Copper is a metal with high thermodynamic stability. When copper is added, it can improve the corrosion resistance of nickel in reducing media, especially in hydrofluoric acid, but reduce the corrosion resistance of nickel in oxidizing medium. And oxidation resistance in the air.
The most commonly used nickel-copper corrosion resistant alloy contains 28% to 34% of Cu. China has Ni68Cu28Fe (Monel 400,Alloy 400) and Ni68Cu28Al (Monel K500,Alloy 500) two grades, the corresponding foreign alloy is Monel.
Ni68Cu28Fe is the most resistant, nickel-copper alloy with the most comprehensive application and the best comprehensive performance. It is equivalent to the international grade Monel 400. It is mainly composed of 30% Cu, 65% Ni and a small amount of Fe (1% to 2%). Single-phase austenite structure.
Ni68Cu28Fe alloy has good corrosion resistance in reducing acid. It is resistant to corrosion in H2SO4 solution at room temperature (30 ° C), air-free, and concentration ≤ 85%, and the corrosion rate is less than 0.2 mm/a. When the concentration of H2SO4 solution is higher than 85%, when the oxidation is oxidized, the corrosion rate increases sharply, reaching 3 to 4 mm/a. When there is air in the H2SO4 solution, the corrosion rate also increases, and the maximum value occurs when the concentration of H2SO4 is 5%. Increasing the temperature of H2SO4 will reduce the corrosion resistance of the alloy. In 60 °C, 95 °C, air-free H2SO4, the corrosion limit concentration of the alloy is reduced to ≤65%. In the H2SO4 solution at boiling temperature, the corrosion resistance is reduced to ≤15%.
Ni68Cu28Fe alloy is not resistant to oxidizing acids such as concentrated sulfuric acid and nitric acid, and to sulfates, oxidizing salts of Cu2+ and Fe3+, and molten salts of KCl and NaNO3.
Among the chloride salts, sulfide salts, nitrates, acetates and carbonates, the NCu28-2.5-1.5 alloy has good corrosion resistance, and the corrosion rate in the hot concentrated salt is generally less than 0.0025 mm/a. Therefore, many important equipment in the vacuum salt industry are made of this alloy.
Ni68Cu28Fe alloy is extremely resistant to corrosion in natural water, distilled water, natural seawater, acid mine water, nuclear reactor high purity water, and pitting corrosion and stress corrosion are rarely found.
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