) give eqs.

(7) to (9), respectively.

= dQf = 1 At-1/2

^t

2 (7)

_ dQ^ _

1 At-^/^

3 (B)

_ dQf _ At-^

^t (9)

The exponent of t gives the slope (m) in the log i - log t plot. Thus one can obtain the following results:

for parabolic m = -1/2

for cubi c m = -2/3

for logarithmic m = -1

-0-
Appendix 2-1

Current transient for mono layer- type oxi'de film

Assuming a monolayer film to nucleate randomly without subsequent twodimensional or thickness growth, the rate of film coverage is proportional to

the area of uncovered sites (first-order rate).

Integration of eq. (10) yields:

6=1- exp(-kt)

(10)

(11)

Anodic current consists of that for active dissolution at uncovered sites,

i and that for film formation, i^. The following relations exist for i , and i^:

o do

(13)

From (10) to (13), the exponential decay of total anodic current can be obtained;

' =

^'d ^ u

= (id + qf

k) (1-e)

o o = (ij + q^k) exp(-kt) (14)

1

i ^

1f

k:

o ^f=

i^otation

total anouic current

active dissolution current at uncovered sites

active dissolution current per unit area

anodic current for film formation

rate constant (dimension: sec"^)

amount of charge required for oxide film formation per unit area

fraction of film covered area to total area

-9-
REFERENCES

M. Prazak and V. Cihal, Corr. Sci ., 2, 71 (1962).

E. A. Lizlov, Corrosion , 22, 297 (1966).

E. A. Lizlov and A. P. Bond, J. Electrochem Soc , 118 . 22 (1971 ).

E. A. Lizlov and A. P. Bond, J. Electrochem Soc , 1 22 , 719 (1975).

Pv. J. Brigham, Corrosion , 28, 177 (1972).

k. J. Brigham and E. W. Tozer, Corrosion , 29^, 33 (1973).

M. B. Rockel, Corrosion , 29, 393 (1973).

T. H. Rnodin, Corrosion , 12^, 123t and 465t (1956).

G. J. Barnes, A. W. Aldey and R. C. Jerner, J. Electrochem. Soc , 119 , 684 (1972).

J. B. Lumsden and R. W. Staehle, Scr. Met. , 6, 1205 (1972).

J. Besson and G. Drautzburg,, Electrochim. Acta , 3^, 158 (1960).

A. A. Pozdeeva, E. I. Antonovskaya and A. M. Sukhotin, Corr. Sci . , 5^, 149

(1966).

L. L. Wikstrom and K. Nobe, J. Electrochem. Soc . , 116 , 525 (1969).

J. VJ. Johnson, C. H. Chi, C. K. Chen and W. J. James, Corrosion , 26 , 238 (1970).

W. D. Robertson, J. Electrochem. Soc , 98, 94 (1951 ).

I'l. J. Pryor and M. Cohen, J. Electrochem. Soc , 100 , 203 (1953).

G. H. Cartledge and R. F. Sympson, J. Phys. Chem ., 61, 973 (1957).

E. A. Lizloves, J. Electrochem. Soc , 114 , 1015 (1967).

A. Weisstuch and C. E. Schell, Corrosion , 28, 299 (1972).

J. Kruger and J. R. Ambrose, National Bureau of Standards Report No. NBSIR

74-583, Technical Summary Report No. 5, Contract NAONR 18-69, NR 036-082,

1974.

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

J. R. Ambrose and J. Kruger, J. Electrociiem. Soc , 121 , 599 (1974).

-10-
M. Magayama and \A. Cohen, J. Electrochem. Soc , 109 , 781 (1962).

N. Sato and M. Cohen, J. Electrochem. Soc . , 111 , 512 (1964).

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

Electrochemical Engineering, Vol. 3, ed., P. Delahay, p. 123, Interscience,

New York.

M. Pourbaix, Atlas of Electrochemical Equilibria at 25°C, pp. 275 and 313,

1966, Pergamon Press.

D. D. Wagman et al . , National Bureau of Standards Technical Note, 270-4

(1969).

P.A.G. O'Hare, K. J. Jensen and H. R. Hoekstra, J. Chem. Thermodynamics

6, 681 (1974).

W. M. Latimer, Oxidation Potential, 2nd ed.. Prentice Hall, New York, 1952.

W. W. Weller, U. S. Bur. Mines Rep. Invest., No. 6787 (1966).

J. Yahalom, L. K. Ives, and J. Kruger, J. Electrochem. Soc , 120 , 384 (1973).

G. H. Cartledge, Corrosion , 24, 223 (1968).

H. Okada, H. Ogana, I. Hoh, and H. Omata, Auger Electron Spectroscopic

Analysis of the Passive Film of Stainless Steels, presented at the

Joint Japan-USA Seminar on Passivity, Honolulu, 1975.
TIME, s

Fig. 1 Anodic current transients following abrasion at 442 mV NHE (A) in a borate buffer solution (pli 8.4) and (B) in a 0.01 M Na2Mo04 - borate

solution. Slopes m = -1/2, -2/3, and -1 on a log-log scale indicate

parabolic, cubic, and logarithmic rate laws for film growth,

respectively.
Fig. 2 Semi logarithmic plots of anodic transients for times less than 2b0 ms following abrasion at 442 mV NHt (A) in a borate buffer solution

(pH 8.4), and (B) in a 0.01 M iMa2Mo04 - borate solution.

-13-

001 O.i I 10 100 1000 Time, sec.

4 Anodic current transients following abrasion in a 0.01 M NaCl - 0.01 M Na2Mo04 - borate solution at different potentials.

-15-

Fig. 6 Optical photomicrographs of (a) active pits in a 0.01 M NaCl - borate

solution and (b) inactivated pits in a 0.01 M NaCl - 0.01 M Na2Mo04

solution. Both (a) and (b) are formed at 442 mV.

-17-

-Z -I 0 1 2 . J 4 6 6 7 8 9 10 II I? 13 K 15 IS

Fig. 8 PotentiaNpH diagrams for Fe and Mo. (From Ref. 26).

-19-
Fig. 9 Potential-pH diagram for Fe in molybdate solutions. (Total molybdate

concentration = lO-^ M) at 298 K. Molybdate ion is assumed to be

electrochemi cal ly inert.
1.8

1.2

-1.8

2 Mo = 10-2 1^

X Fe = 10^ M

Fe(0H)3 -Mo04='"

Fe^**Mo

Fe^* * MoO

Fe + Mo

FeMoOa + Mo

Fe + MoOa'

12 15 pH

Fig. 10 Potential-pK diagrams for a mixed Fe and Mo system. (Total Mo

concentration = 10-^ M and total Fe concentration = lO""* H) at

298 K.

-21-
Fig. 11 EPMA images of pits formed in a 0.01 M NaCl - borate solution at

442 mV NHE. a) secondary electron image, b) area scan for CI.

-22-
. 01 mm

Fig. 12 EPMA images of pits formed in a 0.01 M NaCl - 0.01 M Na2MoO^ - borate solution at 442 mV NHE. a) secondary electron image, b) area scan for CI, c) area scan for Mo.

-23-
! PART II

(To be submitted to the Journal of the Electrochemical Society)

The Effect of a Crevice on Passive Film Growth Kinetics for Iron

John R. Ambrose

Corrosion and Electrodeposi tion Section

National Bureau of Standards

Washington, D. C. 20234

There is usually agreement that the underlying cause of crevice

corrosion is related to the fact that within the restricted geometry of a crevice there is contained a solution in which mass transport of certain

critical species is sufficiently limited so as to inhibit continuation of the

passive state normally displayed by a metal (1-3). This agreement continues

when these critical species are enumerated, although their ranking has been

open to some discussion. They are, in arbitrary order (4):

(1 ) metal cations

(2) bulk solution anions (e.g., chloride ion)

(3) hydrogen ions

(4) dissolved oxygen

Finally, considerable agreement prevails when discussion expands to include

the specific effect that would be manifest when mass transport of these

species is restricted. Without the reaction products from oxygen reduction

within the crevice available to neutralize hydrogen ions produced by

hydrolysis of metal ions, the crevice solution eventually deteriorates to a point where it is difficult to maintain the passive state. Since it is

generally regarded that this passive state results from the presence of a protective surface film, then the properties of this film - its ability to

sustain, repair or even regenerate itself in those environments considered

aggressive - are critical with respect to evaluating susceptibility to crevice

corrosion attack.

Techniques which provide measurements of overall corrosion rates during

crevice attack using electrochemical test methods alone cannot be expected to

allow measurement of film properties. There are too many diffusion limited

steps in the process which contribute to overall reaction kinetics to permit a focus on the most important area - the film, itself - unless measures are

taken to look at the film alone. The work described here is based on results

obtained by observing the film that exists on a metal surface exposed to a crevice. Pure iron exposed to a borate buffer solution to which sodium

chloride could be added was selected since film growth processes within and

without a crevice could be compared.

Recently a technique was developed in this laboratory that utilized

ellipsometry to measure variations in optical properties inside a crevice while

a specimen was potentiostated at a fixed corrosion potential. This approach was shown

-24-
to be useful in measuring relative susceptibility toward crevice corrosion of

a series of ferritic stainless steels as a function of molybdenum content (5).

This earlier developed technique is of value in predicting susceptibl i ty to

crevice attack, but it is difficult to use as a tool for making studies

concerned with developing an understanding of the mechanism of crevice

corrosion because it is difficult to interpret the changes in the

el 1 ipsometric parameters observed during the onset of attack. The work

described here is based on results obtained from a modification of that

technique. Whereas the former procedure involved measurement of ellipsometer

changes which occurred following exposure of a film covered metal surface to a crevice, in this procedure, a bare metal surface was produced

electrochemical ly on iron before ellipsometer measurements were begun. From

this study, it was hoped that some idea of exactly how the restricted geometry

of a crevice affects film growth could be obtained by el 1 ipsometrical ly

measuring film growth and reduction during potentiometric and gal vanostatic

cycl es

.

EXPERIMENTAL

Material used in this study was ferrovac iron, which was mounted in

lucite, then drilled and tapped for electrode assembly. The crevice assembly

shown in Fig. 1 was the same as that used in previous studies (5). However,

for tnis work, the assembly was positioned as shown in Fig. 2, allowing

solution to be drawn into the crevice by capillary action. Within this particular

configuration, el 1 ipsometric measurement was facilliated due to easier optical

allignment of the specimen assembly. Experiments performed utilizing the crevice

assembly were compared to experiments performed under identical conditions

without the presence of a crevice. The distance between the quartz plate and

the metal surface (the crevice thickness) was 0.25 mm in each experiment. This

thickness was assured for each experiment using a teflon shim.

All experiments were performed using solutions prepared from reagent grade

chemicals and distilled water (7 x 10"^ ohm~7 cm~^). A buffer solution containing

9.3 g H3BO3 and 14.6 g Na2B4]07 • IOH2O per liter (pH- 8.4) was used as a stock

solution. In order to evaluate contributions from the presence of chloride ion,

an aliquot of l.On NaCl solution was added to the stock solution to make an overall chloride ion concentration of 10~2[j.

Specimens were polished using SiC metal 1 ographic papers, then rotary

polished v/ith 1 ym diamond paste. Following polishing, specimens were ultrasonically cleaned with ethyl alcohol, rinsed with spectro grade methanol,

dried in air, then mounted for exposure.

Two experimental techniques were used in this study:

a) Potentiodynamic Scan Technique

Following positioning of the specimen, the specimen potential was fixed

at -758 inV SHE until initial el 1 ipsometric measurements were completed. A

ramp function was then applied so that the potential was varied at a fixed rate

of from 400 to 2000 mV/minute to 1242 mV SHE; at this point the potential was returned to -758 mV SHE. Measurements of cell current were recorded. Simultaneous

measurement of decrease in ellipsometer light intensity (proportional to film

thickness) was made using the off-null technique (6).

-25-
b) Gal vanostatic Scan Technique

Following positioning of the specimen, a constant current of -100 yA was passed until initial ellipsometer measurements were completed. Current was

then switched to a constant anodic value of from +100 to +500 yA and continued

until the measured potential reached either 1242 mV SCE, or, in the cases where chloride ion had been added, the potential remained constant or dropped

off. At this point, current was reversed and reduction was allowed to occur

at either -100 (crevice) or -200 yA (non-crevice) until a constant potential

was maintained. Ellipsometer intensity measurements and electrode potential were simultaneously recorded.

