Anodic oxidation of titanium: Mechanism of non-stoichiometric oxide formation

Structurally sensitive in situ methods such as photopolarization and impedance were used to examine the passivation process and the properties of the protective oxide layers on titanium. The kinetics of anodic oxidation and the non-stoichiometry of the surface oxide were correlated. The composition of the anodic film on titanium changes with the relative potential from lower to higher oxidation stages according to TiH₂+TiO→nTi₂O₃·nTiO₂→Ti₅O₉orTi₆O₁₁→Ti₃O₅→TiO₂

The characteristic behaviour of titanium as a member of the valve metal group can easily be seen at higher anodic potentials (approximately +1.5 V (SCE) in 5 mol H₂SO₄ dm^-3) when the electrode is covered with a nearly stoichiometric TiO₂ layer. The semiconducting properties of TiO₂ were investigated using an anodic film stabilized at +2 V(SCE) and it was found that TiO₂, like the lower titanium oxides, is an n-type semiconductor under anodic polarization. The flat-band potential for the stabilized TiO₂ film was -0.2 V, as determined by the two methods in 5 mol H₂SO₄ dm^-3. The donor concentration was 2×10²⁰cm^-3 under the same conditions (a value of 60 was used for the dielectric constant).