The initiation of crevice corrosion in stainless steels

The initiation of crevice corrosion in stainless steels

Corrosion Science, Vol. 39, No. l&11, pp. 1791-1809, 1997 0 1997 ElsevierScienceLtd Printed in Great Britain. All rights reserved 001&938X/97%17.00...

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Corrosion Science,

Vol. 39,

No. l&11, pp. 1791-1809, 1997 0 1997 ElsevierScienceLtd

Printed in Great Britain. All rights reserved 001&938X/97%17.00+0.00

Pn: soolo-938x(97poos4

THE INITIATION

OF CREVICE CORROSION STEELS

N. J. LAYCOCK,*

IN STAINLESS

J. STEWART+ and R. C. NEWMAN*

* UMIST, Corrosion and Protection Centre, PO Box 88, Manchester, M60 lQD, U.K. +Unilever Research, Port Sunlight Laboratories, Quarry Road East, Bebington, Wirral, L63 3JW, U.K. Abstract-There are currently at least four different models for the initiation of crevice corrosion on stainless steels: (1) passive dissolution leading to gradual acidification and general breakdown; (2) inclusion dissolution causing thiosulphate accumulation and assisting breakdown; (3) IR drop within the crevice forcing the metal into the active state; 4) stabilisation of metastable pitting by the occluded crevice geometry. A technique using two coupled electrodes was used to measure crevice corrosion initiation potentials and induction times for 316L Stainless Steel under open circuit conditions in 1 M NaCl with sodium hypochlorite added as an oxidant. The ability of each model to explain the results was tested and the metastable pitting model was found to be the most suitable for these conditions. If metastable pitting occurs equally both inside and outside the crevice area, and any pit within the crevice has a probability (0.. 1) of initiating crevice corrosion, then the induction time is simply the time lapse before random initiation of a pit at a favourable site within the crevice. The role of metastable pitting can also be tested using temperature as a variable: the critical crevice corrosion temperature (CCT) was measured for 904L stainless steel and shown to coincide approximately with a steep increase in the metastable pitting frequency following potential steps at constant temperatures. 0 1997 Elsevier ScienceLtd

Keywords:A. stainlesssteel,B. potentiostatic,C. crevicecorrosion. INTRODUCTION Crevice corrosion shows many similarities with pitting such as an increase in likelihood with increasing potential or chloride concentration, and the existence of critical crevice temperatures (CCT) analogous to the critical pitting temperature. The ranking of alloys in order of crevice corrosion resistance is almost the same as for pitting corrosion’** and so alloy elements, such as molybdenum, must affect both types of corrosion in the same way. Given the similarities between these two forms of localised corrosion, it is not surprising that some authors have considered pitting to be a special case of crevice corrosion,3 and others have thought corroding crevices simply to be large pits.4 Like pitting corrosion, crevice corrosion can be split into initiation and propagation stages. In this case, initiation refers to the transition from passivity to general corrosion within the crevice area and development of an aggressive local chemistry. Propagation is then concerned with the rate of metal dissolution within the crevice and the factors controlling it. A commonly accepted model for initiation of crevice corrosion in stainless steels is the passive dissolution model first described qualitatively by Crolet and Defranoux,’ and then quantitatively by Oldfield and Sutton.6’7 A similar model with a more complete treatment of ionic transport and solution equilibria was later detailed by SharIand’ and by Gartland.’ However, more recent Manuscript received 11 October 1996. 1791

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measurements of the chemistry within crevices during the induction period of crevice corrosion’s’3 have suggested that this model may not be applicable, at least not for all conditions. It may well be that this model applies to the transition from a patch of crevice corrosion to a state of unconditionally stable propagation, rather than to initiation in the proper sense. Passive dissolution model

This mechanism for crevice corrosion of stainless steels in aerated, neutral chloride solutions was described as follows by Oldfield and Sutton.6 Initially, anodic dissolution and the accompanying cathodic oxygen reduction reaction occur both outside and within the crevice area. Therefore, the original oxygen present in the crevice is used up and the crevice becomes a local anode with the passive current within the crevice balanced by oxygen reduction on the passive surface outside the crevice. Metal ions produced within the crevice are only transported slowly out of the crevice, by diffusion and migration, and hydrolysis of these ions leads to a gradual pH decrease within the crevice. Additionally, electroneutrality ensures the migration of Cl- ions into the crevice and development of an aggressive local solution, as in the local acidification of pits. l4 When the environment within the crevice reaches a critical crevice solution composition (CCS), the passive film becomes unstable and breaks down leading to general corrosion within the crevice. Microscopic observation of samples from interrupted tests7 first showed micro-pitting and then coalescence of these pits to produce eventual general corrosion. Lott and Alkire” measured crevice solution composition as a function of time for 304L stainless steel in 0.1 N NaCl and found that in the induction time before initiation of stable crevice corrosion at open circuit, the pH within the crevice was essentially the same as that of the bulk solution (5.5) and that during this time iron was dissolved preferentially from the alloy. Only many hours after general breakdown within the crevice did the local solution have a metal ion composition approaching the alloy composition. These results contradict the Oldfield mode16z7which predicts a general build up of acidity during the induction period due to Cr ion hydrolysis. Nash and Kelly” also showed that even during the early stages of propagation, the pH within crevices on 304L stainless steel was not low enough to cause general breakdown of the passive film. For both a nickel based alloy and 304L stainless steel, Sridhar and Dunn I3 found that the p H did not decrease steadily with time; rather, it decreased suddenly after the increase in current indicating the onset of stable crevice corrosion. Chloride concentration within the crevice showed a similar form of variation, increasing rapidly only after corrosion had initiated. The implication of these results is that crevice corrosion causes a change in crevice solution chemistry, in contrast with models which predict the reciprocal relationship. Thiosuphate

entrapment

Following their measurements of crevice solution chemistry, Alkire and Lott”.” developed and tested a model of crevice corrosion initiation based on dissolution of MnS inclusions within the crevice. Anodically dissolving bulk samples of MnS, they showed that the electrochemical oxidation of MnS produced both thiosulphate ions (SzOs2-) and protons, and that further dissolution of MnS also occurred to produce elemental sulphur (equations (1) and (2)). 2MnS + 3H20 ($ S20:- + 2Mn2+ + 6H+ + 8e-

