Effect of Thiourea during Nickel Electrodeposition from Acidic Sulfate Solutions U.S. MOHANTY, B.C. TRIPATHY, S.C. DAS, and V.N. MISRA The effect of thiourea on the cathodic current efficiency (CE), deposit quality, crystallographic orientations, surface morphology, and polarization behavior of the cathode was investigated during nickel electrodeposition from acidic sulfate solutions for 2 hours at 60 °C. A slight decrease of 3 to 4 pct in the CE was observed, when the concentration of thiourea was increased from 2 to 40 mg dm3. The nickel deposit quality deteriorated significantly at higher thiourea concentrations; the surface morphology deteriorated and the contamination of the nickel deposits increased. The presence of thiourea affected the peak intensities of the crystal planes. Cyclic voltammetric studies on nickel deposition at 25 °C revealed depolarization behavior of the cathode at lower thiourea concentrations, 10 mg dm3; however, a mixed behavior is observed at higher thiourea concentrations. These changes were also observed in the exchange current density (i0) values.
I. INTRODUCTION
ORGANIC additives have been used in electroplating baths, to modify the properties of the deposits. In the nickeldeposition process, organic additives have been used as brighteners, levelers, wetting agents, and stress reducers. Scholter[1] used benzene and naphthalene trisulfonic acids as commercial brighteners during nickel electrodeposition from a Watts bath. Burkhart et al.[2] found that thiouronium salts act as a brightener in aqueous nickel-electroplating baths. Saccharin, p-toluene sulfonamide, sodium m-benzene disulfonate, sodium 1,3,5 napthalene trisulfonate, and O-sulfobenzaldehyde[3] have been used as stress reducers in Watts-type nickel baths. Nayak and Karunakaran[4] obtained bright and smooth electrodeposits of nickel from a Watts bath, using a combination of sodium naphthalene 2-sulfonate and an organic additive. Mohanty et al.[5] investigated the effect of pyridine, 2-picoline, and 4-picoline during nickel electrodeposition from acidic sulfate solutions. They observed that these particular additives do not have a significant effect on the current efficiency (CE); however, they affected the surface morphology and the crystallographic orientation of the nickel deposits. Also, 2- and 4-picoline produced smoother, more compact, and more leveled nickel deposits, as compared to pyridine. Thiourea[6–15] has been used commercially in copper electrorefining and electrowinning, in order to produce smooth and bright copper deposits. Rogers et al.[16] have found that thiourea acts as both leveler and brightener in Watts-type nickel baths. Hoekstra and Trivich[17] have also observed that thiourea, in the concentration range of 50 to 100 mg dm3, is an extremely good brightener and leveler in Wattstype nickel baths. Nevertheless, the role of thiourea in the nickel-deposition process, with respect to CE, deposit charU.S. MOHANTY, Postdoctoral Fellow, is with the Department of Material Science and Engineering, National Cheng Kung University, Tainan, 701, Taiwan, Republic of China. Contact e-mail:
[email protected] B.C. TRIPATHY, Scientist, and S.C. DAS, Head and Deputy Director, are with the Department of Hydrometallurgy, and V.N. MISRA, Director, is with Regional Research Laboratory (CSIR), Bhubaneswar-751013, India. Manuscript submitted November 16, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
acteristics, crystallographic orientations, and the polarization behavior of the cathode from the acidic sulfate solutions, has not been studied in detail. The present article thus investigates the effect of thiourea on the CE, deposit quality, surface morphology, and crystallographic orientations of deposits during nickel electrodeposition from acidic sulfate solutions. In addition, the electrochemical behavior of the electrodeposited solutions, with and without thiourea, was also studied and discussed. II. EXPERIMENTAL The rectangular flow-through cell used in this work was identical to that described in our previous article.[18] Ultrapure water (Millipore Milli Q system) was used for preparing the solutions. The nickel electrolyte was prepared from analytical-grade nickel sulfate (NiSO46H2O), sodium sulfate (Na2SO4), and boric acid (H3BO3). Dilute sulfuric acid (H2SO4) was used to adjust the electrolyte pH to 2.5. The concentration of Ni2 used in the bath was 60 g dm3; the concentrations of Na2SO4 and H3BO3 used were 12 g dm3 each. Calculated amounts of thiourea were added to the electrolytic bath in aliquots from freshly prepared stock solutions of 4 g dm3. For the electrodeposition and polarization studies, the surface of the electrode prior to nickel electrodeposition was polished first with 400- and then with 1200-grade silicon carbide paper, then rinsed with 1 m HCl followed by ultrapure water. All the electrodeposition experiments were conducted for 2 hours at a current density of 200 A m2, by applying current from a regulated power supplier (0 to 30 V, 5A, DC power supply). A precision voltmeter and an ammeter were placed in the cell circuit, to record the potentials and current. The flow rate of the electrolyte was maintained at 1.8 dm3 h1. A thermostat was used for maintaining the electrolyte temperature at 60 °C 1 °C. The pH of the electrolyte was maintained at 2.5, using dilute H2SO4. Stainless steel and lead-antimony (1 to 5 pct Sb) were used as the cathode and anode, respectively. All the potentials were measured against a saturated calomel electrode (SCE). After electrolysis, the cathode was removed from the cell, thoroughly VOLUME 36B, DECEMBER 2005—737
washed with water, and dried. The cathodic CE was calculated from the weight gained by the cathode, following electrolysis. A PHILIPS* PW 1030 X-ray diffractometer was *PHILIPS is a trademark of Philips Electronic Instruments Corp., Mahwah, NJ.