After each experiment, the specimen was removed, repolished, and the

solution changed before a new experiment was initiated. Results reported here are based on the average of three separate experimental determinations.

RESULTS

Potentiodynamic Oxidation Without Chloride Ion

The results of a typical potentiodyanmic scan for iron exposed to the

borate stock solution inside and without a crevice are shown in Fig. 4 and 3,

respectively. A comparison of the results from these two experiments show

two important results: 1) when not exposed to a crevice, iron goes through an active to passive transition at a potential of about 358 mV SHE (Fig. 3).

Film growth begins almost instantly upon initiation of the potential ramp

function; 2) when exposed to a crevice iron does not passivate as it did in

the previous case. As seen in Fig. 4, the current rise changes slope as film

begins to form, but does not drop off as in Fig. 3. Note also, that the

initiation of film growth in measureably retarded within the crevice.

Since the ellipsometer light beam is usually positioned roughly in the

center of the specimen, it was thought that this result might be due to an edge effect, where the current being passed by the counter electrode is

consumed at the edge of the specimen within the crevice; however, reallignment

to look at the specimen edge gave identical results.

Although these results were dramatic, it was felt that evaluation of the

film growth and decay measurements would be more meaningful if the overall

oxidation and reduction rates of the system were fixed. For this reason,

gal vanostatic oxidation and reduction experiments were begun. Log film

thickness vs. log time plots were used to display the data from these

experiments.

Gal vanostatic Oxidation Without Chloride Ion

In Fig. 5 are compared film growth at +100 yA in the borate buffer

solution for iron within and without a crevice. It is clear from this figure

that film growth is relatively constant over the time span of the

gal vanostatic oxidation when a crevice is not present. With the crevice,

however, film growth is initially non-detectable for the first 10-15 seconds,

after which film growth occurs rapidly, almost seeming to "catch up" with the

film thickness which would occur outside a crevice. After about 50 seconds,

growth rates for the two systems appear identical. Log film growth within the

-26-
crevice versus log time is plotted in Fig. 6 for a gal vanostatic oxidation at

+200 yA. On the same plot is included results from Fig. 5 for crevice growth

at +100 yA. It can be seen that although the point in time at which growth

begins is much earlier (at about 6 sec. following start of the scan) the rate

of film growth is nearly the same as in the +100 yA oxidation.

Gal vanostatic Oxidation With Chloride Ion

In Fig. 7 are compared film growth at +100 yA in the borate buffer - lO'^N

NaCl solution for crevice and non-crevice iron. The same phenomonology is

observed as when chloride was absent, but it is clear that the presence of

chloride ion for crevice iron does result in a slight decrease in the rate of

film growth during +100 yA gal vanostatic oxidation.

However, in each case, once the potential exceeds about 400 mV SHE, pits

initiate on each specimen. The potential remains fairly constant for the non- crevice iron, but drops off to about 300 mV SHE for crevice iron.

DISCUSSION

The borate buffer solution was selected for use in this study for

several reasons:

1) Sufficient expertise has been assembled for film growth kinetics on iron in this solution (7,8) to evaluate ellipsometer measurements made

through a quartz disc.

2) The virtial 100% efficiency for electrochemical reduction of films

on iron reported in this solution (9) prevents formation of H2 bubbles

in the crevice which interfere with ellipsometer measurements.

3) Since pH has been thought to be an important parameter in the crevice

solution composition (4), the use of a buffered solution allows some

control over the hydrogen ion variable.

The comparison between film growth rates for iron initially exposed to

the same solution but, in one case, under the kinetic restrictions imposed by

a crevice shows that film growth is significantly retarded within a crevice,

whether chloride ion is present or not. Why is this so? Since the solution

is buffered, OH" concentration is fixed; at the start of a gal vanostatic

oxidation, initial Fe"^^ f]^^x should be the same for an iron surface whether it

is exposed to a crevice or not. Since each experiment was initiated by

passing a constant cathodic current to reduce surface oxide and allow time to

make el 1 ipsometric measurements, it is likely that this pre-treatment

effectively removed all O2 from the crevice. Thus, when current is reversed,

oxidizing at a constant rate of 30 yA/cm^, and with no other oxidizing agent

present, a film will not begin to grow until the solubility product of Fe(0H)2

is exceeded. At that point, the film will grow rapidly as precipitation

occurs. Since precipitation initially occurs from a super saturated solution,

the rate at which this growth initially occurs would be expected to be

independent of anodic current density. However, the time at which growth

begins to occur should be quite dependent upon current density since that

alone controls the Fe'''2 flux passing into the crevice solution. If chloride

ions are assumed to be involved in complexing Fe"''^ ion as has been suggested

by Foley (18), then a precipitated film growth model is further indicated

since film growth within a crevice is reduced in the presence of chloride ions

(Fig. 7).

-27-
One other possibility is that because of the crevice, a portion of the

applied current is consumed by the establishment of a significant iR drop in

the solution and that film will not grow until the actual electrode potential

Eannl --iR) exceeds some critical value. Specimens are being redesigned to

aTTbw measurement of potential within a crevice. Whichever explanation is the

correct one, the net result is the same, following creation of a bare metal

site within a crevice, film growth during the repassi vation process is

retarded during gal vanostatic oxidation.

Once some critical potential is exceeded, however, film growth kinetics

are virtually identical whether growth on the iron occurs within the confines

of a crevice or not. This potential (about 200 mV SHE) happens to coincide

quite well with several other so-called "critical potentials" described in the

literature:

a) Ek, the potential reported by Nagayama and Cohen (9) for iron exposed

to a buffered borate solution (pH=8.4) above which no Fe'''^ ion was detectable

in solution. It is about 200 mV SHE.

b) Ep, the critical potential for pitting of iron in a borate buffer

solution containing lO'^ N chloride ion (8) and is about 200 mV SHE.

c) Formation potential of y FeOOH, expected to be formed by hydrolysis

and precipitation of Fe(0H)2, at a potential of about 200 mV SHE at a pH of 8

(10).

d) A potential shown by Ambrose and Kruger to be about 200 mV SHE where

below this potential film growth on iron in a borate solution containing

chloride ion was the same as if chloride was not present (8). Above this

potential, however, chloride markedly affected film growth.

Of course, the fact that these potentials are remarkably similar can be

attributed to coincidence, but there are many good reasons to suppose that

this should not be so. One very important condition should be pointed out - these gal vanostatic oxidations were performed in a buffered solution. In

non-buffered solutions, crevice pH change would be expected to be proportional

to the total anodic current density less that partial current density being

consumed in film growth. All things being equal, for an anodic current

transfer at 100 yA into a crevice volume of lO"! cm^, and assuming double

hydrolysis of ferrous ion according to:

Fe(H20)j2 %] Fe(H20)5 (0H)+1 ^3 Fe(H,0)4(0H)2

,

k_i k_2

the rate of build-up of hydrogen ions should be approximately 1 pH unit per

second. By the time film growth could be expected to begin according to

solubility product data, the crevice pH would be about 2, the value measured

for steady state crevice corrosion of iron by Bogar and Fujii (11). The

lowering of pH would be expected to retard growth of a precipitated Fe(0H)2

film. In fact, studies by Kruger and Calvert (7) for iron in borate solutions

show this effect (Fig. 8).

-28-
If we now assume that above about 200 mV SHE the film that is formed is an Fe(III) oxide, and that to be protective it must likely be Y-Fe203 , as has

been suggested by many researchers (12), then all that remains between bare

metal and exposure to the solution within the crevice is a thin layer of

Fe(0H)2j thinner because of retarded growth in low pH solution. This is quite

important, because in the presence of chloride ion, protective Fe203 does not

appear to be formed., rather a non-protective y-FeOOH compound, probably as a result of hydrolysis of Fe(OH)J and precipitation of FeOOH from solution (12).

The results thus far obtained point to the fact that in a crevice where

O2 has been depleted, film growth is retarded whether chloride ion is present

or not. It is during this interval of time that processes occur which, in all

probability, set the stage for propagation of crevice attack. If the rate of

formation of film during this period could be accelerated, perhaps by

additions of certain substances known to act as inhibitors, then the expected

consequence of low film growth rates might be averted. For instance, studies

by Kodama and Ambrose (13) have suggested that the beneficial effect of M0O4"

additions is related to the formation of an insoluble FeHo04 compound.

Perhaps impraed resistance to localized corrosion is related to the rate of

formation of such salt films. Such a possibility has been suggested by Beck

in his studies of pitting on titanium (14). Studies into the effect of

alloying Mo and additions of NaMo04 to the solution are now being performed

using the technique employed in this work.

All of the considerations just discussed that affect both the onset time

and the rate of film formation are of importance if there exists spontaneous

film rupture and repair in a crevice as was suggested in a previous report

(5). The experiments performed here show that if film rupture does occur,

then the sequence of events as described here will occur once O2 has been

depleted within the crevice. Of course, O2 will be consumed by

electrochemical reduction at a freely corroding surface as a part of the

overall corrosion reaction. Within a crevice, however, mass transport

restriction limit its replacement. Studies by Shiobara and Morioka (15) and

Iverson (15) point to frequent current oscillations during "incubation"

periods prior to actual progation of pitting and crevice attack. Videm showed

similar breakdown and repair transients for aluminum (17). It is likely then,

that chloride ion does promote spontaneous rupture of the protective film on iron, and, that repassivation phenomena will be controlled by the state of the

solution when such rupture does occur. However, the important stage is the

first one, for if film does not form, then the high metal ion concentration

and low pH conditions created here will invariably affect repassivation

behavior in any future spontaneous rupture event. It is towards elucidating

still further the factors which influence early growth during gal vanostatic

oxidation that our efforts will be directed.

ACKNOWLEDGEMENT

I am grateful to the Office of Naval Research who supported this work

under Contract No. NAOi>IR 18-69, NR 036-082.

-29-
References

I.L. Rosenfeld and I.K. Marshakov, Corrosion, 20_, llljt (1964).

B. F. Brown, Corrosion, 26_, 249 (1970).

E. IlcCafferty, J. tlectrochem. Soc, T21_, 1007 (1974).

W.D. France, Jr., Crevice Corrosion of Metals, GMC Research publ

.

GMR-1105 (1971).

J. Kruger and J.R. Ambrose, National Bureau of Standards Report Uo.

NBSIR 74-583, Technical Summary Report Number 5, Contract NAONR 18-69,

NR 036-083 (1974).

V. Brusic, M.A. Genshaw, and B.D. Cahan, Appl . Optics, £, 1634 (1970).

J. Kruger and J. P. Calvert, J. tlectrochem. Soc, 114, 43 (1967).

J.R. Ambrose and J. Kruger, Proc. Fourth Intl. Cong, on Met. Corr ., N.E. Hamner, Ed., Amsterdam, 1969 (NACE, Houston, 1972), p. 698.

M. Nagayama and M. Cohen, J. Electrochem. Soc , 109 , 781 (1962).

M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions,

CEBELCOR (Pergamon, London,' 1966) , p. 312.

F. D. Bogar and C.T. Fujii, iJaval Research Laboratory Report No. 7690,

RR 022-08-44-5513, 1974.

C. L. Foley, J. Kruger, and C.J. Bechtoldt, J. Electrochem. Soc. 114 , 994 (1967).

T. Kodama and J.R. Ambrose, National Bureau of Standards Report in

print. Technical Summary Report No. 6, 1975.

T.R. Beck, "The Occurrence of Salt Films during Initial Stages of

Titanium Corrosion", presented at Electrochemical Society national

Meeting, Dallas, October, 1975.

K. Shiobara and S. Morioka, J. Jap. Inst. Met., 36_, 385 (1972).

W.P. Iverson, J. Electrochem. Soc, 115 , 61 7 (1968).

K. Videm, The Electrochemistry of Uniform Corrosion and Pitting of

Aluminum, Icjeller Report 1974.

R.T. Foley, "The Role of Chloride Ion in Corrosion - Part 11", final

report, ONB, Contract hooo14-66-C-0025, The American University,

Washington, U.C. (1969).

-30-
Crevice Electrode Schematic

Specimen Electrode Assembly

Fig. 1 Schematic of crevice electrode assembly used in crevice repassivation

kinetic studies [from (5)].