(1)

Crevice corrosion in stainless steels

2H’+MnS

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e Mn2++S+H2

The model considered dissolution of evenly spaced inclusions together with passive dissolution of the steel within the crevice. Transport of ionic species was considered to be by migration and diffusion, and hydrolysis of metal ions within the crevice was ignored (consistent with earlier results which showed that there was little or no pH decrease during the induction period of crevice corrosion”). Initiation of corrosion was taken to occur when the concentrations of chloride and thiosulphate ions exceeded a critical condition for breakdown of the passive film. The model correctly predicted which conditions would or would not lead to crevice corrosion, but predicted induction times were as much as an order of magnitude longer than those measured experimentally. Brossia and Kelly” performed chemical analysis of solution extracted from crevices of 304L stainless steel during the induction period of crevice corrosion, finding that thiosulphate was not present and the dominant sulphur species was sulphide (HS-). This was also shown to be the main product of chemical MnS dissolution and the rate of such chemical dissolution was expected to be much greater than the electrochemical dissolution rate. Nevertheless, sulphide is equally capable of activating metal dissolution’7 and so the thiosulphate entrapment model could be simply modified to become the sulphide entrapment model. IR drop

The JR drop mechanism of crevice corrosion was first proposed by Pickering and Frankenthal18,19 as a result of work on both iron and stainless steel which led to the suggestion that the IR drop caused by the anodic current from the crevice and the solution resistance within the restricted crevice geometry was sufficient to force the local electrode potential from the passive region into the active region and so initiate corrosion. Within this model, the role of localised acidification in crevices is simply to enlarge the active loop of the polarisation curve for the metal in the crevice solution2’ and for iron in acid or alkaline solutions, it has been shown that the active loop in the bulk solution is already sufficient for the IR drop mechanism to operate.20 In other metal/environment systems, such as stainless steel in neutral chloride solution, localised acidification is necessary within the crevice to produce an active loop in the polarisation curve. Chloride ions are thought2’ to raise the current from the crevice area by both increasing the passive current and causing pitting, and they also slightly increase the size of the active loop. In the case of systems where local acidification is required to create a sufficient active loop in the polarisation curve, this model is open to the same criticism as the passive dissolution model, namely that significant local chemistry changes do not occur until after the initiation of crevice corrosion.21 Similarly, Sridhar and Dunn13 have shown for two different systems that the potential at the crevice tip is initially the same as the outside passive surface and does not decrease into the active region until after initiation. Metastable pitting

Measurements of crevice solution chemistry suggest that, rather than a gradual change of local chemistry causing crevice corrosion, the sudden initiation of crevice corrosion causes a sudden change in the local chemistry. Additionally, many reports7*20have included the observation of pitting within the crevice and sulphur species, present in the crevice, are well known to enhance pitting corrosion.‘7 Stockert and Boehni4 have proposed a model

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based on the idea that crevice corrosion is simply a geometrically stabilised form of pitting. Metastable pits are stabilised by the presence of a porous pit cover which provides an ohmic drop sufficient to keep the pit bottom in the active state,22’23or acts as a diffusion barrier helping to maintain a concentrated local chemistry.24 On an open surface, such pits must usually precipitate a salt film in order to survive the eventual collapse of the pit cover.22-26 However, Stockert and Boehni4 suggested that if a metastable pit formed within a crevice, then when its cover broke, the resistive barrier of the crevice geometry was sufficient to stabilise the pit. The nearby area of the crevice would then be exposed to the aggressive pit environment and general breakdown could proceed. The number of metastable pitting transients on crevice free electrodes of 302L stainless steel was measured as a function of potential and a technique was developed which enabled a crevice to be formed on the sample at any time during a given test. The number of pitting transients, AN, between crevice forming and initiation of stable crevice corrosion (i.e. during the induction time) was recorded. Comparison with data from uncreviced electrodes enabled the total number of pitting transients expected after introduction of the crevice, Npot, to be calculated also. It was found that if the crevice was formed later, so that N,,, was lower, the induction time for crevice corrosion increased, but AN remained within a relatively narrow band. The scatter in AN was taken to reflect the stochastic nature of metastable pitting with the induction time simply being the time until a pit initiated at a favourable site within the crevice. Favourable sites were presumed to be the tightest parts of the crevice near a suitable inclusion, e.g. actual contact points between the crevice walls. Suleiman et ~1.~~found that an iron oxide (rust) layer deposited on stainless steel acted as a particularly aggressive crevice, probably because of an anion selective nature aiding localized acidification. It was observed that crevice corrosion initiated in rust covered samples at the same potential as metastable pitting first occurred on rust free samples. This result was regarded as supporting evidence for the metastable pitting model of crevice corrosion described by Stockert and Boehni.4 The first part of this work was designed to obtain data on the initiation of crevice corrosion on 316L stainless steel, and to test the ability of the four possible mechanisms described above to explain these data. Having established that one mechanism was most likely to apply in this case, further experiments were carried out on the initiation of crevice corrosion in a more highly alloyed stainless steel, 904L. For this steel, it was possible to study the effect of temperature on both crevice corrosion and on metastable pitting, and to investigate the relationship between these two phenomena. EXPERIMENTAL

