Synergism of alloying elements and pitting corrosion resistance of stainless steels

The anodic behaviour of the major pure metals of which stainless steels are composed has been studied in chloride-containing aqueous solutions at neutral and acidic pH values, both before and after a nitriding treatment. This suggests a mechanism by which molybdenum and nitrogen favourably influence the pitting resistance of austenitic stainless steels, and that the mechanisms are different at different stages of pit formation. It is shown that nitrogen is able to prevent the transpassive dissolution of molybdenum from the passive film of austenitic stainless steels. X-Ray photoelectron spectroscopy has shown also that the NH₃ ligand is formed in the anodically formed films on nitrided molybdenum as well as in passive films formed on nitrogen-bearing stainless steels. This raises the local pH in the film/substrate interface which leads the electrode reactions to favour the forming of molybdate, a beneficial species which enhances the passivity of stainless steels, instead of forming MoO₃, the loose transpassive product.