Austenitic stainless steels passivity pitting resistance

One of the most effective elements added to austenitic stainless steel, and for that matter even ferritic stainless steel, in order to improve pitting resistance is Mo [8]. Molybdenum, however, is a highly versatile element, existing in the passive film in a number of oxidation states. In the case of the hexavalent state it has been observed in both the cationic and anionic states, namely as molybdenum trioxide and ferrous molybdate. It has most commonly been reported to exist in the quadrivalent state as molybdenum dioxide and oxyhydroxide. 

Pitting corrosion Austenitic staiidess steels owe their corrosion resistance to the formation, from the chrome (Cr) content of the steel, of a passive layer of chromium oxide on the exposed surface. Pitting occurs when the protective oxide film breaks down in small, isolated spots. The rate of attack tends to increase because of the differences in electric potential between the large surrounding passive surface and the active pit. This action is accentuated by the presence of saline solutions. A smooth surface, free of sensitive local minute pits or small depressions, reduces the potential for pitting to commence. The most appropriate quality of stainless for such duties should be selected. 

Stainless steels contain enough chromium to confer corrosion resistance by passivity. The passive layer is not as robust as in the case of titanium or the cobalt chrome alloys. Only the most corrosion resistant of the stainless steels are suitable for implants. These are the austenitic types — 316,316L, and 317, which contain molybdenum. Even these types of stainless steel are vulnerable to pitting and to crevice corrosion around screws. 

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