ELECTRODEPOSITION OF LEAD COATINGS FROM A METHANESULPHONATE ELECTROLYTE

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1 Journal Vyacheslav of Chemical S. Protsenko, Technology Elena and A. Metallurgy, Vasil eva, Felix 50, 1, I. 2015, Danilov ELECTRODEPOSITION OF LEAD COATINGS FROM A METHANESULPHONATE ELECTROLYTE Vyacheslav S. Protsenko, Elena A. Vasil eva, Felix I. Danilov Ukrainian State University of Chemical Technology, Gagarin Ave. 8, Dnepropetrovsk, 49005, Ukraine Vprotsenko7@gmail.com Received 28 July 2014 Accepted 02 December 2014 ABSTRACT Lead coatings were deposited from an electroplating bath on the basis of methanesulphonic acid. Kinetics of lead electrodeposition process was studied by means of linear voltammetry. The electrochemical process of Pb(II) ions discharge was stated to be irreversible; the standard rate constant and transfer coefficient being m s -1 and 0.47, respectively. Coarse-crystalline coatings were shown to deposit from electrolyte containing only lead methanesulphonate and free methanesulphonic acid. An organic additive was proposed to improve the surface appearance and surface morphology of lead deposits. This additive belongs to polyoxyethylene derivatives of naphthol. It inhibits the reaction of Pb(II) ions electroreduction and provides obtaining high-quality uniform lead coatings with a fine crystalline structure. Keywords: lead, electroplating, methanesuphfonate bath, electrodeposits, kinetics. INTRODUCTION Electrodeposited lead and its alloys are widely used in modern industry as antifrictional, protective, solderable coatings in numerous engineering, communications, military and consumer product applications [1-4] as well as in soluble lead-acid flow batteries [5]. Lead electrodeposits are usually obtained from various acid solutions (nitrate, fluoroborate, fluorosilicate, perchlorate, pyrophosphate, acetate, etc.) [1-2], although alkaline baths have been also reported [7]. The common fluoroborate electrolytes used for Pb electrodeposition are very harmful and toxic. Acid aqueous solutions of Pb(II) based on methanesulphonic acid (MSA) seem to be attractive and perspective systems for lead electrodeposition as MSA is considered as a green acid due to its environmental advantages [8]. MSA is known to be far less corrosive and toxic than the usual minerals acids used in different branches of industry [9, 10]. Methanesulphonates of various metals are highly soluble in water, the conductivity of corresponding aqueous solutions is high. In addition, MSA is easily biodegradable [11]. Because of these advantages, electrochemical systems containing MSA and its salts have been shown to be very promising for metal and alloys electroplating. Evidently, the successful development of novel lead electrodeposition processes should be based on serious kinetic studies of electrochemical reactions involved. According to the classification proposed by Winand [12], Pb belongs to the group of normal metals which are characterized by very high exchange current densities; thus, electrodeposition of lead is a fast electrochemical reaction (i.e. reversible process). It was shown recently [13] that the electrodeposition process of lead from nitrate solutions is either mixed ohmic-diffusion or completely ohmic controlled. The polarographic behavior of lead ions in methanesulphonate solutions was reported in work [14]. The electroreduction process on dropping mercury electrode was stated to be reversible and diffusion-controlled. Similar conclusion has been drawn in study [15]. 39

