INTRODUCTION

 Definition of Electrodeposition:

Electrodeposition is an electrochemical process by which metal is deposited on a substrate by passing a current through a conductive bath.

 Objective of Electrodeposition:

    Objects are electroplated to prevent corrosion, to obtain a hard surface or attractive finish, to purify metals (as in the electro refining of copper), to separate metals for quantitative analysis, or, as in electrotyping, to reproduce a form from a mold. Cadmium, chromium, copper, gold, nickel, silver, and tin are the metals most often used in plating (2).

Electrodeposition of Metals:

Electroplating is an oxidation-reduction reaction, where one material gives up electrons (gets oxidized) and the other material gains electrons (gets reduced). The anode is the electrode at which oxidation occurs, and the cathode is the electrode at which reduction occurs (2)

 The process of metal deposition generally consists of several partial processes:

1‑Transport of ions to be deposited from the bulk of the solution to the cathode surface through migration and diffusion.

2‑Charge transfer reaction: the simple metallic Ions are then discharged at the electric double layer.

3‑Electrocrystallization process: the discharged metal atoms are then changed into crystalline state (3).

The electroplating process was found to depend on a great number of factors affecting the nature and properties of deposits, among these factors are current density, conductivity of the solution, presence of addition agents, agitation, temperature and the nature of the base metal (substrate).

Heterogeneous or cluster-based alloys have recently prompted many investigations due to their interesting magnetic properties (1).

Electrodeposition of Cu-Fe Alloys:

Because copper is highly electropositive (noble) character, copper is easily precipitated by metals lower in the electrochemical series such as iron or zinc.

One advantage of iron deposits is that they are soft enough to be machined or otherwise finished, after which they can be casehardened.

The electrodeposition of Cu-Fe alloy is one of the best known examples of anomaly codeposition, in which the electrochemically less noble metal deposits preferentially.

Cathodic Current Efficiency: 

    The ratio of the weight of metal actually deposited to that, which would have resulted if all the current had been used for deposition, is called the cathode current efficiency with respect to metal deposition.

 It is 100% when as much metal is deposited or dissolved, respectively, as would be expected on the basis of Faraday’s Law (3).

Cathodic Polarization:

When no external potential is applied to the electrolytic cell of similar cathode and anode, the potentials of the anode and cathode are equal, at least within a fraction of a millivolt. If the electrodes are connected externally, no current flows. To produce a current, an external potential must be furnished, this potential is called polarization.

At each electrode of the unpolarized cell, anodic and cathodic reactions proceed at equal rates. So that, there is no net change from these exchange currents. Application of the polarizing potential lowers the potential at the cathode, thus accelerating the deposition reaction and retarding the dissolution reaction (anodic reaction). This result in a net cathodic current and corresponding deposition of metal.  At the anode the potential is raised with the opposite consequences. The shift in potential at either electrode is its polarization, overpotential, or overvoltage.

THEORY AND RULES

Plating is governed by Faraday's Laws that state:

1. The weight of a substance formed at an electrode is proportional to the amount of current passed through the cell (at certain limit).

2.      Weights of different substances produced at an electrode by the same amount of current are proportional to their equivalent weights (2).

Cathodic Current Efficiency:

The cathodic current efficiency (F%) was calculated simply according to Faraday’s laws from the relation,

F % = W/W_t x 100

Where (W) is the experimental weight of the deposit and (W_t) is the theoretical weight of the deposit.

The theoretical weight of the individual metal deposited can be calculated as follows:

Where (Q = I x t) is the amount of electricity passed through the cell and expressed in ampere second and Faraday’s constant is 96485.

The partial current efficiencies of the parent metals in the alloy F₁ % and F₂ % were determined from the relations:

F₁ % = W₁ /W_t x 100

F₂ % = W₂/W_t X 100

The total efficiency of the alloy F₁ % is equal to the sum of the partial current efficiencies F₁% and F₂ %.

F_t%= F₁% + F₂%

The composition of the alloy was determined by chemical analysis for the alloy deposit using Inductively Coupled Plasma (ICP) Atomic Emission Spectrometry (ICP‑AES), Model Plasma 400, after dissolving the deposited alloy in diluted aqua regia (3FICL: 1HN0₃) diluted 50% with distilled water (3).

EXPERIMENTAL WORK

All Experiments were carried out at room temperature (23  ± 2 ^oC).

PART I: Cathodic Current Efficiency

Electroplating Bath (cell):

Electroplating is performed in a plating bath, which is usually a non-metallic tank (usually plastic). The tank is filled with electrolyte that has the metal, to be plated, in ionic form. See Tables 1 and 2.

