The electrode potential data of the electrode reaction of titanium shows that its surface is very active and is usually covered with the oxide film naturally formed in the air. Therefore, the excellent corrosion resistance of titanium stems from the fact that there is always a stable, strong adhesion and protective oxide film on the titanium surface. In fact, the stability of this natural oxide film determines the corrosion resistance of titanium. Theoretically, the P/B ratio of the protective oxide film must be greater than 1. If it is less than 1, the oxide film cannot completely cover the metal surface, so it cannot play a protective role. If the ratio is too large, the compressive stress in the oxide film will increase correspondingly, which is easy to cause the oxide film to crack and will not play a protective role. The P/B ratio of titanium varies from 1 to 2.5 according to the composition and structure of the oxide film. From this basic point, the oxide film of titanium can have better protective performance.
When the surface of titanium is exposed to the atmosphere or water solution, it will automatically generate a new oxide film immediately, for example, the thickness of the oxide film is about 1 2 ~ 1.6 nm, and thickens with time, naturally thickens to 5 nm after 70 days, and gradually increases to 8 ~ 9 nm after 545 days. The artificially enhanced oxidation conditions (such as heating, using oxidant or anodic oxidation) can accelerate the growth of the oxide film on the titanium surface and obtain a relatively thick oxide film, thus improving the corrosion resistance of titanium. Therefore, the oxide film formed by anodic oxidation and thermal oxidation will significantly improve the corrosion resistance of titanium.
The oxide film of titanium (including thermal oxide film or anodic oxide film) is usually not a single structure, and the composition and structure of its oxide vary with the formation conditions. Generally, the interface between the oxide film and the environment may be TiO2, while the interface between the oxide film and the metal may be dominated by TiO2. In the middle, there may be transition layers with different valence states, even non-chemical equivalent oxides, which means that the oxide film of titanium has a multi-layer structure. As for the formation process of this oxide film, it can not be simply understood as the direct reaction between titanium and oxygen (or oxygen in the air). Many researchers have proposed various mechanisms. The former Soviet Union workers believed that the hydride was first generated, and then the oxide film was formed on the hydride.