Study of Nickel-Cobalt Alloy Electrodeposition from a Sulfamate ...

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Journal of The Electrochemical Society, 147 (11) 4156-4163 (2000) S0013-4651(00)02-107-8 CCC: $7.00 © The Electrochemical Society, Inc.

Study of Nickel-Cobalt Alloy Electrodeposition from a Sulfamate Electrolyte with Different Anion Additives D. Golodnitsky,a,*,z N. V. Gudin,b and G. A.Volyanukc aSchool of Chemistry, Tel Aviv University, Tel Aviv, 69978, Israel bPolytechnic University, Kazan 420080, Russia cResearch Institute of Aircraft Technique, Kazan 420039, Russia

The experimental results of the present study show that the composition of the electrodeposited nickel-cobalt alloy in sulfamate electrolytes containing anion additives is influenced in a complicated manner by the concentrations of cobalt(II) ion and citric acid, pH, and applied current density. The operating conditions were found under which the increase in the pH of the solution adjacent to the cathode is inhibited. Citrate anions form a wide variety of complexes with nickel and cobalt. Protonated citrate complexes of nickel(II) and cobalt(II) are apt to be involved in the electrochemical alloying process. Acetate anions serve to buffer the sulfamate solutions. Nickel-cobalt electrochemical alloying leads to an increase in the reaction rate of cobalt at the expense of the nickel reaction rate. The data confirm that inhibition of the more noble metal by the less noble one does not depend on the anion composition of the electrolyte. © 2000 The Electrochemical Society. S0013-4651(00)02-107-8. All rights reserved. Manuscript submitted February 28, 2000; revised manuscript received July 26, 2000.

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 H2 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 Ni21 and Co21 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 (E8Ni21 5 20.230 V and E Co 8 21 5 20.270 V vs. NHE), is known as anomalous codeposition. The anomalous behavior is represented by an unexpectedly high cobalt content even at small fractions of the H3O1limiting current in different electrolytes.7 For instance, a typical Watts bath with 1:10 cobalt(II)-to-nickel(II) ratio in the electrolyte can yield an alloy deposit containing 40% cobalt.2 Similar data were obtained for chloride, and acetate electrolytes (at CCo21: CNi21 5 1:15 with 13-16% Co in the alloy),21 sulfamate electrolyte (at * Electrochemical Society Active Member. z E-mail: [email protected]

CCo21:CNi21 5 1:10 with 30-35% Co).19 The recent model of anomalous codeposition of Ni-Fe proposed by Matlosz 17 considers the competitive adsorption of iron and nickel species to be the primary reason for nickel inhibition. A mathematical model of the anomalous deposition of nickel iron-alloys, proposed by Baker and West,18 is compared to steady-state and electrochemical-impedance spectroscopy data on a rotating disk electrode. This model based on the modification of a previously proposed rate law for the adsorption of iron is successful in predicting the changes in alloy composition and the impedance data with the bath concentration, degree of agitation and current density. Although a number of explanations and reviews of this phenomenon have been published,7-20 the mechanism of anomalous codeposition is not well understood. Deposition of Ni-Co alloys has evolved from hard, brittle deposits produced in Watts-type, sulfate and chloride electrolytes to ductile deposits produced in sulfamate electrolytes. Ni-Co platings in sulfamate electrolytes afford good mechanical properties at the high deposition rate (7 to 20 mm/min) compared to Watts or chloride baths.1,2,4,5,9,13,16,17,20,21 In addition, sulfamate electrolytes are known to be more stable to pH changes than sulfate electrolytes.1,2,5,17,20 The principal aim of this investigation was to determine the experimental conditions under which the pHs rise can be inhibited and to clarify the effect of different complexing and buffering agents on the cathode reactions. Experimental Each experiment was carried out on a fresh solution. Solutions were prepared just before each experiment by dissolving the requisite amounts of the metal sulfamates in distilled deoxygenated water. The concentration ranges studied were those normally encountered in industrial plating and electroforming processes. After the solution was transferred to the cell, it was sparged with argon for at least 30 min. An argon atmosphere was maintained over the solutions to inhibit the absorption of oxygen during the experiment. The nickel and cobalt content of the electrolytes was determined spectrophotometrically. Electrodeposition experiments were performed with a three-electrode system consisting of a platinum counter electrode, saturated silver chloride reference electrode, and a platinum rotating disk (RDE) working electrode (area 5 0.28 cm2).The electrode rotation rates varied from 20 to 200 rps. A three-compartment cell was used with the reference electrode connected to a Luggin capillary positioned in the flow field. The ohmic polarization drop did not exceed 10 to 15 mV and was taken into consideration. A P-5848 potentiostat/galvanostat (Gomel Plant of Precise Instruments, Belorussia)

