Mo content-depended competition between Cr₂O₃ enrichment and selective dissolution of CoCrFeNiMo _x high entropy alloys

The corrosion and passivation behavior of the as-cast CoCrFeNiMo_x (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) high entropy alloys (HEAs) are investigated. With the addition of Mo, increasing Mo and Cr riched σ phase precipitates from the primary FCC phase. The corrosion resistance of the CoCrFeNiMo_x HEAs strongly depends on the phase structure and the passive film. Generally, Mo promotes the selective dissolution of the FCC phase and the enrichment of Cr₂O₃ in the passive film, the former deteriorates the corrosion resistance while the latter enhances it. Interestingly, the hypoeutectic Mo_0.6 exhibits a higher corrosion resistance than that of Mo_0.4, eutectic Mo_0.8, and hypereutectic Mo₁. Analysis of surface passive film and the composition of alloy phases implies that the Cr₂O₃ enrichment dominates in Mo_0.6 while selective dissolution governs in Mo_x (x = 0.4, 0.8, and 1.0). Mo content-depended competition between the Cr₂O₃ enrichment and selective dissolution is discussed.