Titanium alloy is an alloy composed of titanium as the base and other elements. Titanium alloy has the advantages of low density, high specific strength, good corrosion resistance, and good processability, making it an ideal structural material for aerospace engineering. In actual production environments, different types of corrosion may occur, mainly including the following categories:

       1. Crevice corrosion

       In the gaps or defects of metal components, the stagnant flow of electrolyte forms an electrochemical cell, leading to localized corrosion. In neutral and acidic solutions, the probability of contact corrosion occurring in the gaps of titanium alloys is much higher than in alkaline solutions. Contact corrosion does not occur across the entire gap surface, but ultimately leads to localized perforation and damage.

       2. Pitting corrosion phenomenon

       Titanium exhibits no pitting corrosion in most salt solutions, which mostly occurs in non-aqueous solutions and boiling high-concentration chloride solutions. In these solutions, halogen ions corrode the passive film on the titanium surface and diffuse into the interior of titanium, leading to pitting corrosion. The diameter of the pitting corrosion holes is smaller than their depth. Certain organic media can also cause pitting corrosion with titanium alloys in halogen solutions. Pitting corrosion of titanium alloys in halogen solutions generally occurs under high-concentration and high-temperature conditions. Additionally, pitting corrosion in sulfides and chlorides requires specific conditions and is limited.

       3. Hydrogen embrittlement

Hydrogen embrittlement (HE), also known as hydrogen-induced cracking or hydrogen damage, is one of the early failure mechanisms of titanium alloys. The passivation film on the surface of titanium and its alloys exhibits high strength, and the sensitivity to hydrogen embrittlement increases with the strength. Therefore, the passivation film is highly sensitive to hydrogen embrittlement.

       4. Contact corrosion

       The passivation oxide film on the surface of titanium promotes the shift of the titanium electrode potential towards a positive direction, enhancing the acid resistance and corrosion resistance of titanium materials in aqueous media. Due to the higher electrode potential of the titanium alloy surface, it inevitably leads to the formation of an electrochemical circuit with other metals in contact, resulting in contact corrosion. Titanium alloys are prone to contact corrosion in the following two types of media: The first type includes tap water, salt solutions, seawater, atmospheric gases, HNO3, acetic acid, etc. In these solutions, the stable electrode potentials of Cd, Zn, and Al are more negative than that of Ti, leading to a 6 to 60 times increase in the rate of anodic corrosion. The second type includes H2SO4, HCl, etc. In these solutions, Ti may be in a passivated state or an activated state. In actual contact corrosion processes, corrosion in the first type of solution is more common. Anodizing treatment is commonly used to form a modified layer on the substrate surface, hindering contact corrosion.