Electronic properties of the passive films formed on CoCrFeNi and CoCrFeNiAl high entropy alloys in sodium chloride solution

Abstract

In the present work, the electronic properties of the passive films formed on equiatomic CoCrFeNi and CoCrFeNiAl high-entropy alloys were investigated in sodium chloride solution. The point defect model (PDM) was employed to study the effect of aluminum addition on the transport of p-type or n-type dopants through the passive films. The crystalline structure of the alloys was determined by X-ray diffraction. CoCrFeNi alloy was comprised of one single face-centered cubic crystalline phase. Aluminum addition promoted the formation of a mixed B2 and BCC structure. X-ray photoelectron spectroscopy (XPS) analysis allowed the examination of the correlation between passive film composition and their electronic properties. The major species in the passive film of the CoCrFeNi alloy were Cr2O3 and Cr(OH)3 while Al2O3 and Al(OH)3 were the predominant species in that of the Al-containing alloy. Electrochemical impedance spectroscopy (EIS) was employed to evaluate the corrosion resistance of the alloys. Aluminum addition promoted the formation of a thicker and more defective passive film. The pitting corrosion susceptibility of the HEAs was dependent on the passive film composition and increased with Al addition. The predominance of n-type defects in the passive film of the CoCrFeNiAl alloy was enhanced upon Al addition. Interaction between passive film composition and the electronic properties of the film played a central role in the corrosion resistance of the high entropy alloys.