How to choose equipment in anhydrous hydrofluoric acid environment?

1. Commonly used is carbon steel: according to relevant literature, the strength should not exceed 480MPa. Corrosion resistance A516>A3>20G>45g>16MnR. It is related to the content of silicon and manganese.

Exquisite fluorosilicic acid is generally used in liquid-phase hydrofluoric acid applications with temperatures below 65 degrees Celsius. If exceeded, corrosion will rapidly intensify. According to its sensitivity to silicon and manganese, it is necessary to control the silicon and manganese content during manufacturing, that is, to control the silicon and manganese content of the welding material (argon arc welding wire has a very small silicon content), control slag inclusion (welding slag contains a large amount of silicon), and clean up the welding slag. A certain design institute stipulates that the hardness of welds and heat affected zones shall not exceed HB234 (HR22). At the same time, it is recommended not to store materials in rough areas that may cause corrosion during use. The surface should be smooth, the structure should be free of liquid accumulation, and the transition should be smooth. Considering the scouring effect of flow velocity on the iron fluoride film, the flow velocity of hydrofluoric acid is generally limited to less than 1m/s, but fortunately below 0.5m/s, and the inlet and outlet pipe diameter of hydrofluoric acid is generally greater than or equal to DN50. Stress corrosion is also an important form of damage, so the equipment must undergo overall stress relief treatment. In operation, special attention should be paid to temperature and concentration. The concentration should be kept above 99.5%, and the water content should be minimized as much as possible to reduce corrosiveness.

Note: The silicon content of A516 in this literature is very low.

2. Stainless steel: Stainless steel is rarely used in hydrofluoric acid applications. The principle is that nickel in stainless steel forms a nickel fluoride protective film with hydrofluoric acid, which is related to the nickel content in stainless steel. Only applicable to AHF, generally used at room temperature. In practical use, 80-90 degrees were used, and severe pitting corrosion and damage occurred above 100 degrees. Stainless steel should be selected with low carbon content (titanium reacts violently with HF, so titanium containing stainless steel is not recommended). The reason is that stress corrosion is prone to occur at slightly higher temperatures, especially for 304 stainless steel. If sufficient stress release is not carried out, this phenomenon is particularly serious.

Firstly, it is necessary to control the appropriate lead salt tin salt ratio. The composition requirements of lead tin alloy coatings for different purposes are different, and the lead salt tin salt ratio in the solution is also different. This should be taken into account. The ratio of anode to cathode area is generally controlled at 1:2.

Fluoroboric acid should maintain a free amount of 100-200g/L to ensure good conductivity, improve dispersibility, and refine crystallization. In order to stabilize the fluoroboric acid solution, a certain amount of boric acid is required. A filter bag can be used to suspend the boric acid in the solution to achieve this.

Be sure not to bring sulfate ions, otherwise lead sulfate precipitation will be generated, making the coating rough, nodular, and hairy.

It is necessary to use a fixed type of glue or peptone to secure the procurement channel. Gel or peptone should be added frequently and sparingly, and after addition, it should be treated by electrolysis.