A niobium nitride diffusion coating has been prepared by plasma surface diffusion alloying method on the surface of AISI 304 stainless steel (304 SS) which is applied to bipolar plates for proton exchange membrane fuel cell (PEMEC). The as-prepared niobium nitride diffusion coating consists of a niobium nitride surface layer (8-9 mu m) and a Nb and N diffusion solid solution subsurface layer (1-2 mu m). The corrosion resistance of the niobium nitride modified 304 SS are investigated and evaluated in simulated PEFMC operating conditions (0.05 M H(2)SO(4) + 2 ppm F(-) solution at 70 C). Results show that the niobium nitride diffusion coating greatly improves the corrosion resistance of 304 SS. Self-corrosion potentials of niobium nitride modified 304 SS increase considerably up to 100 mV and 143 mV (vs. SCE) while corrosion current densities remain low at 0.127 mu A cm(-2) and 0.071 mu A cm(-2) in simulated anodic and cathodic environment, respectively. Interfacial contact resistance (ICR) of niobium nitride modified 304 SS is 9.26 m Omega cm(2) under 140 N cm(-2) compaction force, which is much lower than that of untreated 304 SS, 100.98 m Omega cm(2). After 4h potentiostatic polarization test, the niobium nitride modified 304 SS exhibits sufficiently low ICR values <20 m Omega cm(2). (C) 2011 Elsevier B.V. All rights reserved.