Ellipsometric study combined with electrochemical measurements has been made on anodic oxide films on nickel in neutral sodium borate-boric acid solution in the potential regions of passivity and oxygen evolution. It is found that the oxide film once formed in anodic oxidation is not easily removed by the cathodic reduction, and that the surface treated with the potentiostatic reduction immediately after electropolishing gives good reproducibility in subsequent measurements. The reduced surface thus obtained is used as the optical reference. Films formed in the passive region are assumed to be NiO with optical constant of 2.7-0.25i, and the thickness estimated from ellipsometry increases linearly with rise of the potential from 4 to 13Å. The thickness is about 1.3 times thinner than that calculated from the electrochemically measured anodic film charge assuming the film as NiO.
In the oxygen evolution potential region, the optical surface state is unsteady even after 1hour anodic oxidation. It is shown, however, that the film in oxygen evolution not only undergoes the change of its optical constant but also thickens rapidly with rise of the potential, and that at a certain potential the optical change begins to deviate from the expected direction of film growth. It is concluded that in the oxygen evolution region the film initially converts to a higher nickel oxide, probably Ni₂O₃, with continuation of growth up to a limiting thickness and finally takes a constant thickness independent of the potential.