Influence of electrolyte pH on composition,corrosion properties and surface morphology of electrodeposited Cu

Abstract

Cu–Ni alloys were electrodeposited by the brush plating technique from sulphate/citrate electrolyte at various pH values. The effects of pH on composition and surface morphological properties of alloys were investigated by X-ray fluorescence and atomic force microscopy. The Cu content increased in the composition of Cu–Ni alloy at electrolyte pH 2. The deposition mechanisms of Cu, Ni and Cu–Ni were investigated by cyclic voltammetry. The lower electrolyte pH leads to the higher reduction current density. The corrosion behaviours of the deposits at different pH values were investigated by Tafel analysis and electrochemical impedance spectroscopy. The corrosion studies indicate that the Cu–Ni alloy exhibits better corrosion resistance at higher pH values than at lower pH values. The surface roughness decreases with increasing solution pH and this observation was confirmed by atomic force microscopy measurements.

Keywords

Deposition CV Potentiostatic Corrosion Atomic force microscopy

Introduction

Electrodeposition 1 1,2 of Cu–Ni alloy has attracted widespread interests because of their physical, chemical and mechanical properties. Cu–Ni alloys are commonly used in various sea water applications such as valves, fittings condenser and heat exchanger tubes. In addition, these alloys possess good machinability, and excellent thermal and electrical properties and are particularly resistance to biofoulings. Application of this alloy in industrial practice has since made a number of researchers contribute their work to investigate Cu–Ni alloy with emphases on their structural, compositional and morphological characteristics. The influence of electroplating parameters on alloy composition and structure has been extensively studied, and mechanism for alloy formation has been proposed earlier. 3 3,4 Electrochemical behaviours of Cu–Ni alloys were extensively studied.5 ^– 7 The effects of amino acid on corrosion resistance of Cu–Ni alloy were reported by Badway et al.6 Their studies indicate that the corrosion inhibition was 85% in alloy with the lower nickel content. In electrodeposition process, the bath pH has a significant effect on crystalline structure, deposit morphology, corrosion behaviour and current efficiency. 8 8,9 The aim of the present study is to investigate the influence of electrolyte pH on the composition, corrosion and morphology of the brush electrodeposited Cu–Ni alloy.

Experimental

The electrodeposition of Cu–Ni alloy films was carried out by the brush plating technique 10 10,11 and the electrolyte solution consists of 10 g L⁻¹ CuSO₄, 84 g L⁻¹ NiSO₄ and 59 g L⁻¹ tri-sodium citrate. The depositions were performed at 4 V for 20 min on copper substrate and graphite stylus acts as anode. The depositions were carried out at various solution pH values ranging from 2 to 6. The pH of the electrolytic bath was adjusted to appropriate values by adding H₂SO₄ and KOH solutions. The cyclic voltammetric (CV) studies were carried out for different pH solutions as well as for the individual metal solutions with a scan rate of 10 mV s⁻¹. For CV studies, platinum is used as a counter electrode, saturated calomel electrode is the reference electrode and the copper substrate is the working electrode. X-ray fluorescence system was used to identify the composition of Cu–Ni alloy films. The corrosion behaviours of the coatings were investigated by Tafel analysis and electrochemical impedance spectroscopy techniques using Parstat 2273 instrument. The Tafel analysis was carried out in 3·5 wt-% NaCl solution using three-electrode assembly. Platinum electrode was used as a counter electrode, the reference electrode was saturated calomel electrode (SCE) and deposited alloys were used as working electrodes. Electrochemical impedance measurements were done using the same three-electrode assembly as used for the Tafel analysis in the frequency range 100 kHz to 100 mHz with an amplitude potential of 5 mV. The surface morphologies of the deposits were characterised by atomic force microscopy using a molecular imaging atomic force microscope.

Results and discussion

X-ray fluorescence analyses were carried out for Cu–Ni alloy deposits obtained from the electrolyte pH 2, 4 and 6. The relationship between the electrolyte pH and composition of deposits is shown in Table 1. As a general rule, the variation of pH values should have little effect on the composition of alloys deposited from the electrolyte containing the metal as a simple ion and should have larger effect on the composition of alloy deposited from electrolyte in which parent metal were present as complexes.12 The addition of acid to the electrolyte, the stability of the copper-citrate complex will reduce. Therefore, the efficiency of copper complex reduction at lower pH values was high. Because of easy break of complex, the Cu content increases from 62·47 to 99·44 wt-% and the Ni content decreased from 37·53 to 0·56 wt-% with a decrease in the solution pH from 6 to 2. The copper content decreases in the Cu–Ni alloy composition with increasing electrolyte pH.12 The thicknesses (Table 1) of the Cu–Ni alloy coatings were increased with increasing electrolyte pH. This is due to the fact that the higher hydrogen evolution occurs at the lower electrolyte pH which leads to a decrease in the efficiency of the metal reduction.