 Anodes:

The anodes used were made from pure platinum sheet. Those anodes were cleaned with hot concentrated nitric acid solution, washed with distilled water and finally were rinsed with the electrolyte being used. 

The anode is connected to the positive terminal of the power supply.

Cathodes:

The cathodes used were made from steel sheet (0‑6 mm thickness) of compositions (0.05 C %, 0.01 S %, 0.01 P %, 0.23 Mn %, 0.03 Ni % 0.02 Cr %, 0.04 Cu %, 0.002 Ti %, 0.003 As %, 0.003 Sn %, 0.03 Al %, 0.004 Mg %, 0.002 Zn % and 99.55 Fe %). Before each run, the cathodes were mechanically polished with emery paper, washed with distilled water, rinsed with acetone, dried, weighed and dipped in the electro plating cell.

The cathode is the work piece, the substrate to be plated. This is connected to the negative terminal of the power supply. The power supply is well regulated to minimize ripples as well to deliver a steady

predictable current, under varying loads such as those found in plating tanks.

Electrical Circuits:

The electrical circuit shown diagrammatically in Fig. 1 was used. It consists of the electroplating cell in which the steel cathode was inserted with the Pt anode their dimensions 3.2 cm x 2 cm (6.4 cm² area) and fixed in their appropriate positions. Those two electrodes were connected to a potentiostat type EG & G Model 273A, which was connected to a computer, see Fig.1.

As the current is applied, positive metal ions from the solution are attracted to the negatively charged cathode and deposit on the cathode. As replenishment for these deposited ions, the metal from the anode is dissolved and goes into the solution and balances the ionic potential.

Table-1 Composition of Cu-Fe Platting baths:

All these baths at constant:

Time: t=10 min

Current: I=0.1 A

Table-2 Composition of Cu-Fe Platting baths at different Current Density and Time:

All these baths at constant:

[Cu]=25 g/l

[Fe]=1 g/l

[Na]=70 g/l

PART II:  Cathodic Polarization

In this techniques before each experiment, the electrolytic cell (L) shown in Fig. 2 was cleaned, washed and filled with the test plating solution to the desired level to ensure complete submerge of the electrodes. Then the cleaned electrodes were placed in their respective positions. The potentiodynamic polarization curves were measured under different conditions. The curves were swept from the rest electrode potential (at zero current potential) towards less noble values at scan rate of 1 mV/s.   

The potentiodynamic cathodic polarization measurements were carried out using a steel (working) electrode and platinum (counter) electrode. Each had an area of 1cm². All cathodic potentials were measured relative to an Ag/AgCl saturated reference electrode using the potentiostat/galvanostat connected with a PS computer. All experiments were performed at 25 ± 2 C.  See table-3.

Table-3 Composition of Cu-Fe baths of Cathodic Polarization Curves:

RESULTS AND DISCUSSION

PART I: Cathodic Current Efficiency

1.  Effect of Copper:

Fig.4

Effect of copper (Cu) concentration on the cathodic current efficiency of (Cu-Fe) electrodeposition with constant Sodium Citrate and iron concentrations 50 and 2 gL^-1      at 23 ^oC, t = 10 min and I = 100 mA

2.  Effect of Tri-Sodium Citrate:

Fig.5

 Effect of Sodium Citrate concentration on the cathodic current efficiency of (Cu-Fe) electrodeposition with constant Copper and iron concentrations 20 and 1 gL^-1 at 23 ^oC, t = 10 min and I = 100 mA

3.  Effect of Iron:

Fig.6

Effect of iron (Fe) concentration on the cathodic current efficiency of (Cu-Fe) electrodeposition with constant Sodium Citrate and copper concentrations 50 and 25 gL^-1 at 23 ^oC, t = 10 min and I = 100 mA

4.   Effect of Current Density:

Fig.7

Effect of Current density on the cathodic current efficiency of (Cu-Fe) electrodeposition with constant Copper, Sodium Citrate and iron concentrations 25, 70 and 1 gL^-1 at 23 ^oC, t = 10 min

5.  Effect of Time:

Fig.8

Effect of time on the cathodic current efficiency of (Cu-Fe) electrodeposition with constant Sodium Citrate, iron and copper concentrations 70, 1 and 25 gL^-1 at 23 ^oC, t = 10 min and I = 200 mA

Cathodic Current Efficiency Discussion:

Fig-4 shows that the cathodic current efficiency, F% increase with increasing the concentration of Cu²⁺ ions. This could be attributed to the fact that increasing Cu²⁺ ions content in the bath result due to a decrease in the Cu²⁺ deposition in the diffusion layer and consequently decreases the concentration (diffusion) over potential.

