Pulse electrodeposition and characterization of nano Ni–W alloy deposits

Applied Surface Science 259 (2012) 231–237

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Pulse electrodeposition and characterization of nano Ni–W alloy deposits K. Arunsunai Kumar, G. Paruthimal Kalaignan ∗ , V.S. Muralidharan Advanced Nano Composite Coatings Laboratory, Department of Industrial Chemistry, School of Chemical Sciences, Alagappa University, Karaikudi 630003, Tamil Nadu, India

a r t i c l e

i n f o

Article history: Received 20 February 2012 Received in revised form 5 July 2012 Accepted 5 July 2012 Available online 27 July 2012 Keywords: Nano deposits Ni–W electrodeposits Pulse plating Induced co-deposition Corrosion resistance

a b s t r a c t Pulse plating was employed to obtain Ni–W alloy nanodeposits from ammonical citrate bath. Effects of peak current density, pH, tungstate ion concentration and temperature on the cathodic current efficiency were studied. Ni-32.8 weight (wt)% of W alloy coating was obtained using optimized bath composition. Ni–W solid solution was found in the alloy deposit. Surface morphology studies revealed that the alloy surface was covered by long needle like crystals. XRD patterns of the electrodeposits showed only fcc (nickel) peaks. The hardness of the deposits obeyed Hall–Petch relation. Electrochemical polarization studies revealed that the corrosion potentials of nickel became nobler with W addition and corrosion current density values decreased. Electrochemical impedance studies found that, the increased charge transfer resistances suggesting decreased corrosion current density values. The capacitance values of the double layer decreased with W additions. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction Ni–W nanostructured alloy coatings are known to exhibit superior mechanical and chemical properties than nickel coatings. In 1947 Brenner et al. [1] deposited Ni–W alloys from alkaline sodium citrate bath. Using the same plating bath in 1998, nano crystalline Ni–W alloys were deposited [2]. Direct current plating was used to deposit the nanocrystalline Ni–W deposits [3–11]. At high temperatures, Ni–W and NiW2 alloys were also deposited with increased cathodic current efficiency [12,13]. Crystalline sizes decreased from 7 to 2.5 nm when the wt% of W increased in the alloy. Many plating baths were alkaline (pH 7–9) with the addition of complexing agents such as sulphamate, ammonical citrate [8–14]. Pulse and pulse reverse plating techniques and its applications were reviewed [15]. Using sodium citrate as a complexing agent [16], Ni–W alloy coating was deposited by pulse plating method. A range of 65–140 nm grain sizes were obtained for 15–30 at.% of W in the alloy, when the pH of the bath was adjusted to 7. Ni17 W3 phase was observed in the Ni–W alloy, when the pulse plating bath pH at 6. Crystalline sizes of 11–23 nm were seen in the alloy deposits [17]. Corrosion resistance behaviour of Ni–W electrodeposits was studied in NaCl and H2 SO4 solutions. In the nanocrystalline Ni–W alloy, as the grain size decreased to 15 nm, corrosion current densities were decreased in 3.5% NaCl solution and increased again with the decrease in crystal size upto 5 nm [6,10,11,18,19]. Standard

∗ Corresponding author. Tel.: +91 9443135307; fax: +91 4565 225202. E-mail address: [email protected] (G. Paruthimal Kalaignan).

neutral salt spray tests on nanocrystalline Ni–W coatings demonstrated that, it can effectively shield the base metal steel [20]. Studies on reverse pulse electrodeposited nanocrystalline Ni–W alloy in 3.5 wt% NaCl at pH 3 and 10 revealed that the corrosion rate of the alloy increased with the reduction of grain size in alkaline solution but decreased with the reduction in grain size in acidic solutions [21]. The present communication deals with the pulse electrodeposition of nano Ni–W alloy electrodeposits from citrate complexed alkaline bath (pH 9). The electrodeposits were characterized to analyse their surface morphology and surface structure by using Scanning electron microscopy and XRD techniques. Hardness measurements were also carried out on the electrodeposits. Corrosion resistance properties were evaluated by using electrochemical measurements in 3.5% NaCl solution at pH 7.

