Abstract: Additively manufactured CoCrNi medium entropy alloys have some advantages such as high strength, high ductility, and rapid preparation of complex structural components. However, during the nonequilibrium solidification process, local dislocation enrichment occurs in the microstructure, significantly affecting its corrosion performance and passive film characteristics in marine service environments. The paper focuses on the correlation between microstructure and corrosion behavior after different heat treatments. It showed that as the heat treatment temperature increased from 900 ℃ to 1200 ℃, the dislocation cell structure gradually disappeared and underwent recovery and recrystallization, transforming from columnar crystals rapidly solidified in additive manufacturing to equiaxed crystals. The size of grain decreased and the precipitates gradually coarsened. After heat treatment at 900 ℃, the corrosion potential was the highest, the passivation current density and corrosion current were the smallest, and the corrosion resistance was the best. As the heat treatment temperature increased to 1200 ℃, the corrosion resistance gradually decreased.