Correlation between microstructure and corrosion behaviour of SnBi-graphene oxide composite coatings

In the present study, microstructure-electrochemical property correlation in electrodeposited SnBi-GO composite coatings containing different amounts of graphene oxide has been investigated. The SnBi-GO coatings exhibited uniform and compact morphology for lower GO concentrations, whereas for higher GO concentrations the morphological defects appeared. X-ray diffraction analysis showed the presence of Sn rich and a Bi rich phase in the coatings. Crystallite size of Bi-rich phase decreased whereas for Sn-rich phase it remained nearly unaffected with increasing GO addition. Growth texture analysis showed that the incorporation of GO enhanced growth along the low index planes for the Sn-rich grains and along the high index planes for the Bi-rich grains. Corrosion properties of coatings examined by the potentiodynamic polarisation and impedance spectroscopy methods revealed that the corrosion rate of the coatings decreased with increase in the GO content till an "optimum" addition of GO, after which the corrosion rate significantly increased with further GO addition. Microstructural analysis conducted using the electron backscatter diffraction (EBSD) technique showed that the coating with "optimum" GO amount which yielded the minimum corrosion rate contained highest fraction of low energy low angle grain boundaries which was due to the lowering of Bi dissolution in the Sn grains. Initial decrease in the corrosion rate was attributed to coating compactness, growth of grains along low energy index planes and GO impermeability. Increase in the corrosion rate after the optimum was attributed to increase in the morphological defects and possible galvanic coupling between the GO and the metals in the coatings. The enhanced corrosion properties of SnBi-GO coatings make them a viable material for interconnections used in the electronic components.