Corrosion Resistance of Monel K500 to Strong Acids Such as Hydrochloric Acid and Sulfuric Acid
1. Corrosion Resistance to Hydrochloric Acid
Dilute hydrochloric acid (concentration ≤ 5%) at room temperature: In the absence of air or oxidizing agents, Monel K500 shows low corrosion rate (≤ 0.1 mm/year) and good corrosion resistance. The copper element in the alloy forms a thin, dense copper chloride film on the surface, which can inhibit further corrosion of the matrix. However, if the solution is aerated or contains oxidizing ions (e.g., Fe³⁺, Cu²⁺), the passivation film will be destroyed, and the corrosion rate will increase significantly.
Concentrated hydrochloric acid (concentration > 10%) or elevated temperature: Monel K500 cannot resist corrosion in this environment. When the temperature exceeds 60°C, even dilute hydrochloric acid will cause severe uniform corrosion or pitting corrosion of the alloy, accompanied by hydrogen evolution. In concentrated hydrochloric acid, the corrosion rate can exceed 1 mm/year, leading to rapid thinning or failure of the material.
Industrial application limitation: Monel K500 is not recommended for equipment in direct contact with concentrated hydrochloric acid or hot dilute hydrochloric acid. It can only be used in specific scenarios of sealed, deaerated dilute hydrochloric acid systems at room temperature.
2. Corrosion Resistance to Sulfuric Acid
Dilute sulfuric acid (concentration ≤ 10%) at room temperature: Similar to dilute hydrochloric acid, Monel K500 has good corrosion resistance in deaerated dilute sulfuric acid, with a corrosion rate of less than 0.05 mm/year. The nickel and copper elements in the alloy synergistically form a protective sulfate film, which isolates the matrix from the corrosive medium.
Medium-concentration sulfuric acid (10% < concentration < 70%): The corrosivity of sulfuric acid increases sharply in this range. At room temperature, the corrosion rate of Monel K500 can reach 0.5–2 mm/year; when the temperature rises to 50°C or above, pitting corrosion and intergranular corrosion are prone to occur, which is not suitable for long-term service.
Concentrated sulfuric acid (concentration ≥ 90%) at room temperature: Concentrated sulfuric acid has strong oxidizing properties, which can make the surface of Monel K500 form a dense passivation film composed of nickel sulfate and copper sulfate, thus showing excellent corrosion resistance (corrosion rate ≤ 0.02 mm/year). This is because the strong oxidizing environment stabilizes the passivation film and prevents it from being dissolved by the acid. However, once the temperature exceeds 80°C, the passivation film will lose its protective effect, and the alloy will suffer severe corrosion.
Industrial application tips: Monel K500 can be used for components in room-temperature concentrated sulfuric acid systems (e.g., storage tank linings, pipeline valves), but it must be avoided to contact medium-concentration sulfuric acid or hot concentrated sulfuric acid.
3. Key Factors Affecting Corrosion Resistance
Aeration condition: Oxygen in the acid solution is a key factor accelerating corrosion. Deaeration can significantly improve the corrosion resistance of Monel K500 in dilute acids; on the contrary, aeration will cause localized corrosion.
Temperature: Temperature rise will exponentially increase the corrosion rate. For most acid media, the corrosion rate of Monel K500 doubles for every 20°C increase in temperature.
Impurity ions: The presence of Cl⁻, Br⁻, Fe³⁺ and other ions will destroy the passivation film on the alloy surface, inducing pitting corrosion and stress corrosion cracking.