          Fig-5 shows that the cathodic current efficiency, F% increase with increasing the concentration of Sodium Citrate. This indicates that the more acidic bath increase efficiency.

Fig-6 shows that the cathodic current efficiency, F% increase with increasing the concentration of iron.

Fig-7 illustrates the influence of increasing the deposition current density on the values of F%. The data show that the efficacy increases with increasing currant density.

Fig-8 shows that the cathodic current efficiency, F% increase with increasing the period of the time. This indicates that the more time result in increase the efficiency.

According to the data, promising solution to be selected as an optimum path for further investigation contains:

25 gL^-1 Cupric Acetate and 50 gL^-1 Tri-Sodium Citrate and 2 gL^-1 Iron, bath (Cu-Fe)-2 and the operating values were T at 23 ^oC, t = 10 min and   I = 100 mA.

The optimum path for vary the current density and the time:

(Cu-Fe)-10, which operate at current I=700mA and time t=10min at constant [Cu]=25 gL^-1, [Fe]=2 gL^-1 and [Na]=50 gL^-1  

PART II: Cathodic Polarization Curves:

1.  Effect of Iron:

          Fig 8:

          Potentiodynamic cathodic polarization curves of Cu-Fe alloy electrodeposited from baths with different Iron concentrations at 23°C and scan rate of 1 mV s ^–1 :

                             Fe-1 at: [Fe]=1 g/l

                             Fe-2 at: [Fe]=2 g/l

                             Fe-3 at: [Fe]=5 g/l

                             Fe-4 at: [Fe]=10 g/l

          All at Constant:

                             [Cu]=25 g/l

                             [Na]=50 g/l

2.       Effect of Coupper:

          Fig 9:

          Potentiodynamic cathodic polarization curves of Cu-Fe alloy electrodeposited from baths with different Copper concentrations at 23°C and scan rate of 1 mV s ^–1 :

                             Cu-1 at: [Cu]=10 g/l

                             Cu-2 at: [Cu]=15 g/l

                             Cu-3 at: [Cu]=20 g/l

                   All at Constant:

                             [Fe]=2 g/l

                             [Na]=50 g/l

3.       Effect of Tri-Sodium Citrate:

          Fig 10:

          Potentiodynamic cathodic polarization curves of Cu-Fe alloy electrodeposited from baths with different Tri-Sodium Citrate concentrations at 23°C and scan rate of 1 mV s ^–1 :

                             Na-1 at: [Na]=20 g/l

                             Na-2 at: [Na]=70 g/l

                   All at Constant:

                             [Fe]=1 g/l

                             [Cu]=25 g/l

4.     Effect of Iron, Coupper and (Fe-Cu) Alloy:

          Fig 11:

          Potentiodynamic cathodic polarization curves of Cu-Fe alloy electrodeposited from baths, during electrodeposition of copper and iron as individual metals and as their binary alloy at 23°C and scan rate of 1 mV s ^–1 :

•    Cu-Na: individual metal electrodeposited

from bath containing:

[Cu]=20 g/l  and  [Sodium Citrate]=50g/l

•    Fe-Na: individual metal electrodeposited

from bath containing:

[Fe]=2 g/l  and  [Sodium Citrate]=50g/l

•    Fe-Cu-Na:  alloy electrodeposited

from bath containing:

[Cu]=20 g/l, [Fe]=2 g/l  and  [Sodium Citrate]=50g/l

Cathodic Polarization Curves Discussion:

Typical potentiodynamic cathodic polarization curves of Cu, Fe and Cu-Fe deposition under similar conditions arc shown in Figure 11.

It is obvious that the cathodic polarization curve of Cu lies at less negative potentials than that of Fe indicating that Cu is nobler than Fe in the present system.

The polarization curve of Cu-Fe alloy lies at more positive potentials than those of the parent metals. Anomalous codeposition is observed. Fe, the less noble metal, deposited preferentially.

Figure 8 shows the effect of increasing [Fe]/[Cu] concentration ratio in the bath on the features of cathodic polarization curves of the alloy. The data reveal that increasing this concentration ratio shifts the cathodic polarization curves of Cu-Fe codeposition towards more noble potentials.

Figure 10 shows the effect of increasing [Cu]/[Fe] concentration ratio, which give similar result as [Fe]/[Cu].  Increasing [Cu]/[Fe] concentration ratio shifts the cathodic polarization curves of Cu-Fe codeposition towards more noble potentials.

On the other hand, the polarization curves of Cu-Fe codeposition shift towards the more negative direction with increasing Sodium Citrate concentration as shown in Figure 10.

