High entropy alloys (HEAs) are new concept in alloy design. HEAs contain five or more metallic elements in equimolar or near-equimolar ratios. Although HEAs are multi-component alloys, they form single-phase solid solutions. HEAs have outstanding properties that were not found in conventional alloys such as Fe-base alloys. CoCrFeMnNi alloy, one of the most remarkable HEA, is expected to exhibit high corrosion resistance. The mechanism of corrosion resistance of CoCrFeMnNi alloy could be applied to improve the corrosion resistance of conventional Fe-Cr-Ni stainless steels. Therefore, it is important to clarify the corrosion mechanism of CoCrFeMnNi alloy.

In this study, the equiatomic CoCrFeMnNi alloy was prepared as the specimens. CoCrFeMnNi alloy was made by arc melting. The samples were heated to 1523 K and hot-rolled to thickness of 2 mm. After solution treatment at 1473 K (2h, W.Q.), the surfaces of the specimens were successively polished with a diamond paste down to 1 µm.

Potentiodynamic anodic polarization curves were measured in deaerated 1 M H₂SO₄ at 298 K to elucidate the corrosion behavior of CoCrFeMnNi alloy. As a reference, polarization curves for pure Co, Cr, Fe, Mn, and Ni were measured. The surfaces of the specimen, with the exception of the electrode area (10mm×10 mm), were covered with epoxy resin and paraffin. The measurements were performed in a conventional three electrode cell. The reference electrode was Ag/AgCl (3.33 M KCl) and the counter electrode was a Pt plate.

The results of potentiodynamic anodic polarization curves are shown in Fig. 1. In the case of CoCrFeMnNi alloy, active state, passive, trans-passive, secondary passive, and O₂ evolution regions appeared. Comparing with CoCrFeMnNi alloy and each pure metal, corrosion reactions of CoCrFeMnNi alloy were discussed.