Ellipsometric Analysis of Changes in Surface Oxide Films on Titanium with Anodic and Cathodic Polarization

Abstract

In order to investigate the effect of hysteresis of applied potential on the nature and thickness of surface oxide film, ellipsometric measurements have been performed in-situ on Ti electrodes under anodic and cathodic polarization in 0.5 kmol·m⁻³H₂SO₄. The determination of optical constants for metallic Ti and Ti hydride, which are indispensable for the calculation of thickness and optical constants of the films formed on these materials, has also been performed using the tribo-ellipsometric technique.
In case of simple anodic oxidation started from a corrosion potential, the refractive indices of optical constants for anodic oxide films on metallic Ti decreased and the growth rates of the films per unit voltage increased with increasing potential.
When a Ti electrode covered with an anodic oxide film was polarized to cathodic potentials, a decrease in thickness and changes in optical constants for the film immediately occurred. That is, the refractive index and extinction coefficient of optical constant for the film changed from 2.0 to 1.8 and from 0 to 0.1, respectively, indicating changes in the composition and structure of the film. The formation of hydride layer also occurred simultaneously under the film. The film on the hydride layer disappeared after prolonged strong cathodic polarization.
Furthermore, Ti electrode covered with hydride layer, which was formed by cathodic polarization, was polarized to anodic potentials. The formation of anodic oxide films were found on the hydride layer. The refractive index of optical constants for the anodic oxide films on the hydride layer were smaller than those for the films on metallic Ti.