The passivity of titanium—part III: characterization of the anodic oxide film

Abstract

The thickness of the anodic titanium oxide film formed on titanium in 0.5 M H2SO4 at 22 °C was measured by ellipsometry, and the Forouhi interband single-layer model was used to analyze the data. The anodizing constant was determined experimentally to be 2.75 nm/V, which is in good agreement with literature data, and indicates that the anodizing constants for barrier layers of anodic oxide films formed on a wide variety of metals and alloys lie within the range of 2.2–3.0 nm/V. Using a value for the polarizability of the barrier layer/solution interface obtained via electrochemical impedance spectroscopy (EIS) together with the anodizing constant, the electric field is estimated to be a voltage-independent 1.82 × 106 V/cm. For the anodic oxide film formed on titanium in 0.5 M H2SO4 solution by galvanostatic polarization, the thickness was maintained to be virtually constant even though the oxygen vacancy concentration (donor density) analyzed by the Mott-Schottky relation varied over a wide range, as a function of the film formation rate (current density), with higher donor densities being found for lower current densities. The increase in the donor density leads to a decrease in the modulus of the impedance.