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

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 Δ0 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^-212.6×10^-20