Stainless steels passivity breakdown

K. OsozawaandN. Okato, Effect of Alloying Elements, Especially Nitrogen, on Initiation of Pitting in Stainless Steel, Passivity and Its Breakdown in Iron and Iron-Base Alloys, R.W. Staehle and H. Okada, Ed., National Association of Corrosion Engineers, 1976, p 135-139... 

Metals Affected. Resistance to crevice corrosion can vary from one alloy-environment system to another. Although crevice corrosion affects both active and passive metals, the attack is often more severe for passive alloys, particularly those in the stainless steel group. Breakdown of the passive film within a restricted geometry leads to rapid metal loss and penetration of the metal in that area. 

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

Short-time tests also can give misleading results on alloys that form passive films, such as stainless steels. With Borderline conditions, a prolonged test may be needed to permit breakdown of the passive film and subsequently more rapid attack. Consequently, tests run for long periods are considerably more reahstic than those conducted for short durations. This statement must be quahfied by stating that corrosion should not proceed to the point at which the original specimen size or the exposed area is drastic y reduced or the metal is perforated. 

The nature of the reference electrode used depends largely on the accuracy required of the potential measurement. In the case of breakdown of passivity of stainless steels the absolute value of potential is of little interest. The requirement is to detect a change of at least 200 mV as the steel changes from... 

Figure 13 shows the relationship between the time interval At of passive film breakdown of stainless steel with chloride ions and the logarithms of cumulative probability P(Af) for breakdown at time intervals longer than At. From these results, it is clear that the logarithm of the probability is almost proportional to the time interval, and therefore the cumulative probability for film breakdown follows Poisson s distribution, i.e., the following equation is obtained,... 

XPS was also used for the determination of chlorine in the passive film grown in chlorine containing electrolytes. While chlorine was found in the passive film on pure iron, it was absent for chromium rich stainless steel samples. Chloride content of the passive film is substantially time dependent, increasing with time until film breakdown occurs, and decreasing subsequently [109]. 

Breakdown of passivation and pitting. The local breakdown of passivity of metals, such as stainless steels, nickel, or aluminum, occurs preferentially at sites of local heterogeneities, such as inclusions, second-phase precipitates, or even dislocations. The size, shape, distribution, as well as the chemical or electrochemical dissolution behavior (active or inactive) of these heterogeneities in a given environment, determine to a large extent whether pit initiation is followed either by repassivation (metastable pitting) or stable pit growth.27... 

The processes that we invoked in the past [65] to explain passivity breakdown that is induced by an aggressive anion (e.g., Cl ) are shown in Fig. 33. However, passivity breakdown also occurs in environments that are free of aggressive anions, with notable examples being passivity breakdown on steels and stainless steels in high-temperature pure water. Experimentally, it is found that breakdown occurs only when the corrosion potential is more positive than some critical value. Accordingly, it is reasonable to invoke a charge-transfer reaction as the process that generates cation... 

Reduction in the corrosion resistance of the 316L stainless steel is attributed to the destruction of the passive film in the presence of chloride. The roughest surface of 200 grit showed early passivity breakdown at the highest rate of corrosion and lower breakdown potential compared with the rest of the surfaces. 

Passivity breakdown appears to occur preferentially at local heterogeneities, such as inclusions, grain boundaries, dislocations, and flaws on the passive metal surface. In the case of stainless steels, the passivity breakdown and pit initiation occur almost exclusively at sites of MnS inclusions, and the pitting potential was observed to decrease linearly with the increasing size of MnS inclusions [44]. With metals containing no apparent defects, however, passivity breakdown is likely to occur in the presence of sufficient concentrations of film breaking ions. It is worth noting that any of the localized phenomena is nondeterministic but somehow stochastic. For stainless steels in chloride solution, the passivity breakdown was found to obey a stochastic distribution [45]. 

B.E. Wild, Chloride ion adsorption and pit initiation on stainless steels in neutral media, in R. W. Staehle, H. Okada (Eds.), Passivity and its Breakdown in Iron Based Alloys, NACE, Houston, 1976, pp. 129-130. 

B.E. Wilde, E. Williams, The use of current/voltage curves for the study of localized corrosion and passivity breakdown on stainless steel sin chloride Media, Electrochim. Acta 16 (1971) 1971-1985. 

Some SRB may cause loealized corrosion on stainless steels, nickel alloys, aluminium, zinc and copper alloys. Mechanisms of sulphur-assisted corrosion, with emphasis on Fe- and Ni-based materials, have recently been reviewed by Marcus [6.17]. The review includes the fundamentals of enhanced dissolution, retarding or blocking of passivation, and passivity breakdown. 

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