2. Experimental details Cold rolled mildsteel plates were polished with fine grid paper, degreased with trichloroethylene, cathodically electrocleaned in alkaline solution for 2 min and anodically for 30 s in a solution containing a mixture of 35 g/L NaOH and 25 g/L Na2 CO3 at 30 ◦ C. They were washed in running water and given a dip for 10 s in 5% H2 SO4 solutions. Pulse electrodeposition was carried out using a pulse rectifier (Komal Agencies Mumbai). The prepared mild steel plates (4 × 2.5 × 0.05 cm) were used as cathodes. The pure nickel plate of size 4 × 2.5 × 0.4 cm was used as anode. All chemicals used were of AnalaR grade. The hardness of the electrodeposits was measured by using MHG Everyone Hardness Tester (Hong Kong) on the Vicker’s

0169-4332/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.024

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scale. It had a diamond pyramid of a square base with an angle of 136◦ at the vertex between two opposite faces. The microhardness of the deposits in Kg/mm2 was determined in each case by using the formula HV = 1854 ×

L d2

(1)

where L is the load applied in gm and d the diagonal of the indention (␮m). The normal indentation to the growth surface was noticed. The possible influence of the substrate to the measurement was checked and found to be nil. Scanning electron microscope (SEM) (HITACHI S-570, Japan) was used to characterise the surface morphology of the composite coatings. The deposits were viewed at 20 kV with 30,000 magnifications. For these studies, the electrodeposited panels were cut into 1 cm × 1 cm size, cold mounted, examined and photographed. The deposited surface was subjected to EDAX analysis (relative ratio’s) for the determination of chemical composition of the deposits. The crystalline structure of the plated substrate was identified by X-ray diffraction using Brooker D8 advance X-ray diffractometer operated at Cu K␣ radiation (nickel filtered) at a rating of 40 kV, 20 mA. The scan rate was 0.05◦ per step and the measuring time 15/step. The crystalline size was determined by using the Scherrer equation [22]. D=

0.9 ˇ cos 

(2)

˚ where D is the crystalline size,  is the incident radiation (1.5418 A); ˇ is the corrected peak width at half-maximum intensity and  is the angular position. The cathodic current efficiency of the alloy was calculated as Cathodic current efficiency (%) =

M × 100 ealloy × Q

(3)

where M is the mass of the alloy deposit, ealloy is the electrochemical equivalent of the alloy and Q is the quantity of electricity passed (A/s). The electrochemical equivalent of the alloy was calculated as ealloy

eNi × eW = (eNi × fW ) + (eW × fNi )

(4)

where eNi and eW are the electrochemical equivalents of the constituent metals, fNi and fW are their fractions in the deposits. The density of the alloy was calculated by the consideration of the fraction of constituent metals [23]. The electrodeposits were removed chemically by immersing in 1:1 HNO3 . Then the resulting solution containing nickel and tungsten were determined by using atomic absorption spectroscopy (AAS) (model: Perkin Elmer precisely AAnalyst 800). Electrochemical measurements were carried out using a three electrodes cell assembly. The exposed 1 cm2 area mild steel was used as working electrode. A rectangular size platinum foil and a saturated calomel electrode were used as auxiliary and reference electrodes respectively. The test solution was 3.5% NaCl solution kept at 30 ◦ C. Electrochemical polarization studies were carried out by using Eco-chemie-Potentiostat galvanostat (Auto Lab). Electrochemical impedance measurements were done on the electrodeposits after they attained a steady corrosion potential. The impedance measurements were made in the frequency range of 105 cycles/s to 10 millicycles/s with a sinusoidal perturbation of 10 mV. 3. Results 3.1. Electrodeposition of alloys Table 1 summarises the bath composition and operating conditions of the electrodeposition. Ni–W alloys were electrodeposited on mild steel from ammonical citrate complexing bath. Increasing

Table 1 Composition and conditions of plating bath. Chemicals

Bath composition

Plating conditions

Nickel sulphate Sodium tungstate Tri-ammonium citrate (TAC) Ammonium chloride Dimethyl sulphoxide (DMSO) Sodium lauryl Sulphate (SLS) 2-Butyne 1,4-Diol (BD)

0.15 M 0.15 M 0.30 M 0.20 M 0.06 M 0.80 g/L 50 mg/L

Pulse peak c.d. 3 A/dm2 pH 9 Time 40 min Temperature 70 ◦ C Constant stirring Duty Cycle; on time-50 ms Off time-50 ms

Table 2 Effect of cathodic peak current densities on the % of CCE and % of W in the electrodeposits. Pulse peak current density (A/dm2 )

% CCE

% of W

1 2 3 4

13.25 17.53 50.81 48.75

14.05 24.31 32.25 30.55

Bath composition: NiSO4 0.15 M, Na2 WO4 0.15 M, TAC 0.3 M, DMSO 0.06 M, NH4 Cl 0.2 M, SLS 0.8 g/L, BD 50 mg/L, pH 9, temp 70 ◦ C, time 40 min.

