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The influence of electrolysis parameters on the composition and morphology of Co–Ni alloys
Hydrometallurgy 54 Ž2000. 133–149 www.elsevier.nlrlocaterhydromet
The influence of electrolysis parameters on the composition and morphology of Co–Ni alloys ) L. Burzynska , E. Rudnik ´ Department of Physical Chemistry and Electrochemistry, Faculty of Non-Ferrous Metals, UniÕersity of Mining and Metallurgy, Cracow, Poland Received 20 September 1999; accepted 20 September 1999
Abstract The influence of cathodic current density, the concentration of Co 2q ions in the electrolyte, and additional substances Žsaccharin and sodium lauryl sulfate. on the composition and morphology of Co–Ni alloys were investigated. Research was carried out in a Watts-type bath in the presence of boric acid, as a buffer substance. The circulation speed of the electrolyte was 40 dm3 hy1. Cathodic polarisation curves were determined for parent metals and the Co–Ni alloy. It was established that the presence of additives shifts the cathodic potential of alloy deposition towards more negative values. An increase in the cobalt content in the alloy was observed with decreasing of the cathodic current density and increasing of the Co 2q ions concentration in the bath. Results obtained confirmed the anomalous character of deposition of the Co–Ni alloy. The cathodic current efficiency is dependent mainly on the current density applied, but the direction of changes has not been precisely determined, for it depends on the composition of the electrolyte. Using diffractional X-ray and micro X-ray analyses, it was determined that single-phase deposits with an fcc lattice for the whole investigated range of current density and electrolyte composition were obtained. In the presence of additives there were obtained fine-grained, bright alloys which adhered well to the substrate. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Electrolysis parameters; Co–Ni alloys; Electrolyte
1. Introduction Electrolytic nickel–cobalt alloys are characterised by high strength Ževen at elevated temperatures. w1–3x, hardness w2–6x, and specific magnetic properties w7,8x. These )
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features have determined that this material has many applications, among others, in rocket technology, cosmonautics, sound signal recording, as a material for the electroforming of moulds for die-casting and plastics, as an anticorrosive coating, and also for decorative purposes w1,4,6,9,10x. The cathodic co-deposition of nickel and cobalt is carried out using chloride w11–13x, sulfate w11,14–16x, chloride–sulfate w17–19x, and sulfamate w3,5,20x electrolytes, either with or without the addition of complexing compounds, e.g., pyrophosphate w21x or citrates w12x. Applications of simple and complexing bath in the deposition of the Co–Ni alloys have been extensively described in the literature. However, the process in the chloride–sulfate solutions ŽWatts-type. has been insufficiently studied. The adaptation of the Watts bath w17–19x in the co-deposition of metals results from the comprehensive research of nickel electrodeposition in this type of the electrolyte, and because of the similar properties of cobalt and nickel. The composition of the alloy, and therefore some of its properties Žbrightness, plasticity, etc.. may be regulated by appropriately choosing the bath composition and parameters of electrolysis. The present paper shows the results of laboratory research centred on the deposition of Ni–Co alloys using chloride–sulfate solutions. The objective was to determine the way in which the composition of alloys, cathodic current efficiency and macroscopic properties of deposits are dependent upon the concentration of cobalt and nickel ions, current density and the presence of additional substances.
2. Experimental 2.1. Composition of electrolyte Using data given in literature w17x, we selected the composition of the bath in which the co-deposition of nickel and cobalt was to take place. The composition of electrolytes and process parameters are presented in Table 1, points 1–3. Chloride–sulfate solutions were used. It appears w22x useful to add chloride ions, for these prevent the passivation of anodes and facilitate their dissolution. Measurements were carried out in the presence of boric acid, which fulfilled the role of a buffer substance. The alloy deposition was also conducted in electrolytes with an addition of saccharin and sodium lauryl sulfate. Saccharin lowered the internal stresses of cathodic deposits. Sodium lauryl sulfate, as a surfactant, prevents the incorporation of gaseous hydrogen into the cathodic deposit. The deposition of the alloy was carried out using a bath in which the concentration of nickel ions was one order of magnitude higher than that of cobalt ions. This is connected with the preferential cathodic deposition of cobalt. The cathodic polarisation curves for the deposition of Ni–Co alloy were determined in solutions with compositions given in Table 1, points 1 and 3–5. The polarisation curves for the cathodic individual reduction of Co 2q and Ni 2q ions were also determined. The composition of the baths is presented in Table 1, points 6 and 7. These measurements were carried out in electrolytes with a concentration of cobalt and nickel ions identical with that existing during the recording of the alloy’s polarisation curve. In
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Table 1 Composition of the baths, conditions of Co–Ni alloys deposition and the determination of cathodic polarisation curves for the alloy, nickel, and cobalt No.
Composition of electrolyte
Concentration y3
Ž1.
Ž2.
Ž3.
Ž4.
Ž5.
Ž6.
Ž7.
Process conditions y3
mol dm
g dm
CoSO4P7H 2 O NiCl 2 P6H 2 O H 3 BO 3 Saccharin Sodium lauryl sulfate CoSO4P7H 2 O NiCl 2 P6H 2 O H 3 BO 3 Saccharin Sodium lauryl sulfate CoSO4P7H 2 O NiCl 2 P6H 2 O H 3 BO 3
0.089 0.841 0.405 0.004 0.0003 0.054–0.089 0.841 0.405 0.004 0.0003 0.089 0.841 0.405
25 200 25 1 0.08 15–25 200 25 1 0.08 25 200 25
CoSO4P7H 2 O NiCl 2 P6H 2 O H 3 BO 3 Saccharin CoSO4P7H 2 O NiCl 2 P6H 2 O H 3 BO 3 Sodium lauryl sulfate CoSO4P7H 2 O MgCl 2 P6H 2 O H 3 BO 3
0.089 0.841 0.405 0.004 0.089 0.841 0.405 0.0003 0.089 0.841 0.405
25 200 25 1 25 200 25 0.08 25 171 25
MgSO4P7H 2 O NiCl 2 P6H 2 O H 3 BO 3
0.089 0.841 0.405
22 200 25
Cathodic current density: 1.0–2.0 A dmy2 Circulation of electrolyte: 40 dm3 hy1 Volume of electrolyte: 3 dm3 Cathode substrate: titanium anodes: Ni and Co cathodic current density: 1 A dmy2 circulation of electrolyte: 40 dm3 hy1 volume of electrolyte: 3 dm3 cathode substrate: titanium anodes: Ni and Co cathodic current density: 0.4–2.3 A dmy2 circulation of electrolyte: 40 dm3 hy1 volume of electrolyte: 3 dm3 cathode substrate: steel anodes: Ni and Co circulation of electrolyte: 40 dm3 hy1 volume of electrolyte: 3 dm3 cathode substrate: titanium anodes: Ni and Co circulation of electrolyte: 40 dm3 hy1 volume of electrolyte: 3 dm3 cathode substrate: titanium anodes: Ni and Co circulation of electrolyte: 7 dm3 hy1 volume of electrolyte: 0.5 dm3 cathode substrate: steel anode: Co circulation of electrolyte: 7 dm3 hy1 volume of electrolyte: 0.5 dm3 cathode substrate: steel anode: Ni
order to obtain the same value of the ionic strength of solutions, one of these cations was replaced with Mg 2q ions. Electrolytes with a pH of 4.4 were applied. This value was achieved through the addition of sodium hydroxide. The solutions were prepared using analytical grade reagents Žsupplied by POCh, Poland. and twice-distilled water. The concentration of Co 2q and Ni 2q ions was determined using the method suggested by Langford w23x. This consists in a total determination of cobalt and nickel by means of titration with an EDTA solution in the presence of murexide as the indicator. In a second portion of the solution Co 2q ions are oxidised to Co 3q ions with the simultaneous creation of an ammonia complex. Ni 2q ions do not undergo such a reaction and may be determined using EDTA.
