Determination of critical pitting potentials of stainless steels in aqueous chloride environments

Determination of critical pitting potentials of stainless steels in aqueous chloride environments

El~~tmchimics Acta. 1971, Vol. 16. pp. 1987 to 2003. Pergamon Press. Printed in Northern Irehand DETERMINATION OF STAINLESS OF CRITICAL PITTING POT...

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El~~tmchimics

Acta. 1971, Vol. 16. pp. 1987 to 2003. Pergamon Press. Printed in Northern Irehand

DETERMINATION OF STAINLESS

OF CRITICAL PITTING POTENTIALS STEELS IN AQUEOUS CHLORIDE ENVIRONMENTS*-f-

N. P-ALL and C. Lru Westinghouse Research and Development Center, Pittsburgh, Pennsylvania, U.S.A. Ahatrac-A new method for determinin g the critical pitting potentials, EC, of stainless steels in aqueous chloride environments has been developed, The technique has the advantage of deGning a discrete value of EC which is dependent only on the composition and structure of the metal and the test environment. The irreproducibihty of EC measurements frequently discussed in corrosion literature is readily explained on the basis of these new measurements. Experimental date are presented for Fe-Cr and F&Cr-Mo alloys in a hot sea-water environment. These data suggest that the passivity of the alloys is attributable to an adsorbed layer on the metal surf&e, and that further film growth merely influences the induction time for pit nucleation. R&sum&Mise au point d’une nouvelle m&hode en vue de determiner le potentiel critique d’attaque EC des aciers inoxydables baignant dam une solution aqueuse de chlorure. Cette technique p&e&e l’avantage de d&nir une vaieur distincte de EC, que d6pend seulement de la composition et de la structure du metal ainsi que de I’environnement de l’essai. La non-reproductibilito des valeurs de EC f&quemment discut6e dans la bibliographie relative a la corrosion est ais6ment expliquee B l’aide de ces mesures nouvelles. Des don&es exp&iment&s sont p&se&es pour les aIliages Fe-43 et F&Cr-Mo, dans un bain chaud d’eau de mer. E1le.ssuggerent que la passivite des alhages est attribuable B une couche adsorb& sur la surface motallique et que par con&quent la croissance de fIilm influence simplement le temps d’induction relatif 21la nuclbtion des creux. Zusantmenfat~~--Es wurde eine neue Methode zur Bestimmung des kritischen Potentials ,??.,fiir Lochfrass an rostfreien Stlhlen in w&r&en Chloriden entwickelt. Der Vorteil der Teehnik liegt darin, einen diskreten Wet-t fiir & bestimmen zu kbnnen, der nnr von der Zusamm ensetzung und der Struktur des Metalls und des Versuchsmediums abh%ngt. Aufgrund dieser neuen Messungen wird die in der Literatur oft diskutierte Nichtreprodtuierbarkeit der Messungen von E,-Werten e&l&t. Die Daten aus Versuchen mit Fe-42 und FeCr-Mo in heissem Meerwasser weisen darauf bin, dass die Passivitlt der Legierungen einer adsorbierten Schicht zuzuschreiben ist, und dass weitere Schichtwachstum die Induktionszeit fiir das Einsetzen des Lochfrasses beeinfiusst. INTRODUCTION THE PREVBNTION of pit nucleation and propagation are the major objectives in any attempt to improve the corrosion resistance of stainless steels for use in aqueous chloride environments. The test methods used to evaluate susceptibility to pitting corrosion are not designed to measure corrosion rate, as would be done in studies of general corrosion attack, but instead seek to determine the conditions under which pitting corrosion may or may not occur. Many stainless steels, for example, can be perforated by pitting attack in aqueous chloride environments even though the general corrosion rate is extremely low. The quickest and most frequently used methods for screening the pitting resistance of susceptible alloys are those based on polarization techniques. In particular, the degree of resistance to pitting corrosion is frequently inferred from measurements of critical pitting potentials E,, since, for a given metal/environment system, the aggressive action of anions stimulating the formation of pits is observed only at potentials more positive than a certain critical value .l It would be expected therefore that experimentally determined values of ,??, should meet the following requirements :a a small shift of the potential towards a more positive value than E, should allow * Presented at the 3rd International Conference on Passivity of Metals, Cambridge, England, July 1970; manuscript received 19 August 1970. t The work was supported by the U.S. Department of Interior-Office of Saline Water. 1987

