Corrosion Behavior Of Common Titanium Alloys

Due to various factors such as different types of titanium alloys, different applied loads, different corrosion media and different service conditions, titanium alloys will have different corrosion behaviors in different environments. The common corrosion behaviors of titanium alloys include stress corrosion, crevice corrosion, pitting corrosion, hydrogen embrittlement, etc.

Stress corrosion

Under the joint action of environment and stress, metal materials will produce lag cracks, even lag fracture, which is called stress corrosion cracking (SCC). SCC is actually a kind of brittle fracture under low stress, which is characterized by that the low stress causing SCC is far less than the stress at the time of fracture, and the material does not have large deformation during the whole stress corrosion process.

During the corrosion process of titanium alloy, with the formation of loose layer or passive film on the material surface, a large tensile stress will be generated, so the dislocation will start to move under the action of small external stress. When the local plastic deformation of the material develops to a critical state, SCC micro-strike begins to form. However, due to the passivation of dry titanium alloy, the film properties are relatively stable and not easy to be damaged, so titanium alloy is not easy to occur stress corrosion cracking in most environments.

However, in the chloride aqueous solution with high concentration, low pH value and high temperature, the damaged part of the titanium alloy passivation film may produce hydrogen adsorption due to local acidification, which may lead to fracture and embrittlement of cracks and stress corrosion cracking. The existence of stress cracks and defects in the passive film on the surface of titanium alloy is the premise of stress corrosion. However, although the stress corrosion cracking of titanium alloy will occur when low frequency cyclic load is applied, titanium alloy still has good resistance to stress corrosion cracking under many extreme conditions.

Crevice corrosion

There is a tendency of crevice corrosion under various conditions. When titanium alloy is applied to engineering structural parts, especially fasteners such as valves and pipe joints, it is easy to create crevice corrosion conditions. Through a large number of literature analysis, it is found that crevice corrosion of titanium alloy mostly occurs in halide solution, especially in chloride solution. At the same time, it is also found that titanium alloys are similar to common passive metals

The crevice corrosion mechanism of titanium in chloride is also in line with the autocatalytic theory, that is, the aggregation of H+generated by the hydrolysis of metal ions in the crevice causes the decrease of pH in the crevice and the internal migration of Cl - outside the crevice, further accelerating the dissolution rate of metal in the crevice.

Hydrogen embrittlement corrosion

Titanium and titanium alloys are easy to absorb hydrogen. When the material is in a hydrogen evolution corrosion environment, a small amount of hydrogen absorption on the surface can form hydride, which sharply reduces the impact toughness and elongation of the material. At present, many scholars at home and abroad have discussed the hydrogen embrittlement of titanium in various corrosive media. It is generally believed that all hydrogen embrittlement begins with the destruction of the passivation film on the surface of titanium alloy. Some studies have shown that the conditions for hydrogen embrittlement must be determined.

Zhongyuan Zhengda believes that hydrogen can be absorbed only if there is a mechanism to produce hydrogen, that is, hydrogen must be evolved to absorb hydrogen. Covinaton believes that hydrogen can be absorbed on the surface of titanium alloy only when the temperature is higher than 80 ℃ and there is a mechanism to produce hydrogen. However, at room temperature, due to the small diffusion coefficient of oxygen in titanium alloy, the diffusion of hydrogen in titanium alloy is relatively slow. Therefore, hydrogen is not easy to enter into titanium alloy but stays on the surface, so it will not have a significant impact on the properties of titanium alloy.

Other corrosion

When titanium alloy is used in connection with other metals, the positive potential of titanium alloy will cause galvanic corrosion of other metal materials in contact with it. Therefore, in practical applications, galvanic corrosion is also the focus of attention when titanium alloy is used.

In addition, titanium alloy has poor wear resistance due to its high friction coefficient and difficulty in effective lubrication, and wear corrosion will also occur in corrosive environment. When titanium alloy components are exposed to long-term wear conditions in corrosive media, titanium alloy is also vulnerable to mechanical effects such as friction or erosion in addition to being corroded by corrosive media. The current research results of researchers on the corrosion and wear of titanium alloy in marine environment show that corrosion and wear of titanium alloy are in a "positive interaction" relationship in the process of corrosion and wear, that is, wear and corrosion mutually aggravate the material damage.