2 Journal of Chemical Technology and Metallurgy, 50, 1, 2015 However, the rate of an electrochemical reaction appreciably depends on the nature of electrode involved. The kinetic parameters of Pb(II) ions electrodeposition on the solid electrode may differ from those typical of the discharge on dropping mercury electrode. In this connection, the electrochemistry of Pb(II)/methanesulphonate system has not been fully elucidated. Therefore, the aim of this study is to report the kinetic characteristics of Pb(II) ions discharge on a solid lead electrode and to investigate the main features of lead electroplating in MSA-containing bath. EXPERIMENTAL All solutions were prepared using distilled water and reagent grade chemicals. Lead (II) methanesulphonate was synthesized by the procedure described in detail in [8]. Polarization curves were obtained using Potentiostat Reference 3000 (Gamry). The ohmic potential drop was measured and automatically compensated by means of the built-in IR-compensator of the potentiostat. The working electrode was a platinum plate (S = 2 cm 2 ) on which a Pb-coating (~3 µm) was deposited prior each experiment. All potentials were measured with respect to the saturated Ag/AgCl-electrode and recalculated to a standard hydrogen electrode. The counter electrode was made of lead sheet. Electrodeposition of lead in a galvanostatic mode was performed on copper plate (S = 4 cm 2 ). The current efficiency was determined on the basis of gravimetric measurements. All electrochemical experiments were carried out in a conventional glass three-electrode cell deaerated by blowing with electrolytic hydrogen. The electrochemical cell was thermostated at 298 ± 0.1 K. The morphology of the deposits was investigated by scanning electron microscopy (EVO 40XVP). The samples used in SEM-study were electroplated on the electropolished Cu-foil, the thickness of deposits being about 20 mm. RESULTS AND DISCUSSION As follows from data presented in Fig. 1 (curve 1), the hydrogen evolution reaction in MSA-containing Fig. 1. Voltammograms obtained on Pb electrode in solutions containing (mol L -1 ): free MSA (1), MSA and Pb(II) (2). Scan rate 50 mv s -1. solution occurs on the Pb-electrode at very negative values of electrode potential (< 1 V) due to a high hydrogen overpotential on this metal. Therefore, the current peak at E ~ 0.3 V in Pb(II) containing electrolyte corresponds to the electrochemical reaction of lead ions discharge (Fig. 1, curve 2). An appreciable increase in current density at E < 0.4 V may be associated with the growth of lead electrode surface under the conditions of metal electrodeposition at nonsteady diffusion limiting current. Indeed, formation of fern-shaped coatings was detected during the electrodeposition in this range of electrode potential. An increase in the potential scan rate leads to the shift of peak potential towards more negative values (Fig. 2) which means that the electrochemical process has an irreversible character [16]. The following well-known equation is valid for the peak potential of irreversible electrochemical processes [16]: RT = + [0.78 ln + ln ] (1) nf 0 EP E ks Db where E P is the peak potential (V), E 0 is the standard potential (V), is the transfer coefficient, D is the diffusion coefficient of electroactive species (m 2 s -1 ), k S is the standard rate constant of the discharge reaction (m s -1 ), nf b= v, v is the potential scan rate (V s -1 ), n RT is the number of electrons in a limiting stage of the multielectron discharge reaction, F is the Faraday number, and T 40

3 Vyacheslav S. Protsenko, Elena A. Vasil eva, Felix I. Danilov Table 1. Dependence of current efficiency of lead electrodeposition on the cathodic current density. Current density, -2 Fig. 2. Voltammograms obtained on Pb electrode in solutions containing M free MSA and M Pb(II) at different scan rates (mv s-1): (1) 5, (2) 10, (3) 50, (4) 100, (5) 150, (6) 200. is the thermodynamic temperature. According to Eq. (1), the charge transfer coefficient can be easily calculated from the slope of the linear EP vs. lnv dependence. Plotting the experimental data in this coordinates allowed us to determine the value a = 0.47 which is close to 0.5. From that we suppose that the transfer of the first electron is the limiting stage at the discharge of Pb(II) ions (i.e. n = 1). The following expression for the peak current of an irreversible electrochemical reaction is valid [16]: ip = 0.28nFsc0 π Db (2) where ip is the peak current, n is the number of electron in an overall electrochemical reaction (in our case n = 2), s is the electrode surface area, c0 is the concentration of Current A dm efficiency, % Electrolyte contains M free MSA and M Pb(II) electroactive species in the bulk solution. Then the value of the diffusion coefficient D may be obtained on the basis of Eq. (2) by linearizing the experimental data in the coordinates ip vs. v1/2. The calculated diffusion coefficient proved to be equal to D = m2 s-1. This value of diffusion coefficient is close to that reported previously in study [15]. Inserting the obtained values of a and D into Eq. (1), we calculated the standard rate constant ks = m s-1 (at the standard potential Е0 = V of electrochemical couple Pb(II)/Pb(0)). According to data given in [17], an electrochemical reaction with such a value of standard rate constant may be considered as irreversible under the conditions of linear voltammetry. The current efficiency of lead electrodeposition from methanesulphonate electrolyte is practically 100 % if the current density does not exceed 4 A dm-2 (Table 1). When current density becomes more than 4 A dm-2, the current Fig. 3. SEM images of the surface of Pb coatings deposited from the bath containing M free MSA, M Pb(II) without additive PD (A) and with 0.5 g L-1 additive PD (B). Current density 4 A dm-2. 41

