Investigation of formation of N phase layer and its effect on surface properties of 316 L stainless steel bipolar plate for proton exchange membrane fuel cell

In this paper, the surface microstructure and properties of plasma-nitrided 316 L stainless steel bipolar plate with a thickness of 0.1 mm were investigated. The surface microstructure of the nitrided sample was characterized by SEM, TEM, XRD and XPS. After plasma nitriding, a nitrogen-expand austenite (gamma N phase) layer was formed on the plasma-nitrided sample surface. In gamma N phase layer, a large amount of stacking fault was formed and the superlattice lattice was found owing to serious lattice distortion under high plasma nitriding condition. The hydrophobic property, conductivity and corrosion resistance of the nitrided samples were obviously improved compared with the untreated sample. Under long time or high temperature condition, although a thicker gamma N phase layer was formed, the surface corrosion resistance of the plasma-nitrided samples became worse. Under high hydrogen condition, the plasma-nitrided sample surface exhibited the good corrosion resistance and electrical conductivity. The interfacial contact resistance of the sample was only 8 mW2 (<= 20 mW2), and the corrosion resistance of the sample was 90 times higher than that of the untreated sample. The influence of phase composition and microstructure in nitriding layer as well as surface morphology on surface properties of plasmanitrided sample were analyzed in detail. This work proves that plasma nitriding technology has great advantages in improving surface properties of the bipolar plate.