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2.2. Electrodes Rectangular 5 = 6 cm plates made from stainless steel or titanium Ž99.9%. were used as substrates for the cathode. Prior to each measurement, these were cleaned using abrasive paper of various gradations Žin the following order: 280, 400, 600, 800. and subsequently washed in distilled water and alcohol. The substrates were isolated on one side for determining the polarisation curves. Separate rectangular 5 = 6 cm nickel and cobalt anodes were used. The nickel anodes Ž99.9%. were rolled plates, while the cobalt anodes Ž99.0%. were cast plates. In order to remove the oxidised layer from the surface of the electrodes, the anodes were immersed in a mixture of concentrated acids: HNO 3 , H 2 SO4 , H 3 PO4 , and CH 3 COOH Ž3:1:1:5. at a temperature of 858C–908C for approximately 1 min w24x. The anodes were isolated on one side. The co-deposition of cobalt and nickel was carried out using two nickel anodes and two cobalt ones. One anode made of the appropriate element was applied for defining the cathodic polarisation curves of individual metals. 2.3. Measurement circuit The alloy deposition was conducted in a cuboid vessel made of rigid PVC. The volume of electrolyte was 3 dm3. The cathodic polarisation curves for parent metals were determined in a glass vessel containing 0.5 dm3 of the solution. Within the electrolyser, the electrodes were hung vertically and in parallel with respect to each other. The anodes were placed symmetrically on both sides of the centrally located cathode. The cathode potential was measured with respect to a saturated calomel electrode and the result thus obtained converted with respect to the normal hydrogen electrode. Measurements of potential were carried out using a Luggin capillary, placed at the surface of the electrode in such a way as not to disturb the distribution of the force lines of the electric field. The pH of the bath was monitored with a combined electrode connected with a pH-meter ŽRadelkis.. In order to eliminate the influence of the electric field on the readings of the apparatus, the electrode was placed in a glass housing that did not, however, prevent the free flow of electrolyte. The constant pH value of the bath, which displayed a tendency to alkalify as a result of the cathodic evolution of hydrogen, was maintained by the periodic addition of a few drops of a mixture of diluted HCl and H 2 SO4 Ž10:1.. The flow of electrolyte was kept at a constant and stable level using a peristaltic pump with a circulation speed corresponding to an approximately 13-times exchange of the volume of the solution within 1 h. The electrolyte was pumped from the direction of the anodes in the direction of the cathode. In order to precisely determine the charge flowing through the circuit, independent copper coulometers were used, these being series-connected with the electrodes. The measurement circuit is given in Fig. 1.
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Fig. 1. Diagram of measurement circuit applied for depositing the Co–Ni alloy. 1 — Cobalt anode; 2 — nickel anode; 3 — cathode; 4 — stabilized power supply; 5 — ammeter; 6 — copper coulometer; 7 — resistor; 8 — combined electrode; 9 — pH-meter.
2.4. Determination of polarisation curÕes The polarisation curves for the deposition of nickel, cobalt and their alloy were determined using the galvanostatic method, at a temperature of 21 " 18C. The cathodic polarisation curves were registered after a metallic layer had appeared on the substrate. These layers were obtained carrying on the cathodic process for 1 h at a current density of 0.4 A dmy2 . After that the flow of the electric current was cut off and the potential value read for i K s 0. Next, curves were determined at increasing and decreasing current densities until reproducible values were found. Polarisation curves were registered under a fixed set conditions Žthe electrolyte composition, speed of circulation, temperature.. Measurements were carried out in stationary conditions, i.e., the established potential value was read at a set current density. Partial cathodic polarisation curves for nickel and cobalt were determined on the basis of the composition of alloys. 2.5. Cathodic alloy deposition The deposition of Co–Ni alloys was conducted over a period of 10 to 20 h under galvanostatic conditions, at a temperature of 21 " 18C. The influence of current density on the composition of the cathodic deposit was examined in electrolytes having compositions presented in Table 1, points 1 and 3, while the dependence of the composition of the alloy on the concentration of metal ions was determined in solutions having concentrations given in Table 1, point 2. The cathodic current efficiencies were calculated on the basis of the mass of deposit and coulometric data. Various current densities were used for nickel and cobalt anodes
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for each measurement. Nickel anodes were dissolved by passing 75% of total electric current through them; the remainder dissolved the cobalt anodes. Phase identification of deposits was performed using an X-ray diffractometer ŽDRON 3. and filtered radiation CuK a. Deposited alloys underwent X-ray microanalysis, while cobalt and nickel were analysed quantitatively at points on the surface 900 nm distant from each other. This enabled graphs of changes in concentration for both elements along selected lines to be plotted. An analysis of the surface distribution of alloy constituents was also carried out. Due to the possibility of CoKb and NiK a spectral lines overlapping, CoK a and NiKb radiation was used for measurements. Measurements were carried out with a Philips XL 30 electron scanning microscope with an EDS analyser ISIS model. In order to investigate the microstructure of deposits, a scanning analysis and measurements using the Atomic Force Microscopy method were executed, the latter applying a Nanoscope E apparatus. In the course of electrolysis, changes in the composition of the electrolyte were examined. Samples of the solution were taken at hourly intervals. Cathodic deposits were dissolved in HNO 3 Ž1:1., and their composition subsequently determined applying the Atomic Absorption Spectrometry method ŽPerkin Elmer Atomic Absorption Spectrometer 3110.. Alloys were estimated visually and layers obtained were documented photographically. The brightness of coatings was measured using a Corning-EEL glossmeter. A mirror was adopted as a model surface with 100% brightness. 3. Results and discussion 3.1. Polarisation curÕes Fig. 2 shows the cathodic polarisation curves for the individual deposition of cobalt and nickel, their sum and partial polarisation curves for both metals Žcalculated on the
Fig. 2. Cathodic polarisation curves of cobalt, nickel, their sum and partial curves of cobalt and nickel.
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basis of the composition of alloys.. It was observed that the deposition of the parent metals requires considerable overvoltage. This means that at very low current densities Žto 0.5 A dmy2 ., the flat course of the polarisation curves is observed. However, this phenomenon is characteristic for elements of the iron group w25x. The occurrence of polarisation is connected with the inhibition of one of the intermediate stages of the electrode process. The dependence of the cathode potential on current density in the case of activation polarisation is given by Tafel’s equation: Ei y E0 s h K s a q b log i K
Ž 1.
where Ei is potential of the polarised electrode, E0 equilibrium potential of the electrode, h K cathodic overvoltage, i K cathodic current density, a and b are constants. Fig. 3 is a plot illustrating the dependence of the pure metal deposition potential upon the logarithm of the cathodic current density. It is worth noting that the potential of both electrodes determined at i K s 0 changed with time from the moment the flow of the electric current was cut off. For nickel, the stabilized value does not correspond to the potential calculated using Nernst’s equation. According to data given in literature w26,27x, difficulties with attaining a state of equilibrium are connected with the passive state of the surfaces of nickel and cobalt electrodes. This hypothesis appears credible, since the electrolyte contains dissolved oxygen, which may react with both nickel and cobalt, thereby creating oxidised layers. One cannot, however, exclude the steadying of
Fig. 3. Dependence of the nickel and cobalt cathode potential on the logarithm of the cathodic current density.
