High Rate Anodic Dissolution of Stainless Steel 316 (SS316) Using Nano Zero Valent Iron as Reducing Agent

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1 Journal of Applied Science and Engineering, Vol. 19, No. 1, pp (2016) DOI: /jase High Rate Anodic Dissolution of Stainless Steel 316 (SS316) Using Nano Zero Valent Iron as Reducing Agent R. K. Upadhyay 1 *, Arbind Kumar 2 and P. K. Srivastava 3 1 Department of Mechanical Engineering, Birla Institute of Technology, Deoghar Campus, India 2 Department of Mechanical Engineering, Birla Institute of Technology, Mesra-Ranchi, India 3 Department of Applied Chemistry, Birla Institute of Technology, Deoghar Campus, India Abstract The experiments on electrochemical machining of stainless steel SS316 have been carried out according to designed experimental plan to observe the effect of nano zero valent iron (NZVI) mixed electrolyte solution on material removal rate. Dependence of material removal rate on feed rate in presence of NZVI has been determined and compared with theoretical values. These comparative investigations for MRR are given in tabular format with graphical representation. The significant effect on MRR was observed; when NZVI mixed aqueous NaCl solution was used as an electrolyte due to its high reducing characteristics. The observed experimental results were found to be quite close with theoretical results. Key Words: Nano Zero Valent Iron (NZVI), NaCl Electrolyte, SS316, Material Removal Rate, ECM 1. Introduction *Corresponding author. ritesh.upadhyay@bitmesra.ac.in The electrochemical machining process is best suited for manufacturing sophisticated and precise parts used in various technologically advanced industries like Aerospace, Automotive and Medical and possess much importance in the field of electronics and other high-tech industries for the fabrication of micro components [1]. ECM offers high dissolution rate, with ability to produce highly finished machine surfaces regardless of their hardness. Electrochemical machining is investigated as a very good alternative for machining difficult-to-cut materials at atomic level and to shape sculptured surfaces without tool wear and without inducing residual stress [2 4] even though the potential of ECM while machining of metals and their alloys still remains unexplored due to some problems associated with over voltage, gas evolution conductivity of electrolyte, heat transfer, and variable valencies of work piece metal [5 7]. Stainless steel 316 is the standard molybdenum-bearing grade with good resistance to a wide range of chemicals, chlorides and pitting. Due to improved machinability and outstanding welding characteristics SS316 is extensively used in heavy gauge welded components. The study of ECM performance using different compositions of electrolyte is helpful in efficient working of the process [8]. Therefore, in this direction, nano zero valent iron is considered as reducing agent to prevent the oxidation of metal atom in to higher valence state by utilizing its high reducing power. This work is aimed to study the ECM characteristics of SS316 with aqueous NaCl electrolyte and NZVI mixed aqueous NaCl electrolyte. 1.1 Chemistry of Chromium Although the oxidation states of chromium ranges from -2 up to +6, but only two (+3 and +6) are dominant

2 48 R. K. Upadhyay et al. in the surface due to their stability under most surficial conditions [9]. It has been reported that Cr(III) being the most stable state followed by the divalent state, in aqueous solution. The Latimer diagrams shown below show the variable chromium states in acid and basic solutions [10]. The variable oxidation states of chromium in acidic solutions are represented as follows: The variable oxidation states of chromium in basic solutions are represented as follows: From the above diagrams it is clear that the chromium hexavalent is unstable in acidic conditions, but reduced to the trivalent state, being a powerful oxidant. Thus the most stable oxidation state of chromium is +3. It has been also reported that when stainless steel alloys are subjected to electrochemical machining, part of the chromium metal in the alloy is converted to hexavalent chromium which is toxic in nature and unsafe [11 13]. A group of researchers have studied the electrochemical machining (ECM) characteristics of SS316 in chloride solution and reported that the dissolution characteristics of stainless steels can be controlled by using suitable reducing agent [14,15]. 1.2 Preparation of Nano Zero-Valent Iron Particles NZVI particles are synthesized by reduction method using Sodium borohydride (NaBH 4 )andfecl 3 6H 2 O mixture at ambient conditions [16]. The NaBH 4 is used as a reducing agent in order to promote the reduction of Fe 3+ to produce zero valent iron. The steps followed for the preparation are mixing, separation, washing and drying. 0.2 M of NaBH 4 solution mixed with 0.5 M FeCl 3 6H 2 O and reaction mixture was stirred well. The rapid formation of fine black iron particles were detected immediately after the addition of the NaBH 4 to the FeCl 3 solution. Excessive borohydride was applied in order to accelerate the reaction which ensures the uniform growth of iron particles. These particles were then separated from the solution and then washed 3 times with deionized (DI) water and ethanol. The resulted reaction occurs as: The particle size of nano zero valent iron produced in this study, measured by TEM analysis (model no. JEM- 2010). The average size of NZVI was found between nm. The TEM image of NZVI is shown in Figure Reduction of Cr(VI) to Cr(III) On passing electric current through the solution, positive and negative ions move towards cathode and anode respectively. Aqueous NaCl dissociates as NaCl Na + +Cl and H 2 OasH 2 O H + +OH Further reactions in presence of aqueous NaCl cause the production of soluble CrO 4 2 (dissolution valence 6). However, in presence of NZVI particles in aqueous NaCl, low valence dissolution (consistent with valency = 3) occurred. These results are shown by following chemical reactions. Chromate ions react with hydrogen ions and form water as 2CrO H + Cr 2 O H 2 O 3Fe 0 + Cr 2 O H 2 O 3Fe 2+ +2Cr (OH) 3 +8(OH ) Figure 1. TEM image of nano zero valent iron prticles.

