THE RESPECTIVE EFFECTS OF PASSIVE FILMS AND NON METALLICINCLUSIONS ON THE PITTING RESISTANCE OF STAINLESS STEELS- CONSEQUENCES ON THE PRE-PITTING NOISE AND THE ANODICCURRENT TRANSIENTS

lB. BAROUX and 2n. GORSEJlnstitut National Polytechnique de Grenoble, FranceandUgine Research Center73400 UgineFrance (mailing address)

2CECM-CNRS Vitry15 rue Georges Urbain94407 Vitry/Seine, France

ABSTRACT. For industrial steels, pitting occurs on manganese sulphides, when they are presentin the steel. Adding a slight amount of titanium as alloying element prevents the MnS formation,since titanium is a stronger sulphide former than manganese. Titanium sulphides are more stablein chloride-containing aqueous solutions than MnS. For titanium bearing steels, pits initiateeither on titanium sulphides (in acidic concentrated chloride media), on other metallurgicaldefects, or are the result of some local passive film instabilities, irrespectively of the presence ornot of non metallic inclusions. One shows that the pitting potential-pH dependence is thesignature of the actual pitting sites: as far as sulphides (MnS or Ti-.s) act as pitting sites, a strongdecrease in pitting potential is observed when the pH becomes smaller than a critical valuecorresponding to the formation ofsome sulphur-containing species in the aqueous solution, dueto the dissolution of the sulphides inclusions. Otherwise, no pitting potential/pH-dependence isfound. When polarising a sample below the pitting potential, some pre-pitting events areobserved, resulting in anodic current transients revealing the initiation then the repassivation ofunstable pits. The form of these transients (slow anodic current increase followed by a sharpdecrease, or sharp current increase followed by a smooth decay) is the signature of the type ofpitting site (MnS or other). Moreover, for MnS-containing steel, a strong pre-pitting noise isobserved, exhibiting a pH-dependence similar to the pitting potentials one. The intensity of thepre-pitting noise or, when only isolated events are observed, the occurrence frequency of the pre­pitting events, decreases when the polarisation time increases. Furthermore, potentiostatic agingat constant potential (lower than the pitting potential) decreases the further probability for theoccurrence of stable pits, indicating a beneficial effect on the pitting resistance. This effect isbelieved to be due to the passive film reinforcement and not to a decrease in the available pittingsites density.

161

K. R. Trethewey and P. R. Roberge (eds.), Modelling Aqueous Corrosion, 161-182.© 1994 Kluwer Academic Publishers.

162

1. Introduction

1.1. MOTIVATIONS

It is well known that, on industrial stainless steels, pitting occurs on non metaJlicinclusions [1, 2] and particularly on sulphides [3J, the size of which is of the orderof some micrometers. On another hand, most of the theories of pitting initiationrefer to local passive film breakdown, at the nanometre scale, without any referenceto these metallurgical defects. Considering this discrepancy, some workers suggestedthat pitting occurs at the matrix inclusion boundaries, which has been observed insome cases, but is not a general feature. Moreover the chemical effect of the specieswhich dissolve from the non metallic inclusions is hardly taken into account by thepassive film breakdown theories. It is intended in this work to clear some of thesepoints, by measuring the pitting potential in neutral or acidic chloride media of somespecially designed stainless steels, containing different types of non metaJlicinclusions whose the reactivity regarding the solution pH and the chloride contentare different, and identifying the corresponding pitting sites. Additionally, in orderto modify the passive film, the samples were aged in the corrosive solution either atrest potential or in potentiostatic conditions but below the pitting potential. Theeffect of such treatment on the pitting sensitivity was verified and the anodic currentfluctuations during the polarisation analyzed, then correlated both to the inclusionsproperties or passive film aging. Finally, the results are discussed regarding thesuccessive stages occurring in the formation of stable or unstable pits.

1.2. STUDIED STEELS AND THEIR NON METALLIC INCLUSIONS

The steels under investigation were mainly some annealed AISI 430 type FeCr alloyscontaining also either niobium (steels A and A') or titanium (steels Band B')additions. Their composition is indicated in Table 1. Except for steel A, somealuminium additions were used as deoxidizing agent during the melting process,which lead to Al- 0.030% and induce the presence of some A120 3 inclusions. Let usnote the low sulphur content, which is easily attained with modern steelmakingtechniques and the presence of some stabilizing elements (Ti or Nb) which trap thecarbon and avoid the formation of chromium carbides.For both steels A and A', sulphur is trapped as manganese sulphide, which is

considered as a noxious pitting site. The difference between steels Band B' is theirchromium content; both contain titanium, which also trap nitrogen as titaniumnitrides and sulphur as titanium sulphides. This trapping occurs during thesteelmaking process before the steel solidification. Additionally some solutiontreated 304 and 321 AISI type steels are considered (C and D respectively), in orderto separate matrix and inclusion effects. These two steels are austenitic and nickel­bearing but the latter contains titanium and is MnS-free, whereas the formercontains MnS inclusions.

