Carbon-triggered grain boundary engineering in CoCrFeMnNi HEAs: Decoupling the role of M₇C₃ precipitates on passive film stability and localized corrosion resistance

This study uncovers a non-monotonic effect of carbon on the corrosion behavior of CoCrFeMnNiCx high-entropy alloys (HEAs) in 0.5 mol/L NaCl solution. The microstructure of the HEAs investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscope, the corrosion behaviours were investigated using potentiodynamic polarization, 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 0.5 at.% yields the highest fraction of Cr in the passive film and a significant improvement in corrosion resistance. This is evidenced by a 65% reduction in corrosion current density and a 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.