Titanium is a very corrosion-resistant metal. However, the thermodynamic data of titanium show that titanium is a very unstable metal. If titanium can be dissolved to form Ti2+, its standard electrode potential is very low (-1.63V), and its surface is always covered with an oxide film...
Titanium is a very corrosion-resistant metal. However, the thermodynamic data of titanium show that titanium is a very unstable metal. If titanium can be dissolved to form Ti2+, its standard electrode potential is very low (-1.63V), and its surface is always covered with an oxide film. In this way, the stable potential of titanium is stably biased to a positive value. For example, the stable potential of titanium in seawater at 25°C is about +0.09V. In chemistry handbooks and textbooks, we can obtain standard electrode potentials corresponding to a series of reactions at titanium electrodes. It is worth pointing out that, in fact, these data are not directly measured, but can only be calculated from thermodynamic data, and due to different data sources, it may not be possible to represent several different electrode reactions and different data at the same time. Strange.
The electrode potential data of the electrode reaction of titanium show that its surface is very active and is usually always covered with an oxide film that occurs naturally in the air. Therefore, the excellent corrosion resistance of titanium stems from the existence of a stable oxide film with strong adhesion and good protection on the surface of titanium. In fact, the stability of this natural oxide film determines the stability of titanium oxide film. of 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 cannot completely cover the metal surface, so it is impossible to play a protective role. If this ratio is too large, the compressive stress in the oxide film will increase accordingly, which will easily cause the rupture of the oxide film, and will not play a protective role. The P/B ratio of titanium varies with the composition and structure of the oxide film, and is between 1 and 2.5. From this basic point of view, the oxide film of titanium can have better protective properties.
When the surface of titanium is exposed to the atmosphere or aqueous solution, a new oxide film will be automatically formed immediately. For example, the thickness of the oxide film in the atmosphere at room temperature is about 1.2-1.6nm, and it will thicken with time, and will naturally increase after 70 days. Thickness to 5nm, gradually increased to 8-9nm after 545 days. Artificially enhanced oxidation conditions (such as heating, using oxidants or anodizing, etc.) can accelerate the growth of the oxide film on the titanium surface and obtain a relatively thick oxide film, thereby 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.
Titanium oxide film (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. In general, the interface between the oxide film and the environment may be TiO2, but the interface between the oxide film and the metal may be dominated by TiO. In the middle, there may be transition layers with different valence states, or even non-stoichiometric oxides, which means that the titanium oxide film has a multi-layer structure. As for the process of forming this oxide film, it cannot be simply understood as a direct reaction between titanium and oxygen (or oxygen in the air). Many researchers have proposed various mechanisms. Workers in the former Soviet Union believed that hydrides were first generated, and then an oxide film was formed on the hydrides.
