The corrosion resistance of titanium and its underlying mechanism

Titanium is renowned for its corrosion resistance. Despite the thermodynamic data indicating that it is a highly unstable metal, in reality, titanium exhibits excellent corrosion resistance.

If titanium could dissolve to form Ti2+, its standard electrode potential would be extremely negative (-1.63V). However, the surface of titanium is always covered with a layer of passive titanium oxide film, which steadily shifts the stable potential of titanium towards a positive value. For example, in seawater at 25℃, the stable potential of titanium is approximately +0.09V.

The electrode reaction potential data of titanium indicates that its surface is highly reactive, but it is usually covered with an oxide film. This oxide film is naturally formed in the air and is characterized by its stability, strong adhesion, and strong protective properties. It is the key factor determining titanium's excellent corrosion resistance. Theoretically, the Pilling/Bedworth ratio of a protective oxide film must be greater than 1. The P/B ratio of titanium ranges from 1 to 2.5 depending on the composition and structure of the oxide film, which enables the titanium oxide film to completely cover the metal surface and provide good protection.

When the surface of titanium is exposed to the atmosphere or aqueous solution, a new oxide film is immediately formed. For instance, in room temperature atmosphere, the thickness of the oxide film is approximately 1.2~1.6nm and increases with time. After 70 days, it naturally thickens to 5nm; after 545 days, it gradually increases to 8~9nm. Artificially enhancing oxidation conditions (such as heating, using oxidants, or anodizing) can accelerate the growth of the surface oxide film and obtain a thicker oxide film, thereby improving the corrosion resistance of titanium. Therefore, oxide films formed by anodizing and thermal oxidation both significantly enhance the corrosion resistance of titanium.

The oxide film of titanium (including thermal oxide film or anodic oxide film) typically does not have a single structure, and the composition and structure of its oxide vary depending on the formation conditions. Generally, at the interface between the oxide film and the environment, it may be TiO₂, while at the interface between the oxide film and the metal, it may primarily be TiO. There may be a transition layer with different valence states in between, or even non-stoichiometric oxides, indicating that the oxide film of titanium has a multilayer structure. As for the formation process of this oxide film, it cannot be simply understood as the result of direct reaction between titanium and oxygen.