Effect of Composition Change of Ammonium Sulfide Mother Liquor on Corrosion Resistance of CoCrNi Medium-Entropy Alloy

This work systematically investigated the influence of NH₄⁺ and Cl⁻ concentrations in ammonium sulfide mother liquor on the anti-corrosion property of CoCrNi MEA. The results indicate that with the increase in (NH₄)₂SO₄ concentration, the anti-corrosion performance of the MEA presents a U-shaped trend feature, i.e., its corrosion resistance decreases first and then increases, which is attributed to the comprehensive effects of H⁺ produced by hydrolysis, the changes in solution conductivity and oxygen solubility. Additionally, under high content of Cl⁻ ions (60 g/L), the synergistic influence of Cl⁻ and H⁺ ions significantly increases the number of carrier densities, disrupts the passive film structure and thins its thickness, which remarkably weakens the protective capacity of the film. This causes a rapid increase in pitting sensitivity of the MEA, greatly promoting the occurrence of pitting corrosion.