The Interesting Complicacy of the New Age Metallic Coatings with Carbonaceous Additives

Abstract

Metallic coatings have been traditionally used to protect the underlying substrate against corrosion. Recent reports have illustrated that use of carbonaceous material such as graphene, graphene oxide, carbon nanotubes as additives in conventional coating have led to significant enhancement in the corrosion resistance behaviour of the composite coatings. In this talk, the author will discuss the evolution of microstructure, morphology, phase constitution and corrosion behaviour of electrodeposited ZnCo coatings as a function of the amounts of graphene oxide contained in them. The as-deposited ZnCo-graphene oxide composite coatings were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and energy dispersive X-ray spectroscopy (EDS). XRD based analysis of the coatings revealed that all the coatings contained Zn phase along with intermetallic phase (Zn10. 63Co2.34). A gradual increase in the volume fraction of the intermetallic phase was observed upon graphene oxide addition. Morphological characteristics and changes in the surface roughness were studied using SEM and AFM. A compact and smooth morphology was observed for initial graphene oxide additions while a higher graphene oxide concentration produced surface cracks due to graphene oxide agglomeration in the electrolytic bath. Weight loss measurements of control sample containing only the intermetallic phase revealed that the intermetallic was more corrosion resistant. The coatings were studied using electrochemical impedance spectroscopy (EIS) in 3.5 wt. % NaCl to understand the characteristics of corrosion product layer. Corrosion current in the coatings was found to decrease for initial graphene oxide addition while for higher additions a sudden increase in corrosion rate was observed. The impermeability of the graphene oxide and a relative increase in the volume fraction of nobler intermetallic phase lead to the initial reduction in corrosion rate. While, after an optimum graphene oxide addition which yielded the lowest corrosion rate, the incorporation of agglomerated graphene oxide lead to cracks and surface defects primarily due to non-uniform deposition. The surface morphology was analyzed using SEM after they were exposed to the electroactive media (3.5 wt.% NaCl). A continuous zinc dissolution was observed in the case of pristine ZnCo coatings while formation of pits was evident for maximum graphene oxide addition. Figure 1