163

TABLE 1Steels composition in weight% or in ppm (brackets)

Cr Ni Si Mn Ti Nb (S) (C) (N)

A 16.4 .4 .45 .5 40 240 n.d.

A' 15.7 .4 .45 .7 50 340 340

B 16.8 .4 .45 .4 30 260 140

B' 17.4 .4 .45 .4 30 250 noc 17.6 8.2 .4 1.4 20 530 470

D 17.8 9.3 .4 1.3 .4 30 310 170

The detailed study of the inclusions and precipitates characteristics is out thescope of this paper. Nevertheless their main features, as far pitting initiation isconcerned, are presented in Figures 1 and 2. A specific preparation procedure forthe Scanning Transmission Electron Microscopy (STEM) examinations wasdeveloped for avoiding the sulphur dissolution, which will be described in aforthcoming paper.Concerning steel A', some MnS particles are found around aluminium oxides

(Figure la). Since these oxides have a poorer ductility than the metallic matrix, thecold rolling process provokes some micro-decohesions around the inclusion, wheresome manganese sulphides are often located, leading to the possible formation ofvery noxious microcrevices. This phenomenon is not observed on aluminium-freesteels, where oxides are mainly malleable silicates. However, the art of thesteelmaker consists in avoiding the formation of chromium oxides, which couldproduce a similar but worst effect than aluminium oxides. In every case, some MnSis found closely attached to niobium carbonitrides (Figure 2a). Since thesecarbonitrides precipitate at - 1200 °C, this shows that (for the considered sulphurcontent), the high temperature sulphur solubility is sufficient for the MnSprecipitation to occur in the solid steel (lower than 1200 0C): to our knowledge, thisresult is quite new, since MnS are generally considered to precipitate at the end ofthe steel solidification. Note that very few isolated MnS precipitates are found,probably because Nb(C,N) acts as precipitation sites. This situation contrasts withthe behaviour of steel C (304) for which (i) no carbides may nucleate the MnSprecipitation (ii) sulphides can precipitate in austenite at the delta\gammatransformation temperature.Figures 1b and 2b show the location of titanium sulphides on steel B, i.e. around

the titanium nitrides, embedded in a titanium carbide belt. Detailed examination

164

Figure 1 - NOli-metallic illclusions (before pitting). Scanning electron microscopy (x4000) a: Steel A'.MnS nucleated on an aluminium oxide; b: Steel B; titanium nitride surrounded by sulphur

compounds; c: Steel B; micro-hole in a titanium nitride.

165

suggests that at high temperature an homogeneous titanium carbosulphide beltprecipitates around the titanium nitrides (which formed in the liquid steel). Then,lowering the temperature, titanium sulphides and carbides separate at some pointsas shown on Figure 2b, producing the titanium carbide, TiC, and a titaniumsulphide, identified in some case (using electron diffraction) as the hexagonal phaseTi2S. As an other result, note that aluminium oxides have been identified in the coreof the titanium nitrides (not shown on the presented figures), playing the role ofnuclei for the TiN precjpitation. In some cases, some micro-holes have beenidentified at the center of the titanium nitrides (Figure 2b); they are probably dueto these hard aluminium oxides, which are sometimes removed when polishing thespecimens. One must be careful when examining the samples after pitting and avoidconfusion between such mechanically induced holes and the pitting set up. Isolatedtitanium carbides are also present in the steel, but have no relation with pitsinitiation, except in the case of high potential pitting, as shown later.

Figure 2 - Non metallic inclusions (before pitting), Scanning Transmission Electron microscopy(x18000). a: Steel A'. MnS nucleated on a niobium carbide (or carbonitride); intermetallic (Fe, Nb)

phases are also observed; b: Steel B; titanium nitride surrounded by a titanium carbide, in'which sometitanium sulphides are embedded.