used to examine the nickel deposits, to determine their preferred crystal orientations relative to the ASTM standard nickel. The surface morphology of the deposit was examined by scanning electron microscopy (SEM), using a PHILIPS XL 20 SE microscope. Potentiodynamic polarization measurements were carried out, using a PAR model 273A potentiostat/galvanostat. All the experiments for studying the polarization behavior (both cyclic and linear-sweep voltammetry) were carried out in a glass cell with 100 cm3 of the electrolyte at 25 °C 1 °C. A stainless-steel disk 3 mm in diameter (Austenitic grade 316), and platinum wire, 0.5 mm in diameter, were used as the working and auxiliary electrodes, respectively. An SCE was used as the reference electrode through a Luggin capillary, and all the potentials were reported as such. The cathode potential was scanned in the potential range from 0 to 1100 mV, at a rate of 10 mV s1, using a PAR (model 273A) potentiostat/galvanostat. High-purity nitrogen was used throughout the polarization studies, both to sparge out dissolved oxygen and to maintain an inert atmosphere. From the cathodic polarization (CP) curves for nickel deposition, Tafel slopes were determined and transfer coefficients were calculated, using the following equations: h a b log I
[1]
b RT / an F
[2]
where h is the overpotential (V), b is the Tafel slope (V decade1), a is the transfer coefficient, and I is the current density (mA cm2). The exchange current densities (i0) for nickel deposition were found by extrapolating the Tafel lines to zero overpotential. III. RESULTS AND DISCUSSION A. The CE and Nickel Electrodeposition Potential The results of the study of the effect of thiourea, in the concentration range of 2 to 40 mg dm3, on the CE obtained during the nickel electrodeposition process, are given in Table I.
As noted from the table, a marginal decrease of 3 to 4 pct in CE was observed when the concentration of thiourea was increased from 2 to 40 mg dm3 in the electrolytic bath. The decrease in CE may be attributed to the adsorption of thiourea on the cathode surface, which restricts the electroreduction of Ni2 ions at the cathode. Brown et al.[19] have also studied the adsorption of thiourea on the nickel cathode, and found that it adsorbed on the cathode surface, due to the electronic polarization of the thiocarbonyl group. The adsorption of thiourea has also been reported by Ke et al.[20] and Turner and Johnson,[21] during the electrodeposition of metals like copper from acidic sulfate solutions. They proposed two mechanisms regarding the adsorption of thiourea on the cathode. The first mechanism is termed “structure sensitive” or, more precisely, “chemisorption,” which may require a suitable spacing between metal atoms on the surface of the electrodeposit to hold the organic molecules of a certain size and chemical structure. The other mechanism, which is termed “current density sensitive” or “shape sensitive,” involves preferential adsorption of the organic molecules at high-current density areas, thereby slowing down the growth rate in these areas and allowing rapid growth in low-current density areas (leveling). This preferential adsorption of the organic molecule is independent of the nature of the crystalline structure of the electrode, but depends on the surface contour of the electrode. In our present study, the adsorption of thiourea on the stainless-steel substrate may have proceeded by means of either of these two mechanisms. It can also be seen from Table I that the increased addition of thiourea to the nickel bath seems to have no significant effect on either the nickel electrodeposition potential or CP. A similar observation has been made by Popov et al.,[22] during the electrodeposition of nickel from a Watts bath in the presence of thiourea. B. Surface Quality The nickel deposits obtained from the additive-free bath were found to be smooth, bright, and uniform. The addition of thiourea, in the concentration range of 2 to 5 mg dm3, did not affect the deposit quality. However, an increase in the thiourea concentration to 10 mg dm3 induced roughness in the deposit; finally, at 40 mg dm3, a blackish rough deposit was obtained. The black color in the nickel deposit might be due to the formation of nickel sulfide (NiS) at the cathode surface. Several mechanisms describing the role of thiourea during the electrodeposition of metals like copper
