Effect of annealing temperature on the corrosion behavior of FeCoCrNiMo_0.5 alloy fabricated via selective laser melting: The role of microstructure and passive film

In this study, the effects of annealing temperature (700 ℃, 900 ℃, 1100 ℃) on the microstructure and corrosion behavior of FeCoCrNiMo_0.5 high entropy alloy by selective laser melting were systematically studied. The results show that σ phase precipitates at the boundary of grain boundary or substructure at first. After annealing at 700 ℃, the granular precipitates are densely distributed along the grain boundary. After annealing at 900 ℃, the morphology of the precipitates changed into slender needle shape with uniform distribution. After annealing at 1100 ℃, the size of precipitates increased significantly and the distribution became discrete. The samples annealed at 900 ℃ show the lowest corrosion current density and the widest passivation zone, and the Cr₂O₃/Cr(OH)₃ value in the passivation film is the highest. The loss of passive elements and the formation of micro galvanic couple lead to the preferential dissolution of the matrix around the precipitated phase, and pitting corrosion is initiated here. The chemical corrosion of Cl^- and H⁺ and the galvanic corrosion between the precipitated phase and its surrounding FCC phase accelerate the development of pitting corrosion.