Table 3 Effect of sodium tungstate on % of CCE and % of W in the electrodeposits. Conc. of Na2 WO4 (M)

% CCE

% of W

0.05 0.10 0.15 0.20

11.22 18.45 31.66 29.66

4.09 24.68 32.83 29.06

Bath composition: NiSO4 0.15 M, TAC 0.3 M; DMSO 0.06 M, NH4 Cl 0.2 M, c.d. 3 A/dm2 , SLS 0.8 g/L, BD 50 mg/L, temp 70 ◦ C, time 40 min.

Table 4 Effect of bath pH on % of CCE and % of W in the electrodeposits. pH

% CCE

% of W

7 8 9 10

4.271 9.835 20.82 19.52

16.68 25.78 31.20 28.75

Bath composition: NiSO4 0.15 M, Na2 WO4 0.15 M, TAC 0.3 M, DMSO 0.06 M, NH4 Cl 0.2 M, SLS 0.8 g/L, BD 50 mg/L, c.d 3 A/dm2 , temp 70 ◦ C, time 40 min.

of pulse peak current density from 1 to 4 A/dm2 has increased the % of CCE and % of W in the alloy (Table 2). Beyond 3 A/dm2 , there was only a marginal decrease in % of W in the alloy. Sodium tungstate concentration was changed from 0.05 M to 0.20 M. W alloy was found to be maximum at 0.15 M sodium tungstate concentration in the solution (Table 3). The source of OH− ions was due to the increase of pH in the solution. The % of W in the alloy was found to be 31.20 at pH 9 and then marginally decreased (Table 4). The plating bath temperatures were increased to enhance further mass transport. Increase of temperature has increased the % of CCE and % of W in the alloy (Table 5). The optimized bath composition and conditions of the plating were evaluated. 3.2. Characterization of the deposit 3.2.1. X-ray diffraction The X-ray diffraction patterns for Ni as well as Ni-32.8 wt% of W alloys are presented in Fig. 1. These patterns are having crystalline and fcc structure. The planes, (1 1 1), (2 0 0) and (0 1 2) were predominant for Nickel and (1 1 1), (2 0 0), (2 2 0) planes for Ni–W alloys. JCPDS standards (01 – 087–0712, 65 – 4828) have confirmed the patterns. A similar confirmed XRD pattern has also

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Table 5 Effect of bath temperature on % of CCE and % of W in the electrodeposits. Temperature (◦ C)

% CCE

% of W

40 50 60 70 80

4.32 6.07 28.05 42.42 43.27

7.26 12.01 21.22 30.88 30.96

Bath composition: NiSO4 0.15 M, Na2 WO4 0.15 M, TAC 0.3 M, DMSO 0.06 M, NH4 Cl 0.2 M, SLS 0.8 g/L, BD 50 mg/L, c.d 3 A/dm2 , pH 9, time 40 min.

Fig. 1. XRD Patterns observed for various compositions of Ni–W electrodeposits.

been observed for the electrodeposited Ni–W composite coatings [24]. The crystallite sizes of nickel and its alloys were calculated using Scherrer equation. The calculated crystallite sizes of the nickel deposits were 150.69 nm for (1 1 1), 58.63 nm for (2 0 0) and 79.83 nm for (0 1 2) planes. The crystallite sizes of the alloy deposits were 110.98 nm, 23.55 nm and 37.91 nm corresponding to (1 1 1), (2 0 0) and (2 2 0) planes. The addition of W was reduced the crystallite sizes of all the predominant planes. The crystallite sizes were of the order of nm. 3.3. Scanning electron microscopy When viewed at 30,000 × magnifications, the surface was found to be smooth, uniform and the surface was covered with fine crystallites (Fig. 2a and b). The surface of the alloy was compact and needle shaped grain size as observed in Fig. 2b. A similar morphology was obtained at current densities in the diffusion-controlled region [25]. It was evident that, the W uniform distribution of W over the nickel matrix. The alloy deposits were exhibited as smooth and compact surfaces compared to pure nickel. The EDAX analysis of Ni–W alloy coatings has offered the elemental percentage of Ni as 66.74%, W as 30.99% and O as 2.26%. This confirmed the presence of tungsten in the alloy matrix.

