Corrosion resistance of Monel alloy

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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 hydrofluoric acid contains oxygen and oxidizing salts, it increases the corrosion rate of Monel; therefore, during operation, the hydrofluoric acid in the equipment must be 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 diaphragms that are exposed to high flow rates. In hydrofluoric acids of various concentrations, Monel maintains good corrosion resistance at temperatures below 120°C. High-silicon Monel has a lower corrosion rate in some hydrofluoric acids compared to regular Monel. 3. Alkalis: In NaOH with a concentration of less than 75% and a temperature of less than 135°C, the corrosion rate of Monel is less than 0.01 mm/year; at lower concentrations and temperatures, this rate is often even lower, at less than 0.0025 mm/year. General characteristic: The corrosivity of caustic soda increases as the temperature rises. 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 solution, Monel exhibits better corrosion resistance than pure nickel, but it can only be used at concentrations below 3%.   4. Sulfuric acid: Monel exhibits a very low corrosion rate in reducing sulfuric acid environments. It remains resistant to corrosion 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%, sulfuric acid is oxidizing, and its high oxidizing power leads to a significant increase in the corrosion rate. However, at low temperatures, the corrosion resistance of high-silicon Monel to sulfuric acid at various concentrations is significantly improved; this is because 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 exhibits excellent corrosion resistance at low concentrations and at lower temperatures in air-free hydrochloric acid. 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 in hydrochloric acid at room temperature, with a concentration of <15% and a gas content of <5%, as well as in hydrochloric acid at higher temperatures that is free of air and oxidizing agents. Monel K500 has better resistance to hydrochloric acid corrosion than Monel 400, especially in boiling hydrochloric acid.   6. Phosphoric acid: In phosphoric acid, Monel alloy is resistant to corrosion by phosphoric acid that is free of oxygen and contains oxidizing agents, 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. Introducing air will accelerate the corrosion of Monel alloy, especially at high temperatures. 7. Corrosion at high temperatures Tests in air and oxygen have shown 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 contains CuO and Cu2O, while the inner layer is rich in Ni and is 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/year.   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. Monel 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.