Effect of Al content on the corrosion behavior and mechanism of Al_xCoCrFeNi high-entropy alloys

Al addition is known to improve the tensile strength and high-temperature oxidation behavior of the Al_xCoCrFeNi high-entropy alloy (HEA). However, the corrosion behavior and mechanisms associated with these HEAs remain unclear. The corrosion resistance of structural materials is as important as their mechanical properties. In this study, Al_xCoCrFeNi HEAs with different molar ratios of Al (x = 0.0, 0.5, 1.0, and 1.5) and various crystal structures (FCC, FCC + BCC, and BCC phases) were prepared, and their corrosion behavior and mechanisms were studied, which revealed a competitive relationship between the Al and Cr passivation films. The thickness of the Al passivation film increased with increasing Al content, which was found to be the main reason for the decrease in the corrosion resistance of the Al_xCoCrFeNi HEAs. The corrosion type of the Al_xCoCrFeNi HEA changed from pitting corrosion to selective corrosion with increasing Al content, and the corroded elements changed from (Co, Cr, Fe, and Ni) to (Al and Ni) and then (Al, Ni, and Co). Accordingly, the corrosion mechanisms associated with the Al_xCoCrFeNi HEAs were elucidated.