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mixed potentials as a result of the course of the corrosive process under hydrogen depolarisation. Nevertheless, the rate of this process is negligible, this due to the low concentration of hydrogen ions in the electrolyte ŽpH s 4.4.. The cobalt electrode potential determined empirically at i K s 0 equalled the equilibrium potential. It appears that this fact is connected with the composition of the electrolyte within which the polarisation curve was registered. This contained 1.7 mol dmy3 of chloride ions. The effectiveness of action of chloride ions depends not only on their concentration in the solution, but also on the ratio of metal ions concentrations to those of chloride ions. From the data presented in literature w28x, it is known that these anions adsorb on the surface of the electrode and thereby block the secondary reactions. Data given in Fig. 3 indicate that at the current densities from 0.33 to 0.98 A dmy2 for cobalt and from 0.14 to 1.44 A dmy2 for nickel, the potential of the cathode is a linear function of the logarithm of the cathodic current density. It seems that the process ran under activation control during the investigated conditions. It was observed that an increase in the current density was accompanied by a deviation from this linearity, which in fact may be indicative of the growing participation of concentration polarisation w29x. During the co-deposition of the metals, there was observed a certain decreasing of the rate of the cathodic reduction of Ni 2q ions in relation to that of pure element ŽFig. 2.. This caused the shifting of the partial curve for nickel towards more negative potentials. At the same time, it was observed that at the current densities higher than 0.5 A dmy2 , the cathodic reduction of Co 2q ions rate is increased. This means that the deposition of alloy alters the course of polarisation curves for both constituents. The sum of polarisation curves determined empirically for the parent metals makes it possible to draw the predicted polarisation curve for the alloy ŽFig. 2.. Their course indicates that at the current density higher than 0.7 A dmy2 , the nickel fraction is significantly higher than the cobalt one. This fact determines the rate with which the content of metal ions in the bath is to be replenished so as to maintain their concentration at a constant level and thereby obtain a cathodic deposit with a uniform composition throughout its volume, independently of the duration of electrolysis. As a result, the dissolution of nickel anodes was carried out at current densities that were three times greater. The choice of another anodic current density would have led to a rapid change in the composition of the electrolyte. Fig. 4 shows cathodic polarisation curves of the Co–Ni alloys registered in the baths with different compositions. The presence of saccharin and sodium lauryl sulfate in the solution causes a considerable shift of the alloy polarisation curve towards more negative potentials in comparison with the curve determined in the electrolyte without additives. The highest inhibition of the cobalt and nickel co-deposition is observed in the case of saccharin. It appears that it is connected with strong adsorption of its molecules on the cathode surface. However, the influence of sodium lauryl sulfate on the process rate is significantly weaker. The addition of a small amount of the surfactant to the bath containing saccharin lowers the disadvantageous influence of the latter — the polarisation curve lies between curves determined in the solutions with one of the additives. The determination of the experimental alloy polarisation curve and of the partial curves for its constituents enables one to precisely define the cathodic current density Žat
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Fig. 4. Cathodic polarisation curves of Co–Ni alloy Žbath composition — Table 1.: Ža. point 3, Žb. point 5, Žc. point 1, Žd. point 4.
fixed bath composition, temperature, circulation. at which it is possible to obtain a material having a desired composition. 3.2. Composition of Co–Ni alloys The influence of current density and the composition of electrolyte on the composition of deposit were studied. 3.2.1. Cathodic current density The dependence of the alloy composition on the cathodic current density is presented in Fig. 5. The plot shows that an increase in this parameter is accompanied by a reduction in the cobalt content in the cathodic deposit. Simultaneously, at the set current density, the content of this metal in the alloy is dependent upon the composition of the electrolyte. The same metal ions concentration was maintained, and therefore the change in the composition of alloys is connected with the presence of additional components. The addition of saccharin and sodium lauryl sulfate brought about a reduction of the cobalt content in relation to the composition of coatings obtained in baths without additives. It follows therefore that these substances facilitate the deposition of pure nickel and hinder that of cobalt. It is known, however, that during the cathodic deposition of pure nickel in the presence of saccharin, the cathodic overvoltage increases w30x. The presence of sodium lauryl sulfate in the bath not only compensates for the unfavourable influence of saccharin, but in addition reduces the alloy deposition overvoltage. What is more, the considerable current efficiency of the cathodic process in the presence of additives in the electrolyte appears to indicate that the hydrogen evolution overvoltage increases, too.
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Fig. 5. Influence of cathodic current density on the composition of Co–Ni alloys Žbath composition — Table 1: Ža. point 3, Žb. point 1..
The observed direction of changes in the composition of the alloy under the influence of alterations of current density is in good agreement with results obtained by other researchers w5,15,17x using solutions of simple salts. 3.2.2. Concentration of Co 2 q in electrolyte The dependence of the composition of Co–Ni alloys on the concentration of Co 2q ions in the electrolyte at a fixed concentration of Ni 2q ions is presented in Fig. 6. The course of the curve indicates that an increase in the content of Co 2q ions in the solution is accompanied by an increase in the fraction of this metal in the deposit. A characteris-
Fig. 6. Influence of Co 2q ions content in electrolyte on the composition of the Co–Ni alloy Žbath composition: Table 1, point 2..
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tic feature is that the CorŽCo q Ni. ratio in the alloy is considerably higher than in the bath, for example, 7% wt. of cobalt in the electrolyte corresponds to a 30% wt. content of this metal in the cathodic deposit. This fact is confirmed by the anomalous character of Co–Ni alloy deposition, where the less noble constituent is deposited preferentially w31x. Data given in literature indicate that this phenomenon occurs only in baths that contain simple cobalt and nickel salts w12x, whereas in electrolytes with an addition of complexing substances there is observed the deposition of alloys with CorŽCo q Ni. ratio equal or substantially lower than in the solution. 3.3. Cathodic current efficiency Fig. 7 shows the dependence of cathodic current efficiency on the current density. This is clearly connected with the composition of the electrolyte. In baths containing simple nickel or cobalt salts without surfactants, complexing or other similar additives, it was observed that a change in current density from 0.4 to 2.3 A dmy2 was accompanied by an increase in current efficiency from 94.5% to 99.0%. In solutions with saccharin and sodium lauryl sulfate, there were observed no changes in the efficiency of the cathodic process in connection with an increase in current density; this equalled 99.7 " 0.1%. From the data given in literature w18x, it is known that in baths having similar compositions within the range of high current densities from 16 to 64 A dmy2 the cathodic current efficiency equals 80%–90%, whereas sulfamate solutions w3x with an addition of sodium lauryl sulfate yielded current efficiencies of 97%–100% within the range of the current densities from 2 to 5 A dmy2 Žthe alloys contained 30%–75% of nickel.. Research carried out indicates that an increase in the concentration of Co 2q ions in the electrolyte causes an increase in the cathodic current efficiency of approximately 1%. Similar results were obtained by Abd El-Rehim et al. w17x in chloride–sulfate baths which did not contain any additional substances.
Fig. 7. Dependence of cathodic current efficiency upon current density Žbath composition — Table 1: Ža. point 3, Žb. point 1..
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3.4. X-ray diffraction analysis The phase composition of alloys was studied applying X-ray diffraction analysis. It follows therefrom that peaks solely related to a solid solution with the face-centred cubic structure are present. This is in agreement with the phase diagram for the Co–Ni system within the researched range of alloy compositions w32x. There are no peaks related to pure metallic phases. This would suggest that single-phase cathodic deposits have been obtained. There exists no possibility of precisely determining the parameter of the lattice — and therefrom the composition of the alloy — on the basis of diffraction patterns obtained. It is a well-known fact that solid Co–Ni solutions of decidedly different compositions have very similar lattice parameters w33x. 3.5. X-ray microanalysis Fig. 8 shows the results of X-ray microanalysis along lines for two samples obtained in electrolyte having the same composition, but at different current densities. At the current density of 2 A dmy2 , the concentration of both nickel and cobalt in the sample had a constant value, while in the case of the sample obtained at a current density of 1 A dmy2 , a change in the content of these elements in an area near the edge of the cathode was observed. This may have been caused by local changes in current density. The distribution of both metals on an 8 P 10 4 = 10 P 10 4 nm surface area in the form of a ‘‘map’’ was determined. An even distribution of nickel and cobalt over the whole surface was observed. Sections with a higher concentration of one element were not observed.
