Electrodeposition of Zn-Co alloy coatings on carbon steel and their corrosion resistance

The development of Zn - Co alloys appeared as alternative for obtaining layers more resistant than the conventional zinc with a minimum increment of cost. The aim of this work is to study the electrodeposition mechanism of the Zn - Co alloy on SAE 1020 carbon steel by cyclic voltammetry using two baths with different [Zn2+]/[Co2+] concentration ratios and also to evaluate the corrosion resistance in 0.1 mol L-1 NaCl of electrodeposits obtained potentiostatically using electrochemical techniques as electrochemical impedance spectroscopy, linear polarization resistance and potentiodynamic polarization curves. The voltammograms showed two anodic peaks attributed to dissolution of zinc rich eta-phases and cobalt, and a cathodic peak related to Zn - Co alloy deposition. The nucleation mechanisms were examined by fitting the experimental data (chronoamperometry) into the Scharifker and Hills nucleation models. Scanning electron microscopy (SEM) showed that the electrodeposits obtained for the [Zn2+]/[Co2+] 9:1 ratio presented a structure in the form of hexagonal platelets in comparison to the electrodeposits with [Zn2+]/[Co2+] 12:1 ratio, which presented cauliflower-shaped structures. On the other hand, the electrochemical techniques proved that the electrodeposits obtained for the [Zn2+]/[Co2+] 9:1 ratio at potential of -1450 mV presented the best anticorrosive properties. Structural and chemical characterization of coatings was accomplished by XRD and XRF.