1. INTRODUCTION:

Electrodeposition of zinc-nickel alloy on mild steel provides good corrosion resistance and mechanical properties. Automobile components which are attacked by highway de-icing salts can be given corrosion protection by zinc -nickel alloy coatings. The metals which can be alloyed with zinc to produce corrosion protection coatings are Ni, Co, Mn, Fe, Al and Cr (1). Of all the Zinc alloy coatings Zinc-Nickel alloy coatings offer greater corrosion resistance (2). Corrosion resistance of zinc –nickel alloy electrodeposits is better than that of other zinc alloys of equal thickness on the basis of cost consideration. (3) Zinc -Nickel alloy coatings are used in electro catalytic water electrolysis, electronic industries and also for plating aircraft components (4).

The present work deals with the study of alloy composition, appearance, and thickness and corrosion resistance of zinc nickel alloy deposit obtained with the bath solution containing benzaldehyde.

2. EXPERIMENTAL:

Elecrodepositition of zinc nickel alloy on mild steel plates was done using the bath solution containing ZnO, NaOH, NiSO₄and disodium salt of EDTA as complexing agent.The pH of the bath solution was 11.5. The optimum concentration of Zinc and Nickel in the bath solution was 5 g/l and 0.5g/l respectively. Mild steel plates of area 6.45 cm²were degreased, washed in running water and dipped in 5% H₂ SO₄solution for 10 seconds to remove the metallic and organic impurities .They were washed thoroughly with de-ionized water and dried. The composition of bath solution is given in Table.1.

Table-1 Bath composition

Bath Constituents

Concentration (g/l)

ZnO

NaOH

NiSO₄

Disodium salt of EDTA

6.223

90.0

2.392

15.0

2.1. Electrodeposition

Electrodeposition was carried out using regulated power supply as a direct current source, a multimeter and a cell (5) Cleaned mild steel plate was used as cathode and electrolytic zinc was used as anode. Experiment was carried out at room temperature, 50°C, 60’C and 70°C, at current densities ranging from 0.5A/dm²to 2.5A/dm² and the duration of elecrodisposition was 30 min and 60 min. The electrodeposits obtained under various conditions were analysed and the optimum temperature, current density and time were determined. Electrodeposition was also carried out with benzaldehyde addition in the bath solution to get more bright deposit and high corrosion resistance. The deposition with varying concentration of benzaldehyde was at optimum temperature, current density, and time as 30°C, 1A/dm² and 30 min respectively.

2.2. Composition analysis

Composition analysis of the electrodeposits obtained using the bath solution containing the concentration of zinc and nickel as 5 g/l and 0.5 g/l in the presence and absence of benzaldehyde was carried out using Atomic Absorption Spectroscopy.

2.3. Surface structure

To know the surface structure, XRD patterns were obtained on the electrodeposit. X– ray diffraction study of the electrodeposit was made using Shimadzu-6000 diffractometer with CuKα radiation. SEM was employed to study crystallite size of the electrodeposit using JEOL-6390 Scanning Electron Microscope.

2.4 Current efficiency

Current efficiency was determined with the bath solution at different current densities ranging from 0.1A/dm²to 2.5A/dm²and at temperature ranging from 30°C to 60°C (4). It was also determined using the bath solution containing benzaldehyde at optimum conditions of 30°C, 1A/dm² and 30 min.

2.5. Corrosion Studies

2.5.1 Tafel Polarisation

Electrochemical measurement was done in a three electrode glass cell using electrodeposited sample as working electrode, saturated calomel electrode as reference electrode and platinum as counter electrode. The instrument used for measurement was electrochemical workstation CHI660C with the scan rate of 1mv/s. The electrodes were cleaned in deionized water (5). Tafel study was carried out with Zn –Ni alloy electrodeposit in the presence and absence of benzaldehyde in 0.05 M H₂SO₄with an exposed area of 0.0314cm.

2.5.2 Electrochemical impedance spectroscopy

Electrochemical impedance measurements were carried out using Frequency Response Analyser. The electrochemical impedance spectra were taken for the electrodeposits in the presence and absence of benzaldehyde in 0.05 M H₂SO_4.The frequency range is from 1Hz to 1,00,000 Hz at the rest potential by applying 5 mv AC voltage. From the Nyquist plots .charge transfer resistance was calculated in 0.05 M H₂SO_4.

3. RESULTS AND DISCUSSION:

3.1 Nature of the electrodeposit

The electrodeposit obtained by using the bath solution containing zinc and nickel as 5g/l and 0.5g/l without benzaldehyde was uniform, dark grey and semi bright.

