Corrosion Behavior of CoCrNi Medium-Entropy Alloy in NH₄Cl Solution with Different pH

The corrosion behavior of CoCrNi MEA in NH₄Cl solution with different pH was studied by electrochemical tests, weight loss method, and surface morphology analysis. The results show that with the decrease in pH value, the protection ability of the passive film weakens, the corrosion rate rises, and the anti-corrosion property of the MEA reduces. The augment of H⁺ promotes the adsorption of Cl⁻ on the passive film, leading to an increase in defect density, as well as an enhancement in the size and number of pits. Moreover, the preferential dissolution of Co and Ni elements results in the occurrence of pits. Low pH inhibits the repassivation process of metastable pitting and enhances the formation possibility of stable pitting.