High Rate Anodic Dissolution of Stainless Steel 316 (SS316) Using Nano Zero Valent Iron as Reducing Agent 49 2. Experimental Details The Schematic diagram of ECM set up is shown in Figure 2.

3 High Rate Anodic Dissolution of Stainless Steel 316 (SS316) Using Nano Zero Valent Iron as Reducing Agent Experimental Details The Schematic diagram of ECM set up is shown in Figure 2. It comprises a low voltage DC (2 50 V) power source, electrolyte supply system, tool and tool feed mechanism, work and work holding system. The machining was performed in rectangular flow chamber. 2.1 Selection of Workpiece, Tool Material and Electrolyte A rectangular block of dimension 10 cm 5cmmade up of SS316 was selected as workpiece. chemical composition of SS316 is Cr 18%, Ni 14%, Mo 3.0%, Mn 2.0%, Si 0.75%, N 0.10% C 0.08%, P 0.045% and S 0.03%. The tool made up of brass with circular cross section of diameter 12 mm was selected as cathode. A central hole of diameter 4 mm through the tool was used to feed the electrolyte axially in to machining zone. Aqueous NaCl and NZVI mixed aqueous NaCl electrolyte were used to investigate the machining performance. The concentration of NZVI in aqueous NaCl solution was kept constant at 0.25 g/100 cc of solution for each run. The various working parameters of the experiment are summarized in Table Measurement of Material Removal Rate After measuring the initial weight of the work piece it was kept in horizontal position in rectangular chamber, and the gap between tool and workpiece was maintained carefully to avoid the choking. The electrolyte flow rate was maintained at 10 lit/min and the rest of the parameters were set according to Table 1 for each experiment. The electrode was fed continuously towards the work piece during machining and time was recorded. After machining, the cavity was formed on the workpiece. The final weight of the work-piece was taken and material removal rate was calculated as per the following formula: Figure 2. Electrochemical machining set up. Table 1. Experimental details S.N Experimental details 1 Voltage 10 V 2 Feed rate (cm/sec) to cm/sec 3 Electrolyte flow rate 10 lit//min 4 Electrolytes and their concentration Aqueous NaCl (20 g/100 cc of water) and NZVI mixed aqueous NaCl solution (0.25 g/100 cc of solution) 5 Tool material Brass 6 Workpiece SS316 7 Machining time 1.5 minutes

50 R. K. Upadhyay et al. 3. Results and Discussion (1) The machining was performed at different feed rate and corresponding current density was measured.

The plot of current density at different feed rate is shown in Figure 3, the plot of current density with respect to interelectrode gap (IEG) is shown in Figure 4 and corresponding variation of

4 50 R. K. Upadhyay et al. 3. Results and Discussion (1) The machining was performed at different feed rate and corresponding current density was measured. The results reveal that feed rate significantly affects the current passed between tool and workpiece. The effect of feed rate on machining gap and current density is shown in Table 2. The plot of current density at different feed rate is shown in Figure 3, the plot of current density with respect to interelectrode gap (IEG) is shown in Figure 4 and corresponding variation of interelectrode gap with feed rate is shown in Figure 5. Theoretical material removal rate at different current densities was determined by equation 2. current and = density of the material. Theoretical and experimental MRR at the observed value of current densities (25 A/cm 2 to 125 A/cm 2 ) with two different compositions of electrolyte solutions are recorded in Table 3 and corresponding plot is shown in Figure 6. As shown in Figure 6. the MRR in presence of NZVI mixed aqueous NaCl solution is very close to the theoretical MRR compared with that achieved by aqueous NaCl electrolyte due to reducing nature of NZVI which promotes the reduction mechanism of Cr(VI). 4. Conclusions The experimental observation highlights the ECM characteristics of SS316 alloy using aqueous NaCl solution and NZVI mixed aqueous NaCl solution. The role of NZVI during electrochemical machining of SS316 was (2) where F = Faraday s constant = coulombs, I = Table 2. Effect of feed rate on machining gap and current density S.No. Feed rate (cm/sec) Interelectrode gap (IEG) (cm) Current density (A/cm 2 ) Figure 4. Plot of current density with respect to IEG. Figure 3. Plot of current density against feed rate. Figure 5. Plot of interelectrode gap (IEG) with respect to feed rate.

