Influence of pH on the passivation behaviour of 904L stainless steel bipolar plates for proton exchange membrane fuel cells

The electrochemical behaviour and passive film properties of 904L stainless steel in simulated corrosive environment in proton exchange membrane fuel cells (PEMFCs) of pH ranging from 1 to 5 was evaluated by using potentiodynamic curves, EIS, Mott-Schottky approach, and ex-situ surface analytical techniques XPS and secondary ion mass spectrometry (SIMS). The results confirmed that the passive film of 904L stainless steel performs as n-type and p-type semiconductors at potentials changes around the flat band potential. The doping density decreases with increment of pH value, the passive film formed at pH 1 condition present the highest donor and acceptor density. Furthermore, a bilayer structure passive film was observed, it was mainly composed by an outer iron, molybdenum and copper oxide region, whereas the inner chromium and little molybdenum oxide. Moreover, as the pH value increased, signals of nickel oxides or hydroxides were observed in passive film.