Migration of Ion Vacancy in Hydroxylated Oxide Film Formed on Cr: A Density Functional Theory Investigation

A theoretical study of ion vacancy migration properties in hydroxylated oxide film (passive film) formed on Cr surface has been carried out based on density functional theory (DFT). The ion vacancy migration involves a rather complex regime, which includes inner-Cr2O3, outer-Cr(OH)3 layers, and also the Cr/Cr2O3, Cr2O3/Cr(OH)3 and Cr(OH)3/solution interfaces. An atomic model has been proposed to simulate this system, where the diffusion coefficient of ion vacancy has been achieved as well as considering the influence of electric fields within n-p junction and surface space-charge layer. The results show that in the inner-Cr2O3 layer only O2– vacancy could migrate with a self-diffusion coefficient of 3.54 × 10–21 cm2/s at 25 °C; in the outer-Cr(OH)3 layer, only Cr3+ vacancy could migrate with a self-diffusion coefficient of 1.71 × 10–24 cm2/s at 25 °C, which indicates that the outer-Cr(OH)3 layer also contributes to the protection of passive film. At the Cr2O3/Cr(OH)3 interface, the self-diffusion coefficients of anion and cation vacancies are 6.92 × 10–31 and 1.07 × 10–32 cm2/s, respectively, at 25 °C, which are over six orders of magnitude lower than those at other locations and control the growth of both the inner and outer layers of the passive film. Accordingly, factors such as temperature and anodic polarization, which can weaken or even destroy the blocking effect of Cr2O3/Cr(OH)3 interface by increasing the migration rate of ion vacancy, will not only accelerate the growth of passive film but also increase its vacancy concentration and lead to a decrease in compact density.