Evolution of Phase Constitution, Morphology and Corrosion Behavior of ZnCo Coating Containing Graphene Oxide

Abstract

The morphology, phase constitution and corrosion behavior of a pristine ZnCo coating (obtained from an acidic bath) and ZnCo composite coatings containing different amounts of graphene oxide were studied. To achieve this, electrochemical impedance spectroscopy, potentiodynamic polarization studies, scanning electron microscopy, X-ray diffraction, atomic force microscopy, contact angle measurement and weight loss measurements were conducted. It was observed that the morphology, phase constitution and corrosion resistance of the coatings were highly sensitive to the amount of graphene oxide contained in the coatings. For lower graphene oxide amounts, a compact and smooth morphology was observed, whereas higher graphene oxide content produced a non-uniform morphology with cracks on the coating surface essentially due to the deposition of agglomerated graphene oxide in the coating matrix. All the coatings contained a mixture of Zn phase and Zn10.63Co2.34 intermetallic phase. The volume fraction of the nobler intermetallic phase increased with an increase in the graphene oxide amount. The corrosion rate of the coatings decreased with the initial addition of graphene oxide to reach a minimum after which it increased with continued addition of graphene oxide. The initial reduction in the corrosion rate was attributed to the enhancement in the coating compactness and smoothness with the addition of graphene oxide, impermeability of the graphene oxide and enhancement of the relative volume fraction of the intermetallic phase. The enhancement of the corrosion rate after the optimum, which gave the lowest corrosion rate, was due to the increase in the morphologic roughness, cracks and surface defects in the coatings primarily due to non-uniform deposition of agglomerated graphene oxide in the coating matrix. Surface defects did not allow the formation of a continuous passive protection layer.