N. PES~ALL and C. Lru

1988

nucleation and growth of pits ; a small shift of potential in the negative direction should cause repassivation of the pits and inhibition of the corrosion process. It is clearly evident that accurate and reproducible determinations of A?&values should contribute significantly both towards an understanding of the nature of the pitting process and towards the search for effective methods of eliminating pitting. Several studies have been reported on the critical pitting potentials of iron-base alloys in halide solutions .14 The experimental conditions and procedures which were used to obtain the & values have varied widely and many discrepancies have arisen. Even single crystals exhibit considerable scatter.a Schwenk3 has shown, however, that the most reliable values of pitting potentials may be obtained using a potentiostatic technique in which a constant potential is applied for a given time and in which a constant surface condition is used for each sample. This conclusion was based on the observation of the pitting induction-time/potential relationship replotted in Fig. 1, and the dependence of such a relationship on sample surface condition.

Potential, FIG.

mV (she)

1. Induction-timelpotentialrelationship for a Fe+17.6 Cr-10.3 Ni-O.35 Si-O.07 C steel, in 1 M NaCl at 25°C (after SchwedP).

The necessity for stipulating induction time and surface condition when comparing E,, values is obviously undesirable and it is the purpose of this paper to present a potentiostatic method of determining unique E, values of stainless steels that are dependent only on the composition and structure of the metal and the test environment. The modified technique not only helps to account for the irreproducibility of E, measurements frequently discussed in corrosion literature, but also provides further insight into the mechanism of passivity.

Criticalpitting potentialsof stainlesssteelsin aqueous chlorideenvironments EXPERIMENTAL

1989

TECHNIQUE

Alloy preparation All alloys were levitation-melted in an argon atmosphere and cast into slab molds l/4 in. x 1 in. x l-1/2 in. The sample analyses are given in Table 1. The surfaces of the cast slabs were ground to remove imperfections and provide good rolling surfaces before homogenizing 16 h at 1200°C in an argon atmosphere. The TABLE 1. ALLOY

Identity

Nominal composition at-*%

F233 F230 F231 F232 F389 F390 F294 F234 F315 F307

COMPOSITIONS

Fe25 Cr Fe-25 Cr-1 MO Fe-25 Cr-2 MO Fe-25 Cr-3 MO F-12.8 Cr Fe-16 Cr Fe-21.2 Cr Fe-30 Cr Fe-31.6 Cr Fe-41.8 Cr

wt- 0% Fe-23.7 Cr

Fe-23-5 Cr-1.74 MO Fe-23.4 Cr-3.45 MO Fe-23.2 Cr-5.13 MO Fe12 Cr Fe-15 Cr Fe-20 Cr Fe-28-5 Cr Fe-30 Cr Fe-40 Cr

Analysed composition of samples wt-0% Fe-23.5 Cr-O+0094C Fe-23.2 Cr-1-75 Moo.012 C Fe-23.4 0-3.5 Mo-O.019 C Fe-23.2 Cr-5-07 Mo-O.011 C Fe1 1.8 Cra.006 C Fe-l 4.6 Cr-O-0039 C B-20.5 Cr-O-0065 C Fe-28.4 Cr-O.OO35C Fe-29.6 Cr-O-0063 C F-39-5 Cr-OGO95C

slabs were hot-rolled at 1090°C to l/16 in. thickness. Oxide scale was subsequently removed by sand-blasting before a final heat treatment was made at 1050°C for 1 h in argon followed by quenching into water. Disks were spark-cut from the resulting strip for polarization measurements. All alloys exhibited an equiaxed recrystallized grain structure. Polarizationand E, measurements The present data were obtained using a modified version of the cell described by Greene.’ One modification allowed accurate and reproducible location of the flat circular samples relative to a platinum counter-electrode of similar shape and dimensions as the sample. By passing a continuous stream of high purity nitrogen* through a fritted glass bubbler located symmetrically beneath the sample and counterelectrode, it was possible to de-aerate the solution continuously, to prevent corrosionproduct accumulation in the vicinity of the specimen surface, and to prevent bubbles adhering to the specimen surface. The most important modification, however, was the provision which was made for controlled scratching of the specimen surfaces during polarization. This was achieved using a silicon-carbide crystal embedded in the end of a tapered glass capillary tube, which could be drawn across the specimen surface by manual operation through one of the polarization cell ports. The purpose of the capillary tubing was to provide a facility for studying the effects of blowing gases, such as nitrogen, directly into pits as they formed on the specimen surface.E In order to avoid both solution contamination and crevice effects, which result in irreproducible polarization curves, particular care was taken in designing the specimen holder. Initial attempts to use epoxy-mounted specimens were found to be unreliable. Figure 2 illustrates a specimen-holder design that has proved highly * High purity nitrogen,pre-purifiedgrade, suppliedby Matheson Co., Inc.

