Growth kinetics of multi-oxide passive film formed upon the multi-principal element alloy AlTiVCr: Effect of transpassive dissolution of V and Cr**

Abstract

The growth kinetics of the surface film formed upon the multi-principal element alloy AlTiVCr under anodic polarisation in 0.6 M NaCl was investigated using atomic emission spectroelectrochemistry (AESEC). The AESEC charge balance analysis revealed that thickness of the barrier layer of the passive film upon the alloy: (1) increases linearly with the increase in anodic potential during potentiodynamic polarisation, and (2) increases logarithmically with exposure time during potentiostatic polarisation. This is consistent with the assumptions of the point defect model, despite the film being a multi-oxide film with transpassive dissolution of V and Cr. The X-ray photoelectron spectroscopy (XPS) analysis suggested that the growth of the film was predominantly due to TiO2 during anodic polarisation. The electric field was found to decrease with enrichment of TiO2 in the barrier layer. The Mott-Schottky analysis revealed that the diffusivity of oxygen vacancies increased with the increase in fraction of TiO2 in the film, which subsequently led to the increase in the growth rate of the barrier layer during transpassive dissolution. The present work is a discrete effort towards understanding the growth behaviour of the passive film experiencing complex and competing interfacial electrochemical processes, upon a multi-principal element alloy.