Titanium can be alloyed with aluminum, vanadium, iron, and molybdenum to produce strong, lightweight alloys. Titanium alloy has the advantages of low density, high specific strength, good corrosion resistance, and good process performance, and is an ideal aerospace engineering structural material. In the actual production environment, different types of corrosion will occur, mainly in the following categories:
1. Crevice corrosion
In the gaps or defects of metal components, local corrosion is caused by the stagnation of electrolytes to form an electrochemical cell. In neutral and acidic solutions, the probability of contact corrosion in titanium alloy gaps is much greater than that in alkaline solutions, and contact corrosion does not occur. In the entire gap surface, but eventually, lead to partial perforation failure.
2. Pitting phenomenon
Titanium has no pitting corrosion phenomenon in most salt solutions. It mostly occurs in non-aqueous solutions and boiling high-concentration chloride solutions. The halogen ions in the solution corrode the passive film on the surface of titanium and diffuse into titanium to cause pitting corrosion, the pitting hole diameter is less than its depth. Some organic media will also have to pit corrosion with titanium alloys in halogen solutions. Pitting corrosion of titanium alloys in halogen solutions generally occurs in high-concentration and high-temperature environments. In addition, pitting corrosion in sulfides and chlorides requires specific conditions that are limited.
3. Hydrogen embrittlement
Hydrogen embrittlement (HE), also known as hydrogen-induced cracking or hydrogen damage, is one of the causes of early damage and failure of titanium alloys. The passive film on the surface of titanium and its titanium alloys has high strength, and the sensitivity of hydrogen embrittlement increases with the increase of strength. increase, so the passivation film is very sensitive to hydrogen embrittlement.
4. Contact corrosion
The passive oxide film on the titanium surface promotes the potential of titanium to move to a positive potential, which improves the acid resistance of titanium materials and the corrosion of water media. Due to the higher potential of the titanium alloy surface, it is bound to cause contact corrosion by forming an electrochemical circuit with other metals in contact with it. Titanium alloys are prone to contact corrosion in the below two types of media: the first is tap water, salt solution, seawater, atmosphere, HNO3, acetic acid, etc. The stable electrode potential of Cd, Zn, and Al in this solution is more negative than Ti, and the anodic corrosion rate increases by 6 to 60 times. The second type is H2SO4, HCl, etc. In these solutions, Ti may be in a passivation state or an activated state. The first solution corrosion is very common in the actual contact corrosion process. Usually, anodizing treatment is used to form a modified layer on the surface of the substrate to hinder contact corrosion.
