Microstructure and corrosion behaviour of NiCo-Carbon nanotube composite coatings

Abstract

Microstructure-corrosion property correlation in electrodeposited NiCo-carbon nanotube (NiCo-CNT) composite coatings has been explored. NiCo-CNT composite coatings were electrodeposited over mild steel substrate by dispersing different amounts of CNTs into the electrolyte bath. CNTs were synthesized by the chemical vapor deposition (CVD) method. Morphological characterization revealed crack free morphology for all the coatings. The coating morphology progressively became finer with increase in the addition of CNTs up to a certain amount of CNT after which the morphology again became rougher. Finer morphology was due to a uniform distribution of CNTs in the coating matrix. Lower amount of CNT and non-uniform distribution of agglomerated CNTs in case of higher concentrations produced the rougher morphology. All the NiCo-CNT composite coating were relatively hydrophobic when compared to the only NiCo coating. Structural characterization revealed that an intimate interaction between the growing metal matrix and CNT promoted coating growth along the low energy (111) direction. Corrosion behaviour of the coatings was examined through potentiodynamic polarization and electrochemical impedance spectroscopy methods. It was observed that there exists an optimum with respect to the amount of CNT in the NiCo-CNT composite coating for achieving highest corrosion resistance performance. This optimum CNT amount was the same that produced finer morphology and preferred growth along low energy planes. Further characterization of the composite coating microstructure revealed that the optimum CNT concentration also produced a microstructure with largest fraction of low energy low angle grain boundaries which also resulted in higher corrosion resistance performance.