ELECTROCRYSTALLIZATION MECHANISM OF ZN-NI ALLOYS IN CHLORIDE ELECTROLYTES

The electrocrystallization kinetics of zinc and nickel alloys in chloride solutions has been investigated for ion ratio Ni2+/Zn(II) between 0 and 3, by means of polarization and electrochemical impedance techniques. During the codeposition, interactions between the two elements occur and both zinc and nickel discharges are inhibited. A reaction path has been developed which involves the slow blocking of the electrode surface by a hydrogenated adsorbate inhibiting the discharges of zinc and nickel ions. The reduction of Zn(II) species occurs in two steps via the intermediate Zn(ads)I and it is slowed down by the codischarge of the Ni2+ ions. The specific feature of the codeposition mechanism is the formation of a mixed intermediate (NiZn)ads+ which catalyses the Ni2+ reduction. This model reproduces to a reasonable extent the potential dependencies of the partial current densities for zinc and nickel discharges and of the relaxation processes observed on the impedance spectra.