Enhancement in corrosion resistance of selected high entropy alloys by incorporation of carbon nanotubes

Abstract

High entropy alloys (HEAs) have attracted considerable interest due to their remarkable structural and functional properties. HEAs generally contain 5–13 principal elements with the concentrations of each component lying in the range of 5-35 at. % and the mixing entropy greater than 1.5R. Though researchers have reported on HEAs as promising corrosion resistance material, one of the challenges limiting the application of HEA for corrosion inhibition is the phase inhomogeneity stemming primarily from the elemental segregation within the HEA matrix. Such microstructural inhomogeneity promotes undesirable galvanic coupling and accelerated corrosion. This work addresses the issue of phase heterogeneity in selected HEA systems through the incorporation of carbon nanotubes in the HEA matrix. Following systems were studied: FeCuCrNiCo-CNT, FeCuMnNiCo-CNT and FeCrMnCoNi-CNT composites. In all the cases, it was observed that the corrosive properties of HEAs were highly sensitive to the CNT volume fraction and at an “optimum” CNT volume fraction - high corrosion resistance was obtained. This in turn was intimately related to the phase constitution, coating morphology and surface oxide chemistry. Key findings of the work are: (a) In the case of electrodeposited FeCuCrNiCo-CNT composite coatings - the enhancement of the corrosion resistance at optimum CNT volume fraction was due to evolution of single phase BCC structure from two phase mixture of BCC and FCC structure, enhancement in the coating compactness, increase in the Cr content in the coatings and formation of stable protective oxides such as Cr2O3, NiO, Co3O4, FeO, (b) in the case of electrodeposited FeCuMnNi-CNT composite coatings - the enhancement in the corrosion resistance at optimum CNT volume fraction was due to formation of single phase BCC structure from a mixture of BCC and FCC phase structure, enhancement in the coating compactness, enhancement in the absorption of Fe in the coatings and formation of stabler protective oxide phases such as FeO, NiO, Co3O4, MnO, (c) in the case of FeCrMnNiCo-CNT electrodeposited coatings - the enhancement in the corrosion resistance at optimum CNT volume fraction was due to evolution of smooth and compact morphology, incorporation of increased Cr amount and formation of stabler oxide phases such as Cr2O3, NiO, MnO, Co3O4, FeO, (d) in the case of mechanically alloyed and spark plasma sintered FeCuCrNiCo-CNT composite ingots, the enhanced corrosion resistance at the optimum CNT incorporation was primarily attributed to the enhancement in chemical and phase homogeneity