Development of a bath for electrodeposition of zinc-cobalt compositionally modulated alloy multilayered coatings

The production of compositionally modulated alloy multilayers (CMAM) of Zn-Co alloys from an acid sulfate bath has been extensively studied and the electrodeposition parameters and conditions optimized to yield layers of high and low cobalt contents. It is shown that a change of deposition mechanism (from anomalous to equilibrium codeposition) occurs and is associated with the formation of Co3+ ions in solution and not to the formation of an electrode film of Co(OH)(2) as traditionally believed. Spectroscopic studies (UV- visible) showed that Co3+ ions are formed on the anode surface due to oxidation of Co2+ ions in the bath, and these trivalent ions prevent high-cobalt-containing Zn-Co alloy coatings. Using a two-compartment cell was found to be very effective in preventing the formation of trivalent cobalt ions in the catholyte portion of the bath and thus changing the codeposition mechanism from anomalous to equilibrium and consequently promoting high levels of cobalt in the deposits. This technique also increased the cathode current efficiency to 98%. By using a computer-aided pulse plating unit and designing different waveforms, various types of Zn-Co CMAM coatings of different thickness and number of layers were electrodeposited. (C) 2003 The Electrochemical Society.