Enhancing corrosion resistance in CoCrFeNiTa high entropy alloys via Mo addition

In this work, the microstructural characteristics and corrosion resistance of Co1.5CrFeNi1.5Ta0.1Mox (x = 0, 0.1, 0.3, 0.5) high entropy alloys (HEAs) fabricated by vacuum arc melting were investigated. To assess the comprehensive corrosion resistance of these alloys in seawater, alkaline or acid rain harsh environments, their electrochemical properties were investigated in solutions of 3.5 wt.% NaCl, 1 M NaOH and 0.5 M H2SO4 at room temperature. As minor Mo addition HEA samples indexed a single FCC phase, the precipitation of segregated Ta into the intercellular phase, with increasing Mo additions evolved Ta-rich and Mo-rich growths for the emergence of tuned morphology of Cr- and Mo-rich σ phase in the Mo0.3 and Mo0.5 samples. By potentiodynamic evaluation, Mo0.3 indicated superior corrosion performance with an excellent stability of the passive film in 3.5 wt.% NaCl solution, while the Mo0.1 sample referenced the best passivation behavior in alkaline 1 M NaOH via the probably protective MoO42− formation in the outer layer of the film, but in acidic 0.5 M H2SO4, the effect of Mo additions was undermined as the Mo0.5 sample was degraded. Furthermore, the protective passive film was optimally enhanced in the Mo0.3 sample for an excellent brine corrosion resistance in contrast to the Mo0 sample from localized corrosion and the aggressive degradation of the Mo0.5 sample by preferential corrosion events at the intercellular regions with the σ phase. However, the homogeneously distributed intercellular regions in both Mo0.1 and Mo0.3 characterized the high resistance to pitting exhibited by the HEA alloy in alkaline and seawater environments. Thus, Mo additions could veritably enhance the corrosion resistance of Co1.5CrFeNi1.5Ta0.1Mox HEAs for the development of novel structural alloys with high corrosion performance.