3.1.1. Effect of benzaldehyde on the nature of the electrodeposit

The electrodeposits obtained by using the bath solution containing zinc and nickel as 5g/l and 0.5g/l with benzaldehyde of concentration 0.5g/l was uniform, light grey and more bright than the deposits with other concentrations of benzaldehyde. The nature of the deposits is given in table. 2

Table.2 Effect of benzaldehyde on the nature of the eletrodeposit

Concentration of benzaldehyde (g/l)	Nature of the deposit
0.10	Uniform, light grey and less bright
0.25	Uniform, light grey and less bright
0.50	Uniform, light grey and more bright
0.75	Uniform, light grey and moderately bright
1.00	Uniform, light grey and moderately bright

3. Composition analysis

Composition of the electrodeposit obtained with 0.5g/l benzaldehyde and the electrodeposit without benzaldehyde as determined by Atomic Absorption Spectroscopy is given in table.3 When compared with electrodeposits without benzaldehyde it is observed that there is only less variation in the percentage composition of nickel in the electrodeposit with benzaldehyde in the bath solution.

Table.3.Composition of the electrodeposit given by AAS

Metals in the deposit

Composition (%)

Without C₆H₅CHO

With C₆H₅CHO

89.17

10.83

89.00

11.00

3.3. Current efficiency

The variation of cathode current efficiencies with cathodic current densities at room temperature for 30 min with the bath solution containing 0.5g/l Ni and 5g/l Zn without benzaldehyde addition is given in Table.4. From the result it is observed that the current efficiency is high at current density of 2.5 A/dm².The current efficiency is low at 0.5A/dm² which may be due to the strong complexation of EDTA with the metal ion present in the bath.

Table .4 Variation of cathode current efficiency with current densities.

Current density A/ dm^2.	Time (min)	Temperature ⁰C	Current efficiency
0.5	30	30	12.98
1.0	30	30	21.81
1.5	30	30	25.6
2.0	30	30	54..79
2.5	30	30	68.48

3.3.1. Effect of benzaldehyde on Current efficiency

Current efficiency with benzaldehyde addition determined at optimum current density of 1A/dm² is given in table.5. The results show that there is a linear increase in the current efficiency of the bath solution as the concentration of benzaldehyde is increasing from 0.1 to 1.0 g/l. On comparing the current efficiency at 1A/dm² of the bath solution containing benzaldehyde with that containing no benzaldehyde the former current efficiency is greater.

Table.5 Effect of concentration of benzaldehyde on Current efficiency

Concentration of C₆H₅CHO g/l	Time min	Temperature °C	Current efficiency
0.10	30	30	33.60
0.25	30	30	36.09
0.50	30	30	37.33
0.75	30	30	45.90
1.00	30	30	48.86

3.4. Surface structure

3.4.1. XRD Pattern

XRD pattern obtained on the electrodeposit with the bath solution containing Ni and Zn as 0.5g/l and 5.0g/l respectively is shown in figure.1. The pattern reveals that reflections were obtained from (330,441) planes -Ni₃ Zn₂₂ phase along with (502) of zinc. Along with these intense signals corresponding to Zn(OH)₂ were also seen_. This is mainly due to the alkaline nature of the bath. From the pattern, crystallite size was calculated using Debye Scherer equation and it is found to be 165.79 nm. The results were compared with standard values obtained from JCPDS data card and the structure is found to be face centered monoclinic.

Figure1. XRD Pattern of the deposit without benzaldehyde addition

XRD patterns obtained with benzaldehyde of concentration 0.1 to 1 g/l are given in figure.2 The patterns reveal that reflections are obtained from (132,131,401,214,351) planes of δNi₃Zn₂₂with 0.1g/l benzaldehyde (313,401,422,224 and 351) planes of δNi3Zn22with 0.25 g/l benzaldehyde, (513,401,422 and 224) planes of δNi₃Zn₂₂with 0.5 g/l (513,401,131,241,351) planes of δNi3Zn22with 0.75g/l benzaldehyde (313,131,401,241,351) planes of δNi₃Zn₂₂phase with 1 g/l benzaldehyde. From the pattern, crystallite size is determined to be 643 nm. The results obtained are compared with the standard values obtained from JCPDS data card and the structure is found to be monoclinic.

Figure 2. XRD Pattern of the electrodeposit with benzaldehyde

A to E-with 0.1, 0.25, 0.50, 0.75, 1.0g/l benzaldehyde

3.4.2. SEM Analysis

Scanning electron microscopic examination of the alloy deposit with benzaldehyde of concentration 0.1 to 1.0 g/l concentration is shown in figure.3 The SEM photographs show small and large crystallites of rectangular and triangle shape covering the surface. The surface is smooth, uniform, bright and the size of the crystallites is 1.3 μm

Figure 3. SEM photographs of the electrodeposit.

a to e-with 0.1,0.25,0.50,0.75,1.0g/l benzaldehyde

3.5. Corrosion studies

3.5.1. Tafel plots

Corrosion rate of the mild steel plate and coated sample were determined by tafel studies. Tafel polarization plot taken in 0.05M H₂SO₄ is shown in figure 4. Corrosion rates of the mild steel plate and coated sample in 0.05M H₂SO₄ are given in table 6. On comparing the corrosion rate values of mild steel plate and Zn – Ni alloy electrodeposited plate, the electrodeposited plate has corrosion rate two times less than the corrosion rate of mild steel.

