Microstructure and corrosion properties of NiCo-graphene oxide composite coatings

Microstructure-electrochemical behaviour correlation was investigated for electrodeposited Ni rich, NiCo-Graphene oxide composite coatings as a function of the amount of graphene oxide (GO) which was varied by changing the concentration of GO dispersed in the electrolytic bath. Corrosion behaviour of the coatings examined through potentiodynamic polarisation and electrochemical impedance spectroscopy methods revealed that the corrosion rate of the coatings was sensitive to the amount of GO present in the coating. With increasing GO content, the corrosion rate first decreased to a minimum value followed by a sudden increase to values higher than the corrosion rate value exhibited by the pristine coating. Microstructural examination of the coatings conducted using the electron backscatter diffraction method revealed that the "optimum" content of GO which yielded the minimum corrosion rate produced a unique microstructure with highest fraction of low angle grains boundaries and lowest fraction of incoherent twins boundaries when compared to all other composite coatings. Enhancement in the corrosion rate value beyond the optimum GO content was attributed to the galvanic coupling between GO and the metal matrix.