Corrosion behavior of AlCoCrFeNix high entropy alloys

In this study, the effects of three different nickel concentrations on the microstructure, hardness, and corrosion properties of AlCrFeCoNix (x = 1.0, 1.5, 2.0) high entropy alloys (HEA) were investigated. The alloys were obtained by arc melting from high-purity raw materials and subjected to heat treatment at 1125 °C for 5 h. Microstructural characterization revealed a dendritic morphology for the equimolar and AlCrFeCoNi1.5 alloys, while the AlCrFeCoNi2 alloy presented a hypoeutectic microstructure. Differences in microstructure and chemical composition were observed between the dendritic and interdendritic regions, with clear segregation of Cr in the interdendritic areas. The increase in Ni content and the heat treatment resulted in the formation of the FCC phase, consequently reducing the alloys' hardness. The corrosion properties of HEA were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy methods in a 3.5% NaCl solution. The results show high resistance to localized corrosion of the alloys, evidenced by the low values of corrosion current densities, and high pitting potentials. The protective film's formation depends on Ni content, which is also related to FCC/BCC ratio and phase compositions. The heat treatment generally results in a more homogeneous, thicker, and less defective oxide film.