Study of Nickel‐Cobalt Alloy Electrodeposition from a Sulfamate Electrolyte with Different Anion Additives

Nickel-cobalt alloys exhibit a spectrum of physical properties that have led to the widespread use of these materials in a variety of high-technology applications. The recent emergence of microstructure- and microsystem-fabrication by electroplating through thick three-dimensional complex-shape electroformed molds illustrates the potential for new challenging applications of these alloys. Magnetic recording tapes, composite coatings, and devices for photothermal conversion of solar energy are only a few examples of nondecorative uses of nickel-cobalt electrodeposition. 1-6 The magnetic, mechanical, and corrosion properties of Ni-Co deposits are dictated by the structure and alloy composition. These parameters, in turn, are affected by processing variables such as plating-bath chemistry, pH, temperature, and applied current density. It is well established that the electrodeposition of iron-group alloys is followed by a local pH rise near the electrode surface. This pH rise is favored when H 2 is evolved simultaneously with alloy deposition. 7-11 In addition it was found that in the absence of boric acid in the electrolyte, the oxygen content in electrodeposited nickel and nickel-iron alloys increases with increasing applied current density. 11 This is explained by the surface precipitation and occlusion of hydroxides in the growing deposit resulting from an increase in the pH of the solution adjacent to the cathode (pHs). A near-electrode pH rise influences the reduction of cations that is supposed to be preceded by dehydration or decomposition of Ni 21 and Co 21 complexes. The importance of knowing the surface concentration of the reacting species in the electrochemical reaction, including that of the hydronium ion in the near-electrode layer, was realized long ago. 12 Deposition of Ni-Co alloys with predictable properties, therefore, depends in large part on understanding the effects of electrode polarization and near-electrode phenomena. The kinetics of single iron-group metal deposition has been studied by many authors and excellent reviews are available. 7-11,13,14 The deposition of cobalt is greatly favored over the deposition of nickel. 10,11,14,15-20 This behavior, the opposite of that which would be predicted from thermodynamics alone (E8 Ni21 5