Microstructure-Electrochemical Behaviour Correlation for Nico-GO and Nico-Mwcnts Composite Coatings

Abstract

NiCo coatings are widely used in sensors, actuators and as protective coatings. Recent studies have shown that addition of foreign particles like Al2O3, SiC and TiO2 promotes grain randomization and refinement resulting in improved corrosion resistance performance. In the present work, NiCo coating containing varying amount of graphene oxide (GO) and multi-walled carbon nanotubes (MWCNTs) were electrodeposited over mild steel substrate. Microstructure and corrosion behavior of these composite coatings were then studied as a function of the volume fraction of the secondary additives. NiCo-GO & NiCo-CNT composite coating were characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) for phase identification, morphology and composition respectively. Potentiodynamic polarization studies and electrochemical impedance spectroscopy (EIS) studies in 3.5 wt. % NaCl solution revealed that the corrosion behavior of the coatings was very sensitive to the concentration of GO/ MWCNTs in the coating matrix. With continuous increase in the amount of GO/CNT in the coatings, the corrosion resistance first increased followed by a decrease, indicating possibility of an optimum with respect to the amount of the additives for achieving high corrosion resistance performance. On analyzing the electrolytic bath solution, we observed that nickel ions (Ni+2) showed preferential deposition onto the graphene oxide sheets resulting in a compositional heterogeneity near and far from the graphene-metal interface in the electrodeposited coating microstructure. This compositional segregation led to a change in the relative abundance of low angle and high angle boundaries in the coating microstructure. Electron backscattered diffraction (EBSD) technique was used to study the type of grain boundaries, strain in the grain and average grain size (area) of the samples (prepared using focused ion beam FIB). The data revealed that the samples exhibiting maximum corrosion resistance (optimum CNT/ GO concentrations) also had the highest fraction of low angle grain boundaries (LAGBs) with no additional strain in the matrix. Further increase in GO concentration resulted in more grains with high misorientation angle. In case of CNTs addition, the coatings were found to be hydrophobic in nature with a grain growth along low energy (111) direction. Figure 1