Several Localized Corrosion Characteristics Of Titanium Materials

1.1 crevice corrosion Titanium crevice corrosion resistance is particularly strong, only in a few chemical media crevice corrosion. Titanium crevice corrosion and temperature, chloride concentration, pH value and the size of the crevice has a close relationship. According to the relevant information, the wet chlorine temperature above 85 ℃ is easy to produce crevice corrosion. For example, some plants in the cooler before a packed tower directly cooled to make the wet chlorine temperature down to 65 ~ 70 ℃, and then into the titanium cooler to improve resistance to crevice corrosion, the effect is also very significant. Practice has proved: reduce the temperature is to prevent crevice corrosion is one of the effective methods, in the high temperature sodium chloride solution has also occurred in the titanium crevice corrosion. In short, for easy to produce crevice corrosion of parts and components, such as sealing surfaces, tube plate and tube expansion parts, plate heat exchanger, tower plate and tower contact parts as well as tower fasteners should be used Ti-0.2Pd and other titanium alloys, in the design should be avoided to avoid the appearance of crevices and stagnant flow area. Such as tower fasteners should try not to use bolt connection. Pipe plate and tube with expansion and sealing welding structure is better than simple expansion, for the flange sealing surface, should not be used asbestos gasket, should be used Teflon film wrapped asbestos gasket.

1.2 High temperature corrosion

Titanium corrosion resistance at high temperatures, depending on the characteristics of the medium and the performance of their own surface oxide film. Titanium in the air or oxidizing atmosphere, as a structural material can be used to 426 ℃, but at about 250 ℃, titanium began to absorb hydrogen significantly, in a completely hydrogen atmosphere, when the temperature rises to 316 ℃ or more, titanium hydrogen brittle. Therefore, in the absence of extensive testing, titanium should not be higher than 330 ° C for the use of chemical equipment, from the absorption of hydrogen and mechanical properties and other considerations, the use of full titanium pressure vessel temperature shall not exceed 250 ° C, heat exchanger titanium tubes with the use of the upper limit of the temperature of 316 ° C.
1.3 Stress corrosion

In addition to a few kinds of media, industrial titanium stress corrosion resistance is very good, the phenomenon of stress corrosion caused by titanium equipment damage is still rare. Industrial blunt titanium only in fuming nitric acid, some methanol solution or some hydrochloric acid solution, high-temperature hypochlorite, the temperature of 300 ~ 450 ℃ of molten salt or NaCl-containing atmosphere, carbon disulfide, n-hexane and dry chlorine gas and other media to produce stress corrosion. Titanium in nitric acid with the increase of NO2 content and the decrease of water content, the tendency of stress corrosion rupture gradually increases. In anhydrous nitric acid containing 20% free NO2 titanium stress corrosion tendency to reach large. In concentrated nitric acid containing more than 6.0% NO2 and less than 0.7% H2O, even at room temperature, industrial pure titanium will occur stress corrosion rupture. China in 98% concentrated nitric acid in the use of titanium equipment has occurred in serious stress corrosion and explosion. Industrial pure titanium in 10% hydrochloric acid solution, there is stress corrosion rupture sensitivity, in containing 0.4% hydrochloric acid plus methanol solution of titanium stress corrosion.