1990

N.

hSSALL

and C. LIU

Teflon

g”Tket

, Sample

disk

,

Palycarbona+3

Electrical

connexion4-

to potentiostat

Stainless

steel

/../

_-

Teflon 0-Rmg

Polycorbonale

FIG. 2. Specimen holder used for polarization measurements.

successful in the present work. The holder has the advantage of allowing independent sealing of the specimen and the sample holder itself, as well as accommodating specimens with non-parallel surfaces. In use, the specimen holder exposes only the sample, polycarbonate and Teflon to the electrolyte. All measurements were made in high temperature synthetic sea water which was prepared by dissolving sufficient “Sea Salt”* in distilled water to approximate a normal concentration (3.5 %) sea-water solution. Before using the sea-water in the polarization experiments, the solution was acidified with sulphuric acid to a pH of approximately 4-O and subsequently boiled to remove free carbon dioxide and Measurements were made in the de-aerated sea water at pH dissolved oxygen. 7-3 & 0.2 and 90 f O*l”C. The samples, which were in the form of flat disks (O-495 in. dia.), were mechanically polished to either a 3/O emery or 1 ,um diamond finish and then cleaned in alcohol and dried before mounting in the sample holder, exposing a surface area of 1 cma. The entire sample f%cture was rinsed ultrasonically in distilled water and quickly inserted into the polarization cell at 90°C. Before anodically polarizing the samples at a constant rate of 10 mV/min, they were allowed sticient time (ca 30 min) to reach an equilibrium rest potential. The voltage/current relationships were automatically recorded on an X-Y recorder_ At selected values of potential, usually 5-50 mV intervals, the potential was maintained constant with the potentiostat and the specimen surface gently scratched with the silicon-carbide crystal. At each of these fixed potentials, the current/time relationships were automatically recorded until the specimen repassivated. This procedure was continued until the repassivation time at a given potential exceeded about 1 h. The new experimental technique is based on the idealized relationships, presented in Fig. 3. The latter illustrates how the potential dependences of repassivation time and induction time might be expected to define a single value of E, for a given alloyenvironment combination. * “Sea Salt” is a product of Lake Products Co., Inc., St. Louis, Missouri; its formula, based on ASTM-l 141-52, is 58-49 Y0 NaCl, 2646 % MgCl, . 6Ht0, 9-75 % Na,SO,, 2-765 % CaCl,, l-645 Y0 KCl, 0.477 % NaHCOs, 0,238 % KBr, O-071 % H*BO,, O-095 0A SrCl, .6H,O, O-007 % NaF. The synthetic sea-water uses 41-953 g/l of the salt.

Critical pitting potentials of stainless steels in aqueous chloride

environments

1991

The induction time for pitting at a constant potential, for example, would be expected to be increased by smooth, homogeneous surfaces and rapid stirring of the electrolyte. In contrast, if the protective film on a passive stainless steel is removed at a potential below E, (as by scratching, for example), one would expect repassivation time/potential relationships of the type illustrated in Fig. 3. As the amount of “protective film” which is removed is decreased, the repassivation time at constant potential would be expected to decrease. The hypothetical potential designated & in Fig. 3 is the potential below which the passive state becomes unstable and general corrosion takes place.

General corrosion

pitting

Smwther surface Greater stirring

FIG.

3. Hypothetical repassivation-time/potential and induction-time/potential relationships for a passive metal in an aqueous chloride environment.

In this paper, experimental results are reported which exhibit the general relationships shown in Fig. 3 and which allow comparisons to be made between the commonly used potentiokinetic methods of determining E, values and the new “scratch method”. RESULTS Several specimens of FeCr and Fe-Cr-Mo alloys have been evaluated by means of anodic polarization measurements, using both the conventional potentiokinetic scanning technique and the modified method in which the specimen surface is periodically scratched. Representative data for the Fe-Cr-Mo alloys are presented in Figs. 4-8. In Fig. 4, the repassivation-time/potential relationships for samples of Fe-25Cr2Mo are presented. The numbers associated with each experimental point represent the maximum current @A) obtained during the scratching process at any given potential. It is evident that E, is defined most readily using very light scratches.

