The Role of Nitrogen to Hydrogen Ratio in Evolution of γN Phase and Surface Properties of Plasma-Nitrided 316L Stainless Steel Bipolar Plate

This study investigated the impact of the nitrogen-to-hydrogen ratio on the evolution of the surface phase composition and properties of plasma-nitrided 316L stainless steel bipolar plates. The surface microstructure and phase composition of the plasma-nitrided sample were characterized using x-ray diffraction (XRD) and scanning electron microscopy (SEM) with electron backscattered diffraction (EBSD). After plasma nitriding, a uniform gamma N phase layer formed on the sample surface under high hydrogen conditions. As hydrogen content increased, the thickness of the gamma N phase layer also increased because hydrogen increases sputtering, removes the oxide film, and accelerates the diffusion of nitrogen atoms into the sample surface. After plasma nitriding, the hydrophobicity, conductivity, and corrosion resistance of the sample were considerably improved with the increasing hydrogen content in the nitriding atmosphere. When the N2-H2 ratio was 1:7, the interfacial contact resistance and corrosion current density of the sample were reduced to 8m Omega/cm2 and 1 mu A/cm2, respectively. The relationship between the evolution of the gamma N phase and the surface properties of the 316L stainless steel bipolar plate under different nitrogen-to-hydrogen ratio conditions is discussed in detail.