5 High Rate Anodic Dissolution of Stainless Steel 316 (SS316) Using Nano Zero Valent Iron as Reducing Agent 51 Table 3. Theoretical and Experimental MRR at different current densities SN. Feed rate (cm/sec) Current density (A/cm 2 ) Theoretical MRR (cm 3 /s) Experimental MRR with aqueous NaCl electrolyte (cm 3 /s) %error Experimental MRR with NZVI mixed aqueous NaCl electrolyte (cm 3 /s) %error Figure 6. Plot of material removal rate against current density. investigated and the reduction reaction of Cr(VI) has been explored. The experimental results reveal that the electrochemical machining of SS316 in presence of NZVI mixed aqueous NaCl solution proves to be very effective in improving the material removal rate when compared with the machining in presence of aqueous NaCl only, for the same machining conditions. The rates of improvement in MRR are (15.2, 14, 15.0, 13.8, 12.0)% for feed rates of (0.002, , , 0.008, 0.015) cm/sec respectively. Hence, it is concluded that the MRR is improved due to reducing effect of NZVI which promotes the reduction of Cr(VI) to Cr(III). References [1] Bhattacharyya, B., Doloi, B. and Sridhar, P. S., Electrochemical Micro-Machining: New Possibilities for Micro-manufacturing, J. Material Processing Technology, Vol. 113, No. 1, pp (2001). [2] Kozak, J., Rajurkar, K. P. and Makkar, Y., Selected Problems of Micro Electrochemical Machining, Journal of Materials Processing Technology, Vol. 149, No. 1, pp (2004). doi: /j.jmatprotec [3] Rao, S. and Padmanabhan, G., Effect of Process Variables on Metal Removal Rate in Electrochemical Machining of Al-B4C Composites, Archives of Applied Science Research, Vol. 4, No.4, pp (2012). [4] Mukherjee, S. K., Kumar, S., Srivastava, P. K. and Kumar, A., Effect of Valency on Metal Removal Rate in Electrochemical Machining of Aluminium, J. Material Processing Technology, Vol. 202, pp (2008). doi: /j.jmatprotec [5] Datta, M., Anodic Dissolution of Metals at High Rates, IBM Journal of Research and Development, Vol. 37, No. 2, pp (1993). doi: /rd [6] Neto, J. C. D. S., Silva, E. M. D. and Silva, M. B. D., Intervening Variables in Electrochemical Machining, Journal of Material Processing Technology, Vol. 179, pp (2006). doi: /j.jmatprotec [7] Landolt, D., Muller, R. H. and Tobias, C. W., High Rate Anodic Dissolution of Copper, J. Electro Chemi. Soc, Vol. 116, p (1969). doi: / [8] Wang, C. B. and Zhang,W. X., Nanoscale Metal Particles for Dechlorination of TCE and PCBs, Environ. Sci, Technol, Vol. 31, pp (1997). doi: /es970039c [9] Fendorf, S. and Li, G., Kinetics of Chromate Reduction by Ferrous Iron, Environmental Science & Technology, Vol. 30, No. 5 pp (1996). doi: /es950618m

6 52 R. K. Upadhyay et al. [10] Shupack, S. I., The Chemistry of Chromium and Some Resulting Analytical Problems, Environmental Health Perspectives, Vol. 92, pp (1991). doi: / ehp [11] Mount, A. R., Howarth, P. S. and Clifton, D., The Electrochemical Machining Characteristics of Stainless Steels, J. Electrochem. Soc, Vol. 150, No. 3, D63 D69 (2003). doi: / [12] Singh, R., Misra, V. and Singh, R. P., Synthesis Characterization and Role of Zero-valent Iron Nanoparticle in Removal of Hexavalent Chromium from Chromium-spiked Soi, J. Nanopart, Vol. 13, pp (1992). doi: /s y [13] Yang, J. E., Kim, J. S., Ok, Y. S., Kim, S. J. and Yoo, K. Y., Capacity of Cr(VI) Reduction in an Aqueous Solution Using Different Sources of Zero Valent Irons, Korean J. Chem. Eng, Vol. 23, pp (2006). doi: /s [14] Chen, S., Yue, Q., Gao, B., Li, Q., Xu, X. and Fu, K., Adsorption of Hexavalent Chromium from Aqueous Solution by Modified Corn Stalk: A Fixed Bed Column Study, Bioresource Technology, Vol. 113, pp (2011). Retrieved from tech; doi: /j.bior tech [15] Fendorf, S. E. and Zasoski, R. J., Chromium (III) Oxidation by -MnO Characterisation, Environmental Science & Technology, Vol. 26, pp (1992). doi: /es00025a006 [16] Sun, Y. P., Li, X. Q., Cao, J., Zhang, W. X. and Wang, H. P., Characterization of Zero-valent Iron Nanoparticles, Adv. Colloid Interface, Vol. 120, pp (2006). doi: /j.cis Manuscript Received: Jul. 02, 2015 Accepted: Oct. 26, 2015

CONTROL OF H 2 GAS EVOLUTION AT CATHODE DURING ELECTROCHEMICAL MACHINING OF IRON BY USING PALLADIUM BASED MEMBRANES

CONTROL OF H 2 GAS EVOLUTION AT CATHODE DURING ELECTROCHEMICAL MACHINING OF IRON BY USING PALLADIUM BASED MEMBRANES R.K Upadhyay 1, Arbind Kumar 2, P.K Srivastava 3 1 Department of Mechanical Engineering