N. PESSALLand C. LIU

1992

lo* Repassivation

1

Pitting

ID3

; lo2 i=

ICI

:.E

1 c 1 L 0

M

!

i

1

t

60

90

120

150

Potential,

,I,,

I

180

210

xl

270

300

3M

3Kl

mV (see)

FIG. 4. Repassivation-time/potential induction-time/potential relationships for Fe-25 Cr-2 MO (at-%) in de-aerated synthetic sea-water at 90°C.

As the severity of the scratch increases, as indicated by the larger currents, the repassivation time at a given potential increases. It is noticeable that the scatter of the data also increases with heavy scratching. The potential range a-b in Fig. 4 represents the scatter of four pitting potentials obtained using samples with a 3/O emery surface finish and employing a conventional scanning technique at a polarization rate of 10 mV/min. Point c was obtained by holding the potential of a specimen with a 3/O surface finish at +250 mV until pitting occurred after approximately 1000 s. Figures 5, 6 and 7 illustrate similar relationships for Fe-25Cr, Fe-25Cr-1Mo and Fe-25Cr-3Mo alloys, respectively. It is interesting to note that for the binary alloy, the well-defined value of 1F,, obtained using very light scratches, is significantly lower (ca 100 mV) than that obtained using the conventional scanning technique (a-b). In contrast, Fig. 6 shows that with the addition of 1 at-% molybdenum, the value of E, is closely approximated by the conventional scanning method. The corresponding data for the alloy Ft+25Cr-3Mo, Fig. 7, continue this trend but also illustrate the marked dependence of induction-time/potential relationship on the condition of the surface. In order to show the independence of the scratch technique on surface finish, as distinct from induction-time measurements, the scratch test was used on samples

Critical pitting potentials of stainless steels in aqueous chloride environments

104

1993

1

I I

Repassivation

Pitting

IO3

u) 2 g 10 ‘F

3Al Surface Finish ‘\ 10

\ r-\--a a

‘,”

t

I Potential,

mV (see)

FIG. 5. Repassivation-time/potential induction-time/potential relationships for Fe-25 Cr-1 MO (at- %) in de-aerated synthetic sea-water at 90°C.

of the Fe-25Cr-3Mo alloy with three different surface finishes ; 1 pm diamond, 3/O emery and etched. * The results, Fig. 8, clearly indicate that the value of E, obtained by the new method is essentially independent of these surface finishes. Figure 8 also shows that the value of E,, under the present experimental conditions, is not affected by purging the sea water with oxygen rather than with nitrogen. A summary of the experimental data for Fe-Cr-Mo alloys is presented in Fig. 9. The pitting potentials obtained using the potentiokinetic method are illustrated by the broken curves. The large amount of scatter obtained using samples with a 1 pm diamond surface finish and the wide discrepancy between pitting potentials obtained using identical scanning procedures with different surface finishes are clearly illustrated. Samples which were etched before continuous-scan polarization measurements exhibited pitting potentials within the scatter of the data for samples with a 3/O surface finish, as shown in Fig. 9. In contrast, the values of EC obtained using the scratch method were essentially independent of surface finish and reproducibly lower than the values obtained using the potentiokinetic method. Although qualitatively similar, the increasing deviation * 15 o/0HNO.,

5 y0 HF by volume at 80°C for 5 min.

N. PESSALLand C. Lru

lW4 104

Repassivalion

Pitting

IO3

m pi lo2 E ‘F

310 Surface Finish 10

‘E I

1

,

I

I

DO -70

-40

-10

,

20

!

I

\

xl

80

Potential.

110

I:

140

c /

170

:

m

,

230

!