-31-
1 %

Fig. 2 Photograph of crevice electrode assembly positioned for repassi vation

measurements

.

-32-
Fig. 3 Film growth and anodic current as measured during a potentiodynamic

scan. The experiment was performed using iron exposed to borate

buffer solution without a crevice and tlie potential was driven from

-7138 mV SHE to 1242 mV SHE at a scan rate of 1000 mV/min.

-33-
<

£

CVJ 6

]

0.2 nm

I minute

Fig. 4 Film growth and anodic current as measured during a potentiodynamic

scan. The experiment was performed using iron exposed to borate

buffer solution inside a crevice and the potential was driven from

-758 mV SHh to 1242 mV SHE at a scan rate of 1000 mV/min.

-34-
0.01 10 100 SECONDS

Fig. 5. Galvanostatic oxidation of iron at 100 yA (33 yA/cm ) from -758 mV to

+1242 mV SHE both within (circles) and without (triangles) a crevice.

Log film growth in nm is plotted vs. log time in seconds. The solution

is the borate buffer (pH = 8.4).

-35-
E

1 1

+ 100 /i,A —

. — + 200/XA ^

/a

a/

1 10 Time, s

Fig. 6. Galvanostatic oxidation of iron at 100 yA and 200 yA (33 and 67 yA/cm ) from -758 mV to +1242 mV SHE within a crevice. Log film growth in nm is plotted vs. log time in seconds. The solution is the borate

buffer (pH = 8.4).

-36-
E

c

10 100 1000

SECONDS

Fig. 7. Galvanostatic oxidation of iron at 100 yA (33 yA/crti^) from -758 mV to

+642 mV in a borate buffer (pH = 8.4) - 0.01 N NaCl solution with a crevice as compared to galvanostatic oxidation of iron with a crevice

but no chloride ions present (Fig. 5.)

-37-
(To appear in the proceedings of the Japanese-American Symposiun on "Passivity and its Breakdown on Iron and Iron Base Alloys" which will be

published by N.A.C.E. in January 197G).

Chemical Breakdown of Passivity

Jerome Kruger

National Bureau of Standards

Washington, D.C. 20234

"By gnawiiig through a dyke even a rat may drown a nation ''-E. Burke

Chemical breakdown of passivity leads to the various forms of

localized corrosion--pitting , crevice corrosion, intergranular attack. It is, therefore, of crucial importance to be able to understand the

processes that produce these most damaging forms of corrosion and thereby

develop ways to control them. This brief, personal, and hence opinionated

examination of chemical breakdown of passivity will concentrate attention

on the passive film and what happens to it so that breakdown results.

Once breakdown has occurred and localized attack is proceeding, the scope

of this discussion will be exceeded. Therefore, the problems associated

with crevice attack or those of an actively growing pit such as propagation rates, morphology, potential, and composition distributions in

pits will not be considered. There are a number of useful discussions

(1-5) of these topics of crucial importance once chemical breakdown of the

passive film has occurred.

To further limit the scope of this discussion, a clear definition of

chemical breakdown must be spelled out. For the sake of brevity and to

focus the discussion, I will define chemical breakdown as those processes

brought about by chemical alteration of the film or the environment that

produce changes in the passive film so that it can no longer effectively

prevent damaging anodic currents from flowing. One way to destroy the

effectiveness of the passive film is by mechanical disruption. When this

is done by chemical means, as subsequent discussion will consider, it will

fall within the scope of the discussion. When mechanical breakdown occurs

by the introduction of externally applied stresses, it will be considered

purely mechanical breakdown and will be described elsewhere (6). Also,

excluded from this discussion are those activation processes that occur

when a passivated metal surface potential is brought into the

transpassive region or into the active region (below the Flade potential).

A discussion of these modes of activation is given by Hoar (7).

., THEORETICAL MODELS

A number of models have been proposed to describe the events leading

to passive film breakdown and the subsequent pitting or crevice attack.

Most of these are listed by Hoar (7,8). In order to be successful a model

must, of course, explain the phenomenology of breakdown. While perhaps not

all inclusive and in some cases controversial, the following phenomena are usually considered to be necessary requirements for chemical breakdown to

occur:

1) A certain critical potential Ep must be exceeded, called by some

the critical potential for pitting (9-10).*

*The situation is more complex. Pourbaix (11) has pointed out that there exists

a potential Ep, which he calls the protection potential. This potential more

active than E^ is the potential below which breakdown once initiated can be

stopped. Work by Pessel and Liu (12) indicates that when E^. is determined in

a way that eliminates the time for breakdown to occur, C-=Ep.

-38-
2) Damaging species (Cl" or the higher atomic weight halides) are needed

to initiate and propagate breakdown.

3) There exists an induction time separating the initiation of the

breakdown processes by the introduction of conditions conducive to breakdown

and the completion of the process when pitting or crevice attack commences.

4) Breakdown occurs at highly localized sites.

The models for breakdov^n that must satisfy the above requirements can be

divided into three groups: 1) adsorbed ion displacement, 2) ion migration or penetration, and 3) chemico-mechanical . As Hoar (8) has pointed out there are common elements in all of the models, arid if a true model for breakdown is ever achieved, it probably will contain elements of all three groups. The

individual groups will be discussed in terms of how well they satisfy the

phenomenological requirements.

1 ) Adsorbed ion displacement models

Two mechanisms have been suggested under this general model. First,

there is the mechanism suggested by Kolotyrkin (13) and Uhlig (14,15). This

model considers the passive film to be an adsorbed film (probably a monolayer)

of oxygen. Breakdown occurs when a more strongly adsorbing damaging anion (I

will use tne cnloride ion which is highly polarizable and hence capable of

adsorption) displaces the oxygen forming the passive film. Once the Cl~ ion

is adsorbed at the surface, breakdown commences. When other anions are present they can compete with the Cl~ for sites and inhibit breakdown (14,15).

Direct evidence that this occurs has been provided by Rozenfeld and

Maksimchuck (18) using radioactive Cl". The model _explains the critical

pitting potential as that potential above which Cl" adsorbs on the surface

under consideration. The effectiveness of ions such as Cl" in promoting

breakdown is explained by their being more readily adsorbed than oxygen in the

competition for surface sites. The effect of damaging ion concentration on induction times can perhaps be explained by this mechanism, but such important

influences on induction time as thickness of the passive film (16) and heat

treatment of the passive film (16,17) can not be readily accounted for by this

model. Finally, this mechanism is consistent with localized attack since

adsorbed ion displacement will probably occur at discrete sites, defects,

inclusions, etc., where displacement may be more likely.

The second model that can be considered as falling under the general

heading of an adsorbed ion displacement model is that described by Hoar and

Jacob (8). It nas, however, elements in common with the ion migration models

also. In this model a small number (3 or 4) of halide ions jointly adsorb on the surface of the passive film around a lattice cation. The probability of

formation of such a high energy complex is small and thus requires a high

activation energy for formation. Once formed, however, the halide ions will

readily remove the cation from the passive film lattice (this model assumes a three dimensional passive film). The film is thus made thinner at the site

where the complex first formed and the stronger anodic field at the thinned

site will rapidly pull another cation through, v/here it will meet more halide

ions, complex with them and thereby enter the solution. Thus, once started,

the passive film is locally thinned and breakdown proceeds, using Hoar's word

"explosively"

.

-39-
This model like the Uhlig one requires a critical potential where

adsorption takes place. It also requires strongly adsorbing anions. Because

of the difficulty of forming the transitional complex, an induction time is

required, and in fact it was developed to explain the dependence of the

induction time on Cl" concentration, temperature, and potential. Also, it is

consistent with the localized nature of breakdown. This model does not appear

to be applicable to all systems because Ambrose and Kruger (17) have shown

that incorporation of Cl" in the passive film lattice rather than the

formation of a complex on the outside of the film accelerates breakdown for

passivated ion. Hoar (19) has also found that the proper functional

dependence of induction time with temperature found for stainless steel > which

was the basis for his model, does not hold for iron where he proposes a penetration model .

"

'

2) Ion migration or penetration models

The theoretical models that can be grouped under this heading all

require the penetration of damaging anions through the passive film, the

breakdown process being complete when the anion reaches the metal -film

interface. Most of these models consider the passive film to be three

dimensional. The models differ widely, however, in the mode of penetration.

At one extreme is a model that is quite old and was suggested by Evans

(20) and co-workers. More recently, Richardson and Wood (21,22) have revised

the pore mechanism and conclude that breakdown of the passive film does not occur at all because there always exist pores or defects in the film that

allow instantaneous penetration and hence instant localized corrosion. One

consequence of this is that there is no induction time for the breakdown

processes to take place because there are no breakdown processes. Richardson

and Wood account for incubation by asserting that during the beginning stages

of pitting the currents are so low at the small bare metal sites that it

appears the pitting has not been initiated, although it actually has been

going on from the instant that the surface was exposed to damaging anions. In

this way it satisfies the phenomenological requirement for the existence of an induction period. The other phenomena this model must contain are also

satisfied. Thus, since the pores, bare or "weak spots", are discrete sites,

attack will be localized. The critical potential is the potential for

adsorption at the bare metal at the base of the pores, and the role of the

damaging anion is the same, and as mysterious, as it is for the other models.

At the other extreme of the types of penetration models are those models

involving migration of the damaging anion through a lattice, via defects, or

via some sort of ion exchange process. The processes which involve ion

migration in a lattice can occur in a variety of ways. Hoar et al (23)

suggest anion entry without exchange to produce "contaminated" passive films.

The site vyliere this occurs becomes a path of high conductivity where high

cation currents and [lence the initiation of pitting are possible. Pryor and

co-workers (24,25) also propose the production of an anion contaminated film

but suggest that cation vacancies are created which lower ionic conductivity

and promote breakdown. Another ion migration scheme can involve exchange of

0 or OH with the damaging anion (16). Eq. 1 shows that such a process can

-40-
also create anion vacancies that will further enhance the migration of

damaging anions to the film metal interface.

The anion lattice migration models have no difficulty in satisfying the

phenomenology of breakdown. The critical potential is, as with all the

models, the potential required for adsorption, although in this case it is on the outer interface of the passive film. The induction time is the time

required for the migration of the damaging anion through the passive film, and

since this migration may be via discrete defects the localized nature of

breakdown would be expected. The special role of certain damaging anions is

not accounted for any better than it is by the other models thus far

considered.

3) Chemico-mechanical models

This group of theoretical models which involves chemically induced

mechanical disruption of the passive film is perhaps the newest. One version

was briefly described by Hoar (7). He postulated that the adsorption of a damaging anion on the surface of the passive film lowers the interfacial

surface tension (surface free energy) at the solution interface until

"peptization" occurs. This "peptization" results from the mutual repulsion of

the adsorbed anions. When the repulsive forces are sufficient, the passive

film cracks and damaging anions can attack the metal exposed.

Sato (26) has extended this concept of chemical -mechanical breakdown by

providing a more detailed examination of the model. He has suggested that

high fields could lead to mechanical rupture of thin films by electrostriction

pressures exceeding the compressive fracture strengths of the film. The

so-called film pressure this generates is given by:

where p = film pressure

p

= atmospheric pressure

e = film dielectric constant

E = electric field

Y

= surface tension

L = film thickness

It is possible that those ions considered to be "aggressive" in

promoting localized corrosion attack increase film pressure by decreasing the

surface tension term in equation [2] through adsorption. Unlike the other

theories we have considered, this theory does not consider the critical

potential for breakdown to be related primarily to adsorption but instead to

be the "critical potential above which the film pressure exceeds the critical

compressive stress of film breakdown. "(26) Also, this model differs from the

other models described in that, although as with the others the special role

-41-
of aggressive anions such as chloride is not clear, this model goes further

and would appear to rule out any special role at all. This is so because zero charge potentials are similar on mercury and probably are likewise similar on oxides as well (27). The existence of an induction period is also difficult

to explain using the mechanical breakdown model. Finally, the localized

nature of attack is consistent with the model if one mode of breakdown

involves rupture at discrete sites as Sato does suggest.

