Evolution of surface oxide chemistry and its effect on the electrochemical degradation behaviour of AlCoFeCuNi high entropy alloy coating incorporated with multiwalled carbon nanotubes

AlCoFeCuNi high entropy alloy (HEA) coatings containing different volume fractions of carbon nanotubes (CNTs) were electrodeposited over mild steel substrate. Phase constitution, morphological evolution, electrochemical corrosion properties and surface oxide chemistry of the coatings were investigated as a function of their CNT content and correlated. With initial incorporation of the CNTs, the surface morphology of the composite coatings became progressively finer and compact till an optimum CNT volume fraction. Higher volume fractions of CNTs led to their agglomeration which yielded rougher and defective coating morphology. Corrosion rate of the HEA coating was highly sensitive to the CNT content and at an optimum CNT volume fraction HEA-CNT composite coatings (HEA_CNT-3 coating produced from electrolyte with 10 mg/L of CNT) with lowest corrosion rate was obtained (polarization resistance: Rp = 1178.85 Ω-cm2) while the highest corrosion rate was observed for the pristine HEA coating (Rp = 225.84 Ω-cm2). The coating with highest CNT volume fraction (HEA_CNT-6 coating produced from electrolyte with 35 mg/L of CNT) exhibited the polarization resistance value of 281.80 Ω-cm2. Contact angle measurement revealed that with increase in CNT content, coating hydrophobicity increased monotonically. Analysis of the surface oxide layer of the exposed samples exhibited increase in the relative content of Co3O4, CuO, NiO, AlO(OH) oxides with the presence of CNTs. This work shows that an optimum volume fraction of CNT which produces relatively compact morphology and facilitates evolution of stabler surface oxides leads to significant enhancement in the corrosion resistance of AlCoFeCuNi HEA-CNT composite coating.