Effect of some plating parameters on the electrodeposition of Zn-Co alloysfrom 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 pla tes, fine-grained, white-grey and with metallic lustre and were deposited o n to a steel substrate from an acidic citrate (Zn-Co)-1 bath which contains : CoSO4.7H(2)O 0.26 mol.l(1) ZnSo(4).7H(2)O 0.09 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 codeposi tion in which the less noble metal, Zn, deposits preferentially to the more noble one. A comparison between the experimental and theoretical (calculat ed) polarization curves during codeposition was utilized to provide an expl anation of the anomalous phenomenon. Under the chosen condition of current density (2.0 A dm(2)), duration (20 min.) and temperature (25 degrees C), p H = 5, the current efficiency of the alloy plating (f(Zn-Co) = 62%) was muc h lower than those observed for either of the parent metals (f(Co) = 89% an d F-Zn = 97%). The Co% in the alloy increases with increase of either its c ontent in the bath or of the bath temperature. On the other hand. Co% in th e alloy increases with decreasing current density or decreasing pH. X-ray d iffraction analysis showed the presence of two phases for the alloy codepos ited from (Zn-Co)-1 bath (hexagonal and monoclinic). Superimposing a.c. on d.c. during Zn-Co codeposition greatly improves the uniformity and the leve lling of the deposit.