Pulse and direct current plating of Ni–W alloys from ammonia–citrate electrolyte

Surface & Coatings Technology 200 (2006) 4201 – 4207 www.elsevier.com/locate/surfcoat

Pulse and direct current plating of Ni–W alloys from ammonia–citrate electrolyte M.D. Obradovic´a, G.Zˇ. Bosˇnjakovb, R.M. Stevanovic´a,T, M.D. Maksimovic´c, A.R. Despic´a a

Institute of Chemistry, Technology and Metallurgy-Department of Electrochemistry, University of Belgrade, Njegosˇeva 12, P.O. Box 473, 11001 Belgrade, Serbia and Montenegro b Military Technical Institute YA, Katanic´eva 15, 11001 Belgrade, Serbia and Montenegro c Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, P.O. Box 3503, 11000 Belgrade, Serbia and Montenegro Received 10 November 2004; accepted in revised form 27 December 2004 Available online 8 February 2005

Abstract The effect of the potential of deposition and electrode rotation rate on current efficiency and partial current density of nickel deposition from ammonia–citrate electrolyte with and without tungstate ions was investigated. In addition, the effect of pulse deposition parameters, such as frequency and ratio between pause and deposition time, t p/t c, on current efficiency and composition of Ni–W alloys was investigated. Partial current densities of tungsten and nickel deposition were calculated from experimental results and compared with those in direct current (dc) deposition at the same average current density, as well as with those calculated using theoretical equations for the pulse current (pc) deposition process controlled by mass transport. Obtained partial current densities of tungsten are in fair agreement with those predicted by theory in both cases, as a function of dimensionless pulse period and as a function of the t p/t c ratio. That confirms that the tungsten deposition process is controlled by mass transport. Dependences of the partial current densities of nickel deposition on the rotation rate as well as on the pc variables (frequency and pause to pulse ratio) were attributed to changes in surface concentration of ammonia and the pH. D 2005 Elsevier B.V. All rights reserved. Keywords: Nickel–tungsten alloys; Ammonia–citrate electrolyte; Pulse electrodeposition

1. Introduction The electrochemical reduction of tungstates or molybdates from aqueous solutions produces very thin layers consisting of multivalent oxides and/or hydroxides [1], at which hydrogen overvoltage is lowered so that all processes other than hydrogen evolution are hindered [2]. Further reduction of tungsten or molybdenum ions or surface species to metallic state does not occur. However, the presence of ions of the iron-group metals in the tungstate or molybdate solution render not only an increase in current efficiency of the oxide formation [3,4], but also leads to a reduction of tungsten or molybdenum species to metallic state simultaneously with deposition of the irongroup metal [5]. That phenomenon, comprising that one of T Corresponding author. E-mail address: [email protected] (R.M. Stevanovic´). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.12.013

the metallic alloy components cannot be electrodeposited autonomously from aqueous solutions, but do deposit with another metal to form a metallic alloy is termed, according to Brenner’s classification, induced deposition [6]. Induced deposition of tungsten or molybdenum aided by the presence iron-group metal ions has been reported in a large number of scientific works [7–21]. The mechanism of deposition was given different explanations due to a pronounced sensitivity of the alloy composition and structure on deposition conditions. In metallurgically obtained Ni–W alloy the dominant phase should be solid solution of W in Ni, as indicated by the phase diagram in Fig. 1 [22,23]. However, it is well known that the phase structure of electrodeposited alloys may significantly differ from that of alloys obtained by thermal equilibration. So far, no more than about 50 mol% of tungsten in the Ni–W alloy could be obtained by electrodeposition [12]. X-ray diffraction data of the not

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Fig. 1. Ni–W phase diagram; w(W)—weight fraction and x(W)—mole fraction of tungsten in the alloy.

