Indian Journal of Chemical Technology Vol. 17, September 2010, pp. 381-385 Electrolytic deposition of Zn-Mn-Mo alloys from a citrate bath Renu Rastogi* & Archana Pandey Department of Chemistry, Brahmanand PG College, Kanpur 208 001, India Email: renurast765@rediffmail.com Received 13 July 2009; revised 25 May 2010 Zn-Mn-Mo alloys were electrodeposited from a citrate bath and the influence of various parameters such as current density, 2.0-6.0 Adm -2 ; temperature, 20-35ºC;, 2.0-2.15; duration of deposition 20-50 min and concentration of the constituent metals were studied. The electrolytic bath consists of zinc sulphate (20-35 gl -1 ), manganese sulphate (50-80 gl -1 ), ammonium molybdate (3-6 gl -1 ), citric acid (5 gl -1 ) and starch (1 gl -1 ). Semi bright, light gray adherent deposits were generally obtained containing 88.98-99.82% Zn, 0.06-8.70% Mn and 0.10-2.32% Mo. The variation of cathode polarization with current density or the under the conditions was studied. Morphological studies have also been done. Keywords: Electro deposition, Citrate bath, Current density, Zn-Mn-Mo alloy, Polarization In the last decade electro deposition techniques are attracting increasing attention of researchers for the deposition of new thin film alloys since these open up a new possibility in controlling material properties. Moreover, electrochemical deposition offers real advantages to prepare identical thin films with large surface area and a great variety of structures and properties of thin films 1. There are numerous early reports in the literature on electro deposition with ultrafine structures 2. The study of electro deposition of alloys has been increasing because of the properties that can be obtained from these alloys. A number of alloys are found to enhance some important properties of pure metals such as corrosion resistance and hardness among others. Application of thin film of metallic alloys as a coating on base metal minimizes the consumption of rare or expensive alloying elements. Electro depositon of molybdenum alloy produces hard and protective surface coatings that prevent corrosion and their components are non toxic. Molybdenum is one of the several common metals that cannot be deposited alone from aqueous solution. It requires another metal to stimulate its deposition 3. The two important uses of molybdenum are as an alloy in stainless steels and in alloy steel. The ability of molybdenum to withstand extreme temperatures without significantly expanding or softening makes it useful in applications that involve intensive heat variations, including the manufacture of aircraft parts, electrical contacts, industrial motors and filaments. Molybdenum can be implemented both as an alloying agent and as a flame-resistant coating for other metals. Many binary alloys containing Zn, Mn or Mo such as Zn-Fe electrodeposited from sulphate bath 4, Ni-Mo 5 electrodeposited from citrate bath and Zn-Mn 6 from alkaline pyrophosphate bath are reported. Satisfactory deposits of alloy containing Zn electrodeposited from acetate 7 bath has also been reported. A ternary Al-Mn-Mo alloy electrodeposited from Lewis acidic medium is also reported 8. Zn-Co alloys from chloride electrolytes were studied on steel substrates by Lodhi et al. 9. A corrosive study on electro deposition of Ni-Mn by Ananth and Ananthi was also reported 10. But no attention has been given to incorporate molybdenum with zinc and manganese. This idea encouraged the authors to investigate the possibility of electro deposition of Zn-Mn-Mo alloy from a citrate bath under various plating conditions of electrolysis. Experimental Procedure The electro deposition was carried out by electrolyzing about 225 ml of the solution consisting of sulphates of manganese and zinc, ammonium molybdate, citric acid (5 gl -1 ) and starch (1 gl -1 ) for 30 min. Details of electrolytic cell are given elsewhere 11. The deposited alloy film was washed with distilled water and dried at 110ºC. It was then peeled off carefully from the cathode and weighed. A fixed amount of the alloy was dissolved in sulphuric acid for analysis. Zinc was estimated titrimetrically using biphenyl benzedrine as an indicator and standard colorimetric methods 12 were used to estimate their molybdenum and manganese contents by colorimeter (systronics).
