Evidence for the rate determining step at the solution—oxide film interface in the anodic growth of thin oxide films at platinum and nickel electrodes in aqueous solutions

Abstract

The rates of growth of anodic oxide films at Pt in acid and alkaline solutions and Ni in alkaline solutions are compared. In acid solutions, the rates are pH independent. In alkaline solutions, they are affected by pH. The exchange current densities, i 0, in alkaline solutions increase one decade as pH increases one unit. The dependence of i 0 on pH is not expected for the model of high field assisted formation of films with either the step at the metal—oxide film interface or a step within the film as rate determining. It is suggested that a process at the oxide film—solution interface is rate determining with OH − as reaction species. In acid solutions, too, this step is rate determining but with H 2O as reacting species. The possibility that this step can be rate determining is usually overlooked in analyses of growth of anodic films.