annealed Ni–W electrodeposits often point to a nanocrystalline or amorphous structure [24–26]. Some results indicated homogeneity ascribable to solid solution of W in Ni [27,28], while intermetallic phases are difficult to detect [29,30]. In a complex process of alloy formation by induced electrochemical deposition different non-equilibrium structures and species are likely to appear. Some layered structures [9] as well as small quantities of oxide [27] and carbide [31] species were also detected in the electrodeposits. In our recent communications we discussed the effect of the deposition conditions (composition and pH of the ammonia–citrate electrolyte, current density (c.d.), hydrodynamic conditions and quantity of the deposit) on the composition and structure of Ni–W alloys [27,32]. In this work, the square wave pulsating current (pc) was employed for Ni–W alloy deposition. Hence, additional process parameters of pc (frequency—f, and current pause time to cathodic deposition time ratio—t p/t c) could be varied independently. Several studies of pc deposition of tungsten and molybdenum with iron-group metals were reported in literature [24,33–35]. The models, describing the nature of the influence of pc parameters on the alloy composition, pertain to a particular combination of metals. Some authors found higher content of tungsten and molybdenum in the pc deposited alloys than in the deposits obtained by dc at the same average c.d. The increase was ascribed to the chemical reduction of tungstates [24] or molybdates [33] during the current pause interval (t p) by hydrogen evolved during the pulse. Other authors [34], studying deposition of Ni–Mo alloys from an electrolyte containing an excess of nickel,

suggested that increase in molybdenum content upon application of pc can be due to a higher instantaneous partial limiting c.d. of molybdenum. The effect was noticeable for sufficiently short pulse periods (b10 s) and for a codeposition process controlled by mass transport. The aim of this work was to investigate the effect of the pc parameters on the deposition of Ni–W alloys from the ammonia–citrate electrolyte and compare it to corresponding dc deposition at one and the same average c.d. The effects of the pc frequency and the t p/t c ratio on the composition of Ni– W alloys as well as on the current efficiencies, of both metal deposition and hydrogen evolution, were analyzed. Dependencies of partial current densities of tungsten, nickel deposition and hydrogen evolution on the pc frequency and the t p/t c ratio were obtained and compared with those calculated using theoretical model for the pc deposition process controlled by mass transport [36].

2. Experimental The Ni–W alloys were electrochemically deposited from an ammonia–citrate electrolyte composed of 0.075 mol dm3 NiSO4d 6H2O, 0.20 mol dm3 Na2WO4d 2H2O, 0.314 mol dm3 H3C6H5O7d 2H2O, at pH 8.15 adjusted by addition of ammonia (1.3 mol dm3). Pure nickel was electrodeposited from a solution of the same composition and pH but in the absence of the tungstate salt. The working electrode was a rotating disc electrode (rde) bTacusell ControvitQ with a replaceable gold disc. A commercial saturated calomel electrode (SCE) was used as a reference and platinum wire as a counter electrode. Deposition was carried out at the rotation rate of 1000 rpm, except when the

M.D. Obradovic´ et al. / Surface & Coatings Technology 200 (2006) 4201–4207 Table 1 Pulsating current parameters

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3. Results and discussion

f (Hz)

t p/t c

t c (ms)

t p (ms)

j c (mA cm2)

0 (dc) 2.5 5 10 25 25 25 25

dc 1 1 1 0.2 1 2 5

dc 200 100 50 33.3 20 13.3 6.7

dc 200 100 50 6.67 20 26.6 33.3

70 140 140 140 84 140 210 420

effect of the rotation rate was investigated, in the range 625 to 2500 rpm. The temperature was maintained at 35 8C. Alloys have been deposited potentiostatically (in the potential range between 1.15 and 1.70 V vs. SCE) as well as galvanostatically with a constant and a pulsating average current density of 70 mA cm2. A range of pulse parameters has been chosen for obtaining optimal coating quality (Table 1). The frequency of the pulsation, f, was 2.5, 5, 10 and 25 Hz (pulse period was 0.40, 0.20. 0.10 and 0.04 s, respectively) at t p/t c=1. At constant f=25 Hz the t p/t c ratio was 0.2, 1, 2 and 5. The samples prepared at 25 Hz and t p/t c ratio 10 were very thin and non-compact (current efficiency was less than 0.01) while those prepared at 50 Hz and t p/ t c=1 were dendritic and loose. Hence, they were not considered. Deposition was carried out using a Stonehart BC 1200 potentiostat/galvanostat driven by PAR 175 universal programmer. For the determination of the ohmic drop a Hewlett-Packard oscilloscope 54501A was used. All values of potential were corrected for the pseudo-ohmic polarization. Pure nickel and alloy depositions were carried out up to two constant total quantities of electricity 4 and 21 C cm2. After deposition, the deposits were completely dissolved using anodic linear sweep voltammetry into a solution containing 1.00 mol dm3 NaCl and 0.01 mol dm3 HCl, at a sweep rate of 0.1 mV s1. The recorded voltammograms have been integrated to establish the quantity of deposit in terms of the equivalent charge obtained upon dissolution. The current efficiency was determined from the charge passed during the deposition and the charge obtained upon dissolution. The content of nickel was determined by atomic absorption analysis of the solution, while the content of tungsten was calculated from the difference between dissolution charge of the alloy and the determined content of nickel. All the electrolytes were prepared from reagent grade chemicals and high-purity water (bMilliporeQ, 18 MV resistivity). Before each experiment, the disc electrode was polished with silicon carbide polishing papers P800, P1200, P1200, P4000 (bBuehlerQ), washed in an ultrasonic bath, polished further with an aqueous suspension of Al2O3 of the grain sizes of 1, 0.5 and 0.3 Am (bBuehlerQ). Finally, the electrode was washed again by high-purity water in the ultrasonic bath.

