Impedance parameters of nitrided 304L stainless steel in an acidified sulphate solution

Electrochemical measurements were performed on 00Cr18Ni10 (AISI 304L) stainless steel before and after plasma nitriding in a 80% N2+20% H2 gas mixture at 400 or 425 °C, resulting in the formation of a supersaturated solid solution of nitrogen in austenite (S-phase), and at 550 °C giving nitride precipitates in the diffusion zone. Polarisation curves and impedance measurements showed that in a solution of 0.1 M Na2SO4 acidified to pH 3.0, the anodic reactivity of the materials increased in the sequence: unnitrided<nitrided at 400 °C<nitrided at 550 °C. Impedance data were fitted to the equivalent circuit R0(Q1[R1(Q2R2)] in which Q1 and R1 were assigned to the constant phase element (CPE) and the electrolyte resistance in the pores of the surface film, respectively. The latter parameters suggested that the porosity of the film on nitrided steel was higher than that on the untreated steel. The anodic behaviour of the nitrided material was explained by structural factors, such as distortion of the S-phase or chromium depletion, and also by a possible formation of NH4+, NO2− or NO3− from the nitrogen introduced.