The Passive Oxide Films on Cobalt in Weakly Alkaline Solution

Abstract

Anodic oxide film formed on cobalt in sodium borate solution at pH=11.0 have been investigated by coulometry and potentiometry. The potential change during galvanostatic oxidation exhibits three plateaus before the potential reaches a steady state value for oxygen-evolution reaction. In the first plateau Co(OH)₂ is formed on cobalt. Further oxidation at constant current leads to a potential rise which accompanies the formation of CoO layer between the cobalt metal and the Co(OH)₂ film. In the second plateau a reaction of CoO·nH₂O to Co₃O₄ occurs with a coulomb equivalent to the reaction. This Co₃O₄ film is oxidized to Co₂O₃·mH₂O in the third plateau, the potential of which is 0.575V (she) not depending on the current densities and probably corresponds to the equilibrium potential of Co₃O₄/CoOOH. This change of Co₃O₄ to Co₂O₃·mH₂O, however, does not complete during the potential arrest at the third plateau. From the galvanostatic-cathodic reduction of the films formed at constant potential and at constant current, the primary passive film formed in the potential range from -0.455V to +0.22V (she) is estimated to be CoO·nH₂O, the secondary passive film in the range from +0.22V to +1.1V to, be mainly Co₃O₄, and the transpassive film at potentials more positive than 1.1V to be Co₃O₄ and Co₂O₃·mH₂O. In the potential region of the secondary passivity (0.2V to 0.45V), both CoO·nH₂O and Co₃O₄ are formed under potentiostatic conditions and the ratio in amount of Co₃O₄ to CoO·nH₂O increases with rising potential. In this potential region, CoO·nH₂O is first formed and then gradually changes to Co₃O₄ at constant potential.