PITTING CORROSION OF STAINLESS STEELS: THE IMPORTANCE OF BEING A METALLURGIST B. BAROUX1,2, D.GORSE3, R.OLTRA 4 (1) LTPCM, INP GRENOBLE, FRANCE (2) ARCELOR, STAINLE SS STEEL DURABILITY RESEARCH MANAGER (3) CECM-CNRS VITRY, FRANCE (4) LRRS, UNIVERSITY OF DIJON, FRANCE In « Critical Factors in Localized Corrosion IV » Ed. S. Virtanen, P. Schmuki, and G. S. Frankel proceedings ECS 202nd meeting, Salt Lake City, 2002 . p335 et seq.

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PITTING CORROSION OF STAINLESS STEELS: THE IMPORTANCE OF BEING A METALLURGIST 1,2

3

B. BAROUX , D.GORSE , R.OLTRA

4

(1) LTPCM, INP GRENOBLE, FRANCE (2) ARCELOR, STAINLE SS STEEL DURABILITY RESEARCH MANAGER (3) CECM-CNRS VITRY, FRANCE (4) LRRS, UNIVERSITY OF DIJON, FRANCE

ABSTRACT It is intended in this paper to emphasize some metallurgical evide nces which have to be taken into account when attempting to model the effect of sulfide inclusions on pitting corrosion resistance of stainless steels. A recently published model assuming that pitting sensitivity is due to a Cr depletion around the sulfide is criticized as failing to meet those evidences. Then, Taking advantage of a lot of consistent results already published in the literature and of recent works, it is suggested that the role of the sulfides in pit initiation is simply to provide sulfur species (more or less easily following the sulfur stability), which can deposit on the passive film around the sulfide, resulting in a poorer resistance to pit nucleation.

INTRODUCTION It is obvious to say that pitting resistance of industrial stainless steels depends on their metallurgical properties

1,2

. The Chromium content plays of course a major role, but the

phenomena can be rather complex, depending on the pitting stage under consideration (initiation, propagation, repassivation). Other oxidisable elements, such as Silicon, which is present at the level of some tenth weight % in industrial AISI 430 or 304 type steels, can also enter the passive film and improve the pitting resistance. Some elements which do not enter the passive film may have detrimental or beneficial effects on the pitting resistance. The detrimental effect of Sulfur is well known and form the subject of a large part of the following comments. The beneficial effects of Ni in 304 or 316 steels, Mo in 434 or 316 steels are also well known but, despite the numerous hypotheses which have been proposed on the basis of academic works, the reasons of these effects are not so clear . Following the case, they might for instance (i) favour the pit repassivation by changing the solution composition in an initiated pit (for instance, by preventing a too large pH decrease inside the pit) or (iii) combine in the aqueous solution with these sulphur species, and then decrease their noxious effect.

3,4,5

The microstructure is a relevant factor as well. The steel generally contains some non metallic phases such as precipitates (Cr carbides in ferritic steels for instance) or inclusions (oxides, sulfides, etc.). The inclusions, which are formed during (or at the end of) the melting process, do not generally produce any significant segregation at their interface with the metallic matrix. This no longer the case for precipitates (the chromium carbides for instance), which are formed in the metal solid state. However, non metallic inclusions can be unstable in the corrosive 2

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medium, which is thinked of certain sulfides and even oxides in some cases . Furthermore, since their ductility is generally not the same that the metallic matrix one, hot and cold rolling may produce some micro-decohesions at the metal-inclusion interface, which behave as micro-crevices and may be some preferential pitting sites, depending mainly on the cold rolling ratio. Anyway, even if pitting was also observed on pure FeCr alloys 7 , it is now well established that, on industrial steels, it mainly initiates on non metallic inclusions (e.g. fig. 1,2 4).

Fig 1. Initiation sites observed in metastable pitting conditionson diamond polished FeCr specimen immersed in NaCl 0.02M aqueous solution pH 6.6 at 23°C and polarized 100mV below the pitting potential 8

. From left to right: (a) Pitting on a scratch on Pure Fe17Cr steel7 . (b) pitting around a sulfur free inclusion on an industriallike Fe17CrNb steel. (c) pitting around a complex eutectic inclusion on an industrial-like Fe17CrTi.

Fig 2. Effect of

(Fe,Zr) inclusions on pitting of industrial-like Fe17CrZr specimen. Several heats with

different Zr content were obtained from laboratory heats after forging, hot rolling, cold rolling to 1mm thick sheets then final annealing . δZr is the Zr excess available in the solidified alloy to form (Fe,Zr) intermetallic phases, after Carbides and nitrides formation (In these steels, Zr carbides and nitrides precipitate before the complete steel solidification).

This experiment illustrates the role of intermetallic (Fe,Zr) inclusions as

pitting sites. From left to right: (a)

Number of pits per area unit observed by SEM X3000 after 8 days immersion in aerated NaCl 0.02M aqueous solution. (b) Pitting potential measured vs SCE on mechanically polished specimen in deaerated NaCl 0.02M aqueous solution pH 6.6 at 23°C using a 100mV/minscan rate.

