Enhanced passivity of austenitic AISI 304 stainless steel by low-temperature ion nitriding

Low-temperature ion nitriding introduces interstitial nitrogen into the austenitic stainless steel matrix. An enhanced passivity of the nitrided stainless steel was detected by anodic potentiodynamic polarization in a 0.1 M H2SO4 + 0.05 M HCl electrolyte. Capacitance measurements by electrochemical impedance spectroscopy revealed a 3-4 nm thick oxide film on the surface of nitrided specimens that is two to three times thicker than on blank specimens. X-ray photoelectron spectroscopy analyses and Auger depth profiles indicate increased chromium uptake into the oxide film on the nitrided surface, supposedly through the kinetically stable oxide-metal interface enriched with nitrogen and nickel. Several possible mechanisms that influence passivation of the nitrided stainless steel may operate simultaneously. Among these mechanisms, austenite strengthening by interstitial nitrogen near the oxide/alloy interface associated with structural defects like dislocation branches and vacancies are most likely the explanation for the enhanced passivity of the nitrided stainless steel. (C) 2001 American Vacuum Society.