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2.Pitting potentials and pitting sites

2.1. MEASUREMENT OF THE PITTING POTENTIALS

The pitting potentials were measured on mechanically polished samples (SiC grade1200, under water) using a procedure and an experimental set-up which aredescribed elsewhere [4]. The electrolyte was a deaerated chloride solution (NaCI0.02 M to 0.5 M), with a pH adjusted by HCI additions from pH 6.6 (near neutral)to pH 3 (which is above the critical pH below which the passive film could suffergeneral breakdown). The temperature was kept constant (23·C). The samples wereaged 24 h in air after polishing, then immersed for 15 min in the corrosive solutionbefore beginning the potentiokinetic scan (100 mV min-I). The "Elementary pittingprobability" per area unit Gj(V) was determined using a statistical method [4], as afunction of the electrode potential V. A conventional pitting potential Vpit wasobtained, for which Gj(Vpil) = 0.1 cm-2•

2.2. EFFECT OF pH AND CHLORIDE CONTENT ON THE PITTING P01ENTIALS

Figure 3a shows the effect of pH for steels A' and B in NaCI (0.02 M). No pH­dependence was observed for steel B. At the opposite, a sharp pitting potentialdecrease was evidenced when the pH is lowered under a critical value pHc rangingbetween 4.5 an 5. Since the main difference between the two steels was the presenceor absence of MnS, and that titanium sulphides are known to have a better stabilityin aqueous electrolytes than MnS, one can assume that this decrease was due to thepH-assisted MnS dissolution. Note that the same phenomenon is observed (Figure3b) with the two austenitic steels C (MnS-containing) and D (MnS-free).The effect of the solution pH on the MnS dissolution was studied in details by

Eklund [5], as it can produce some harmful sulphur bearing species (from the pitsinitiation viewpoint). For the studied pH values, the pH-assisted reaction to beconsidered should be:(0) MnS + H+ - Mn2++ HS-.

The existence and the numerical value of the observed threshold pH will bediscussed in a forthcoming paper. Now, it is suggested that in chloride-containingenvironments, the overall reaction (0) splits into a set of two chloride assistedreactions:(1) MnS + H3o+ + CI- - HS- + H20 + MnCI+ followed by(2) MnCI+ + CI- - Mn2+ +2CI-Figure 3b also shows the results obtained for steels A an B'. The slight difference

between steels Band B' may be attributed to the difference in chromiumconcentration. The strong difference between steels A and A' may be due both tothe difference in chromium concentration and sulphur contents, but also, from ourindustrial experience, to the noxious effect of aluminium oxides present in steel A'and around which MnS are found.

400

-os. 300~

B• O.02M •

• BOOSM

.~200

~ •6.6

7 6 5 4 3

pH

Figure 3 - Effect of the solution pH on the pitting potentials. a: Steels A' and B in [NaCl] = 0.02 M;b: Steels A, B' (FeCr steels) and C, D (FeCrNi steels); [NaCl] = 0.02 M; c: Effect of the solution

chloride content dependence for a titanium-containing steel.

168

The effect of solution chloride content was also investigated between 0.02 M and0.5 M. Figure 3c shows that for high chloride concentrations, a pitting potential/pH­dependence is found even for titanium-containing steels, suggesting that titaniumsulphides are not so stable in such electrolytes. It is suggested that the simplifiedmodel proposed for the MnS dissolution is also valuable for titanium sulphides,replacing equations (I) and (2) by:(1') Ti2S + H30+ + 6CI- - HS- + H20 + 2TiCI/ + 6e­

which is strongly chloride and potentia] assisted, followed by:(2') TiCh+ + CI- - Ti+4 + 4CI-

In case of the formation of TiSz instead of Ti2S, reaction (1') should be replaced by:(1") TiSz + 2H30+ + 3CI- - 2HS- + 2H20 + TiCh+

The CI- pitting potential dependence is shown in Figure 4. For as neutral pH (6.6)the results are consistent with the logarithmic law previously found [4] for thissample preparation procedure and one finds Vpit= canst -120 mV.Jog[CI'). Thingsbecome more complex for MnS-containing steels when pH<pHc (steel C at pH 3 onFigure 4a, whatever the chloride content), or even for MnS-free steels for acidichighly concentrated chloride solutions (steel B at pH 3 and 0.5 M NaCI on Figure4b), evidence that once again the determining pitting mechanisms are influenced bythe dissolution of inclusions.

