Pitting Corrosion of Nickel in Borate and Phosphate Solutions

The pitting corrosion of nickel covered with films potentiostatically formed at various potentials for 3.6 ks in borate and phosphate solutions of pH 8.42 and pH 11.50 was investigated in 0.5 kmol/m³ NaCl solution at a constant potential of 0.395 V (SHE) higher than a critical pitting potential of 0.295 V in the same solution. The time, τ, obtained from the logarithmic plots for pitting current (⁠i_p⁠) or amount of electric charge passed (⁠Q_p⁠) vs time curves was found to be the transition time from pit nucleation to pit growth. The τ value was a linear function of potential in the passive region and showed a rapid increase at potential in the secondary passive region, irrespective of anion species and solution pH in which a film was formed. From the linear relationship between r and total film thickness in the passive region, it was proposed that a passive film formed in the solutions consists of two layers: an outer layer of Ni(QH)₂ and an inner layer of NiO. Thereby, the pit nucleation was assumed to be the period that continues until the inner layer is perforated by attack of Cl²⁺ ions. The τ value, independent of anion species and solution pH at a constant film thickness, may be explained in terms of an ion selective property of nickel film, which is anionic selective below pH 13.0. The mechanism of its breakdown proposed in the present work was the same as that for iron described previously, whereas the kinetics of pit growth was much different from that for iron.