Sato points out in his description of his mechanical breakdown theory

that whether a pit nucleates or dies depends not only on breakdown but also

on film reformation. This concept, the interplay between breakdown and

repair, underlies the last model of the chemico- mechanical group to be

discussed. Videm (28) has recently proposed that it is the interplay between

the breakdown (although he does not specifically say so, this could be

mechanical as Sato has suggested) and repassi vation that determines whether a passive film will suffer permanent breakdown that leads to pitting. Videm

proposes that the critical pitting potential is determined by the relative

rates of breakdown and repassi vation. Another very recent variation of

dynamic breakdown-repair and theory is that of Ambrose and Kruger (29). It

differs from Videm in that it supplies more details about the breakdown

process. It proposes that breakdown results from penetration by the

aggressive anion to the metal -film interface where localized film rupture

could be affected by promoting formation there of pockets of non-protective

low density material similar to that produced as a corrosion product during

active pit growth. As the pockets expand upon continuing formation at the

metal-oxide interface, further stress is placed on the film directly atop the

pockets, eventually resulting in fracture of the film. In iron, for instance,

this low density material was identified as y-FeOOH (30). The special role of

the aggressive anions is that they influence the rate of repassi vation.

Recent direct measurements of repassi vation (31,32) have verified this. The

critical potential can influence a number of factors, adsorption of the

aggressive anion as in other mechanisms, the rate of repassi vation » and the

mechanical properties of the film. The mechanism supports the existence of an induction period because penetration of anions, a time dependent step, is

involved. Finally, because penetration is part of the model , the attack would

be expected to occur at discrete sites.

INITIATION PROCESSES

It is evident from the discussion of the various theoretical models that

even some of the widely different models have elements in common. For

example, virtually all of them require the existence of an adsorption step.

There is, however, a high probability that no one model is applicable to all

systems. Thus, it will be very difficult to match experimental results to

models in an orderly way. Instead, my approach will be to select only a few

results described in the literature to emphasize contrasting points of view.

It is anticipated that by doing so a clearer definition of the problems will

be achieved and experimental v^ork will be stimulated to provide new insights.

In conformity with the scope described earlier the discussion of the

kinetics of the initiation process will confine itself to the induction period

that leads to breakdown and active pitting. The role of potential will then

be considered. Finally, a brief discussion of the influence of alloy

-42-
composition, structure and environmental factors as they affect the initiation

process will be given.

Kinetics of Ini tiation--The Induction Period

The only kinetics that need to be considered in chemical breakdown, as defined earlier, are the kinetics of the processes occurring during the

induction or incubation period that takes place prior to the significant

increase in current that marks the onset of pitting or crevice attack. A

number of conflicting ideas, experiments, and explanations of these

experiments exist to explain this induction period. Three representative

groups will be described. A representative from the group of experiments such

as those of Uhlig (14,15) and Rozenfeld (18) which use the competitive

adsorption model for interpretation is not included because no induction

periods were reported.

a) Mechano-Chemical Breakdown - A very extensive study of the breakdown

on pure Al has been made by Videm (28). His main conclusions were as follows:

1) He found, as Bogar and Foley (33) had also shown for an Al alloy,

that above E there exists an incubation period before pitting starts, when

Cl~ is added^to solution containing a filmed Al surface. The induction time

decreased exponentially with potential and a ten-fold increase in chloride

concentration reduced the time to initiate a pit by a similar factor.

2) Because he could not detect CI" by autoradiography, except at sites

where pitting attack had taken place, and because he could not detect

impedance changes after Cl~ soaking, he concluded that theories for initiation

of pitting of Al requiring contamination of oxide films [see (25)] by gradual

chloride pick-up and subsequent change in conductivity are not valid.

3) He was able to measure regular current pulses during anodic

polarization which he ascribed to the development of local defects (see Fig.

1). The pulses were found both with and without the presence of CI" in the

experimental solution, but the nature of the pulses changed markedly and

abruptly when pitting was initiated at the end of the induction period. This

result led Videm to conclude that the Richardson and Woods (21) suggestion

that there is no pit initiation step because pitting starts immediately at

small flaws in the passive film is invalid.

4) The role of the chloride is not promoting breakdown per se , but in

preventing repair during a dynamic breakdown-repair type of mechanism.

Similar conclusions were reached for Al by Zahavi and Metzger (34).

These results of Videm 's thus point to a non-penetration dynamic

breakdown and repair theory. It does not explain, however, how a system

predisposed to pit reaches the stage vyhere there is sufficient chloride

available at the initiation sites to stop repassi vation effectively. For

crevice attack where the crevice helps concentrate Cl~, his results would be

understandable.* In the absence of a crevice, however, it is difficult to see

*Wilde's (4) admonition that the kinetics of pitting corrosion and crevice

corrosion are different and that one must disentagle the two to assess susceptibility to localized attack is pertinent to this point.

-43-
why at some definite time prior to pitting the Cl~ ions should suddenly start

to prevent repassivation unless they concentrate in the film or somehow

locally in the environment.

b) Transitional complex formation - An experimental representative of

this group is that of Hoar and Jacob (8). Their model has been described

earlier. Their experimental data for austenitic stainless steel showed that

1/t, where i is the induction period, was an approximate measure of the rate

of breakdown. A plot of log x vs. log Cp-,- showed that the breakdown rate is

proportional to the n-th power of Cp-,- with n having values between 2.5 and

4.5. They found an activation energy of CO k/mole and t to be dependent on potential. Their interpretation of the values found for n was that 2.5 to 4.5

halide ions form a "transitional complex" which once formed (because of the

high activation energy, a difficult and seldom event) will proceed to break

down the film in the way described in the section on models. Because of the

high activation energy and the requirement for 2.5 to 4.5 halide ions to form

a complex. Hoar and Jacob rule out an ion migration model for stainless steel.

However, for other systems their model may not be valid. For example, Engell

and Stolica (36) found a value of 1 for n for mild steel, and the induction

time potential independent. This could mean a penetration model where

individual aggressive anions were penetrating independently of each other.

Foster and Hoar (19) and also Ambrose and Kruger (17) propose a penetration

model for the passive film on iron. Other values of n have, however, been

found. For Al Bogar and Foley (33) found values of n between 4 and 11 vyhile

Videm found a value of 2. Obviously, one must be careful in using a simple

interpretation such as Hoar's and Jacob's to explain the meaning of n.

c) Penetration - All of the penetration models explain in a very direct

way the existence of an induction period. This period is the time required

for the damaging anion to travel through the passive film. An experiment

directed at providing a test of this idea was given by McBee and Kruger

(16) who measured the time to breakdown as a function of passive film

thickness. As Fig. 2 shows the thicker the passive film, the longer the time

to breakdown. Unfortunately as Fig. 2 also shows they were not able to

establish a functional relation between thickness and breakdown time, since

three functional relationships fitted the data equally well. To further show

that penetration is involved in the breakdown of the passive film on iron,

Ambrose and Kruger (17) showed that by removing part of the passive film by

cathodic reduction and then regrowing the film in the presence of Cl~ one could repassivate, but the time for breakdown was shortened. They interpreted

this as evidence for the penetration model because the regrowth step

incorporated CI" ions. These ions therefore had a shorter distance to

penetrate, and the time for breakdown was shortened. Finally, Ambrose and

Kruger showed that annealing the passive film on iron at 65°C for one hour and

then bringing the system back to room temperature increased the induction time over 100-fold. Therefore, they reasoned the defects in the film whereby

penetration proceeds were removed by the annealing. Similar results for Ta were found by Vermilyea (37), but opposite results were found for stainless

steels by Okamoto (38)* (annealing shortened the induction time). These

*If the films on stainless steel are amorphous [see (39)], the annealing could

have caused crystallization producing more diffusion paths. On the other hand,

if the films are crystalline, annealing could destroy diffusion paths by grain

growth and destruction of lattice vacancies.

-44-
results plus others (17,38) which show that longer passive film growth times

lengthen the induction period, do not rule out the transitional complex model

because the production or reduction of defects in the film can also be

effected on the film surfaces and thereby affect the adsorption of damaging

anions at that surface.

Two main experimental problems exist for the penetration model.

First, micro breakdown events (see 28,40) as indicated by small current or potential pulses (Fig. 1) may continuously exist throughout the induction

period. If so why is penetration needed? There exists contradictory evidence

of CI' entry into the film lattice. Ell ipsometric spectroscopy (16),

inipedance measurements (24,41), and autoradiography (42) have indicated

gradual CI' entry. Other work, however, using autoradiography (28) and

impedance measurements (28,43) show no CI up-take by the film.

Critical Potential for Breakdown

There are vast numbers of experiments and discussions in the literature

on the factors or underlying causes of the existence of a critical potential

for breakdown, E . I will avoid a discussion of its practical significance

(see 25,35,11) methods of measurement (see 44,12) and I will detail briefly

only those ideas or findings that relate to its role in initiation of

breakdown. An incomplete listing of different explanations of the origin of

E follows:

c

a) Er determined by potential of zero charge - Almost all of the models

require as their first step the adsorption of damaging anions. Roughly, the

potential above which such adsorption takes place is the potential of zero charge (pzc). Thus, since the adsorbed ion displacement models require the

damaging anion to adsorb and thereby displace the ions forming the passive

film, such an explanation of E would be reasonable, and the effect of such

variables as the nature of the fiietal , concentration of anions, and temperature

all affect E and pzc (see 45,46). The transition complex model also could

use the relation of pzc to E , not for a bare metal but for the passive

film-solution interface. LiKewise, for the penetration models, Ambrose and

Kruger (17) have suggested the relationship of pzc to E . Because it is

difficult to measure the pzc for solid electrodes (45) Especially wlien

adsorption may coincide with the initiation of breakdown processes, there have

been no good experimental verifications that E is related to the pzc.

Galvele (47) has pointed out that E and pzc may be related if E is the

potential to initiate pitting, but they are not related if E is the potential

to maintain pitting.

A major problem with relating pzc to E is that the pzc for many anions,

both aggressive and non-aggressive alike, arE similar on mercury and probably

are similar on oxides as well (27).

b) Er determined by film pressure to fracture film - Sato (26) has

identified E v/ith the critical potential above which the film pressure

exceeds the Critical compressive stress of film mechanical breakdown. The

film pressure, p, resulting from both el ectrostriction and interfacial effects

-45-
is given in Eq. [2]. Using the above assumption Sato has been able to derive

the relationship between the E and the concentration of anions in solution as follows:

8TTkTr»-

dl e(e+l)E [3]

MA

where a,„ is the activity of the salt y,., r^- is the adsorption density of

A , k i's the Boltzman constant, T the temperature, E the electric field

strength, and e the film dielectric constant.

He was able to calculate from [3] using the results of Leckie and Uhlig (15)

for co-existing anions that the density of adsorption of the aggressive Cl~

ion on passive stainless steel is of the same order as that for the inhibitive

OH", N0^~, and SO. ' ions. One problem v/ith the relation of E to film

pressure is that the special role of aggressive anions such as CI" is

uncertain because they have about the same polarizabil ity as oxygen ions and

only slight changes in dielectric strength (a few percent) might occur if they were to replace the oxygen ions. This would be particularly true at the low

concentrations of Cl~ to be expected in passive films undergoing breakdown

c) Er is determined by repassi vation kinetics - This concept of E is

based on kinetic considerations. Some studies (28,29,34,38) have found 'that a competition between repassi vation and breakdown of films on Al is important in

determining initiation of final breakdown. Videm (28) proposed that the value

of E is determined by the relative rates of these two processes. He based

this on his experiments that showed a narrow potential region where repair and

breakdown were competitive. Below this region, repair predominated and above

it, breakdown was the major process. Because potential can affect

repassivation kinetics (31,32) (see Fig. 3) and because, as Videm and others

(47,34,38,29,49) suggest, initiation of localized attack depends on a dynamic

balance between breakdown and repair, the kinetic control of E must

definitely be considered. More quantitative details are still^lacking,

however, and the problem remains of finding a way to concentrate damaging

anions before final breakdown occurs when no crevice or "occluded cell"

situation exists.

d) Er. is determined by the potential of formation of an unprotective

film - One possible factor determining E which may not apply to all cases may

be that it is the potential above which an unprotective film may form or

deposit. Ambrose and Kruger (17) suggest that this may be the case for iron._

They found that the E they measured in pH 8.4 borate solutions containing Cl~

was both equal to the^pzc for iron found by Lorenz and Fischer (50) and the

equilibrium potential for the reaction that produces in a series of steps the

non-protective y-FeOOH (51). It was proposed that y-FeOOH wai^formed after

penetration by Cl' had produced an ion conductive path for Fe which would

produce the non- protective y-FeOOH above E . The role played by E in

producing a non- protective film in the presence of Cl" is shown dramatically

in Fig. 4. Whether this sort of explanation of E can apply to a large

variety of situations remains to be shown. ^

(48).

