Cathodic Reduction of Anodic Oxide Films Formed on Titanium

Cathodic reduction behavior of the anodic oxide film on titanium has been investigated by using ellipsometry combined with electrochemistry. In acidic sulfate solution, the anodic oxide film reductively dissolves into the solution as Ti(III) ion resulting in the thinning of its thickness without any significant change of the optical property of the remaining film. In neutral phosphate solution, the anodic oxide film absorbs hydrogen in the hydrogen evolution potential region, resulting in a change of the optical property without thinning its thickness. The amount of hydrogen absorbed per unit volume of the film does not depend on the film thickness but on the cathodic potential. The composition change estimated from measurements of anodic charge during the hydrogen release process indicates that the hydrogen absorption begins to occur at about −0.25V ( vs. RHE) and that the anodic film changes in its composition from to at −0.9V. The hydrogen absorption induces a decrease of the refractive index and an increase of the extinction index of the anodic film.