Cathode Process in Nickel-Cobalt Alloy Deposition from Sulfamate Electrolytes—Application to Electroforming
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Cathode Process in Nickel-cobalt Alloy Deposition from Sulfamate Electrolytes—Application to Electroforming By D. Golodnitsky, N.V. Gudin & G.A. Volyanuk Electroforming is a specialized application of electroplat- -0.230 and -0.270 V vs. NHE, respectively. Accordingly, it ing for the production of finished components and unique would be expected, thermodynamically, that Ni, the more articles that cannot be made by any other method. noble metal, would deposit preferentially; however, the re- Sulfamate electrolytes for Ni-Co deposition form highly verse is found to be true. The discharge rate of the more noble efficient and stable solutions, but their wide use for component is inhibited, and this causes the appearance of the electroforming has been hampered by the fact that the less noble component at a much higher ratio in the deposit mechanism of Ni+2 and Co+2 codeposition is as yet imper- than in the electrolyte. Anomalous codeposition of binary fectly understood. Study was made of the effect of electro- iron-group alloys has been widely discussed by many inves- lyte composition and operating conditions on the Ni-Co tigators,6-20 but the mechanism is far from being understood. alloy constitution and the mechanism of its electrodepo- It was assumed that formation of the less noble metal is sition. It was shown that the Faradaic efficiency of the favored in aqueous solution and that metal hydroxides (MOH+) deposition is characterized by a complicated dependence are the important charge-transfer species. Matulis et al.11 on pH with a maximum of 98.5 percent at pH 3.5 to 3.8. suggested that at pH above 4, nickel deposition occurs mainly The concentration of cobalt in the deposit decreases by as through the discharge of NiOH+ ions. Dahms and Croll13 much as five to seven percent for a pH rise from 2 to 5.2. showed that anomalous deposition occurred when the hydro- A minor increase of cobalt concentration in the electro- gen limiting current was exceeded. A mathematical model lyte is followed by a steep rise in the cobalt content of the for anomalous codeposition of nickel-iron on a rotating disk alloy. At a ratio of cobalt to nickel of 0.1, the alloy contains has been developed by Hessami and Tobias.15 Sasaki and 45 percent cobalt; at equal cation concentrations, the Talbot9 found that this model was unable to characterize fully alloy contains 73 percent Co. This is because of the faster either Ni-Co or Co-Fe electrodeposition. With minor changes kinetics of Co+2 reduction. By rotating-disk-electrode to the hydrolysis constants, however, the model predictions and chronopotentiometric methods, it was demonstrated greatly improve the fit for the Ni-Co results. Contrary to the that the rate-determining step of alloy deposition is the results presented above, Glasstone12 found that the potentials electrochemical reaction, complicated by adsorption. It is of Ni+2, Co+2 and Ni-Co deposition are pH independent and believed that the codeposition of Ni+2 and Co+2 is explained that the Ni-Co reduction potential lies between the Ni+2 and by the formation of heteronuclear surface complexes with Co+2 potentials. A strong case can be made for the hypothesis sulfamate anion as the bidentate ligand. The alloy struc- that depolarization and overpolarization effects on alloy ture and physical and mechanical properties, such as deposition depend on the potential of zero charge, on the state hardness, internal stress, tensile strength, elongation and of the cations in the electric double layer and adsorbed ad- thermal stability were also studied. An optimum was atoms.17-19 found between alloy characteristics and operating condi- Deposition of Ni-Co alloys has evolved from hard, brittle tions. Parts having complex shapes (molds, miniature deposits produced in Watts-type sulfate and chloride electro- nozzles and other electronic and aircraft components) lytes to ductile deposits produced in sulfamate electrolytes. were produced by Ni-Co electroforming. Mechanical and physical properties are determined by alloy Recent developments suggest that the engineering and composition, which in turn is controlled by the electrodepo- electroforming applications of nickel and nickel-alloy plat- sition variables. Considering that Ni-Co platings in sulfamate ing are becoming increasingly important. Composite coat- electrolytes afford good mechanical properties at a high ings, electroplated carbon/graphite fibers, electroformed deposition rate (7 to 20 µm/min), compared to Watts or molds, printed-circuit boards—these are only a few ex- chloride baths,2,21-27 it was of interest to estimate the possible amples of nondecorative uses of nickel. Electroforming is a importance of sulfamate-anion effect on the cathode process. specialized application of the electroplating