-46-
Alloy Composition, Structure, and Environmental Factors

It is not possible within a limited space to discuss the many complex

and contradictory results concerned with the effect of alloy composition and

structure and environmental factors that influence breakdown initiation

processes. Two excellent reviews iiave been given by Smialowska (1,2). Instead,

I will describe briefly how some of these factors relate to the theoretical

models for breakdown.

1 ) Alloy composition

Kolotyrkin (3) points out in his review of pitting that the tendency

to breakdown decreases with an increase in the content of N, Ni , Cr, and

Mo, especially for the latter two. The main effect of these beneficial

alloying elements is to shift E in a noble direction. This explains

phenomenological ly why they are beneficial but does not actually tell us how they work. One way in which they can retard initiation processes is

by producing a passive film that is more difficult to penetrate because it

provides fewer diffusion paths. Hoar (51) has suggested that some of

these beneficial alloying elements can do this by producing amorphous

(glassy) films. McBee and Kruger (39) have found experimental evidence

for this when Cr is added to Fe and also have found reduced rates of

penetration (52). Another way, however, in which these alloying elements

can affect breakdown is by affecting repassivation kinetics. Thus, Mo

which has not been found in the films on alloys containing it (53), is

known to be beneficial in retarding breakdown. Measurements by Ambrose

and Kruger (29) indicate that its addition to an alloy does, however,

increase the rate of repassivation, perhaps by its entrance into the

environment.

2) Alloy structure

Only the surface structure of alloys will be considered here. Three

kinds of possible variations in surface structure have been studied.

First, Kruger has looked at the role of crystal lographic orientation on

breakdovyn tendency for iron (54.) As Fig. 5 shows, the tendency to

pit goes up as the surface approaches the {110} orientation. He also

found that pitting density varied with crystal lographic orientation (55).

Another example is the single crystal work by Smialowska and colleagues

(56,57) on Fe-16 Cr. They found that nucleation occurred at crystal subboundaries as well as at other sites where there were not necessarily any

metal inhomogenieties

.

The second surface variation can occur by the introduction of

defects by abrasion, cold work or other deformation processes.

Contradictory results have been found. Thus, Rozenfeld (58) found that

removing cold work by annealing an 18 Cr-9 Ni-Ti steel increased the

number of pits while Randak and Trautes (59) found that cold work

increased the number of breakdown sites for an 18 Cr-8 Ni steel.

Finally, surface heterogenieties play a big role in initiating

breakdown. Smialowska and co-workers (57,60,61) have found extensive

evidence for the nucleation of pits at inclusions such as sulfides or

-47-
chromium oxide for a number of alloys. Bond and co-workers (62) showed

that very small amounts of segregated metallic impurities in quite pure

aluminum single crystals could produce breakdown nucleation sites.

The obvious, but sometimes contradictory, effect of surface

structure or breakdown can be explained by all the theoretical models

discussed because it is reasonable to expect adsorption, penetration,

complex formation, or repassi vation kinetics to be affected by surface

heterogenieties

.

3) Environmental factors

These factors can include solution composition (including pH) and

temperature. There are large numbers of experiments in the literature

(see 1,3) and the results are contradictory. Generally, however,

breakdown tendency increases with temperature and concentration of

damaging anions while it is not strongly dependent on pH (63). The nature

of the anions in solution play a large role (15) especially if competitive

anion adsorption situations are possible. There is some controversy as to

what consitutes a damaging anion. Thus, Hoar (7) considers the C10,~ ion

as damaging as Cl~ whereas Uhlig has found it to be inhibitory (15,T8).

With regard to models, again all models will depend on environmental

consiaerations . A Detailed examination of how all the various and

contradictory results in the literature best fit the various models is

outside the scope of this discussion.

CRITICAL QUESTIONS

Based on the foregoing discussion the following non-inclusive list of

critical questions can be posed:

1) What makes a damaging anion damaging?

2) What happens when a damaging anion reaches an expoded metal surface?

3) Are the values obtained for induction times consistent with any of

the penetration models?

4) How does the damaging anion concentration build up during the

breakdown of uncovered (non-creviced) surfaces before corrosion products

form?

5) Is the pzc always related to E^?

6) Does the effect of alloying on breakdown show itself by altering

the nature of the film, the environment, or the exposed metal surface?

7) What is the role of repassi vation in breakdown?

8) Are significant changes in film pressure affected by damaging anions?

-48-
9) Are the pulses observed for some passive surfaces related to

breakdown and repassivation events?

ACKNOWLEDGEMENT

I arn grateful to the Office of Naval Research, which supported th

work under contract NAONR 18-89 NRO 36-082. I also wish to acknowledge

many stimulating and helpful discussions with Dr. John R. Ambrose, whos

work has served as a basis for many of the ideas presented here.

-49-
REFERENCES

Z. Szklarska-Smialowska, Corrosion, 27, 223 (1971).

Z. Sklarska-Smialowska, "Localized Corrosion," Staehle, Brov^n,

Kruger, and Agrawal , Editors, National Ass. Corr. Eng., Houston,

p. 312 (1974).

Ya.M. Kolotyrkin, Corrosion, 19., 261 (1963).

P. Forchhammer and H. J. Engell, Waksoff u. Korr. , 20, 1 (1969).

W. Schwenk, Corrosion, 20, 1291 (1964).

R. W. Staehle - Paper on mechanical breakdown presented at JapaneseAmerican Conference on Passivity, Honolulu (1975).

T. P. Hoar, Corrosion Science, 7^, 355 (1967).

T. P. Hoar and W. R. Jacob, Nature, 216, 1299 (1967).

M. Pourbaix, L. Kleinzack-Mathieu CI. Mertens, J. Meunier, CI. Vanlengenhaghe,

L. de Munck, L. Laureys, L. N. Nellmans, and M. Warzu, Corrosion Science,

3, 239 (1963).

H. Kaesche, Z. physik Chem., N.F., 34, 87 (1962).

M. Pourbaix, Ref. 2, p. 12.

N. Pessall and C. Liu, Electrochemica Acta, 16, 1987 (1971).

Ya.M. Kolotyrkin, J. Electrochem. Soc, 108, 209 (1961 ).

H. Bohni and H. H. Uhlig, J. Electrochem. Soc, 116, 906 (1969).

h. P. Leckie and H. H. Uhlig, J. Electrochem. Soc, VIS, 1262 (1966).

C. L. McBee and J. Kruger, Ref. 2, p. 252.

J. R. Ambrose and J. Kruger, Proc. 4th Int. Cong, on Metallic Corrosion,

NACE, Houston, p. 698 (1972).

I. L. Rozenfeld and V. K. Maksimchuk, Z. Physik Chem., 215, 25 (1960).

T. P. Hoar and D. J. Foster, quoted in Ref. 2, p. 114.

V. R. Evans, L. C. Bannister and S. C. Britton, Proc Roy. Soc, A131 , 367 (1931).

J. A. Richardson and G. C. Wood, J. Electrochem. Soc, 120 , 193 (1973).

-50-
J. A. Richardson and G. C. Wood, Corrosion Science, 10, 313 (1970).

T. P. Hoar, D. C. Hears, and G. P. Rothwell, Corrosion Science, 5^, 279

(1965).

M. A. Heine, D. S. Keir, and M. S. Pryor, J. tlectrochem. Soc, 212, 29

(1965).

M. J. Pryor, Ref. 2, p. 2.

N. Sato, Electrochemica Acta, 19, 1683 (1971).

P. Delahay, "Double Layer and Electrode Kinetics," Interscience, Nevj

York, p. 23 (1965).

K. Videm, Kjeller Report KR-149, Institutt for Atomenergi, Kjeller

Norway (1974).

J. Kruger and J. R. Ambrose, NBS Report NBSIR 74-583, September (1974).

C. L. Foley, J. Kruger, and C. J. Bechtoldt, J. Electrochem. Soc, 114 , 994 (1967).

J. R. Ambrose and J. Kruger, Proc. 5th Int. Cong, on Metallic Corr.,

NACE, Houston, p. 406.

J. R. Ambrose and J. Kruger, J. Electrochem. Soc, 121, 599 (1974).

F. D. Bogar and R. T. Foley, J. Electrochem. Soc, 119, 462 (1962).

J. Zahavi and M. Metzger, Ref. 2 p. 547.

B. E. Wilde, Ref. 2, p. 342.

H. Engell and N. Stolica, Z. Physik Chem., N.F., 20, 113 (1959).

D. A. Vermilyea, J. Electrochem. Soc, 104, 485 (1957).

G. Okamoto, Corrosion Science, 13_, 471 (1973).

C. L. Hcbee and J. Kruger, Electrochemica Acta, 17_, 1337 (1972).

W. P. Iverson, J. Electrochem. Soc, 115 , 617 (1968).

M. J. Pryor, Corrosion Science, 10, 313 (1970).

B. Verkerk, Proc UNESCO Int. Conf. on Radioisotopes in Scientific

Research UNESC0/BS/RIC44.

G. C. Wood, Ref. 2, p. 10.

S. Smialowska and M. Czachor, Ref. 2, p. 353.

J. O'M. Bockris and A. K. N. Reddy, "Modern Electrochemistry," Vol. 2,

Plenum Press, New York, p. 706 (1970).

-51-
S. D. Argade and E. Gileadi in "Electrosorption," E. Gileadi, Ed.,

Plenum Press, New York, p. 87 (1967).

J. R. Galvele, S. M. de Micheli, I. L. Muller, S. B. de Wexler, and

I. L. Alanis, Ref. 2, p. 580.

H. S. Parker and C. A. Harding, J. American Ceramic Soc, 53., 583 (1970).

K. J. Vetter and H. H. Strehlow, Ref. 2, p. 240.

W. J. Lorenz and H. Fischer, Electrochemica Acta, 1J_, 1597 (1966).

T. P. Hoar, J. Electrochem. Soc, V\7_, 17C (1970).

C. L. McBee and J. Kruger, work presented at Japanese-American Conf. on Passivity, Honolulu (1971).

J. B. Lumsden and R. W. Staehle, Scripta. Metall., 6, 1205 (1972).

J. Kruger, J. Electrochem. Soc, 106, 736 (1959).

J. Kruger, J. Electrochem. Soc, HO, 654 (1963).

M. Janik-Czachor and Z. Szklarska-Smialowska, Corrosion Science, 8, 215

(1968).

A. Szummer, Z. Szklarska-Smialowska, and M. Janik-Czachor, Corrosion Science,

8, 827 (1968).

I. L. Rozenfeld, Korrzija i Zachita Metal lov, Lokanyje konosionnyje, Izd.

Metallurgia (1970).

A. Randak and F. W. Trautes Werkstoff word. Korr., 21, 97 (1970).

Z. Szlarska-Smialowska , A. Szummer, and M. Janik-Czachor, But. Corr. J.,

5, 159 (1970).

M. Smialowski, Z. Szklorska-Smialowska, M. Rydicik, and A. Szummer, Corrosion Science, 9, 123 (1969).

A. P. Bond, G. F. Boiling, and H. A. Domian, J. Electrochem. Soc, 113 , 773 (1966).

W. Schwenk, Werstoff U. Korr., 19, 791 (1968).

-52-
Fig. 1. Periodic potential fluctuations on A--a mild steel surface

in 0.1% NaCl solution and B--after ^% NaN02 has been added

to the NaCl solution. Note how the inhibitor NaN02 reduces

the pulse intensity. From Iverson (40).

-53-
2. Plots of time to breakdown as a function of film thickness

for iron in decreated 8.4 buffered borate solution containing

0.005 N NaCl . a) /time, b) In time and c) time. From (16).

-54-
Fig. 3. Effect of potential repassivation rate as shown by changes

in film thickness, x, for Ti alloy in 1 N NaN03 (dashed

line) and in 1 N NaCl (dotted line) at -222 and +706 mV

(SHE) at 25°C. From (32).

-55-
Fig. 4. The effect of potential on film growth in the absence and

presence of chloride ion (10"^ M) in boric acid-sodium

tetraborate buffer solution (pH = 8.4). Potentials were

• ' above E| (at -0.11 V). Film thickness was measured by

ell ipsometry . is as defined as E- in the text. From

(17). .

