Corrosion stability of Zn-Co alloys deposited from baths with high and low Co content - The influence of deposition current density

Electrochemically deposited Zn-Co alloys under various deposition conditions were investigated using atomic absorption spectroscopy (AAS) for determination of chemical composition, anodic linear sweep voltammetry (ALSV) and X-ray diffraction (XRD) for phase structure determination, SEM and AFM for surface morphology analysis, and polarization measurements for determination of corrosion properties. The influence of deposition current density and composition of deposition solution on the phase structure, morphology and corrosion properties of Zn-Co alloys were studied. The plating baths used were with low metal concentration, free of additives and deposition was carried out at room temperature, since the aim of the work was to investigate only the influence of different deposition current densities using economically favorable deposition parameters. It was shown that the ratio of cobalt to zinc ions in the plating bath strongly affects the chemical content and phase structure, as well as the morphology and corrosion stability, of Zn-Co alloys. The single-phased Zn-Co alloys showed the best corrosion stability. The alloys deposited at the highest current density from plating bath with the lower ratio of cobalt to zinc ions, as well as the one deposited at the intermediate current density from bath with higher ratio of cobalt to zinc ions, exhibited the lowest corrosion rates. It was also shown that the deposit having smaller crystallite grains (obtained at higher current density), among two alloy deposits having the same chemical content and phase structure, yielded lower corrosion rate. (C) 2010 Elsevier B.V. All rights reserved.