Titanium oxide film

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The electrode potential data for the electrode reactions of titanium indicate that its surface is highly reactive; it is usually covered by an oxide film that forms naturally in air. Therefore, titanium’s excellent corrosion resistance stems from the presence of a stable, strongly adherent oxide film on its surface, which provides excellent protection. In fact, it is the stability of this natural oxide film that determines titanium’s corrosion resistance. When the surface of titanium is exposed to the atmosphere or an aqueous solution, a new oxide film is formed immediately. For example, at room temperature, the thickness of the oxide film in the atmosphere is 1.2–1.6 nm, and this thickness increases over time – it rises to 5 nm after 70 days and to 8–9 nm after 545 days. Artificially enhancing oxidation conditions (such as heating, using oxidants, or anodization) can accelerate the growth of the surface oxide film and result in a thicker oxide film, thereby improving the corrosion resistance of titanium. The oxide film of titanium is usually not a single structure; the composition and structure of its oxides vary depending on the conditions under which it is formed. Under normal circumstances, TiO2 may be present at the interface between the oxide film and the environment, whereas TiO is likely to predominate at the interface between the oxide film and the metal. Moreover, there may be transition layers with different valence states in between, or even non-stoichiometric oxides, indicating that the titanium oxide film has a multi-layer structure.