Nanocrystalline Ni–Co alloy coatings: electrodeposition using horizontal electrodes and corrosion resistance

Nanocrystalline Ni–Co alloy coatings containing 0–45 wt% Co were electrodeposited using horizontal electrodes in a modified Watts bath. Different techniques including scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, microindentation, and potentiodynamic polarization were used to characterize the alloy coatings. Properties of the alloy coatings were investigated as a function of the cobalt ion concentration (Co²⁺) in the bath. It was observed that the alloy codeposition exhibits anomalous behavior. Co content in the alloy coatings increases with increasing Co²⁺ in the bath and with electrolyte agitation. Morphology and grain size of alloy coatings are greatly affected by Co content. By increasing Co content, surface morphology of the alloy coatings changes from pyramidal to spherical. Microhardness of the alloy coatings increases with increasing Co content mainly due to decreasing grain size that follows the Hall–Petch relation. In addition, Ni–17 wt% Co alloy exhibits better corrosion resistance compared to pure Ni and other Ni–Co alloy coatings. The higher corrosion resistance of Ni–17 wt% Co coating is discussed based on its phase structure, grain size, and preferred orientation.