Corrosion resistance and basic electrochemical characteristics of titanium alloy
1.1 Corrosion resistance of titanium alloy
Titanium alloy is widely recognized as a "marine metal" and an ideal material for ships due to its excellent corrosion resistance. A large number of marine engineering practices and scientific experiments have shown that titanium alloys have excellent corrosion resistance in pure seawater, with almost no uniform corrosion occurring, and excellent resistance to pitting and crevice corrosion. China's 16 year sea trials in the South China Sea, East China Sea, and Yellow Sea have shown that titanium alloy materials have almost no significant corrosion phenomenon in fully immersed, tidal range, and splash zone seawater. Three years of real sea exposure tests in the deep-sea environment of 800-1000m in the South China Sea have also confirmed that titanium alloys have excellent resistance to uniform corrosion in deep-sea environments.
The excellent corrosion resistance of titanium alloy materials in seawater environment is mainly attributed to the following five reasons:
(1)
(The relative reactivity between titanium metal and oxygen is extremely fast, surpassing that of aluminum, nickel, and stainless steel, which determines that the surface of titanium alloys is difficult to exist in a metallic state in an aerobic environment;
(2)
Titanium alloys can spontaneously form dense oxide films in both atmospheric and aqueous environments, without the presence of loose hydroxide layers, and their oxide films have excellent shielding and blocking properties;
(3)
The oxides TiO, TiO2, and Ti2O3 formed on the surface of titanium metal are very stable and have excellent resistance to chloride ion erosion;
(4)
The titanium surface oxide film is formed by epitaxial growth mechanism, with strong interfacial bonding force, a volume ratio of 1.76, in a compressive stress state, and few film defects;
(5)
The oxide film on the surface of titanium has excellent self-healing properties, and it will quickly re passivate and enter a stable state after being corroded by environmental media or damaged by external forces. These excellent characteristics determine that titanium alloy is a natural corrosion-resistant metal material.
Therefore, it is generally believed that engineering equipment made of titanium alloy can avoid corrosion problems, which leads to little consideration of corrosion and protection issues of titanium alloy structures in marine engineering design. This consistent thinking can bring serious engineering safety hazards, especially for equipment used in extreme environments such as deep sea for a long time, its service safety issues cannot be ignored.
In fact, although titanium alloy materials have excellent corrosion resistance, they still face environmental damage problems such as performance degradation and corrosion failure in deep-sea environments under extreme environmental factors, mechanical loads, and coupled effects of microbial communities. For example, a study by Beijing University of Science and Technology found [10] that when the pH of seawater is less than 2, the stability and corrosion resistance of titanium alloy passivation films significantly decrease; Sulfur ions and fluoride ions have a significant impact on the stability of titanium alloys, especially when the fluoride ion concentration exceeds 0.005mol/L, serious corrosion problems will occur in titanium alloys.
The US Navy found stress corrosion cracking in high-strength titanium alloy 13V-11Cr-3Al during actual sea exposure tests in the Pacific Ocean from 700 to 2000 meters, and the stress corrosion sensitivity of the weld and heat affected zone was higher than that of the base metal. According to the research of Harbin Engineering University, Institute of Metals, Chinese Academy of Sciences, Northeastern University and other institutions in China, with the increase of deep-sea hydrostatic pressure, the stress corrosion sensitivity of titanium alloy increases. The stress corrosion of GR5 titanium alloy under 20MPa hydrostatic pressure is in the form of hydrogen induced cracking. With the increase of strength, the stress corrosion sensitivity of titanium alloy increases. In addition, in marine equipment engineering structures, complex structures such as electric couples, gaps, and welding are inevitable, which further exacerbates the complexity and destructiveness of local corrosion damage to titanium alloys.
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