Principles of corrosion resistance of titanium

Titanium is a very corrosion resistant metal. However, the thermodynamic data for titanium show that titanium is an extremely thermodynamically unstable metal. If titanium can be dissolved to form Ti2+, its standard electrode potential is very negative (-1.63V), and its surface is always covered with a dull titanium oxide film. This makes the stabilizing potential of titanium steadily positive, e.g., the stabilizing potential of titanium in seawater at 25℃ is about +0.09V.

Titanium electrode reaction potential data show that its surface is very active, usually covered with a layer of oxide film, which is naturally generated in the air, it is this layer of stability, strong adhesion, protective oxide film, determines the excellent corrosion resistance of titanium. Theoretically, the Pilling/Bedworth ratio of the protective oxide film must be greater than 1. If the ratio is less than 1, the oxide film cannot completely cover the metal surface and does not play a protective role. If this ratio is too large, the pressure stress in the oxide film increases accordingly, easily causing the rupture of the oxide film, and also cannot play a protective role. Titanium P / B ratio with the composition of the oxide film and the structure of different, between 1 ~ 2.5.

Exposure of titanium surface in the atmosphere or aqueous solution, immediately generate a new oxide film, for example, at room temperature in the atmosphere oxide film thickness of about 1.2 ~ 1.6nm, and with the extension of time and thickening, 70 days after the natural thickening of 5nm, 545 days after the gradual increase of 8 ~ 9nm. artificially strengthen the oxidation conditions (such as heating, oxidizing agent or anodic oxidation, etc.) can accelerate the surface of the oxide film growth and get a thicker oxide film. Growth and get thicker oxide film, thus improve the corrosion resistance of titanium. Therefore, anodic oxidation and thermal oxidation of the oxide film generated, will significantly improve the corrosion resistance of titanium.

Titanium oxide film (including thermal oxidation film or anodic oxidation film) is usually not a single structure, the composition and structure of the oxide with the generation of conditions and changes. In general, TiO2 may be present at the interface between the oxide film and the environment, while TiO may dominate at the interface between the oxide film and the metal. In between, there may be transition layers with different valence states, or even non-chemical equivalent oxides, which means that the oxide film of titanium has a multilayer structure. As for the process of generating this oxide film, it can not be simply understood as a result of the direct reaction between titanium and oxygen.