2.3. PITTING SITES

The procedure used to provide evidence of the pitting sites was as follows. First thesamples were diamond polished in order to obtain a surface micrographicallyobservable with the Scanning Electron Microscope (SEM). Next, they were aged for24 h in air, then immersed for 15 min at rest potential in the test solution asdescribed above. Last, the potential was quickly raised up to pitting potential, toobtain an anodic current of the order of few microamps. The samples were thenexamined ex-situ, in order to observe the pitting sites (Figure 5). For steel A',whatever the pH or the chloride content, pits initiate either around aluminiumoxides (Figure Sa), where some MnS are located, or on MnS inclusions (Figure 5b)around Nb(C,N) or (seldom) isolated. This confirms the preceding hypothesis. Forsteel B in NaCI(O.5 M) solution, pitting generally occurred at the TiN boundary(Figure 5c), where titanium sulphides were present. This shows clearly that for suchchloride concentrations, titanium sulphides act as pitting sites and become unstablewhen the potential increases. For lower chloride concentration (NaCI 0.02 M), thesituation is not so clear. In some cases, pits were observed at the center of thetitanium nitrides but it was probably due to the preexisting mechanical polishinginduced micro-holes which were noted above. Otherwise, pits could initiate directlyon the metallic matrix, with no direct relation to non metallic inclusions. In the twosituations, however, titanium sulphides do not seem to act as pitting sites,which isconsistent with the absence of any pitting potential/pH-dependence.

169

450

• A: pH=6.400 DC: pH=3.

350• C: pH=6.6

"""'.uulZl

--- 0> 300E •---0-> 250 0

200

~150O,QI 0,02 0,1 0,5

NaCI (mole)

450

o B : pH=3.

400 0 • B: pH=6.

iii' 350U~ •> 300E 0

---0-> 250

•200

~ 0

150

0,01 0,02 0,1 0,5

NaCI (mole)

Figure 4 - Effect of the chloride content on the pitting potential for pH 6.6 and 3. a:MnS bearingsteels (A and C); b: MnS-free steel (B).

170

Figure 5 - Scanning electron microscopy on pilled samples. a: Steel A'; pilling at the boundary of analumina inclusion; b: Steel A'; pilling on a manganese sulphide; c: Steel B; pilling at the boundary of

a titanium nitride.

171

3. The fluctuations of anodic current below the pitting potential

3.1. THE PRE-PITIING EVENTS

Recording the anodic current i(t) during polarisation below the conventional pittingpotential (Figure 6a) provides two types of information. First the average anodiccurrent decreases with time, corresponding probably to the onset of an improvedpassivity. Second, some oscillations of this anodic current are observed, which wereshown by SEM observations than above, to correspond to initiated and repassivatedpits. The problem of analysing the pre-pitting noise is to separate its signal from theaverage anodic current decay which corresponds to the passive film reinforcement.For this purpose well known image analysis techniques [7] were used. The result wasto subtract from the signal its baseline (corresponding to the average anodic current)without cutting the characteristic fluctuations frequencies. The baseline wasdetermined by associating to each point [t,i(t)] the point [t, il(t)= min{i(tl),t'=t-V2to t+ ./2}] 'where • is an arbitrary time constant. The frequency 11. should besmaller than the characteristic frequencies of the pre-pitting noise. Figure 6b showsthe typical reduced signal obtained using for steel A' using this method.For MnS-containing steels (A or AI), the pre-pitting events are very numerous

and produce what we call a "pre-pitting noise" (Figure 6b). In contrast, for MnS-freesteels, the anodic events are much less numerous and well separated, as shown onFigure 6c. It is therefore logical to relate the so called "pre-pitting noise" to thedissolution and repassivation of manganese sulphides.

3.2. THE INDIVIDUAL ANODIC EVENTS

For MnS-containing steels, the current/time signature of the individual pre-pittingevents is found to increase linearly with t2, followed by a sharp current decrease(Figure 7a), which is different from the linear i(t) variation previously found byStewart and Williams [8J. The peak amplitude at 200 mV SCE is of the order of 0.1JLA to 1 JLA.For the MnS-free steels, the shapes of the i(t) transients are quite different (type

II, see Figure 7b) and the peak intensity is smaller (10 to 100 nA at 200 mV SCE).In a neutral NaCI(0.02 M) solution, the transient is characterised by a sharp increasefollowed by a slow current decay, as expected for a local passive film breakdownfollowed by the passive film healing. Let us note that Riley, Wells and Williams [10],using some microelectrodes, also observed this form of events on a 304 type steelcontaining MnS, but at the scale of some pA. It is believed that this sort of eventscharacterise the breakdown of the passive film itself and cannot be found on largeelectrodes because they are hidden by the current generated by the numerous MnSlocal dissolution. Attention is now given to what happens on MnS-free steels inacidic chloride concentrated solution, where pits are found to initiate on thetitanium sulphides. One should notice that in some cases, particularly in some

