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The current research on the corrosion resistance of high entropy alloys (HEAs), mainly focuses on the influence of element composition and percentage difference. Considering the almost consistent passivation range with different corrosion resistance contributed by element addition, it inevitably exists obvious differences the passive film properties. The effect of passive film on corrosion behavior of HEA is worthy to be investigated in detail. Considering the microstructure transformation detected by electron probe X-ray microanalysis (EPMA) and surface feedback effect measured by approach curve of scanning electrochemical microscopy (SECM), the passivity of Al x (CoCrFeNi) 100-x (x = 0–20) HEAs in 0.5 M H 2 SO 4 solution has been investigated by cyclic potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), Mott-Schottky analysis and X-ray photoelectron spectroscopy (XPS). The results demonstrate that the passive film is thick and dense at low Al content (5%), which shows excellent corrosion resistance. However, with the increase of Al content, the corrosion resistance of passive film deteriorates gradually. XPS are adopted to detect film composition to explain the reason of film property change. The results suggest that there are more oxidized states of Al and less oxidized states of Cr in the passive film of HEAs with Al addition. The dominate semiconductor characteristics of Al x (CoCrFeNi) 100-x with Al addition change from p-type to n-type. All results indicate that the addition of Al has a significantly influence on the composition and properties of passive film on the Al x (CoCrFeNi) 100-x alloys surface. • The corrosion resistance and semiconductor characterization of passive film on Al x (CoCrFeNi) 100-x was studied. • The phase constituents of Al x (CoCrFeNi) 100-x change from single FCC phase to BCC phases as more Al is being added. • A small amount of Al without phase transformation is beneficial to stabilize a thick and compact passive film. • The passive film semiconductor characteristic of Al x (CoCrFeNi) 100-x with Al addition is predominant from p-type to n-type. [ABSTRACT FROM AUTHOR]

Although the mechanical properties of CoCrFeNi system high entropy alloys (HEAs) could be enhanced with the increase of Al content, it is inevitable to produce a gradual transformation of multiphase microstructure with chemical segregations. It will lead to the change of work function and the appearance of localized corrosion. The present work investigates the correlation between the corrosion behavior and the microstructure of Al x (CoCrFeNi) 100−x (x = 0–20) HEAs in 0.5 M H 2 SO 4 solution by immersion test, electrochemical measurement, SKPFM and SEM. The microstructure of CoCrFeNi system HEAs with increasing the content of Al has been transformed from single FCC phase to mixture of FCC and BCC duplex phases, and then to BCC phase. The detected value of work function gradually decreases with Al addition. The addition of a little amount of Al which retains the single FCC phase structure is beneficial to improve the corrosion resistance of Al-containing CoCrFeNi system HEAs, and the addition of excessive Al elements which promotes the formation and growth of BCC phase weakens the corrosion resistance of Al-containing CoCrFeNi system HEAs. • The relationship between the microstructure and corrosion resistance of Al x (CoCrFeNi) 100−x was systematically studied. • The microstructure of Al x (CoCrFeNi) 100−x changes from single FCC phase to FCC + BCC, and then to BCC phase. • Little Al addition without phase change improves the corrosion resistance, while excessive Al addition weakens. [ABSTRACT FROM AUTHOR]