| dc.description.abstract | High entropy alloys (HEA) are composed of five or more alloying elements in a nearly equi-atomic ratio. HEAs exhibit high oxidation and corrosion resistance behaviour. In this work, HEA coatings were electrodeposited over mild steel substrate from an aqueous electrolyte bath. Microstructure-corrosion property correlation for as-deposited pristine HEA and HEA-graphene oxide (GO) composite coatings was examined. Four different HEA coating systems were investigated: CrNiCoFeCu, CrNiCoFeMn, AlNiCoFeCu, and AlCrNiCoFeCu HEA coatings. Corrosion behaviour and passive film constitution of CrNiCoFeCu and CrNiCoFeMn coatings were compared, and it was observed that the formation of relatively stable protective Cr, Co, and Mn oxide corrosion products on the surface of the CrNiCoFeMn coating enhanced the corrosion resistance of this coating. In contrast, the formation of Fe, Cr, and Cu oxide corrosion products on the surface of CrNiCoFeCu coating were de-stabilized by the inter-dendritic segregation of the Cu-rich phase leading to the lower corrosion resistance of the CrNiCoFeCu coating. In the case of AlNiCoFeCu and AlCrNiCoFeCu coating, the effect of Cr on the evolution of oxide phase in Al-containing HEA coatings was compared. The formation of a denser and more stable protective oxide layer on the surface of AlCrNiCoFeCu HEA coating resulted in better corrosion resistance performance compared to the AlNiCoFeCu HEA coating, which had a lesser stable and defective protective oxide layer. It was observed that Cr facilitates the formation of other metallic oxides in the passive film, which enhanced its ability to reduce ionic diffusion and improve the corrosion resistance of the Cr containing AlCrNiCoFeCu HEA coating. In the case of CrNiCoFeCu HEA-GO composite coatings, with GO addition, the corrosion current density and corrosion rate reduced. while, the polarization resistance increased, indicating an enhancement in the corrosion resistance property of the CrNiCoFeCu HEA-GO coatings with increase in the GO content. The microstructural characterization of the coatings showed that GO addition into the CrNiCoFeCu matrix resulted in two distinct microstructural changes; one was increase in the Cr-rich phase, and the other was the formation of Cu-rich and Cr-rich layer over the coating surface which can facilitate the formation of the protective oxide film that can hinder the penetration of the electroactive media. These factors, along with the impermeability imparted by GO resulted in enhancement in the corrosion resistance of the CrNiCoFeCu HEA-GO coatings. In the case of CrNiCoFeMn HEA-GO composite coating, morphology of the coating showed that the relative smoothness and compactness of the coatings increased with GO additions. A significant improvement in the corrosion resistance in terms of reduction in the corrosion current density and corrosion rate and increased corrosion potential and polarization resistance was recorded for the GO containing CrNiCoFeMn HEA coatings, which implied enhancement in the corrosion resistance performance of the coatings. Microstructural characterization of CrNiCoFeMn HEA coatings revealed that the GO addition resulted in distinct microstructural changes; with GO addition, the microstructure transformed from a nearly homogenous microstructure to a microstructure containing FeCoNi rich regions embedded in an Mn-Cr rich matrix. The formation of a strongly oxidizing matrix capable of forming relatively stable protective oxide layers and impermeability imparted by the GO were accounted for the observed enhancement in the corrosion resistance of the CrNiCoFeMn HEA-GO composite coatings as compared to the pristine CrNiCoFeMn HEA coating. In the case of AlNiCoFeCu HEA-GO composite coatings, the as-deposited AlNiCoFeCu HEA coating exhibited a granular morphology, which became finer and relatively more compact with increasing in the GO amount. Structural characterization revealed a mixture of BCC and FCC phases, with a fraction of the FCC phase increasing with GO. The coatings' electrochemical properties showed that AlNiCoFeCu HEA-GO composite coatings' corrosion rate progressively decreased with increase in the GO content. Microstructural characterization revealed a highly Al-rich matrix phase and Co, Ni, Cu, and Fe containing dendritic phase in the coating microstructure for the pristine coating. With the addition of GO, the coating microstructure progressively became more compositionally homogeneous. Al distribution between the matrix and dendritic phases became more uniform. Microstructural homogenization reduced the extent of galvanic coupling between phases and uniform distribution of Al, which can form stable and protective alumina phase along with the impermeability properties of GO enhanced the corrosion resistance performance of the AlNiCoFeCu HEA-GO composite coatings when compared to pristine AlNiCoFeCu HEA coating. In the case of AlCrNiCoFeCu HEA-GO composite coatings, the corrosion resistance of the AlCrNiCoFeCu HEA-GO composite coatings was higher than the AlCrNiCoFeCu HEA coating without GO. Corrosion resistance gradually increased with increase in the GO content in the coating. Detailed microstructural characterization revealed that GO addition facilitated microstructural and compositional homogeneity, eliminating localized corrosive attack due to elemental segregation induced galvanic coupling, thereby increasing the corrosion resistance for the AlCrNiCoFeCu HEA-GO composite coatings. | en_US |