Fig. 2. (a) SEM for electrodeposited Ni. (b) SEM for electrodeposited Ni-32.8% W alloy.

3.5. Electrochemical characterization 3.5.1. Potentiodynamic polarization studies Potentiodynamic polarization studies were carried out at a potential of ±400 mV away from the OCP at scan rate of 5 mV/s. Potentiodynamic polarization curves obtained for various composition of Ni–W electrodeposits are given in Fig. 3. The linear segments of anodic and cathodic polarization curves were extrapolated to corrosion potentials to determine the corrosion current densities. The slopes of the linear segments of anodic and cathodic polarization curves gave anodic and cathodic Tafel slopes. The addition of W in the electrodeposits has caused the corrosion potentials to become nobler. Corrosion current densities have decreased with increase of % of W in the alloy (Table 6).

Table 6 Parameters derived from potentiodynamic polarization curves for various compositions of Ni–W electrodeposits.

3.4. Microhardness

Electrodeposits

Ecorr mV vs SCE

The microhardness of the electrodeposits (kg/mm2 ) was determined. The alloy was found to have higher microhardness than nickel. The increase in wt% of W in the alloy has increased the hardness of the deposits.

Ni Ni + 4.1 wt% W Ni + 25.7 wt% W Ni + 32.8 wt% W

−525 −498 −456 −398

Tafel slopes mV/decade Anodic

Cathodic

70 85 90 70

75 90 95 90

icorr (␮A/cm2 )

18.02 6.13 5.93 1.86

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0 wt % W 4.1 wt % W 25.7 wt % W 32.8 wt % W

-2

-3

log z (z / ohm cm-2 )

3.0

-4 i / A cm-2

0 wt % W 4.1 wt % W 25.7 wt % W 32.8 wt % W

3.5

-5

-6

2.5

2.0

1.5

-7

1.0

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1 0.5

E/V

-1

0

1

2

3

4

5

log f (f / frequency)

Fig. 3. Potentiodynamic polarization curves obtained for various compositions of Ni–W electrodeposits in 3.5% NaCl solutions.

Fig. 5. Bode plots obtained for various compositions of Ni–W deposits.

Fig. 4. Nyquist plots (Z vs Z ) obtained for various composition of Ni–W electrodeposits.

Fig. 6.  vs log f plots for various composition of Ni–W electrodeposits.

3.5.2. Electrochemical impedance studies Electrochemical impedance spectroscopy was measured by applying an AC potential to an electrochemical cell and measuring the current through it. The real part of the impedance (Z ) and the imaginary part of impedance (Z ) were plotted in X-axis and Y-axis respectively. (Nyquist plot). Fig. 4 presents the Nyquist plots obtained for various compositions of Ni–W electrodeposits. Increase the wt% of W in the alloy caused an increase in the impedance at all frequencies and an inductive loop was also observed. The real part of the impedance (Z ) was observed at

highest frequency corresponds to RS + Rct (solution resistance + charge transfer resistance) and the impedance at very low frequency corresponds to Rct . Fig. 5 presents the Bode plots obtained for various composition of Ni–W electrodeposits. The plots of  (phase angle) vs log f obtained for various compositions of Ni–W electrodeposits are shown in Fig. 6. Two separate humps were seen for 4.1% W alloy. For other alloy deposits, the two humps were merged to give a single hump. Table 7 has summerised the parameters derived from electrochemical impedance measurements. Both Bode and Nyquist methods have given higher

Table 7 Parameters derived from electrochemical impedance spectra for various compositions of Ni–W electrodeposits. Electrodeposits

Ni Ni + 4.1 wt% W Ni + 25.7 wt% W Ni + 32.8 wt% W

Rct ( cm2 )

icorr (␮A/cm2 )

Cdl (␮F/cm2 )