Fig. 8. Distribution of nickel and cobalt along a line on the surface of alloys. The inset Žv . shows the place of cutting out the samples from the cathode Žbath composition — Table 1, point 1.: Ža. i K s1 A dmy2 , Žb. i K s 2 A dmy2 .
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3.6. Scanning analysis The alloys underwent scanning electron microscope analysis. Fig. 9 shows the example microphotographs. These would suggest that the microstructure of deposits is dependent mainly on the composition of the bath. The current density Žwithin the researched range. is a less significant factor. Spherical growths are clearly visible on the surfaces of alloys obtained in baths that did not contain additional substances. The presence of saccharin brought about a strong levelling of the surface. This is especially visible in Fig. 10, which shows the results of analysis using the AFM method. A similar morphology was observed in nickel deposits obtained from Watts-type baths with the addition of saccharin w34x. It would appear, therefore, that the change in the microstructure of alloys is the result of a specific adsorption of particles of this compound on the surface of the cathode, which leads to the deposition of metal in the hollows and thereby to the levelling of surface. Moreover, scanning analysis shows that irrespective of the place at which the investigations were carried out Žin areas taken about 1 mm from the edge and in the middle of the cathode., the composition Žlocal analysis. and morphology of alloys are identical.
Fig. 9. Micrographs of surface of Co–Ni alloys Žbath composition — Table 1: Ža. point 3; i K s 0.5 A dmy2 , Žb. point 1, i K s1 A dmy2 ..
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Fig. 10. AFM analysis result Žbath composition: Table 1, point 1; i K s1 A dmy2 ..
3.7. Macroscopic properties A significant influence of the additional components of the electrolyte on the macroscopic properties of Co–Ni alloys was observed. Deposition was carried out in a solution containing solely simple cobalt and nickel salts ŽTable 1, point 3., in an electrolyte with an addition of saccharin ŽTable 1, point 4., and in a bath containing saccharin and sodium lauryl sulfate ŽTable 1, point 1.. The alloy obtained in the electrolyte without additives was grey, mat, cracked, and separated itself from the both types of the cathode substrate. The addition of saccharin lowered the internal stresses of the deposit w35x — the alloy compact adhered to the substrate. The coating was silvery, but its whole surface was covered with numerous pits. From data given in literature regarding the cathodic reduction of pure nickel w30x, it appears that the presence of saccharin increases the overvoltage of deposition of this metal. This leads to the inhibition of the nuclei of crystallisation growth, and thereby to the brightening of deposits Žwhen the dimensions of crystallites are smaller than the length of visible light waves, i.e., 400 nm. w34x. Simultaneously, saccharin reduces the overvoltage of hydrogen evolution w30x, and this results in numerous pits on the cathode surface. A low sodium lauryl sulfate addition prevents their formation. In Table 2, we
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Table 2 Results of measurements of brightness of Co–Ni alloys No.
Additional substance
Quantity of light reflected, %
1 2 3
Without additives Saccharin Saccharinqsodium lauryl sulfate
0 59 34
have presented the results of measurements of the brightness of alloys obtained in baths with the same concentration of metal ions without additional substances and in their presence. The results thus obtained indicate that the presence of sodium lauryl sulfate does, however, reduce the brightness of the surface. The appearance of surfaces of alloys obtained is given in Fig. 11. 3.8. Changes in the composition of electrolyte During the 10–20 h deposition of Co–Ni alloys there were observed no changes in the concentration of cobalt and nickel ions in the electrolyte within the scope of sensitivity of the method applied. This was facilitated by the considerable volume of the solution, the application of great speeds of the solution flow, and also by the appropriate
Fig. 11. Appearance of surfaces of alloys Žbasic bath composition: Table 1, point 3. Ža. i K s 0.4 A dmy2 ; Žb. with an addition of saccharin Ž1 g dmy3 ., i K s 0.5 A dmy2 ; Žc. with an addition of saccharin Ž1 g dmy3 . and sodium lauryl sulfate Ž0.08 g dmy3 ., i K s1 A dmy2 .
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choice of rate of nickel and cobalt anodes dissolution Žmetal ions should be added to the electrolyte in the same ratio as they are consumed in cathodic reduction.. The maintenance of a constant composition of the bath leads to the deposition of alloys having a uniform composition throughout their whole volume. 4. Conclusions From research carried out and data given in literature it appears that the composition of Co–Ni alloys may be controlled through the selection of the following parameters. Ž1. Cathodic current density — an increase in this parameter brings about a decrease in the cobalt content of deposited material in solutions of simple salts. Ž2. Concentration of cobalt ions — an increase in the concentration of Co 2q ions in solutions of simple salts Žat a fixed concentration of Ni 2q ions. results in the alloy being enhanced with this constituent; the CorŽCo q Ni. ratio in the deposit is considerably greater than in the bath. Ž3. The addition of saccharin and sodium lauryl sulfate to solutions of simple salts is conducive to the deposition of an alloy having an increased nickel content. The following dependence of the cathodic current efficiency on parameters of electrolysis was observed: 1. Current density — when this parameter increases, the direction of change cannot be uniquely determined, this is because it is dependent on the composition of the bath. 2. Concentration of cobalt ions — an increase in their concentration exerts a slight influence on the efficiency of the cathodic process. 3. The presence of saccharin and sodium lauryl sulfate brings about the deposition of an alloy with the current efficiency of 99.7% at the current densities to 2 A dmy2 . The macroscopic properties of alloys are to a significant extent dependent upon the composition of the electrolyte Žthe presence of substances reducing internal stresses of the deposit and of the surfactant.. Further research will center on obtaining a composite with Co–Ni matrix strengthened by dispersive particles. 5. List of symbols Ei E0 a, b iK hK
Potential of the polarised electrode Equilibrum potential of electrode Constants Cathodic current density Cathodic overpotential
Acknowledgements Research was financed by the Polish Committee for Scientific Research under grant No. 7 T08 018 12.
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References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x w18x w19x w20x w21x w22x w23x w24x w25x w26x w27x w28x w29x w30x w31x w32x w33x w34x w35x
W.T. McFarlen, Plating 57 Ž1970. 46–50. H. Oettel, Neue Hute ¨ 21 Ž7. Ž1976. 416–420. D.W. Endicott, J.R. Knapp, Plating 53 Ž1966. 43–60. W.R. Wearmouth, K.C. Belt, Trans. IMF 52 Ž1974. 114–118. L. Winkler, Jahrbuch Oberflachentechnik, 1970, pp. 244–254. ¨ H. Benninghoff, Metall 3 Ž1979. 295–296. S.N. Srimathi, S.M. Mayanna, B.S. Sheshadri, Surf. Technol. 16 Ž1982. 277–322. I. Tsu, Plating 48 Ž1961. 1207–1210. W.R. Wearmouth, Trans. IMF 60 Ž2. Ž1982. 68–73. A. Hart, Metals Edge — Advanced Materials News 18 Ž1990. 15–19. A. Riesenkampf, Z. Fura, Arch. Metall. 20 Ž1. Ž1975. 3–15. C. Fan, D.L. Piron, Acta 41 Ž10. Ž1996. 1713–1719. N. Fenineche, C. Coddet, A. Saida, Surf. Coat. Technol. 40 Ž1990. 75–81. A.M. Abd El-Halim, Surf. Technol. 23 Ž1984. 207–213. V. Lainer, J. Vielichko, Vestn. Mashinostr. 40 Ž2. Ž1960. 73–76. R. Srivastava, S. Nigam, Surf. Technol. 10 Ž1980. 343–348. S.S. Abd El-Rehim, A.M. Abd El-Halim, M.M. Osman, J. Appl. Electrochem. 15 Ž1985. 107–112. J. Przyłuski, J. Bielinski, M. Kucharski, Surf. Technol. 9 Ž1979. 179–185. ˜ S.S. Abd El-Rehim, A.M. Abd El-Halim, M.M. Osman, Surf. Technol. 22 Ž1984. 337–342. G.M.C. Lee, W.J.D. Jones, IMF 55 Ž2. Ž1977. 70–72. V. Sree, T.L. Rama Char, J. Electrochem. Soc. 108 Ž1. Ž1961. 64–70. W.I. Lajner, N.T. Kudriawcew, Podstawy Galwanostegii, Vol. I, PWT, Warsaw, 1955, p. 314. K.E. Langford, Met. Finish. 52 Ž1954. 71–73. W.J. Tegart, The Electrolytic and Chemical Polishing of Metals in Research and Industry, Pergamon, London, 1956, p. 96. A.T. Vagramyan, Elektroosazhdenie Metallov, Izd. Akad. Nauk. USSR, pp. 134–144. M.A. Zhamagortsyan, L.A. Uvarov, A.T. Vagramyan, Elektrikhimiya 1 Ž1. Ž1965. 20–22. M.A. Zhamagortsyan, A.A. Yavich, A.T. Vagramyan, Elektrokhimiya 9 Ž9. Ž1973. 1394–1396. M.A. Zhamagortsyan, A.A. Yavich, A.T. Vagramyan, Elektrokhimiya 11 Ž1. Ž1975. 103–107. M. Stern, A.L. Geary, J. Electrochem. Soc. 104 Ž1. Ž1957. 56–63. O.K. Galdikene, B.J. Ukyalene, Y.Y. Matulis, Liet. TSR Mokslu. Akad. Darb., Ser. B 4 Ž59. Ž1969. 65–72. A. Brenner, Electrodeposition of Alloys, Academic Press, New York, 1963. T.B. Massalski, Binary Alloy Phase Diagrams, ASM Metals Park, OH, 1986, pp. 782–783. A. Taylor, J. Inst. Met. 77 Ž1950. 585–594. R. Weil, R. Paquin, J. Electrochem. Soc. 107 Ž2. Ž1960. 87–91. D. Mockute, Chemija 1 Ž1995. 17–31.
