ELECTROCHEMICAL SYNTHESIS OF POLYPYRROLE (PPy) and PPy METAL COMPOSITES ON COPPER and INVESTIGATION OF THEIR ANTICORROSIVE PROPERTIES Sibel Zor, Hatice Özkazanç Kocaeli University, Department of Chemistry, Kocaeli 4138, Turkey Abstract Copper, which has a wide range of applications in industry, is generally protected by corrosion inhibitors. Benzotriazole is the most common corrosion inhibitor for copper and its alloys in aqueous media and has been widely used for nearly 5 years. Despite being a good corrosion inhibitor, benzotriazole is quite toxic. Therefore, the interest to the polymeric coatings, which are harmless to the environment, has been increased. Polypyrrole (PPy) is one of the most studied conducting polymers due to high conductivity and stability. In this study, firstly copper was passivated by cyclic voltammetry in oxalic acid (H 2 C 2 O 4 ) medium, in order to obtain more compact and adherent PPy films. PPy was coated on the passivated copper surface in oxalic acid medium and then, after doping cations onto this PPy coating, PPy metal (zinc and nickel) composites were obtained by using cyclic voltammetry. The effect of PPy and PPy metal (1 and 3 cycles), composite films on the corrosion of copper in.1 M H 2 SO 4 were investigated by potentiodynamic polarization and impedance measurements. Also, the surface morphologies of the composites were examined by scanning electron microscopy (SEM). It was found from the obtained results that zinc and nickel-doped polymeric composite films were more effective on protection of copper from corrosion. Keywords: Conducting polymers, polypyrrole, corrosion, impedance. 1 Introduction Copper is used as a reactive metal in various electronic devices since it has a relatively low specific resistance and high electron movement. Especially it has been preferred as an intermediate connection material instead of aluminium in microprocessors [1]. Copper, which is commonly used in electronically industry, is generally protected by corrosion inhibitors. For the copper and its alloys in aqueous media, the most commonly used corrosion inhibitor is benzotriazole (Figure 1) [2, 3]. 1
(a) (b) Figure 1: Corrosion inhibitors used for the corrosion protection of copper; a) 1H Benzotriazole, b) 3- Amino-1,2,4-triazole [3]. However, harmful properties of benzotriazole have increased the number of studies on the new type of coatings which are friendlier to the environment [4]. In recent years, successful results have been obtained in the studies based on improving the corrosion protection behaviour of the oxidative metals like copper coated with conducting polymers. Polypyrrole (PPy) is one of the most commonly used conducting polymers due to being synthesized easily and having high conductivity level and environmental stability [5]. In this study, copper electrode surface was coated with PPy in oxalic acid medium by electrochemical method after applying a pre-passivation process. Then, various metal cations (Zn 2+ and Ni 2+ ) were doped onto the PPy film by again using electrochemical method. Corrosion protection behaviours of the obtained PPy, PPy Zn and PPy Ni composite coatings in.1 M H 2 SO 4 was investigated by anodic polarization and impedance measurements. Surface morphologies were examined by SEM. 2 Method All chemicals were provided from Merck. Pyrrole was used as monomer and oxalic acid was used as dopant. 1-2 M ZnCl 2 and 1-2 M NiCl 2 were used for preparation of cation doping..1 M sulphuric acid (H 2 SO 4 ) was used as corrosive medium. Copper was used as working electrode while saturated calomel electrode (SCE) was used as reference electrode and platinum (Pt) electrode was used as counter electrode. Cylindrical shaped copper electrodes were coated with a thick polyester block, leaving one of the base areas of the copper electrodes uncoated. Electrodes with a surface area of.785 cm 2 were obtained. Before synthesis process, the electrode surfaces were polished by using a mechanical polisher with emery papers having various thicknesses (18-4-8-12) and then, washed with tap water, distilled water and acetone, respectively. Square plate copper (1x1cm) electrodes were used to obtain the SEM images of the uncoated copper, PPy coated copper and copper coated with PPy metal composites. Coatings were formed over the plates by using the same processes applied on the cylindrical electrodes. Reference 6 Potentiostat/Galvanostat/ZRA device was used in electrochemical measurements (cyclic voltammetry, potentiodynamic, impedance measurements). For fitting the curves, Echem analysis and ZSimpWin3.21 were used. For graphical drawings, Origin Pro 8.6 programme was used. 2
