Effect of Some Plating Parameters On The Electrodeposition Of Zn-Co Alloys From Aqueous Citrate Baths

Abstract

Preliminary experiments were curried 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)-l bath which contains: CoSO4.7H2, O 0.26 mol.l1 ZnSO4.7H2O 0.09 mol.l−1, citric acid 0.1 mol.l1 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 dm2), duration (20 min.) and temperature (25 °C), pH = 5, the current efficiency of the alloy plating (fzn-Co = 62%) was much lower than those observed for either of the parent metals (fco = 89% and fzn = 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)-I bath (hexagonal and monoclinic). Superimposing a.c. on d.c. during Zn-Co codeposition greatly improves the uniformity and the levelling of the deposit.