ON THE QUESTION OF NITRATE FORMATION BY N-CONTAINING AUSTENITIC STAINLESS-STEELS

Since nitrates and nitrites are powerful corrosion inhibitors, the pos sibility that alloyed nitrogen may be capable, at suitable anodic pote ntials, 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 cons idering the stability of NO3- ions on the surface of pure Cr, polarize d 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 t hat reduction of NO3- occurred at all potentials studied, contrary to conventional interpretations using the Pourbaix diagram for the N-2-H2 O system. Specifically, nitride was found to be formed on Cr under con ditions leading to the exposure of the metal surface to the electrolyt e. This resulted in strong evidence that nitrogen alloyed with austeni tic stainless steels does not oxidize to NO3-. A mixed surface nitride was formed on the surfaces of four types of austenitic stainless stee l 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-r ay photoelectron spectrum with that formed by N-containing austenitic stainless steels. The anodically inhibitive role of the surface nitrid e formed by the reduction of NO3- ions was also found to be identical with the role of N alloy additions to austenitic stainless steels. Thi s work contributes new insights into the role of alloyed N in the corr osion behavior of austenitic stainless steels and also offers a new mo del for understanding the mode of operation of nitrates as corrosion i nhibitors in ferrous alloys.