382 INDIAN J CHEM. TECHNOL., SEPTEMBER 2010 The cathode current efficiency for the alloy deposition was calculated under different plating conditions from the deposit composition by the usual method 13. The of the electrolytic solution was measured with meter (systronics) and was adjusted to the desired value with dilute sulphuric acid or ammonia. The cathode potential corresponding to the hydrogen scale were determined to an accuracy of +0.001 volt against a standard saturated calomel electrode using an agar-agar bridge drawn into a capillary of approximately 0.1 cm diameter and making a proper contact with the cathode. The required cathode potential was obtained by subtracting the standard potential of the calomel electrode combination. The difference between the potential attained with and without a definite flow of current gave the value of the cathode polarization (η) at a particular plating conditions. Morphological studies were done with the help of microphotographs. Results and Discussion The cathode potential-current density curves for Mn, Mo and Zn and binary Mn-Zn, Mn-Mo, Mo-Zn alloys are shown in Fig. 1. Their relative positions indicate the feasibility of ternary alloy formation of these metals from a citrate bath which is fairly stable under various plating conditions. The operating parameters for depositing alloy films from baths with a wide range of composition under various plating conditions are given in Table 1. Effect of current density The alloy films were generally smooth, semi bright, adherent and light grey at comparatively lower current densities but at higher current densities rough, dark grey and uneven deposits with less adherences were formed. An increase in current density causes the cathode potential to become more negative. The discharge of ions occurs slowly at comparatively low densities and so the rate of the growth of nuclei becomes higher than the rate at which new ones form. As the current density is increased the rate of formation of fresh nuclei increases and deposits become finer grained. At very high current densities, the metal ions in the vicinity of the cathode are removed more quickly, with the result the migration and diffusion of the ions do not supply fresh ions in sufficient quantities, leading to the formation of rough deposits. The amount of molybdenum and manganese in the deposits increase, whereas that of the zinc decreases on increasing current density as shown in Table 2. This might be due to the gradual utilization of more current by molybdenum and manganese in comparison to zinc. Effect of An increase in increases the percentage of manganese and molybdenum in the alloy but decreases that of zinc as shown in Table 3. At lower current densities and deposits were smooth, semi bright, light grey and adherent in nature and become dark gray and uneven on increasing the or current density. A variation in may affect the equilibrium Fig. 1 Cathode potential curves for Zn, Mn and Mo, their binary and ternary alloy at 25 C and of 2.05. Bath composition (gl -1 ): Citric acid, 5.0; starch, 1.0 and (1) ZnSO 4, 30.0(2) MnSo 4, 60.0(3). Ammonium molybdate, 4.0 (4) ZnSO 4, 30.0 and MnSO 4, 60.0(5) ZnSO 4, 30.0 and ammonium molybdate, 4.0(6) MnSO 4, 60.0 and ammonium molybate: 4.0. Table 1 Operating conditions Bath composition Range Optimum Crystalline ZnSO 4.7H 2 O 20.0-35.0 gl -1 30.0 gl -1 Crystalline MnSO 4.H 2 O 50.0-80.0 gl -1 60.0 gl -1 Crystalline (NH 4 ) 2.MoO 3 3.0-6.0 gl -1 4.0 gl -1 Crystalline C 6.H 8 0 7.H 2 O 5.0 gl -1 5.0 gl -1 Starch 1.0 gl -1 1.0 gl -1 2.0-2.15 2.05, ºC 20-35 25 Duration of deposition, min 20-50 30 Current density, Adm -2 2.0-6.0 4.0 Anode Pt Pt Cathode Stainless steel Stainless steel Agitation Nil Nil Deposition composition, % Zn 88.98-99.82 99.30 Mn 0.06-8.70 0.40 Mo 0.10-2.32 0.30