3.1. Direct current deposition of nickel and Ni–W alloy from the ammonia–citrate solution The dependence of the current efficiency of nickel and Ni–W alloys deposition on deposition potential is shown in Fig. 2. Driving the potential from 1.15 V vs. SCE in the negative direction (region II in Fig. 2) is seen to result in a relatively fast increase of the current efficiency of nickel deposition. In the potential range from 1.20 to 1.31 V vs. SCE the current efficiency of nickel deposition increases somewhat slower and attains a maximum value of about 0.25 (region III). In the potential range from 1.31 to 1.50 V vs. SCE the current efficiency of nickel deposition decreases (region IV). In the low polarization c.d. range nickel deposition from the ammonia–citrate complex proceeds as an activation controlled two-step reduction reaction, with Ni(I) species as an intermediate, which is a general characteristic of the irongroup metals regardless of the species from which metal deposition takes place [37–40]. Increase in current efficiency in that potential range (region II) should be due to increased coverage of the surface with Ni(I). At more negative potentials (region III here) diffusion/reaction control takes over, because of the low concentration of the electroactive nickel ammonia–citrate complex species and low rate of its formation from other complexes [32]. The investigation of citric and ammonia complex of Ni(II) indicated that the type and concentration of the complex compounds depend mainly on pH [41] and ammonia concentration [11–13,32]. The dependences of c.d. on rotation rate of the rde at two constant deposition potentials of 1.27 (region III) and of 1.32 V vs. SCE (region IV) are presented in Fig. 3. The increase of the rotation rate leads to a decrease of the nickel deposition c.d. That, as well as the fact that no well defined

Fig. 2. Current efficiencies (g I) for electrodeposition of nickel and of Ni–W alloys from ammonia–citrate electrolyte as functions of deposition potential (all values are corrected for ohmic drop); Q dep=4 C cm2; x dep=1000 rpm.

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Fig. 3. The effect of rotation rate on partial current densities for nickel deposition and for hydrogen evolution during the potentiostatic deposition of nickel; Q dep=4 C cm2.

limiting current density was found on the polarization curves [32] indicates that the process is not controlled by diffusion. In this case nickel is deposited primarily from the Ni– ammonia–citrate complex [32]. Increase in surface concentration of ammonia arises from splitting of hydrogen from the ammonium ion. After detachment of a proton, released ammonia reacts with Ni–citrate complex to increase concentration of the Ni–ammonia–citrate complex. Such a local increase in the surface concentration of the former, leading to the increase of the nickel deposition rate, should be more pronounced at low rotation rates or at a still electrode (Fig. 3). Similar dependencies of current efficiency on the current density and the electrode rotation rate were obtained for electrodeposition of copper from ammonia–citrate electrolytes. The results were explained considering that the reduction of the cupric complex occurs in two steps coupled by the Cu(I) intermediate being able to diffuse away into the bulk of solution [42]. Other authors explained such rather unusual behavior of nickel by the presence of the surface passivating species [39,40]. Various passivating species as Hads, Ni(OH)ads and Ni(OH)2 are mentioned in the literature [38]. The local pH increase, as the consequence of the hydrogen evolution reaction on growing nickel deposits, would favor the formation of Ni(OH)2 and its subsequent adsorption, arresting further growth. Partial c.d. of hydrogen evolution during nickel deposition at a constant deposition potential of 1.27 V (region II) increases with rotation rate (Fig. 3), while at more negative potentials it slightly decreases with rotation rate. Such an influence of rotation rate (at 1.32 V) on the hydrogen evolution reaction may be explained by the effect of the passivation processes. It is known that the ammonium ions act as the primary donors of protons for the hydrogen evolution reaction. On the other hand, at more negative potential (at 1.32 V) and high cathodic c.d.s one cannot