Table I. Effect of Thiourea on CE, CP, Sulfur Contamination, NOP, and Crystallographic Orientations of the Nickel Electrodeposits Crystallographic Orientation of Nickel Deposits [Thiourea] (mg dm3) 0 2 5 10 20 40
CE (pct)
CP (V)
Sulfur Contamination (pct)
96.0 95.5 94.5 94.0 93.4 92.0
0.86 0.86 0.86 0.87 0.87 0.87
— 0.35 0.42 0.48 0.88 1.82
738—VOLUME 36B, DECEMBER 2005
Relative Peak Intensities (I/I0) (pct) NOP (mV)
(111)
(200)
(220)
(311)
178 166 158 140 130 90
62 22 17 20 100 100
100 100 100 100 50 25
— 5 10 1 10 11
— 8 10 2 12 9
METALLURGICAL AND MATERIALS TRANSACTIONS B
have been proposed by authors;[21,23,24] these support the formation of NiS at the cathode, including the following: (a) (b) (c) (d)
the oxidation of thiourea to form H2S and sulfur (S),[23] the hydrolysis of thiourea to form urea and H2S,[21] the reduction of thiourea to NH4CN and H2S,[21] and the decomposition of thiourea to elemental S.[24]
The formation of NiS at the cathode, which resulted in the black color, might have occurred as a result of any of these mechanisms. The sulfur content in the electrodeposits was confirmed by analyzing the deposits using ICP-AES. It was found that by increasing the thiourea concentration, the contamination of the electrodeposits increased; the thiourea concentration reached 1.82 pct, in the presence of 40 mg dm3 of thiourea in the electrolyte (Table I). This may be due to either the inclusion of sulfur in the form of precipitated black NiS or the decomposition products of thiourea, as suggested by the mechanisms mentioned earlier. This observation is consistent with those made by other authors.[25,26] C. Deposit Morphology and Crystallographic Orientations The morphology and the crystallographic orientations of the nickel deposits in the presence of thiourea were determined by SEM and X-ray diffraction techniques, respectively. Typical SEM micrographs are shown in Figure 1. The data on the various crystallographic orientations of the nickel deposits are noted in Table I. The nickel deposits obtained from the additive-free solution consist of sharp-
edged, round crystallites of varying sizes (2 to 15 m) (Figure 1(a)), which were randomly oriented and had compact morphology, as reported earlier.[27] This morphology corresponds to a crystal orientation in the order (200)(111). Nakamura et al.[28] also observed both a (200) diffraction peak of strong intensity and (111) and (220) peaks with relatively weak intensities. The addition of 2 mg dm3 of thiourea initiates growth in the direction of the (311) and (220) planes, and results in a preferred crystal orientation in the order (200) (111) (311) (220). This produces a less compact deposit that consists of elongated, sharp-faceted grains scattered uniformly over the surface (Figure 1(b)). Increasing the concentration of thiourea to 5 mg dm3 does not change the preferred crystal orientations, but there is a decrease in the peak intensities of the (220) and (311) planes. Increasing the concentration of thiourea produced a completely different morphology, which consisted of roundshaped crystallites of varied sizes with pores (Figure 1(c)). A further increase in the thiourea concentration, to 20 mg dm3, however, changed the preferred crystal orientations to (111) (200) (311) (220), which corresponds to a deposit morphology that consists of round-shaped crystallites 2 to 4 m in size (Figure 1(d)) that had a greater degree of compactness between them. But higher concentrations of thiourea, i.e., 30 mg dm3, did not affect the order of crystal orientations significantly; however, a reduction in the peak intensities of the (200) plane was observed, as shown in Table I. This produces a morphology that shows a cauliflower type of growth (Figure 1(e)). However, at 40 mg dm3, cauliflower-like crystals are seen in clusters (Figure 1(f)) and the surface looked more flat. A similar observation was made by Takei.[29] This change in the deposit morphology may be attributed to the varied degree of adsorption of thiourea on the cathode. D. Polarization Studies The CP of the Ni2 reduction on the stainless-steel substrate in the presence of thiourea was investigated by recording current-potential curves using the cyclic voltammetry technique. A typical I-V profile of nickel electrodeposition on the stainless-steel substrate in the presence of thiourea is shown in Figure 2. The peak B in this figure suggests that the electrodeposition of nickel is preceded by hydrogen evolution, which is consistent with other literature reports.[30–34] It can also be seen from the voltammogram that the peak current of hydrogen evolution reaction (HER) increases when the concentrations of thiourea in the bath are increased. A distinct hump P is observed at around 0.72 V, at 40 mg dm3 of thiourea in the electrolytic bath. This hump occurs around the potential where the reduction of the S0 to S2 state has been reported.[35] S 2e S2
