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

Abstract

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-Cr 2 O 3 , outer-Cr(OH) 3 layers, and also the Cr/Cr 2 O 3 , Cr 2 O 3 /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-Cr 2 O 3 layer only O 2− vacancy could migrate with a self-diffusion coefficient of 3.54 × 10 −21 cm 2 /s at 25 °C; in the outer-Cr(OH) 3 layer, only Cr 3+ vacancy could migrate with a self-diffusion coefficient of 1.71 × 10 −24 cm 2 /s at 25 °C, which indicates that the outer-Cr(OH) 3 layer also contributes to the protection of passive film. At the Cr 2 O 3 /Cr(OH) 3 interface, the self-diffusion coefficients of anion and cation vacancies are 6.92 × 10 −31 and 1.07 × 10 −32 cm 2 /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 Cr 2 O 3 /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.

Microstructure and corrosion behavior of Cr and Cr–P alloy coatings electrodeposited from a Cr(iii) deep eutectic solvent

“…8a. On the other hand, since the Cr coating is rich in Cr 2 O 3 , which is anion- selective, 46,47 Cl À can favorably permeate into the coating and also corrodes the Cr layer. Sheet-like particles are hence formed on the Cr surface.…”

Structural, Magnetic, Electronic, Defect, and Diffusion Properties of Cr2O3: A DFT+U Study

“…To our knowledge, there is only one theoretical investigation available where the growth of the chromium passive layer in a Cr 2 O 3 bulk has been investigated with the DFT approach. 25 Here Yu et al 25 have investigated vacancy diffusion pathways in Cr 2 O 3 bulk along one direction, the z axis of the cell. The calculations were performed without taking into account the magnetism, with a pure DFT approach.…”

First Principles, Explicit Interface Studies of Oxygen Vacancy and Chloride in Alumina Films for Corrosion Applications

“…We propose that their V O charge assignment may need to be re-examined. 50 For completeness, we also examine another main hypothesis of aluminum oxide film depassivation which concerns Cl − substituting for OH − on Al 2 O 3 surfaces (Fig. 1e).…”