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-2 0% Ni-6%, Mo, and Fe-20% Cr-20% Ni-6% Mo-0.2% N in deaerated 0.1 M hyd rochloric acid (HCl) + 0.4 M sodium chloride (NaCl) was Investigated u sing electrochemical and x-ray photoelectron spectroscopic (XPS) analy ses, Electrochemical analyses showed molybdenum and nitrogen improved passivation characteristics through an apparent synergism. Evidence wa s found of a compositional reorganization of SS in the atomic layers o f the alloy immediately below the passive film. Nickel and molybdenum appeared to become enriched in proportions that suggested molybdenum-n ickel intermetallic bonding. This was augmented by alloyed nitrogen, w hich strongly governed the elemental enrichment process. The possible nature of the bonding of these elements was reviewed with respect to t he Engel-Brewer model of intermetallic bonding. Variable-angle XPS and electrochemical polarization analysts in deaerated 0.1 M HCl was perf ormed on a MoNi4 radio frequency (RF) sputtered thin film that simulat ed the commonly observed composition of such a sublayer alloy for a ni trogen-bearing SS. To simulate nitrogen segregation, electrochemical d eposition of nitrogen was performed on thin films of MoNi4. Following polarization of nitrided and non-nitrided films, variable-angle XPS sh owed the alloy surface underwent further changes resulting from nickel dissolution. The end-point composition of the alloy in each case belo w the passive film corresponded closely with known stable and intermed iate intermetallic phases.