Role of Oxide Defects in the Anodic Oxidation of Nickel

Abstract

The galvanostatic oxidation of nickel electrodes in pH 2.8 requires the presence of a prior oxide film in order to avoid the active nickel dissolution region. Either the 6–8Å film on electropolished nickel or that formed during the early stages of anodic passivation can serve as an appropriate starting surface for galvanostatic experiments. Anodic charging of such electrodes gives a distinct transition region over a potential range of almost 1V. The current efficiency for nickel dissolution during the potential transient is high (>80%) with only a small portion of the charge contributing to oxide growth. This situation persists even at high charging rates and/or high solutionpH's where it appears that chemical dissolution of the oxide is not playing an important role. The results suggest that the standard high field oxide growth mechanism is not operative during galvanostatic oxidation of nickel. The results are best explained in terms of a defective oxide film where the change of potential during the transient is associated with an increase in the state of film perfection. Anodic charge consumption would be mainly due to a continuous breakdown and repair of the oxide film at defect sites, most of the charge going towards during inefficient film repair. The increase in anodic potential with increasing perfection of the film is interpreted in terms of an overvoltage effect, i.e., the area over which breakdown and repair is occurring decreases with increasing film perfection and therefore the anodic current density increases. At potentials > +0.4V, the rate of increase in oxide film perfection decreases and an eventual steady state is reached. The influence of potential of anodization and solution pH on nickel dissolution, oxide repair, and oxide perfection is discussed. The defect model of oxide film development is shown to apply equally well during potentiostatic oxidation of nickel, the film reaching an almost steady thickness very soon after the potential step anodization with the continual long‐term decrease of anodic current with time of anodization being due to a localized increase in oxide film perfection.