Role of Cu in corrosion resistance of CoCrCuFeNi medium-entropy alloys: Importance of compositional change and thickening of oxide films

In oxide films on CoCrCuFeNi medium-entropy alloys with Cu contents of 0–‍5 at%, Cu existed as CuO, and the concentration increased with increasing Cu content in the alloys. Active dissolution in HCl was suppressed by the addition of Cu. The pitting potential in NaCl increased with the addition of 1 at% Cu, but it decreased with the addition of 3 at% Cu or more. The role of Cu was clarified by relating the active dissolution rate and pitting potential. In addition, the effects of Mo and W are discussed.