Lower corrosion resistance of the nitrocarburized layer formed on two supermartensitic stainless steel types

The aim of this study was to verify the pitting corrosion behavior in the surface layers obtained by plasma nitrocarburizing at 400 and 450 °C/5 h on two types of super-martensitic stainless steel, namely micro-alloyed (Nb-SMSS) and unalloyed (SMSS). The results reveal that in all the nitrocarburized layers, a discontinuous, thin layer measuring less than 5µm in thickness exhibits a microhardness exceeding 950 HV0.05, for the two steels. Furthermore, the structure of the surface layer is a combination of expanded austenite (γN), expanded martensite (α`N), ε-Fe2-3 N, cementite (θ-Fe3 C), and traces of CrN. The surfaces exhibit poor corrosion resistance across all layers, which can be attributed to localized micro-galvanic corrosion between the iron nitride (ε-Fe2-3 N) and expanded austenite (γN), since they are known to have higher corrosion resistance, as well as expanded martensite ((α`N) and cementite (θ-Fe3 C), which have lower corrosion resistance. This corrosion process initiates after the dissolution of the surface layer in a 3.5% NaCl solution, subsequently leading to substrate corrosion.