Electrochemical Behavior of Metallic Coatings Containing Carbonaceous Additives

Abstract

Coatings have been traditionally used for protection against corrosion. Therefore, substantial efforts have been focused on engineering the microstructure and morphology of coatings to enhance their corrosion resistance performance. It has been shown recently that incorporation of carbonaceous additives such as graphene, graphene oxide, and carbon nanotubes (CNTs) into conventional metallic coatings has produced composite coatings with significantly enhanced corrosion resistance behavior. It is believed that the observed enhancement in the corrosion resistance is essentially due to the inertness and impermeability of graphene/graphene oxide (GO) and hydrophobicity of the CNTs. In this thesis, it is shown that in addition to the above factors, microstructural alterations in the metallic coatings due to the incorporation of carbonaceous additives also play a major role in deciding the corrosion properties of the coatings. The electrodeposited coating systems that have been explored in the present work are: NiCo-graphene oxide, NiCo-carbon nanotube, ZnCo-graphene oxide, ZnCo-carbon nanotube, and ZnMn-graphene oxide composite coatings. In all the cases, the corrosion rate was found to be very sensitive to the amount of additive present in the coatings. With increasing GO/CNT content, the corrosion rate first decreased to a minimum value followed by sudden increase to values higher than the pristine coating. In the NiCo-graphene oxide composite coating work, microstructural examination of the coatings revealed that the “optimum” content of GO which yielded minimum corrosion rate produced a unique microstructure with highest fraction of low angle grains boundaries and lowest fraction of incoherent twin boundaries when compared to all other composite coatings. In the NiCo-CNT composite coating work, the corrosion rate was found to be sensitive to the amount of the additive and an optimum concentration of CNT in the coating matrix yielded high corrosion resistance behavior. This CNT concentration produced microstructure with coating growth along the low energy (111) direction with largest fraction of low energy low angle grain boundaries. In the ZnCo-GO composite coatings work, it was shown that an optimum addition of graphene oxide in ZnCo coatings yields high corrosion resistance performance. Decrease in the corrosion rate was attributed to impermeability of the graphene oxide and enhancement in the relative volume fraction of the Zn10.63Co2.34 intermetallic phase. In case of the ZnCo-CNT composite coatings, the electrochemical measurements showed that the corrosion rate of the ZnCo-CNT composite coatings decreased for initial addition of CNTs to reach a minimum for an “optimum” CNT concentration, after which it increased with further excess CNT addition. The enhancement in the corrosion rate was attributed to compact surface morphology and highest intermetallic (Zn10.63Co2.34) phase fraction. In the case of ZnMn-GO composite coatings the corrosion resistance was again observed to be sensitive to the amount of GO contained in the coatings. It was observed that the incorporation of GO produced alteration in the growth texture of the Zn0.73Mn0.27 intermetallic phase in the coating matrix which enhanced the corrosion behavior of the ZnMn-GO composite coatings. Key conclusions derived from the work presented in this thesis are: (a) there exists an optimum concentration of the carbonaceous additives at which the corrosion resistance of the composite coatings is maximum. Before the optimum, alteration in coating microstructure and phase constitution govern the electrochemical behavior. Microstructural alterations such as increase in low angle grain boundaries and increase in the fraction of relatively stable phase decrease the corrosion rate, (b) morphology of the composite coatings tends to get smoother with lesser surface defects with increase in the additive amount till the optimum. Beyond the optimum, the morphology becomes rough with defects and cracks which appear primarily due to the agglomeration of the additives. Decrease in surface roughness increases the corrosion rate till the optimum and increase in surface defects for higher additive additions increases the corrosion rate, (c) the galvanic coupling between carbonaceous additives and the metallic matrix becomes prominent at higher additions of carbonaceous material due to substantial increase in cathode/anode area ratio. This galvanic coupling deteriorates the corrosion rate at higher additions. At lower carbon additive amounts the galvanic coupling is not very prominent because of small cathode/anode ratio. At lower carbonaceous additions, the changes in the coating microstructure is prominent in determining the corrosion rate.