Anodic oxide films formed on nickel in Na2SO4 solutions have been examined by electrochemical and surface-analytical techniques (RHEED, OKα X-ray emission spectroscopy, Auger electron spectroscopy, and replica electron microscopy). The passive oxide film is composed of 9–12 A˚ of NiO with a defect character which determines Its stability towards breakdown. While the overall film thickness is hardly influenced by the anodic formation conditions, the defect character of the film (and therefore its stability) changes dramatically with varying time and potential of anodization. The defect character also depends on the epitaxy between the oxide overgrowth and the underlying nickel metal, as seen in the transpassive region where the stability of the oxide film decreases as the oxide epitaxy degenerates from single crystal to polycrystal. Thick oxide films can be anodically grown on nickel in pH 7.65 borate buffer solution by polarizing into the oxygen evolution region. Oxide growth current efficiency is very low with most of the charge consumed by O2 evolution. Auger analysis in conjunction with ion sputtering indicates that the thick oxide films are porous in nature, and that their thickness is highly dependent on the substrate nickel orientation. The porosity is on too fine a scale to be resolved by replica electron microscopy.

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National Association of Corrosion Engineers
1982
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