日本金属学会誌

J. Japan Inst. Metals, Vol. 40, No. 2 (1976),
pp. 124-132

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

Toshiaki Ohtsuka1, Kiyokatsu Kudo2 and Norio Sato2

1Graduate School, Hokkaido University, Sapporo
2Faculty of Engineering, Hokkaido University, Sapporo

Abstract:

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 μ A/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 μ A/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.


(Received 1975/09/1)

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