Grain boundary engineering in Ni-carbon nanotube composite coatings and its effect on the corrosion behaviour of the coatings

Abstract

Coatings have been traditionally used for protection against corrosion. One possible way to enhance the corrosion resistance performance of coatings is by grain boundary engineering (GBE). This study illustrates the role of optimum amount of carbon nanotubes (CNTs) in modifying the grain boundary constitution in nickel-CNT composite coatings thereby enhancing its corrosion resistance performance. Compact and crack free Ni-CNT composite coatings were electrodeposited on mild steel substrates using electrolyte bath with different concentrations of dispersed CNTs (4, 6, 8, 10, 20, 30, 40, 50, 70 and 100 mg/L). Corrosion performance of coatings exposed to 3.5 wt.% NaCl medium was studied by potentiodynamic polarisation and electrochemical impedance spectroscopy techniques. Among the 10 different Ni-CNT composite coatings, relatively low corrosion rates were observed for two distinct CNTs concentrations. Microstructural characterization conducted using the electron back scatter diffraction method (EBSD) suggested that the grains in all coatings grew with 〈110〉 orientation. Ni-CNT3 coating (from 8 mg/l of CNT in electrolyte) which showed low corrosion rate (corrosion current density of 0.98 µA/cm2) primarily contained very high fraction of low angle grain boundaries. Ni-CNT8 coating (from 50 mg/l of CNT in electrolyte) which exhibited the lowest corrosion rate (corrosion current density of 0.66 µA/cm2) contained high fraction of Σ3 coherent twins. Ni-CNT10 coating (from 100 mg/l of CNT in electrolyte) which exhibited the highest corrosion rate (corrosion current density of 14.08 µA/cm2) contained random high energy high angle grain boundaries (HAGBs) along with high energy symmetrical and asymmetrical tilt boundaries.