2021年8月8日星期日

The difference between manual and machine polished titanium rods

Titanium is a very corrosion-resistant metal. The thermodynamic data of titanium shows that titanium is a very thermodynamically unstable metal. If titanium can be dissolved to generate Ti2+, its standard electrode potential is very negative (-1.63V), and its surface is always covered with a passive 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 manuals and textbooks, standard electrode potentials corresponding to a series of titanium electrode reactions can be obtained. It is worth pointing out that, in fact, these data are not directly measured, but often can only be calculated from thermodynamic data, and due to different data sources, several different electrode reactions may be displayed at the same time, and different data may not appear. Strange.
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 formed in the air. Therefore, the excellent corrosion resistance of titanium stems from the existence of a stable, adherent, and particularly protective oxide film on the surface of the titanium. In fact, it is the stability of this natural oxide film that determines the titanium surface. Corrosion resistance, including titanium and titanium alloy titanium rods, titanium wires, titanium plates, etc., have strong corrosion resistance. Of course, the corrosion resistance of various grades is different. We talked about it in the previous content of the website I won’t say much today. In theory, 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, so it is impossible to protect it. If this ratio is too large, the compressive stress in the oxide film will increase correspondingly, which will easily cause the oxide film to rupture, and it will not provide protection. The P/B ratio of titanium is between 1 and 2.5 depending on the composition and structure of the oxide film. From this basic point of analysis, the titanium oxide film 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 1.2~1.6nm, and it will thicken with time, and it will naturally increase to 5nm after 70 days. Gradually increase to 8-9nm after 545 days. Artificially intensified oxidation conditions (such as heating, using oxidants or anodic oxidation, etc.) can accelerate the growth of the surface oxide film and obtain a thicker 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. Now our customers have made many similar products with our titanium rods and wires, which shows that this is a feasible way.
Titanium oxide film (including thermal oxide film or anodic oxide film) is usually not a single structure, and its oxide composition and structure vary with the production conditions. Under normal circumstances, 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 other words, the surface of the titanium rod we produce is TiO2 under normal conditions, and the interface between the metal and the oxide film is TiO. Of course, the titanium wire, titanium plate, and titanium forgings are the same here, and the surface of the titanium alloy rod is more complicated. However, whether it is a pure titanium rod, a titanium alloy rod, or a titanium alloy wire, there are transition layers with different valence states in the middle, and even non-chemically equivalent oxides, which means that the oxide film of titanium materials has a multilayer structure. As for the formation of this oxide film, it cannot be simply understood as a direct reaction between titanium and oxygen (or oxygen in the air).

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