Overall corrosion
The uniform corrosion occurs on the surface of titanium sample or workpiece, forming a layer of corrosion products with uniform thickness, which is tightly attached to the titanium surface, and generally does not expand inward with the extension of time, but there are exceptions. In many corrosive media, the corrosion performance of titanium is as good or better as that of other metals with protective layer (such as aluminum). The corrosion of titanium is usually electrolytic, so there is a certain relationship between corrosion and electrode potential and electromotive current. The anodic and cathodic polarization also has a strong impact on the corrosion mechanism and speed. The potential of titanium depends largely on the insulating property of the oxide film. Therefore, 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 insulation performance of the oxide film will contribute to the improvement of the corrosion resistance. On the contrary, any factor that reduces the effective protective capacity of the oxide film, whether mechanical or chemical, will sharply reduce the corrosion resistance of titanium.
Local corrosion
The corrosion of titanium under most conditions is local, and the degree of corrosion at one point is significantly different from that at another point. Crack corrosion, hole corrosion, stress corrosion cracking, etc. are all local corrosion. The crevice corrosion often occurs at the flange or hem and between the crevices near the accumulation, and the crevices are too small or too large to occur. Cavity corrosion is a kind of corrosion occurring in the opening, which is easy to occur in the presence of CI -, Br -, I - plasma. Stress corrosion cracking (SCC) is a kind of corrosion of workpiece or sample under the joint action of tensile stress and corrosion environment.
abrasion
The corrosion form of the sample or workpiece in the corrosive flowing medium is accelerated due to the mechanical action of the fluid. The fluid can take some or all of the corrosion products away, expose the new surface, and accelerate the corrosion.
Contact corrosion of dissimilar metals
Contact corrosion of dissimilar metals is also called galvanic corrosion. In a corrosive environment, place two kinds of metal or structural parts with different potentials. In the case of electric short circuit, the metal with low potential will corrode.
H2 absorption or H2 brittleness
Under normal conditions, titanium and titanium alloys always contain H2. If H2 is extracted from the material, when the extraction amount exceeds the solid solution limit, brittle hydrides will be formed, resulting in hydrogen embrittlement.
Under most conditions, the corrosion of titanium and titanium alloys is local. At the same time, the corrosion degree of one point is very different from that of another point. Therefore, the quantitative evaluation of corrosion can only be based on a large number of statistical materials, not on the results of several samples. Another serious problem in evaluating corrosion is what to take as the standard. Quality loss is rarely used, and corrosion degree is mostly judged by strength loss, surface appearance change or perforation. Generally speaking, the corrosion process of titanium and titanium alloys is slow. Unless they are completely unfit for the conditions. In order to correctly evaluate the service performance of titanium, it is usually necessary to carry out tests for decades or even years. In many occasions, titanium and titanium alloys corrode rapidly at first, then slow down, and often only slightly at last. However, in some cases, titanium alloy will change after a period of time, and the structure and properties will change dramatically.
Therefore, the short-term use test is not completely reliable. There are many fast use test methods, but generally speaking, the faster the test speed, the lower the reliability of the results.
Titanium is one of the most thermodynamically unstable metals. Its standard electrode potential is -1.63V. The surface is always covered with a thin and dense TiO2 film. Therefore, the stability potential of titanium and titanium alloys is positive. For example, the stability potential of titanium in 25 ℃ seawater is about 0.09V. The electrode potential is mostly calculated from thermodynamic data, and different data may appear due to different data sources, which is normal.
There is always a thin oxide film on the surface of titanium and titanium alloys that is naturally formed in the air. Its excellent corrosion resistance is due to the existence of a stable, strong adhesion and good protection oxide film on its surface. The corrosion resistance of this protective film can be expressed by P/B ratio. If the P/B value is greater than 1, it can be protected. Otherwise, the corrosion resistance is low, but it can not be greater than 2.5. If it is greater than this value, the compressive stress in the oxide film will increase, which is easy to cause the oxide film to crack, and the corrosion resistance will decline. The best value is 1~2.5.
Titanium will form an oxide film immediately in the atmosphere or aqueous solution. The thickness of the film formed in the atmosphere at room temperature is 1.2 nm~1.6 nm, and increases with time, increasing to 5 nm after 70 days, and 8 nm~9 nm after 545 days. Artificial enhanced oxidation conditions, such as heating, adding oxidant or anodizing, can accelerate oxidation, increase film thickness and improve corrosion resistance.
Generally, the oxide film on the surface of titanium and titanium alloy is not a single structure, and its composition and structure are related to the formation conditions. Generally, the interface between the oxide film and the environment is mostly TiO2, while the interface between the oxide film and the metal may be mainly TiO2, and the middle is a transition layer with different valence states, or even a non-chemical equivalent oxide, which means that the oxide film on the surface of titanium and titanium alloys is a complex multi-layer structure. As for their formation process, it cannot be simply understood as the direct reaction of Ti and O2. Some researchers have proposed various formation mechanisms, and Russian scholars believe that hydride is generated first, and then a pure oxide film is formed on the hydride.