I(A) 3 Experimental 3.1 PPy and PPy Metal Composite Coating on Copper Main problem in the studies related with the electropolymerization of copper in acidic medium is the oxidation of the copper before the oxidation of the monomer. In order to overcome this problem, the copper surface is passivated before the polymerization process [1, 2, 4-6]. For the pre-passivation of the copper electrode, cyclic voltammogram in.1 M oxalic acid solution between -.5 and 1 V potential range at 2 mv/s scan rate with 5 cycles is given in Figure 2.,1,8,6,4,2, -,2 -,6 -,4 -,2,,2,4,6,8 1, 1,2 E,V(SCE) Cu (pasive) Figure 2: Cyclic voltammogram of the Cu electrode in.1 M H 2 C 2 O 4 between -.5 and 1 V potential range at 2 mv/s scan rate (5 cycles). It was observed that this voltammogram, which is obtained during the passivation for the copper electrode in oxalic acid media independent from the monomer, was very similar with the voltammogram obtained by the other researchers. Passivation mechanism was attributed to the formation of the insoluble products (cuprous oxide, Cu(O x ), Cu(O x ) 2 2- etc.) on the surface. These products lead to the passivation of the electrode by the formation of a thin copper oxalate layer, avoiding the dissolving of the metal without stopping the electrochemical polymerization process [5]. In order to prepare the PPy film on the copper electrode with cyclic voltammetry technique, 3 cycles were performed in.1 M oxalic acid +.1 M pyrrole solution between -.5 and 1 V (SCE) potential range at 2 mv/s scan rate. Finally, an adherent black thin PPy film formation was observed on the electrode surface (Figure 3). Resulting from the oxidation of the pyrrole, a new irreversible peak is observed in I-V curves around.6 V. When there is pyrrole in the oxalic acid solution, the shape of the first cycle slightly changes with the existence of the pyrrole and an oxidation peak is observed at.v. In the following scans, the decrease in the current after the corrosion peak to 2 ma and slowly increase after.6 V is related with the polymerization process of pyrrole. In the next cycles, oxidation-reduction peak of pyrrole is observed between -.4 and.5 V. After 3th scan, the current decreased since the aluminium surface was coated with PPy with increasing scan rate [2]. 3
I(A),12,8,4, -,4 (b) Cu(pasive)-PPy -,6 -,4 -,2,,2,4,6,8 1, 1,2 E, V(SCE) Figure 3: Sequenced polycyclic voltammogram for Cu (passive) electrode in.3 M H 2 C 2 O 4 +.1 M pyrrole at a potential range between -.5 and 1. V (SCE) (3 cycles), 5 mv/s scan rate. After PPy coated electrode was washed with pure water, PANi Zn coating was obtained in 1-2 M ZnCl 2 solution between -.5 and -1 V potential range at 5 mv/s scan rate with 3 cycles by using cyclic voltammetry technique. PPy Ni coating was obtained by using the range between. and -.5 V under the same conditions with PPy Zn. The potential ranges were determined considering the reduction potentials of the metal cations. Half reduction reactions of these cations and the standard electrode potentials are given in the equations 1 and 2. Zn 2+ (aq) + 2e - Zn(s) E = -.76 V (1) Ni 2+ (aq) + 2e - Ni(s) E = -.V (2) Photo images of the passivated copper electrode and copper electrode coated by PPy and PPy Ni composite films are given in Figure 4. (a) (b) (c) Figure 4: Photo images of (a) passivated copper electrode, (b) PPy and (c) PPy Ni coated copper electrode. 4
log I (A/cm 2 ) log I (A/cm 2 ) log I(A/cm 2 ) log I (A/cm 2 ) 3.2 Anodic polarization measurement results of PPy and PPy metal composites formed on the passivated copper For the passivated and non-passivated copper electrode in oxalic acid medium, anodic current potential curves obtained in.1 M H 2 SO 4 solution between -. and 1 V are given in Figure 3a. After corrosion potential, when the potential is increased on positive direction the dissolving rate for both passivated and non-passivated copper electrode increases and exhibits similar behaviours. Corrosion potentials for non-passivated and passivated copper are -78 and -81 mv, respectively. These values show that the cuprous oxalate passive layer provides a low protection on passivated copper [1]. Anodic polarization curve of the copper electrode coated with PPy after passivation is shown in Figure 3b. It is seen that the corrosion potential shifts to anodic direction (- 59 mv) and has less anodic current density. 