172

~

30

:<20~

10

00 200 400 600 800 1000 1200 1400

Time (sec)

1.6 ~1.4

12

:<~0.8

0.6

0.4

02

0

24

20

16:<~ 12

4~ Lo-l-_._--+--_---<-__-<-_ _....-

o 200 400 600 800 1000 1200 1400Time (sec)

Figure 6 - Pre-pitting evellts during a polarisation at 200 mV SeE; [NaCl] = 0.02 M, pH =6.6.a: Typical anodic current variations for MnS-containing steels; b: Typical pre-pilling noise for MnS­containing steels after substracting the anodic current baseline; c: Typical anodic micro-events for

titanium bearing steels.

173

conditions near irreversible pitting, type I individual events are observed even onMnS-free steels.

parabolic

~

. ... _ 10 to IDOnA

aL-_=- J....-_-:J~ tb

~......L.... ~____.,~ t

Figure 7 - Forms of the individual anodic events. a: MnS-containing steel; b: MnS-free steel.

3.3. ANALYSIS OF THE PRE-PITTING NOISE FOR MnS-CONTAINING STEELS

A simple way for analysing the reduced signal corresponding to the pre-pitting noise(Figure 6b) is to build up a "distribution function" N(I) which counts, for a giventime period T (-8 min), the number of events for which the anodic intensity i is largerthan I. This function N decreases with 1 (Figure 8a). We see that the pre-pittingnoise increases with the electrode potential and the solution chloride content.Moreover, the solution pH acts in the same way as for the pitting potentials, sincea discontinuity of N between pH 4 and 5 is once more evidenced (Figure 8b). Thepre-pitting noise also decreases with the polarisation time (Figure 8c)Furthermore, the form of the individual events permits the analysis of the pre­

pitting noise using the method proposed by Gabrielli et aJ.[9]. These authors studiedsome series of birth and death events with a parabolic growth law between birth anddeath. The birth frequency and the lifetime 6 of the events are assumed to followa Poissonian law (birth frequency: A, death frequency: p,). This situation can bemodelled as follows:

with:

i(t)=E i,.(t),.

i,.(t)=K(t-t.i.H(t-t,.).[1-H(t-t,. -6,.))

(1)

(2)

where K is a constant and H the step function (H(x)= 0 when x<O and 1 whenx>O). Then, the average current is <i> =2KA/p,3 and the power spectral density

-- 0.005 M 200 mV

........ 0.02 M 200 mY

_._.- 0.02M 100mV

2,8

2,8

2,4

2,4

-- pH=3.0

- .. - .. pH=4.0

....... pH=5.2

---- pH=6.6

2

2

.... ::.-:::'":'-:.;._ .. - ..

174

140

120

100 \

\- 80 \

Z 60 \

\

40

20a

00 0,4 0,8 1,2 1,6

I (pA)

160

140

120

100 ,.~

~ '.\

Z 80.\ ,\ '..~

,60 .,

'.'40

..":' .......~...........

20 b00 0,4 0,8 1,2 1,6

I (pA)

80

-<>- pH=3.0

------- pH=6.6

604020

140

120

1 100

lIJ 80

! 60Z40

20 C

o ~--~--=~==::=;:::.!===:=;==---<o

Polarisation time (min)

Figure 8 - The distribution function N(l) for a MnS-containing steel (A') polarized at various polential(200 and 100 mV SCE) in some NaCl aqueous solutions. Measurements are made in the time

periode [tp,tP+ T] where T-8 min. a: Effect of electrode potential and solution chloride content; pH6.6, tp=16 min; b: Effect of the solution pH; [NaCl]: 0.02 M, 200 mV SCE, tp=8 min; c: Effect of

the polarisation time tp and of the solution pH on N(J=0.5 pA); [NaCl] 0.02 M, 200 mV SCE.

