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 γ N phase and its effect on conductivity and corrosion resistance of plasma-nitrided 316 L stainless steel bipolar plate was investigated. The results shows that a certain thickness of uniform γ N phase layer was formed after plasma-nitriding treatment. As the increasing in plasma-nitriding time, the thickness of γ N phase layer and nitrogen atoms in γ N phase layer was increased gradually. As the plasma-nitriding time was larger than 10 h, a large amount of cracks was formed in γ N phase layer because the nitrogen atoms were diffused into γ phase to cause the serious lattice distortion. When the plasma-nitriding time was reached to 24 h, the Fe4N compound was found on the sample surface. After plasma-nitriding treatment, the conductivity and corrosion resistance of the sample was obviously enhanced compared with the untreated sample. With the increasing in the thickness of γ N phase layer, the interfacial contact resistance (ICR) and corrosion current of the sample was gradually reduced. The ICR of the sample was reduced to 7 mΩ under 10 h condition. However, the formation of Fe4N compound and high roughness contributed to the increase of ICR of the sample under 24 h condition, its value was reached to 15 mΩ. When the plasma-nitriding time was larger than 10 h, the corrosion resistance of the sample became poor. The formation of cracks in γ N phase layer and the high surface roughness resulted in the degradation of corrosion resistance of the sample. Under 5 h condition, the comprehensive properties of the sample were the best. The ICR and corrosion of the sample were current conductivity and corrosion resistance of the sample were 15 mΩ and 5.1 μA cm−2, which were low 4 times and 15 times compared with the untreated sample.