Nyquist plot

Bode plot

Nyquist plot

Bode plot

Nyquist plot

Bode plot

715 970 1877 2569

782 952 1202 2483

21.9 19.56 10.69 6.65

20.1 19.93 16.69 6.88

206 136 62 45

188 139 97 47

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Rct values. Capacitance of double layer decreased for the alloy compared to nickel and increase of wt% of W in the alloy decreased Cdl values. The increase of W addition offered the alloy deposits enhanced corrosion resistance. 4. Discussions Metal tungstates like NiWO4 were formed from solutions containing divalent cation complexes of tungstates. Complexes of tungstates with citrates, oxalates and tartarates are known [26,27]. The complexes have the general formula [(WO4 )(Cit)(H)x ]x−5 where (cit) stands for triply deprotonated anion of citric acid concentration; x is the number of protons. The electrochemical reduction of WO4 2− ion occurs as WO4 2− + 8H+ + 6e− → W + 4H2 O

(5)

The standard potential is given by [28] E rev = 0.049 − 0.0788 pH + 0.0098 log [WO4 2− ]

(6)

AtpH9, [WO4 2− ] = 0.1 M,E rev = −0.679 VvsSHE

(7)

2− ]

The reduction of [WO4 to W depends mainly on potential. At pH 9, the complex may be of the form [(WO4 )(Cit)(H)]4− . This complex undergoes reduction to W alloy as [(Ni)(HWO4 )(Cit)]2− + 3H2 O + 8e− → NiW + 7(OH− ) + Cit3− (8) (or) [(Ni)(Cit)] + [(HWO4 )(Cit)]4− → [(Ni)(HWO4 )(Cit)]2− + Cit3−

(9)

In the present study, the deposited Ni–W alloy had higher nickel concentration in the presence of NH4 Cl at a pH range between 7 and 9. The % of CCE was increased with pH and then degreased. In NH4 Cl solutions, it existed an equilibria NH4 OH  NH4 + + OH−

(10)

The pKa value is 9.25. This shows that at pH 9, 36% of NH4 OH was remained undissociated. 67% of NH4 + ions were available for complexation. In the Ni–W alloy deposition, the reduction of [(Ni)(HWO4 )(Cit)]2− required higher current densities and higher pH. This explains the observed higher % of CCE with the increase of pH up to 9. The increase of mass transport by stirring was also favoured deposition. It was also increased the availability of the nickel complex at the interface. Thus the electrodeposition of Ni–W alloy occurred from a complex containing nickel and tungstate ions stabilized by citrates. In presence of NH4 Cl, the nickel ammonical citrate was formed. The formation of ternary complex containing ammonia was favoured the alloy formation. In metallurgically obtained Ni–W alloy, the dominant phase is a solid solution of W in nickel [29]. However, it is well known that the phase structure of electrodeposited alloys may significantly differ from that of alloys obtained by thermal equilibrium. Noncrystalline and amorphous Ni–W alloy deposits were also observed (loc.cit). Homogenity in the Ni–W alloy was ascribed to solid solution of W in nickel [30,31]; intermetalic phases were difficult to detect [32]. In the complex process of alloy formation by induced electrodeposition, different non-equilibrium structures and species are also likely to occur. In the present study, X-ray diffraction patterns have revealed only fcc nickel. Binary alloy phase diagram of Ni–W has revealed the formation of solid solution in nickel matrix up to 13 atomic wt% of W. Beyond 13 atomic wt% of W, metastable solid solution between Ni4 W and Ni were reported earlier [12]. In earlier study [17], it was reported that the appearance of 3 peaks at 50◦ , 60◦ and 90◦ . 2 in the diffractograms of Ni–W coating was revealed to the Ni17 W3 phase. It was also confirmed using Vegard’s law.

235

In the present study, there is a possibility of co-existence of non-equilibrium solution along with the meta stable solid solution in the W alloy. The observed planes of (1 1 1), (2 0 0) and (2 2 0) were suggested the existence of Ni17 W3 phase. In an earlier study [33], the diffraction peak appeared at 50.4◦ was related to Ni4 W phase. This diffraction peak was not observed in the present study. There are reports that, the Ni–W alloy coating consisted of amorphous and crystalline phases [34,35]. The intensity of (1 1 1) peak was greater than the other planes in Ni–W alloy suggesting the existence of (1 1 1) texture. The development of this texture was associated with the preferred growth along with (1 1 1) orientation, because of lower strain associated in this direction. The Bragg peaks were broadened in the alloy compared to pure nickel. This broadening was increased with wt% of W in the alloy. This is due to the reduction in crystallite size with W addition. The formation of non-crystalline (amorphous) structure was not favoured as seen from the appearance of well-defined sharp peaks. Surface morphology of electrodeposited nickel and Ni–W alloy revealed that the surface of nickel deposits was smooth with uniform distribution of finer grains. On the alloy surface, long needle shaped grain sizes were also seen. The grain size was in the range between 50 nm and 80 nm. The EDAX analysis has revealed the % of W in the alloy as 30.99%. Hardness of a deposits either bulk or coating becomes a vital parameter governing the mechanical properties of the materials. The Hall–Petch (H–P) mechanism governs the hardening phenomenon in microcrystalline material. The Hall–Petch equation relates the hardness H of a metal grain size as H = H0 +

