Dissolution from electrodeposited copper-cobalt-copper sandwiches

The electrochemical behaviour of electrodeposited Co-Cu/Cu multilayers from citrate electrolytes was investigated using cyclic voltammetry and stripping techniques at a rotating ring disc electrode. Copper and cobalt-copper alloy sandwiches were deposited from an electrolyte containing 0.0125 M CuSO₄, 0.250 M CoSO₄ and 0.265 M trisodium citrate at two different pHs, 1.7 and 6.0. The Cu/Co-Cu/Cu sandwich is representative of a single layer in a Co-Cu/Cu multilayer deposit, which is known to exhibit unusual physical and magnetic properties. Results from cyclic voltammetry and detection of dissolving species at the ring showed that cobalt is stripped from a Cu/Co-Cu/Cu sandwich even when a copper layer as thick as 600 nm covers the Co-Cu alloy. Scanning electron microscopy showed that cobalt can dissolve from the deposit easily because the copper layer covering the Co-Cu alloy is porous. A separate series of experiments with Cu/Co-Ni-Cu/Cu sandwich showed that cobalt does not dissolve from these deposits because the addition of nickel stabilises cobalt in the Co-Ni-Cu alloy.