Abstract
Surface analytical studies of high nitrogen austenitic stainless steels exposed to deaerated 0. 1M HCl have revealed that nitrogen alloying additions influence the composition of salt layers and the passive film/alloy interface. In this study we employ electrochemical techniques and variable angle X-ray Photoelectron Spectroscopy (XPS) to examine the passive films formed on a series of austenitic stainless steels, Fe18Cr8Ni, Fe18Cr8Ni0.2N, Fe20Cr20Ni, Fe20Cr20Ni6Mo and Fe20Cr20Ni6Mo0.2N, in acidic chloride aqueous solution. In addition, several other model alloys, Fe19Cr, Fe19Cr9Ni, Fe19Cr2.5Mo, and Fe19Cr9Ni2.5Mo, were examined before and after electrochemical surface nitriding, a technique proven to have an effect analogous to N alloying. It was shown that nitrogen, nickel and molybdenum additions independently and in certain combinations stimulate selective dissolution of iron, resulting in a significant enrichment of chromium beneath the passive film. In most cases, the formation of ferrous molybdate in the outer layers of the protective passive layer formed on stainless steels was seen to be most strongly enhanced with the addition of nickel. This was seen to be true to a lesser extent for additions of nitrogen to Ni and Mo containing alloys due to formation of an efficient mixed nitride alloy surface barrier to active dissolution, limiting the availability of ferrous ions and molybdate for salt layer formation. 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.
