Corrosion and passivation behaviour of as-cast and heat-treated AlCoCrFeNiX0.5 (X=Mo, Ta) high entropy alloys in 3.5 wt.% NaCl solution

In the present study, the corrosion behaviour and passivation characteristics of AlCoCrFeNi and AlCoCrFeNiX0.5 (X = Mo, Ta) high entropy alloys after casting and heat treatment were investigated. The alloys were produced by vacuum arc melting, followed by heat treatment at 1200°C for 5 h and rapid cooling in water. Electrochemical corrosion tests were conducted using cyclic potentiodynamic polarization and electrochemical impedance spectroscopy, focusing on corrosion resistance and passive film behaviour. The results revealed a remarkable improvement in the corrosion resistance of the base AlCoCrFeNi alloy after the heat treatment process. The as-cast Ta-containing alloy exhibited the highest pitting potential and the largest passive range among the studied alloys at room temperature. The analyses of the passive films revealed that Al oxide was the dominant oxide in passive films for all the alloys, but its porous nature negatively affected the corrosion resistance of the alloys having high fractions of Al oxide in their passive films.