Corrosion behavior of (Al7.5Co21.9Cr10.9Ti5.0Fe21.9Ni32.8)100-xCux high-entropy alloys in 0.5 M H2SO4 solution

Abstract

Effect mechanism of Cu content on the microstructure and corrosion behavior of (Al_7.5Co_21.9Cr_10.9Ti_5.0Fe_21.9Ni_32.8)_100-xCu_x (x = 0.5, 2.5, 5.0, 10.0, 15.0) high-entropy alloys (HEAs) in 0.5 M H₂SO₄ solution is discussed. The microstructure is composed of dendritic FCC phase and spherical ordered L1₂ phase for all the HEAs. For the HEAs with 0.5 and 2.5 at.% Cu addition, the dominant diffraction peak is (111). When Cu addition is increased to 5.0, 10.0 and 15.0 at.%, the dominant diffraction peak is transformed from (111) to (200), which suggests that the corrosion resistance will be reduced when Cu addition is higher than 5.0 at.% since for alloys with face-centered cubic (FCC) structure (111) is the close-packed plane. For the HEA with 2.5 at.% Cu, almost no element segregation between the spherical ordered L1₂ phase and the surrounding FCC matrix can be detected. However, for the HEAs with higher Cu content, Cu atoms segregate to interdendritic region to form L1₂ phase with strong Cu enrichment. Galvanic corrosion between L1₂ phase and surrounding FCC matrix will be generated, which will impair the corrosion resistance of interdendritic region. No pitting can be observed around the single TiN inclusions. Pitting corrosion only initiates at the interface between the outer TiC in the compound type inclusion and the surrounding FCC matrix. For the HEA with 2.5 at.% Cu, more bound water in the passivation film of can be found and the content ratio of the mixture of Cr and Ni oxides is higher than other HEAs. Immersion and electrochemical tests showed that the HEA with 2.5 at.% Cu displays the best corrosion resistance with the highest R_f = 5.8708 × 10⁴ Ω·cm² and the lowest i_corr = 4.38 × 10⁻⁷ A/cm².