RASTOGI & PANDEY : ELECTROLYTIC DEPOSITION OF Zn-Mn-Mo ALLOYS FROM A CITRATE BATH 383 Table 2 Effect of current density on deposit composition at 25 o C and a of 2.05 Current density (Adm -2 ) Metal in deposit, (%) Cathode current efficiency (%) Current (%) utilized for the deposition of 2.0 0.20 0.10 99.70 42.09 0.17 0.05 41.87 57.91 3.0 0.24 0.12 99.64 41.35 0.20 0.06 41.09 58.65 4.0 0.30 0.40 99.30 41.09 0.25 0.20 40.64 58.91 5.0 0.48 1.06 98.46 38.07 0.37 0.48 37.22 61.93 6.0 0.60 1.18 98.22 37.61 0.46 0.52 36.63 62.39 Bath composition (g/l): Manganese sulphate, 60.0, Zinc sulphate, 30.0, Ammonium molybdate, 4.0, Citric acid, 5.0, Starch, 1.0 Table 3 Effect of temperature and on deposit composition at current density 4.0 Adm -2 Metal in deposit, (%) Cathode current efficiency, (%) Current (%) utilized for the deposition of 20 2.05 0.88 2.32 96.80 47.64 0.85 1.21 45.58 52.36 25 2.05 0.30 0.40 99.30 41.10 0.25 0.20 40.65 58.91 30 2.05 0.24 0.22 99.54 31.18 0.15 0.08 30.95 68.82 35 2.05 No Deposition 25 2.0 0.16 0.20 99.64 30.09 0.10 0.07 29.92 69.93 25 2.10 0.40 1.12 98.48 41.84 0.34 0.46 41.04 58.16 25 2.15 1.40 3.92 94.68 59.41 1.66 2.76 54.99 40.59 Bath composition: Same as given in Table-2 between various metal complexes formed with citric acid present unequally with corresponding changes in the deposit composition. Effect of time of deposition Table 4 demonstrates the effect of time of deposition on alloy composition. It has been observed that the manganese and molybdenum contents increase as the time of deposition is increased from 20 to 50 min. Zinc on the other hand, shows a reverse behaviour probably because of decrease in diffusion of the zinc ions. On increasing time of deposition deposits become rough as the rate of growth of nuclei exceeds the rate of formation of new nuclei. Effect of temperature Comparatively more fine grained, bright, adherent deposits were generally produced at temperatures greater than 25ºC. The amounts of manganese and molybdenum decrease, whereas zinc increases. On the other hand at a temperature below 25 C, a reverse behaviour is noticed as is evident from the results given in Table 3. With increase in temperature no deposition was shown which might be due to increase in diffusion of zinc ions with increase in temperature. At temperature >30 and of 2.05 no deposition were observed. Effect of metal concentration The metal percentage in the deposit increases by increasing its concentration in the electrolytic bath (Table 5). Increasing the zinc or molybdenum concentration at any particular current density led to more fine grained, smooth, light grey highly adherent deposits. However, dark grey rough, less adherent deposits were formed on increasing manganese concentration in the electrolytic bath. On increasing molybdenum and manganese concentration in bath percentage of molybdenum and manganese increases and zinc decreases but when zinc concentration increases percentage of zinc and molybdenum increases and manganese decreases. This might be due to more nucleation of molybdenum ions. An increase in the concentration of particular metal ions in the electrolyte might facilitate the nucleation of that ion in the deposit as a result of which its concentration increases. This is evident from an examination of the distribution of total current utilized by different ions for their deposition. Cathode current efficiency Cathode current efficiency was always found to be less then 100% because of the simultaneous discharge of hydrogen ions along with the metallic ions. The total current efficiency increases with increasing or decreasing temperature of the solution (Table 3). This indicates that relatively more current is utilized for the deposition of the alloy than for the discharge of hydrogen ions provided by the ionization of