260

mV (see)

FIG. 6. Repassivation-time/potential and induction-time/potential relationships for Fe-25 Cr-1 MO (at- %) in de-aerated synthetic sea water at 90°C.

between the curves b and c in Fig. 9 observed as the molybdenum content is reduced clearly demonstrates the importance of determining equilibrium A??,values in screening new alloy systems for corrosion resistance. In order to investigate further the large divergence between the E, values for the Fe-25Cr alloy obtained using the scratch method and using the potentiokinetic technique, a series of Fe-Cr alloys, ranging in composition from 12-40 wt.- % Cr, was also investigated. The experimental results, Fig. 10, clearly indicate that the Fe-Cr and Fe-Cr-Mo alloy systems exhibit distinctly, different relationships between the scratch-test and potentiokinetic E, values. For the ternary alloys, the two different experimental methods indicate only slight differences in E,, values but for the binary Fe-Cr alloys, the scratch method consistently indicates significantly lower E, values. Critical pitting potentials can also be determined galvanostatically, since under galvanostatic conditions pitting corrosion sometimes occurs at a stationary anodic potential corresponding to the value of E,. s3 However, in many cases thegalvanostatic method is an inferior means of characterizing alloys because periodic oscillations of the potential prevent unambiguous determinations of E,. In the present work it was felt worth while to make a few galvanostatic measurements in order to correlate them with the E, values defined by the scratch method. The results of such measurements are presented in Figs. 1 I and 12.

Critical pitting potentials of stainless steels in aqueous chloride environments

0.75/A

Repassivation

Pitting

\ 4m dibmo+

I I

1

&6d I ;

1995

\

:I

Heavy Scratches

‘7; I’

;I I ’ I 1 I : I I ’ II :

1;

1 !I

:I I; ,.,.:/ II I

1,

56

/

/ ’

;I t\

3/o Surface Finish

T-tg

1’ 60,’

!l

\ i \ ;

tight Scratches

~5 11

’ ’ 260 290 320



3%)

1

380 410 Potentiaf,

I

I

440 470 mV (scel

I

I

I

I

I

500

530

560

590

6ZU

FIG. 7. Repassivation-time/potential and induction-time/potential relationships for Fe-25 O-3 MO (at- %) in de-aerated synthetic sea water at 90°C.

Figure 11 illustrates the potential/time relationship for an Fe-25 at- % Cr alloy at four current levels. It is evident that E, determined by the scratch method is in excellent agreement with the galvanostatic measurements at 10 and 30 mA. In contrast, Eb determined by the conventional potentiokinetic method is distinctly high. Similar data in Fig. 12 confirm the excellent correlation between the scratch method and the galvanostatic method. However, apart from the time-consuming nature of galvanostatic measurement, Figs. 11 and 12 demonstrate the difhculties of interpretation that can arise when using the galvanostatic method, as a result of the wide oscillations of potential. The scratch test, which defines E, to within a few mV is obviously a more useful technique in evaluating the relative pitting resistances of a wide range of alloys. The scratch test has also been used to investigate the protective potentials, Ep, discussed by Pourbaix and co-workers .g They observed that repassivation occurred at a particular potential, ED, which was generally several hundred mV less than E,, if the direction of polarization was reversed, after pitting a specimen by means of anodic polarization. On the basis of Fig. 3, however, it would be expected that under equilibrium conditions, Ep should be identical to the value of E,. Such an interpretation has been confirmed by the results of three consecutive anodic polarization measurements on an Fe-25 at-% Cr alloy, as shown in Fig. 13.

1996

N.

and C. Llu

PIWALL

l& Repassivation

1.8b



5.d

:

Pitting

2.8 9 2-50

10:’

;i I! I’

’ I ’ I

2.3 d “l 1oi d .E

,

’ ; ; I I

: 1cI

I

/’

; 1 Fc,4* a ,h

I

I

I’

I 3.50

,I I’

0.50

/ 1’ 1

1.5,0’5.t, ,0.5> \, ,

I ml ml 320 350

3/O Surface finish Continuous 5can

‘I II

1 I :E I c I

*

/

i

380 410 440 470 Poteniial, mV (see)

5M)

5%

1 560

0 5w

FIG. 8. Repassivation-time/potential and induction-time/potential relationships for Fe25 Cr-3 MO (at- “/,) in de-aerated synthetic sea water at 90°C.

