Microscopic investigation of pitting corrosion in plasma nitrided austenitic stainless steel

Abstract

UNS 31603 austenitic stainless steel was nitrided using different techniques, and pitting corrosion resistance was analysed in a chloride solution. All nitriding techniques, LEII, PI· and convectional DC nitriding produced a nitrided layer called S phase which is corrosion resistant. Pits morphology and layer structure was investigated using optical and electronic microscopy, SEM-FIB, EDS, and a 3D reconstruction of a pit was assessed using FIB tomography. It was concluded that pits are initiated in MnS inclusions and a channel was generated passing through the nitrided layer, connecting the steel with the electrolyte. Base alloy dissolution was observed beneath the nitrided layer.