Experimental Investigations of Catalytic Effect of Cu2+

Published Date: 16 Jan 2018

Experimental Investigations of Catalytic Effect of Cu²⁺ During Anodic Disolution of Iron in NaCl Electrolyte

R.K Upadhyay¹, Arbind Kumar² and P.K Srivastava³

Abstract:

Taguchi’s orthogonal array L₉ has been effectively used to study the effect of process parameters such as voltage, feed rate and electrolyte concentration on material removal rate in context of two different types of electrolyte namely aqueous NaCl solution and electrolyte solution containing Cu²⁺ ions. The results indicated that Cu²⁺ has a catalytic effect on the anodic dissolution of iron, which restrict the oxidation of Fe²⁺ to Fe³⁺ and increases the dissolution rate during machining. The experimental results were analyzed using analysis of variance (ANOVA) method to investigate the significance and percentage contribution of individual process parameters on performance characteristics.

Key Words: Electrochemical Machining, Aqueous NaCl, Cu²⁺, Parameters, Oxidation, Material Removal rate.

Introduction:

Electrochemical machining (ECM) has got an industrial importance due to its capability of controlled atomic level metal removal¹. It is an anodic dissolution process based on electrolysis, where the application of a more traditional process is not convenient. ECM has been successfully employed in aerospace, automobile industries and now gaining much importance in the electronics and other high-tech industries for the fabrication of micro components^2-3. Mask less and through mask electrochemical micromachining techniques have been successively used thin films and foils of materials those are difficult to machine by other methods^4-5. Electrochemical machining is low voltage (5-25 volt) machining process which offers high metal removal rate and also capable to machine hard conductive materials into complicated profiles without any thermal damages thus suitable for mass production work with low labor requirements^6-7. The dissolution rate is highly reliant on the selection of electrolytes and its current carrying capacity. On increasing the concentration of electrolyte solution dissolution rate also increases but excess concentration allows the crystal formation, which may damage the accessories of ECM and reduce the volume of electrolyte in flow pipes. The conductivity of electrolyte depends not only on the concentration but also on ionic interaction. Thus, the current carrying process done by the base electrolyte is small, but H⁺ and OH^- ions produced in electrolysis of water play important role^8-9. The achievement of higher dissolution rate in ECM is a strong research base which is possible by change in composition of electrolyte solution to promote catalytic effect during dissolution¹⁰.

During electrochemical machining of iron at low current density it has been observed that Fe⁺ cation formed very easily but it is highly unstable and immediately oxidizes into Fe²⁺ state. Increase in current density leads to simultaneous production of Fe²⁺ and Fe³⁺, at higher current density apparent valence of iron increases above three¹¹. Therefore, to stabilized Fe²⁺ in the aqueous solution is a challenge during dissolution.

EXPERIMENTAL SET-UP AND PRINCIPLE OF ECM:

Fig 1 Experimental set-up

ECM is an anodic dissolution process works on the principle of Faradays law. While machining of iron in presence of aqueous NaCl electrolyte solution the following chemical reactions are observed¹².

Reactions at Cathode:

Na⁺ + e^-  Na

Na + H₂O  NaOH + H⁺

2H⁺ + 2e^-  H₂

It shows that only hydrogen gas will evolve at cathode.

When pure iron is being machined electrochemically the following reactions would occur^13-14.

Fe  Fe²⁺+ 2e^-

Fe²⁺ + 2Cl^-  FeCl₂

Fe²⁺ + 2(OH)^-  Fe(OH)₂

FeCl₂ + 2(OH)^-  Fe(OH)₂ + 2Cl^-

2Cl^- Cl₂ + 2e^-

2FeCl₂ + Cl₂  2FeCl₃

H⁺ + Cl^-  HCl

2Fe(OH)₂ + H₂O +O₂  2Fe(OH)₃

Fe(OH)₃ + 3HCl  FeCl₃+ 3H₂O

FeCl₃+ 3NaOH  Fe(OH)₃ + 3NaCl

It shows that during electrochemical machining of iron in NaCl electrolyte, iron is removed as Fe(OH)2 and precipitated as sludge while sodium chloride is recovered back. Due to further reaction, formation of Fe(OH)3 is also possible Which, confirms the existence of iron in +2 and +3 states during dissolution.

