Plasma nitriding of 316L stainless steel in two different N2-H2 atmospheres - Influence on microstructure and corrosion resistance

Abstract

Plasma nitriding of austenitic stainless steel is of high technological importance due to its ability to improve the surface hardness, wear and corrosion resistance. Therefore, such process presents a significant potential as a treatment of materials for biomedical applications. The resulting properties obtained on the nitrided layer depend on the nitriding parameters used, such as temperature, gases partial pressures, process time, among others. In this work, austenitic stainless steel AISI 316L was nitrided in a DC plasma using two different N2-H2 atmospheres: 25% N2 and 20% N2. A detailed analysis of surface microstructure has been carried out and related to corrosion behaviour in two saline solutions. The results were compared with the 316L unnitrided (UN). The chemical composition of the nitrided layer was determined by GDOES. The phases present in the nitrided layer were analysed by X-ray diffraction (XRD) and the phases in the surface of nitrided layer were analysed by grazing incidence X-ray diffraction mode (GIXRD). The corrosion behaviour of the samples was studied by an electrochemical corrosion test in 3 wt% NaCl and Ringer's solutions. The GDOES (Glow Discharge Optical Emission Spectroscopy) analysis shows a higher N concentration of in the surface of 25% N2 samples, named HN. The phases observed, by GIXRD, in HN samples were expanded austenite γN, CrN, Fe3N and Fe4N and, in the 20% N2 samples, named LN, only peaks of expanded austenite and Fe4N were observed. The corrosion results showed that LN present better corrosion resistance than HN, this could be due to the absence of chromium nitrides in the surface of LN as a consequence of lower N percentage in the nitriding atmosphere.