Hydrogen embrittlement of titanium containers

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1. Why are the severe accidents that occur in titanium equipment used in the chemical industry almost always caused by hydrogen embrittlement? How can hydrogen embrittlement be prevented? This is because titanium readily absorbs hydrogen; even in environments with moderate temperatures, the fast diffusion rate of hydrogen can cause titanium to become embrittled and develop intergranular microcracks. The sources of hydrogen include the following: a. Impurity elements contained in titanium materials – hydrogen ； b· When the hydrogen concentration in the environment reaches a certain level, titanium in a high-temperature state or titanium whose surface passivation layer has been damaged is prone to hydrogen-induced embrittlement; c· Hydrogen-induced embrittlement can occur due to incorrect material selection, i.e., using titanium in environments where it is not suitable, or as a result of improper structural design (such as the presence of gaps or electrochemical corrosion occurring when in contact with different metals), which leads to localized corrosion; d· Hydrogen-induced embrittlement can also result from inadequate processing techniques (such as incorrect acid cleaning formulations or excessive acid cleaning times, insufficient protection during welding, or contamination of the titanium surface with iron during manufacturing). Measures to prevent hydrogen embrittlement in titanium equipment are as follows: a. Use titanium materials with low hydrogen content. b. Preventing hydrogen absorption during processing: It is necessary to avoid the inclusion of iron particles on the surface of titanium materials during processing operations such as cutting, stamping, rolling, and welding. Heat treatment and heating processes must take place in furnaces with a slightly oxidizing atmosphere. For some titanium components with complex structures, where it is difficult to achieve gas-shielded welds on the back side, measures must be taken to prevent hydrogen absorption during welding. c• Choose an appropriate operating environment: In environments with dry or wet hydrogen at temperatures ranging from 71 to 316°C, the presence of a certain amount of oxygen and moisture can prevent hydrogen absorption. Titanium generally does not absorb hydrogen or does so very slowly in oxidizing media, neutral media, weakly reducing media, or reducing acids containing oxidants; however, hydrogen absorption-induced embrittlement can occur when the surface is contaminated, there are surface defects, local corrosion takes place, or under abnormal operating conditions. Titanium is prone to hydrogen embrittlement in environments subjected to general or localized corrosion. d. The hydrogen resistance can be improved through surface treatment (high-temperature oxidation, anodization, etc.). Corrosion-resistant titanium alloys are used to enhance the corrosion resistance of titanium and prevent hydrogen-induced embrittlement. 2. In which media is titanium prone to stress corrosion cracking? What are the factors that cause stress corrosion cracking? Practice has shown that in environments where stainless steel suffers from stress corrosion failure, titanium generally does not experience stress corrosion cracking. However, it has been gradually discovered that titanium suffers from stress corrosion cracking in media such as fuming nitric acid, methanol, triethylene glycol, high-temperature chlorides, liquid dinitrogen tetroxide, molten metal salts, carbon tetranitride, uracil, organic solvents, hydrogen, and bromine vapor. The factors that cause stress corrosion cracking are as follows. (1) Environment: This factor includes the type and concentration of ions, the pH value of the solution, temperature, conductivity, additives, and flow rate. (2) Mechanical factors: Tensile stress must be present in the system where stress corrosion cracking occurs. Tensile stress arises from residual stresses and service stresses generated during the manufacturing process. Stress corrosion cracking typically occurs only when the stress intensity factor of the system is greater than the critical stress intensity factor, and the strain rate exceeds a certain value. (3) Metallurgical factors: In industrially pure titanium, the susceptibility to stress corrosion cracking increases as the oxygen content rises.