Effects of saccharin and anions (SO₄ ²⁻, Cl⁻) on the electrodeposition of Co–Ni alloys

The electrodeposition of Co–Ni alloys is investigated to examine the mechanistic effects of chloride, sulfate and saccharin on the resulting alloy composition, deposition current efficiency and partial current densities of the cathodic reactions. When deposition is carried out in chloride, sulfate or mixed sulfate-chloride solutions without saccharin, the influence of hydrogen evolution (HER) becomes dominant and deleterious at higher overpotentials, leading to very low metal deposition current efficiency and the formation of a hydroxide/oxide film on the substrate. This problem is significantly reduced when saccharin is added to the mixed sulfate-chloride plating bath and to a lesser extent the sulfate-only solution. Although saccharin is ineffective in suppressing H⁺ reduction at low overpotentials, it is very effective at inhibiting H₂O reduction at high overpotentials and enabling metal deposition to more easily occur. The system follows anomalous behavior at all current densities both in the absence and presence of saccharin, although it approaches normal behavior as the current density increases toward −1,000 A m⁻² due to Co(II) reduction being mass transfer-controlled.