process con- It is our purpose to contribute to elucidation of the effect of cerned with the fabrication of complex parts and components sulfamate electrolyte composition on the codeposition of that cannot be made by any other method. Modern applica- Ni+2 and Co+2, to investigate the mechanical properties of the tions of electroforming are diverse and may be categorized as alloy, and to optimize the bath composition and operating follows: (1) tools, including molds and dies, diamond-cutting parameters for electroforming of parts having complex shapes bands, (2) mesh and foil products, such as filters and razor and other components. screens; and (3) other products, such as space mirrors, metal optical parts, bellows, radar and waveguides.1-5 The possibili- Experimental Procedure ties for innovation in this area are far from being exhausted. Each experiment was carried out in a fresh solution. Solu- Nickel-cobalt alloys are widely used for electroforming, tions were prepared just before each experiment by dissolv- owing to their magnetic and high tensile properties. ing the requisite amounts of the metal sulfamates in distilled, From a theoretical point of view, Ni-Co alloy plating is deoxygenated water. The concentration ranges studied were interesting, as it exhibits anomalous codeposition, that is, the those normally encountered in industrial plating. Boric acid less noble metal deposits preferentially to the more noble was used to adjust the pH. After each solution was transferred one. The standard equilibrium potentials of Ni and Co are to the cell, it was sparged with argon for at least 30 min. An February 1998 65 rinsed with a solution consisting of a 4:2:1 volume ratio of sulfuric acid, nitric acid and water, respectively. This was followed by a thorough rinse with distilled deionized water. The platinum disk was cleaned by immersion in 0.5 M NH2SO3H and sweeping from -0.2 to 1.3 V for five min. Copper plates had been previously cleaned in alkaline solu- tion, etched in 15-percent H2SO4 for 15-20 sec and thor- oughly rinsed with distilled deionized water. The bath tem- perature varied from 22 to 60 ±2 °C. The measurements of surface pH were performed galvanostatically by the use of a nickel-hydrogen reversible electrode28 and calculated from Eq. (1). pHs = E/0.058 (1) Fig. 1—Surface pH (1) and pH bulk (2,3) vs. current density in Ni-Co +2 +2 sulfamate solutions (mmol): 1, 2 - Ni 1120, Co 70; H3BO3 328, NaCl 69; where E is the electrode potential determined from potential 3 - Ni+2 1120, Co+2 70, NaCl 69 at 25 °C. drop curves at 0.02 sec after current interruption. The formation of Ni+2 and Co+2 complexes with sulfamic argon atmosphere was maintained over the solutions to acid has been studied by nuclear magnetic relaxation (NMR). inhibit the absorption of oxygen. The nickel and cobalt The investigations were conducted over a wide range of content of the electrolytes was determined spectrophoto- reagent concentrations and solution acidity. metrically. Metal distribution was studied in a 150-mL Haring-Blum Electrodeposition experiments were performed with a cell and calculated from Eq. (2). three-electrode system consisting of a platinum counter- electrode, saturated silver chloride (SCE) reference electrode A = log L/log M (2) and a platinum rotating disk (RDE) working electrode (area = 0.28 cm2). The electrode rotation rates varied from 20 to where L is the ratio of cathode spacing and M is the ratio of 200 rpm. A three-compartment cell was used with the refer- deposit weights on the near and far electrodes. According to ence electrode connected to a Luggin capillary positioned in Chin,29 at ideal metal distribution, log M = 0, A = ∞; when the flow field. The ohmic polarization drop did not exceed 10 there is no secondary distribution, A = 1 and, in the absence to 15 mV and was taken into consideration. A potentiostat/ of deposit on the far electrode, A = 0. galvanostat was used to control the potential in the deposi- Tensile tests were carried out at room temperature on a tions. Potentiodynamic measurements were performed at a constant crosshead machine, type MP-05, at a speed of 3.33 slow sweep rate of 400 mV/min. To determine partial cur- x 10-5 cm/sec on 200- µm-thick samples. The deposits were +2 +2 rents of Ni and Co reduction and H2 evolution, 7 to 10 electroformed nonadherently on flat stainless steel cathodes equidistant points were selected in the overall polarization and removed mechanically, care being taken not to deform curve. Each potentiostatic electrodeposition corresponding the specimens severely while removing them. Hardness was to the chosen points was terminated after the amount of measured with a microhardness tester, using a 100-g load. charge passed into the solution was approximately equal to Indentations were made on the 50-µm-thick deposits. Inter- 1.0 C. The actual value of the total charge ac- cumulated was mea- Table 1 sured by a coulometer.