neglect a participation of water molecules and citric ions [32]. Unlimited supply of water molecules should be expected to cause independence of the hydrogen c.d. on the rotation rate. However, the observed slight decrease in the hydrogen c.d. should be considered further. Increased alkalinity arising upon detachment of hydrogen ions from the water molecules should favor formation of the above mentioned nickel hydroxide passivating species. Stability of such a hydroxide film should be decreasing in increasing presence of complexing ions i.e. ammonia at a still electrode or at low rotation rates. The dependence of the current efficiency of alloy deposition, from ammonia–citrate electrolyte in the presence of tungstate, on the deposition potential is also shown in Fig. 2. It is known that in the pH region between 1.5 and 9.5, tungstate and citrate ions form joint complex species of a formula [(WO4)p (HCit)q Hr ](2p+3qr), where HCit stands for triply deprotonated citric acid (C6H5O73), while p, q, and r can have different values, the r-value increasing with decreasing pH [43]. The protonated tungstate–citrate complexes with more than one proton participate in the electrochemical reduction to lower valence states of tungsten and their concentration is affected not only by pH but also by the presence of the ammonium ions [32]. At potentials more positive than 1.15 V (region I) a layer of mixed tungsten oxides (from W(VI) to W(IV)-oxide) is formed which prevents nickel deposition and catalyzes hydrogen evolution. In region II the lower valence tungsten oxides at the surface still affect nickel deposition to render porous and non-compact nickel deposit. It is seen that current densities for the alloy deposition are much smaller than those for the deposition of pure nickel in the same potential range. It is at the potential at which current efficiency of nickel deposition attains a maximum, that compact deposits are obtained both of nickel alone and of the alloy, the current efficiency of the latter also reaching a maximum (region III). Going more negative than 1.31 V results in a decrease of both current efficiencies (region IV). The effect of the rate of rotation on partial c.d. of nickel deposition and hydrogen evolution is the same as that obtained without the presence of tungstate in solution. Contrary to that the increase of the rotation rate was found to lead to an increase of the partial c.d. of tungsten. The j vs. x 1/2 plot being linear indicates that the reduction of the protonated tungsten–citrate ions is diffusion controlled [32]. 3.2. Influence of pulsating current on alloy composition and deposition efficiency Generally, pc deposition regime provides for an enhancement of mass transfer during the pulse and hence offers the possibility of employment of higher amplitude cathodic c.d., j c, i.e. of attaining higher instantaneous deposition c.d. than in direct current (dc) deposition [33,36,44,45]. The pulse limiting c.d., j pL (defined as the c.d. at which the surface concentration of the reducible ion becomes zero at the end

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of pulse), is higher than the corresponding limiting c.d. under direct current conditions, j L. An exact solution for both mass transport and electrochemical kinetics equations for rde under pc conditions is difficult to obtain [44,45]. A simplified solution of mass transport equation can be obtained by neglecting the convection term and modifying the boundary condition to make it state that concentration of reacting ion attains concentration in the bulk at the distance from the electrode equivalent to twice the Nernst diffusion layer thickness. In that way, Viswanathan et al. [36] found a numerical series solution which gives the ratio of the average limiting c.d. during the pulse to the limiting c.d. for dc deposition 

 jpL ¼ ðjL Þ

1

Pl

Cn n¼1 G

1 ½expðk2n sc Þexpðk2n sÞ ½1expðk2n sÞ

ð1Þ

where C n /G and k n are the coefficient and the eigenvalue of the series in Eq. (1), respectively. Dimensionless cathodic c deposition time, sc ¼ Dt , and dimensionless period, s ¼ DT d2 d2 are defined as the ratios between each of the corresponding pc time variables (t c and T) and the square of the Nernst diffusion layer thickness, d=1.61D 1/3m 1/6x 1/2, D being the diffusion coefficient, m the coefficient of kinematics viscosity and x the electrode rotation rate. The thickness of the pulsating diffusion layer increases during the pulse and the end value depends on the duty cycle, s c/s, i.e. on the ratio of the cathodic deposition time to the pulse period, t c/T. Expressing entire period as sum of deposition (On) time—t c and pause (Off) time—t p, (T=t c+t p), enables calculation of the average c.d., j av, as j c sc jc tc jc jav ¼ ¼ ¼ s tc þ tp 1 þ tp =tc

