Influence of different layer microstructures induced by different gas compositions on corrosion behavior of plasma nitrided stainless steel

AISI 316 austenitic stainless steels have been plasma nitrided using a dc glow discharge unit in order to investigate the influence of gas composition on microstructure and corrosion behavior of treated samples. Corrosion properties of untreated and plasma nitrided 316 steels have been evaluated using anodic polarization tests in 1 N H2SO4 solution. Qualitative evaluation was carried out using surface analyses such as SEM, EDX, XRD and GDS before and after the corrosion tests. The results showed that the untreated sample suffered localized pitting corrosion under the testing conditions. Nitriding in different gas compositions resulted in different surface structures that affect the corrosion behavior of the modified layer. Increasing of H2 to gas mixture results in improving nitriding efficiency but only in the case of sample nitrided at 723 K and N2/H2 with ratio of 50/50 a precipitate-free single phase nitrided layer of nitrogen expanded austenite (γN phase) was produced, which considerably improved the corrosion properties of the austenite stainless steel. Nitriding at these conditions due to formation of a surface layer without defects and higher thickness allows obtaining a significant improvement of corrosion resistance even at higher treatment time and temperatures.