[ 384 INDIAN J CHEM. TECHNOL., SEPTEMBER 2010 Table 4 Effect of duration of electrolysis on deposit composition at 25 o C, 2.05 and current density 4.0 Adm -2 Time (min) Metal in deposit, % Cathode current efficiency % Current (%) utilized for the deposition of 20 0.24 0.16 99.60 32.25 0.16 0.06 32.03 67.75 30 0.30 0.40 99.30 41.10 0.25 0.20 40.65 58.91 40 0.34 0.64 99.02 42.20 0.29 0.31 41.59 57.80 50 0.42 1.28 98.50 45.98 0.39 0.58 45.01 54.02 Bath composition: Same as given in Table 2. Table 5 Effect of metal concentration in the bath on deposit composition at 25 C, 2.05 and current density 4.0 Adm -2 Concentration of metal Metal in deposit, % Cathode current compound gl -1 efficiency % Current (%) utilized for the deposition of Bath composition gl -1 Ammonium molybdate 3.0 0.23 0.33 99.44 37.52 0.18 0.15 37.19 62.48 MnSO 4 : 60.0 4.0 0.30 0.40 99.30 41.10 0.25 0.20 40.65 58.90 ZnSO 4 : 30.0 5.0 0.38 0.46 99.16 42.70 0.33 0.23 42.14 57.30 Citric acid:5.0 6.0 0.42 0.50 99.08 45.62 0.49 0.27 44.86 54.38 Starch:1.0 Manganese sulphate 50.0 0.27 0.35 99.38 36.96 0.20 0.15 36.61 63.04 (NH 4 ) 2 MoO 3: 4.0 60.0 0.30 0.40 99.30 41.10 0.25 0.20 40.65 58.90 ZnSO 4 : 30.0 70.0 0.32 0.45 99.23 43.00 0.28 0.23 42.49 57.00 Citric acid:5.0 80.0 0.35 0.51 99.14 46.89 0.33 0.28 46.28 53.11 Starch:1.0 Zinc sulphate 20.0 0.25 0.50 99.25 30.38 0.16 0.18 30.04 69.62 MnSO4: 60.0 25.0 0.27 0.45 99.28 32.69 0.18 0.17 32.04 67.31 (NH4)2MoO3:4.0 30.0 0.30 0.40 99.30 41.10 0.25 0.20 40.65 58.90 Citric acid:5.0 35.0 0.37 0.31 99.32 44.25 0.33 0.16 43.76 55.75 Starch:1.0 solvent. Further it is significant to note that at any given and temperature the efficiency decreases with increasing current density and the time of deposition (Tables 2 and 4). It is due to the consumption of metal ions present in the bath and their consequent depletion in the vicinity of the cathode. However, the total efficiency at any particular and temperature of the bath shows a rising trend with increasing concentration of molybdenum, manganese or zinc in the electrolytic solution (Table 5). Cathode polarization TTable 6 illustrates the variation of cathode polarization with temperature and at different current densities. The cathode polarization becomes more negative with increasing current density and in the ranges 2.0-6.0 Adm-2 and 2.0-2.15 respectively. The above changes in cathode polarization with current density might therefore, be due to hydrogen over voltage. Further, with increasing current density the diffusion of metal ions becomes insufficient to keep pace with their discharge at the cathode with the Table 6 Variations in cathode polarization with temperature and at different current densities. Cathode polarization (v) at current densities (Adm -2 ) 2.0 3.0 4.0 5.0 6.0 20-0.275-0.408-0.524-0.600-0.678 25-0.266-0.367-0.457-0.546-0.623 30-0.225-0.342-0.437-0.505-0.562 35-0.210-0.328-0.415-0.488-0.525 2.0-0.234-0.309-0.433-0.462-0.541 2.05-0.266-0.367-0.457-0.546-0.623 2.10-0.271-0.405-0.495-0.578-0.628 2.15-0.315-0.445-0.540-0.611-0.710 Bath composition: Same as given in Table 2. result that more hydrogen ions get simultaneously discharged which shifts the cathode polarization to more negative values as the current density is increased (Fig. 2). On increasing the of the bath, availability of hydrogen ions decreases at a given current density and thus the polarization shifts to more negative values. On the other hand, it becomes less negative as the temperature is raised from