Journal of The Electrochemical Society, 147 (11) 4156-4163 (2000)

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S0013-4651(00)02-107-8 CCC: $7.00 © The Electrochemical Society, Inc.

Table I. Effect of operating conditions on the change of bulk pH in Ni-Co sulfamate electrolyte containing different buffering and complexing agents.

Cathode current density (A/m2)

The amount of charge passed through the solution, Ah/m3

Initial pH bulk

0300 0500 1000 0300 0300 0300

03000 03000 03000 03000 03000 20000

3.00 3.00 3.00 4.50 5.40 3.20

0.11 0.36 1.40 0.40 — 0.95

was used to control the potential in the depositions. Potentiodynamic measurements were performed at a slow sweep rate of 400 mV/min. In order to determine partial currents of Ni21 and Co21 reduction and hydrogen evolution, seven to ten equidistant points on the overall polarization curve were selected. Each potentiostatic electrodeposition corresponding to a chosen point was terminated after the amount of charge passed into the solution was approximately equal to 1.0 C. The actual value of the total charge accumulated was measured by a coulometer. Mechanically polished, copper M00 disks were used as cathodes in quantitative studies. After 50 mm deposition, the samples were washed and dried. An X-ray fluorescence (XRF) spectrophotometer (Kevex Instruments) in connection with OMICRON software was used for compositional analysis of the electrodeposited alloys. The calibration of the XRF was verified spectrophotometrically. Pure metals were used as standards. The partial current densities of the alloy components were calculated from the XRF data using Faraday’s law. The current density of the side reaction was obtained by substracting the sum of the metal deposition partial current densities from the total current density at the same potential. For each set of electrodeposition conditions a duplicate or triplicate run was performed. Prior to each experiment, the glassware and disk were pretreated. Glassware was 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 20.2 to 1.3 V for 5 min. Copper disks had been previously cleaned in alkaline solution, etched in 15% H2SO4 for 15-20 s and thoroughly rinsed with distilled deionized water. The bath temperature varied from 22 to 60 6 208C. In Ref. 22 it was found that the growing deposit of nickel, whose permeability for H is well known, is reversible to proton in the hydrogen-laden medium. Construction of nickel-hydrogen reversible electrodes is simple. The foil of nickel is sealed into a glass tube through which electrical contact is made with the highly sensitive voltmeter. Hydrogen that is evolved simultaneously with nickel and alloy deposition is generated on the electrode. This is followed by a dramatic pH rise in the near-cathode surface. The electrode was polarized galvanostatically for 5 to 15 min depending on the current density. During measurements of the potential between nickel and saturated silver chloride reference electrode the electronic interrupter was used to assure the momentary disconnection of the circuit. The transient of potential is recorded. The surface pH was calculated from Eq. 122 pHs 5 E/0.058

pH bulk change in electrolyte with additives: H4Cit H3BO3 NaAc (125 mM) 1 NaAc (640 mM) (500 mM) (500 mM) 0.03 0.03 0.16 0.02 0.37 0.56

