The anodic oxide film on titanium has been studied by ellipsometry and SEM observation. Ex situ multiple-angleof-incidence and in situ ellipsome_tric measurements allow the complex refractive index to be estimated at n = 2.3-2.9i for the titanium substrate and at n = 2.1-0.03i for the anodic oxide film at wavelength 546.1 nm. The anodic oxide film thickness increases linearly with potential in a range from -0.55 to 7.5V (RHE) at the rate of 2.8 nm V -' in phosphate solutions ofpH 1.6-12.1, 2.5 nm V-' in 0.1M HC1 solution, and 2.4 nm V-' in 0.1M H2SO4 solution. At potentials more positive than 7.5V, the film breaks down, leading to the formation of a thick oxide film probably due to an increased ionic current through the breakdown sites. The film composition is estimated to be TiO2(H20)l.4 or TiO0.6(OH)2.8, which suggests the presence of hydroxyl bridge in its bonding structure.
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Ohtsuka et al. (1985) studied this question.