Ni coating on 316L stainless steel using cage plasma treatment: Feasibility and swelling studies

Abstract A modified cage plasma nitriding technique is used to coat a Ni film onto a 316L substrate as a protective layer for austenitic steels in molten fluoride salts. The coating is achieved by deposition of Ni atoms sputtered from a Ni cage with a 316L substrate floated at the same potential as the cage to reduce plasma sputtering on the substrate. The Ni coating is porosity-free and precipitation-free and forms a γ-(Fe, Ni) phase due to interdiffusion from the substrate. Beneath the interdiffusion layer exists a thick γ-phase layer mainly affected by long-range nitrogen diffusion along the grain boundary. This layer develops grain boundary Cr nitride precipitates. Ion irradiation on the polished cross-section of the plasma-treated sample shows no void swelling in the surface-deposited Ni layer, very limited void formation in N diffusion layer, and significant void swelling in bulk unaffected by nitrogen. The studies demonstrate the feasibility of the technique for Ni coating and show good swelling resistance of the coating layer and N diffusion-affected layer. Mechanisms are discussed.