Corrosion of titanium and titanium alloys

Uniform corrosion occurs on the surface of titanium samples or workpieces, forming a layer of corrosion products with a uniform thickness that adheres closely to the titanium surface and generally does not expand inward over time, but there are exceptions. In many corrosive media, titanium's corrosion performance is as good or better than other metals with protective coatings, such as aluminum. The corrosion of titanium is usually electrolytic in nature, so there is a certain relationship between corrosion and electrode potential and electrodynamic current. Anodic and cathodic polarization also have a strong influence on corrosion mechanisms and rates. The electrical potential of titanium depends largely on the insulating properties of the oxide film. Therefore, the characteristics of the titanium surface oxide film play a decisive role in its corrosion resistance. Any factors that can improve the density, thickness, and insulation properties of the oxide film will help improve corrosion resistance. On the contrary, any factor that reduces the effective protective ability of the oxide film, whether mechanical or chemical, will cause the corrosion resistance of titanium to drop dramatically.

In most cases corrosion of titanium is localized in nature, with the extent of corrosion at one point being significantly different than the extent of corrosion at another point. Crevice corrosion, cavity corrosion, stress corrosion cracking, etc. are all localized corrosion. Crevice corrosion usually occurs between flanges or folds and between gaps near build-up. This will not happen if the gap is too small or too large. Cavitation corrosion is a type of corrosion that occurs in openings. This type of corrosion can easily occur in the presence of CI-, Br-, I-plasma. Stress corrosion cracking is a kind of corrosion that occurs in workpieces or specimens under the combined action of tensile stress and corrosive environment.

The form of corrosion of samples or workpieces in corrosive flowing media is accelerated by the mechanical action of the fluid, because the fluid can take away some or all of the corrosion products, expose new surfaces, and accelerate corrosion.

Dissimilar metal contact corrosion is also called galvanic corrosion. In a corrosive environment, placing two metal or structural components with different electrical potentials. If an electrical short occurs, the metal with the lower potential will corrode.

Suck H2 or H2 Crisp

Under normal circumstances, titanium and titanium alloys always contain H2. If H2 is extracted from a material, brittle hydrides will form when the extracted amount exceeds the solid solution limit, leading to hydrogen embrittlement.

In most cases, the corrosion of titanium and titanium alloys is localized, and the degree of corrosion at one point is greatly different from the degree of corrosion at another point. Therefore, quantitative evaluation of corrosion can only be based on a large amount of statistical material rather than the results of a few samples. Another serious issue in assessing corrosion is what to use as a standard. Mass loss is rarely used, and the degree of corrosion is mostly judged based on strength loss, changes in surface appearance, or perforations. Generally speaking, titanium and titanium alloys corrode slowly. Unless you are completely unfit for the conditions. To properly evaluate the performance of titanium, testing is often conducted over dozens of days or even years. In many cases, titanium and titanium alloys corrode quickly at first, then slow down, and finally only weak corrosion often occurs. But in some cases, titanium alloys will undergo transformation after a period of time, and the structure and properties will change dramatically. Therefore, short-term use tests are not entirely reliable. There are many rapid testing methods, but generally speaking, the faster the test, the less reliable the results.

Titanium is one of the thermodynamically extremely 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 tends to be positive. For example, the stable potential of titanium in seawater at 25°C is about 0.09V. The electrode potential is mostly calculated based on thermodynamic data. Due to different data sources, different data may appear, which is normal.

There is always a thin oxide film on the surface of titanium and titanium alloys, which is naturally generated in the air. Its excellent corrosion resistance comes from the stable, strong adhesion and good protective oxide film that always exists on its surface. The corrosion resistance of the protective film is expressed by the P/B ratio. Only when the P/B value is greater than 1 does it have a protective effect. Otherwise, the corrosion resistance will be low, but it cannot be greater than 2.5. If it is greater than this value, the compressive stress in the oxide film increases, which can easily cause the oxide film to rupture and reduce corrosion resistance. The optimal value is 1~2.5.

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 grows with time. It will increase to 5nm after 70 days and can be thickened to 8nm~9nm after 545 days. . Artificially strengthening oxidation conditions, such as heating, adding oxidants or anodizing, can accelerate oxidation, increase film thickness, and improve corrosion resistance.

The oxide film on the surface of titanium and titanium alloys is generally not a single structure, and its composition and structure are related to the formation conditions. Usually, the interface between the oxide film and the environment is mainly TiO2, while the interface between the oxide film and the metal may be mainly TiO2, with transition layers of different valence states or even non-stoichiometric oxides in the middle. This means that the surface oxide film of titanium and titanium alloys has a complex multi-layer structure. As for their formation process, it cannot simply be understood as a direct reaction between Ti and O2. Several researchers have proposed multiple formation mechanisms. Russian scholars believe that hydride is generated first, and then a pure oxide film is formed on the hydride.