Passive film formed on nickel in a neutral solution

A bright nickel was passivated in the pH 8·39 boric acid-borate buffer solution. The oxide formation reactions were mainly investigated by means of polarization experiments, alternating anodic and cathodic polarization, potential decay experiments and colorimetric analysis. From the results, the chemical composition and structure of passive films formed on nickel were discussed. The experimental results are summarized as follows.1. A considerable amount of nickel ions dissolved during the anodic formation of passive films. The dissolution ceased because of the formation of NiO₂. 2. The passive films on bright nickel has a duplex structure consisting of NiO and Ni₃O₄. At higher potentials, NiO₂ was produced on them. NiO and Ni₃O₄ were formed directly from Ni, and NiO₂ was transformed from Ni₃O₄. 3. NiO and NiO₂ were reversibly produced and reduced, but Ni₃O₄ was very difficult to be reduced. 4. It was observed that there was a reversible charging and discharging layer at the potential where Ni₃O₄ formed. The electric capacity was calculated to be about 100 μF/cm², assuming that roughness factor was 2. It would be reasonable to think that a space charge was established on nickel surface.