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Titanium is a very corrosion-resistant metal. However, the thermodynamic data for titanium show that titanium is a very thermodynamically unstable metal. If titanium can dissolve to produce Ti2, its standard electrode potential is very low (a 1.63V), its surface is always covered with an oxide film. This stabilizes the stable potential of titanium to a positive value, for example, the stable potential of titanium in sea water of 251 is about 0.09V. In chemistry manuals and textbooks, we can obtain standard electrode potentials corresponding to a series of titanium electrode reactions. It is worth pointing out that in fact these data are not measured directly but are often only calculated from the thermodynamic data and because different data sources may indicate several different electrode reactions and different data simultaneously Strange.

The electrode potential data for the electrode reaction of titanium show that its surface is very active and is usually always covered with an oxide film naturally formed in the air. Therefore, the excellent corrosion resistance of titanium from the titanium surface there will always be a stable, strong adhesion, particularly protective oxide film is actually the natural oxidation of this film determines the stability of titanium Corrosion resistance. Theoretically, the P / B ratio of the protective oxide film must be greater than 1. If it is less than 1, the oxide film can not completely cover the metal surface, and thus it is impossible to protect it. If this ratio is too large, the compressive stress in the oxide film correspondingly increases, which easily causes the rupture of the oxide film and also fails to provide a protective effect. Titanium P / B ratio with the oxide film composition and structure of different between 1-2.5, from this basic point of analysis, the titanium oxide film can have better protection performance.

The surface of the titanium is exposed to the atmosphere or aqueous solution and a new oxide film is automatically generated immediately. For example, the thickness of the oxide film in the atmosphere at room temperature is about 1.2-1.6mn and thickens with time, naturally thickening to 70% 5nm, 545 days later gradually increased to 8-9nm. Artificially enhanced oxidation conditions (such as heating, the use of oxidants or anodic oxidation, etc.) can accelerate the growth of the titanium surface oxide film and get a relatively thick oxide film, thereby enhancing the titanium corrosion resistance. Therefore, anodized and thermally oxidized oxide film, will significantly improve the corrosion resistance of titanium.

An oxide film of titanium (including a thermal oxide film or anodic oxide film) is usually not a single structure, and the composition and structure of the oxide thereof vary depending on the formation conditions. In general, the interface between the oxide film and the environment may be Ti02, however, the interface between the oxide film and the metal may be dominated by TiO. In the middle there may be transitional layers of different valence states, or even non-stoichiometric oxides, indicating that there is a multilayer structure in the titanium oxide film. As for this layer of oxide film formation process, can not simply be understood as titanium and oxygen (or oxygen in the air) from the direct reaction. Many researchers have proposed various mechanisms. Former Soviet workers believed that hydride was first formed and then an oxide film was formed on the hydride.