0.01 0.01 0.13 0.20 0.30 0.43

Results and Discussion It is rather surprising how often one finds it stated in the literature that the presence of a buffer guarantees equality of the surface and bulk pHs.12 A good experimental example of the contrary situation can be found in Ref. 24. The presence of several cations can make the situation even more complex. In order to eliminate or to diminish the increase of the nearcathode pH solution complexants and buffering additives, such as boric acid (H3BO3), sodium acetate (NaCH3COO-NaAc), citric((HOOCCH2)2C(OH)COOH-H4Cit), glycolic (HOCH2COOHHGly), and oxalic (HOOCCOOH-HOxal) acids have all been examined as additives to the sulfamate electrolyte. Changes in bulk pH and surface pH vs. polarization in Ni-Co sulfamate electrolyte (pH 3) with several different additives are shown in Table I and Fig. 1. Even though the accuracy of pH values obtained near the electrode is not fully satisfactory, the rise in surface pH was clearly detected. Regardless of the electrolyte composition, the H3O1 concentration decreases with increase in current density and pHs reaches the pH of formation of nickel and cobalt hydroxides. This is followed by the formation of powdery deposits as a result of the presence of occluded hydroxides. The sharp acidity drop is most pronounced at high cathode current densities. Whereas the pH in the bulk of the sulfamate solution increases up to 3.4, the surface pH rises to 10 at i 5 500 A/m2. Addition of H3BO3 is followed by a slight stabilization of pHs. Tilak et al.25 were the first to suggest that the lowering of pH results from the complexation of boric acid and nickel. The composition of the complex formed is Ni[B(OH)4]1, and the proton is a product of the complexation reaction. Formation of similar cobalt

[1]

where E is the electrode potential determined from potential drop curves at 0.02 s after current interruption. The formation of Ni21 and Co21 complexes with citric acid has been studied by nuclear magnetic resonance (NMR). The investigations were conducted over wide range of reagent concentrations and solution acidity. The experimental details can be found elsewhere23

Figure 1. Plots of pH surface vs. current density in Ni-Co sulfamate solution with anion additives, (mM): 1, Ni21 1120, Co21 70; 2, Ni21 1120, Co21 70, H3BO3 656; 3, Ni21 1120, Co21 70, NaAc 500; 4 Ni21 1120, Co21 70, NaAc 500, HCit 125, at 258C.

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complexes was suggested in Ref. 26. However, boric acid provides insignificant buffer capacity at low (up to 500 mM) concentration and increasing the concentration of boric acid is followed by its precipitation, especially at room temperature. In sulfamate electrolytes containing glycolic and oxalic acids the local pH rise in the nearelectrode layer was found to vary from 7.2 to 11.5, indicating that the dissociation rate of the acids cannot compensate the local pH change for more than about a 100-fold increase in the cathodic-current density (from 5 to 500 A/m2). Marked improvement in the stability of the pH of the solution adjacent to the cathode (pHs 8.2-8.4) in the wide current-density range from 50 to 1000 A/m2 was found in a sulfamate solution, containing 500 mM sodium acetate and 125 mM citric acid.27 Formation of Ni21 and Co21 acetate and citrate complexes.—The basic constituent of electrolytes under investigation was nickel sulfamate Ni(NH2SO3)2, a salt of a strong monobasic sulfamic acid NH2SO3H which is similar in structure to sulfuric acid with one hydroxyl group replaced by the amino group. It has been recently demonstrated that there is no formation of nickel or cobalt sulfamate complexes in the bulk of electrolyte or that any complexes formed are weak and unstable.20,28 Nickel(II) and cobalt(II) complex formation with acetate and citrate-anions was studied by nuclear magnetic resonance (NMR). The descending character of dependence of the relaxation efficiency coefficient (Ke1) of Ni21 vs. concentration of acetate anions (CAc2) indicates the formation of nickel acetate complexes. Linearity of the Ke1/CA2 c2 vs. Ke1 curve was found only for n 5 2, where n is the ligand number. This points to Ni(CH3COO)2 complex composition. The value of pK for complex formation, calculated according to Ref. 29, is 6.8 6 0.8. Nickel concentration-independent Ke1 suggests the absence of binuclear complexes. Similarly, a Co(CH3COO)2 complex with pK 7.8 6 0.8 was found. Citrate anions form a wide variety of complexes as a result of their three carboxyl and one hydroxyl groups. In addition, a feature peculiar to citrate nickel and cobalt complexes may be interelectron repulsion of the electron-donating groups. This may be followed by the occurrence of a nonbonded carboxyl group capable of protonation and surface coordination. These peculiarities enhance the lability of citrate complexes and are reflected in the kinetics and mechanism of electrode processes in sulfamate-based electrolytes, containing acetate and citrate additives. The complex formation of Ni21 and Co21 with citric acid has been studied.30-32 1:1 complexes with different degrees of protonation were found, and 1:2 complexes when the ligand is in excess. All these investigations were carried out at low concentrations (10 mM and less) of the metal ions and ligand.

