Engineering Applications of Monel Alloy

Thread Content

1 Seawater environment: Why is the corrosion rate of Monel in still seawater 0.0025 mm/year? The corrosion rate of high-silicon Monel alloys at different flow rates is less than 0.020 mm/year, with values that are quite similar; this makes them highly suitable for use in flowing seawater. They can resist corrosion both in still seawater and in seawater with high flow speeds, and can therefore be used for corrosion-resistant components in various marine environments, including ships, materials for offshore platforms, docks, drilling platforms, shallow-sea oil fields, sea bridges, submarine pipelines, and more. 2 Hydrofluoric acid: Monel is resistant to corrosion in hydrofluoric acid at all concentrations and temperatures. When oxygen and oxidizing salts are present in hydrofluoric acid, the corrosion rate of Monel increases; therefore, during operation, the hydrofluoric acid in the equipment must be kept isolated from air. Monel is less affected by flow velocity; therefore, it is more suitable for use in equipment and components such as pumps, valves, and bubble caps that operate under conditions of high flow velocity. In hydrofluoric acid at various concentrations, Monel exhibits good corrosion resistance below 120°C. High-silicon Monel has a lower corrosion rate than ordinary Monel in certain types of hydrofluoric acid. 3 Alkalis: In NaOH solutions with a concentration of less than 75% and a temperature below 135°C, the corrosion rate of Monel is less than 0.01 mm/year; at even lower concentrations and temperatures, this rate often drops below 0.0025 mm/year. General characteristics: The corrosiveness of caustic soda increases with rising temperature. Monel alloy can be used in high-temperature, high-concentration caustic soda. The corrosion behavior in KOH is essentially similar to that in NaOH. In NH4OH solutions, the corrosion resistance of Monel is superior to that of pure nickel; however, it can only be used at concentrations below 3%. 4. Sulfuric acid: Monel exhibits a very low corrosion rate in reducing sulfuric acid media; it remains corrosion-resistant in sulfuric acid at 30°C without air, as well as in sulfuric acid with a concentration of less than 85%. When the concentration is above 85%, due to the oxidizing nature of sulfuric acid, its high oxidizing strength causes a sharp increase in the corrosion rate. However, high-silicon Monel exhibits a significant improvement in corrosion resistance against sulfuric acids of various concentrations at both high and low temperatures, and this is due to the fact that the alloying element silicon enhances the alloy’s corrosion resistance. In a dilute sulfurous acid medium, Monel exhibits excellent corrosion resistance, making it suitable as a structural material for certain applications in the paper industry. 5 Hydrochloric acid: In hydrochloric acid, Monel is highly resistant to corrosion in dilute concentrations and at lower temperatures, in hydrochloric acid devoid of air. However, at the boiling temperature, this alloy is not corrosion-resistant even when the hydrochloric acid concentration is very low. Therefore, it is specified that Monel alloy can only be used at room temperature in hydrochloric acid with a concentration of <15% and without air present, or at higher temperatures in hydrochloric acid with a concentration of <5% again without air or oxidizing agents. Monel K500 has better resistance to hydrochloric acid corrosion than Monel 400, especially in boiling hydrochloric acid. In phosphoric acid, Monel alloy is resistant to corrosion by phosphoric acid that contains no oxygen or oxidizers at temperatures below 105°C. Tests show that in phosphoric acid at temperatures below 50°C, the corrosion rate of Monel is <0.05 mm/a ； In phosphoric acid at concentrations below 105%, the corrosion rate is <0.25 mm/a. Charging with air accelerates the corrosion of Monel alloy, especially at high temperatures. High-temperature corrosion tests in air and oxygen show that Monel has a fairly good oxidation resistance, but it is lower than that of pure nickel. At this point, the outer layer of the Monel alloy surface contains CuO and Cu2O, while the inner layer is rich in Ni and essentially pure NiO. The maximum temperature at which Monel alloy can operate in air is generally around 600°C. In high-temperature steam (whether dry or wet), Monel can be used at temperatures below 485°C. In high-velocity steam, Monel also exhibits excellent erosion resistance. Stressed Monel alloy can be used at temperatures below 455°C; above this temperature, especially between 470–500°C, intergranular corrosion occurs. Under dry steam at 150–250°C, after testing for 1000–1150 hours, the corrosion rate of Monel alloy was less than 0.026 mm/a. 8 Salt: In neutral and caustic salts, such as chloride salts, sulfide salts, nitrates, acetates, and carbonates, Monel exhibits excellent corrosion resistance; even in hot concentrated salt solutions, its corrosion rate is typically less than 0.0025 mm/year. 9 Molten salts and liquid metals: Resistant to corrosion by molten NaOH and KOH (at temperatures below 500°C). In molten caustic and neutral salts used for heat treatment at high temperatures (below 600°C), such as chloride salts and nitrates, Monel exhibits excellent corrosion resistance, almost on par with pure nickel. In fluoride-based mixed molten salts at 816°C, Monel shows no significant corrosion. It also has good corrosion resistance in molten Na and NaK; no corrosion was observed in molten NaK at 300°C. It can resist the corrosion of liquid mercury, which can reach 100°C.