The corrosion behaviour of equiatomic CoCrFeNi and the high entropy alloys CoCrFeNiX (X=Al, Cu, Sn)

Currently, High Entropy Alloys (HEAs) are attracting interest from many researchers for alloy design and the development of alloy properties, by creating and exploring alloys made with multiple metallic elements. Many HEA systems thus far reported reveal various outstanding material properties, such as mechanical properties, corrosion properties, electrical properties, etc., which could potentially support current and future applications. Furthermore, the unexpected microstructure of HEAs is a great motivation for further research to identify their properties leading to the selection of appropriate applications in industry.
In corrosion studies, most of the work that has been reported has been carried out on HEAs based on the CoCrFeNi system, with additional alloying elements. This system was selected as the main alloy system for study here, with additions of inexpensive metallic elements, Al, Cu, and Sn. The CoCrFeNi-based alloy and three equiatomic HEA alloys, CoCrFeNiAl, CoCrFeNiCu, and CoCrFeNiSn, were produced by vacuum arc melting. Corrosion behaviour has been assessed in three different solutions representing different conditions and pH; 0.6 M NaCl, 0.6 M H2SO4, and 0.6 M NaAlO2 solutions. Chloride solution was also used to observe tribocorrosion resistance. In addition, phase composition, chemical composition, microstructure, hardness, and density were examined.
These results indicate that the alloying elements have an effect on the microstructure of the CoCrFeNi system. The addition of a fifth alloying element, Al, Cu, or Sn, can transform the structure from a single FCC phase to dual phase. The second phase, Al-Ni-rich, Cu-rich, and Ni-Sn-rich phases respectively, is formed in each HEA system. In the study of corrosion behaviour, CoCrFeNiSn shows the highest passivity of all the tested specimens in 0.6 M NaCl solution, stemming from Cr2O3 and SnO2 based oxide films. In acidic and alkaline solutions, the best corrosion properties with the highest corrosion and transpassive potentials, Ecorr and Et, which decrease the corrosion sensitivity are observed in CoCrFeNiSn for 0.6 M H2SO4 solution and CoCrFeNiCu for 0.6 M NaAlO2 solution, in both cases occurring as a consequence of the high quantity of protective Cr2O3 film covered on the alloy surfaces. In addition, it can be found that CoCrFeNiAl has the widest passive region under both acidic and alkaline solutions, resulting from the formation of Al2O3 or a mixed (Cr, Al)2O3 film on the sample surface. CoCrFeNiSn has higher hardness than CoCrFeNi, resulting from the dual phase structure, with high hardness in the Ni-Sn-rich phase, but also ductility in the CoCrFeNi phase which provides a degree of toughness improvement and seems to resist crack propagation, perhaps contributing to CoCrFeNiSn having better tribocorrosion resistance than CoCrFeNi in chloride conditions. From all the results outlined above, the creation of high entropy CoCrFeNiX alloys with inexpensive metallic elements (X) such as Al, Cu, and especially Sn, reveals a positive effect on the development of corrosion properties in various solutions. These finding can be a significant spur to drive further development of these materials for diverse industries in the future.