Figure 2. Distribution of cobalt(II) species in a sulfamate solution, containing (mM): Co21 70, NaAc 500, HCit 125; at 258C. 1, Coaquo; 2, CoH2Cit; 3, CoHCit2; 4, Co2Cit242; 5, CoAc2.

Our study of the reactions of cobalt(II) and nickel(II) with citric acid at high reagent concentrations comparable with those used in industrial electrolytes and over a wide range of solution acidity23 showed the formation of NiH2Cit, NiHCit2, Ni2Cit(Hcit)32, Ni2Cit242, CoH2Cit, CoHCit2, Co2Cit242 complexes. We also found the species NiCo(HCitCit)32 and NiCo(Cit)242 at pH > 6 and 1:1 metal-to-citric acid concentrations.23 The pK values of these complexes were 19 and 27.7, respectively. High-pH stable Ni-Co sulfamate electrolyte containing 500 mM sodium acetate and 125 mM citric acid contains, in addition, Ni(H2O)6, NiAc2, Co(H2O)6, and CoAc2 complexes. In order to simulate the actual operating conditions, the concentrations of all the possible nickel and cobalt species were calculated from the equilibrium equations and constants. The distribution of complexes with pH is shown in Fig. 2 and 3. It is clear that above pH 4 CoHCit2 is prevalent cobalt species, and above pH 6 the concentration of binuclear cobalt complexes dramatically increases. Nickel, however, was found to be only partially bound by citrate and acetate anions. This is due to its high concentration (1.1 M) in the electrolyte. Study of the electrode processes.—Factors investigated in Ni-Co deposition included electrolyte concentration, current density, pH, and temperature. The various compositions of sulfamate electrolytes presented in Table II were examined. The concentration of Ni21 varied from 160 to 1400 mM, CCo21 was 10 to about 600 mM, CNaAc 70 to 500 mM, and CH4Cit 18 to 500 mM. Alloy films were deposited potentiostatically on the platinum rotating disk electrode (RDE) from nickel (CNi21 5 1120 mM), cobalt (CCo21 5 70 mM), and NiCo (no. 2, Table II) sulfamate baths at a pH of 3.0. As the electrode rotation rate (v) is increased from 24 to 200 rad/s, a 0.12-0.13 V decrease in the cathode polarization (DE) is observed at a current density of 315 A/m2 for Ni and Co electrodeposition at 258C. For alloying, however, this DE decrease (within the same range of electrode rotation rates) was found to be only 0.05 V. The extrapolated i-v1/2 straight line does not pass through the origin (Fig. 4). This indicates that the deposition proceeds under mixed activation and mass-transport control. Also there is generally a deviation of the Tafel slope at high cathodic polarization. This cannot be explained by diffusion limitations alone, since the experimental results were taken at a small percentage (less than 40%) of the limiting current for Ni, Co, and Ni-Co electrodeposition. Moreover, an increase of rotation rate by a factor of eight caused the cathode current density to rise (over 20.7 V) only by a factor of 1.5 to 2. We believe that this phenomenon may be related to the preceding chemical reaction (a descending curve of i?v21/2 2 i dependence) and/or to the formation of an adsorbed film onto the electrode surface. To study the kinetics of Ni-Co electrodeposition, the method of partial polarization curves on a stationary electrode was used. As the bath temperature rises from 25 to 608C, the alloy and metal current

Figure 3. Distribution of nickel(II) species in a sulfamate solution, containing (mM): Ni21 1120, NaAc 500, HCit 125; at 258C. 1, NiH2Cit; 2, NiHCit2; 3, Ni2Cit(HCit)32; 4, NiAc2; 5, Niaquo.


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Table II. The composition of sulfamate-based electrolytes for Ni-Co alloy deposition. Electrolyte composition, (mM) No. of electrolyte 01 02 03 04 05 06 07 08 09 10 11

Ni (NH2SO3)2

Co (NH2SO3)2

NaAc

H4Cit

1170 1120 1083 0954 0596 1120 1120 1120 0160 0640 1440

037 070 108 238 594 070 070 070 010 040 090

500 500 500 500 500 500 500 500 071 286 643

125 125 125 125 125 050 250 500 018 072 161

(Co21)-to-(Ni21) ratio

Total concentration of salts (mM)