ð2Þ

Obviously, at one and the same average c.d., a higher t p/ t c ratio (shorter duty cycle) implies a thinner pulsating diffusion layer and higher pulse c.d. The content of tungsten in pc prepared deposits and current efficiency for electrodeposition of the Ni–W alloys are plotted as functions of frequency, f (inverse period, f=1/ T), at t p/t c=1 (in Fig. 4a and c), as well as of the t p/t c ratio, at f=25 Hz (in Fig. 4b and d). The tungsten content in the deposited alloys is found to be in the range between 11 and 25 mol%. These values are smaller than those for the dc deposited alloy, except for the alloy plated at the frequency of 25 Hz and at t p/t c=2. The increase in frequency and in the t p/t c ratio is seen to lead to an increase of the tungsten content of the deposit, except for the cases of the maximum values of frequency and pause to pulse ratio. On the other hand, it is seen, from Fig. 4c and d, that current efficiency for alloy deposition slightly decreases upon increasing frequency and significantly decreases upon increasing t p/t c. Assuming two electrons exchanged per nickel and six per each tungsten ion discharge, partial current densities during

Fig. 4. Tungsten content in the deposits and current efficiency for electrodeposition of Ni–W alloys as functions of: frequency at t p/t c=1 (a and c), and the t p/t c ratio at f=25 Hz (b and d), respectively; j av=70 mA cm2; Q dep=21 C cm2; x dep=1000 rpm.

the pulse, j p, can be calculated from the data given in Fig. 4, using the following equations: j ðWÞp ¼

 3gI jav xðWÞ  1 þ tp =tc ð1 þ 2xðWÞÞ

ð3Þ

jðNiÞp ¼

 gI jav ð1  xðWÞÞ  1 þ tp =tc ð1 þ 2xðWÞÞ

ð4Þ

  jðH2 Þp ¼ ð1  gI Þjav 1 þ tp =tc

ð5Þ

The calculated values of partial current densities for pc deposition, j p(i), divided by the corresponding partial current densities in dc deposition, j p(i)/j dc(i), are shown in Fig. 5 as functions of the dimensionless pulse period, s (at t p/t c=1), for each of the species (nickel, tungsten and hydrogen). The same partial current ratios, j p(i)/j dc(i), are plotted in Fig. 6 against t p/t c ratio, at s=0.142 ( f=25 Hz). Theoretical values of j pL/j L calculated using Eq. (1) with coefficients C n /G and eigenvalue k n for n up to 10 [36] are also given in Figs. 5 and 6. Characteristic values of kinematics viscosity coefficients (m=1.0106 m2 s1), and diffusion coefficient (D=6.91010 m2 s1) for reducible ions of tungsten in the ammonia–citrate electrolyte [8] were taken for calculations. The diffusion coefficient for nickel ions in neutral solution of D=6.71010 m2 s1 was assumed [39,40]. It should be noted that these authors state

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Fig. 5. Partial pulse c.d.s of deposition of both metals and of hydrogen evolution divided by corresponding partial c.d.s in the dc deposition, as functions of frequency or dimensionless pulse period, s (at t p/t c=1): dots— obtained from experimental results and calculations according to Eqs. (3)– (5) and full line—theoretical values of the ratio, j pL/j L, calculated using Eq. (1).

that D value may be affected by formation of a porous hydroxide layer due to pH changes. It may be seen from Figs. 5 and 6 that the calculated current ratios for tungsten are in a fair agreement with the theoretically predicted in both cases, when plotted as function of dimensionless pulse period as well as of the t p/t c ratio. That confirms that the tungsten deposition process is controlled by mass transport. The deviation from the assumed model is obtained only at the highest t p/t c ratio, at which compact deposit is still obtained despite the fact that the amplitude c.d. exceeds 400 mA cm2. The deviation is probably a consequence of a decrease of the surface concentration of the electroactive protonated tungstate– citrate complex caused by the increase of surface pH due to the application of high c.d. in the pulse. It is known that an increase of pH leads to a decrease of the concentration of the electroactive protonated tungstate–citrate complex [43]. However, there is also a possibility that increased surface pH causes noticeable passivation of the surface, which hinders the process in a similar manner as already discussed for dc hydrogen evolution along the deposition of pure nickel (c.f. Section 3.1). Here, in the case of pc, deactivation may proceed not only during the deposition time (t c), but in addition for the period of the relatively long pause (absence of polarization for t pN30 ms). The calculated ratios j p(Ni)/j dc(Ni) plotted against the t p/ t c ratio (Fig. 6) are also in fair agreement with the theoretical predictions for a process controlled by mass transport. However, the calculated ratios for nickel increase with dimensionless period (Fig. 5) at constant cathodic current in the pulse, while the theoretical value decreases. This indicates that deposition of nickel is more complex than would be if the process was controlled by plain mass transport. This result, obtained under the applied pc regime,