The influence of electrolysis parameters on the composition and morphology of Co–Ni alloys ) L. Burzynska , E. Rudnik ´ Department of Physical Chemistry and Electrochemistry, Faculty of Non-Ferrous Metals, UniÕersity of Mining and Metallurgy, Cracow, Poland Received 20 September 1999; accepted 20 September 1999
Abstract The influence of cathodic current density, the concentration of Co 2q ions in the electrolyte, and additional substances Žsaccharin and sodium lauryl sulfate. on the composition and morphology of Co–Ni alloys were investigated. Research was carried out in a Watts-type bath in the presence of boric acid, as a buffer substance. The circulation speed of the electrolyte was 40 dm3 hy1. Cathodic polarisation curves were determined for parent metals and the Co–Ni alloy. It was established that the presence of additives shifts the cathodic potential of alloy deposition towards more negative values. An increase in the cobalt content in the alloy was observed with decreasing of the cathodic current density and increasing of the Co 2q ions concentration in the bath. Results obtained confirmed the anomalous character of deposition of the Co–Ni alloy. The cathodic current efficiency is dependent mainly on the current density applied, but the direction of changes has not been precisely determined, for it depends on the composition of the electrolyte. Using diffractional X-ray and micro X-ray analyses, it was determined that single-phase deposits with an fcc lattice for the whole investigated range of current density and electrolyte composition were obtained. In the presence of additives there were obtained fine-grained, bright alloys which adhered well to the substrate. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Electrolysis parameters; Co–Ni alloys; Electrolyte
1. Introduction Electrolytic nickel–cobalt alloys are characterised by high strength Ževen at elevated temperatures. w1–3x, hardness w2–6x, and specific magnetic properties w7,8x. These )
Corresponding author
0304-386Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 9 9 . 0 0 0 6 0 - 2
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features have determined that this material has many applications, among others, in rocket technology, cosmonautics, sound signal recording, as a material for the electroforming of moulds for die-casting and plastics, as an anticorrosive coating, and also for decorative purposes w1,4,6,9,10x. The cathodic co-deposition of nickel and cobalt is carried out using chloride w11–13x, sulfate w11,14–16x, chloride–sulfate w17–19x, and sulfamate w3,5,20x electrolytes, either with or without the addition of complexing compounds, e.g., pyrophosphate w21x or citrates w12x. Applications of simple and complexing bath in the deposition of the Co–Ni alloys have been extensively described in the literature. However, the process in the chloride–sulfate solutions ŽWatts-type. has been insufficiently studied. The adaptation of the Watts bath w17–19x in the co-deposition of metals results from the comprehensive research of nickel electrodeposition in this type of the electrolyte, and because of the similar properties of cobalt and nickel. The composition of the alloy, and therefore some of its properties Žbrightness, plasticity, etc.. may be regulated by appropriately choosing the bath composition and parameters of electrolysis. The present paper shows the results of laboratory research centred on the deposition of Ni–Co alloys using chloride–sulfate solutions. The objective was to determine the way in which the composition of alloys, cathodic current efficiency and macroscopic properties of deposits are dependent upon the concentration of cobalt and nickel ions, current density and the presence of additional substances.
2. Experimental 2.1. Composition of electrolyte Using data given in literature w17x, we selected the composition of the bath in which the co-deposition of nickel and cobalt was to take place. The composition of electrolytes and process parameters are presented in Table 1, points 1–3. Chloride–sulfate solutions were used. It appears w22x useful to add chloride ions, for these prevent the passivation of anodes and facilitate their dissolution. Measurements were carried out in the presence of boric acid, which fulfilled the role of a buffer substance. The alloy deposition was also conducted in electrolytes with an addition of saccharin and sodium lauryl sulfate. Saccharin lowered the internal stresses of cathodic deposits. Sodium lauryl sulfate, as a surfactant, prevents the incorporation of gaseous hydrogen into the cathodic deposit. The deposition of the alloy was carried out using a bath in which the concentration of nickel ions was one order of magnitude higher than that of cobalt ions. This is connected with the preferential cathodic deposition of cobalt. The cathodic polarisation curves for the deposition of Ni–Co alloy were determined in solutions with compositions given in Table 1, points 1 and 3–5. The polarisation curves for the cathodic individual reduction of Co 2q and Ni 2q ions were also determined. The composition of the baths is presented in Table 1, points 6 and 7. These measurements were carried out in electrolytes with a concentration of cobalt and nickel ions identical with that existing during the recording of the alloy’s polarisation curve. In
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Table 1 Composition of the baths, conditions of Co–Ni alloys deposition and the determination of cathodic polarisation curves for the alloy, nickel, and cobalt No.
Composition of electrolyte
Concentration y3
Ž1.
Ž2.
Ž3.
Ž4.
Ž5.
Ž6.
Ž7.
