Plasma nitriding of the inner surface of stainless steel tubes

Stainless steels, especially the austenitic grades, present a good corrosion resistance but poor mechanical properties. Nitriding can increase the surface hardness but typically at the expense of corrosion resistance. Besides, nitriding of inner surfaces is not straight forward, as control of the plasma there is challenging. This work has focused on the development of active screen plasma nitriding processes of inner geometries of the widely used AISI 316L austenitic stainless steel, with preservation of the corrosion resistance. The structure and composition of the nitrided surfaces were studied and corrosion resistance of the inner surfaces assessed. As a result of the study, a relation has been established between plasma treatment parameters, geometrical features and the corrosion resistance. Additionally, electro-tribological studies have been performed. It has been observed that wear resistance improved and that ECR (Electrical Contact Resistance) is a good indicator of the wear phenomena occurring along the test and potentially could be a useful and simple monitoring tool of the status of the surface and assist in defining safe and dependable operational lives. As part of the study, an equivalent electric circuit is proposed. In conclusion, treatment conditions for AISI316L inner tube surfaces have been defined, with corrosion resistance preservation and improved tribological performance, while ECR monitoring has been found insightful and informative of the evolution of the later.