REPORT SUMMARY

PART I:

Electrodeposition of Cu-Fe Alloys (Cathodic Current Efficiency):

The aim of Electrodeposition of Cu-Fe alloys is to produce highly corrosion resistant alloy plated steel sheets. The electrodeposition of Cu-Fe alloy is one of the best known examples of anomaly (irregularity) codeposition, in which the electrochemically less noble metal (Fe) deposits preferentially.  Sodium Citrate was used as medium of all solutions cells.

Five different experiments were done to produce the best corrosion resistant alloy plated steel sheets:

1.     Effect of Copper:  [Cu] = 10,25g/l. At constant [Fe] = 2g/l, [Na] = 50g/l.

2.     Effect of Iron: [Fe] = 1,10 g/l. At constant [Cu] = 25g/l, [Na] = 50g/l.

3.     Effect of Tri-Sodium Citrate:  [Na] = 20,70 g/l. At constant [Cu] = 25g/l,

      [Fe] = 1g/l.

4.     Effect of Current Density:  I= 100,300, 500, 700 mA. At constant [Cu] = 25g/l,      [Fe] = 1g/l, [Na] = 70g/l.

5.     Effect of Time: t= 5, 50 min. At constant [Cu] = 25g/l, [Fe] = 1g/l,

       [Na] = 70g/l.

PART II:

Cathodic Polarization Curves:

The aim of this technique is to measure the current when applying the potential at different solutions conditions.  When no external potential is applied to the electrolytic cell, the potentials of the anode and cathode are equal (no current flow).

Where as, to produce a current, an external potential must be furnished (this potential is called polarization).

 Four different experiments were done to study the cathodic polarization curve:

1.     Effect of Fe, Cu and Fe-Cu alloy:

      Solution contain: [Cu] = 20g/l, [Fe] = 2g/l, [Na] = 50g/l.

2.     Effect of Cu:

Solutions contain: Vary [Cu] = 10, 15,20g/l. At constant [Fe] = 2g/l,              [Na] =50g/l.

3.      Effect of Fe:

Solutions contain:  Vary [Fe] = 1,2,5,10g/l. At constant [Cu] = 25g/l,         [Na] = 50g/l.

4.     Effect of Tri-Sodium Citrate:

Solutions contain:  Vary [Na] = 20, 70 g/l. At constant [Cu] = 25g/l,   

 [Fe] =1g/l.

CONCLUSION

PART I:

Electrodeposition of Fe-Cu alloy (Cathodic Current Efficiency):

According to the data, promising solution to be selected as an optimum path for further investigation contains:

25 gL^-1 Cupric Acetate and 50 gL^-1 Tri-Sodium Citrate and 2 gL^-1 Iron, bath (Cu-Fe)-2 and the operating values were T at 23 ^oC, t = 10 min and   I = 100 mA.

For vary the current density and the time, the optimum path:

(Cu-Fe)-10, which operate at current I=700mA and time t=10min at constant [Cu]=25 gL^-1, [Fe]=2 gL^-1 and [Na]=50 gL^-1  

Percentage Efficiency (F%) increases with increasing the concentration of Sodium Citrate. This indicates that the more acidic bath increase efficiency.

PART II:

Cathodic Polarization Curves:

The cathodic polarization curve of Cu lies at less negative potentials than that of Fe indicating that Cu is nobler than Fe. 

The polarization curve of Cu-Fe alloy lies at more positive potentials than those of the parent metals. Anomalous codeposition is observed, Fe the less noble metal, deposited preferentially.

Increasing [Fe]/[Cu] concentration ratio in the bath shifts the cathodic polarization curves of Cu-Fe codeposition towards more noble potentials. As the increasing of [Cu]/[Fe] concentration ratio do.

          On the other hand, the polarization curves of Cu-Fe codeposition shift towards the more negative direction with increasing Sodium Citrate concentration.

REFERNCES

1.     J. Electrochem. Soc., Structure, Composition and Morphology of Electrodeposited Co Fe Cu Alloy. Vol. 144, No. 10, Oct. 1997.

2.     Al-Betar, A. Some Selected properties of Electroplating. KFUPM, Chemistry Department.

3.     Al-Garawi, Raid. Electrodeposition of Copper, Iron and Their Binary Alloy. KFUPM, Chemistry Department.

4.     Khaled, M. and M. Emad. Electroplating of Co Fe Alloys from Aqueous Acetate Baths. 2000, 41.

5.     M. Emad. Electrodeposition of Metals. Taken in part from the thesis of M. Emad.

6.     “Electrochemistry,” Microsoft Encarta Encyclopedia. 1993-1996.