Understanding the evolution of microstructure, phase homogeneity, electrochemical characteristics and surface chemistry of electrodeposited MnCrCoFeNiCu high entropy alloy coating with reinforcement of carbon nanotubes

Abstract

MnCrCoFeNiCu high entropy alloy (HEA) coatings with varying volume fractions of carbon nanotube (CNT) were electrodeposited on mild steel. Morphology, phase constitution, wettability, surface oxide chemistry and corrosion of coatings were examined. Coating morphology became finer with initial CNT incorporation and then transformed to rougher surface for higher CNT volume fractions. Pristine HEA coating contained mixture of body centre cubic (bcc) and face centre cubic (fcc) phases. With the CNT addition phase homogenization happened and for the HEAC3 coating (from electrolyte with 5 mg/L concentration of CNT) single phase bcc microstructure was obtained. The phase mixture however re-evolved for the higher CNT incorporation due to CNT agglomeration and reduction in the metal-CNT interfaces. Water contact angle was 127° for HEA_C3 coating. All the HEA-CNT coatings exhibited higher corrosion resistance than the pristine HEA coating, corrosion rate trend was however not monotonic with CNT volume fraction and maximum corrosion resistance was observed for the HEA_C3 coating. Surface oxide chemistry of exposed sample showed stabler oxide over the HEA-CNT coatings when compared to the oxides of the pristine HEA coating. Higher corrosion resistance of the HEA_C3 coating was due to compact morphology, hydrophobicity, phase homogenization and evolution of stable surface oxides.