Breakdown of passive film on nickel in borate solutions containing halide anions

The effect of Cl⁻, Br⁻ and I⁻ anions as aggressive agents on the anodic behaviour of nickel electrode in deaerated Na₂B₄O₇ solutions have been investigated by galvanostatic polarization technique. Lower concentrations of the halide anions have no effect on the mechanism of nickel passivation. An increase in the halide anions concentration causes oscillation of the potential in the oxygen evolution region. This could be attributed to the destruction of the passivity by halide anions and repassivation of the film by anodic current and/or OH⁻ anions. Higher aggressive anion concentrations cause breakdown of the passive film and initiated pitting corrosion. As the temperature increases, the breakdown potential is shifted towards the more negative direction. On the other hand, as the pH of the solution increases, the breakdown potential is shifted toward more positive direction, indicating increased protection of the passive film. The activation energy, , of the oxide film formation in the presence of Cl⁻ anions was calculated and was found to be 21 kJ/mol.