The structure and semiconducting behavior of passive films formed on nickel base alloy G3 in borate buffer solution after polarization at different times and potentials were studied by XPS analysis and capacitance measurement in this paper. The results indicated that the formed passive films had double layer structure, i.e., outer layer hydroxide as p-type semiconductor and inner layer oxide as n-type semiconductor. Due to the series connection of two space charge capacitances, the measured Mott-Schottky plots were non-linear. A non-linear fitting method was proposed to determine the carrier densities in the outer and inner layers. The carrier densities of passive film formed at different polarization times and potentials were also discussed. The results indicated that both the outer and inner layers were compact and could protect the substrate simultaneously. (C) 2011 Elsevier B.V. All rights reserved.