Abstract
Ellipsometric and electrochemical techniques are used to study the anodic oxidation of titanium in aqueous and nonaqueous electrolytes. Transparent oxide films with a refractive index of 2.48 grown on electropolished surfaces exhibit ideal valve‐metal behavior up to a thickness of approximately 30 nm. Beyond this thickness the refractive index and low‐frequency dielectric constant of the film both decrease, but thicknesses of 100 nm can be achieved prior to complete breakdown under suitable experimental conditions. Open‐circuit transients are used to determine the relation between the current density and the electric field in the oxide film. At an anodization current density of 325 μA/cm2 the field in the film is found to be 0.484 V/nm, and the value of B in the dependence of current density on field, 
, is found to be 22.1 nm/V. There is some evidence that the current is controlled by the effective field in the oxide film and hence is sensitive to field‐induced variations in the dielectric constant. Electrostriction is found to affect titanium oxide to about the same extent as other valve‐metal oxides. If the current is made cathodic prior to film breakdown, the insertion of hydrogen into the film causes the initially transparent oxide to absorb light. If the hydrogen that enters the film is assumed to be uniformly distributed, a value of 0.1 is calculated for the optical extinction coefficient when the hydrogen concentration reaches its limiting value. There is evidence that the distribution of hydrogen within the oxide film is not uniform, and that it may penetrate only the outer 10% of thicker films that have not broken down. The alteration in film structure resulting from breakdown produces marked changes in both the cathodic process and the optical behavior.