Pitting of stainless steels

Unfortunately, there is no general theory that will explain all the forms of localised attack that occur with the variety of metal/environment systems encountered in practice, e.g. the mechanism of the pitting of stainless steels in Cl -containing solutions is quite different from the dezincification of brass in a fresh natural water. Nevertheless, many of the following factors play an important part in most forms of localised attack ... 

Freiman, L. 1. and Kolotyrkin, Ya. M., Pitting Corrosion of Aluminium in Solutions of Sodium Perchlorate and Perchloric Acid , Zashch. Melal, 2, 488 (1966) C.A., 65, 19674d Novakovskii, V. M. and Sorokina, A. N., Comparative Electrochemistry of Stress Corrosion and Pitting of Stainless Steels in Chloride Solutions , Zashch. Melal, 2, 416 (1966) C.A., 65, 18152g... 

Pitting of stainless steels can usually be avoided by correct specification of steel type, and type 316 is the normal choice where pitting is at all likely. 

Potential-time relationships have been widely used for studying film formation and film breakdown, as indicated by an increase or decrease in the corrosion potential, respectively. May studied the corrosion of 70/30 brass and aluminium brass in sea-water and showed how scratching the surface resulted in a sudden fall in potential to a more negative value followed by a rapid rise due to re-formation of the film conversely, the pitting of stainless steel in chemical plant may be detected by a sudden decrease in potential... 

G.T. Burstein and P C. Pistorius, Surface Roughness and the Metastable Pitting of Stainless Steel in Chloride Solutions, Corrosion, Vol 51, 1995, p 380-385... 

The same phenomenon was also observed by Beck and Chan [112] in their investigation of the pitting of stainless steel in a chloride aqueous medium. Analysis in terms of the theory outlined in Sect. 6.3.1.(b) predicted that the critical velocity for repassivation varied as r 4/3. 

G.S. Frankel, L. Stockert, F. Hunkeler, H. Boehni, Metastable pitting of stainless steel. Corrosion 43 (1987) 429-436. 

R.C. Newman, W.P. Wong, H. Ezuber, A. Gamer, Pitting of stainless steels by thiosulfate ions. Corrosion 45 (1989) 282-287. 

Bemhardsson, S., Mellstrdm, R., and Brox, B., Limiting Chloride Contents and Temperatures with Regtird to Pitting of Stainless Steels." Paper 85, NACE CORROSION/80 Conference, Anaheim, CA, 1980. 

Pitting of stainless steel (304 SS) was investigated by means of a SVET in combination with potential monitoring by Isaacs (1989). The in situ measurement of localized currents made it possible to follow the initiation, propagation, and repassivation of pits with a time resolution of a few minutes. Since the initiation of a pit leads to a decrease in the open circuit potential (OCP), while repassivation of a pit leads to an increase in the OCP, the potential transients proved to be a suitable complimentary method to monitor the activity of the sample. The behavior of stainless steel (in ferric chloride solution) was compared with that of iron (in a dilute chloride and sulfate solution). In the case of iron, it was observed that once started pits did not repassivate and finally spread due to a depassivation of the area next to the pit. 

Garner. A., Newmann, R. C. (1991), Thiosulfate pitting of stainless steels, NACE Corrosion/91, Paper No. 186. 

Pitting of Stainless Steels in Oxygen Saturated NaCI Solutions—90 C During 30 Day Exposure ... 

Sulfate-reducing bacteria have also been reported to be responsible for pitting corrosion on stainless steels in aqueous environments. The mechanisms proposed are mostly related to iron and steel. However, a different mechanism has been proposed in which the role of thiosulfate in the microbial pitting of stainless steel has been emphasized [137]. The same authors have also demonstrated clearly that SRB-induced pitting corrosion of stainless steel is unlikely to occur in a uniformly anaerobic SRB medium, whereas it will occur when the anaerobic sites are coupled to an oxygen cathode [136]. 

In contrast to the situation for aluminum, electrochemical studies on the pitting of stainless steels are fi aught with contradictions because of the effect of experimental factors. Several investigations have used breakthrough potential or breakdown potential, as a measure of pitting susceptibility. As pointed out earlier, the current increases rapidly at the breakdown potential (Fig. 4.24), this is considered as an indication of a breakdown of the passive, corrosion-resistant steel film and initiation of pitting. 

Impurities such as chloride gradually build up in amine systems until a steady-state concentration is reached. Since most amine systems contain some stainless steel, it is of interest to know what chloride levels can cause pitting of stainless steels in an amine environmenL Limited information is available in the literature. Experiments reported by Seubert and Wallace (1985) indicate little or no pitting tendencies with 304 SS exposed to DGA solutions containing up to 4,000 ppm chloride. Based on these experiments, the maximum acceptable chloride level for DGA plants containing type 304 SS was set at 1,000 ppm. 

On the other hand and in relation with the role of manganese sulphide in pitting of stainless steels already mentioned, R.C Al-kire, performed SECM experiments to demonstrate the role of adsorbed sulphur species near sttlphide inclusions like a driving force for pitting of Nickel. 

Chromium is obviously the major element which controls pitting of stainless steels, but it is not the one. Other alloying elements (such as Ni, Mo, and Cu) or impurities (such as S) are known to play important roles, but these roles are often complex and not always fully... 

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