KH (d)

1/2

(11)

where, H0 and KH are experimental constants, which vary with metal to metal. H0 is the value characteristic of dislocation blocking and is related to the friction stress. KH takes account for the penetrability of the boundaries to moving dislocations and is related to the number of available slip systems. For nano nickel and nickel based nano crystalline alloys until a limited size (14 nm for nickel), the H–P relation was obeyed [36]. For Ni–W alloys deposits obtained from modified citrate bath, a breakdown of H–P relationship was seen for grain sizes < 8 nm (loc cit). When boric acid addition, the break down was reduced from 8 to 4.5 nm. Microhardness of the electrodeposits was found to increase with W incorporation. It was observed for electrodeposited nickel [37], the Vickers hardness increased from 140 to 650 as the grain size reduced from 10 ␮m to 10 nm. Though H–P relation was followed (Fig. 7) the behaviour was non-linear. In the case of nanocrystalline alloy, two different factors may be responsible for strengthening namely solid solution strengthening and grain boundary hardening. It was shown that, the solid solution strengthening effect of W was about an order of magnitude smaller than this intrinsic hardness of nickel and much smaller than the grain size distribution [38]. It was shows that, solid solution strengthening was not responsible but grain structure contributes enhanced microhardness. The observed non-linear H–P relation ship may be due to the increase in grain boundary content in the alloy and the dislocation behaviour had little influence on the yield stress of the crystal. For the finest nanostructured metals, the deformation mechanism remains unclear although grain boundary sliding [39] or Coble Creep [40] had been postulated. From the electrochemical theory of corrosion, it is known that, corrosion current densities are obtained by extrapolation of linear segments of anodic and cathodic polarization curves to corrosion potentials. Stern and Geary [41] equation correlates the

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600

(b)

550 500 450

Hardness (Kg/mm 2 )

400 350 300 600

(a) 0.06

550

0.07

0.08

(Crystalline Size)

-1/2

0.09

(nm)

0.10

0.11

1/2

500 450 400 350 300 -5

0

5

10

15

20

25

30

35

Wt% of W Fig. 7. Hall–Petch plot obtained from hardness measurements for (a) electrodeposited Ni. and Ni–W alloy (b) crystalline size of Ni and Ni–W alloy corresponding to (1 1 1) XRD diffraction peaks.

potential current relationship close to corrosion potential under linear relationship conditions. Rct = Charge transfer resistance Rct

ba bc K (12) = = 2.303(ba + bc )icorr icorr

where ba , bc are the anodic and cathodic Tafel Slopes. In the present study, higher Rct values were observed from impedance measurements with increased wt% of W in the alloy. This suggests that the addition of W to nickel has improved the corrosion resistance. Based on the impedance measurement data, experimentally calculated icorr values were decreased with addition of W (Table 7). Potentiodynamic polarization was carried out at a scan rate of 5 mV/s. The electrodeposits were subjected to polarization on either side and the surface of the deposits underwent changes. This surface may not be the same surface as observed in open circuit potential conditions. Corrosion current densities were calculated by extrapolation of the linear segments on the polarization curves to Ecorr (OCP). Electrochemical impedance measurements were made at open circuit potential with a perturbation of ±10 mV. The Rct and Cdl values were obtained corresponding to that of the surface at Ecorr (OCP). Hence, icorr values obtained from both the methods did not agree; however, the observed values are in the same order. Increase of W content has decreased the icorr values in both the methods. Cdl values obtained from impedance measurement studies were nearly agreed with each other. Cdl values were decreased with wt% of W addition in the alloy. The impedance spectra of Ni–W alloy deposits were different from that of the pure nickel deposits. The Nyquist plots for Ni–W alloy containing 25.7 wt% of W and 32.8 wt% of W had exhibited inductive behaviour. The Bode plots for Nickel and   Ni–W alloys revealed that up to 4.1 wt% of W the slopes of log Z  vs log f were same in the frequency range of 1–200 c/s. The deposit containing 25.7 wt% of W exhibited a different  slope than pure nickel. Two linear segments are seen in the log Z  vs log f plot for 32.8 wt% of W alloys in the frequency ranges of 5 × 10−1 c/s to 5 c/s and 10–100 c/s. The phase angle  vs log f graphs revealed that, two humps for pure nickel electrodeposits were suggesting two different time constant processes. For pure nickel one capacitive behaviour was seen. For 4.1 wt% of W alloy, two capacitive peaks were seen. These merged into a broad peak covering 5 orders of frequency were seen for

