Two‐Layer Formation of Passivating Films on Cobalt in Neutral Solution

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Abstract
Layer structure and thickness of passivation oxide films on cobalt have been investigated at stationary and transient states in solution of 0.30 mol dm^−3 boric acid‐0.075 mol dm^−3 sodium borate mixture of pH 8.4 by means of chemical analysis and in situ ellipsometry combined with electrochemical measurements. A two‐layer structure composed of an inner CoO and an outer CO3ΔO4 is found in a potential range more positive than 0.75V (vs.the hydrogen electrode at the same solution). The nonstoichiometry changes from Δ＝0 to 0.33 with increasing anodic potential. The outer CoΔO4 layer thickness increases linearly with potential, whereas the inner layer thickness is not a simple function of potential, but depends on the film formation procedure. A step‐wise potential increase gives an initial rapid growth of the outer Co3ΔO4 layer followed by a gradual thickening of the inner CoO layer. The transient film growth behavior is interpreted in terms of a field‐assisted ion diffusion mechanism, and the self‐diffusion coefficient in the inner CoO layer is estimated to be D/cm sec^-1 = 6.5 X 10^-21～2.6×10^-20

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