Table.6 Effect of benzaldehyde on the thickness of the deposit

Concentration of C₆H₅CHO( g/l )	Thickness (nm)
0.10	557.52
0.25	649.97
0.50	661.45
0.75	721.22
1.00	620.56

Figure.4 Tafel plots of mild steel plate and electrodeposited

sample

Tafel polarization plots of Zn–Ni alloy deposit from the bath solution with varying concentration of benzaldehyde are given in figure.5,6,7,8 and 9. Corrosion rates of the electrodeposited samples in 0.05M H₂SO₄ are given in table.7 The benzaldehyde added to plating bath increases polarization and also adsorbs on the electrode surface and then blocks the high energy sites for the crystallization of metal. There is anomalous co-deposition of Zn- Ni alloy along with the precipitation of Zn(OH)_2. Benzaldehyde adsorbs on Zn(OH)₂ precipitated layer. The corrosion of the electrodeposited alloy is prevented by the adsorbed aldehyde (6).The corrosion rate of the deposit is decreasing with the increase in the concentration of benzaldehyde. It is also found that corrosion rate of the deposits with benzaldehyde addition is less than that obtained without benzaldehyde addition in the bath solution.

Table.7.Effect of benzaldehyde on corrosion rate in .05M H₂SO₄

Concentration of C₆H₅CHO (g/l)	Corrosion rate in 0.05M H₂SO_{4 (}mm/yr₎
0.10	0.8652
0.25	0.7574
0.50	0.6583
0.75	0.6088
1.00	0.3962

Figure.5 Tafel plot of the electrodeposit with 0.1g benzaldehyde

Figure.6 Tafel plot of the electrodeposit with 0.25g benzaldehyde

Figure. 7 Tafel plot of the electrodeposit with 0.5g benzaldehyde

Figure. 8 Tafel plot of the electrodeposit with 0.75g benzaldehyde

Figure. 9 Tafel plot of the electrodeposit with 1.0g benzaldehyde

3.8 Electrochemical Impedance spectroscopy

Impedance diagrams obtained for the mild steel plate and Zn –Ni alloy coated sample in 0.05M H₂SO ₄ are shown in figure.10 The charge transfer resistance (Rct) values of mild steel plate and coated sample are given in table.9 From Rct. values it is observed that the electrodeposited plate has greater corrosion resistance than mild steel plate.

Table.8 . Rct values of mild steel plate and coated sample.

Sample

Charge transfer resistance (Rct) in 0.05M H₂SO₄ (kΩ)

Mild steel

0.379

Coated sample

0.608

Figure.10 Impedance diagram of mild steel plate and electrodeposited sample

Table. 9. Rct values of the coated sample with benzaldehyde addition

Concentration of C₆H₅CHO (g/l)	Charge transfer resistance (Rct) in 0.05M H₂SO₄ (kΩ)
0.10	0.982
0.25	1.118
0.50	1.141
0.75	1.162
1.00	1.286

4. CONCLUSION:

Electrodeposition of zinc –nickel alloy on mild steel was studied in alkaline bath solution with EDTA as complexing agent. A good electrodeposit obtained with zinc-nickel alloy having the composition of nickel as 10.83% shows that the deposit has high corrosion resistance. Greater value of current efficiency with benzaldehyde addition shows that the electrolytic bath is stable. Smaller grain size of the deposit shows that the coating is porous free. Corrosion studies of the deposits show that the electrodeposited sample with benzaldehyde addition has greater corrosion resistance than the sample without benzaldehyde addition on mild steel plate.

5. REFERENCES:

1. Hsin-Yi H su and Chao-Chen Yang Z Naturoforch, 58b, 1055 – 1062 (2003)

2. Hwa Young Lee,Sung Gyu Kim Metal Processing Research Center,Korea Institute of Science and Technology, P.O.Box131, Cheongryang, Seoul 130-650, South Korea

3. Srivastava R D and Mukergee RC 1976 J.Appl.Eletcrochem.6.321

4. S. Shivakumara et al Dept of chem. Sciences. Kuvempur Univ, Shankaraghatta 577451, India

5. V. Raveendran and V. S. Muralidharan Portugaliae Electrochemica Acta 25; (2007): 391-399

6. Visalakshi Ravindran, Ph. D thesis, Alagappa University, Karaikudi, India (1996).

7. Hsin-Yi H su and Chao-Chen Yang Z Naturoforch, 58b, 1055 – 1062 (2003)

8. Hwa Young Lee,Sung Gyu Kim Metal Processing Research Center,Korea Institute of Science and Technology, P.O.Box131, Cheongryang, Seoul 130-650, South Korea

9. Srivastava R D and Mukergee RC 1976 J.Appl.Eletcrochem.6.321

10. S. Shivakumara et al Dept of chem. Sciences. Kuvempur Univ, Shankaraghatta 577451, India

11. V. Raveendran and V. S. Muralidharan Portugaliae Electrochemica Acta 25; (2007): 391-399

12. Visalakshi Ravindran, Ph. D thesis, Alagappa University, Karaikudi, India (1996).

Received on 23.04.2010 Modified on 12.05.2010

Asian J. Research Chem. 3(4): Oct. - Dec. 2010; Page 1044-1049