0.032 0.062 0.100 0.250 1.000 0.062 0.062 0.062 0.062 0.062 0.062

1190 1190 1190 1190 1190 1190 1190 1190 0170 0680 1530

All the electrolytes contain 69 mM NaCl.

efficiencies increase. Analysis of partial polarization curves showed that the rate of reduction of nickel and cobalt complexes grows by a factor of eight. Within the range of current densities studied (20 to 1000 A/m2), the alloy current efficiency increases with increasing current density and reaches 82% at i from 600 to 1000 A/m2 at 558C. (Fig. 5). The cobalt content as a function of applied current density goes through a maximum at about 200 A/m2 over all temperature and pH ranges under investigation. This may be due to the concentration limitations of the reduction of complexes of cobalt occurring at lower current density, as compared to the reduction of nickel complexes. Changes in partial current efficiencies are strongly dependent on acidity. The sharp increase of the current efficiency of the alloy from 34.5 to 89.1% as a result of pH change from 2 to 4 is seen at i 5 50 A/m2 (Fig. 6). The nickel current efficiency increases from 27 to 74%, and the partial polarization curves shifted toward positive values. The cobalt current efficiency dependence on pH is complicated. It goes through a maximum in the vicinity of pH 3 at 50A/m2 and gradually decreases with increase in pH at 500 A/m2. Cobalt content in the alloy falls from 34 to 14% as pH increases (Fig. 6) and the cathode polarization increases by a factor 1.6. It should be emphasized that in the pH range from 3 to 4 the rate of cobalt deposition on alloying does not change. According to the distribution curves (Fig. 2 and 3), besides the aquo-complexes of nickel and cobalt, different citrate complex species, such as NiH2Cit, NiHCit2, and CoH2Cit, CoHCit2 are present at pH between 2 and 5. The respective equilibrium constants (pK) are 2.5, 4.5, 3.0, and 3.7. In solutions

Figure 4. Levich plots of i vs. v1/2 in nickel-cobalt sulfamate electrolyte at pH 3.1, T 5 258C, and polarization potential, V:1, 20.65; 2, 20.7; 3, 20.8; 4, 20.9.

of higher acidity the nickel-citrate complexes are less stable, thus the concentration of free citrate anions depends on the equilibrium constants of the nickel complexes. The formation of more stable complexes of nickel between pH 3 and 4 is followed by a decrease in the concentration of of nonbonded citrate anions. This, in turn, shifts the equilibrium of the complexation reactions toward formation of cobalt aquo-complexes and current efficiency of cobalt increases. As the pH is further raised, more strong complexes of both metals are formed. It was impossible to determine the order of the cathodic reactions of Ni21 and Co21 with respect to the hydroxyl ion because of the continuous compositional change of the electrolyte as the pH varies. The quality of deposits in the vicinity of bulk pH 2.0 was unacceptable owing to rapid hydrogen evolution that interfered with regular crystal growth. This resulted in porous and dull deposits. At pH 2 to 4 the deposits became less stressed and the further pH rise did not influence the quality of the alloy. The change of sodium acetate concentration in sulfamate electrolyte from 125 to 1000 mM does not substantially affect the alloy composition and current efficiency. The order of the cathodic reaction of Co21 and Ni21 codeposition with respect to acetate anions was found to be 0.18 and 0.25, respectively. The study of the effect of citric acid concentration on the cathode processes was performed in electrolytes no. 2 and no. 6-8 at pH 3 and 55°C. The experimental data are presented in Fig. 7. The increase in citric acid concentration in sulfamate-based solution is followed by a sharp decrease in partial current efficiencies and, consequently, in the current efficiency of the nickel-cobalt alloy (by 15 to 35% depending on current density). This may be explained by decreased activity of

Figure 5. Plots of the (1) current efficiency and (2) Ni-Co alloy composition vs. current density in sulfamate electrolyte no. 2 at pH 3 and T 5 558C.

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Figure 6. Plots of current efficiency (1, 2) and composition of the Ni-Co alloy (3, 4) vs. pH in electrolyte no. 2 at T 5 558C and i (A/m2): 1, 4-50; 2, 3-500.

