Electrochemical behavior of SnNi-graphene oxide composite coatings

It has been illustrated by researchers that graphene and graphene oxide can be embedded into metal matrix to produce highly corrosion resistant composite coatings. This paper investigates the effect of graphene oxide (GO) on microstructure and corrosion behavior of Sn-rich, SnNi-GO composite coatings. SnNi-GO composite coatings were electrodeposited on mild steel substrate by dispersing chemically synthesized GO into the electrolyte bath. Amount of GO in the composite coatings was varied by changing the concentration of GO in the electrolyte bath used for the electrodeposition. Morphological characterization of the coatings revealed the presence of rod shaped features in a flat matrix. Volume fraction of the rod-shaped features progressively increased with the addition of GO into the coatings. Structural characterization revealed the presence of Sn rich phase along with Ni₃Sn₄, Ni₃Sn₂ and Ni₃Sn intermetallic phases in all the coatings. Crystallite size of the Sn-rich phase decreased significantly due to the addition of GO into the coatings. Corrosion behavior of the coatings was examined through potentiodynamic polarization and electrochemical impedance spectroscopy methods. A significant improvement in the corrosion resistance in terms of reduction in corrosion current and corrosion rate and increase in polarization resistance was noted in the case of SnNi coating containing GO. Corrosion resistance of the coatings increased with increase in the GO content. Microstructural characterization of the cross-section coating sample performed using transmission electron microscopy (TEM) technique revealed differences between the microstructures of coating containing different amounts of GO. All the coatings with GO contained almost pure Sn grains with Ni present at the grain boundaries. Size of the Sn sub-grains within the larger Sn grains, however, decreased with increase in the GO content in the coatings. Smaller Sn sub-grains and presence of Ni at the grains boundaries led to quicker formation of the corrosion products that are SnO and NiO with increasing GO content in the coatings eventually yielding the highest corrosion resistance value for the coating containing maximum amount of GO.