Effect of Chloride Ion on the Localized Breakdown of Nickel Oxide Films

Abstract

The effect of chloride ion on the passivation of nickel is studied and a mechanism for pit initiation on electrodes anodized in Cl−‐containing solutions is proposed. Although Cl− does not directly influence the nickel dissolution reaction, , it does interfere with formation and thereby retards passivation. This is manifested by an increase in the anodic passivation charge upon stepping the potential into the passive region. At a constant anodic potential, both the extent of retardation of oxide formation and the probability for pitting increase with increasing [Cl−]. Under conditions where pitting will occur, the anodic current falls substantially during the first 30–60 sec (after the potential step) but then begins to increase as pitting occurs. The original low bulk concentration of Cl− (≤0.1M) was not considered sufficient to cause pitting and this suggests a localized enrichment in Cl− during the 30–60 sec current decay. The oxide film is viewed as undergoing continuous breakdown and repair at localized areas and when sufficient Cl− has accumulated in some areas, repair of the oxide is not possible. For a constant bulk [Cl−], the pitting probability increases sharply with increasing potential of anodization; however, the corresponding extent of oxide retardation actually decreases. This increased pitting probability is explained in terms of the higher rate of nickel dissolution (from oxide‐free areas where the film has broken down) at the higher anodic potential being able to effectively concentrate Cl− in localized areas of the surface. The presence of Cl− ion in the anodizing solution does not result in any change in average oxide film thickness or substantial Cl−incorporation within the film.