1-1 1-2 1-3 1-4 1-5 1-6 1-7 Cu 1-8 Cu(pasive) -,2,,2,4,6,8 1, E,V(SCE) (a) Cu and Cu(passive) 1-1 1-2 1-3 1-4 1-5 1-6 1-7 -,2,,2,4,6,8 1, 1,2 E,V(SCE) Cu(pasive) Cu(pasive)-PPy (b) Cu(passive)-Cu PPy 1-1 1-2 1-3 1-4 1-5 1-6 1-7 -,2,,2,4,6,8 1, 1,2 E,V(SCE) Cu(pasive) Cu(pasive)-PPy Cu(pasive)-PPy+1 cycle Zn Cu(pasive)-PPy+3 cycle Zn (c) Cu(passive)-PPy Zn (1 and 3 cycles) 1-1 1-2 1-3 1-4 1-5 1-6 1-7 1-8 -,2,,2,4,6,8 1, 1,2 E, V(SCE) Cu(pasive) Cu(pasive)-PPy Cu(pasive)-PPy+1 cycle Ni Cu(pasive)-PPy+3 cycle Ni (d) Cu(passive)-PPy Ni (1 and 3 cycles) Figure 5: Anodic current potential curves for (a) Passivated and non-passivated copper, (b) PPy, (c) PPy Zn (1 and 3 cycles) and (d) PPy Ni (1 and 3 cycles) coated copper obtained after our at 5 mv/s scan rate. For PPy Zn and PPy Ni coatings and for 1 cycles, the corrosion potentials are - 26.4 and -49.2 mv, respectively. For 3 cycles, these potentials are -46.5 and -49.2 mv, respectively. The corrosion potentials shift to positive direction with increasing amount of cations. Each of the coatings on the copper electrode causes a change in the corrosion potential and reduces the anodic current values at the same time (Figures 5b, c and d). 5
Corrosion of copper in sulphuric acid solution occurs with two anodic reactions. In the first reaction, Cu 2 O film forms and hydrogen ions diffuse (Eq. 3). In the second reaction, Cu 2 O dissolves and turns into Cu +2 form (Eq. 4). Cathodic reaction is the reduction of the oxygen as given in Equation 5 (y is the doping level of the polymer film in the given mechanism) [1]. Anodic reactions: 2Cu + H 2 O Cu 2 O + 2H + +2 e - (3) Cu 2 O + 2H + 2Cu +2 + H 2 O + 2 e - (4) Cathodic reactions: O 2 + 4H + + 4e - 2 H 2 O (5) (PPy y+ yc 2 O 2-4 ) n + 2nye - 2- (PPy) n + nyc 2 O 4 (6) When PPy coated copper electrode is immersed into the corrosive solution, oxalate anions are formed as a result of the redox reaction between the solution and the coating. Oxalate anions and Cu +2 ions together form the insoluble copper oxalate, resulting with the passivation of the metal [7]. These coatings formed on the copper surface cause a barrier effect and avoid the dissolving of the copper. 3.3. AC Impedance Measurement Results Nyquist diagrams obtained after AC impedance measurements performed at various immersion times (, 1, 5, 24, 48 and ours) starting from the first moment that the passivated copper, PPy and PPy metal coated copper were exposed to the corrosive medium are seen in Figure 6. Polarization resistance values of the curves, which were obtained just after the immersion of the passivated copper and the copper coated with PPy into the corrosive solution, are 318.1 and 544.61 Ω, respectively (Figures 6a and b). With increasing immersion time, decrease in the polarization resistance also increased. At the end of the 72 th hour, these values decreased to 182.46 and 79.145 Ω, respectively. Warburg impedance observed in the Nyquist curves of the passivated copper are due to the diffusion of the products formed after corrosion. Polarization resistance measured just after the immersion of the PPy Zn composite films into the corrosive medium is 1131.473 for 1 cycles zinc doping, while it is 1283.22 for 3 cycles. These bigger values compared with the PPy coating show that zinc increases the barrier effect of PPy (Figures 6c and d). The increase observed in the polarization resistance with increasing immersion time can be explained by the fact that the solution passes through the areas on the permeable PPy without zinc coating and increase the dissolving of the copper metal. An increase in the polarization resistance is observed after nickel doping onto PPy film (Figures 6e and f). Polarization resistance is determinate as 656.54 for 1 cycles nickel doping while it is determined as 1616.82 for 3 cycles. It was observed for the PPy Ni composite films that their polarization resistance decreased with increasing immersion time in the corrosive medium. 6
