Effect of surface pretreatment on the galvanostatic oxidation of nickel

Abstract

The open-circuit, cathodic reduction and anodic charging characteristics of oxide films on both electropolished and electropolished + HNO 3 etched nickel electrodes have been studied in aqueous Na 2SO 4 solutions. While the oxide film thicknesses are approximately the same, the film on etched nickel dissolves much more rapidly than that on electropolished nickel. To galvanostatically passivate etched nickel at charging rates in the range 20–200 μA cm −2 in pH 2.8 Na 2SO 4, the rate of oxide chemical dissolution has to be substantially decreased and this is achieved by lowering the electrolyte temperature from 25°C to 5°C. The transient anodic passivation charge thus observed for etched nickel is ∼ 2 x larger than that for electropolished nickel. With either pretreatment, the current efficiency for oxide formation is low (∼ 20%), the majority of the transient charge (∼ 80%) accounted for by Ni 2+ in solution. Chemical dissolution of the oxide alone cannot explain the observed charging differences between etched and electropolished nickel since the individual charges and oxide formation current efficiencies are not influenced by anodic charging rate or solution aggressiveness. The results are best explained in terms of a continual breakdown and repair of the oxide film at defect areas during anodic charging, with most of the charge consumed by inefficient film repair. The influence of the oxide defects and the reason for the differences in behaviour between electropolished and etched nickel are discussed.