Determination of Fe²⁺ and Fe³⁺ ions in electrolyte solution:

The electrolyte solution containing Fe⁺² and Fe⁺³ ions was collected. Fe⁺² ions were determined directly by titrating a known volume of iron electrolyte solution with K₂Cr₂ O₇ in acidic medium (HCl).

Cr₂O₇ ^2- + 6Fe⁺² + 14H⁺ = 2Cr⁺³ + 6Fe ⁺³ + 7H₂O

Internal indicator N- phenyl anthranilic acid was used to mark the end point. Fe⁺³ ions were determined after all the Fe⁺³ ions are reduced into Fe⁺² ions with SnCl2 in presence HCl in hot.

Sn⁺² + 2Fe ⁺³ = Sn⁺⁴ + 2Fe⁺²

The solution was then cooled and excess SnCl₂ was removed by adding HgCl₂ solution.

2Hg⁺² + Sn⁺² +Cl^- = Sn⁺⁴ +Hg₂Cl₂ (white ppt)

Titration of known volume of standard solution was done using standard solution of K₂Cr₂O₇ in acidic medium. From the volume of K₂Cr₂O₇ used, the total amount of Fe⁺² and Fe⁺³ ions was determined. The amount of Fe⁺³ ion was determined by subtracting amount of Fe⁺² which is determined earlier.

Material removal rate during electrochemical machining is greatly influenced by dissolution valence. As the dissolution valence decreases MRR increases. In this paper an approach is made to enhance the electrochemical dissolution of iron through control of valency (transition) therefore, in this direction, use of electrolyte solution containing Cu2+ is suggested. The dissolution limit of iron by Cu²⁺ ions can be is justified by considering the standard electron potential E° for Cu2+, Fe/Fe²⁺and Fe/Fe³⁺ described as follows¹⁵.

Cu2+ + 2e- Cu E° = +0.34V Fe2+ + 2e- Fe E° = -0.44V Fe3+ + e- Fe2+ E° = +0.77V

As E° for Cu2+ Cu is more positive than Fe2+ Fe, Cu2 +will oxidize Fe to Fe2+. However, as E° for Cu2+ Cu is less positive than Fe3+ Fe2+, Cu2+ will not oxidize Fe2+ to Fe3+. Making electrolyte solution:

250 gramsof NaCl was mixed with400 gramsof CuSO4 in10 litersof water. The mixture is stirred well for 2 minutes then heated until it loses its green color. The crystals of sodium sulphate (Na₂SO₄) and copper chloride (CuCl₂) were removed by filtering the solution and thi the solution thus obtained was saturated solution of Na₂SO₄ containing Cu²⁺ ions which participates in anodic dissolution process.

MACHINING CONDITIONS:

Following machining parameters are selected on the basis of performance characteristics,

Table1: Machining conditions for analysis

Machining Conditions	Specifications
Constant Parameters
Inter electrode gap	0.0010 (cm)
Power Supply	2-50 V DC
Electrolyte flow rate	8 l//min
Electrolyte type	NaCl aqua solution mixed with Cu2+
Electrolyte concentration	5.0 M
Tool Material	Brass
Work piece	Iron
Machining time	2 minutes
Variable Parameters
Applied voltage	10V - 18V
Feed Rate	0.0001 – 0.0005 cm/s
Electrolyte concentration	10 – 30 g/lit

SELECTION OF MACHINING PROCESS PARAMETERS

Table 2 shows machining parameters and selected levels for experimental procedure

Table 2: Process parameter and their levels

Parameters	Code	Levels
		1	2	3
Voltage (V)	A	10	14	18
Feed Rate (cm/sec)	B	0.0001	0.0003	0.0005
Electrolyte Concentration (g/lit)	C	10	20	30

