By employing Mott-Schottky analysis in conjunction with the point defect model (PDM), we compared donor density and donor diffusion coefficients in the passive films formed on the surface of nanocrystallized bulk 304 stainless steel (NB304ss) and cast 304 stainless steel (304ss) in 0.05 mol/L H2SO4 + 0.25 mol/L Na2SO4 solution. The donor density at the metal/film interface of the NB304ss was lower than that at the metal film interface of the cast 304ss. Based on the Mott-Schottky analysis, an exponential relationship between donor density and formation potentials of the passive films on the NB304ss and the cast 304ss was built up. The results showed that the donor diffusion coefficients in the passive film formed on the surface of NB304ss was lower than that in the cast 304ss. The lower donor density and the lower diffusion coefficient restrained the electrochemical reaction in the passive film and improved the stability of the passive film. That is the reason why the passive film formed on the NB304ss was more protective. (c) 2006 Elsevier Ltd. All rights reserved.