On the question of nitrate formation by N-containing austenitic stainless steels

Since nitrates and nitrites are powerful corrosion inhibitors, the possibility that alloyed nitrogen may be capable, at suitable anodic potentials, of oxidizing to either of these species was considered. While no spectral evidence has been found of NO₃^- formation by N-containing stainless steels, it was decided to clarify this issue further by considering the stability of NO₃^- ions on the surface of pure Cr, polarized over a wide range of potentials from -930 to 1300 mV with respect to a saturated calomel electrode (SCE) in 0.1 M HCl + 0.5 M NaNO₃ (pH 1.1). Using X-ray photoelectron spectroscopy analysis, it was observed that reduction of NO₃^- occurred at all potentials studied, contrary to conventional interpretations using the Pourbaix diagram for the N₂-H₂O system. Specifically, nitride was found to be formed on Cr under conditions leading to the exposure of the metal surface to the electrolyte. This resulted in strong evidence that nitrogen alloyed with austenitic stainless steels does not oxidize to NO₃^-. A mixed surface nitride was formed on the surfaces of four types of austenitic stainless steel by reducing NO₃^- ions at the cathodic potential -700 mV (SCE) in 0.1 M HCl + 0.5 M NaNO₃ solution. This nitride exhibited an identical X-ray photoelectron spectrum with that formed by N-containing austenitic stainless steels. The anodically inhibitive role of the surface nitride formed by the reduction of NO₃^- ions was also found to be identical with the role of N alloy additions to austenitic stainless steels. This work contributes new insights into the role of alloyed N in the corrosion behavior of austenitic stainless steels and also offers a new model for understanding the mode of operation of nitrates as corrosion inhibitors in ferrous alloys.