Low-temperature carburized high-alloyed austenitic stainless steels in PEMFC cathodic environment

Abstract Austenitic stainless steels are promising materials for bipolar plates in polymer electrolyte membrane fuel cells (PEMFC). In this study, we explore the possibility to apply low-temperature carburized high-alloyed (high Cr, Ni, Mo) steels as bipolar plate materials. The electrochemical properties and the microstructural characteristics of the modified surfaces were analyzed by optical microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Auger electron spectroscopy. The carburizing treatment (Kolsterising®, Bodycote) induced the formation of a carbon-stabilized expanded austenite layer. The amount of carbon dissolved in the metal matrix was dependent on the alloy composition, in particular on the amount of Mo. Additionally, surface carbides were found at the surface of low-Ni containing steels. The interfacial contact resistance (ICR) measured in a simulated PEMFC was reduced after carburizing as a consequence of thinner oxide layer and change in oxide composition. Potentiodynamic tests revealed nobilitation of all the steels when carbon was present in solid solution. The occurrence of surface carbides, however, was highly detrimental to the corrosion properties. The ICR of carburized materials in aged condition was higher than the corresponding untreated alloys. The presence of localized corrosion in carburized materials could be responsible for this effect.