A niobium carbide diffusion layer with a cubic NbC phase surface layer (similar to 6 mu m) and a Nb and C diffusion subsurface layer (similar to 1 mu m) is fabricated on the surface of AISI 304 stainless steel (304 SS) bipolar plate in a proton exchange membrane fuel cell (PEMFC) using plasma surface diffusion alloying. The electrochemical behaviour of the niobium carbide diffusion-modified 304 SS (Nb-C 304 SS) is investigated in simulated PEMFC environments (0.5 M H2SO4 and 2 ppm HF solution at 80 degrees C). Potentiodynamic, potentiostatic polarisation and electrochemical impedance spectroscopy measurements reveal that the niobium carbide diffusion layer considerably improves the corrosion resistance of 304 SS compared with untreated samples. The corrosion current density of Nb-C 304 SS is maintained at 0.058 mu A cm(-2) and 0.051 mu A cm(-2) under simulated anodic and cathodic conditions, respectively. The interfacial contact resistance of Nb-C 304 SS is 8.47 m Omega cm(2) at a compaction force of 140 N cm(-2), which is significantly lower than that of the untreated sample (100.98 m Omega cm(2)). Moreover, only a minor increase in the ICR of Nb-C 304 SS occurs after 10 h potentiostatic tests in both cathodic and anodic environments. (C) 2013 Elsevier B.V. All rights reserved.