(PSD) of the reduced signal is:

<i>2 24n4",4 +18n~1J2+51J4wOO=-.81J2•_....:J:.....-._-"--'-----'-A (4n:J'2+1J2)3

175

(3)

where f is the frequency. Applying the Fast Fourier Transformation technique (FFT)to the measured signal i(t) allows to obtain the Power Spectral Density (PSD) andthe birth and death frequencies Aand p, by fitting this equation. The result evidencesa good agreement with the model (Figure 9). However, further work is needed toobtain some reliable values of Aand p" since they are believed to strongly dependon the value of. chosen for determining the i(t) baseline.

~

~ 10.1•I

~~

~ 10.16Q.

1O.a0L..---,-~~...........L..-_~ ..........~........._~--,-~.......L_~~~.........J

10.3 1O.a 10.1 100 101

Frequency (Hz)

Figure 9 - Typical PSD versus frequency diagram for an unitary acquisition period (512 s), f<2 Hz.The high frequency noise is due to the limited sampling (4 points per sec.). Polarisation at 200 mV

SeE; [NaCl] = 0.02 M; pH = 6.6; J=0.023 Hz; /-1-=0.43 Hz; PSD(O) =1.33 x 1(113 A2 S.

4. Aging effects

It was seen above that the current fluctuation density for MnS-containing steels wasa decreasing function with polarisation time. A first explanation is that the pittingsites having initiated an unstable pit become inactive after the pit repassivation,leading to a decrease of the available pitting sites and then of the further pit

176

generation rate. However, it was observed that aging potentiostatically a MnS-freesteel decreases the number of pre-pitting events as well. Since in this case the pre­pitting events are very rare, the explanation presented above does not hold. It isbelieved that aging rather increases the resistance of the passive film as the followingresults also suggest.Figure 10 shows the effect of a pre-polarisation treatment (1 hour in the test

solution at various potentials VPOl) on the pitting resistance. One can see that suchpre-polarisation increases the pitting potential (dVpi/dVpol-O.S), irrespective of thetype of sulphides present in the steel. The main conclusion of this experiment is thatpre-polarising a sample in the corrosive solution under conditions where pitting doesnot occur improve its corrosion resistance, which is clearly related with a passivityreinforcement. As there is no evidence of passivation of the inclusions (pitting sites),it is clear that the passive film also plays a role in the pitting initiation.

• B (pH 6.6)

500• A (pH 6.6)

••

~ ./

~ •>' 400•

•

300 -'---+------4------1------+------'-400 -200 0

V pol (mV/SeE)

200

Figure 10 - Effect ofpolarising the samples for 1 h at various potentials Vpel on the pitting potentialVpir [NaCl] = 0.02 M; pH = 6.6.

However, increasing the pre-polarisation time from 1 to 16 hours reveals that theeffect of the aging time is not the same for the two type of steels (Figure 11). A 16h potentiostatic aging is more beneficial for MnS-free steels than for MnS-containingones. It is believed that the dissolution of sulphur species counteracts in the lattercase the beneficial effect of the passive film reinforcement. Furthermore, in the firstcase (MnS-free steel), the pitting probability law uJ(V) exhibits a bimodal behaviour,with a first set of pits initiated at "low" potential (type 1) and a secund one (type 2)at very high potential (> 1.1 V SeE). A detailed examination was carried out with

177

diamond polished samples, in order to characterise the corresponding initiation sites.The anodic current versus potential curves were recorded in the two cases and thepotentiokinetic scan stopped at 50 p,A before making SEM examination. It wasfound that high potential "pitting" does not correspond to a sharp current increaseand cannot be classified as "classical" pitting. The sample examination shows (Figure13) that these "pits" initiate at the center of Ti nitrides, which are known to beunstable in oxidative or very anodic conditions. More precisely, pits initiate on themicro-holes which are produced by the mechanical polishing by removing the oxidespresent in the core of the titanium nitrides. Excluding this artefact, a 16 h pre­polarisation at 200 mV SCE in the corrosive medium itself seems to inhibit anyfurther pitting initiation (at least when characterised by the potentiokinetic methodwhich was used in this study). Such a result strongly motivates fundamental studiesof the relationship between passive film modifications occurring during the ­polarisation and pitting initiation mechanisms [6].Figure 12 shows the effect of aging the samples for 24 h at rest potential before

the pitting potential measurement in NaCl(O.02 M) at pH 6.6. The rest potentialevolution during this aging was recorded and found to be similar for MnS-containingand MnS-free steels (from nearly -330 mV SCE at time zero to nearly -150 mV after24 h). This rest/potential aging do not produce noticeable pre-pitting events (whichwould produce some rest potential fluctuations), at least in these experimentalconditions. However it slightly increases the pitting potential, a little bit more for theMnS-free steels than for the MnS-containing ones. This improvement should bedirectly related to reinforcement of the passive film.Finally, it is shown that for the 24 h aged MnS-free steels, the elementary pitting

