Effect of thiourea on the structure and corrosion properties of electrodeposited ZN-SN

Pulse electrodeposition was applied to deposit Zn-Sn coatings from acidic sulfate-gluconate baths with polyethylene glycol (PEG), hexadecyltrimethylammonium bromide (CTAB) and thiourea as additives. The influence of different concentrations of thiourea on the voltammetric behavior, surface morphology and corrosion properties of the coatings was investigated by cyclic voltammetry, field emission scanning electron microscopy (FESEM) and potentiodynamic polarization techniques respectively. Thiourea as a grain refiner was added to the plating solution in different concentrations (0.015 M, 0.030 M, 0.045 M and 0.060 M). Cyclic voltammetry results showed that increase in concentration of thiourea does not change the deposition potential of the zinc-tin on glassy carbon electrode. FESEM results showed that increasing the thiourea content up to 0.045 M in the plating bath reduced the average grain size of the deposits drastically from 738 nm to 244 nm. However further addition to 0.060 M thiourea in the composition increased the average size to 290 nm. The lowest corrosion current density was achieved for the bath with 0.045 M thiourea.