The sample was l&t polarized at 10 mV/min to a potential of - 130 mV and then the current/time relationships recorded as the specimen surface was gently scratched. The current ranges observed after scratching at 10 mV intervals are shown in curve 1 of Fig. 13, with no repassivation observed after 3130 s at -90 mV. Curve 2 represents a subsequent potentiokinetic polarization at 10 mV/min that was allowed to proceed until breakdown occurred without scratching at + 120 mV. However, as soon as the pitting current reached ca 600 PA the potential was rapidly reduced by manual control to -100 mV. As shown in Fig. 13, the pit repassivated in 41 s at - 100 mV. The continuous scan was then repeated, curve 3, and the potential rapidly reduced to -95 mV as soon as a pit had been nucleated. In this case the sample failed to repassivate after 3180 s at -95 mV. Curves 2 and 3 thus indicate that EP lies between -100 and -95 mV. Since the scratch test defined E, between -100 and -90 mV, these experiments suggest that Ep and E, are, in fact, interchangeable, if changes of solution concentrations are prevented from developing in the pits. This is achieved by rapid stirring of the solution. DISCUSSION

The experimental observations reported in this paper provide strong evidence for the concept that a given metal/environment system can be characterized by a

Critical pitting potentials of stainless steels in aqueous chloride environments

1997

700 ,

400 3 -s >m E

Etchedor3/O

(scratch

test1

.foce

ftnlsh

D Etched Surface

-mLl

1

FIG.

3

2

MO Addition

to Fe - 25 Cr.at4

9. Critical pitting potentials of Fe-Cr-MO alloys in de-aerated synthetic sea water at 90°C.

(scratch

10 FIG.

15

20

25 Cr addition

N to Fe, wtt3

35

test)

40

45

10. Critical pitting potentials of Fe-Cr alloys in de-aerated synthetic sea water at 90°C.

unique critical potential, E,, above which pits will nucleate and below which pits will not nucleate. The most attractive method of determining accurate E, values would appear to consist of rapid anodic polarization during which extremely small areas of the specimen surface are removed at frequent intervals (scratch experiments). At the

N. PESSALLand C. LIU

1998

Fe-25 at- % cr

,-<(continuous _... ;. ._

.__

scan)

___-.--

__~_

-200

10

15 Time,

FIG.

25

min

11. Galvanostatic measurements on Fe-25 Cr (at- %) in de-aerated synthetic sea water at 90°C. I pm diamond surface finish.

Fe-ZSCr-3Mo(at-%)

Fe-40 Wt - 96 Cr

5

10

Time,

min

15

20

25

FIG. 12. Galvanostatic measurements on Fe-40 Cr (wt-%) (1 pm diamond surface finish) and Fe-25 Cr-3 MO (at-%) (3/O emery surface finish) in de-aerated synthetic sea water at 90°C.

potential E,, the surface will fail to repassivate and an abrupt increase in current will closely define the required potential. This technique contrasts with conventional scanning methods for determining E, which are dependent upon the polarization rate and surface treatment.

Critical pitting potentials of stainless steels in aqueous chloride environments 10+

i-

I

1

I

I

1999

I

NR-3180 a

NR-:

-90 mV)

Scrathes \

a g lo-( I % u

r

y:

ro

0

/ t =347min t=35min

lo-’

I -500

p304min

! -MO

!

,

-100 0 Potential, mV (see)

, 1M)

3k

so0

FIG. 13. Consecutive anodic polarization measurements on Fe-25 Cr (at-%) in de aerated synthetic sea water at 90°C. It is interesting to consider the data obtained in the present work from the point of view of current concepts of the mechanism of passivation. The main points that must be explained are the following. 1. For a given metal/environment system, a unique potential, E,,, exists that separates pitting tendency from repassivation tendency. 2. The induction time for pitting is strongly dependent on surface finish and potential. 3. Repassivation time of an active site depends upon the geometry of the site, and the potential. 4. Under the conditions of the present work, E, obtained by the scratch method is less noble than corresponding values obtained by the potentiokinetic method. In particular, these differences E,, are quite large for binary Fe-Cr alloys (ca 100 mV) but small for ternary F*Cr-Mo alloys (t20 mv). 5. Transient current spikes occur during potentiokinetic anodic polarization at potentials both higher and lower than E,, 6. When repassivation occurs after scratching the current/time curve is often characterized by an abrupt drop followed by a slow, near-exponential current decay.

2ooo

N.

PESSALL

and C. LIU

The above observations can be rationalized by invoking aspects of several proposed mechanisms of pit nucleation and passivation.**l”-la For example, the data are consistent with the concept that i?, represents the potential at which the competitive adsorption between “protective ions” such as the 0% of water molecules, and “aggressive ions” such as Cl- is in equilibrium. Equation (1) has been used by Kolotyrkin I1 to express such a reversible depassivation process,

+ x Cl- + M(Cl),-”

+ llz H20.

