Corrosion mechanism of an equimolar AlCoCrFeNi high-entropy alloy additively manufactured by electron beam melting

Abstract

High-entropy alloys (HEAs) have emerged as a class of structural alloys with various attractive properties, and their application in additive manufacturing, which enables unprecedented thermal history and geometrical complexity, is promising for realising advanced materials. This study investigates the corrosion behaviour and passive film characteristics of an equimolar AlCoCrFeNi HEA additively manufactured by electron beam melting (EBM). Potentiodynamic polarisation in a 3.5 wt% NaCl solution revealed that the bottom part of the EBM specimen shows better corrosion performance than a conventionally prepared cast specimen in terms of both corrosion and passivation current density, while a continuous increase in the current density without any apparent passivity was observed during the anodic polarisation of the top part. The electrochemical impedance spectroscopic study indicated significant differences in the passive film characteristics between the specimens, and revealed an enhanced charge-transfer resistance and the formation of a more protective passive film of the bottom part. The elemental redistribution, in particular, the enrichment of Cr in the B2 phase during the post-melt high-temperature exposure of the alloy during EBM, was responsible for the improved stability of the passive film, retarding the selective dissolution of the B2 phase in the bottom part. These findings indicate that the microstructural evolution caused by ‘in situ annealing’ during the EBM process significantly influences the corrosion behaviour of the HEA.