Passivation behavior of CoCrNiZr medium-entropy alloy in the sulfuric acid solutions

Abstract The passivation behavior of a medium-entropy alloy (MEA) CoCrNiZrx (x = 0.1, 0.2 and 0.3) in 0.1 M H2SO4 solution was investigated. The roles of Zr in alteration of microstructure, electrochemical characteristics, and passive film chemistry of CoCrNi alloy were evaluated. With the addition of Zr element, the phase structure changes from a single face-centered cubic (FCC) phase to a two-phase structure, with the appearance of Zr-rich HCP phase. The addition of Zr accelerates the degradation process of MEAs in 0.1 M H2SO4, resulting in the increase of anodic current and carrier density and decrease of the film resistance as determined by the electrochemical impedance spectroscopy and Mott-Schottky results. The preferential dissolution of the Zr-rich phase (high thermodynamic activity), the weak compactness of the Zr oxides, the depletion of the Cr oxides, and the increase in carrier density are responsible for the degradation of the Zr-alloying CoCrNi alloy in 0.1 M H2SO4.