Cathode process in nickel-cobalt alloy deposition from sulfamate electrolytes : Application to electroforming

Electroforming is a specialized application of electroplating for the production of finished components and unique articles that cannot be made by any other method. Sulfamate electrolytes for Ni-Co deposition form highly efficient and stable solutions, but their wide use for electroforming has been hampered by the fact that the mechanism of Ni +2 and Co +2 codeposition is as yet imperfectly understood. Study was made of the effect of electrolyte composition and operating conditions on the Ni-Co alloy constitution and the mechanism of its electrodeposition. It was shown that the Faradaic efficiency of the deposition is characterized by a complicated dependence on pH with a maximum of 98.5 percent at pH 3.5 to 3.8. The concentration of cobalt in the deposit decreases by as much as five to seven percent for a pH rise from 2 to 5.2. A minor increase of cobalt concentration in the electrolyte is followed by a steep rise in the cobalt content of the alloy. At a ratio of cobalt to nickel of 0.1, the alloy contains 45 percent cobalt; at equal cation concentrations, the alloy contains 73 percent Co. This is because of the faster kinetics of Co +2 reduction. By rotating-disk-electrode and chronopotentiometric methods, it was demonstrated that the rate-determining step of alloy deposition is the electrochemical reaction, complicated by adsorption. It is believed that the codeposition of Ni +2 and Co +2 is explained by the formation of heteronuclear surface complexes with sulfamate anion as the bidentate ligand. The alloy structure and physical and mechanical properties, such as hardness, internal stress, tensile strength, elongation and thermal stability were also studied. An optimum was found between alloy characteristics and operating conditions. Parts having complex shapes (molds, miniature nozzles and other electronic and aircraft components) were produced by Ni-Co electroforming.