Anodic Oxidation of Cobalt in Neutral and Basic Solution

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Abstract
The anodic oxidation and anodic oxide films of cobalt have been studied in borate buffer solutions in the pH range from 7 to 11. The anodic polarization curve shows the active dissolution, primary passivity, secondary passivity, tertiary passivity, and transpassivation. It is also shown that the anodic oxide film is hydrated oxide of CoO in the primary passive region, bilayered oxide CoO/Co304 in the secondary passive region, and bilayered oxide of CoO/Co203 in the tertiary passive and transpassive regions. The dissolution current of the anodic oxide film in the secondary and tertiary passive regions is much smaller than that in the primary passive region. By cathodic reduction the outer Co203 layer is first converted to Co3O4 and then reduced further to hydrated Co2+ ions before the inner CoO layer is reduced to metallic cobalt

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