-56-
( Ml )

• PITTING O NON PITTING

(100) < no)

Fig. 5. Stereographic triangle showing the effect of orientation of

iron grains on tendency of pit. From (54).

-57-
PART IV

(To be published by Surface Science as proceedings

of the 3rd International Conference on El 1 ipsometry

)

Qualitative Use of Ell ipsometry

to Study Localized Corrosion Processes

J. Kruger and J. R. Ambrose

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

Localized corrosion processes (see [1] for a detailed description) such

as pitting, intergranular corrosion, crevice corrosion, and stress corrosion

involve many aspects that are not amenable to a quantitative study by

ell ipsometry. For example, a protective film undergoing the breakdown that

leads to pitting probably involves such complications as a continually varying

refractive index, the possibility of a changing medium, and the formation of

other films on top of the original film present at the inception of pitting.

Quantitative characterization of the thicknesses and complex refractive

indices of all the films involved is questionable under the best of

circumstances, but within the current state of the art is out of the question

for localized corrosion processes. This paper will describe certain

qualitative uses of ell ipsometry that can, however, provide valuable insights

into localized corrosion processes.

By qualitative ell ipsometry we mean the use of the ellipsometer in such

a way that we do not determine the optical constants of the film on the

surfaces studied nor do we attempt to determine film thickness with any

accuracy, but we do use rough estimates of film thickness. Instead, changes

in A and \i) can be used in the following ways:

1) To measure the time during the course of a process when significant

changes occur as indicated by abrupt changes in A and ii) and to

correlate the A,ij; changes with others, e.g., electrochemical ones.

2) To observe variations of A and ti; as a function of wavelength under

different environmental conditions in order to detect regions of the spectrum

where special effects are observed.

3) To determine what rate laws are operative by measuring variations

of thickness (for thin (<5 nm) films, assumed proportional to a) with

time.

We will describe the application of some of these "qualitative" uses of

ellipsometry to the study of a) the initiation of pitting, b) the initiation

of crevice corrosion, and c) repassivation kinetics as applied to stress

corrosion.

-58-
Initiation of Pitting

The process of pit initiation involves a change in the protective

ability of a passive film on a metal surface at discrete sites so that

extensive metal dissolution occurs. Such a description does not assume any of

the many proposed mechanisms for pit initiation [2] at these sites. Once

metal dissolution begins the initiation process is over and the pit

propagation stage is underway with its usually accompanying deposition of

non-protective corrosion products on top of the growing pit. Ellipsometry is

especially applicable to the initiation stage where subtle changes in the

protective film can be observed upon introduction of the damaging species

(usually chloride ions) that cause film breakdown. During pit propagation,

the usually accompanying surface roughening and corrosion product deposition

makes the application of ellipsometry more difficult.

We will describe three ways in which ellipsometry can be applied

qualitatively to pit initiation, 1) to make induction time measurements, 2) to

observe changes in the passive film during the induction time, and 3) to study

the effects of potential. All of these studies used to illustrate the

application of qualitative ellipsometry to pitting are concerned with the

pitting of passivated iron surfaces by chloride ions.

Induction time measurements

These studies were originally described in a paper by Ambrose and Kruger

[3] where more complete experimental details are given. In brief, the

experimental procedure involved potentiostatical ly polarizing pure iron

surfaces in sodium borate-boric acid solutions at potentials that promoted the

formation of a passive film. When the thickness of the passive film was no longer changing (for all practical purposes), measured amounts of solutions

of chloride ions were added to the solution already in the experimental cell.

The onset of pitting was then observed both by ellipsometric and current

density measurements. Fig. 1 shows the effect of chloride ion concentration

on induction time, defined as the time required before pitting is initiated by

passive film breakdown at discrete sites. The initiation of pitting is

signalled both optically and electrochemical ly but at two different times t-, and tg. The first indication of breakdown at t-] is signalled by an increase

in current which corresponds to a rapid increase in the rate of metal

dissolution. The second indication at t2 involves a rapid change in A (a

decrease) and probably corresponds to the formation of a corrosion product

film at the breakdown site. This is suggested by the fact that if sufficient

time is allowed one can observe the deposited corrosion products visually

without the aid of the ellipsometer. The electrochemical ly measured induction

time t] is dependent on a number of factors such as concentration (see Fig.

1), temperature of initial passive film formation, time allowed for passive

film growth prior to Cl' introduction and others, but t2 - t] was found to be

approximately constant (around 1 min.) It indicates that such deposition is

not coincidental with breakdown.

These qualitative ellipsometric studies carried out at one wavelength,

546.1 nm, provided the following data that are useful in developing an understanding of the localized corrosion processes of pitting:

-59-
a) No detectible optical changes due to thinning or refractive index

variations (when using A = 546.1 nm) occur in the passive film prior

to the initiation of pitting.

b) Pit initiation is not simultaneous with corrosion product

deposition but requires its own induction period of around 1 min.

The implications of these useful results revealed by qualitative

ellipsometry on the mechanism of pitting are given elsewhere [4].

Changes in the passive film leading to pitting

Probing more deeply into the just described work by Ambrose and Kruger,

McBee and Kruger [5] sought to determine whether optical changes could be

detected during the time before breakdown (the induction time) if

ell ipsometric measurements were made using a light source that permitted

measurements at a number of wavelengths in the visible region of the spectrum.

They found for the same system as that of Ambrose and Kruger that for a few

wavelengths, changes in A and ij; (more markedly in a) could be detected during

the induction period. Fig, 2 shows that some of these changes in A were significantly outside the range of experimental errors.

Since changes in A and i|j occur during the induction period at a few

wavelengths, it is possible to follow these changes as a function of time by

working at any one of these wavelengths. Fig. 3 shows that, indeed,

continuous variations in A and i) occur during the induction period until

pitting is initiated (at the cross-hatched area in Fig. 3). Unambiguous

determination of the optical constants for the passive film, and the changes

in these constants with the addition of the chloride, were not made for reasons given elsewhere [6]. However, calculations were made which show that

the changes in the film brought about by the chloride addition can alter the

refractive index, n, and the absorption coefficient, k, at the affected

wavelengths, while producing no changes at the other wavelengths studied.

Fig. 3 also shows other valuable details about the events occurring in

connection with the induction period. First, it shows that if chloride is

removed from the environment the passive film will eventually recover its

original a and \i) values. Second, the time to breakdown and the time to recover depend strongly on the thickness of the passive film originally

exposed to the chloride. Finally, the extent of the optical change,

^passive " ^breakdown* is inversely proportional to the thickness of the

passive film originally exposed to the chloride.

Thus, these results illustrate how, by just following a and changes

without attempting to calculate optical constants or thicknesses, one can learn valuable insight into the subtle processes leading to pitting. The

bearing that these results have on the mechanism of film breakdown is beyond

the scope of this paper but is given by McBee and Kruger [5].

-60-
Effect of potential

Thus far, we have described studies where the surface observed was held

at a potential above that necessary to initiate pitting, the critical pitting

potential, E^.. Many workers [7] believe that pits will not initiate at

potentials below E^. Others [8] believe that if one waits long enough pitting

will initiate below E^. This disagreement may arise because of the use of

different techniques for measuring Ep. El 1 i psometry, however, does reveal

that one can distinguish entirely different optical behavior between

passivated surfaces exposed to chloride well below and well above most

commonly accepted values of E^.

First, it can be shown (Fig. 4) that passive film growth starting with a bare iron surface proceeds in an identical fashion in solutions containing

chloride to that observed for solutions without damaging ions when the

potential at which the film is grown is below E^. When, however, the

potential is above E^,, chloride in the aqueous solution promotes rapid

limiting thickness). This type of growth is similar to that observed at time

t2 and attributed to the deposition of a nonprotective corrosion product film

as described earlier. Therefore, ellipsometry shows that potential has a profound effect on events occurring after pit initiation occurs.

Secondly, potential can also affect the processes occurring prior to

breakdown when chloride in introduced. Above E,-, as described earlier, the

introduction of chloride produces changes in the ellipsometric parameters,

especially A, at only a few wavelengths. Below E^, however. Fig. 5 shows that

approximately equal changes in A occur at every wavelength when chloride

ions are introduced. This essentially uniform decrease in the overall A

values, except perhaps at 590 nm, accompanied by mainly insignificant changes

in the \i) spectrum indicates an increase in film thickness. An increase in

film thickness can be caused by either the adsorption of chloride on top of

the film or further growth of the passive film. It is also possible that both

of the above mentioned processes take place concurrently. Calculations show

that the magnitude of the changes in A and \i) require a change in film

thickness of at least 1.5 nm. This is too large to be accounted for by only

chloride adsorption. At any rate, for surfaces at potentials below E^j

calculations show that the ellipsometric spectra indicate changes in film

thickness (usually by a downward shift in A and insignificant changes in

rather than changes in the film properties, as would be indicated by irregular

movements in the A and ip values at discrete wavelengths.

These ellipsometric studies thus reveal that potential may affect much

more than the kinetics of pit initiation. They reveal that subtle changes in

the passive film itself are affected by the potential at which it is exposed

to chloride, and these changes must be related to the tendency for the film to

break down when the potential is above Ep.
Initiation of Crevice Corrosion

Once it starts, crevice corrosion bears some similarities to pitting

corrosion, in that both forms of localized corrosion propagate because of

restricted mass transport of reactants or reaction products. In the case of

pitting, mass transport is restricted because of corrosion products deposited

on top of the pit and, in the case of the crevice, because of the existence of some material that restricts mass transport, e.g. gaskets, another piece of

metal, a deposit of dirt, etc. Ambrose [9] has described how one can apply

ellipsometry to the study of processes occurring inside a crevice by creating

a crevice with a glass plate on top of the metal surface to be studied (Fig.

6). Use of such transparent windows for visual observation of a crevice is

not new [10,11], 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 changes in composition of aqueous media as well.

One can only hope to gain qualitative indications of the changes because the

system is extremely complicated from an ell ipsometric point of view. Besides

the complications of the bottom surface of the glass plate which can be a surface for the adsorption of films and the changing index of retraction of

the solution in the crevice, there exist the possibilities of passive film

growth or dissolution, the deposition of corrosion products on the glass or

metal and the effects of metal roughening once crevice corrosion has

commenced. We will show, however, that this application of ellipsometry can provide additional information that helps in the interpretation of

electrochemical results by being able to detect small changes occurring early

in the chain of events leading to crevice corrosion and being able to

determine at what time these changes occur.

The device used to simulate a crevice in these studies is shown in Fig.

7 and consists of a Teflon adapter which serves to position a circular pyrex

glass plate against the surface of a polished metal specimen. The thickness

of the pyrex 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

(Figo 6). 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. Using this

device, as well as looking at surfaces without a crevice where solution

conditions are changed to simulate the conditions existing in the solution

within a crevice, useful insights can be gained into the processes leading to

crevice corrosion in two areas,

a) the effect of deoxygenation and

b) the effect of the crevice.

-62-
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 [12]. Therefore, el 1 ipsometric measurements can examine the

effect of oxygen depletion on the protective film on the metal in the absence

of a crevice. For a metal with absorbing films such as stainless steel, A is more sensitive than ^ to changes in film thickness [13]. Both parameters are affected by film optical property changes. Results for 304 stainless steel

exposed to l.ON NaCl without a crevice, in the open circuit mode, are shown in

Fig. 8. Oxygen depletion results in the expected shift in corrosion potential

toward more negative values with the A-values remaing virtually unchanged

throughout the exposure period, increasing by only 0.08° 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, i|j is increased by 0.260° over the

exposure period.

These data indicate that in a deoxygenated solution where the

potential is changing slowly (9.4 mV/minute), the ellipsometric parameters A and are rather constant with only very slight thinning or change in the

nature of the film on the stainless steel substrate having occurred. Based on the reaction:

O2 + 2H2O + 4e" ^ 40H"- (1

)

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

magnitude decrease in oxygen concentration. However, after about 100 minutes

there is an increase in the rate of potential change accompanied by a significant increase in the values of ij while A remains essentially constant.

This would appear to indicate that a change in the nature of the film has

occurred, but because A does not change there appears to be no significant

thinning. Since micropits were observed at this stage, such behavior

indicates a localized breakdown in passivity.

