Titanium is a very corrosion resistant metal. However, the thermodynamic data of titanium show that it is a thermodynamically very unstable metal. The standard electrode potential of titanium is very low (-1.63 V) if it can dissolve to generate Ti2+, and its surface is always covered with an oxide film. This makes the stable potential of titanium steadily biased towards positive values, e.g. the stable potential of titanium in seawater at 25°C is about +0.09 V. In chemistry manuals and textbooks, we can obtain standard electrode potentials corresponding to a range of titanium electrode reactions. It is worth pointing out that in practice these data are not directly measured, but often only calculated from thermodynamic data, and it is not surprising that several different electrode reactions may be represented and different data appear at the same time due to different data sources.
The electrode potential data of the electrode reaction of titanium shows that its surface is very active and is usually always covered with an oxide film that is naturally generated in air. Therefore, the excellent corrosion resistance of titanium stems from the fact that there will always be a stable, highly adherent and particularly protective oxide film on the surface of titanium, and it is actually the stability of this natural oxide film that determines the corrosion resistance of titanium. Theoretically, the P/B ratio of a protective oxide film must be greater than 1. If it is less than 1, the oxide film cannot completely cover the metal surface and therefore cannot play a protective role. If the ratio is too large, the compressive stress in the oxide film will increase accordingly, which will easily cause the oxide film to rupture and will not play a protective role. The P/B ratio of titanium is between 1 and 2.5 depending on the composition and structure of the oxide film, and from this basic point of analysis, the oxide film of titanium can have relatively good protective properties.
When the surface of titanium is exposed to the atmosphere or aqueous solution, a new oxide film will be generated immediately and automatically, for example, the thickness of the oxide film is about 1.2~1.6nm in room temperature atmosphere and thickens with time, naturally thickening to 5nm after 70 days and gradually increasing to 8~9nm after 545 days. The growth of oxide film can be accelerated and a thicker oxide film can be obtained, thus improving the corrosion resistance of titanium. Therefore, anodic oxidation and thermal oxidation of the oxide film generated will significantly improve the corrosion resistance of titanium.
The oxide film of titanium (including thermal or anodic oxide films) is usually not a single structure, and the composition and structure of the oxide varies with the conditions of generation. In general, at the interface between the oxide film and the environment may be TiO2, however, at the interface between the oxide film and the metal may be dominated by TiO And there may be transition layers of different valence states or even non-chemical equivalent oxides in between, which indicates the existence of a multilayer structure of the titanium oxide film. As for the process of generation of this oxide film, it cannot be simply understood as a direct reaction between titanium and oxygen (or oxygen in air). Many researchers have proposed different mechanisms, and workers in the former Soviet Union have suggested that hydrides are first generated, and then oxide films are formed on top of the hydrides.
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