Fig. 1—Typical SEM micrographs of the nickel electrodeposits obtained in the presence of thiourea during the electrodeposition of nickel for 2 h: (a) blank, (b) 2 mg dm3, (c) 5 mg dm3, (d) 20 mg dm3, (e) 30 mg dm3, and ( f ) 40 mg dm3. METALLURGICAL AND MATERIALS TRANSACTIONS B
E 0SCE 0.71V
[3]
This sulfur might have been formed due to the decomposition of thiourea, as per the mechanisms described by Monev et al.[23] and Sultanu and Bursuc.[24] The I-V data was used to determine the nucleation overpotential (NOP) through the method described here. The NOP is defined as the difference between the nucleation potential (ENu), denoted by point C, and the crossover potential (E), which represents the point of zero current during VOLUME 36B, DECEMBER 2005—739
Fig. 2—Typical voltammograms showing the effect of varying concentrations of thiourea during electrodeposition of nickel from a bath composed of Ni2 60 g dm3, Na2SO4 12 g dm3, H3BO3 12 g dm3 with a scan rate of 10 mV/s: (a) blank, (b) 10 mg dm3, and (c) 40 mg dm3.
the reverse scan, as shown in the cyclic voltammogram in Figure 2. For each of the systems, the data are noted in Table I. The NOP value obtained for nickel electrodeposition in the absence of thiourea is 178 mV. The addition of thiourea in the concentration range of 2 to 40 mg dm3 shifts the NOP value toward more positive potentials, i.e., the NOP values progressively decrease as the thiourea concentration increases in the electrolyte. A similar decrease in NOP in the presence of thiourea has also been reported for other metals.[36,37] From the linear sweep voltammograms in Figure 3, it can be observed that, at a thiourea concentration of 10 mg dm3, depolarization of the cathode occurs. However, at elevated thiourea concentrations ( 20 mg dm3), mixed behavior is observed: depolarization occurs up to a potential of 980 mV (vs SCE), and a slight increase in CP is observed above 980 mV (vs SCE). The depolarization of the cathode at lower concentrations of thiourea could be attributed to both the adsorption of thiourea and its electroreduced products and their reaction with the Ni2 ion. Han et al.[26] has reported that partial Ni2 ions complexes with the electroreduced product of thiourea in the form of Ni [CS(NH2)2]2 and the formation of this complex favored the nickel deposition process, thereby causing the depolarization of the cathode. The CP data for nickel electrodeposition on stainless-steel and nickel substrates were used to calculate the electron transfer kinetic parameters, i.e., the Tafel slope (b), transfer coefficient (a), and exchange current density (i0), as mentioned in Section II. These data are included in Table II. The addition of thiourea up to 10 mg dm3 affects the Tafel slope only slightly, about 120 mV decade1. However, at a high concentration of thiourea (i.e., 40 mg dm3), the Tafel slope observed on both stainless-steel and nickel substrates is found to be in the range of 140 to 166 mV, which suggests that the chargetransfer mechanism of the deposition process is quite complex. This is similar to observations made by other authors.[38,39] The transfer coefficient (a) remains essentially unaffected by the presence of thiourea at low concentrations (2 to 10 mg dm3), but it decreases significantly at higher concentrations (20 to 40 mg dm3) which is indicative of a more complex electrode process. The addition of thiourea to the nickel bath, 740—VOLUME 36B, DECEMBER 2005
Fig. 3—Linear sweep voltammograms showing the effect of varying concentrations of thiourea during the electrodeposition of nickel on a stainless-steel substrate from a bath composed of Ni2 60 g dm3, Na2SO4 12 g dm3, and H3BO3 12 g dm3, with a scan rate of 10 mV/s: (1) blank, (2) 5 mg dm3, (3) 10 mg dm3, and (4) 20 mg dm3.