These ellipsometric studies suggest that, contrary to what has been

proposed in the literature [13], depletion of oxygen alone does not lead to a uniform film thinning and loss of passivity over the whole surface, a behavior

characteristic of crevice corrosion attack. Instead oxygen depletion 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

When a crevice is present not only is there oxygen depletion but also a change in the concentration of other species in the aqueous solution in the

crevice. Using the apparatus shown in Fig. 6, ellipsometric observations of

the metal surface in a crevice produced results of the sort shown in Fig. 9.

With the surface held at the initial immersion corrosion potential (+42 mV) , A values decreased markedly during the first 20 minutes of exposure (6A = -0,12°) before increasing to a maximum value (6A = +.60°), after about 1450

minutes. The ii values fluctuated a great deal but, in general they decreased

-63-
during the first 200 minutes (6A = 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

that the A values started to decrease sharply. It is worth noting that, while

large optical changes occur, virtually no electrochemical changes are observed

until the current increase signals the onset of crevice attack. The optical

changes are those that presumably lead to the initiation of this attack.

It is obvious from Fig. 9 that optical results completely different from

those obtained without a crevice (Fig. 8) 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 i|j is opposite to that observed in oxygen depletion where a crevice was not

present, and apparently processes other than oxygen depletion are playing a predominant role. Other studies [12] indicate that these processes are leading to a marked change in the concentration of hydrogen ions, chloride

ions and metal ions in the crevice solution.

Before attempting to interpret the el 1 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 assuming no adsorption takes

place on theplate and the A measured will not differ from that resulting from

optical conditions at the metal surface. The value of \p 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 such an experiment.

•In order to determine empirically the effect of increase of the solution

refractive index on A and i|; outside of a crevice, ellipsometric measurements

were made on a stainless steel surface, with a crevice, in both IN NaCl and

2.5N Nad solutions, the latter having a higher refractive index. As Table 1 shows, the measured value of A with the crevice in place decreased markedly in

value in the more concentrated solution. The value of \i) increased slightly.

Thus, based on the changes in A shown in Fig. 9, the initial stages of crevice

corrosion are consistent with an increase in crevice solution index of

refraction, i.e., an increase in the concentration of CI" and metallic ions.

Because i) is difficult to interpret in the presence of a crevice for the reasons given above and because of the greater fluctuations in the values

measured, we cannot judge whether the i) 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 overal

1

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 non- crevice oxygen depletion studies, and is

characteristic of surface roughening and/or film growth. Notably, this

-64-
decrease comes at the same time that the current measurements indicate an increased rate of corrosion where presumably metal roughening and/or corrosion

product deposition occurs.

The effect of a crevice on the corrosion rate and ellipsometric

parameters of a Ti 8AI-IM0-IV alloy was also studied. This alloy, which is

not susceptible to crevice corrosion in a similar environment at room

temperature, exhibited none of the ellipsometric variations associated with

the onset of crevice corrosion attack on 304 stainless steel when its surface

was held at the open circuit corrosion potential while exposed to the crevice

created by the glass plate. In fact. A, and i were relatively constant

over the entire period of exposure.

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 measurements 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 events initiated during these changes can be

developed using measurements in special environments without a crevice. The

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

a chloride solution can be described as follows:

1) The initial decrease in A may correspond to the concentration

of metal and chloride ions leading to an increase in the refractive

index of the solution in the crevice.

2) The subsequent increase in a may correspond to film thinning.

3) The final decrease in A may correspond to both metal roughening

and corrosion product deposition.

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

ellipsometric and current measurements over the entire exposure period

indicates that none of the crevice solution concentration effects which were

detected for the susceptible 304 stainless steel had occurred.

Repassivation Kinetics

This last to be described application of ellipsometry seeks to measure

the rate at which a protective film grows on a surface once the previous film

is removed. This phenomenon is of great importance to stress corrosion

cracking (SCC) where films are ruptured when a metal is stressed, and the rate

at which the surface is repassivated determines the length of time the

unprotected metal can be attacked by an environment. To measure this

repassivation process, Ambrose and Kruger [14] developed a technique called

"triboellipsometry". The technique uses a small abrasive polishing wheel in

-65-
the environment to be studied to remove the protective film on a metal

surface. This abrasion step leaves the metal surface in a coldworked state,

but, since the tip of a growing stress corrosion crack is continually

undergoing plastic strain, the growth rate of a film on the abraded surface is

probably closer to that which occurs at the crack tip than the rate for a strain free surface. The polishing wheel is then quickly withdrawn, and

ellipsometric and electrochemical current transients are recorded on an oscilloscope. The current is a measure of both the film growth and the metal

dissolution that occur during repassivation. The ellipsometer measures mainly

film regrowth.

While there are other electrochemical techniques [15, 16] for measuring

repassivation kinetics by using current decay measurements, the tri boel 1 ipsome trie technique allows one to determine how much of the current is

involved in film repair and how much is involved in metal dissolution. It

cannot do this quantitatively because it is difficult to measure the amount of

bare surface produced by abrasion so that the current density is difficult to

determine. Moreover, exact measurements of film thickness by ellipsometry on the abraded surface are difficult. (One can, however, measure changes in A and \i) and relate these changes to thickness changes that are reasonable

estimates even when the optical constants of the film are not well known

[17]). To overcome the difficulties of inexact current density and film

thickness measurements, a parameter has been defined [18, 19] that allows

one to compare the repassivation behaviors of a given metal in different

environments and thus use it as a diagnostic tool for estimating which

environment would have the greater likelihood to promote stress corrosion.

This parameter, Rp, has been defined in a manner that is convenient for

triboellipsometric measurements as the ratio

where i-*. is the total anodic current which flows for a given time interval, t,

to a surface made bare by film removal (through rupture or by other means) and

x^ is the thickness of the film at time t. Rp can also be expressed as

where Qj is the total charge involved in the anodic process, Qa is the charge

involved in metal dissolution, is the charge involved in the film

formation, and K is a constant which depends on the surface area, and the

nature of the film.

R (2)

R, (3)

-66-
The value of Rp is a measure of the effectiveness of the repassivation

process after film removal. If repassivation is ineffective at first, Qj

increases rapidly, resulting in a large value of Rp. On the other hand,

effective repassivation, where Qf does not differ greatly from Qj, yields a small value Rp. Since Rp is measured as a function of time, Ambrose and

Kruger examined [18] what happens to dRp/dt under different hypothetical

situations which could exist at a crack tip. A more detailed and rigorous

analysis is given else- where [19].

Case 1 dRp/dt = 0. This occurs when = Q^j. Here SCC susceptibility

is highly unlikely. From Equation (3), it can be seen that Rn = K and dR^/dt

= 0. ^

^

Case 2 dRp/dt > 0. This reflects the situation when Q^j > Qf and R K Qjj/Qf so that dR/dt is positive. If » Qfj dR/dt can become a large positive

number and SCC may be unlikely because the extensive dissolution involved may

result in considerable blunting of the crack tip.

Case 3 dRpdt < 0. This can happen where dx/dt > dQj/dt. By

differentiating tne expression for Rp in Equation 2.

dRp ^ dQ^/dt _ Q^dx/dt

(4)

dt ~ ?~

It can be seen that, since Qj is always greater than Qf, when dx/dt > dQj/dt,

dRp/dt will have a negative value. The significance of dRp/dt < 0 can not be

explained until two other important parameters determined by triboell ipsometry

are examined; dx/dt, the rate of film growth, and tp, the time when dx/dt

becomes small (arbitrarily defined as the point in time where dx/dt < 0.001

nm/msec). Using these two parameters, in addition to Rp and dRp/dt, their

possible values in various environments for a given metal known to meet

mechanical and metallurgical requirements for SCC can be compared as shown in

Table II.

Using the analysis shown in Table II, a highly qualitative but useful

guide for comparing SCC behavior of a metal in an environment where

susceptibility is known with its behavior in an unknown environment can be

developed. Table II also provides insights into the process occurring at a crack tip or during crack initiation. It allows, for example, an estimation

of the effectiveness of a protective film from the value of dRp/dt. It also

points out that a high rate of film formation is not a sufficient condition

for nonsusceptibility because the ability of the film to prevent dissolution

(its protective quality) is equally important (Condition A). Table II further

indicates that the formation of a good protective film is not sufficient to

prevent SCC if dx/dt is small (Condition B).

-67-
Fig. 10 shows the application of the techniques and analysis just

described to the study of the repassi vation behavior of a low carbon steel in

three environments: a) 4N NaN03 solution at 90°C where this steel is

suscentible to SCC, b) IN NaN02 at 25°C where it is not susceptible to SCC,

and c) IN NaCl at 25°C where it is not susceptible to SCC but where extensive

corrosion takes place. Table III gives the various repassivation parameters

for the low carbon steel in the above environments.

The three different kinds of behaviors shown in Fig. 10 and in Table III

provide examples of three of the conditions described in Table II. These

examples were described a number of years ago by Evans quoted in [20] before

techniques existed that could clearly distinguish them. He proposed that

"chemical factors in the liquid mainly decide whether attack at any point will

(a) heal up [condition C existing in NaNOpJj (b) remain localized developing

into a crack [Condition D existing in NaCT], or (c) spread out [condition A

existing in NaN03], leading to general attack." The fact that

triboellipsometry can distinguish these three kinds of behavior during the

first few tens of milliseconds of a repassivation event points to its great

usefulness in gaining an understanding of phenomena that involve

repassivation.

-68-
References

[I] M. G. Fontana and N. D. Green, "Corrosion Engineering",

McGraw-Hill Book Co., New York (1967).

[2] T.P. Hoar, Corrosion Sci., 7, (1967), 341.

[3] J.R. Ambrose and J. Kruger, Proc. 4th International Congress

on Metallic Corrosion, 1969, Nat'l. Assn. of Corrosion Eng.,

Houston, Texas (1972), p. 698.

[4] J. Kruger, Proceedings of Japanese American Conference on the

Passivity and Breakdown of Passivity of Iron Base Alloys, to

be published by Nat'l. Assn. Corrosion Eng., Houston.

[5] C.L. McBee and J. Kruger, "Localized Corrosion", N.A.C.E.-3,

Staehle, Brown, Kruger, Agarwal , Editors, Nat. Ass. Corrosion

Eng., Houston, (1974), p. 252.

[6] C.L. McBee and J. Kruger, Sur. Sci., 16, (1969), 340.

[7] H. Bohni and H.H. Uhlig, J. Electrochemical Soc, 1J[6_, (1969),

906.

[8] M.J. Pryor, "Localized Corrosion", N.A.C.E.-3, Staehle, Brown,

Kruger, Agarwal, Editors, Nat. Ass. Corrosion Eng., Houston,

(1974), p. 2. .

[9] J.R. Ambrose — to be submitted to the Electrochem. Soc.

[10] R.B. Hears and U.R. Evans, Trans. Faraday Soc. 30 (1934), 417.

[II] U.R. Evans, J. Inst. Metals 30, (1923), 239.

[12] I.L. Rosenfeld and I.K. Marshakov, Corrosion 20 (1964), 115t.

[13] J.F. Bates, Corrosion 29 (1973), 28.

[14] J.R. Ambrose and J. Kruger, Corrosion, 28 (1972), 30.

[15] J.C. Scully and T.P. Hoar, 2nd Int. Congress of Metallic Corrosion, New York 1963, Nat. Ass. Corrosion Eng., Houston (1966),

p. 184.

[16] N.D. Tomashov and L.P. Vershinina, Electrochimica Acta, 15, (1970),

501.

[17] J. Kruger, "Advances in Electrochemistry and Electrochemical Engineering"

Vol. 9, R.M. Muller, E., John Wiley and Sons, New York (1973), pp. 249-

250.

69
[18] J.R. Ambrose and J. Kruger, Proc. 5th International Congress

on Metallic Corrosion, 1972, Nat'l. Assoc. of Corrosion Eng.,

Houston, Texas, (1974), p. 406.

[19] J.R. Ambrose and J. Kruger, J. Electrochem. Soc. 121 ("1974),

599.

[20] R.N. Parkins and R. User, 1st Int. Congress on Metallic Corrosion,

Butterworths London (1962), p. 289.

