Improved tribocorrosion resistance by addition of Sn to CrFeCoNi high entropy alloy

Among the high entropy or complex concentrated alloys (HEAs/CCAs), one type of system is commonly based on CoCrFeNi, which as an equiatomic quaternary alloy that forms a single phase FCC structure. In this work, the effect of Sn in an equiatomic quinary system with CoCrFeNi is shown to lead to a great improvement in hardness and resistance to tribocorrosion. The addition causes a phase transition from a single FCC phase in CoCrFeNi to dual phase in CoCrFeNiSn with an Ni-Sn intermetallic phase, and a CoCrFeNi FCC phase. The presence of both the hard intermetallic and this ductile phase helps to resist crack propagation, and consequent material removal during wear. In addition, the high polarization resistance of the passive film formed at the surface and the high corrosion potential of the Ni-Sn phase contribute to preventing chloride corrosion attack during corrosion testing. This film is tenacious enough for the effect to persist under tribocorrosion conditions

The corrosion behaviour of CoCrFeNi-x (x = Cu, Al, Sn) high entropy alloy systems in chloride solution

The corrosion properties in NaCl solution of four equiatomic HEAs of the CoCrFeNi system adding Al, Cu and Sn are investigated. These alloys are processed by vacuum arc melting and assessed via the Potentiostat method. The properties were compared with two standard stainless steels. The results indicate that CoCrFeNiSn possesses the best passivation in this solution, explained by the alloy phases and presence at the surface of elements in oxidation states corresponding to stable oxide films. The other systems show a range of behaviours attributable to their different microstructures and varying potential for stable oxide formation