METHOD

All electrochemical experiments used a potentiostat made by ACM Research together with a sweep generator made by Thompson instruments. Data were recorded digitally using a 386 DX computer fitted with a Keithley DAS 8 A/O data acquisition card used as an analog to digital converter in conjunction with Keithley Easyest LX software. In coupled electrode tests, experiments were designed such that currents were below 2 PA and could be measured using a Keithley model 614 electrometer giving low background noise levels. A saturated calomel electrode (SCE) was used as the reference electrode and all potentials quoted here refer to this scale. An approximately 10 mm length of 1 mm diameter platinum wire was used as the counter electrode when needed. All test solutions were made from analytical grade chemicals and de-ionized water.

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316L stainlesssteel* To test models of crevice corrosion initiation stainless steel electrodes were coupled together in various oxidising solutions and the net current flow between them was measured, together with their coupled potential. The electrodes were coupled using a potentiostat configured as a zero resistance ammeter, although currents were measured using an electrometer (Fig. 1). Naturally aerated 1 M NaCl was used as the base solution and sodium hypochlorite (NaOCl) was added to raise the oxidising power. The NaOCl was provided as a 10 wt% solution with NaOH @H 13). To prepare a given test solution, after addition of the desired amount of NaOCl to the base NaCl solution, the pH was adjusted to 7 using a few drops of 1 M HCl. At this pH, the NaOCl is partly converted to hypochlorous acid (HOCl) and chlorine (Clz) as shown below. The best measure of oxidising power is the total available Cl2 level. Solutions were used containing 0, 8 and 20 ppm available CIZ. The duration of each test was less than 2 h and the oxidising power of the solution did not decrease significantly during this time. This was demonstrated by consecutive tests in the same solution which showed the same potential-time behaviour for the stainless steel electrodes (Fig. 2). H+ + OCl- + HOC1

(3)

HOC1 + H+ + Cl- + Clz + Hz0

(4)

AID converter

cell potential output

I

computer display

potentiostat

jvired as ZRA

electrometer

H-M I

I

Fig. 1. Experimental arrangement for 2 electrode couple tests with monitoring of the current using an electrometer. * Only nominal compositions were available for the alloys used in this investigation.

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ct al.

moo

0 Time (s)

Fig. 2. Potential time variation for 3 16L (high S) electrode couples (crevice/crevice free) in 1 M NaCl solution with 8 ppm of available chlorine as an extra oxidant. Results are shown for two consecutive tests in the same solution, each on fresh electrodes.

316L

steel was obtained as 6 mm diameter rod which was cut and machined into 50 mm long cylindrical electrodes with one rounded end. Rubber O-rings of 4 mm internal diameter were used as crevice formers when required and the contact area was estimated as a 0.5 mm wide strip around the rod. 25 mm lengths of each electrode were immersed, such that with 2 O-rings on one electrode and none on the other, the surface area within the crevices was approximately 1% of the total surface area. Some tests were carried out using two coupled electrodes without crevice formers, but most tests involved one electrode without crevices coupled to an electrode with two O-rings fitted to it. All electrodes were freshly longitudinally abraded with 240 grit Sic paper and rinsed with deionised water. The current resolution in these tests meant that only events (e.g. metastable pits) greater than 100 nA peak current could be detected. Stable crevice corrosion was indicated by a net anodic current from the creviced electrode, together with a persistent decrease in the couple potential. The crevice corrosion initiation potential, EC,,was defined as the peak potential reached prior to the persistent decrease, and the induction time, Tind, was the time from the start of the test until the onset of stable corrosion as Anodic polarisation data were also obtained for this steel in 1 M NaCl defined for EC,,,. both with and without addition of 5 mM Na2S203 at a sweep rate of 0.5 mV s-‘, using 25 mm lengths of rod, polished to 240 grit. stainless

904L stainless steel Anodic polarisation curves were obtained using 4.5 mm diameter rod electrodes of 904L with 2 crevice forming O-rings fitted to each rod, in 1 M NaCl at controlled temperatures from 0” up to 65°C + 2°C. The test solution volume was 200 cm3 and the solutions were deaerated using nitrogen gas. Electrodes were placed in the solution so that a surface area of 5 cm2 was immersed and a waterline was present on the sample. Samples were used for many

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tests, but were re-abraded with 240 grit SiC paper and rinsed with de-ionised water prior to each test. Once placed in the cell, samples were quickly stepped from their rest potential (approximately - 450 mV) to - 500 mV and then the potential was swept in the anodic direction at 0.5 mV s-i until the anodic current exceeded 100 PA cmp2. The crevice corrosion breakdown potential, Eb, was defined at a sustained anodic current density of 10 PA cme2 and three measurements of Eb were made for each experimental condition. Potentiostatic data on metastable pitting of 904L were obtained as a function of temperature in 1 M NaCl. To obtain low noise data, some 904L rods were machined until their diameter was only 1 mm and abraded with successively finer Sic paper down to a 1200 grit finish. These electrodes were rinsed with de-ionised water and used with only about 3 mm immersed in solution. After careful positioning in the cell, which was open to the air, the sample potential was stepped to the test potential and the current was recorded for the next 350 s at 9 Hz. 300 mV was chosen as the test potential for experiments at various temperatures from 2 to 3O”C, since previous work28 had shown metastable pitting rates reach a maximum at this potential. Temperatures below room temperature were reached by cooling the electrochemical cell and test solution (approximately 200 cm3) in a large beaker of ice. Once the desired temperature was reached, the cell was removed from the ice beaker and the sample was introduced. The temperature was measured using a thermometer and was found to rise by no more than 2°C during any given test (temperatures were recorded as the mid-point of this range). For all tests, the first 50 s of data following the potential step were ignored since the total current was often too high for resolution of individual pitting events. The remaining 300 s of data were analysed by plotting in 50 s intervals and counting all distinguishable pitting events (the minimum resolvable event was ~20 nA).

