Selective leaching and surface properties of CoNiCr-based medium-/high-entropy alloys

Abstract This study investigated the selective leaching and surface properties of FCC-structured CoNiCr, FeCoNiCr, and FeCoNiCrPd medium-/high-entropy alloys (M/HEAs) for the feasibility assessment of their potential biomedical applications. The selective leaching results revealed that the concentrations of Co and Ni ions released from CoNiCr, FeCoNiCr, and FeCoNiCrPd M/HEAs were higher than those of the Cr, Fe and Pd ions. The relatively high selective leaching rates of Co and Ni ions were due to the amounts of metallic Co and metallic Ni near the surface of these M/HEAs were comparable to their corresponding oxidation states. On the other hand, most Cr and Fe near the surface of these M/HEAs were in their highly passive oxidation states, rather than in their corresponding metallic states. Compared to CoNiCr and FeCoNiCr MEAs, FeCoNiCrPd HEA is more suitable for practical biomedical applications because of its superior corrosion resistance and extremely low selective leaching rates. The excellent surface properties of FeCoNiCrPd HEA were caused by the fact that its surface primarily contained highly inert metallic Pd and PdO oxide, even though the amount of the Pd is only 20% in equiatomic FeCoNiCrPd HEA.