however, increases the i0 value at low concentrations (2 to 20 mg dm3) of thiourea, but a progressive decrease in i0 value is observed with an increase in the thiourea concentration from 20 to 40 mg dm3 in the bath. The increase in the value of i0 may be attributed to both the adsorption of thiourea and the reaction of its electroreduced products with Ni2 ions, which thereby depolarizes the cathode. A decrease in i0 at high thiourea concentrations may be due to the excess adsorption of thiourea molecules, which block the active sites of the cathode surface, thus reducing the rate of electron transfer for the electroreduction of Ni2 ions.
IV. CONCLUSIONS The following conclusions could be drawn from the study. 1. An increase in the concentration of thiourea to 40 mg dm3 causes a decrease of 3 to 4 pct in the CE. 2. A significant deterioration of the nickel deposit occurs when the concentration of thiourea in the electrolytic bath is increased; at 40 mg dm3, a blackish rough deposit is obtained. The morphology of the nickel deposits is also affected strongly at elevated thiourea concentrations. 3. An increase in the thiourea concentration in the nickel electrolytic bath progressively increases the sulfur content in the nickel deposit. METALLURGICAL AND MATERIALS TRANSACTIONS B
Table II. Effect of Thiourea on the Kinetic Parameters Substrate Stainless steel
Nickel
[Thiourea] (mg dm3)
Tafel Slope (b) (mV decade1)
Transfer Coefficient ( )
Exchange Current Density (i0) (mA cm2)
0 2 5 10 20 30 40 0 2 5 10 20 30 40
101 119 120 122 139 140 142 126 119 122 120 148 156 166
0.59 0.49 0.50 0.48 0.42 0.42 0.41 0.47 0.49 0.48 0.50 0.40 0.38 0.35
6.3 104 6.5 104 6.7 104 7.0 104 6.0 104 5.8 104 5.4 104 6.8 103 6.9 103 7.0 103 7.2 103 6.4 103 6.0 103 5.6 103
4. The crystal orientations are strongly affected by the presence of thiourea, but the observed variations do not follow a particular pattern. 5. Thiourea depolarizes the nickel-ion-reduction process on the stainless-steel substrate, up to concentrations of 10 mg dm3; however, a mixed behavior is observed at higher thiourea concentrations.
ACKNOWLEDGMENTS One of the authors (USM) thanks CSIR for granting him a research fellowship. USM, BCT, and SCD thank the director of the Regional Research Laboratory (CSIR) for his permission to publish this article. Finally, the authors also thank everyone associated directly or indirectly with this work.
REFERENCES 1. M. Schotter: U.S. Patent, 1934, 1972, p. 693. 2. B. Burkhart, A. Ofing, V. Schundermann, U. Scroeder, and K. Glaser: Ger Offen., 1994 (patent written in German). 3. Frederick A. Lowhenheim: Modern Electroplating, John Wiley & Sons, New York, NY, 1974, pp. 39 and 325. 4. B. Nayak and K. Karunakaran: J. Appl. Electrochem., 1982, vol. 12, pp. 323-28. 5. U.S. Mohanty, B.C. Tripathy, P. Singh, and S.C. Das: J. Appl. Electrochem., 2001, vol. 31, pp. 579-83. 6. Z.D. Stankovic: Erzmetallurgy, 1985, vol. 38, p. 361. 7. P. Cofre and A. Bustos: J. Appl. Electrochem., 1994, vol. 24, pp. 564-68. 8. Z.D. Stankovic and M. Vukovic: Electrochim. Acta, 1996, vol. 41, pp. 2529-35. 9. D.R. Turner and G.R. Johnson: J. Electrochem. Soc., 1962, vol. 109, p. 918. 10. D.F. Suarez and F.A. Olson: J. Appl. Electrochem., 1992, vol. 22, pp. 1002-10. 11. M. Alodan and W. Smyrl: Electrochim. Acta, 1998, vol. 44, pp. 299-309. 12. S. Mendez, G. Andreasen, P.S. Chilardi, M. Figueroa, L. Vazquez, R.C. Salvarezza, and A.J. Arvia: Langmuir, 1998, vol. 14, p. 2515.
