Investigation of evolution of γ _N phase and its effect on conductivity and corrosion resistance of plasma-nitrided 316 L stainless steel bipolar plate for proton exchange membrane fuel cell

Abstract

In this work, the evolution of γN phase and its effect on conductivity and corrosion resistance of plasma-nitrided 316L stainless steel bipolar plate was investigated. The variation of γN phase layer and the phase composition in γN phase was analyzed by using SEM with EBSD, XRD and XPS. The results shows that a certain thickness of uniform γN phase layer was formed after plasma-nitriding treatment. As the increase of plasma-nitriding time, the thickness of γN phase layer and the content of nitrogen atoms in γN phase was increased gradually. When the plasma-nitriding time was reached to 24h, the Fe4N compound was found on the sample surface. As the plasma-nitriding time was larger than 10h, a large amount of was formed in γN phase layer owing to serious lattice distortion. After plasma-nitriding treatment, the conductivity and corrosion resistance of the sample was obviously enhanced. With the increasing in the thickness of γN phase layer, the interfacial contact resistance (ICR) of the sample was reduced and the corrosion resistance of the sample was improved. However, the ICR of the sample was significant increased when the Fe4N compound was formed on the sample surface under plasma-nitriding time of 24h condition. When the plasma-nitriding time was larger than 10h, the corrosion resistance of the sample became poor. The formation of crack in γN phase layer and the high surface roughness resulted in the degradation of corrosion resistance of the sample. Under plasma-nitriding time of 5h condition, the comprehensive properties of the sample were the best. The conductivity and corrosion resistance of the sample were enhanced 4 times and 15 times compared with the untreated sample. This work can provide a theoretical reference for the design of 316L stainless steel bipolar plates for proton exchange membrane fuel cells.