EXPERIMENTAL

RESULTS

AND DISCUSSION

This study tested the applicability of different possible mechanisms for the initiation of stable corrosion in relatively shallow crevices on 316L stainless steel under open circuit conditions. However, the initiation of crevice corrosion is potential dependent and, in naturally aerated solutions, the potential of stainless steel rises only slowly so that induction times for crevice corrosion may be very long (days/weeks). Addition of extra oxidant to the solution increases the rate of potential rise without drastically affecting the general form of the potential behaviour, and induction times can be significantly reduced. The initiation potential for crevice corrosion of 316L stainless steel in 1 M NaCl was measured using a coupled electrode technique and with the potential raised by the addition of sodium hypochlorite (NaOCl) to the test solution. The oxidising power of these solutions is described by the available chlorine content, but at the neutral pH of these tests, the active oxidising agent is hypochlorous acid, HOCl. HOC1 + 2e- + H+ + Hz0 + Cl-

(5)

Figure 3 shows E vs t data for different oxidant concentrations and it can be seen that E crev was a more consistent parameter than iind. That is, by decreasing the oxidant concentration, rind was lengthened, but EC,, was distributed over a relatively narrow range (Fig. 4). This observation is inconsistent with any initiation mechanism which proposes a gradual, time dependent development of critical conditions within the crevice cavity.

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.l

iz 5:

20 ppm Cl 00.

: > g -0.1 2 g 5 P -0.2

0 ppm Cl

-.3 0

500

1000

2000

1500

2500

Time (s) Fig. 3.

E vs I plots for two electrode couples (one with 2 crevice formers and one without) stainless steel in 1 M NaCl with various available chlorine levels.

of 3 16L

6 [IT8 ppm Cl n 12 ppm Cl 5

El 20 ppm Cl

4

I

0 -60

-70

SO

-50

4

L

-30

-20

-10

0

10

E (mV vs SCE) Fig. 4. The distribution of crevice corrosion initiation potentials (I?,,,) for three different oxidant concentrations. A nominal value of X mV includes all measured values within the range Xi 5 mV.

Passive dissolution model The passive current is essentially independent of potential and therefore the gradual acidification proposed by Oldfield and Sutton6” should depend only on time and not on potential. In fact, as shown by Figs 3 and 4, crevice corrosion initiation appears to depend

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more on potential than time. E vs t data for crevice free, coupled electrodes in 1 M NaCl, 8 ppm available Cl solution were digitally recorded and smoothed to remove fluctuations. Using a D/A converter connected to the potentiostat, this potential programme was then applied to single electrodes in de-aerated 1 M NaCl solution and the passive current monitored (Fig. 5). The results were similar for electrodes with and without crevice formers, with steady state passive currents of approximately 2-3 uA cmP2 and, up until the onset of unstable pitting or crevice corrosion at about 1200 s, there is no indication that an increased anodic current is coming from the crevice area. The crevices used in this work had a contact diameter of 0.5 mm, which gave a maximum crevice diffusion length of 0.25 mm. In a conducting solution such as 1 M NaCl, the effects of migration on ionic transport are negligible and current from the crevice is carried by diffusion of metal ions to the bulk electroiyte.29 A one-dimensional finite difference model (Appendix A) was used to calculate the concentration of metal ions at the crevice centre as a function of time assuming that the passive current, ipass,was uniform throughout the crevice but varied with time as in Fig. 5. Figure 6 shows the results of these calculations for the metal ion concentration at the crevice centre, with assumed crevice heights of 1 and 10 urn. Importantly, the peak metal ion concentration was reached after about 40 s compared to crevice corrosion induction times for these conditions of approximately 1500 s. The maximum metal ion concentration was -0.54 M for a crevice height of 1 urn, which corresponds to 0.1 M Cr3+ assuming stoichiometric dissolution. The pH of the crevice solution is controlled by hydrolysis of Cr3+ and the critical crevice solution, according to Oldfield and Sutton,’ has a pH of 1.65. Assuming a hydrolysis constant3’ of 10-3.8, and neglecting activity coefficients, this requires a Cr3+ concentration of more than 1 M. However, by the time crevice corrosion actually initiated in these tests, the calculated metal ion concentration is approximately 0.1 M giving a pH of 2.7, which is too high to cause depassivation. In addition, visual examination of crevice corroded samples showed that corrosion had not initiated at the centre of the crevice, 30

p?-

1 -

crevice free

--

with crevices

20

E : 3. ._

10

I

I

I

800

1200

1800

Time (s) Fig. 5.

Passive current density as a function of time in de-aerated I M NaCI, for an applied potential simulating the E vs r behaviour in 8 ppm Cl solutions.