Ni21 and Co21 cations in the presence of the high concentration of a complexing agent in the electrolyte. The increase of the CH4Cit in the electrolyte has almost no influence on the alloy composition at 50 A/m2; however, at high current density (500 A/m2), the cobalt content of the deposit increases by about 10%. As a result, the quality of deposits is impaired; the coatings become very bright and extremely brittle. Increasing the concentration of citric acid is followed by increase in cathodic polarization, but the Tafel slope, which is 118 6 5 mV and 108 6 5 mV for Ni21 and Co21 reduction, respectively, is not affected. According to the distribution curves of the complex species vs. concentration of the citric acid, presented in Fig. 8, it can be seen that in solutions with CH4Cit higher than 150 mM the cobalt cations are completely bonded by citrate anions, while for nickel the aquo-complexes are predominant. Thus, the zero order of the nickel reduction with respect to Cit- was to be expected. However, both cathode reactions were found to be of the negative first order (20.85 for Ni21 and 20. 80 for Co21 codeposition). We attribute this to the conversion of the nickel aquo-complexes in the bulk of the solution to citrate complexes in the vicinity of the cathode as a result of the near-electrode hyperalkalinity and increase in concentration of the citrate anion near the electrode. The distribution of complex nickel and cobalt citrate species at pH 3.5 and 8.2, calculated from NMR measurements, is shown in Table III. If one takes into account the decrease in cation concentration in the near-electrode area as compared to that in the bulk (as a result of diffusion limitations), the real

Figure 8. Distribution of nickel and cobalt species in a sulfamate solution, containing (mM): Ni21 1120, Co21 70, NaAc 500, HCit 125 at pH 3.5. 1, Niaquo; 2, CoH2Cit; 3, CoHCit2; 4, NiH2Cit; 5, NiHCit; 6, Coaquo.

concentration of the three and four negatively charged nickel and cobalt complexes should be higher than calculated. Therefore, the negative order of the cobalt and nickel cathode reaction with respect to citrate anion may be associated with the direct participation of nickel and cobalt citrate complexes in the reduction. As the net concentration of the cations increases, the alloy becomes enriched with cobalt, the concentration of which rises from 10 to 32 wt %. This effect is more pronounced at low current densities. Cathodic polarization of alloy deposition decreases substantially as the net cation concentration is varied from 170 to 1530 mM (electrolytes no. 2, 9-11, Table II). At a cathode potential of 20.6 V vs. silver chloride electrode the rate of Co21 and Ni21 reduction increased by factors of 8.3 and 3.5, respectively. The anomalous behavior of iron-group metals on alloying can best be appreciated from the study of the effect on alloy deposition of the CCo21:CNi21 ratio. The corresponding composition profile is shown in Fig. 9. From this figure it is obvious that the content of cobalt is always higher than that expected from the cobalt concentration in the bath. At CCo21:CNi21 5 0.1, the deposit contained 40% cobalt, while at a ratio of 0.5 the cobalt content was 73% at 558C and 500 A/m2. Thus we have, for Ni-Co alloys, an unambiguous indication of the inhibition of deposition of the more noble metal and promotion of the deposition of the less noble metal. The data are in a good agreement with other authors.7-20 Beyond anomalous codeposition it was found that the resulting composition of the Ni-Co system is very sensitive to the applied current. The extent to which the current efficiency and cobalt content are affected by a current density depends strongly on the cobalt to nickel concentration ratio in the electrolyte. At a cobalt(II) concentration of 37 mM cobalt current

Table III. Effect of pH on the distribution of citrate nickel and cobalt complexes. Nickel(II) and cobalt(II) citrate complexes

Figure 7. Plots of Ni-Co alloy current efficiency (1, 3) and cobalt content (2, 4) vs. concentration of citric acid in sulfamate electrolytes no. 2, 6-8 at pH 3.0, 558C and i (A/m2): 1, 4-500; 2, 3-50.

