Nickel-copper alloy contains 70% nickel and 30% copper, which is Monel alloy. Monel alloy has very good corrosion resistance to non-oxidizing acids, especially hydrofluoric acid. Hydrofluoric acid of various concentrations can be used at room temperature. Boiling point can be used in 10% concentration of hydrofluoric acid. Nickel-copper alloy has good corrosion resistance to hot concentrated lye, but it is not as good as pure nickel. Nickel-copper alloys are resistant to seawater, low-concentration hydrochloric acid, and various organics, but not resistant to oxidizing acids and other strongly oxidizing solutions, lava salts, molten metals, and high-temperature sulfur-containing gases.
Monel alloy is mainly used in hydrofluoric acid system. The valve has full Monel alloy valve, that is, the valve body and internal parts are Monel, and the valve body is carbon steel internal part is Monel. The full Monel valve is used in the occasions where the temperature of the hydrofluoric acid regeneration tower is high (149ºC) and the water content is large. In the case of low temperature and low water content, a carbon steel valve body Monel internal valve can be used. This is because the hydrofluoric acid will produce iron fluoride (FeF2) embroidery on the carbon steel surface. When the temperature is not high, it will act as a protective film. When the temperature is higher than 121--149ºC, the rust will begin to exfoliate and increase corrosion.