Measurement of MRR

The initial weight of the work piece was taken for calculation of MRR. Keeping the flow rate constant at 15 lit/min and the rest of the parameters are set according to table 1 for each run. Work piece was kept horizontal, and cylindrical electrode was used for machining. Gap between

tool and workpiece was maintained carefully to avoid the choking. The electrode was fed continuously towards the work piece during machining and time was recorded. After machining, the cavity was formed on the work-piece. The final weight of the work-piece was taken and material removal rate was calculated as per the following formula:

MRR= ………………. (1)

EXPERIMENTAL PROCEDURE:

The design resulted in total of eighteen experiments, which are performed at 10V-18V supply voltage, 10-30 g/lit electrolyte concentration and 0.0001-0.0005 cm/sec feed rate as the values for the control variables. The responses measured are Material removal rate (MRR) Scheme of the experiments is as shown in Table 3.

Table 3: Taguchi L₉ OA for MRR

Expt. No	A	B	C	MRR (cm3/sec)
				In presence of aqueous NaCl electrolyte solution	In presence of electrolyte solution containing Cu2+ ions
1	10	0.0001	10	0.0220	0.0251
2	10	0.0003	20	0.0391	0.0445
3	10	0.0005	30	0.0627	0.0721
4	14	0.0001	20	0.0370	0.0428
5	14	0.0003	30	0.0595	0.069
6	14	0.0005	10	0.0614	0.0718
7	18	0.0001	30	0.0536	0.063
8	18	0.0003	10	0.0600	0.0711
9	18	0.0005	20	0.0653	0.0774

RESULTS AND DISCUSSION

Analysis of Variance (ANOVA) when machinating in presence of NaCl electrolyte solution: Percentage contribution of each parameter on material removal rate during electrochemical machining of iron in aqueous NaCl electrolyte solution is shown in table 4 and represented graphically in figure 2.

Table 4: ANOVA for MRR [NaCl as electrolyte]

Source	DF	Sum of Squares	Mean of Squares	F-Value	P Value	Contribution
Voltage(V)	1	0.000506	0.000506	20.42	0.006	28.3%
Feed Rate(cm/sec)	1	0.000983	0.000983	39.68	0.001	55%
Electrolyte Concentration(g/lit)	1	0.000175	0.000175	7.06	0.045	9.8%
Error	5	0.000124	0.000025			6.9%
Total	8	0.001788
S = 0.0049774 R-Sq = 93.07% R-Sq(adj) = 88.91%

Fig 2. Contributions of the parameters when machining in presence of aqueous NaCl electrolyte solution

Regression Equation:

MRR= -0.01096 +0.002296Voltage +64.0FeedRate +0.000540Concentration. ………(2)

The equation (2) shows that Feed rate is dominant factor affecting MRR. The graphs shown in figure 3 are plotted from the regression equation (2).

Fig 3. Main Effects Plot for SN ratios (NaCl electrolyte solution)

Figure shows the main effect plot of the MRR depicting the effect of various machining parameters on MRR. As seen from the plot obtained, the MRR increased with increase in both voltage and feed rate. This is due to the fact that with increase in voltage the current increases in the inter electrode gap thus increasing the MRR. Feed rate is another important parameter. Increase in feed rate results in decrease of the conducting path between the workpiece and the tool hence resulting in high current density thus enhancing the rapid anodic dissolution. An overall increase in the MRR was also observed with increase in the concentration as the larger number of ions associated with the machining process which increases the machining current and thus results in higher MRR.

Effects of selected process variables (i.e. Voltage, Feed rate and Concentration) on material removal rate (MRR) at different sets of conditions while machining in presence of aqueous NaCl solution are shown in figure 4(a), 4(b) and 4(c).

Fig. 4(a) Effects of Voltage on material Fig. 4(b) Effects of Feed rate on material

removal for different Concentration, removal for different Voltage,

Feed rate= 0.0001 cm/sec. Concentration = 20 g/lit.