Process conditions y3
mol dm
g dm
CoSO4P7H 2 O NiCl 2 P6H 2 O H 3 BO 3 Saccharin Sodium lauryl sulfate CoSO4P7H 2 O NiCl 2 P6H 2 O H 3 BO 3 Saccharin Sodium lauryl sulfate CoSO4P7H 2 O NiCl 2 P6H 2 O H 3 BO 3
0.089 0.841 0.405 0.004 0.0003 0.054–0.089 0.841 0.405 0.004 0.0003 0.089 0.841 0.405
25 200 25 1 0.08 15–25 200 25 1 0.08 25 200 25
CoSO4P7H 2 O NiCl 2 P6H 2 O H 3 BO 3 Saccharin CoSO4P7H 2 O NiCl 2 P6H 2 O H 3 BO 3 Sodium lauryl sulfate CoSO4P7H 2 O MgCl 2 P6H 2 O H 3 BO 3
0.089 0.841 0.405 0.004 0.089 0.841 0.405 0.0003 0.089 0.841 0.405
25 200 25 1 25 200 25 0.08 25 171 25
MgSO4P7H 2 O NiCl 2 P6H 2 O H 3 BO 3
0.089 0.841 0.405
22 200 25
Cathodic current density: 1.0–2.0 A dmy2 Circulation of electrolyte: 40 dm3 hy1 Volume of electrolyte: 3 dm3 Cathode substrate: titanium anodes: Ni and Co cathodic current density: 1 A dmy2 circulation of electrolyte: 40 dm3 hy1 volume of electrolyte: 3 dm3 cathode substrate: titanium anodes: Ni and Co cathodic current density: 0.4–2.3 A dmy2 circulation of electrolyte: 40 dm3 hy1 volume of electrolyte: 3 dm3 cathode substrate: steel anodes: Ni and Co circulation of electrolyte: 40 dm3 hy1 volume of electrolyte: 3 dm3 cathode substrate: titanium anodes: Ni and Co circulation of electrolyte: 40 dm3 hy1 volume of electrolyte: 3 dm3 cathode substrate: titanium anodes: Ni and Co circulation of electrolyte: 7 dm3 hy1 volume of electrolyte: 0.5 dm3 cathode substrate: steel anode: Co circulation of electrolyte: 7 dm3 hy1 volume of electrolyte: 0.5 dm3 cathode substrate: steel anode: Ni
order to obtain the same value of the ionic strength of solutions, one of these cations was replaced with Mg 2q ions. Electrolytes with a pH of 4.4 were applied. This value was achieved through the addition of sodium hydroxide. The solutions were prepared using analytical grade reagents Žsupplied by POCh, Poland. and twice-distilled water. The concentration of Co 2q and Ni 2q ions was determined using the method suggested by Langford w23x. This consists in a total determination of cobalt and nickel by means of titration with an EDTA solution in the presence of murexide as the indicator. In a second portion of the solution Co 2q ions are oxidised to Co 3q ions with the simultaneous creation of an ammonia complex. Ni 2q ions do not undergo such a reaction and may be determined using EDTA.
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2.2. Electrodes Rectangular 5 = 6 cm plates made from stainless steel or titanium Ž99.9%. were used as substrates for the cathode. Prior to each measurement, these were cleaned using abrasive paper of various gradations Žin the following order: 280, 400, 600, 800. and subsequently washed in distilled water and alcohol. The substrates were isolated on one side for determining the polarisation curves. Separate rectangular 5 = 6 cm nickel and cobalt anodes were used. The nickel anodes Ž99.9%. were rolled plates, while the cobalt anodes Ž99.0%. were cast plates. In order to remove the oxidised layer from the surface of the electrodes, the anodes were immersed in a mixture of concentrated acids: HNO 3 , H 2 SO4 , H 3 PO4 , and CH 3 COOH Ž3:1:1:5. at a temperature of 858C–908C for approximately 1 min w24x. The anodes were isolated on one side. The co-deposition of cobalt and nickel was carried out using two nickel anodes and two cobalt ones. One anode made of the appropriate element was applied for defining the cathodic polarisation curves of individual metals. 2.3. Measurement circuit The alloy deposition was conducted in a cuboid vessel made of rigid PVC. The volume of electrolyte was 3 dm3. The cathodic polarisation curves for parent metals were determined in a glass vessel containing 0.5 dm3 of the solution. Within the electrolyser, the electrodes were hung vertically and in parallel with respect to each other. The anodes were placed symmetrically on both sides of the centrally located cathode. The cathode potential was measured with respect to a saturated calomel electrode and the result thus obtained converted with respect to the normal hydrogen electrode. Measurements of potential were carried out using a Luggin capillary, placed at the surface of the electrode in such a way as not to disturb the distribution of the force lines of the electric field. The pH of the bath was monitored with a combined electrode connected with a pH-meter ŽRadelkis.. In order to eliminate the influence of the electric field on the readings of the apparatus, the electrode was placed in a glass housing that did not, however, prevent the free flow of electrolyte. The constant pH value of the bath, which displayed a tendency to alkalify as a result of the cathodic evolution of hydrogen, was maintained by the periodic addition of a few drops of a mixture of diluted HCl and H 2 SO4 Ž10:1.. The flow of electrolyte was kept at a constant and stable level using a peristaltic pump with a circulation speed corresponding to an approximately 13-times exchange of the volume of the solution within 1 h. The electrolyte was pumped from the direction of the anodes in the direction of the cathode. In order to precisely determine the charge flowing through the circuit, independent copper coulometers were used, these being series-connected with the electrodes. The measurement circuit is given in Fig. 1.
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Fig. 1. Diagram of measurement circuit applied for depositing the Co–Ni alloy. 1 — Cobalt anode; 2 — nickel anode; 3 — cathode; 4 — stabilized power supply; 5 — ammeter; 6 — copper coulometer; 7 — resistor; 8 — combined electrode; 9 — pH-meter.
2.4. Determination of polarisation curÕes The polarisation curves for the deposition of nickel, cobalt and their alloy were determined using the galvanostatic method, at a temperature of 21 " 18C. The cathodic polarisation curves were registered after a metallic layer had appeared on the substrate. These layers were obtained carrying on the cathodic process for 1 h at a current density of 0.4 A dmy2 . After that the flow of the electric current was cut off and the potential value read for i K s 0. Next, curves were determined at increasing and decreasing current densities until reproducible values were found. Polarisation curves were registered under a fixed set conditions Žthe electrolyte composition, speed of circulation, temperature.. Measurements were carried out in stationary conditions, i.e., the established potential value was read at a set current density. Partial cathodic polarisation curves for nickel and cobalt were determined on the basis of the composition of alloys. 2.5. Cathodic alloy deposition The deposition of Co–Ni alloys was conducted over a period of 10 to 20 h under galvanostatic conditions, at a temperature of 21 " 18C. The influence of current density on the composition of the cathodic deposit was examined in electrolytes having compositions presented in Table 1, points 1 and 3, while the dependence of the composition of the alloy on the concentration of metal ions was determined in solutions having concentrations given in Table 1, point 2. The cathodic current efficiencies were calculated on the basis of the mass of deposit and coulometric data. Various current densities were used for nickel and cobalt anodes
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for each measurement. Nickel anodes were dissolved by passing 75% of total electric current through them; the remainder dissolved the cobalt anodes. Phase identification of deposits was performed using an X-ray diffractometer ŽDRON 3. and filtered radiation CuK a. Deposited alloys underwent X-ray microanalysis, while cobalt and nickel were analysed quantitatively at points on the surface 900 nm distant from each other. This enabled graphs of changes in concentration for both elements along selected lines to be plotted. An analysis of the surface distribution of alloy constituents was also carried out. Due to the possibility of CoKb and NiK a spectral lines overlapping, CoK a and NiKb radiation was used for measurements. Measurements were carried out with a Philips XL 30 electron scanning microscope with an EDS analyser ISIS model. In order to investigate the microstructure of deposits, a scanning analysis and measurements using the Atomic Force Microscopy method were executed, the latter applying a Nanoscope E apparatus. In the course of electrolysis, changes in the composition of the electrolyte were examined. Samples of the solution were taken at hourly intervals. Cathodic deposits were dissolved in HNO 3 Ž1:1., and their composition subsequently determined applying the Atomic Absorption Spectrometry method ŽPerkin Elmer Atomic Absorption Spectrometer 3110.. Alloys were estimated visually and layers obtained were documented photographically. The brightness of coatings was measured using a Corning-EEL glossmeter. A mirror was adopted as a model surface with 100% brightness. 3. Results and discussion 3.1. Polarisation curÕes Fig. 2 shows the cathodic polarisation curves for the individual deposition of cobalt and nickel, their sum and partial polarisation curves for both metals Žcalculated on the
Fig. 2. Cathodic polarisation curves of cobalt, nickel, their sum and partial curves of cobalt and nickel.
