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Application of TA10 titanium alloy in ammonium chloride evaporator

Industrial pure titanium has strong corrosion resistance in room temperature ammonium chloride solution, but there is a tendency for crevice corrosion in high temperature and high concentration ammonium chloride solution. A pure titanium ammonium chloride evaporator in a chemical plant in China suffered severe corrosion after using 8Mo and had to be scrapped. This is a large evaporator with a diameter of 1000mm x 5000mm, with an ammonium chloride concentration of 15% -20%, a temperature of 110 ℃ -120 ℃, and a pH (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， Other impurities are 0.1 for each individual and 0.4 for the total. The equipment ran for less than 8Mo, and severe corrosion occurred at the edge of the lower tube plate in the middle heating chamber, as well as the welds at the pipe and tube plate connections; The entire inner side above the lower vertebral body is corroded and partially penetrated, with a corrosion pit depth of 6mm and a width of 80mm-100mm. Industrial pure titanium is not suitable because severe crevice corrosion can occur in high concentration high-temperature NH4Cl solution.
Analysis testing
Qiu Honglin and others from Baoji Nonferrous Metals Processing Plant conducted metallographic and scanning electron microscopy analysis on corrosion. The metallographic analysis of the corroded area did not reveal any abnormal structural changes, nor did they see the presence of hydrides accompanied by crevice corrosion. It is still an equiaxed structure.
PSEM-500X scanning electron microscope was used to analyze the corroded area, and they found that the surface was honeycomb like but without cracks. Energy spectrum analysis revealed that the corroded 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 area contains 0.020% H, which is (150-200) ppm higher than the uncorroded area. Obviously, the titanium in the corroded area exhibits obvious hydrogen absorption; The corroded area is uneven and honeycomb like, with severe corrosion. The corrosion products consist of 58.207% Ti, 34.205% Cl, and other elements. The corrosion products contain a large amount of Cl ions, exhibiting typical crevice corrosion characteristics. The corrosion of the tube plate is located at the lower end of the heating chamber, and there is accumulation in the corroded area.
Repair and application of TA10 titanium alloy
Qiu Honglin and others first cleared the accumulated material, and for this purpose, titanium spray pipes were installed below the lower pipe 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 lengthening it from 50mm to 400mm and forming a smooth large radius transition angle to eliminate accumulation and liquid; Avoiding the effects of liquid stagnation dead corners and pits, the flowability of NH4Cl solution has been improved, and corrosion only occurs after 16Mo operation of the equipment, which is about twice the service life compared to the original equipment.
*The improvement is to use TA10 titanium alloy to manufacture NH4Cl evaporator, which is a Ti-0.3Mo-0.8Ni alloy with a composition (mass%) of Mo0.2-0.4 Ni0.6-0.9、Fe-0.30、C-0.08、N0.03、H0.015、O0.25， The other impurities are 0.1 for each individual, totaling 0.4, while the remaining Ti is a low-strength titanium alloy. Operation shows that there is no crevice corrosion, indicating 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.

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