The influence of sodium gluconate on nickel and manganese codeposition from acidic chloride-sulfate baths

Nickel and manganese were codeposited from acidic chloride-sulfate solution in the presence and without sodium gluconate as complexing and buffering agent. Equilibrium pH-dependent distribution of soluble species in the baths was calculated. Deposition of metals was studied by cyclic voltammetry and Hull cell tests. Deposits were obtained in potentiostatic (-1.6 to -1.7 V vs. Ag/AgCl) and galvanostatic (4-8 A/dm(2)) conditions. It was found that the buffering action of gluconate ions improved the quality of the deposits, and compact Ni-Mn with a grid of fine crack layers was produced. The additive inhibited the formation of nonmetallic inclusions represented by decreased oxygen content in the deposits. Layers produced at constant potentials contained 67-79 % Ni, 1-8 % Mn, and 15-30 % O. Under galvanostatic conditions, the average compositions were in the range of 18-80 % Ni, 2-48 % Mn, and 17-42 % O. Despite of the bath composition, the main cathodic reaction was hydrogen evolution resulting in very low amounts of the cathodic deposits.