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basis of the composition of alloys.. It was observed that the deposition of the parent metals requires considerable overvoltage. This means that at very low current densities Žto 0.5 A dmy2 ., the flat course of the polarisation curves is observed. However, this phenomenon is characteristic for elements of the iron group w25x. The occurrence of polarisation is connected with the inhibition of one of the intermediate stages of the electrode process. The dependence of the cathode potential on current density in the case of activation polarisation is given by Tafel’s equation: Ei y E0 s h K s a q b log i K
Ž 1.
where Ei is potential of the polarised electrode, E0 equilibrium potential of the electrode, h K cathodic overvoltage, i K cathodic current density, a and b are constants. Fig. 3 is a plot illustrating the dependence of the pure metal deposition potential upon the logarithm of the cathodic current density. It is worth noting that the potential of both electrodes determined at i K s 0 changed with time from the moment the flow of the electric current was cut off. For nickel, the stabilized value does not correspond to the potential calculated using Nernst’s equation. According to data given in literature w26,27x, difficulties with attaining a state of equilibrium are connected with the passive state of the surfaces of nickel and cobalt electrodes. This hypothesis appears credible, since the electrolyte contains dissolved oxygen, which may react with both nickel and cobalt, thereby creating oxidised layers. One cannot, however, exclude the steadying of
Fig. 3. Dependence of the nickel and cobalt cathode potential on the logarithm of the cathodic current density.
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mixed potentials as a result of the course of the corrosive process under hydrogen depolarisation. Nevertheless, the rate of this process is negligible, this due to the low concentration of hydrogen ions in the electrolyte ŽpH s 4.4.. The cobalt electrode potential determined empirically at i K s 0 equalled the equilibrium potential. It appears that this fact is connected with the composition of the electrolyte within which the polarisation curve was registered. This contained 1.7 mol dmy3 of chloride ions. The effectiveness of action of chloride ions depends not only on their concentration in the solution, but also on the ratio of metal ions concentrations to those of chloride ions. From the data presented in literature w28x, it is known that these anions adsorb on the surface of the electrode and thereby block the secondary reactions. Data given in Fig. 3 indicate that at the current densities from 0.33 to 0.98 A dmy2 for cobalt and from 0.14 to 1.44 A dmy2 for nickel, the potential of the cathode is a linear function of the logarithm of the cathodic current density. It seems that the process ran under activation control during the investigated conditions. It was observed that an increase in the current density was accompanied by a deviation from this linearity, which in fact may be indicative of the growing participation of concentration polarisation w29x. During the co-deposition of the metals, there was observed a certain decreasing of the rate of the cathodic reduction of Ni 2q ions in relation to that of pure element ŽFig. 2.. This caused the shifting of the partial curve for nickel towards more negative potentials. At the same time, it was observed that at the current densities higher than 0.5 A dmy2 , the cathodic reduction of Co 2q ions rate is increased. This means that the deposition of alloy alters the course of polarisation curves for both constituents. The sum of polarisation curves determined empirically for the parent metals makes it possible to draw the predicted polarisation curve for the alloy ŽFig. 2.. Their course indicates that at the current density higher than 0.7 A dmy2 , the nickel fraction is significantly higher than the cobalt one. This fact determines the rate with which the content of metal ions in the bath is to be replenished so as to maintain their concentration at a constant level and thereby obtain a cathodic deposit with a uniform composition throughout its volume, independently of the duration of electrolysis. As a result, the dissolution of nickel anodes was carried out at current densities that were three times greater. The choice of another anodic current density would have led to a rapid change in the composition of the electrolyte. Fig. 4 shows cathodic polarisation curves of the Co–Ni alloys registered in the baths with different compositions. The presence of saccharin and sodium lauryl sulfate in the solution causes a considerable shift of the alloy polarisation curve towards more negative potentials in comparison with the curve determined in the electrolyte without additives. The highest inhibition of the cobalt and nickel co-deposition is observed in the case of saccharin. It appears that it is connected with strong adsorption of its molecules on the cathode surface. However, the influence of sodium lauryl sulfate on the process rate is significantly weaker. The addition of a small amount of the surfactant to the bath containing saccharin lowers the disadvantageous influence of the latter — the polarisation curve lies between curves determined in the solutions with one of the additives. The determination of the experimental alloy polarisation curve and of the partial curves for its constituents enables one to precisely define the cathodic current density Žat
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Fig. 4. Cathodic polarisation curves of Co–Ni alloy Žbath composition — Table 1.: Ža. point 3, Žb. point 5, Žc. point 1, Žd. point 4.
fixed bath composition, temperature, circulation. at which it is possible to obtain a material having a desired composition. 3.2. Composition of Co–Ni alloys The influence of current density and the composition of electrolyte on the composition of deposit were studied. 3.2.1. Cathodic current density The dependence of the alloy composition on the cathodic current density is presented in Fig. 5. The plot shows that an increase in this parameter is accompanied by a reduction in the cobalt content in the cathodic deposit. Simultaneously, at the set current density, the content of this metal in the alloy is dependent upon the composition of the electrolyte. The same metal ions concentration was maintained, and therefore the change in the composition of alloys is connected with the presence of additional components. The addition of saccharin and sodium lauryl sulfate brought about a reduction of the cobalt content in relation to the composition of coatings obtained in baths without additives. It follows therefore that these substances facilitate the deposition of pure nickel and hinder that of cobalt. It is known, however, that during the cathodic deposition of pure nickel in the presence of saccharin, the cathodic overvoltage increases w30x. The presence of sodium lauryl sulfate in the bath not only compensates for the unfavourable influence of saccharin, but in addition reduces the alloy deposition overvoltage. What is more, the considerable current efficiency of the cathodic process in the presence of additives in the electrolyte appears to indicate that the hydrogen evolution overvoltage increases, too.
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Fig. 5. Influence of cathodic current density on the composition of Co–Ni alloys Žbath composition — Table 1: Ža. point 3, Žb. point 1..
The observed direction of changes in the composition of the alloy under the influence of alterations of current density is in good agreement with results obtained by other researchers w5,15,17x using solutions of simple salts. 3.2.2. Concentration of Co 2 q in electrolyte The dependence of the composition of Co–Ni alloys on the concentration of Co 2q ions in the electrolyte at a fixed concentration of Ni 2q ions is presented in Fig. 6. The course of the curve indicates that an increase in the content of Co 2q ions in the solution is accompanied by an increase in the fraction of this metal in the deposit. A characteris-
Fig. 6. Influence of Co 2q ions content in electrolyte on the composition of the Co–Ni alloy Žbath composition: Table 1, point 2..
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tic feature is that the CorŽCo q Ni. ratio in the alloy is considerably higher than in the bath, for example, 7% wt. of cobalt in the electrolyte corresponds to a 30% wt. content of this metal in the cathodic deposit. This fact is confirmed by the anomalous character of Co–Ni alloy deposition, where the less noble constituent is deposited preferentially w31x. Data given in literature indicate that this phenomenon occurs only in baths that contain simple cobalt and nickel salts w12x, whereas in electrolytes with an addition of complexing substances there is observed the deposition of alloys with CorŽCo q Ni. ratio equal or substantially lower than in the solution. 3.3. Cathodic current efficiency Fig. 7 shows the dependence of cathodic current efficiency on the current density. This is clearly connected with the composition of the electrolyte. In baths containing simple nickel or cobalt salts without surfactants, complexing or other similar additives, it was observed that a change in current density from 0.4 to 2.3 A dmy2 was accompanied by an increase in current efficiency from 94.5% to 99.0%. In solutions with saccharin and sodium lauryl sulfate, there were observed no changes in the efficiency of the cathodic process in connection with an increase in current density; this equalled 99.7 " 0.1%. From the data given in literature w18x, it is known that in baths having similar compositions within the range of high current densities from 16 to 64 A dmy2 the cathodic current efficiency equals 80%–90%, whereas sulfamate solutions w3x with an addition of sodium lauryl sulfate yielded current efficiencies of 97%–100% within the range of the current densities from 2 to 5 A dmy2 Žthe alloys contained 30%–75% of nickel.. Research carried out indicates that an increase in the concentration of Co 2q ions in the electrolyte causes an increase in the cathodic current efficiency of approximately 1%. Similar results were obtained by Abd El-Rehim et al. w17x in chloride–sulfate baths which did not contain any additional substances.
