Nitriding of 316 stainless steel in molten fluoride salt by an electrochemical technique

Electrochemical nitriding of type 316 stainless steel (316 SS) in molten fluoride salt (LiF–KF, FLiK) containing Li3N was investigated. Formation of chromium nitride CrN and iron nitride α-Fex(x>8)N was confirmed. Cyclic voltammogram measurements and electrolysis treatment were carried out using an Al–Li alloy reference electrode at 873 K. After potentiostatic treatment at 1.0 V vs. Li/Li+ for 100 min, the specimen was characterized by SEM, XPS, EPMA, and XRD. SEM observation revealed that the initially smooth surface changed into a rugged structure covered by a wrinkle structure about 1–2 μm in width after electrolysis. XPS analysis revealed that chromium nitride (CrN) was selectively formed at the surface layer. According to the EPMA analysis, the treatment promoted nitrogen diffusion from the surface to a depth of 35 μm. CrN was formed from the surface to a depth of 2 μm. It was determined that iron nitride α-Fex(x>8)N was formed from there to a depth of 35 μm as an inside diffusion layer, but neither Fe2N nor Fe4N was detected. Formation of CrN and α-Fex(x>8)N was also confirmed by XRD measurement and XPS analysis. The formation of chromium nitride and iron nitride was discussed in terms of thermodynamics.