Thickness and Layer Structure of Anodic Passivating Films on Cobalt in Neutral Solution

The thickness and layer structure of the passive film formed on cobalt by potentiostatic oxidation in a borate-buffer solution of pH 8.42 have been studied by ellipsometry, electrochemical techniques and chemical analysis. The primary passive film is formed in a potential region from −0.05 to +0.37 V (vs SHE) where the steady dissolution current of cobalt is about 2 μA/cm2 almost independent of the potential. This film is composed of CoO with the complex refractive index N=2.3−0.1i and the thickness which increases from 20 to 25 Å with rising potential. The secondary passive film is formed at potentials more positive than 0.4 V, where the steady dissolution current is less than 0.1 μA/cm2. It consists of an inner layer of CoO with the refractive index N=2.3−(0.1∼0.3)i and an outer layer of oxide of either Co3O4 or Co2O3. Transition from Co3O4 to Co2O3 occurs in a potential region from 0.72 V to 0.95 V. The refractive index of the outer layer was estimated as N=3.2−0.5i for Co3O4 and N=3.2−0.95i for Co2O3. The thickness of the outer layer increases almost linearly with the potential up to 37 Å at +1.15 V and remains constant in a potential region more positive than +1.15 V, where the transpassive dissolution occurs. The inner layer of CoO also increases with potential but has a maximum thickness of about 40 Å at +0.8 V around which the transition from Co3O4 to Co2O3 occurs.