Anticorrosion Properties of the Low-Temperature Glow Plasma Nitriding Layer on AISI 904L Austenitic Stainless Steel in Hydrofluoric Acid Obtained at Various NH3 Pressures

A low-temperature (400 °C) glow plasma nitriding layer on AISI 904L austenitic stainless steel was obtained at various NH3 pressures and studied using electrochemical method, X-ray diffraction, and scanning Kelvin probe. The pressure of NH3 dominated the microstructure of the nitriding layer. The saturation degree of γN controlled corrosion performance and microhardness. Insufficient NH3 pressure (<100 Pa) resulted in discontinuous nitride caking coverage, whereas excessive NH3 pressure (>100 Pa) facilitated the transformation of the nitriding layer to harmful nitrides (CrN) due to a localized overheating effect caused by the over-sputtering current.