Normal and anomalous codeposition of Zn-Ni alloys from chloride bath

The codeposition of Zn-Ni alloys from chloride bath has been studied by means of potentiostatic electrodeposition in the potential range -700 to -1100 mV vs Ag/AgCl, where both normal and anomalous codeposition occurs. Deposition of alloys of different composition, morphology and structure, depending on the cathodic potential, was found. Analysis of the partial current densities showed that the production of nickel rich alloys in the potential range -700 to -900 mV is due to the underpotential reduction of zinc, driven by nickel ion discharge. Morphological and microstructural analyses showed that these alloys have the face-centred-cubic structure of nickel (alpha phase) and that the addition of zinc in the nickel lattice causes internal stresses in the deposits, which are prevalently amorphous. At potentials more negative than -910 mV, corresponding to the equilibrium potential of the zinc rich gamma phase deposition, the rate of deposition of the alpha phase decreases and the further increase in deposit zinc content leads to the formation of the gamma phase, with a decrease in internal stress. In this range of potential, zinc and nickel reduction can occur separately, according to their respective exchange current densities.