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N. J. Laycock et al 0.6 1

crevice height = 1 pm

crevice height = 10 Brn 0 1

I

0

50

100

,

1

150

200

Time (s) Fig, h, Calculated variation of the metal ion concentration at the crevice centre as a function of time. The input passive current varied as shown in Fig. 3.1J

where the pH would have been lowest, but near the crevice mouth. One interpretation of this is that the IR drop increased into the cavity and neither metastable pitting nor propagation of crevice corrosion were possible at the relatively low potentials in the centre of the crevice. Thiosulphate entrapment In the thiosulphate entrapment model of Lott and Alkire’5*‘6 the dissolution of MnS inclusions is potential dependent and so the existence of a critical potential could be accommodated within the framework of this model. The 316L used for these crevice corrosion tests contained a high amount of sulphur (0.028%) and significant build-up of sulphur species due to MnS dissolution could be expected in the crevice. A low sulphur (< 0.003%) 3 16L was also obtained and polarisation date for both steels in sodium sulphate solution are shown in Fig. 7. Following the work of Suter et a[.,31 the increased passive current from the high S steel was presumed to be caused by anodic MnS dissolution, but the results of Fig. 7 emphasise the irreproducible nature of this effect. Above about 400 mV, MnS dissolution contributes several PA crnA2 to the current. but inclusion dissolution appears to be random in the Same way as pit initiation. At the potentials typical of crevice corrosion initiation ( from - 50 to + 50 mV), there is no consistent difference between the passive currents from the two steels, which is in good agreement with the results of Suter et a[,31Inclusion dissolution at these potentials could be predominantly chemical, and would consequently have been unnoticed in these tests. Nevertheless, the effect of a low MnS dissolution current was modelled using a procedure similar to that for the passive dissolution calculations and assuming oxidation of MnS according to the reaction scheme of equation (1) (above). Using this MnS dissolution current in place of the passive current and considering the transport of thiosulphate ions instead of metal ions, the thiosulphate concentration at the crevice centre was calculated for a constant current of 0.25 PA cm-’ (Fig. 8) and this gives a maximum thiosulphate

Crevice corrosion in stainless steels 50 45 -

--

0.026 s

-

0.003 s

1801

: :

40 -

:

:

35 -

:

f :

c;;z 30 E a 25 2

20 15 10 50 , -1

I 0.5

I 0.0

I 1.0

, 0.5

I 1.5

E (V vs SCE) Fig. 7.

Anodic polarisation curves for high sulphur and low sulphur 3 16L stainless steel in 0.1 M Na2S04 (two tests per alloy).

6

1 crevice height = 1 pm

crevice height = 10 pm

01 0

60

100

150

200

Time (s) Fig. 8. Computer simulation of the increasing thiosulphate concentration at the centre of the crevice given a constant average current density of 0.25 pA cm-* due to MnS dissolution.

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concentration of 5 mM, although this takes no account of contributions from chemical dissolution. Lott and Alkire’5,‘6 considered crevice corrosion to initiate at a critical concentration of chloride and thiosulphate ions, but whether this occurred by general breakdown or pitting was not discussed and Brossia and Kelly12 have now shown that sulphide, rather than thiosulphate, is the main sulphur species present in corroding crevices of 304 stainless steel. In fact, the effect of all reduced sulphur compounds on pitting is to stabilise the growth of metastable pits such that stable pitting occurs more readily,‘7-32 but thiosulphate is more aggressive than sulphide.” Polarisation curves were measured for the high sulphur steel in 1 M NaCl solutions both with and without 5 mM thiosulphate (Fig. 9). Thiosulphate had little or no effect on the pitting potential for this steel, but even without thiosulphate, stable pitting occurred almost immediately after the start of metastable pitting. The high S content of this steel suggests that large MnS inclusions would be present and the transition to stable pitting is thought to be assisted by large inclusion cavities, and possibly by the supply of sulphur species from the inclusion itself, which could explain why the pitting potential (Fig. 9) is approximately the same as the initiation potential for crevice corrosion; dissolution of large inclusions produces large cavities and high thiosulphate concentrations, meaning that pit initiation sites are effectively crevices in their own right as suggested by others.3,4 Higher concentrations of thiosulphate could have been present in crevices due to chemical dissolution and locally increased chloride levels would also be expected, so that although general breakdown by this mechanism seems unlikely, enhanced pitting within the crevice cannot be ruled out, particularly for lower S steels. IR drop mechanism

The polarisation

data of Fig. 8 show the absence of any active loop for 3 16L in the bulk

1000 --

5 mM thiosulphate

-

no thiosulphate

loov5

a5 lo._

0.1 -0.7

I

I

I

I

I

-0.5

-0.3

-0.1

0.1

0.3

E (V vs SCE) Fig. 9.

Polarisation

curves for high and low sulphur 316L stainless without 5 mM Na&07.

steel in

1 M NaCl with and

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solution, even with the addition of thiosulphate. If the total current from the crevice is assumed to cause the IR drop at the crevice centre, then taking a passive current density of 3 uA cm-2 and a solution resistivity of 10 R cm,3o the maximum IR drop in a 0.025 cm deep. A 1 urn high crevice can be estimated as approximately 20 mV. This shows that the normal passive current in the bulk solution is not enough to produce significant potential variation within the crevice, and since no active corrosion region exists above the corrosion potential, initiation by this mechanism must require the development of a locally aggressive solution. The most aggressive local chemistry has already been shown to develop at times much shorter than the observed induction times for crevice corrosion (Fig. 6). Metastable pitting There is an obvious similarity between EC,, as measured in this work, and commonly measured pitting potentials. It is unlikely that at the times where crevice corrosion initiated in these tests, the local chemistry was sufficient to cause general breakdown of the passive film. Nevertheless, increased chloride and thiosulphate concentrations in the crevice could help to increase the likelihood of pitting, although 5 mM thiosulphate has been shown to have little effect (Fig. 9). If we make the simplest assumption that the probability of pitting within the crevice is equal to the probability of pitting on the exposed steel surface, then a knowledge of metastable pitting rates on the open surface should enable calculation of the number of pits occurring within the crevice area, c$ Stockert and Boehni.4 Fig. 10 shows the comparison of E vs t behaviour for crevice/crevice free coupled electrodes against crevice free/crevice free couples. In both cases, metastable pitting is indicated by fluctuations in the potential that began at approximately - 130 mV. The pitting potential measured in

.I

-. 0

I

I

I

I

I

500

1000

1500

2000

2500

Time (s) Fig. 10. Comparison of E vs t behaviour for different types of electrode couple; creviced to crevicefree, and crevice free to crevice free.