4 Journal of Chemical Technology and Metallurgy, 50, 1, 2015 Table 2. Kinetic parameters of Pb(II) electrodeposition reaction. Electrolyte k S, m s -1 D, m 2 s -1 Without PD With 0.5 g L -1 PD Electrolytes contain M free MSA and M Pb(II) efficiency diminishes as a result of reaching the limiting current density of metal deposition reaction and the gaseous hydrogen starts to evolve on the cathode. Intensive formation of dendrites is observed on the deposits surface at these elevated values of current density. It should be noted that the electrodeposition of lead from aqueous solutions containing only free metanesulphonic acid and lead methanesulphonate results in obtaining coarse-crystalline coatings which demonstrate a tendency to the formation of fern-shaped films. In order to improve the surface appearance of deposits and obtain high-quality Pb coatings, one can apply special organic additives and surfactants [6]. We have chosen an organic additive PD [5] which allows depositing high-quality lead coatings from methanesulphonate bath under study. The additive PD is a polyoxyethylene derivative of naphthol. The improvement in deposit quality on addition of PD could be seen by eye as it significantly increases the reflectivity of the deposit; the lead coatings become smooth and uniform without any burnings on the surface. Fig. 3 represents the SEM images of Pb coatings obtained from methanesulphonate electroplating baths. Lead deposit obtained from the bath, which does not contain organic additive, shows high roughness with irregular crystallites. The introduction of PD into the plating bath leads to diminishing the crystallites size and smoothing the surface. One can conclude that the additive PD controls the size of the lead crystals and also the rate of nucleation of further crystallites, thereby limiting crystallite growth and giving a substantially more uniform deposit. PD is a water soluble polymer which can adsorb on the electrode surface during electrodeposition and, consequently, inhibit the electrochemical reaction and provide the formation of fine-crystalline coatings. In order to confirm these assumptions, we obtained a series of linear voltammograms. These curves are not given in 42 the paper. They are very similar to those presented in Fig. 2 at different sweep rates in electrolyte containing additive PD and determined the main kinetic characteristics of Pb(II) electroreduction using the algorithm described above. As can be seen from the obtained data (Table 2), the addition of PD to the electroplating bath has a very little effect on the charge transfer coefficient and diffusion coefficient, whereas the standard rate constant of Pb(II) ions discharge appreciably decreases. Thus, the additive PD inhibits selectively the discharge of Pb(II) ions in the methanesulphonate bath and leads to the formation of high-quality uniform coatings with a fine crystalline structure. CONCLUSIONS Lead electrodeposition from methanesulphonate electrolytes was shown to proceed irreversibly; the standard rate constant and transfer coefficient being equal to m s -1 and 0.47, respectively. The introduction of additive PD (i.e. a polyoxyethylene derivative of naphthol) into the lead plating bath results in a decrease in the rate of Pb(II) ions electroreduction process. Additionally, the application of this organic additive allows improving the surface morphology of lead coatings obtained from the methanesulphonate bath. REFERENCES 1. A.T. Kuhn, The Electrochemistry of Lead, Academic Press, London, N.V. Parthasaday, Practical Electroplating Handbook, Prentice-Hall, Englewood Cliffs, NJ, C.S. Chen, C.C. Wan, Y.Y. Wang, Electrodeposition of tin-lead alloy on a rotating disk electrode in methane sulphonic acid solutions, Trans. Inst. Metal Finish., 76, 1998, F.I. Danilov, E.A. Vasil eva, T.E. Butyrina, V.S. Prot-

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