Table 1

Composition of Cu–Ni alloy electrodeposits from X-ray fluorescence studies

Sample no.	Electrolyte pH	Cu, wt-%	Ni, wt-%	Thickness, μm
1	2	99·44	0·56	8·6
2	4	80·30	19·69	12·5
3	6	62·47	37·53	15·5

Cyclic voltammetric studies

The CV studies were carried out for sulphate/citrate electrolyte in the potential range −1·6 to +1·0 V. The curves of the individual metal components, Cu and Ni are presented in Fig. 1 and those recorded for the Cu–Ni alloys are shown in Fig. 2. Figure 1a shows the CV for Cu and the anodic peak A1 was observed at 0·2 V(SCE) and it is related to the CuO formation as per the following reaction13 (1) The formation of CuO layer results in complete passivation of the surface and the broad passive region extends up to 0·5 V, where oxygen evolution commences. In the cathodic part, the peak A2 was observed at −0·3 V and it corresponds to the reduction of CuO to Cu (1). One smaller peak A3 at −1·1 V was seen and was ascribed to the deposition of soluble Cu (II) species.14

Figure 1

Cyclic voltammetry curve for individual a Cu and b Ni system

Figure 2

Cyclic voltammogram of Cu–Ni alloy deposits from sulphate/citrate electrolyte: a pH 2; b pH 4; c pH 6

Ni shows the anodic peak B1 at 0·2 V (Fig. 1b) which can be related to NiO formation13 (2) The peak B1 is followed by a broad passive region extending up to 0·4 V, when the transpassive oxidation of NiO and oxygen evolution takes place.7 In the cathodic region, the reduction peak B2 appears around −1·1V(SCE) and it is related to the reduction of NiO to Ni (II).

The CV recorded for the Cu–Ni alloys deposited at pH 2, 4 and 6 shows similar behaviour except more reduction current (Fig. 2). When the electrolyte pH decreased from 6 to 2, the hydrogen ion concentration is increased, so more the hydrogen evolution will occur at electrolyte pH 2. The more hydrogen evolution leads to the higher current at electrolyte pH 2. The Cu–Ni alloy shows the one anodic peak C1, obviously related to the CuO as well as NiO and this can be seen in Fig. 1. There is no separate peak for the CuO and NiO, because the oxidation potentials of CuO and NiO are very close which leads to the overlay of two peaks, so only one oxidation peak can be obtained. In the cathodic region, two peaks are observed at C2 and C3. The peak C2 is related to the reduction of Cu and C3 is related to the reduction of Cu–Ni alloy. There is a characteristic cross over between the current during cathodic and anodic sweep, which suggests the presence of nucleation and growth process. These features are common for the CV curve at pH 2, 4 and 6 and the only difference is in the deposition current which gets decreased in the order of pH 2, 4 and 6. The anodic peak is at 172 mV for pH 6 and it gets shifted to 614 mV for the solution with pH 2. On the other hand, the variation of solution pH affects the intensities of reduction peaks C2 and C3. The intensity of C2 decreased and C3 increased with increasing solution pH. This suggests that the formation of Cu–Ni alloy phase increases and copper phase decreases at pH 6. This result agrees with that obtained by X-ray fluorescence analysis.

Based on our studies, the alloy formation can be described by the following equation (3)

Tafel analysis

The corrosion behaviours of the Cu–Ni alloy deposited at various solution pH were investigated in 3·5 wt-% of NaCl solution (Fig. 3). The corrosion current I_corr, corrosion potential E_corr, corrosion rate and polarisation resistance R_p were calculated by using Tafel analysis and are listed in Table 2. The I_corr decreases exponentially for Cu–Ni alloys deposited at electrolyte pH from 2 to 6 and shifts the corrosion potential to less negative values. The I_corr value of Cu–Ni alloy deposited at pH 2 is 17·8 μA cm⁻² and it decreases to 11·07 μA cm⁻² for the Cu–Ni alloy deposited at pH 6. The E_corr values shifted from −330 to −256 mV cm⁻² for Cu–Ni alloy deposits when the electrolyte pH is increased from 2 to 6.

