Abstract
High entropy alloys (HEAs) are a newly developed family of multi-component alloys that consist of various major alloying elements, including copper, nickel, aluminum, cobalt, chromium, iron and others. Each element in the alloy system is present in between 5 and 35 atom %. The crystal structures and physical properties of HEAs differ completely from those of conventional alloys.First, this investigation discusses the corrosion resistance of the Al0.5CoCrCuFeNiBx alloys with various amounts of added boron. The potentiodynamic polarization curves and electrochemical impedance spectra (EIS) clearly revealed that the Al0.5CoCrCuFeNi alloy was more resistant to general corrosion than type 304 stainless steel (higher corrosion potential, lower corrosion current density) in acid solution. However, the general corrosion resistance of Al0.5CoCrCuFeNiBx alloy decreased as the concentration of boron increased. The cyclic polarization curve of the Al0.5CoCrCuFeNi and Al0.5CoCrCuFeNiB0.2 alloy were referred to as a pseudo-passive curve; however, the passive regions of the Al0.5CoCrCuFeNiB0.6 and Al0.5CoCrCuFeNiB1.0 alloy were totally disappeared and referred to active curve due to the increasing effect of boron. Surface morphological and chemical analyses verified that the production of CrB2 for the boron-containing HEA was reducing the Cr2O3 passive film on the alloy surfaceThe AlxCrFe1.5MnNi0.5 alloys exhibited a wide passive region, which extended ＞ 1000 mV in acidic environments. The potentiodynamic polarization and EIS of the AlxCrFe1.5MnNi0.5 alloys, obtained in H2SO4 solutions, clearly revealed that the corrosion resistance decreased as the concentration of aluminum increased. Moreover, the alloying of aluminum in the CrFe1.5MnNi0.5 alloys decreased the pitting potential (Epit) and impaired the pitting resistance in chloride environments. The Nyquist plots of the Al-containing alloys had two capacitive loops, which represented the electrical double layer and the adsorptive layer. Scanning electron microscopy (SEM) micrographs revealed that the general and pitting corrosion susceptibility of the AlxCrFe1.5MnNi0.5 alloys increased as the amount of aluminum in the alloy increased.The EIS clearly revealed that the polarization resistance (Rp) values of the AlxCrFe1.5MnNi0.5 (x=0, 0.3, 0.5) alloys were markedly lower than that of the 304 stainless steel in chloride environments. At passive potential, the corresponding current declined with the anodizing time accounting, causing passivity by the growth of the multi-component anodized film in H2SO4 solution. X-ray photoelectron spectroscopy (XPS) analyses revealed that the surface of anodized Al0.3CrFe1.5MnNi0.5 alloy formed aluminum and chromium oxide film which was the main passivating compound on the alloy. This anodic treatment increased the corrosion resistance in the EIS measurements of the CrFe1.5MnNi0.5 and Al0.3CrFe1.5MnNi0.5 alloys by two orders of magnitude in chloride environment. Accordingly, the anodic treatment of the AlxCrFe1.5MnNi0.5 alloys optimized their surface structures and minimized their susceptibility to pitting corrosion.