A comparative study on the electrochemical behaviour of amorphous and nanocrystalline cobalt–tungsten electrodeposited coatings

Abstract

Amorphous and nanocrystalline cobalt–tungsten coatings were electrodeposited from a citrate-ammonia bath on copper substrates. Both coatings showed a nodular surface morphology, but a microcrack network was observed in the amorphous coating. The cyclic voltammograms of both deposits revealed anodic and cathodic low-current plateaus around the open circuit potential, exhibiting a passive behaviour. Mott–Schottky analysis showed that the passive films exhibit n-type semiconductivity behaviour and that formed on the amorphous coating showed higher crystal defects. Electrochemical impedance spectroscopy revealed that the amorphous coating has higher corrosion resistance than the nanocrystalline one at both open circuit and anodic potentials. This was attributed to the higher pore resistance of passive film formed at the open circuit potential and more chemical stability of the amorphous coating which reduces its dissolution at the anodic potential. The plugging of the microcrack network in the amorphous coating by corrosion products eliminated the negative effect of microcracks.