Plasma Nitrided Type 349 Stainless Steel for Polymer Electrolyte Membrane Fuel Cell Bipolar Plate—Part II: Nitrided in Ammonia Plasma

Austenitic 349 stainless steel was nitrided in an $NH3$ plasma. A low interfacial contact resistance was obtained with the nitrided steel. Glancing angle X-ray diffraction suggests that the nitrided layer is very thin and possibly amorphous. X-ray photoelectron spectroscopy (XPS) studies show that the nitrided layer is composed of mixed oxides and nitrides of $Fe3+$ and $Cr3+$⁠. Contaminations of V and Sn were also observed, though their influence on the as-nitrided surface conductivity is not clear. The nitrided samples were investigated in a simulated polymer electrolyte membrane fuel cell (PEMFC) environment, and showed excellent corrosion resistance. The XPS depth profile indicated that the passive film, which formed on the plasma-nitrided steel in the PEMFC anode environment, is composed of mixed oxides and nitrides, in which chromium oxide/nitride dominates the surface chemistry. No V or Sn was detected on the surface after the polarization tests. For the PEMFC bipolar plate application, nitridation in $NH3$ plasma is a promising surface treatment approach, though more research is needed to investigate the influence of the plasma density and substrate bias on the surface conductivity and performance of the nitrided steel in PEMFC environments.