probability ~(V) does not vary exponentially with V any more, such as for unagedsteels or aged steels; this reveals possibly a passive film modification which cannotbe observed on the MnS-containing steel due to the preponderant effect of the localMnS dissolution. Such non-exponential ~(V) variations have already been observedon steels containing MnS when the scan rate is lowered under a threshold value,suggesting that for slow scanning the passive film can be modified during thepotentiokinetic test. In a previous report [4], it was noticed that in potentiostaticconditions, the pit generation rate (PGR) g=d~/dt was not only an exponentialfunction of the electrode potential but also a decreasing function of time, what onemodels by g(V) = const.exp(V/cI».exp(-t/'t), where cI> is a characteristic potentialdifference and 't(V) is a characteristic time. Assuming, for the sake of simplicity, that't does not vary with V in the appropriate potential range, and considering apotentiokinetic scan with scan rate s, one may integrate g = sd~/dV with respect todV, which gives:

178

(1) For s>cll!t:

(2) For s<~/t:

exp(1--.t)Vm=const.-~-. S't

1--.t ~S't

((-.t-1)V]]

m=const. ~~-1 . 1-ex~ S't~

S't

(4)

(5)

In the first case, the EPt> varies exponentially with V. In the secund case, the w(V)variations shown on Figure 14 are well described. Furthermore, this model explainsthe existence of a critical scanning rate ~/t, as evidenced in reference [4].

5. Discussion and concluding remarks

5.1. TIlE RESPECTIVE EFFECTS OF TIlE INCLUSIONS AND OF THE PASSIVEFILM

It is now well accepted that, for industrial steels, pits initiate on non metallicinclusions. On the other hand, more sophisticated theories of pits initiation deal withthe passive film breakdown mechanisms, with no reference to such metallurgicalheterogeneities. The question of the relevance of such theories for industrial steelsis then debatable.Manganese sulphides are known to be active pitting sites due to their poor

stability in chloride-containing media at anodic potentials. Titanium additionsincrease the pitting resistance due to the better stability of titanium sulphides. Thepresent study shows that these sulphides can also act as pitting sites for high chloridecontents. When pits initiate on sulphide inclusions, a critical pH is then observedbelow which the pitting potential sharply decreases. This critical pH should berelated the reaction kinetics of the sulphur species, irrespectively of the nature ofthe dissolving sulphides (MnS or TizS). The dissolution of sulphides producessulphur bearing species which can interact with the surrounding passive film,favouring its local breakdown when the electrode potential is high enough.Even when the non metallic inclusions play the major role in the pits initiation

mechanism, the effect of the surrounding passive film properties cannot be ruledout. Aging the tested samples at rest potential in the corrosive solution, or atconstant potential but below the pitting potential, clearly increases the pitting

179

100 0 •0 0 •0 0 •0 0 •0 ° •0 0 •C'i 1 0

°0a 0 ° •0 ° ..

~ 0 ° •0 ° •

~ 0 ° Type 10 •~0

~., ..•

r.::I 0,1 0 ° •0

0 •0

o A': t =15min • B': t =15 minp p

°A':t=16h • B': t =16h

0,Q10 500 1000 1500

V (mV/SCE)

Figure 11 - Effect of a 16 h ageing at constant potential (steel A') 150mV, steel B' 200 mv.· on theelementary pitting probabilities G1(V). [NaCl] = 0.02 M; pH = 6.6.

00 • •Do • •0'0 • •D° • • •og • •0 • .' .'Do ; ••00 • •D° • •Do0 • ••8 0 • •0° •

'0 • •0

0 • • 0 A': t A= 15 min0 ° A': t = 24hA• B': tA= 15 min• B':t=24h

10

C'i1

a~

~~ 0,1r.::I

0,Q1o 500

V (mV/SCE)1000 1500

Figure 12 - Effect of rest potential ageing on the elementary pitting probabilities G1(V).[NaCI] = 0.02 M; pH = 6.6.

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resistance. The contribution of the passive .film should be studied under conditionswhere sulphides do not act as pitting sites.