N. J. Laycock et ul.

1804

potentiodynamic tests (Fig. 9) was very similar to E;,,,, measured in open circuit tests, but in open circuit pitting tests the magnitude of potential fluctuations increased without ever becoming a permanent potential decrease within the 2500 s duration of the tests. Either, stable pitting did not occur in open circuit tests, or, more likely, pitting under these conditions necessarily goes through cycles of propagation and repassivation as found in galvanostatic tests by Suleiman.” The current between the coupled electrodes was also measured during these tests and an example of the results for crevice free/crevice free couples is shown in Fig. Il. Current transients in these results represent metastable pitting events occurring randomly on both electrodes, although separation of individual events is difficult because of the relatively large sample area giving many overlapping events. Still, an attempt was made to count individual events and Fig. 12 shows the metastable pitting rate, h,, as a function of time (the detection limit was 0.1 uA and rates were averaged over 900 s intervals). If pitting events are randomly distributed in space and time, but the average rate is a constant both in and outside the crevice and any pit initiated within the crevice area has a probability, p (0.. 1) of initiating crevice corrosion, then the expected distribution of E,,,, can be calculated from a knowledge of h, and E vs t behaviour (Appendix B). Figure 13 shows the predicted distribution of EC,,, for two values of p in an 8 ppm chlorine solution, compared with the experimentally measured values for all solutions. The effect of decreasing p is to widen the distribution and to shift the modal initiation potential to higher values. Ap value of 0.5 produces a good fit of the calculated values with the experimental ones, although the experimental distribution is somewhat sharper than predicted. A possible explanation is that the number of metastable pits below the detection limit increases sharply with potential near - 30 mV, and so causes

3-

2-

l-

"

o-

-1 -

-2 -

-3 : 0

I 500

I 1000

I 1500

I 2000

I 2500

Time (s) Fig. 11. The current between identical, crevice free coupled electrodes of 3 16L stainless steel in 1 M NaCl with 8 ppm chlorine.

1805

Crevice corrosion in stainless steels

_

0.15 -

‘;tn Q) ;;;

u

0.10 -

F P .Z n 0.05 -

00

!

I

I

400

0

I

800

f

1600

1200

Time (s) Fig. 12. Metastable pitting rate as a function of time as measured for crevice free, coupled electrodes of 316L stainless steel in 1 M NaCl with 8 ppm chlorine (data from three tests).

13 12 Lu X c" 0 'Z 2 ._ .f

11 10

Elmodel(p=l) U model(p=

0.5)

0 experimental

9 6 7 6

B

6

s’

4 3 2 1 0 -140

-110

-60

-50

-20

10

40

E (mV vs SCE) Fig. 13. Comparison of the observed distribution of crevice corrosion initiation potentials with the distribution expected for a metastable pitting model of crevice corrosion initiation. The model distributions are for the E vs r behaviour in 8 ppm chlorine.

most crevices to initiate near this potential. Alternatively, it might be that only metastable pits which reach a critical size can initiate crevice corrosion, but are then almost certain to do so 0, = 1). A more detailed study of metastable pitting data would be needed to test for the existence of such a critical size, but if the number of pits reaching this size increased sharply above 30 mV, this would concentrate the observed EC,, distribution at this potential.

N. J. Laycock et al.

1806

Despite slight differences between the predictions of the metastable pitting model and the experimental crevice corrosion data, this model shows by far the best agreement with experiment out of the four models considered. The critical dependence of crevice corrosion initiation on potential, its independence of time, and the scatter in the results are explained, with no requirement for changes in the crevice chemistry, or potential, which have been shown not to OCCU~.‘~I2 Metastable

pitting and the effect of temperature

on crevice corrosion

The critical crevice corrosion temperature (CCT) is the temperature below which stable crevice corrosion will not occur at any potential for a given alloy, and is a common method of alloy ranking. 34,35The CCT was measured for 904L and found to be between 15 and 20°C for these particular experimental conditions (Fig. 14). If metastable pits initiate crevice corrosion and the CCT is the lowest temperature at which crevice corrosion occurs, the CCT should correspond to the lowest temperature at which metastable pitting is possible. This assumes that crevice corrosion propagation is possible if metastable pitting is possible, which seems reasonable since crevices can propagate at relatively low current densities. An applied potential of 300 mV was found to maximise metastable pitting rates28 and the number of metastable pits in a fixed time interval at this potential is shown as a function of temperature in Fig. 14. Metastable pitting appears at temperatures as low as 0°C some 15-20°C below the measured CCT. However, Fig. 14 does show a sharply increasing number of observable pits in the region of the CCT. It could be that the CCT measurement did not allow enough time

W”

0

-5

0

5

10

15

20

25

30

35

40

T (“(7 Fig. 14. Breakdown potential (@lo pA cmm2) as a function of temperature for creviced and crevice-free electrodes of 904L stainless steel in 1 M NaCI. The CCT is defined as indicated. Each data point is the average of three measurements; error bars represent the standard deviation. Also shown is the total number of metastable pits (> 20 nA) in the time interval 5G350 s following a potential step to 300 mV at the specified temperature for 904L in 1 M NaCl.