USC;OMM-NBS-DC 70
Table I - The Variation of a and i/; as a Function of Solution Concentration

NaCl Solution Refractive A i> 6a 6\i) Concentration Index 1.0>2.5 1.0-^2.5

l.ON 1.3432 113.86 29.38

2.5 1.3567 111.70 29.41 -2.16 +.03'

71
002 1 c

•r" 1 3 4J C o

CLLU <—

<*- OJ o O

C

iA < (/) •r- i- 00 c

<u •r— 4J O<_) ^rO

ITJ J- 0) XJ

<a o

Q. •r- -M

OJ Ol

sz E OJ o «o o

•r—C CO

4-> •r— >

to C_)

«/) «3 4-> o

a.C <+- OJ

(U <u o: E

c: se

<+- o O jc o u

•r- 1— •r- c: > o C O 3 (/) LU •-> £

si

C c

a. o E c o •r- o c

ID 1 (O

4-> +J

»—I

01C

l-H s: (I) E

-U c

_J o >

<: •r- •1— •r- 1— o J3 > E 1 Q. 0)

•f— 0) •r-

+-> > «4-

u s-

•r- r—

c 4-> c s-

•r—

O o O o o •p.

u 3

fd • OJ (U o C

L. to CO +J s- O) oo c fO

4->

O o (U o o M a> •r- S- • to S- (-> S- E n3 o E cn 3

4-' o. c: S> cn s- 3 »o o CO o o (= O) o ra o 4-> ^- •r— +J o C 00 • fO -M C M0) •f— o c O O) 3 E •r- a •r- +-> +3 3 >> c </) CO lO C •r- CO t. r—

rO E 3 ra o i*- O) o o» O o +5 *f— o 1/) O O fO T3 -o O) cn-r- M— 10 3 CL C CO OJ > OJ C r—

CO •r— E CD +-) •1-C c E •r- c a -(-> ^ S- 3 +J 3

cn -o > 00

a o Oa S- +-> • r- •r- Ol C «+- cu •1-

O O) o +j 3 X O) -M c: O- o > o o -a O o oo

00 »o +J r— S- "r— > CO > •1— CL 4-> • +J c c: cn 3 3 M- 3 cn 3

4-> o> c: .a 0)

C j3

U -!-> -a JO

0)

X 3 O > c. • +J CO -M

C

-!-> c CO 3^ M- •rO 3 CO

C o O 'r- +-> •r— •r- 3 •<— s- 4-> X o; >> o +-> i- -a 1— CO Q. • O) x> (U (tJ o E if- -Q O 1 a; 3 E >>.e S_ C +-> 00 o; 2 o cn Q- S- o o; o •r— S- OJ •r- O

Q. _j ^- O- > +-> o

CM

X

-o

-3oo oAA X -M AA 3OAA 3

73

4-> -o

onAA 3 C

X U-> oV

•u I AA

a:

CO

X|+J X 4J

4->VoV

X -tJ -a ro X -M oro AA 3 OAA

V 3 C a:

a I AA 3 • 4->

OJCOc

&.

4->

+3o

t-cnEoccc • cn o. a; -M _3

4-> •4->

72
TABLE III - Repassivation Parameters for Low Carbon Steel

for Nitrate, Nitrite, and Chloride Solutions. From [18]

/ mcoul/cm^ x p dx/dt

^ nm ' dRp/dt (msec) (nm/msec)

4W NaNOa at 90 C >2.75 + 750 0.00425

IN NaN02 at 25 C 0.25 TOO 0.012

IN NaCl at 25 C >8.0 + >500 "0

73
(0oE

0) ro0o

0)

4) 5

COoX

lA

0)oa0 cE

dJE Oo

to

(U 0)

4-> cn

S- 03 to c -a

0) s_

a ea

4-> o +-> M- 3 T3 -C

fO fO

C

s> •r- CD cn

C +J E c c s <u •1

o +J o cu o

r—

cu S- o

"O o <+- o If- +3

to >> c c c O) o -o 2 to 4J o O c a s. o Q. o rtJ 0)

•r— c M O vo 4J . fO "1— •<-> 0) 1—1

+J o 1- CM c E S-i—1

to Z3

u 03 E to o E c o o O •r- o 4-> <+- c u. C fO 

" c -o o

•r- a»o

i~ r— o ^ f— o

CO

0)>u

to X)o -— to ^ m o o »4- -r- C O C 4J 5 O 3 O M -r- r— -O U 4-> O ^

(U

U (/I ro 4-13 (U

*l- -O J- S- <U O O) ^ I- «4- <u 4-> «+- a»

••- ja H- < • CO UJ -—a:

t>o co >

+J «^

n3

O "O . S- r-

(T3 + +J

CU +-> S- fO

a» 4J

to c

Q. toc c o

•rO

74
i

m

DC

He

•

mi -

i -

a

2

z

i

-

—

mZ

tnZ

m

m

mim

(Ti

lO

0^

ro

X

E

z:

UJ

>

<

(

_IOr7 - SSDd V)

OJ -o

•r" c

i- o O)

c: o

4-> +J f—

1

o o LD

i- "o c OJ 1 1 cn i.cu E -C o

o 03 2 S-

•r- +J CO

n3 +3 C cn • s_ c 3 c O) o o O QJ

<4- •p— -C

-M (/) a> (0 XJ > >

r— to c o •r- s i-

•r- l/l o

(T3 +-> 0) 1 j- JD 4-> cu O (T3 C_) o

i_ < O)

(/) O O) ^ Ol -O 4-> 3 OJ 4-> r— a -o c

O) •r— OJ ro > s_ S- +->

1 OJ o fO +3

<1 I+- L>

<+- O)

OJ "o -a SI c o

4-> •pTM E

c

C •p— LO (/) >>

0) o 3 XI

CD LU o

i- t. CO o s_ CU

0) <c 4-

J3 > jC ^-

(= +J •r— CU -o U O JZ o +-) to O C7) CU

s- E c

n3 a» -p

(O 'oj >

t/)> -o OJ 00 fO <

u. f03

(U Q. o

x: c < >+- I— o o

75

CM

CD
-I- 1 1 1 I -I 0 20 40 60 80 100 120 140 160 ISO

TNiiC. min

Fig. 3 Modulations in A and i) at 410 nm for prebreakdown and recovery

processes are indicated for films of four thicknesses, 5.5,

'4.0, 3.0, and 2.0 nm. (From [5]).

• 76
ssauvioiLj:^

o

E SCU O)

00 "+- o; HS- 3

Cl O OJ C -l-> fd rO

s- cu o

U XJ C (T3 QJ S- CO +-> Ol

(tJ +->

OJ E

=3

t-> -r- -o c o

•I— CO

I

+-> -r- O fO

s-CD a

•r- E s- I— o

•I- J3

<4- C

E T- O

to

•r- CM +J I E OOJ — 4J>

O

OL E

O

4- -r- O

CU U -r- (D S- ^- o

4- r—

O) -Eo

I Q.

-a O CD

• s- O 3

to

fO OJ

> to o to

OJ fO E

OJ u

S- -r- s +->

oo E

E

a; +j

.

o —

I

^ I oo

• I I TO +-> ra E

II — O

s_ Q.UJ ' E O • o .— >,

•I- <1> s_

4-> J3 »-> 3 0) r- -O E

o c o

to (TJ to

77
400 440 480 520 560 600

WAVELENGTH , nm

Curve A represents the values of a obtained at -100 mV (SHE) for

{221} Fe in borate solution. Curve C shows the changes in the

spectrum that occur when chloride is added to the solution. The

star indicates that the spectra are not similar at 590 nm. (From [5]). 7p
o

0)

a

•r—S- +J

(UEo

toQ.OO

(0u

M-ucO -r- x: -oE W O>

UJ

m

O)o> •>- cuS- 4-> CD

(U •-

If- O) O N

•I— r— S0) n} -a r- o o 2 a.

79
lA • 0) O T3

+-> fOE

-a

(D O)

00U r1/1 Z3

•I- o

-a

a

VI +-> fO c

1— QJ cn E

-a

Q- Sro OJ a +j

ro O)E c o O to I— Q.

»f- -r- CU rI— rOl <U JZ <ii M iCU O S U Q}

•IU

•M -r- ra > E (1) OJ X_

.z: o a

80
(UE <

+->

i~ o O) +J CD

(U

O E

03 • t. M- I—

OJ

O

O Cl 03

S- o

<u +J

M o O) u • E C r— o Z3

(/)<+- c

Q. •r- •r— 03 OJ

CO

a

'cu 03 > c »« (U o r— J03 O co -(-> O

•r— +J o

03c O !-> O) Q.TCD 2

>> c X O r- o •1- <U

to <U

a o *-> S- t/J c S- o O «/1

+-> 3 U to

CO ^ c O -M ..- OJ o +J

ItC CO O 03

OJ M- -O J-

«4-

C

O

UJ «o

81

00
E

(U (/I sz to I— LU -oca: c l/T M > •> to E ^ CM O C PO +

S- 4-> < O fO

tO "O

i~ 0) 0)

OJ E -l-> +J T- (O

QJ +J ->-> E I/) (O o o

S- I r- 03 C7) +-> D. O C

<U 4-

4->

O

O)E c o o

4-> o

CL

to

•rO

r—C

r— 13 OJ 4-

C

CD

ra

U e O. o

to

<U fd • 

o I—

•r- -M

O > e 03

OJ O) rr

s_ s- L> iZ3

O fO o • c o o c

4-> o

cu

toO r—

L. (U O

82

cn

cn
o 100 200 300

time, msec

E

c3

E

/

X

Rr

M 1

1

•0

100 200 300

tiiTM, ms«c

E

c\

3

E

1-

0*1

10 20

J-o

30

t ime,mse,c

Fig. 10 Change in thickness, x, and repassivation ratio, R , after removal

of abrasion wheel of triboel 1 ipsometric apparatus ?or low carbon

steel in (a) UafiO^ at 90C at open circuit potential -288 mV, in

(b) IN NaN02 25C at open circuit potential -180 mV, and in (c)

IN NaCl at 25C at open circuit potential -298 mV. (From [14]).

83
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. ]. miHI.K A TION OK RI-'.I'OR T NO. BIBLIOGRAPHIC DATA

SHEET NBSIR 75-916

2. Gov 't Actf.ssioll No.3. Rcc ipicnl *s Ac cession No.

4. TITLH AND SUIHTITLlv

1 MC rxU Ic Ul rdobiVC rillll Ui UW LM iNlllCLICo uMU riupClLICO III Ol<lv-b Corrosion and Crevice Corrosion Susceptibility

5. Publication Date

s October 1975

6. Performing Organization ( ode

7. AUTHOR(S)

J. Kruger, J. R. Ambrose, and T. Kodama

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

S'. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3120448

1 1. *--ontraci/ Cirant No. NAONR 18-59, NR036-082

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

Office of Naval Research

Department of the Navy

Arlington, VA 22217 !

1^ It7r\^ r\r ti It r\f\r t fir 1-^^ r i niA 1 lyjjc (ji ixcpori ot i t.rioci ^""'"''^ Interim

9/74-1 0/75

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT {A 200-word or less factual summary of most si^ificant information. If document includes a significant

bibliography or literature survey, mention it here.) This report consists of four parts as follows: Part I - A study of the influence of

molybdate ions on the repassivation kinetics of iron in chloride solutions. Results

indicate that molybdate ions precipitated as ferrous molybdate in pits inhibit' propagation but not initiation. Part II - Using a technique that allows el 1 ipsometric

studies to be made in a crevice, this work has found significantly lower film growth

rates in a crevice on iron as compared to the rates observed outside the crevice.

Part III - A review of the current state of our understanding of chemical breakdown of

passivity is given with a listing of critical questions that remain to be resolved. A

short bibliography on the subject is included. Part IV - A review of the qualitative

applications of ellipsometry to study localized corrosion that have been developed

at NBS is given.

. % ... „

'

t

.t, .5 •

.

' 1

f

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)

Chemical breakdown of passivity; Chloride^ Crevice corrosion; Ellipsometry; Iron;

Molybdenum; Repassivation kinetics; Stress corrosion

18. AVAILABILITY , QT Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO, OF PAGES

1 For Official Distribution. Do Not Release to NTIS

UNCLASSIFIED 85

1

' Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402. SD Cat. No. CI3

20. SECURITY CLASS

(THIS PAGE)

22. Price

1 Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151 UNCLASSIFIED $ 5.00

USCOMM-DC 29042-P74

ll