Breakdown of Oxide Films on Nickel

Abstract

Oxide films have been formed on nickel by either electropolishing in or anodizing at different potentials and times in pH 2.8 . The 6–8Å film of on electropolished nickel breaks down rapidly on open circuit at pH 2.8 with a poorly defined potential arrest structure. In contrast, several distinct potential arrests are observed during open‐circuit breakdown of the oxide films on anodized nickel, the number of arrests and their duration being dependent on the condition of film formation and indicative of film stability. The results are interpreted in terms of the anodic formation of a nickel oxide film ≈ 12Å thick with a small area (<5%) covered by defective oxide. Breakdown begins at the defect sites by chemical dissolution of the oxide. Oxide undermining is responsible for removal of most of the oxide and starts when the underlying nickel metal at the defect sites has been exposed. Large variations in oxide film stability are associated with very small changes in the over‐all character of the film since the small coverage with defective oxide plays the major role in film breakdown.