Fig.4(c) Effects of Concentration on material removal for different Feed rates, Voltage= 14 V

NaCl electrolyte tend to promote the oxidation of Fe²⁺ to Fe³⁺ during the dissolution process the maximum MRR obtained during machining of iron in aqueous NaCl solution recorded was 0.0653 cm³/sec. Although the higher concentration of NaCl is favorable for better MRR but excess concentration allows the crystal formation which reduces the volume of electrolyte in flow pipes and also affects the dissolution rate.

Analysis of variance when machining in presence of electrolyte solution containing Cu²⁺ ions

Percentage contribution of each parameter on material removal rate during electrochemical machining of iron in electrolyte solution containing Cu²⁺ ions is shown in table 5 and represented graphically in figure 5.

Table 5 ANOVA for MRR [electrolyte solution containing Cu²⁺ ions]

Source	DF	Sum of Squares	Mean of Squares	F-Value	P Value	Contribution
Voltage(V)	1	0.000812	0.000812	24.86	0.004	31.8%
Feed Rate(cm/sec)	1	0.001362	0.001362	41.69	0.001	53.3%
Electrolyte Concentration(g/lit)	1	0.000217	0.000217	6.65	0.050	8.5%
Error	5	0.000163	0.000033			6.4%
Total	8	0. 002555
S = 0.0057157 R-Sq = 93.61% R-Sq(adj) = 89.77%

Fig 5. Contributions of the parameters when machining in presence of electrolyte solution containing Cu²⁺ ions

Regression Equation:

MRR = -0.0157 +0.002908Voltage +75.3FeedRate +0.000602Concentration. .……. (3)

The equation (3) shows that voltage is dominant factor affecting MRR. The graphs shown in

figure 6 are plotted from the regression equation (3).

Fig 6. Main Effects Plot for SN ratios (electrolyte solution containing Cu²⁺ ions)

The oxidation of Fe²⁺ in to Fe³⁺ is restricted due to the presence of Cu²⁺ in electrolyte solution which promotes the higher dissolution rate during machining. The influence of selected process variables i.e. Voltage, Feed rate and Concentration on material removal rate at different sets of conditions in presence of electrolyte solution containing Cu²⁺ ions are shown in figure 7(a), 7(b) and 7(c) respectively.

Fig. 7(a) Effects of Voltage on material Fig. 7(b) Effects of Feed rate on material

removal for different Concentration, removal for different Voltage,

Feed rate= 0.0001 cm/sec. Concentration = 20 g/lit.

Fig. 7(c) Effects of Concentration on material removal for different Feed rates, Voltage= 14 V.

The maximum MRR obtained during machining of iron in presence of Cu² electrolyte solution containing Cu²⁺ ions was 0.0774 cm³/sec, which is 18.5% more when compared with aqueous NaCl electrolyte.

CONCLUSION

The electrochemical characteristics of iron in aqueous NaCl solution and electrolyte solution containing Cu²⁺ ions has been analyzed experimentally to investigate the influence of process parameters on MRR. The Process parameters such as voltage, feed rate, Electrolyte concentration, were successfully controlled. The different combinations of these parameters were used for the experimentation in order to determine their influence on MRR. The experiment was performed by varying all parameters in combination as per L₉ orthogonal array. The experimental observations support the conclusion that the presence of Cu²⁺ ions in electrolyte solution restrict the further oxidation of Fe²⁺ to Fe³⁺ and enhance the low valence dissolution of iron during machining. Design of experiments and analysis of variance helped in identifying the significant parameters affecting MRR. The best combination of the parameters are Voltage= 18 V, Feed Rate=0.0005 cm/sec and electrolyte Concentration = 20 g/lit when using a solution containing Cu²⁺ ions as electrolyte. The maximum MRR obtained was 18.5 % higher when compared with aqueous NaCl electrolyte for the same set of working conditions.

Acknowledgement:

I express my sincere thanks to Department of Applied Chemistry BIT Extension Centre Deoghar for their cooperation to conduct the experiments in order to observe the catalytic behavior of Cu²⁺ ions.

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