In many corrosive media, the corrosion performance of titanium is as good as or better than that of other metals (such as aluminum) with a protective layer. The corrosion of titanium is usually electrolytic, so there is a certain relationship between corrosion and electrode potential and electromotive current. The characteristics of the oxide film on the titanium surface play a decisive role in its corrosion resistance. All factors that can improve the compactness of the oxide film, increase the thickness of the oxide film, and improve the insulating properties of the oxide film are all conducive to the improvement of corrosion resistance. On the contrary, any factor that reduces the effective protection ability of the oxide film, whether it is mechanical or chemical, will make the corrosion resistance of titanium drop sharply. Generally, there are the following types of corrosion:
1. Local corrosion
The corrosion of titanium under most conditions is local in nature, and the degree of corrosion at one point is quite different from that at another point. Crevice corrosion, cavitation corrosion, stress corrosion cracking, etc. are localized corrosion. Crevice corrosion mostly occurs at flanges or folds and in crevices near deposits, and it will not occur if the crevice is too small or too large. Cavitation corrosion is a type of corrosion that occurs in open pores. Stress corrosion cracking is a kind of corrosion that occurs when the workpiece or sample is under the combined action of tensile stress and corrosive environment.
2. Abrasion
The corrosion form of the sample or workpiece in the corrosive flowing medium, due to the mechanical action of the fluid, the corrosion is accelerated, because the fluid can take away part or all of the corrosion products, expose new surfaces, and accelerate corrosion.
3. Dissimilar metal contact corrosion
It is also called galvanic corrosion. In a corrosive environment, two metals or structural parts with different potentials are placed. In the case of an electrical short circuit, the metal with a low potential will corrode.
The surface of titanium and titanium alloys always has a thin layer of oxide film that is naturally formed in the air. Its excellent corrosion resistance comes from the existence of a stable, strong adhesion and good protection oxide film on the surface. . Titanium will immediately form an oxide film in the atmosphere or aqueous solution. The thickness of the film formed in the atmosphere at room temperature is 1.2nm~1.6nm, and it will increase with time. It will increase to 5nm after 70 days and 8nm~9nm after 545 days. . To enhance the corrosion resistance, methods of artificially strengthening oxidation conditions, such as heating, adding oxidizing agents or anodic oxidation, can be used to accelerate oxidation and increase film thickness.
