Abstract The present work investigates the corrosion resistance of as-cast FeCoCrNiMn high-entropy alloy in borate buffer solution. Compared with 304 and 316L stainless steels steel, as-cast FeCoCrNiMn high-entropy alloy exhibited excellent passivation ability. This should be attributed to homogenization of alloying elements. Highly corrosion-resistant elements in as-cast FeCoCrNiMn alloys, for example, Cr, Co and Ni, could promote the formation of protective passive film in borate buffer solution. Moreover, combined effect of high mixing entropy and sluggish diffusion also could improve the corrosion resistance. The formation mechanism of the passive films on the surface of as-cast FeCoCrNiMn alloys satisfied the point defect model.