Modifying a Stainless Steel for PEMFC Bipolar Plates via Electrochemical Nitridation

AISI446 stainless steel was electrochemically nitrided at room temperature. X-ray photoelectron spectroscopy (XPS) analysis indicated that the nitrided steel was covered with surface ammonia and a layer of nitrides (mainly of mixed chromium nitrides). The nitride layer for 4h nitrided steel at -0.9V was about 2.5nm thick. Dominating oxides appear on the steel's surface, so nitrogen incorporated oxides is a suitable term to describe the nitrided surface. The nitrided surface showed very low interfacial contact resistance (ICR) and excellent corrosion resistance in simulated polymer electrolyte membrane fuel cell (PEMFC) environments. The excellent stability of the nitride steel was confirmed by XPS depth profiling before and after testing in the PEMFC environments. Electrochemical nitridation provides an economic way for modifying the steel's surface to approach the U.S. Department of Energy 2015 goal for bipolar plates.