Anodic Oxidation of Cobalt in Neutral and Basic Solution

The anodic oxidation and anodic oxide films of cobalt have been studiect in borate buffer solutions in the pH range from 7 to 11. The anodic polariza-tion curve Shows the active dissolution, pr imary passivity, secondary passivity, tertiary passivity, and transpassivation. It is also shown that the anodic oxide film is hydrated oxide of CoO in the pr imary passive region, bilayered oxide CoO/Co804 in the secondary passive re gmn, and bilayered oxide of CoO/Co20:~ in the tertiary passive and transpasslve regions. The dissolution current of the anodic oxide film in the secondary and tertiary passive regions is much smaller than that in the pr imary passive region. By cathodic reduction the outer Co203 layer is first converted to Co~O4 and then reduced further to hydrated Co S+ ions before the inner CoO layer is reduced to metallic cobalt. Early this century, it was reported that cobalt was passivated chemically in concentrated nitric acid and electrochemically in alkaline solutions (1). Later