Mechanism Of Deep Sea Stress Corrosion Of Titanium Alloy
Mechanism of deep sea stress corrosion of titanium alloy
Stress corrosion can occur suddenly, and it is a local corrosion failure form with great harm without obvious change in appearance. Once the stress corrosion crack occurs, it will cause local stress concentration and brittle cracking, and finally lead to the fracture of the material at a far lower yield stress. Several factors and mechanisms affecting stress corrosion are introduced below.
1. Residual stress
40% of titanium alloy accidents are related to stress corrosion cracking caused by residual stress introduced in the process of forging, processing, welding and heat treatment.
2. Surface texture
In the marine environment, the oxide film formed on the surface of titanium alloy can slow down its own corrosion. However, in the high concentration of chloride and low oxygen environment, such as pitting, crevices, cracks and other occluded areas, the oxide film will be damaged, and adsorptive hydrogen will be generated at the same time, leading to stress corrosion cracking. The surface state and microstructure of titanium alloy have great influence on the probability and rate of stress corrosion.
3. Alloy elements
Alloying elements are also the key factors affecting the stress corrosion sensitivity of titanium alloys. When alloy elements form oxides, these oxides will be embedded in titanium matrix in the form of dispersed clusters. If the alloy elements are evenly distributed in each phase, the oxide clusters will also be evenly distributed in the matrix, and eventually a stable passivation film will be produced. It can be seen that the treatment method causing uneven distribution of alloy elements,
4. Anodic dissolution and hydrogen-induced cracking
Generally speaking, the stress corrosion cracking mechanism of high-strength materials can be divided into anodic dissolution type and hydrogen-induced cracking type. The deep-sea stress corrosion cracking of titanium alloy is also no exception. According to the anodic dissolution mechanism, the nucleation and propagation of stress corrosion cracks are affected by the continuous dissolution capacity of the anode [39]. The reaction process is characterized by the anodic dissolution of Ti, the precipitation of H and the formation of Ti hydride. The reaction formula is:
The influence of environmental pressure on the distribution of hydrogen concentration at the crack tip of cracked titanium alloy in the deep sea environment shows that hydrogen accumulates at the crack tip. The greater the pressure, the more serious the situation is, and the more likely to lead to crack propagation.
Ti Gr-12 titanium alloy in seawater, - 1500mV (Ag/AgCl) cathodic polarization potential, 2 × Hydride layer and stress corrosion cracking at 10-7s-1 strain rate
Stress corrosion is one of the most important risks faced by titanium alloys in the deep sea. Understanding the mechanism of stress corrosion and the causes of stress corrosion can not only reduce the impact of stress corrosion on titanium alloys, but also have great significance for the application research of titanium alloys in the deep sea.
