EFFECT OF CHLORIDE-ION ON THE LOCALIZED BREAKDOWN OF NICKEL-OXIDE FILMS

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, Ni → Ni2+ + 2e, it does interfere with NiO 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. © 1979, The Electrochemical Society, Inc. All rights reserved.