Figure 3

Tafel curve for Cu–Ni alloy deposits at a pH 2, b pH 4 and c pH 6 in 3·5 wt-% NaCl solution

Table 2

Corrosion resistance data for Cu–Ni alloy deposits at different pH values

pH	E_corr, mV cm⁻²	I_corr, μA cm⁻²	Corrosion rate, mpy	R_p, Ω cm⁻²	C_dl, μF cm⁻²	R_ct, Ω cm⁻²
2	−330	17·8	7·7	1·5	13 470	786
4	−307	16·1	7·0	2·2	1861	4675
6	−256	11·0	4·8	2·9	570·8	9194

On the other hand, the corrosion rate for Cu–Ni alloy deposited at pH 6 was very low compared with Cu–Ni alloy deposited at pH 2 and 4. The polarisation resistance of Cu–Ni alloy also showed the same trend like corrosion rate. In the anodic region of Tafel plot, the formation of passivation can be seen when the electrolyte pH increases. This is due to the increase in nickel content in the deposits, when the electrolyte pH is increased from 2 to 6. This leads to the formation of NiO passive layer deposits. In general, the formation of passive layer always leads to the better corrosion resistance. From this study, it is suggested that the Cu–Ni alloy deposited at pH 6 has better corrosion resistance than the other deposits.

Electrochemical impedance spectroscopy studies

The impedance data of Cu–Ni alloy deposited at pH 2, 4 and 6 are presented in Fig. 4. The interception of real axis in the Nyquist plots at higher frequency is ascribed as electrolyte bulk resistance R_s and at lower frequency the interception of the real axis is ascribed to the charge transfer resistance R_ct. The Nyquist plots showed single semicircle for deposits at all pH values. This is due to short exposure time. The double layer capacitance C_dl and charge transfer resistance R_ct values were determined from electrochemical impedance spectroscopy measurement and summarised in Table 2. The C_dl value for Cu–Ni alloy deposited at pH 2 was 13·47 mF cm⁻² and it decreases to 570·8 μF cm⁻² for the deposits at pH 6. The charge transfer resistance values increased from 786 to 9194 Ω cm⁻². In principle, higher value of R_ct and lower value of C_dl indicates better corrosion resistance of coatings. Therefore, these values suggest that the Cu–Ni alloy deposited at pH 6 has a better corrosion resistance.

Figure 4

Nyquist diagrams of Cu–Ni alloy deposits at a pH 2, b pH 4 and c pH 6 in 3·5 wt-% NaCl solution

Morphology studies

Figure 5 depicts atomic force microscopic images showing surface morphology of the Cu–Ni alloy obtained at pH 2, 4 and 6. It indicates that the surface of the films deposited at pH 6 (Fig. 5c) appeared to have a uniform distribution of granular shaped grains with crack free and very smooth surface. The average grain size varied between 40 to 80 nm. The average surface roughness R_a of the sample is 0·08 μm. For the film deposited at pH 4 (Fig. 5b), the surface morphology slightly disturbs the uniform distribution with agglomeration. The average grain size ranges from 100 to 150 nm and R_a value is 0·11 μm. For the deposits at pH 2 (Fig. 5a), the surface morphology was uneven and more roughness is observed. The average grain size ranges from 250 to 300 nm and R_a value is 0·21 μm. It can be concluded from these results that average distributed grains with agglomeration of Cu–Ni alloy deposits are obtained at electrolyte pH 6 compared with other deposits at lower electrolyte pH. The smaller grain improves strong binding with adjacent grains and this will produce compact and smooth films of alloy coatings. The lower grain size and roughness facilitate compact and smooth Cu–Ni alloy coatings and this enhances the corrosion resistance properties of coatings.

Figure 5

Surface morphology of Cu–Ni alloy deposits at a pH 2, b pH 4 and c pH 6

Conclusion

The effects of solution pH on Cu–Ni alloy electrodeposition from sulphate/citrate electrolyte and the composition, corrosion, morphology and mechanism of the deposits were investigated. Composition analysis showed that the nickel content increases up to 37·53 wt-% in alloy deposits with increasing electrolyte pH. Cyclic voltammetry studies clearly shows the formation of Cu–Ni alloy system. Tafel analysis and electrochemical impedance spectroscopy on all deposits indicate that the Cu–Ni alloy deposited at pH 6 exhibited better corrosion resistance behaviour compared to other deposits. Polarisation studies indicate that R_p increases up to 2·9 Ω cm⁻² and I_corr reduces to 11·07 μA cm⁻² when pH is increased from 2 to 6. Furthermore, the high R_ct (9194 Ω cm⁻²) obtained from Nyquist plots also confirms the above fact. Our studies indicate that the nickel rich Cu–Ni alloy deposits exhibit better corrosion behaviour than the other deposits. Surface morphology of the alloy deposited at pH 6 indicates very smooth and uniform deposits.

Footnotes

Acknowledgements

This research was supported by DRDO fund under grant no. ERIP/ER/0502128/M/01/1049.

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Influence of electrolyte pH on composition,corrosion properties and surface morphology of electrodeposited Cu–Ni alloy