Also, the polarization resistance increases with increasing amount of nickel doped onto PPy. The polarization resistances obtained by nickel doping on PPy surface are close to the values obtained for zinc doping. This shows that the zinc and nickel doped PPy films have similar protective effects. Photo images of the passivated copper and PPy, PPy Ni coated copper in corrosive medium for ours are given in Figures 7a, b and c. It is seen that the corrosive solution passing through the permeable PPy film in PPy and PPy Ni layers causes swelling on the coatings. 14 12 1 8 6 4 2-2 Cu (pasive) h 5 1 15 2 3 35 4 5 (a) Cu(passive) (a) 5 35 Cu(pasive)-PPy 3 2 15 1 5 h 5 1 15 1 2 3 4 5 6 (b) Cu(passive)-PPy 5 2 15 5 5 75 1 3 2 15 Cu(pasive)-PPy+3 döngü Zn h 5 1 5 15 1 5 Cu(pasive)-PPy+1 döngü Zn 2 4 6 8 h (c) Cu(passive) Zn (1 cycles) Cu(pasive)-PPy+1 döngü Ni 35 h 3 2 5 5 1 1 2 3 4 5 6 7 8 (e) Cu(passive) Ni (1 cycles) 1 5 4 35 3 2 15 1 5-5 5 5 1 2 4 6 8 1 (d) Cu(passive) Zn (3 cycles) 5 75 1 Cu(pasive)-PPy+3 döngü Ni 2 4 6 8 (f) Cu(passive) Ni (3 cycles) h Figure 6: Nyquist curves of the passivated and then PPy, PPy Zn (1 and 3 cycles), PPy Ni (1 and 3 cycles) coated copper in.1 M H 2 SO 4 for various immersion times. 7
(a) Cu(passive) (b) Cu(passive) PPy (c) Cu(passive) PPy Ni(3 cycles) Figure 7: Photo images of the passivated copper and PPy, PPy Ni coated copper electrodes in.1 M H 2 SO 4 after ours of immersion time. 3.4. SEM studies of the PPy and PPy Metal composites In Figure 8a, SEM micrograph of the uncoated copper electrode is given. The marks after emery process are seen on the surface. SEM micrographs obtained after pre-passivation process over uncoated copper electrode are given in Figure 8b. After passivation, oxide layers which are thought to result from cuprous oxalate products like cuprous oxide, Cu(O x ) and Cu(O x ) 2 2- are seen [1]. In the SEM micrographs, which are obtained after the passivated copper surface was coated with PPy, it is seen that there are PPy based polymeric particles. These particles are microspherical and pellet shaped (Figure 8c) [4]. Also, it is seen that the PPy layer covered the whole surface. In Figure 8d, it is seen that PPy film is completely coated with zinc and the coating is quite homogeneous. Local particles which are thought to be zinc oxide can be seen. In Figure 8e, it was determined that the morphology of the PPy coated surface changed significantly after coating with nickel and the nickel coating enhanced more homogeneous film formation. It is observed that both zinc and nickel coatings formed in the shape of a thin net. Homogeneous dispersion of zinc and nickel formed on the PPy coating increased the barrier effect of the PPy film. (a) (b) (c) (d) (e) Figure 8: SEM micrographs of (a) non-passivated, (b) passivated copper, (c) PPy, (d) PPy Zn and (e) PPy Ni coated copper electrode. 8
4. Results In this study, corrosion behaviours of PPy, obtained by a pre-passivation of copper electrode surface and electrochemical process and corrosion behaviours of PPy metal composites by doping various metal cations onto PPy were investigated. Lower anodic current values of PPy and PPy metal composites compared with the passivated copper showed that these coatings on the copper surface provide a barrier effect and avoided the dissolving of the copper. Higher polarization resistance values of PPy metal composites compared with PPy coating show that doping zinc and nickel was more effective in covering the PPy film. In the SEM images, zinc and metal cations in the doped PPy film exhibit a homogeneous dispersion. In the time-dependent impedance measurements, polarization resistance decreased as a result of the adsorption of the corrosive solution onto cation doped and undoped PPy films. Obtained results show that the order of corrosion protection effect on copper electrode is PPy Zn PPy Ni PPy. Acknowledgement This study was supported by the Research Fund of the Kocaeli University (Project No 28/29). References [1] B. Duran, M.C. Turhan, G. Bereket, A.S. Saraç, Electrochim. Acta 55 (29) 14-112. [2] L.M. Martins dos Santos, J.C. Lacroix, K.I. Chane-Ching, A. Adenier, L.M. Abrantes, P.C. Lacaze, J. Electroanal. Chem. 587 (26) 67-78. [3] B. Trachli, M. Keddam, H. Takenouti, A. Srhiri, Corros. Sci. 44 (22) 997-18. [4] A.M. Fenelon, C.B. Breslin, Electrochim. Acta 47 (22) 4467-4476. [5] M.I. Redondo, C.B. Breslin, Corros. Sci. 49 (27) 1765-1776. [6] P. Herrasti, A.I. del Rio, J. Recio, Electrochim. Acta 52 (27) 6496-651. [7] B. Duran, G. Bereket, C.M. Turhan, S. Virtanen, XII. International Corrosion Symposium, Eskişehir Osmangazi University, 347-357, 6-9 September (21). 9