Influence of nitrogen on electrochemical passivation of high-nickel stainless steels and thin molybdenum-nickel films

The influence of molybdenum and nitrogen on passivation of the nickel-bearing austenitic stainless steels (SS) Fe-20% Cr-20% Ni, Fe-20% Cr-20% Ni-6%, Mo, and Fe-20% Cr-20% Ni-6% Mo-0.2% N in deaerated 0.1 M hydrochloric acid (HCl) + 0.4 M sodium chloride (NaCl) was Investigated using electrochemical and x-ray photoelectron spectroscopic (XPS) analyses, Electrochemical analyses showed molybdenum and nitrogen improved passivation characteristics through an apparent synergism. Evidence was found of a compositional reorganization of SS in the atomic layers of the alloy immediately below the passive film. Nickel and molybdenum appeared to become enriched in proportions that suggested molybdenum-nickel intermetallic bonding. This was augmented by alloyed nitrogen, which strongly governed the elemental enrichment process. The possible nature of the bonding of these elements was reviewed with respect to the Engel-Brewer model of intermetallic bonding. Variable-angle XPS and electrochemical polarization analysts in deaerated 0.1 M HCl was performed on a MoNi4 radio frequency (RF) sputtered thin film that simulated the commonly observed composition of such a sublayer alloy for a nitrogen-bearing SS. To simulate nitrogen segregation, electrochemical deposition of nitrogen was performed on thin films of MoNi4. Following polarization of nitrided and non-nitrided films, variable-angle XPS showed the alloy surface underwent further changes resulting from nickel dissolution. The end-point composition of the alloy in each case below the passive film corresponded closely with known stable and intermediate intermetallic phases.