Evolution of Passivity for the Multi-Principal Element Alloy CoCrFeNi with Potential, pH, and Exposure in Chloride Solution

The evolution of passivity of the multi-principal element alloy (MPEA) CoCrFeNi was studied as a function of potential, pH, and exposure duration in 0.1 M NaCl. It was shown that CoCrFeNi exhibits excellent passivity irrespective of pH, revealing a multi-oxide passive film enriched with Cr(III) oxide. Electrochemical impedance spectroscopy suggests that the passive film thickness and polarization resistance increase with increasing pH and exposure duration, whereby the growth behavior of the passive film was consistent with the assumptions of the point defect model. X-ray photoelectron spectroscopy analysis suggested that the fraction of Co(II) and Ni(II) oxides in the passive film, and their contributions to the passivity of the alloy, increased with increase in pH of the electrolyte. The present work explores the complex synergy between composition, thermodynamics, and kinetics on the resultant passivity of a MPEA.