is in line with uncommon effect of the electrode rotation rate on dc potentiostatic deposition of nickel (Fig. 3) as well as with those obtained in our earlier study of Ni–W alloy deposition from ammonia–citrate electrolyte [32]. The decrease in frequency (increase in dimensionless period in Fig. 6) as well as the decrease in the electrode rotation rate (Fig. 3) should both lead to a decrease of the mass transport of nickel species. However, growth of the diffusion layer also leads to simultaneous local increase in concentration of ammonia which reacts with Ni–citrate to increase concentration of reducible Ni–ammonia–citrate complex. The presence of ammonia should also disturb the stability of the surface oxide species. Hence, it appears that the ammonia governs the nickel deposition process. The calculated values of the ratio j p(H2)/j dc(H2) according to the Eq. (5) are presented as a function of dimensionless pulse period (Fig. 5), although the reaction of hydrogen evolution during dc nickel and alloy deposition is not controlled by mass transport. However, the dependence of the ratio on the dimensionless pulse period shows a relatively slow decrease of the hydrogen evolution current with increase of that variable (decreasing of frequency). This is similar to the effect of the rotation rate (Fig. 3) and points out to passivation processes. Considering that the pc parameters affect the partial currents of tungsten and nickel deposition in a different manner, one can conclude that the two processes are independent of each other or, in terms used by Landolt [46], that the codeposition of nickel and tungsten from ammonia– citrate electrolyte is a non-interactive codeposition. The presence of passivating species on the surface affects nickel deposition but does not significantly affect deposition of tungsten. It is possible that the protonated tungstate–citrate complexes react with oxide species of nickel forming a joint complex from which tungsten is reduced. This is in line with the work of Landolt et al.

Fig. 6. Partial pulse c.d.s of deposition of both metals divided by corresponding partial c.d.s in the dc deposition, as functions of the pause to pulse ratio, t p/t c, (at s=0.142, f=25 Hz): dots—obtained from experimental results and calculations according to Eqs. (3)–(5) and full line—theoretical values of the ratio, j pL/j L, calculated using Eq. (1).

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[18–20] on reduction of molybdenum in which they concluded that the reaction occurs in two steps via an intermediate to which the formula [NiCitMoO2] ads is tentatively assigned. At the same, time nickel deposition proceeds independently of molybdenum. However, the process of formation of that complex is very fast so that it is difficult to record it. In literature, however, there is also an opinion that in the bulk of solution there is a presence of a ternary complex [Nip2+(WO42)q (HCit3)r ]2( pq1,5r) [11–13], from which tungsten is deposited as metal. The effect of pulse parameters on the structure and morphology of Ni–W alloys deposits is subject current investigation and will be presented at another instance.

4. Conclusions The tungsten content in alloys electrodeposited by pc from the ammonia–citrate electrolyte, within the range of parameters used, was found to be between 11 and 25 mol%. The comparison of the obtained effect of the pc parameters on partial current densities of nickel and tungsten deposition with the effect predicted in theory for the pc deposition controlled by mass transport indicates the following. The nickel is deposited as the two-step reduction reaction under a reaction/diffusion control, because of local concentration of ammonia (arising from splitting of hydrogen from the ammonium ion) resulting in a change in surface pH, strongly influencing the concentration of the nickel electroactive species. Under the conditions of alloy deposition the reduction of tungstate is proven to be a diffusion controlled reaction. The obtained results indicate that, during deposition of the alloy, partial currents of tungsten and nickel deposition are independent of each other, which indicates a noninteractive codeposition. At low overpotentials a layer of tungsten oxides formed at the electrode surface prevents nickel deposition and catalyzes hydrogen evolution.

Acknowledgements Financial support for this work by the Ministry of Science and Environmental Protection of Serbia (Contract No. H-1821) is gratefully acknowledged.

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