Effect of some plating parameters on the electrodeposition of Zn-Co alloys from aqueous citrate baths

Preliminary experiments were carried out in order to electrodeposit a sound and satisfactory Zn-Co alloy from citrate baths. Highly adherent Zn-Co plates, fine-grained, white-grey and with metallic lustre and were deposited on to a steel substrate from an acidic citrate (Zn-Co)-1 bath which contains: CoSO 4 .7H 2 O 0.26 mol.l -1 , citric acid 0.1 mol.l -1 and tri sodium citrate 0.19 mol.l -1 at pH = 5.0. The Zn-Co alloys codeposited from the citrate bath belong to the anomalous type of codeposition in which the less noble metal, Zn, deposits preferentially to the more noble one. A comparison between the experimental and theoretical (calculated) polarization curves during codeposition was utilized to provide an explanation of the anomalous phenomenon. Under the chosen condition of current density (2.0 A dm -2 ), duration (20 min.) and temperature (25°C), pH = 5, the current efficiency of the alloy plating (F Zn-Co = 62%) was much lower than those observed for either of the parent metals (F Co = 89% and F Zn = 97%). The Co% in the alloy increases with increase of either its content in the bath or of the bath temperature. On the other hand, Co% in the alloy increases with decreasing current density or decreasing pH. X-ray diffraction analysis showed the presence of two phases for the alloy codeposited from (Zn-Co)1 bath (hexagonal and monoclinic). Superimposing a.c. on d.c. during Zn-Co codeposition greatly improves the uniformity and the levelling of the deposit.