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

An austenite 349 stainless steel was nitrided via nitrogen plasma. Glancing angle X-ray diffraction patterns suggest that the nitrided layer is amorphous. X-ray photoelectron spectroscopy analysis indicated that the plasma nitridation process produced bulk-type nitrides in the surface layer. In general, the nitrided layer was composed of iron oxide in the outer layer and chromium oxide in the inner layers. Contaminations of vanadium and tin were detected in the as-grown nitrided layer; these dissolved away after polarization. The influence of these contaminants on the corrosion resistance of the nitrided layer in polymer electrolyte membrane fuel cell (PEMFC) environments is not considered significant. The nitrided sample had a much higher contact resistance than the bare one and the contact resistance increased with the nitriding time. The high interfacial contact resistance values can be related to the thicker oxide film after plasma nitridation. The corrosion resistances obtained for the 1 h nitrided and bare stainless steels in simulated PEMFC environments were similar. The outmost nitrided layer dissolved after polarization in the PEMFC environments leaving a passive film (modified with nitrides), similar to that of bare stainless steel under the same conditions. The passive film thickness was 3.7 nm for nitrided steel in PEMFC cathode environment and 4.2 nm for nitrided steel in PEMFC anode environment.