NiH2Cit NiHCitNi2Cit(HCit)32 Ni2Cit242 CoH2Cit CoHCit2 Co2Cit242

Relative concentration (%) at pH 03.5 16.7 20.5 00.0 00.0 56.0 36.0 00.3

08.2 00.0 03.4 27.6 05.1 00.0 01.1 98.9


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the curve were distinguished, indicating preceding chemical reactions and a possible adsorption stage. The slope of the first ascending portion was found to increase with increasing concentration of the citric acid, pointing to the possible participation of citrate anions in the cathodic reaction. The second ascending portion can be associated with the participation of protons in the chemical reaction. On the basis of the experimental data we suggest the following mechanism of cathodic reduction of nickel(II) and cobalt(II) complexes from sulfamate electrolytes containing acetate and citrate anion additives. The electrodeposition of nickel-cobalt alloys followed by a local pH rise near the electrode surface results in the formation of strong citrate complexes, Ni2Cit(Hcit)32 and Co2Cit242, that are electrochemically inactive. These complexes are involved in the chemical reactions Ni2Cit(HCit)32 r Ni(HCit)2 1 NiCit22 Co2Cit242 Figure 9. Plot of Ni-Co alloy composition (3, 4) vs. CCo21-to-CNi21 ratio in sulfamate electrolyte at pH 3.0, T 5 558C, and current density 1, 50 and 2, 500 A/m2.

efficiency decreases with increase of current density. In the concentration region of 72-108 mM the cobalt current efficiency vs. current density goes through a maximum. Further increase of cobalt(II) concentration in the electrolyte up to about 600 mM is accompanied by the increase of its content in the alloy almost proportionally to the current density. This intricate character of the dependence is presumably related to the change in composition of metal complexes, which are formed in the bulk of the electrolyte and in the near-electrode layer and to diffusion limitations. In Ref. 33 the authors state that the inhibiting effect on the codeposition of the more noble metal is generally strongest when the reaction rate of the less noble metal is kinetically controlled, and it diminishes as the limiting current is reached. Partial polarization curves of Ni21 and Co21 codeposition are shown in Fig. 10a and b. As was expected, increase of cobalt(II) concentration in the electrolyte by a factor of 16 is accompanied by the positive (0.2 V) shift of the cobalt reduction potential. The opposite effect (20.5 V) was found for Ni(II) codeposition. Changes in temperature, pH and bath composition do not influence the Tafel slope, which is 108 6 5 mV and 112 6 5 mV for Co21 and Ni21 reduction, respectively. The cathode reactions were found to be first order with respect to Ni21 and Co21 concentrations (Fig. 10, insets). From the Tafel slope values it is expected that Ni-Co alloy deposition is a twostage process, where the rate-determining step is the acquisition of the first electron. Additional support for this assumption is the large apparent activation energy of alloy deposition (15.5 kcal/mol). In this case, the equation of Ni21 and Co21 reduction takes the form i 5 k CM21 (CH4Cit)21 exp(2azFE/RT)

1 H2O r

Co(HCit)2

1

CoCit(OH)32

[3] [4]

Reactions 3 and 4, associated with the ascending portion of the it1/2 vs. i curve (Fig. 12), are assumed to precede the codeposition process. The singly charged protonated citrate complexes may adsorb on the electrode surface either through a delocalized proton of citrate, or via substitution by bridging through adsorbed 19 NH2SO2 3 . Further increase in cathodic polarization would be followed by conversion of the chelate Ni(HCit)2 and Co(HCit)2 complexes to the surface bridge-structure complex with an internal delocalized proton [Mx-H-Cit-Ni(II)2]ss and [Mx-H-Cit-Co(II)2]ss. The second ascending portion of the it1/2 vs. i curve (Fig. 12) could be

[2]

where CM21 is the concentration of the dissolved metal ion, and CH4Cit is the concentration of citric acid. Curves of the potential drop following current interruption and switching chronopotentiograms34 in diluted electrolytes (5 mM of net metal concentration at pH 3.5) were investigated in order to clarify the mechanism of Ni-Co deposition (Fig. 11 and 12). The long vertical portion on the potential vs. time curve observed after current interruption can be attributed to a high degree of activation limitation of the reaction. In addition, a constant-potential segment occurs on the potential-drop curves. This can be explained by the formation of an adsorbed film on the electrode surface. The length of the constant-potential step increases with increase of polarization time (Fig. 11a), but does not depend on the cathode potential change from 20.5 to 20.75 V (Fig. 11b). At 20.8 V the length of this step decreases, but the step does not disappear even at 21.0 V. On the plots of it1/2 vs. i (where i is the current density and t is the duration of the pulse) obtained from switching chronopotentiograms (Fig. 12), two ascending and one descending portions of

Figure 10 (a) Partial polarization curves of Co21 reduction in electrolytes no. 1-4 at pH 3.0, and T 5 558C. (b) Partial polarization curves of Ni21 reduction in electrolytes no. 1-4 at pH 3.0 and T 5 558C.

