Microstructural, Morphological and Electrochemical Effects of Graphene Oxide Incorporation in Tin-Cobalt Composite Coatings

Abstract

Effect of incorporation of graphene oxide (GO) on the evolution of morphology, phase constitution, microstructure and corrosion properties of SnCo-GO composite coating was investigated. SnCo-GO composite coatings containing different amounts of graphene oxide were electrodeposited over mild steel substrate using electrolyte with different concentrations of dispersed graphene oxide (0.0625, 0.125, 0.1875, 0.25, 0.3125, 0.5, 0.625 g/L). Morphological examination revealed that lesser additions of GO increased the coating compactness and uniformity whereas higher amounts of GO produced surface defects in the composite coatings. Structural characterization revealed the presence of primarily two different intermetallic phases in the coating microstructure: Co2Sn phase and Co3Sn2 phase. It was observed that the phase fraction of the relatively more inert Co3Sn2 phase increased monotonically with increase in the amount of GO in the coatings. Microstructural investigations revealed that in the pristine SnCo coating the Co2Sn phase is present at the grain boundaries of the Co3Sn2 phase grains. Incorporation of GO altered the coating microstructure considerably leading to the formation of a layered microstructure where the elongated Co2Sn phase grains were present beneath laterally elongated grains of the Co3Sn2 phase. Electrochemical analysis performed using the Tafel polarization and impedance spectroscopy measurements showed that the corrosion properties of the coating were highly sensitive to the amount of GO present in the coating. Initial addition of GO increased the corrosion resistance till an optimum GO concentration for which highest corrosion resistance was achieved. Further addition of GO beyond the optimum lead to monotonic decrease in the corrosion resistance to values which were lower than the pristine SnCo coating. Initial increase in the corrosion resistance was attributed to uniform distribution of GO in the coating, increase in the uniformity and compactness of the coatings, increase in the fraction of the relatively more inert Co3Sn2 phase in the coating microstructure and formation of a layered microstructure in which the exposed coating surface contained the relatively more inert Co3Sn2 phase. Decrease in the corrosion resistance beyond the “optimum” was due to increase in the coating surface defects due to the deposition of agglomerated GO and galvanic coupling due to increase in the cathodic (GO) to anodic (metal) phase area.