(1)

m The direction in which the above reaction proceeds will be dependent upon both the Cl- concentration and the metal potential. However, at a constant Cl- level, the reaction will proceed to the right at E > E,, with metal dissolution; at E < E, the reaction will proceed to the left with metal passivation ; at E = E, the reaction will be at equilibrium. The competitive adsorption mechanism can also be invoked to explain the difficulties encountered in making precise E, determinations of passive stainless steels. Thus, at potentials above E,, with the protective film intact, a finite time (induction time) is required for sufficient aggressive anions, presumably Cl- ions, to accumulate at a given point on the metal surface to cause permanent film rupture by a mechanism similar to (1). Hoar and Jacobla suggest that this process involves 34 Cl- ions which jointly adsorb on the oxide film surface around a lattice cation with the resultant formation of a transitional complex of high energy and low probability of formation. Once formed, the complex separates from the film and the cation goes into solution; at increasing potentials more noble than B,, the rate at which the accumulation of Clions can occur is accelerated and the induction time for pitting correspondingly reduced. The effect of surface finish on induction time to pitting can also be explained by the presence of fewer “weak points” on smooth surfaces at which the critical concentration of Cl- ions can be attained. The induction time is correspondingly longer for the 1 ,um diamond surface finish, so that the potentiokinetic polarization method tends to give anomolously high values of E, for samples with very smooth surface finishes. If we now consider the scratch method, in which the protective film is broken at potentials less than E,, it is reasonable to assume that the repassivation process will be hindered, but not stopped, by the presence of Cl- ions at the bare metal surface. However, the higher the specimen potential, or the greater the area of bare metal exposed, the greater will be the affinity of the Cl- ions for the metal and the greater will be the time needed to accomplish repassivation, One of the notable observations of this work is the large difference between E, values for Fe-Cr alloys obtained with the scratch method and with the potentiokinetic method. Contrasting with this are the corresponding small differences observed for Fe-Cr-Mo alloys. These observations clearly reflect large induction times for

Critical pitting potentials of stainless steels in aqueous chIoride environments

pitting in Fe-Cr possible

alloys but very short induction

explanations

for such phenomena

times for FeCr-Mo

During sometimes

alloys.

Two

are:

(i) Thinner oxide films exist on Fe-Q-MO alloys than on Fe-Cr (ii) Narrower pores or faults occur in the oxide Hms on Fe-Cr

FeCr-Mo

2001

alloys. alloys

than

on

alloys. potentiokinetic observed.

anodic

In the particular 10-

polarization

experiments, current spikes are in Fig. 14, a current transient

case illustrated

-5 Pitting

Transient CE>E,l

I N 5 g

lo-

-6

1% (scratch 13

FIG.

71 40 3

14: Potentiokinetic

I -MO anodic

II I -MO -100 Potential. mV (see) pohrization

I 0

curve

syntheticsea water

at

test) I 1W

for Fe-25 Cr 90°C.

i

I)

(at-%) in de-aerated

was observed at a potential above E, (defined by scratch test). Transient current spikes are usually attributed to local pitting accompanied by rapid repassivation.la A possible explanation for such rapid repassivation at E > E, is that the fault in the oxide layer is small enough to prevent suBcient Cl- accumulation for pit propagation_ In this discussion, it has been assumed that metal passivity is attributable solely to an adsorbed film, while the presence of a coherent oxide on top of the adsorbed layer merely serves to slow down the rate at which aggressive ions can reach the 12

N. F'ESSALL and C. LIU

2002

adsorbed layer and cause breakdown of passivity. FrankenthaP4-ls has made a similar suggestion. While this interpretation is not the only one possible, it is consistent with the observed manner in which specimens tend to repassivate after the surface has been activated by scratching. Figure 15 illustrates a typical current/time curve during /

Rapid Repassivati~

1

,

-.

Scratch

1o-7 o

FIG.

25

I

1 50

75

1Do

125

1; 0 Time.

175

200

225

250

275

300

s

15. Current/timerelationships for Fe-40 Cr (wt- %) after scratchingthe specimen surfaceat -30 mV (see) in de-aeratedsyntheticsea water at 90°C.

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