Abstract
The influence of Mo and N on the passivation of Ni-bearing austenitic stainless steels Fe20Cr20Ni, Fe20Cr20Ni6Mo and Fe20Cr20Ni6Mo0.2N in deaerated 0.1M HCl + 0.4M NaCl was investigated with the aid of electrochemical and X-ray Photoelectron Spectroscopic (XPS) analysis. Electrochemical analyses showed that Mo and N improved passivation characteristics through an apparent synergism. Evidence was found of a compositional reorganization of stainless steels in the atomic layers of the alloy immediately below the passive film. Ni and Mo appear to become enriched in proportions which suggest Mo-Ni intermetallic bonding. This is augmented by alloyed N, which strongly governs the elemental enrichment process. The possible nature of the bonding of these elements is discussed with respect to the Engel-Brewer model of intermetallic bonding. Variable angle XPS and electrochemical polarization analysis in deaerated 0.1M HCl was then performed on a M0Nİ4 RF sputtered thin film which simulates the commonly observed composition of such a sub-layer alloy for a N-bearing stainless steel. In order to simulate N segregation, electrochemical deposition of nitrogen was performed on thin films of M0Ni4. Following polarization of nitrided and non-nitrided films, it was found by variable angle XPS that the alloy surface underwent further changes due to Ni dissolution. The end point composition of the alloy in each case below the passive film corresponded closely with known stable and intermediate intermetallic phases.