higher W alloys. A simple Randle’s equivalent circuit consists of solution resistance, which is in series with double layer capacitance. This double layer capacitance is parallel to charge transfer resistance. The present results suggested that, the above Randles equivalent circuit is not applicable to this alloy corrosion. The low frequency loop may be inductive or capacitive. If there is an adsorbed intermediate species covering the surface, then that situation can be represented as an inductance [42]. The low frequency inductive loop observed in the Nyquist plots is due to the precipitated Ni(OH)2 . The Ni2+ ions were formed on the surface by the dissolution of the metals from the grain boundaries reacted with OH− ions. The adsorbed (NiOH)ads intermediate was responsible for the charge storage behaviour and the appearance of the inductive loop [43,44]. The appearance of two time constants for the alloy in the Bode plots suggest that, the surface of alloy was covered by the precipitated corrosion product. The Ni–W alloy had Ni17 W3 phase, which was nobler than pure nickel [17]. This caused the dissolution of nickel from the alloy. The decrease of Cdl values was seen when the wt% of W increased in the alloy. Increase of W addition has favoured the formation of Ni17 W3 phase and dissolution of nickel. The adsorbed Ni(OH) caused a decrease in Cdl values. The impedance diagrams of two capacitative responses with very similar frequency characteristics were reported for galvanized steel covered with thin films of chromates [45] and molybdates [46]. 5. Conclusions Ni–W electrodeposits were obtained by using pulse plating method from ammonical citrate bath. The optimized plating bath have given alloy electrodeposits with 32.8 wt% of W. The alloy was obtained from the reduction of nickel tungstate complex containing citrates. The addition of NH4 Cl favoured the ternary complex containing ammonia. The Ni–W alloy deposits surface were found to be homogeneous and had a solid solution of W in nickel. The structure of the electrodeposits revealed only fcc peaks. Surface morphology revealed that the surface of nickel deposits was smooth with the distribution of finer grains. On the alloy surface, long needle shaped grains were seen. SEM photographs for the Ni–W alloy deposit grain size were in the range of 50–80 nm. Addition of W in the alloy increased the hardness and decreased the crystallite size. Hardness variation with crystal size did not follow linear Hall–Petch relationship. Corrosion resistances of nickel deposits increased with W addition. Acknowledgements The authors thank CSIR, New Delhi for awarding CSIR Emeritus Scientist Scheme. The authors have also acknowledged the Head of the Department of Physics, Alagappa University, Karaikudi for the assistance rendered in XRD measurements. References [1] A. Brenner, P. Burkhead, E. Seegmiller, Journal of Research of the National Bureau of Standards 39 (1947) 351–383. [2] T. Yamasaki, P. Schlossmacher, K. Ehrlich, Y. Ogino, Nanostructured Materials 10 (1998) 375–388. [3] M. Donten, H. Cesiulis, Z. Stojek, Electrochimica Acta 45 (2000) 3389–3396. [4] O. Younes, E. Gileadi, Electrochemical and Solid-State Letters 3 (2000) 543–545. [5] O. Younes, L. Zhu, Y. Rosenberg, Y. Shacham-Diamond, E. Gileadi, Langmuir 17 (2001) 8270–8275. [6] M. Obradovi, J. Stevanovi, A. Despi, R. Stevanovi, J. Stoch, Journal of the Serbian Chemical Society 66 (2001) 899–912. [7] O. Younes, E. Gileadi, Journal of the Electrochemical Society 149 (2002) C100–C111. [8] M. Donten, Z. Stojek, H. Cesiulis, Journal of the Electrochemical Society 150 (2003) C95–C98. [9] K.R. Sriraman, S. Ganesh Sundara Raman, S.K. Seshadri, Materials Science and Engineering A 418 (2006) 303–311.

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