1,Why is it that almost all the bad accidents occurred in titanium equipment used in chemical industry are caused by hydrogen embrittlement rupture? How to prevent hydrogen embrittlement rupture?
Because titanium is very easy to absorb hydrogen, even at low temperatures, due to the hydrogen diffusion rate is large, it is easy to make titanium hydrogen embrittlement and produce intergranular microcracks. The sources of hydrogen are as follows.
a. Hydrogen, an impurity element contained in titanium;
b) Titanium at high temperature or with damaged surface passivation film is susceptible to hydrogen embrittlement due to a certain concentration of hydrogen in the environment.
c - Due to the selection of the wrong material, the use of titanium in the environment should not be used titanium; or due to structural design is not reasonable (such as the existence of gaps or contact with dissimilar metals to produce galvanic coupling corrosion) caused by localized corrosion, resulting in hydrogen embrittlement;.
d - Inadequate processing and manufacturing processes (such as the wrong pickling formula or excessive pickling time, insufficient protection during welding and iron contamination of titanium surfaces in the manufacturing process, etc.) causing hydrogen embrittlement.
Measures to prevent hydrogen embrittlement of titanium equipment are as follows:
a. Selection of titanium materials containing less hydrogen.
b. Prevent hydrogen absorption in the manufacturing process: to avoid cutting, stamping, rolling, welding and other processing and manufacturing process of titanium surface embedded in iron particles, heat processing and heat treatment heating must be carried out in the micro-oxidizing atmosphere of the heating furnace; for some of the complex structure of the titanium equipment, the back of the welded joints is difficult to realize the gas protection to prevent the welding of the pollution of hydrogen absorption.
c- Select the appropriate use of the environment: when the temperature in the 71 ~ 316 ° C of dry hydrogen and wet hydrogen in the environment, such as containing a certain amount of oxygen and moisture can prevent hydrogen absorption. Titanium in oxidizing media, neutral media, weakly reducing media or reducing acids containing oxidizing agents, hydrogen absorption usually does not occur or very slowly; but when the surface is contaminated, surface defects, localized corrosion occurs or abnormal working conditions occur, hydrogen absorption embrittlement may occur. Titanium is prone to hydrogen embrittlement in environments where total or localized corrosion occurs.
d. Through the surface treatment (high temperature oxidation, anodizing treatment, etc.) can improve the hydrogen resistance. The use of corrosion-resistant titanium alloys to improve the corrosion resistance of titanium to prevent hydrogen embrittlement.
2, titanium in which medium is easy to produce stress corrosion rupture? What are the factors that cause stress corrosion rupture?
Practice shows that in the stainless steel stress corrosion damage environment, usually titanium does not produce stress corrosion rupture. However, titanium has been found in fuming nitric acid, methanol, three gas ethylene, high temperature chloride, liquid nitrogen tetroxide, molten metal salts, carbon tetrazolium, uracil, organic solvents, hydrogen and bromine vapors, and other media in the occurrence of stress corrosion rupture.
Factors contributing to stress corrosion rupture are listed below.
(1) Environment The factors include the type and concentration of ions, the solution of the positive H value, temperature, conductivity, additives and flow rate.
(2) Mechanical factors in the occurrence of stress corrosion rupture system must exist tensile stress. The tensile stress comes from the residual stress and working stress generated in the process of manufacturing, usually only when the stress intensity factor of the system is greater than the critical stress intensity factor, and the strain rate exceeds a certain range, stress corrosion rupture occurs.
(3) Metallurgical Factors Industrial pure titanium increases the susceptibility to stress corrosion rupture as the oxygen content increases.
