Influence of Alloying Elements on the Electrochemical Behavior of Cantor High Entropy Alloys in Chloride Solutions

FeCoCrNi-based high entropy alloys (HEAs) are often used as prototype model alloys and have been developed broadly through modifying or replacing constituents with other elements to serve in extremely harsh environments such as in nuclear and marine engineering. Most previous studies were focused on the microstructure, mechanical properties, and oxidation behavior of FeCoCrNi-based HEAs. To date, very little work has been published on the corrosion behavior of HEAs in aqueous environments. In this study, the corrosion behavior of Cantor HEAs, including Fe₂₀Co₂₀Cr₂₀Ni₂₀Cu₂₀ (H4Cu₂₀), Fe₂₀Co₂₀Cr₂₀Ni-₂₀Cu₁₅Al₅ (H4Cu₁₅Al₅), and Fe₂₀Co₂₀Cr₂₀Ni₂₀Cu₁₀Al₁₀ (H4Cu₁₀Al₁₀), also known as H4C alloys, are investigated in aerated 3.5 wt% NaCl solutions at room temperature via electrochemical measurements. A common stainless steel (UNS S30403), and the original Cantor HEA (Fe₂₀Co₂₀Cr₂₀Ni₂₀Mn₂₀, H4Mn₂₀) are also evaluated as comparisons. Results confirmed that the addition of Al into the FeCoCrNi-based HEA improves the general corrosion resistance of Cantor HEAs. Among all five alloys, H4Cu₁₀Al₁₀ has the best general corrosion resistance with the slowest cathodic kinetics. The main types of corrosion on these HEAs were interdendritic and pitting corrosion after anodic polarization. Polarization experiments and post-characterization revealed that although the H4C alloys exhibit low pitting potential and low resistance to pitting initiation, the formed localized corrosion was interdendritic corrosion rather than big pits. Overall, results indicate that there is no significant advantage of using Cantor HEAs over common stainless steel in terms of corrosion consideration in aqueous chloride environments.

Multi-principle element alloys (MPEAs) represent a new alloy development philosophy, where the base alloy has significant atom fractions of several elements.¹ Among MPEAs, high entropy alloys (HEAs) are defined as alloys containing 5 or more principle elements.² In 2004 Cantor et al. introduced Fe₂₀Co₂₀Cr-₂₀Ni₂₀Mn₂₀(H4Mn₂₀), a 5-element equimolar HEA.³ It was found that this alloy formed a single FCC solid solution and solidified dendritically. The chemical compositions of the dendritic region appeared to contain equal concentrations of all elements while the interdendritic region showed an increase in Cr and Mn concentrations.³ The addition or modification of alloying elements in HEAs can be done to customize them for desired applications. However, certain mixtures of alloying elements can create adverse property effects. For instance, adding passive elements (such as Cr or Mn) can form stable passive layers on an alloy, but can also aid carbide formation and promote elemental segregation within the alloy leading to the formation of additional phases.^4–8 Previous studies on FeCoCrNi-based HEAs focused mainly on the microstructure ^3,9–14, mechanical properties ^11,12,15, and oxidation behaviors.^16–19