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Hydrofluoric acid, hydrogen fluoride, and metals: Let’s first provide a brief overview of metals and their resistance to acids. 1. Monel alloy is a nickel-copper alloy that is resistant to alkalis and non-oxidizing acids; it is particularly suitable for use with hydrofluoric acid. 2. Hastelloy B is a nickel-molybdenum alloy that is resistant to alkalis and non-oxidizing acids; it is especially suitable for use in hydrochloric acid, sulfuric acid, and nitric acid. 3. Hastelloy C is a nickel-molybdenum-chromium alloy with superior properties compared to Hastelloy B; it is particularly suitable for use in aqueous solutions of hydrochloric acid and sulfuric acid at low concentrations and at room temperature. 4. Inconel is a nickel-based superalloy that exhibits high strength and toughness over a wide temperature range, from low temperatures to 1100 degrees Celsius, as well as certain corrosion resistance. Detailed explanation: 1) Nickel, Monel, and Inconel alloys. Monel exhibits excellent corrosion resistance to hydrofluoric acid with a concentration of over 50% at room temperature; to prevent damage to the passivation layer, it is also necessary to limit the flow rate of the material and any stirring activity. For anhydrous to 50% hydrofluoric acid, a certain degree of corrosion occurs when the temperature exceeds 80°C. Cold-formed or welded Monel products will develop stress corrosion cracks if effective stress relief is not applied. Do not use Monel 60 electrodes during welding, otherwise the weld will be corroded by HF. The presence of oxidizing agents and reducing sulfur compounds increases the corrosion rate of all these materials. Nickel, Inconel, and copper-nickel alloys containing 30% nickel have poorer resistance to hydrofluoric acid than Monel. Nickel, Monel, and Inconel alloys exhibit good corrosion resistance to gaseous HF at temperatures above 600°C, even in the presence of trace amounts of water vapor. The Inconel 825 alloy belongs to the NiCrFe-based family of alloys; its equivalent grade in China is 0Cr21Ni42Mo3Cu2Ti. In pure hydrofluoric acid, its use is generally not recommended; however, it exhibits good corrosion resistance in mixed acids containing fluorosilicic acid and sulfuric acid. Yet, in mixtures of sulfuric acid and hydrofluoric acid, as the acid temperature rises and the water content increases, its corrosion resistance declines. For example, under conditions where the sulfuric acid content is around 65%, the hydrofluoric acid content is around 17%, the water content is around 18%, and the acid temperature is below 100 degrees Celsius, this material maintains good corrosion resistance. But once the acid temperature and water content exceed these levels, the rate of corrosion increases significantly. Since Inconel 600 is a NiCr-based alloy, its equivalent grade in China is 0Cr15Ni75Fe. It possesses resistance to sulfuric acid and high-temperature hydrogen fluoride gas. 2) Hastelloy is also used in environments with HF present; in high-temperature liquid HF environments, Monel alloy and Hastelloy are widely employed. Both Hastelloy C276 and Hastelloy C4 belong to the NiCrMo-based alloy category; they possess excellent resistance to sulfuric acid and hydrofluoric acid, as well as good machinability and weldability, making them ideal corrosion-resistant materials. The corresponding domestic grades for these two alloys are OOCr16Ni60Mo16W4 and OOCr16Ni63Mo16Ti. 3) Copper, Bronze: Copper can be used in hydrofluoric acid with a concentration of 60–90% at room temperature, although moderate corrosion occurs when the temperature reaches the boiling point. Bronze also has acceptable corrosion resistance at room temperature; it is sometimes used to manufacture components for hydrofluoric acid pumps, but it is not suitable for higher temperatures. 4) Nickel-copper coating on the surface of ordinary carbon steel is a new process. The HF corrosion resistance of 20 steel after surface nickel-copper plating, as estimated under laboratory conditions, is approximately 0.17 mm/a, which is close to that of Monel alloy. However, this process cannot penetrate to great depths. 5) Titanium and tantalum: Hydrofluoric acid at all concentrations causes severe corrosion to these two metals. 6) Impervious graphite and carbon can be used for anhydrous hydrofluoric acid. Due to its properties, its voltage resistance is low, generally below 0.4 MPa. They are generally made as graphite heat exchangers, and can also be used as linings. When lining, phenolic putty is commonly used as a binder.