The Effect of Cu Addition on the Corrosion Resistance of High Entropy Alloys

Abstract

Molybdenum (Mo) has been used as an additive element to improve the corrosion resistance of stainless steels. However, Mo addition sometimes results in the increase of cost. This study focused on copper (Cu) because Cu is also known as an element which improves corrosion resistance of stainless steels. However, Cu addition readily causes the formation of Cu-rich phases in the stainless steels. And, the existence of Cu-rich phases is harmful for corrosion resistance and hot workability. Therefore, Cu content in stainless steels is usually limited below 3 mass%. In this study, the principal of high entropy alloys was applied to fabricate the Fe-alloys containing high concentration of Cu. High entropy alloys (HEAs) are alloys produced by mixing five or more kinds of elements, and it is known that the contribution of configuration entropy to Gibbs free energy becomes large, and a single solid solution phase is generated. The purpose of this study is to make alloys with high Cu contents and clarify the mechanism of improving corrosion resistance by Cu.Equiatomic CoCrCuFeNi and CoCrFeMnNi alloys were prepared by an arc melting method, and pure metals of Co, Cr, Cu, Fe, Ni, and Mn were used as raw materials. For homogenization, a heat treatment was performed at 1200 ℃ for 2 h, followed by water-quenching. The ingots made by arc-melting were cut out into 1 mm in the thickness. After that, the specimen surface was polished up to 1 µm with a diamond paste. The micro-structures were observed using an optical microscope and a scanning electron microscope, and the chemical compositions and elemental distributions of the specimens were analyzed by energy dispersive X-ray spectroscopy. Their crystal structures were identified by X-ray diffraction. Corrosion resistance was evaluated by potentiodynamic polarization and immersion in HCl was carried in. In polarization, Ag / AgCl (3.33 M KCl)was used for the reference electrode, and a platinum sheet was used for the counter electrode. The potential scan was started from 0.05 V below the open circuit potential, and the scan rate was 23 mV min-1. An immersion test in 1 M HCl was performed at 25℃ for 7 days. The mass loss measured, and corrosion rates were calculated.From EDS analysis, in the case of equiatomic CoCrCuFeNi alloy, Cu-rich phases were generated.Matrix phase was solid solution phase containing 9at.%Cu. Co18Cr27Cu9Fe27Ni19, Co18Cr28Cu7Fe28Ni19, Co18Cr29Cu5Fe29Ni20, Co19Cr29Cu3Fe29Ni20, Co19Cr30Cu1Fe30Ni20 and Co19Cr30Fe30Ni21 were prepared based on the matrix phase composition of equiatomic CoCrCuFeNi alloy. From XRD and EDS analysis, when the concentration of Cu was 5at.% or less, it was a single-phase solid solution. The results of polarization curve measurement of these alloys in 1 M HCl, it was found that Cu reduce the current value of active region. From the immersion test results, the addition of Cu reduced the corrosion rate.