Thermally nitrided stainless steels for polymer electrolyte membrane fuel cell bipolar plates: Part 2: Beneficial modification of passive layer on AISI446

Thermal nitridation of AISI446 mod-1 superferritic stainless steel for 24 h at 1100 °C resulted in an adherent, inward growing surface layer based on (Cr, Fe)2N1−x (x = 0–0.5). The layer was not continuous, and although it resulted in low interfacial contact resistance (ICR) and good corrosion resistance under simulated polymer electrolyte membrane fuel cell (PEMFC) cathodic conditions; poor corrosion resistance was observed under simulated anodic conditions. Nitridation for 2 h at 1100 °C resulted in little nitrogen uptake and a tinted surface. Analysis by SEM, XPS, and AES suggested a complex heterogeneous modification of the native passive oxide film by nitrogen rather than the desired microns-thick exclusive Cr-rich nitride layer. Surprisingly, this modification resulted in both good corrosion resistance under simulated cathodic and anodic conditions and low ICR, well over an order of magnitude lower than the untreated alloy. Further, little increase in ICR was observed under passivating polarization conditions. The potential of this phenomenon for PEMFC bipolar plates is discussed.