Abstract: This study uncovers a non-monotonic effect of carbon on the corrosion behavior of CoCrFeMnNiC_x high-entropy alloys (HEAs) in 0.5 mol/L NaCl solution. The microstructure of the HEAs investigated using X-ray diffraction, scanning electron microscopy, and transmission electron microscope, the corrosion behaviors were investigated using potentiodynamic polarization and electrochemical noise measurements. HEAs exhibited a face-centered cubic (FCC) structure, carbon addition promoting the formation of Cr-rich M₇C₃ carbides. While all alloys undergo active dissolution, a critical carbon content of 0.5at% yields the highest fraction of Cr in the passive film and a significant improvement in corrosion resistance. This is evidenced by 65% reduction in corrosion current density and 35% increase in polarization resistance. The enhancement is mechanistically linked to carbon-induced grain refinement, formation of Cr-rich M₇C₃ carbides, and the development of a more stable passive film with reduced charge carrier density. Electrochemical noise analysis confirms a suppression of metastable pitting events at the optimal carbon level.