5.2. TIIE VARIOUS STAGES OF PITTING INITIATION

It should be noted that the scale at which the pits "embryos" are observed, usingeither electrochemical experiments or electronic microscopy, is much larger that thepassive film thickness. The latter (few nanometres) is referred to as "microscopic"and the former (few micrometers) as "mesoscopic". Figure 14 shows the situation atthe near ultimate stage (size scale - some ten micrometers), just before the pitpropagates irreversibly: the thin metallic cap above the hollow produced by the pitinitiation (with no visible residual inclusion) can still break down and the pitrepassivate! Many elementary steps can intervene between the microscopic and themesoscopic scale, and at each of them repassivation may occur. The current modelsin the field are clearly related to the microscopic steps, which are never observedexperimentally. Indeed, the pit generation rate g, is the product of the microscopicnucleation rate A by the pit survival probability (l-p), p being the repassivationprobability, which includes the effect of all repassivation phenomena, acting from themicroscopic to the mesoscopic scale. The microscopic nucleation rate A is relatedto the potential drop at the film solution interface (namely the Helmoltz potentialdrop VH) by 14]:

(6)

where Z is the valency of the dissolving metallic cations, q the electron charge, k theBoltzmann constant, T the Kelvin temperature and a is the transfer coefficient. ForZ = 3 and a = 0.5, one finds ell = kT/Zaq = 17 mV at room temperature. Theconstant (const) depends on the solution chloride concentration. Following this idea,rising the electrode potential induces pitting when the potential increase isconcentrated in the Helmholtz double layer, which occurs when the Fermi Level inthe passive film is pinned on the electronic surface states at the film/solutioninterface. One has then VH=V-Vo , where Vo is the potential drop throughout thepassive film for the Fermi level pinning to occur. Vo is directly related to the pittingpotential, at least for a given chloride concentration. The effect of sulphur speciesadsorption or electrosorption could be to increase either the density or the energylevel of the electronic surface states, resulting in a Vo decrease, therefore also apitting potentia] decrease. Finally, assuming that Vo increases with the aging time t(following dVoIdt = eIl/.) leads to A=const.exp(V/eil-t/.). For a constant repassivationprobability p, it gives the pit generation rate and the elementary pitting probabilitylaws presented above.However the repassivation probability, Le. the probability for a pit to repassivate,

181

Figure 13 - "Pitting" at titanium nitride in very anodic conditions.

Figure 14 - Typical view of a semi-developed pit. The inclusion responsible for the pit initiation hasdisappeared A thin metallic film is still present and covers a part of the pit. Secondary pits are visible

all around the main hole. The pit may either continue to grow or repassivate if the thin "top" filmbreaks down.

182

either at the microscopic scale (passive film healing) or at the mesoscopic one(passivation of an unstable pit, in which active dissolution previously occurred) is nota constant. It is believed to depend on the metal composition and on the dissolutionkinetics at a depassivated point of the surface. Some alloying elements (as nickel in304 steels) which do not enter the passive film in a significant amount but are knownto slow down the dissolution rate in active conditions, may increase the repassivationrate, which could explain their beneficial effect on the pi tting resistance.Finally, one should notice that the observation of the pre-pitting events, and the

measurement of their birth and death frequencies, provide some information on therepassivation phenomena at the mesoscopic stage. The type of the current transientsform (type I or II) is clearly the signature, not only of the presence or the absenceof dissolving sulphides, but of the repassivation phenomena which occur at this sizescale as well.

References

1. Z. Szklarska-Smialowska, Pitting Corrosion o/metals, NACE, (1986), Houston, Texas,USA

2. J. L Crolet et aI., Mem. Sci. Rev. Met. (France), Nov. 1977, 647.

3. A. Szummer and M. Janik-Czachor, Br. Corr.1., Vol. 9, (1974), 216.

4. B. Baroux, Corr. Sci., Vol. 28, (1988), 969.

5. G.S. Eklund, J. Electrochem. Soc., Vol. 121, (1974), 467.

6. J.P. Petit, L. Antoni, B.Baroux: Proceedings of the European symposium on themodification of passive fIlm, Paris, 1993, in press.

7. D. Lizzarazu: Diplome d'Etudes Approfondies d'Electrochimie, 1992, Institut NationalPolytechnique de Grenoble.

8. J. Stewart and D. E. Williams: Advances in Localized Corrosion, NACE, Orlando, June1987. Proceedings, p13I.

9. G. Gabrielli, F. Huet, M. Keddam, R. Oltra, Corrosion, Vol. 46, (1990), 266.

10. A. M. Riley, D. B. Wells and D. E. Williams, Corr. Sci., Vol. 32, (1991), 1307.