Fig. 7. Dependence of cathodic current efficiency upon current density Žbath composition — Table 1: Ža. point 3, Žb. point 1..
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3.4. X-ray diffraction analysis The phase composition of alloys was studied applying X-ray diffraction analysis. It follows therefrom that peaks solely related to a solid solution with the face-centred cubic structure are present. This is in agreement with the phase diagram for the Co–Ni system within the researched range of alloy compositions w32x. There are no peaks related to pure metallic phases. This would suggest that single-phase cathodic deposits have been obtained. There exists no possibility of precisely determining the parameter of the lattice — and therefrom the composition of the alloy — on the basis of diffraction patterns obtained. It is a well-known fact that solid Co–Ni solutions of decidedly different compositions have very similar lattice parameters w33x. 3.5. X-ray microanalysis Fig. 8 shows the results of X-ray microanalysis along lines for two samples obtained in electrolyte having the same composition, but at different current densities. At the current density of 2 A dmy2 , the concentration of both nickel and cobalt in the sample had a constant value, while in the case of the sample obtained at a current density of 1 A dmy2 , a change in the content of these elements in an area near the edge of the cathode was observed. This may have been caused by local changes in current density. The distribution of both metals on an 8 P 10 4 = 10 P 10 4 nm surface area in the form of a ‘‘map’’ was determined. An even distribution of nickel and cobalt over the whole surface was observed. Sections with a higher concentration of one element were not observed.
Fig. 8. Distribution of nickel and cobalt along a line on the surface of alloys. The inset Žv . shows the place of cutting out the samples from the cathode Žbath composition — Table 1, point 1.: Ža. i K s1 A dmy2 , Žb. i K s 2 A dmy2 .
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3.6. Scanning analysis The alloys underwent scanning electron microscope analysis. Fig. 9 shows the example microphotographs. These would suggest that the microstructure of deposits is dependent mainly on the composition of the bath. The current density Žwithin the researched range. is a less significant factor. Spherical growths are clearly visible on the surfaces of alloys obtained in baths that did not contain additional substances. The presence of saccharin brought about a strong levelling of the surface. This is especially visible in Fig. 10, which shows the results of analysis using the AFM method. A similar morphology was observed in nickel deposits obtained from Watts-type baths with the addition of saccharin w34x. It would appear, therefore, that the change in the microstructure of alloys is the result of a specific adsorption of particles of this compound on the surface of the cathode, which leads to the deposition of metal in the hollows and thereby to the levelling of surface. Moreover, scanning analysis shows that irrespective of the place at which the investigations were carried out Žin areas taken about 1 mm from the edge and in the middle of the cathode., the composition Žlocal analysis. and morphology of alloys are identical.
Fig. 9. Micrographs of surface of Co–Ni alloys Žbath composition — Table 1: Ža. point 3; i K s 0.5 A dmy2 , Žb. point 1, i K s1 A dmy2 ..
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Fig. 10. AFM analysis result Žbath composition: Table 1, point 1; i K s1 A dmy2 ..
3.7. Macroscopic properties A significant influence of the additional components of the electrolyte on the macroscopic properties of Co–Ni alloys was observed. Deposition was carried out in a solution containing solely simple cobalt and nickel salts ŽTable 1, point 3., in an electrolyte with an addition of saccharin ŽTable 1, point 4., and in a bath containing saccharin and sodium lauryl sulfate ŽTable 1, point 1.. The alloy obtained in the electrolyte without additives was grey, mat, cracked, and separated itself from the both types of the cathode substrate. The addition of saccharin lowered the internal stresses of the deposit w35x — the alloy compact adhered to the substrate. The coating was silvery, but its whole surface was covered with numerous pits. From data given in literature regarding the cathodic reduction of pure nickel w30x, it appears that the presence of saccharin increases the overvoltage of deposition of this metal. This leads to the inhibition of the nuclei of crystallisation growth, and thereby to the brightening of deposits Žwhen the dimensions of crystallites are smaller than the length of visible light waves, i.e., 400 nm. w34x. Simultaneously, saccharin reduces the overvoltage of hydrogen evolution w30x, and this results in numerous pits on the cathode surface. A low sodium lauryl sulfate addition prevents their formation. In Table 2, we
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Table 2 Results of measurements of brightness of Co–Ni alloys No.
Additional substance
Quantity of light reflected, %
1 2 3
Without additives Saccharin Saccharinqsodium lauryl sulfate
0 59 34
have presented the results of measurements of the brightness of alloys obtained in baths with the same concentration of metal ions without additional substances and in their presence. The results thus obtained indicate that the presence of sodium lauryl sulfate does, however, reduce the brightness of the surface. The appearance of surfaces of alloys obtained is given in Fig. 11. 3.8. Changes in the composition of electrolyte During the 10–20 h deposition of Co–Ni alloys there were observed no changes in the concentration of cobalt and nickel ions in the electrolyte within the scope of sensitivity of the method applied. This was facilitated by the considerable volume of the solution, the application of great speeds of the solution flow, and also by the appropriate
Fig. 11. Appearance of surfaces of alloys Žbasic bath composition: Table 1, point 3. Ža. i K s 0.4 A dmy2 ; Žb. with an addition of saccharin Ž1 g dmy3 ., i K s 0.5 A dmy2 ; Žc. with an addition of saccharin Ž1 g dmy3 . and sodium lauryl sulfate Ž0.08 g dmy3 ., i K s1 A dmy2 .
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choice of rate of nickel and cobalt anodes dissolution Žmetal ions should be added to the electrolyte in the same ratio as they are consumed in cathodic reduction.. The maintenance of a constant composition of the bath leads to the deposition of alloys having a uniform composition throughout their whole volume. 4. Conclusions From research carried out and data given in literature it appears that the composition of Co–Ni alloys may be controlled through the selection of the following parameters. Ž1. Cathodic current density — an increase in this parameter brings about a decrease in the cobalt content of deposited material in solutions of simple salts. Ž2. Concentration of cobalt ions — an increase in the concentration of Co 2q ions in solutions of simple salts Žat a fixed concentration of Ni 2q ions. results in the alloy being enhanced with this constituent; the CorŽCo q Ni. ratio in the deposit is considerably greater than in the bath. Ž3. The addition of saccharin and sodium lauryl sulfate to solutions of simple salts is conducive to the deposition of an alloy having an increased nickel content. The following dependence of the cathodic current efficiency on parameters of electrolysis was observed: 1. Current density — when this parameter increases, the direction of change cannot be uniquely determined, this is because it is dependent on the composition of the bath. 2. Concentration of cobalt ions — an increase in their concentration exerts a slight influence on the efficiency of the cathodic process. 3. The presence of saccharin and sodium lauryl sulfate brings about the deposition of an alloy with the current efficiency of 99.7% at the current densities to 2 A dmy2 . The macroscopic properties of alloys are to a significant extent dependent upon the composition of the electrolyte Žthe presence of substances reducing internal stresses of the deposit and of the surfactant.. Further research will center on obtaining a composite with Co–Ni matrix strengthened by dispersive particles. 5. List of symbols Ei E0 a, b iK hK
Potential of the polarised electrode Equilibrum potential of electrode Constants Cathodic current density Cathodic overpotential
Acknowledgements Research was financed by the Polish Committee for Scientific Research under grant No. 7 T08 018 12.
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References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x w18x w19x w20x w21x w22x w23x w24x w25x w26x w27x w28x w29x w30x w31x w32x w33x w34x w35x
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