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attributed to the formation of such surface complexes. In addition the possibility of the formation of a heteronuclear complex in the vicinity of the electrode or on its surface cannot be ruled out. We believe that such an intermediate surface complex would favor the electrodeposition of Ni-Co alloy in sulfamate electrolytes containing citrate anions. The reduction of Ni(II) and Co(II) complexes may be schematically presented [Mx-H-Cit-M(II)2]ss 1 e r M1 1 HCit32

[5]

M1 1 e r M0

[6]

As was shown above, a plethora of experimental results of Ni-Co alloy deposition do show its anomalous electrochemical behavior regardless of the anion composition of the electrolyte (Ref. 7, 2, 19, 21; This work, Fig. 9). This has provided further support to the experimental evidence that complexing agents, although improving stabilization of solution and completely bonding the less noble metal, are not sufficiently effective at reducing anomalous codeposition. Taken together, these observations point to the fact that the inhibiting of the discharge rate of the nobler component is likely to depend on the internal structural features of nickel(II) and cobalt(II) species involved in the reduction process. To explain the anomalous codeposition of Ni21 and Co21 let us consider the individual com-

Figure 11. Length of the horizontal segment of the potential drop curves vs. polarization potential in electrolyte no. 2 (5 mM of net metal concentration) at pH 3.5 and T 5 258C. (b) Length of the horizontal segment of the potential drop curves vs. polarization time in electrolyte no. 2 (5 mM of net metal concentration) at pH 3.5 and T 5 258C.

Figure 12. Plots of it1/2 vs. i at pH 3.5 and T 5 258C and 5 mM of net metal concentration; 1, 6.3 and 2, 12.5 mM citric acid.

plexes from the standpoint of crystal-field theory.35,36 Cobalt possesses the electronic structure 3d74s2. Nickel has the electronic structure 3d84s2. In the light of this theory, aquo- and protonated citrate Ni21 and Co21 complexes can be related to coordinative substances, whose structures show little sublevel splitting, namely, highspin complexes. Calculated crystal field stabilization energy for Ni21 in octahedral complexes is 29.3 and 17.1 kcal for Co21.36 High-spin octahedral cobalt(II) complexes with three unpaired electrons are more labile than that of Ni(II). Therefore they are expected to react rapidly, and they have been observed to do so. We believe that the formation of labile high-spin cobalt complexes would explain the preferential reduction of Co21 as compared with Ni21. Moreover, high-spin complexes involved in the reaction would permit a two-step reduction mechanism, which is in complete agreement with the experimental Tafel slopes. Notwithstanding the fact that the actual electrode reactions of electrochemical alloying of the iron-group metals may include additional intermediate steps, we believe that in a first approach, the crystal-field theory permits generalization for anomalous codeposition of nickel(II) and cobalt(II) from different electrolytes. Along with pH change (Dahms and Croll), competitive adsorption (Matlosz) and underpotential deposition the crystal-field theory can be considered as one possible explanation of the anomalous behavior of electrochemical alloying of the iron-group metals. Conclusions The effect of different complexing and buffering agents on the cathode reactions of nickel(II) and cobalt(II) codeposition was studied. The operating conditions under which the pHs rise is inhibited in sulfamate electrolyte were found. It was suggested that protonated Ni(HCit)2 and Co(HCit)2 citrate complexes are involved in alloy deposition. Essentially the role of acetate anions resides in the buffering of the sulfamate solutions. The acetate complexes do not participate directly in the cathode reaction. The experimental results of the present study show that nickel-cobalt electrochemical alloying leads to an increase in the reaction rate of cobalt at the expense of the nickel reaction rate. The data confirm that inhibition of the more noble metal by the less noble one does not depend on the anion composition of the electrolyte. We believe that the anomalous codeposition of Ni-Co alloy can be explained, along with pH change, competitive adsorption, and underpotential deposition, in terms of the crystal-field theory by the preferential reduction of high-spin cobalt (II) complexes. Acknowledgments The authors thank Dr. J. Penciner from the Tel Aviv University for editing of the paper.


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Tel Aviv University assisted in meeting the publication costs of this article.

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