Industrial pure titanium has strong corrosion resistance in ammonium chloride solution at room temperature, but there is a tendency of Crevice corrosion in high temperature and high concentration ammonium chloride solution. The evaporator of pure titanium ammonium chloride in a chemical plant in China was severely corroded after using 8Mo, and had to be scrapped. This is a machine Φ 1000mm × A 5000mm large evaporator with an ammonium chloride concentration of 15% -20%, a temperature of 110 ℃ -120 ℃, and a pH of 4.0-6.0. The final product is ammonium chloride crystals. The evaporator tube plate, tube bundle, and shell are made of TA2 industrial pure titanium, and their impurity content is not more than (%): Fe0.30, C0.10, K0.05, H0.015, O0.25, and other impurities are 0.1 for each individual and 0.4 for the total. The equipment has been running for less than 8Mo, and severe corrosion has occurred at the edge of the lower tube plate of the intermediate heating chamber, as well as the welds at the connection between the pipes and the tube plate; The whole inner side above the lower vertebral body is corroded and partially penetrated. The corrosion pit is 6mm deep and 80mm-100mm wide. Industrial pure titanium is not suitable for use, because serious Crevice corrosion will occur in high concentration high-temperature NH4Cl solution.

Analysis testing

Qiu Honglin and others from Baoji Nonferrous Metals Processing Factory conducted metallographic and scanning electron microscopy analysis on the corrosion. The metallographic analysis of the corrosion site did not reveal any abnormal changes in the structure, nor did they find the presence of hydrides generated by crevice corrosion, which is still an equiaxed structure.

The corrosion site was analyzed using PSEM-500X scanning electron microscopy, and they found that the surface was honeycomb like, but without cracks. Energy dispersive analysis revealed that the corrosion area contained a large amount of Cl, consisting of 58.207% Ti, 34.205% Cl, 4.631% Fe, 2.441% Ca, 1.259% Si, and 0.254% K. The corroded part contains 0.020% H, which is (150-200) ppm higher than the non corroded part. Obviously, the titanium in the corroded part exhibits obvious hydrogen absorption phenomenon; The corrosion part is uneven, honeycomb shaped, and severely corroded. The corrosion product consists of 58.207% Ti, 34.205% Cl and other elements. The corrosion product contains a large amount of Cl ions, which is typical of Crevice corrosion. The tube plate is corroded at the lower end of the heating chamber, and there are deposits at the corroded area.

Repair and application of TA10 titanium alloy

Qiu Honglin and his team first cleared the accumulated material. Therefore, a titanium spray pipe was installed below the lower tube plate to spray on time to reduce scaling. Corrosion also occurred at the connection of the lower cone flange, so they made structural improvements by extending it from 50mm to 400mm and forming a smooth transition angle with a large radius to eliminate accumulation and liquid; Avoiding the effects of stagnant corners and pits in the liquid, the flowability of NH4Cl solution is improved, and corrosion only occurs after 16Mo of equipment operation, which prolongs the service life by about twice compared to the original equipment.

*The improvement is to use TA10 titanium alloy to manufacture NH4Cl evaporator. It is a Ti-0.3Mo-0.8Ni alloy, and its composition (mass%): Mo0.2-0.4, Ni0.6-0.9, Fe-0.30, C-0.08, N0.03, H0.015, O0.25. Other impurities are 0.1 each, 0.4 in total. The rest of Ti is a low strength titanium alloy. Operation shows that no Crevice corrosion is found. It can be seen that the use of TA10 titanium alloy in high temperature and high concentration NH4Cl solution is completely safe and reliable. The amount of titanium tubes and plates used for one evaporator is about 1 ton. Although the price of TA10 titanium alloy material per ton is about 60000 yuan higher than that of industrial pure titanium, its service life is more than 15 times longer than that of industrial pure titanium. Therefore, using TA10 titanium alloy to manufacture NH4Cl evaporators is economically cost-effective. TA10 is a common titanium alloy that can be produced by some titanium processing plants.