Crevice corrosion in stainless steels

1807

for initiation at the lower temperatures; if samples were left long enough at 0°C then crevice corrosion may indeed have initiated. Stocker? and Boehni4 showed that, on average, a similar number of metastable pits was always required before crevice corrosion initiated, Applying this idea to the data of Fig. 14 and assuming metastable pitting rates to be independent of time, initiation at 2°C could be expected to take 5-10 times as long as initiation at 20°C. Since metastable pitting rates actually decay approximately exponentially with time,4 this is probably an underestimate of the longer times required at lower temperatures. In fact, if the metastable pitting rate were to decay to zero without initiation of crevice corrosion, then initiation would never occur. As suggested for crevice initiation on 3 16L, other explanations could involve the number of pits below the detection limit or the existence of a critical pit size necessary to initiate crevice corrosion. Nevertheless, despite the fact that some metastable pitting is detected below the CCT, Fig. 14 shows a close correlation between a significant increase in the metastable pitting rate and the onset of crevice corrosion, providing further evidence that crevice corrosion is indeed initiated by metastable pitting.

CONCLUSIONS The initiation of crevice corrosion on 316L was shown to occur after times an order of magnitude longer than the time taken for the peak metal ion concentration to be reached at the crevice centre. At the times typical of initiation in these tests, modelling of the crevice chemistry suggested that the pH in the crevice was not low enough to cause general breakdown in the crevice. The passive dissolution model of Oldfield and Sutton6’7 does not appear to explain the results in this case. For crevice corrosion to initiate by the IR drop mechanism of Pickering and Frankenthal’8>‘9 an aggressive local chemistry is required within the crevice, since the polarisation curve for 3 16L in the bulk solution shows no active loop. Again, the most favourable time for initiation by this mechanism is inconsistent with the measured induction times. Thiosulphate accumulation, owing to anodic dissolution of inclusions in the crevice, has been suggested as a reason for crevice breakdown by Lott and Alkire,“~i6 but Brossia and Kelly’* have since found sulphide, and not thiosulphate, inside corroding crevices. Crevice corrosion initiated in our tests at potentials much lower than the potentials where electrochemical dissolution of MnS was observed in sulphate solutions. Chemical dissolution of inclusions, possibly enhanced by partial acidification of the crevice, could lead to some thiosulphate accumulation, although not enough to cause general corrosion. Pitting could be enhanced within the crevice, but low (5 mM) levels of thiosulphate were shown to have little effect on the pitting potential of the 316L used for the crevice tests. Stockert and Boehni4 proposed that metastable pits could be stabilised by crevice walls, so that crevice corrosion was simply a geometrically stabilised form of pitting. Analysis of metastable pitting data for 3 16L enabled the calculation of initiation potentials predicted by this model, and the predicted values agree well with the experimental results for this system. The CCT measured for 904L was 15-20°C higher than the lowest temperature at which metastable pitting was observed, but the number of observable metastable pits was increasing rapidly in the region of the CCT. It is suggested that over longer tests, crevice corrosion would occur at all temperatures where the metastable pitting rate does not decay to zero with time.

N. J. Laycock

I808 AcknowledgemPnrs-This Technology scheme.

research

was funded

jointly

et al.

by the EPSRC

and Unilever

Research

under

the Total

REFERENCES I. M.O. Speidel and R.M. 2. 3. 4. 5. 6. 7. 8. 9. IO.

I I. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

27. 28. 29. 30. 31. 32. 33. 34. 35. 36.

