Modelling the anodic corrosion of alloys in acid solutions on the basis of AC impedance and photoelectrochemical measurements

A renewed version of a surface charge approach to describe the impedance response of anodic film growth on passive metals in acidic solutions is presented. It is based on the point defect chemistry, the fact that oxygen vacancies are the main charge carriers in a range of anodic oxides and the suggestion of a constant field strength in the bulk of the barrier layer. High-field transport equations are employed to the point defect motion. A negative surface charge due to accumulation of metal vacancies near the film/solution interface accelerates the oxygen vacancy transport in transient conditions, and a corresponding positive surface charge of oxygen vacancies at the opposite interface is shown to retard it, thus explaining either a capacitive or a pseudoinductive relaxation of the metal/film /electrolyte system under small amplitude ac perturbation. The model is consistent with the anodic passivity of Bi-Sb and Fe-Mo alloys in concentrated acid solutions. Kinetic and structural parameters of the alloy/oxide film/electrolyte system are thereby determined. Specific alloys studied: Bi-40Sb and Fe-20Mo.