Experimental Investigations of Catalytic Effect of Cu2+

Experimental Investigations of Catalytic Effect of Cu2+ 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 components2-3. 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 methods4-5. 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 requirements6-7. The dissolution rate is highly reliant on the selection of electroly tes 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 role8-9. 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 à ¯Ã†â€™Ã‚ ³ Na Na + H2O à ¯Ã†â€™Ã‚ ³ NaOH + H+ 2H+ + 2e à ¯Ã†â€™Ã‚ ³ H2 It shows that only hydrogen gas will evolve at cathode. When pure iron is being machined electrochemically the following reactions would occur13-14. Fe à ¯Ã†â€™Ã‚ ³ Fe2++ 2e Fe2+ + 2Cl à ¯Ã†â€™Ã‚ ³ FeCl2 Fe2+ + 2(OH) à ¯Ã†â€™Ã‚ ³ Fe(OH)2 FeCl2 + 2(OH) à ¯Ã†â€™Ã‚ ³ Fe(OH)2 + 2Cl 2Cl Cl2 + 2e 2FeCl2 + Cl2 à ¯Ã†â€™Ã‚ ³ 2FeCl3 H+ + Cl à ¯Ã†â€™Ã‚ ³ HCl 2Fe(OH)2 + H2O +O2 à ¯Ã†â€™Ã‚ ³ 2Fe(OH)3 Fe(OH)3 + 3HCl à ¯Ã†â€™Ã‚ ³ FeCl3+ 3H2O FeCl3+ 3NaOH à ¯Ã†â€™Ã‚ ³ 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 SELECTION OF MACHINING PROCESS PARAMETERS Table 2 shows machining parameters and selected levels for experimental procedure Table 2: Process parameter and their levels 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= †¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦. (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 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] 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. †¦Ã¢â‚¬ ¦Ã¢â‚¬ ¦(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] 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. .†¦Ã¢â‚¬ ¦. (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 whe n 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. 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