Pedrdzoh, Mater. Performance 31, 59 (1992). M.B. Rockel and M. Renner, Werksfoffi und Korrosion 36. 537 (1984). l.L Rosenfeld and I.S. Danilov, Corros. Sci. 7, 129 (1967). L. Stockert and H. Boehni, Mater. Sci. Forum 44/45, 313 (1989). J.L. Crolet and J.M. Defranoux, Corros. Sci. 13, 575 (1973). J.W. Oldfield and W.H. Sutton, Br. Corros J. 13. I3 (1978). J.W. Oldfteld and W.H. Sutton, Br. Corros. J. 13. 104 (1978). S.M. Sharland, Corros. Sci. 33, 104 (1978). P.O. Gartland, Mathematical Model of Crevice Corroston for Fe-Ni-Cr-Mo Alloys in Chloride Solutions. SINTEF report no. PBS9 - 2231 I9/XAB. S.E. Lott and R.C. Alkire, Corros. SC;. 28. 479 (1988). B.K. Nash and R.G. Kelly, Corros. Sci. 35, 8 17 (1993) C.S. Brossia and R.G. Kelly, in Crirical Factors in Loc~alizedCorrosion II, ed. P.M. Natishan, R.G. Kelly, G.S. Frankel and R.C. Newman. The Electrochemical Society, Pennington NJ, 1995, p. 201. N. Sridhar and D.S. Dunn, Corrosion 50, 857 (1994). J.R. Galvele, J. Elecfrochem. Sot. 123, 464 (1976). SE. Lott and R.C. Alkire, J. Ebctrochem. Sot. 136, 973 (1989). S.E. Lott and R.C. Alkire, J. Electrochem. Sot. 136. 3256 (1989). R.C. Newman, H.S. Isaacs and B. Alman, Corrosion 38, 261 (1982). H.W. Pickering and R.P. Frankenthal, J. Elecfrochem. Sot. 119, 1297 (1972). H.W. Pickering and R.P. Frankenthal, J. Elecfrochem. Sot. 119, 1304 (1972). K. Cho and H.W. Pickering, in Criticai Facrors in Localized Corrosion, ed. G.S. Frankel and R.C. Newman. The Electrochemical Society, Pennington NJ, 1992. p. 407. K. Cho and H.W. Pickering, J. Electrochem. Sot. 137, 3313 (1990). G.S. Frankel, L. Stockert, F. Hunkeler and H. Boehni. Corrosion 43, 429 (1987). G.S. Frankel, in Advances in Localized Corrosion, ed. H. Isaacs, U. Bertocci, J. Kruger and S. Smialowska. NACE, Houston, 1990, p. 137. P.C. Pistorius and G.T. Burstein. Phil. Trans. R. S’oc Lond. A 341, 531 (1992). N.J. Laycock and R.C. Newman, Mater. Sci. Forum 192-194, 649 (1995). N.J. Laycock, M.H. Moayed and R.C. Newman. in Critical Facfors in Localized Corrosion II, ed. P.M. Natishan, R.G. Kelly, G.S. Frankel and R.C. Newman. The Electrochemical Society, Pennington NJ, USA, 1995, p. 68. M. Suleiman, I. Ragault and R.C. Newman, C‘orros SC;. 36. 479 (1994). N.J. Laycock, M.H. Moayed and R.C. Newman, J. Elecrrochem. Sot., in press. J.W. Tester and H.S. Isaacs, J. Electrochem. Sot. 122, 1438 (1975). CRC Handbook of Chemistry and Physics, ed. D.R. Lide. CRC Press, Boca Raton FL, 1974. T. Suter. T. Peter and H. Boehni, Mater. Sci. Forum 192-194, 25 (1995). R.C. Newman and E.M. Franz, Corrosion 40, 325 (1984). R.C. Newman and M.I. Suleiman, Corros. Sri. 36, 1657 (1994). C.W. Kovach and J.D. Redmond, Corrosion ‘93, Paper 267. NACE, Houston TX, 1993. M.O. Speidel and R.M. Pedrazolli, Corrosion ‘92, Paper 398. NACE, Houston TX, 1992. D.R. Cox and V. Isham, Poinf Processes. Chapman and Hall, London, 1980.

APPENDIX

A

The crevice was taken to be a local anode from the start and cathodic reactions were ignored. The crevice itself was further simplified by division into two equal parts. each being from the centre to one edge, and transport of species was presumed to occur from the centre outwards by diffusion through 100 identical elements of width x. The passive current, i,,,,, was assumed to be identical for each element but to vary with time as in Fig. 5 and the metal

Crevice corrosion in stainless steels

1809

was assumed to dissolve stoichiometrically with an average valence of 2.2 and an effective diffusivity of 2 x lo-’ cm2 s-‘. Initially the concentration of metal ions throughout the crevice was zero and i,,,, = ib,,. A concentration of metal ions, C, builds up in each element within the crevice due to the passive dissolution, and the flux into an element due to this effect, Jr.,, (mol cm -’ s- I), is given by equation (Al). At time intervals of At, Cj is calculated assuming a linear concentration gradient between each element. This allows the use of Fick’s First Law and is valid so long as At > x2/D (the characteristic time for diffusion from each element). In the first element outside the crevice mouth, the metal ion concentration is fixed at zero and a concentration gradient along the crevice is set up, with the diffusion flux into an element, Jdie(mol cmp2 s-’ ), given by equation (A2). The total flux into an element is the sum ofJdiffandJpass and the calculation of Cj follows simply for elements of unit width, length x and heighty. The height of the crevice gap (y) is difficult to measure, but can be estimated as on the order of the surface roughness, i.e. J pass = ipasslnF JdiN = (D/.X)(Cj_l

(Al)

- 2Cj - Cj+l)

642)

A smooth curve was fitted to the passive current density for crevice free electrodes (Fig. 5) and used as the input ipa,, for the finite difference calculations.

APPENDIX

B

Regression analysis of the plot in Fig. 12 gives an equation with the form of equation (Bl) where a and b are constants, and the expected distribution of induction times for crevice corrosion initiated by metastable pitting can now be calculated. If any metastable pit initiating in the creviced area (fraction CLof the total) has a probability, p (O..l), of initiating crevice corrosion then the crevice initiation frequency, h-,, is given by equation (B2). &, = at + b A, = a&

@I)

032)

A modified time scale must now be used such that h, is constant with time and initiation can be considered a Poisson process. 36The transformation of the time axis is accomplished as follows, where N is the number of stable crevice initiation events. Obviously, kc = dN/df and eqns (Bl-2) can be combined to give an equation with the form of equation (B3), where m and care constants. dNJdt = mt + c

(B3)

N = 0.5m[t + (c/m)]* - c2/(2m)

(B4)

If a modified time parameter, T is now defined as in equation (BS), then dN/dT is a constant and crevice initiation can be considered a Poisson process with a constant rate, k. From eqns (BC6) it can be seen that k = 0.5 m T = [t + (c/m)]*

N = kT - 2/(2m) dN/dT = k

(B5) (W

(B7)

For n experiments an exponential distribution of initiation times on the modified time scale is now obtained. The expected initiation times, Ti, can be calculated using equation (B8), for i= I...n. Tl = (l/k)ln(l

- [i/(n + l)])

(B8)

Using equation (B5), these initiation times can be converted back into real times, ti and the corresponding initiation potentials. Ecrcy, can be obtained from the E vs t curve for the relevant solution.