PASSIVITY OF HIGH-NITROGEN STAINLESS ALLOYS - THE ROLE OF METAL OXYANIONS AND SALT FILMS

Surface-analytical studies of high-nitrogen austenitic stainless steels exposed to de-aerated 0.1 M HCl have revealed that nitrogen alloying additions influence the composition of salt layers and the passive film/alloy interface. It was shown that nitrogen, nickel and molybdenum additions stimulate selective dissolution of iron, resulting in a significant enrichment of chromium beneath the passive film. The build-up of a protective ferrous molybdate layer was seen to be most strongly enhanced with additions of nickel and, to a lesser extent, nitrogen. While the primary kinetic barrier to anodic dissolution of high-nitrogen stainless steels is a chromium-oxide-based passive film, it appears that a mixed nitride surface layer and an ultra-thin layer of ferrous molybdate act as secondary kinetic barriers.