Thermally Nitrided Stainless Steels for Polymer Electrolyte Mebrane Fuel Cell Biopolar Plates: Part I, Model Ni-50Cr and Austenitic 349(TM) Alloys

Thermal nitridation of a model Ni-50Cr alloy at 1100°C for 2 h in pure nitrogen resulted in the formation of a continuous, protective CrN/Cr ₂N surface layer with a low interfacial contact resistance. Application of similar nitridation parameters to an austenitic stainless steel, 349™, however, resulted in a discontinuous mixture of discrete CrN, Cr₂N and (Cr,Fe)₂N_1-x (x = 0-0.5) phase surface particles overlying an exposed γ austenite-based matrix, rather than a continuous nitride surface layer. The interfacial contact resistance of the 349™ was reduced significantly by the nitridation treatment. However, in the simulated PEMFC environments (1 M H₂ SO₄ + 2 ppm F^- solutions at 70°C sparged with either hydrogen or air), very high corrosion currents were observed under both anodic and cathodic conditions. This poor behavior was linked to the lack of continuity of the Cr-rich nitride surface formed on 349™. Issues regarding achieving continuous, protective Cr-nitride surface layers on stainless steel alloys are discussed.