Corrosion of titanium alloy structural parts
1. Corrosion performance
Titanium has good corrosion resistance because a protective oxide film is tightly adhered to its surface. Once the film is damaged, it can be instantly and automatically repaired in the presence of oxygen. However, in actual use, corrosion may occur when environmental conditions change (such as increase in temperature, increase in concentration of a certain medium, contact with other corrosive metals, etc.). Under the coating, pitting corrosion can occur due to cracking or failure of the coating. For titanium alloys with a passivation film on the surface, due to insufficient oxygen supply in static or low-flow seawater, the effect of the passivation film on the surface of the titanium alloy is reduced, and pitting corrosion may also occur. Titanium fasteners are also resistant to corrosion by pure non-oxidizing acids (such as hydrochloric acid, dilute sulfuric acid, etc.), hydrofluoric acid, high-temperature dilute phosphoric acid and room temperature concentrated phosphoric acid.
Titanium alloy fasteners, like other easily passivated metals, are prone to crevice corrosion and hydrogen embrittlement, but crevice corrosion usually occurs in high-temperature solutions containing halogen ions. Titanium alloy fasteners are subject to galvanic corrosion when combined with more inert metals, and the greater impact is brittle intergranular fracture corrosion caused by hydrogen absorption from sacrificial anodes or cathodic protection.
2. Several possible corrosion situations
⑴Stress consumption breaking (SCC)
Stress corrosion cracking is one of the biggest concerns for fastener technicians. Different titanium alloy materials have different sensitivities to stress corrosion. For example, For instance, Ti-5111 is practically unaffected by SCC in room temperature seawater climate, yet the break strength of Ti-6Al-4V will be decreased in seawater, particularly when this material is utilized When titanium fasteners are exposed to seawater and are subjected to continuous tensile force, stress corrosion cracking will occur.
⑵Hydrogen embrittlement fracture
Hydrogen produced by seawater corrosion or marine cathodic protection systems can cause fasteners to break due to hydrogen embrittlement. Although titanium alloy is suitable for ordinary marine conditions, hydrogen will be generated on the surface when it is combined with active metals. When the temperature is below 80°C, hydrogen will not diffuse into the titanium metal. When the temperature is above 80°C, hydrogen embrittlement may occur. .
⑶ Galvanic corrosion
Titanium alloys can be safely joined to corrosion-resistant metals such as inert stainless steel and graphite-reinforced composites. However, when used in aluminum components, local corrosion around the titanium fasteners often causes fractures. Galvanic corrosion can be avoided by selecting appropriate materials. For example, wear-resistant thread coatings are generally non-conductive. By reducing the area of titanium fasteners, the galvanic current is effectively reduced, which actually limits the galvanic corrosion of titanium alloy fasteners.
⑷Crevice corrosion
Crevice corrosion of titanium alloy fasteners (titanium standard parts) is often caused by geometric reasons, material reasons, and environmental reasons. Due to the geometric shape of titanium alloy fasteners, a certain gap is naturally formed between the fasteners, bolts and nuts. Factors such as the material state of titanium alloy fasteners, the type and content of alloy elements, and surface finish will all affect the behavior and process of crevice corrosion in titanium alloy materials. Crevice corrosion of titanium alloys may occur in some environmental factors such as specific media and temperatures, and is most likely to occur in solutions containing chloride ions. In an environment with a temperature below 85°C, no crevice corrosion of titanium alloys was found in the laboratory stage, but the synchronous potential monitoring test in the crevice showed that as the temperature increases, the potential in the crevice also becomes negative when crevice corrosion occurs. It shows that the driving force of crevice corrosion is increasing.