RASTOGI & PANDEY : ELECTROLYTIC DEPOSITION OF Zn-Mn-Mo ALLOYS FROM A CITRATE BATH 385 Bath composition (gl -1 ) Table 7 Summary of the deposit morphology Current density (Adm -2 ) Morphology of the deposits Zinc sulphate, 30.0; Manganese sulphate, 4.0 25 2.05 Smooth, light grey, compact, deposit 60.0; Ammonium molybdate, 4.0; citric acid, 5.0; starch, 1.0 As for Fig.1 6.0 25 2.05 Dark grey, uneven, crystalline deposit. As for Fig.1 4.0 25 2.00 Smooth grey, fine grained deposit As for Fig.1 4.0 25 2.15 Blackish grey, uneven crystalline deposit. 4.0 20 2.05 Blackish grey deposit with large grains. 4.0 25 2.05 Smooth, grey, deposit with smaller grains. Except zinc sulphate (35.0) 4.0 25 2.05 Dark grey, fine grained compact deposit Except manganese sulphate (70.0) 4.0 25 2.05 Even, fine grained, uniform grey deposit Except ammonium molybdate (5.0) As for Fig.1 Except time of deposition (20 min) 4.0 25 2.05 Even, fine grained, compact deposit with a few scattered gathering of grains of large size. Except time of deposition (40 min) 4.0 25 2.05 Dark grey uneven, crystalline deposit compact with scattered gathering of grains of large size. Conclusion From the results of the experiments it may be concluded that Zn-Mn-Mo alloys are electrodeposited from citrate bath after optimizing various parameters such as current density, temperature and. A smooth, semi bright, light grey adherent deposits are obtained under optimum conditions. Fig. 2 Variation of cathode polarization with logarithm of current density at different temperatures and (1), 2.05 and temp. 20 C; (2), 2.05 and temp. 25 C (3), 2.05 and temp. 30 C (4), 2.05 and temp. 35 C (5), 2.15 temp. 25 C (6), 2.10 and temp. 25 C (7), 2.0 and temp 25 C. 20-35ºC which may be due to a higher rate of diffusion of the depositing ions. Morphology The morphological features of the alloy plates were studied by micro photographs and illustrated in Table 7. It appears that light grey, even; compact and fine grained deposits are formed at comparatively low, current density and at high temperatures. Acknowledgement Authors thank Dr. V K Dwivedi, Principal, Brahmanand P.G. College, Kanpur, India for providing the necessary facilities. References 1 Osaka T, Electrochim Acta; 45 (2000) 3321. 2 Cavallothi P L & Lecis N, Surf Coat Technol; 105 (1998) 232. 3 Bremen A, Electrodeposition of alloys, Vol. I (Academic Press, New York), 1963, 78 4 Karahan I H, J Mater Sci, 42 (2007) 10160. 5 Pavlov M & Marozova N, Protection of Metals, 43 (2007) 459 6 Sylla D, Sayall C, Gadouleaiu C, Rebere J C & Refait P, Surf Coat Technol, 200 (2005) 2137. 7 Inamadar A J, Mujawar S H, Barman S R, Bhosale & Patil P S Semi Cond Sci Technol, 23 (2008) 085013 8 Tsuada T & Hussey C L, J Electrochem Soc, 152 (2005) 620. 9 Lodhi Z F, Mol J M C & Hamer W J, Electrochim Acta, 52 (2007) 5444. 10 Ananth N V & Ananthi P, Int J Hydrogen Energy, 33 (2008) 5779. 11 Shukla R K, Jha S K D & Srivastava S C, J Appl Electrochem, 11 (1981) 697. 12 Vogel A I, A Text book of quantitative inorganic analysis, 3 rd edn (ELBS Longmans Green and Co. Ltd., London), 1961, 787. 13 Srivastava S K; Kashyap R & Srivastava S C, Indian J Technol, 25 (1988) 225.