Determination of the diffusivity of cation vacancy in a passive film of Ni using Mott-Schottky analysis and in-situ ellipsometry

Abstract

The diffusivity of cation vacancies in a passive film formed on Ni in a pH 8.5 buffer solution was estimated based on the Point Defect Model with high field migration equations. The parameters required to calculate the diffusivity of the cation vacancies include the flux of cation vacancies, the acceptor density, and the thickness of the passive film as functions of the film formation potential and they were measured from potentiostatic polarization, a Mott-Schottky analysis, and in-situ ellipsometry, respectively. The passive current density, proportional to the flux of the point defects, linearly increased with an increase of the passivation potential, while the acceptor density was reduced with the potential. The thickness of the passive film appeared to increase very rapidly and the growth became stable in thousands of seconds, and increased linearly with an increase of the potential. The dependence of all parameters investigated in this study on the passivation potential corresponded well to the theory. The diffusivity of the cation vacancies in the passive film of Ni was determined to be 2.7×10 −18 cm 2 s −1