Among all the inclusions which are present in the steel, the sulfides and particularly the Mn Sulfides are undoubtly the more detrimental. For instance, in free machining steels (such as AISI

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303 type steel which contain 0.2% to 0.3% Sulphur which forms some numerous and large size Manganese sulfides), The Mn sulfides improve dramatically the steel machinability but also decrease dramatically its pitting resistance. Standard AISI 430 or 304 grades contain less than .03%S, which leads to less numerous and smaller sulfides. The grades used for long products (bars and wires) contain typically .02%S in order to insure a sufficient machinability; but at the opposite, the grades used for flat products (sheets and plates) generally contain less than 10 to 30 ppm S, showing a better pitting resistance. For AISI standards 430,434,304,316, and more generally for all Ti free stainless steels, Sulphur is combined with Mn to form manganese sulfides MnS, which act as pitting sites in several environments. Following the sulphur content of the steel and the metallurgical processing, MnS are found either isolated or stuck to other inclusions (generally oxides), or even combined in complex inclusions. Chromium may also substitute to Mn in sulfides , the Cr content of which depends on the Mn content of the steel. The higher the 9

chromium content in the sulfide , the higher the pitting resistance , at least when other metallurgical parameters are not modified by the Mn lowering. In Ti bearing steels (Ti > some 0.1% weight or more), the MnS formation is prevented, since Ti sulfides (Ti xS) are formed at higher temperature during the melting process. Ti sulfides are considered as less harmful than MnS for pitting corrosion, but are also able to act as pitting sites in some conditions

10, 9,11

.

Three basic characteristics of manganese sulfides are worthy to notice, as far as they can influence the pitting initiation process: (i) They are electronic conductors, but less than the surrounding metal, leading to pit initiation at the inclusion/matrix interface. Their conductivity should depend on their exact chemical composition. (ii) They are polarised together with the surrounding metal, but should be unstable in the potential range where the metal is passive, and then readily dissolve, providing some local chemical and electrochemical conditions which differ from the ones which prevail on other parts of the surface. (iii) Their size morphology, together with 12

their composition, play an important role on their ability to promote pitting . Rapid solidification, or laser surface melting

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provide the removal or at least the redistribution of the Mn sulfides,

resulting in a strong pitting resistance improvement. It should also be noticed that all the sulfides present at the steel surface do not promote pits initiation, what some authors explained by the differences existing in their composition or morphology (of the metal/sulfide interface). However the question is not so clear, since the number of MnS acting as pitting sites is much smaller than the total number of Mn sulfides at the steel surface. In our opinion, the competition between sulfide dissolution and local modifications in the passive film properties could result in a low probability stochastic initiation process, which might account for the small number of sulfides which are found to initiate pitting. Some authors underlined the importance of the pitting test procedure for the relation between the MnS dissolution 12

kinetics and the pitting sensitivity , which evidences once more the absence of any "intrinsic" pitting potential. Last, the recognition that manganese sulfides are probably the least resistant pit initiation sites in standard industrial stainless steels motivated a severe control of the steel microstructure, 4

either by lowering the Mn content (which has unfortunately also some undesirable secondary effects) or, better, by adding some small amounts of Ti as alloying elements, preventing the formation of Mn sulfides. However, this way of improving the pitting resistance is itself limited, since in the absence of MnS, pits can initiate on other inclusions, namely Ti sulfides, oxides, silicates, or possibly on the passive surface itself, as the electrode potential or the chloride concentration increase. The precise role of Mn sulfides in pits initiation has been extensively studied in the past (e.g. ref.

14,15,16

). The first idea to be proposed was that MnS dissolution provokes locally the

formation of a virgin metal surface. This micro-area is exposed to an acidified and sulphur species enriched environment produced by the sulfide dissolution. When the solution near from the microarea has reached a certain composition, the contacting metal can no longer repassivate and the metal starts to dissolve 15. Following other workers, the complete MnS dissolution is not needed, as the pits preferentially initiate at the metallic matrix/MnS interface. Sulfur containing species coming from sulfides dissolution are believed to be responsible for the detrimental effect of these inclusions. In the same way, the anti-repassivating effect of thiosulfate ions possibly have a determining effect17,18. These models keep however opened the questions of (i) the MnS dissolution mechanisms, (ii) the nature and the effect of the dissolved sulphur species which form during this dissolution and (iii) the role plaid by the passive film, either close to or possibly onto the inclusion.

FEW EXPERIMENTAL EVIDENCES The effect of Titanium additions

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In the following, the steels under investigation are AISI 430 type containing also either Nb (steel A) or Ti (steel B) additions which trap the carbon and avoid the formation of chromium carbides. Ti also trap the sulfur in the form of Ti sulfides which locate around Titanium nitrides, embedded in a Ti carbide belt.. In steel A, some Aluminium was added as deoxidising agent during the melting process, leading to Al~ 0.030% in the final product and to the presence of some Al2O3 inclusions in the microstructure. In this steel, Sulfur is trapped under the form of Manganese sulfides which are found either isolated, or closely stuck to Aluminium oxides or Nb carbonitrides.

Cr

Si

Mn

A' 15.7

.4

.45

B

.4

.45

16.8

Ti

.4

Nb

(S)

(C)

(N)

.7

50

340

340

30

260

140

Table 1. Steels composition in weight% or in ppm (brackets) The pitting potentials were measured for all these steels using the potentiokinetic method in NaCl aqueous media, the pH of which were varied from 3 to 6.6. The results for steels A and B

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in NaCl (0.02M) are shown on figure 3a. No pH dependence is observed for steel B. At the opposite, for steel A, a sharp pitting potential decrease is evidenced when the pH is lowered under a critical value pHc ranging between 4.5 an 5. Since the main difference between the 2 steels is the presence or absence of MnS, and that Ti sulfides are known to have a better stability in aqueous electrolytes than MnS, one can assume that this decrease is due to the pH assisted MnS dissolution.. The effect of the solution chloride content between 0.02M and 0.5M was also investigated. Fig. 3c shows that for high enough chloride concentrations, a pitting potential pH dependence is found even for Ti containing steels, suggesting that Ti sulfides are not so stable i n such electrolytes. The discontinuity of the pitting potential vs pH variations should then be related to the dissolution of sulphur species, which occurs easily for MnS containing steels (whatever the steel matrix composition) but only for high enough chloride contents for Ti bearing steels. Let us note that the critical pH which is found (4.5