METALLURGICAL AND MATERIALS TRANSACTIONS B
13. G. Fabricius, K. Kontturi, and G. Sundholm: Electrochim. Acta, 1994, vol. 39, pp. 2553-57. 14. L. Muresan, S. Varvara, G. Maurin, and S. Dorneanu: Hydrometallurgy, 2000, vol. 54, pp. 161-69. 15. M. Alodan and W. Smyrl: J. Electrochem. Soc., 1998, vol. 145, pp. 957-63. 16. G.T. Rogers, M.J. Ware, and R.V. Fellows: J. Electrochem. Soc., 1960, vol. 107, pp. 677-81. 17. J.J. Hoekstra and D. Trivich: J. Electrochem. Soc., 1961, vol. 62, p. 111. 18. B.C. Tripathy, P. Singh, and D.M. Muir: Metall. Mater. Trans. B, 2001, vol. 32B, pp. 395-99. 19. H. Brown: Trans. IMF, 1969, vol. 47, p. 63. 20. B. Ke, J.J. Koekstra, B.C. Sissons, Jr., and D. Trivich: J. Electrochem. Soc., 1959, vol. 106, pp. 382-85. 21. D.R. Turner and G.R. Johnson: J. Electrochem. Soc., 1962, vol. 109, pp. 798-803. 22. B. Popov: Kem. Ind., 1982, vol. 31 (9), p. 455. 23. M. Monev, E.M. Baumgartner, and J.C. Raub: Metalloberflaeche, 1997, vol. 51 (5), p. 328. 24. N. Sultanu and B. Bursuc: Al. L Cuza, Lasi Sect, 1984, vol. 1b, p. 3019. 25. A.V. Petrauskas and A. Bodnevas: Issled. Obl. Elektro. Osozdeniya Metal, 1973, p. 189. 26. Q. Han, K. Liu, J. Chen, and X. Wei: Int. J. Hydrogen Energy, 2003, vol. 28, pp. 1207-12. 27. U.S. Mohanty, B.C. Tripathy, P. Singh, and S.C. Das: J. Electroanalytical Chem., 2002, vol. 526, pp. 63-68. 28. Y. Nakamura, N. Kaneko, M. Watanabe, and H. Nezu: J. Appl. Electrochem., 1994, vol. 24, pp. 227-32. 29. T. Takei: Nippon Kagaku Kaishi, 1973, vol. 10, p. 1863. 30. J.P. Hoare: J. Electrochem. Soc., 1986, vol. 133, pp. 2491-94. 31. R.K. Dorsch: J. Electroanalytical Chem., 1969, vol. 21, pp. 495-508. 32. M. Fleischmann and A.S. Reintjes: Electrochim. Acta, 1984, vol. 29 (1), pp. 69-75. 33. J. Horkans: J. Electrochem. Soc., 1979, vol. 126, p. 1861. 34. M. Pushpavanam and K. Balakrishnan: J. Appl. Electrochem., 1996, vol. 26, pp. 283-90. 35. E. Deltombe, N. Dezoubov, and M. Pourbaix: Atlas of Electrochemical Equilibria in Aqueous Solution, 1st ed., Pergamon Press, London, 1966. 36. E.E. Farndon, F.C. Walsh, and S.A. Campbell: J. Appl. Electrochem., 1995, vol. 25, pp. 574-83. 37. D.J. Mackinnon, J.M. Brannen, and R.M. Morrison: J. Appl. Electrochem., 1998, vol. 18, pp. 252-56. 38. R.C.V. Piatti, A.J. Arvia, and J.J. Podesta: Electrochim. Acta, 1969, vol. 14, pp. 541-60. 39. T. Hurlen: Electrochim. Acta, 1975, vol. 20, pp. 499-503.
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