Although titanium flanges have good corrosion resistance, they may still be subject to corrosion and stress corrosion in some environmental media, among which hot salt stress corrosion is the most important one. Tests have shown that, except for pure titanium, almost all titanium flanges have the same degree of hot salt stress corrosion tendency under high temperature, stress and working environment with halides (such as NaF, NaCl, NaBr, NaI). For most alloys, the hot salt stress corrosion sensitive temperature range must be 288-427°C. The corrosion tendency is related to metallurgical factors such as alloy composition and processing history. High-alumina high-oxygen alloys and b-processed or b-treated coarse-grained Widmanstatten structures are more sensitive to stress corrosion.
The cause of metal embrittlement caused by hot salt stress corrosion is considered to be related to hydrogen embrittlement. Under high temperature and stress, halides are hydrolyzed to generate HCl gas, and HCl further reacts with titanium to generate hydrogen, namely
NaCl+H20—HCl+NaOH
2HCl+Ti—TiCl2+2H
In addition to hot salt stress corrosion, titanium flanges have a certain degree of stress corrosion tendency in red fuming nitric acid, N204, methanol solutions containing hydrochloric acid and sulfuric acid. Aqueous solutions of %NaCl may reduce corrosion fracture life.
The stress corrosion tendency of titanium flange is related to alloy composition and heat treatment. Increasing the content of aluminum, tin and oxygen will accelerate the effect of stress corrosion. On the contrary, adding b stabilizing elements to the alloy, such as aluminum, vanadium, group, silver, etc., can relieve stress corrosion.
Some titanium flanges still have the tendency of liquid metal embrittlement. For example, molten cadmium and titanium will cause cadmium embrittlement, and mercury has a similar effect. Above 340°C, silver can promote corrosion and cracking of alloys such as Grade 6 （TA7 in China）.
