Electrochemical polymerization and characterization of polypyrrole on Mg–Al alloy (AZ91D)

Synthetic Metals 161 (2011) 360–364

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Electrochemical polymerization and characterization of polypyrrole on Mg–Al alloy (AZ91D) Metehan C. Turhan, M. Weiser, Manuela S. Killian, B. Leitner, S. Virtanen ∗ Department of Materials Science, WW4-LKO, University of Erlangen-Nuremberg, Martensstraße 7, 91058 Erlangen, Germany

a r t i c l e

i n f o

Article history: Received 12 July 2010 Received in revised form 25 November 2010 Accepted 30 November 2010 Available online 23 December 2010

a b s t r a c t We report the formation of polypyrrole (PPy) coatings on Mg alloy AZ91D from aqueous solutions of a dicarboxylic organic acid salt by cyclic voltammetry (CV) method. The polymeric films of PPy are directly coated onto AZ91D substrates without any pretreatment or intermediate steps. The electrochemically formed PPy films were characterized by Fourier Transform Infrared Spectroscopy (FT-IR), X-Ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS). Electrochemical impedance spectroscopy (EIS) measurements demonstrate a good corrosion protection.

Keywords: Magnesium Magnesium alloy AZ91D Conductive polymer Corrosion Biocoating Salicylate Cyclic voltammetry FT-IR XPS ToF-SIMS

1. Introduction In recent decades, many types of conducting polymers such as polyaniline (PANI), polythiophene (PT), polypyrrole (PPy) and their derivatives have been explored to be promising candidates to carry coating technology of materials one step forward in the new century. These smart materials have attracted great interest in the development of actuators [1], biosensors [2], fuel or bio-fuel cells [3,4], supercapacitors [5–7] and light emitting diodes (LED) [8]. These coatings on widely used metals have been mostly reported to reduce the corrosion rates, as summarized in a review [9]. On the other hand, critical works [10,11] have recently reported that the control of corrosion with conducting polymers is not favoured under practical conditions, an overview is provided in a recent review [12]. However, these materials are found to increase the corrosion resistance in practical applications when they are applied together with other layers in a sandwich like structure (e.g. conversion coating, second primer layer, and polyurethane topcoat), the release of the dopant anions [13] from conducting polymers

∗ Corresponding author. Tel.: +49 9131 852 75 77. E-mail address: [email protected] (S. Virtanen). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.11.048

© 2010 Elsevier B.V. All rights reserved.

can then inhibit corrosion. Among the conductive polymers, PPy is found to be one of the most suitable candidates for corrosion protection because of its relatively easy preparation from aqueous solution and stability at oxidized state. Investigations including electrochemical polymerization of pyrrole on active metals such as iron [14], copper [15], aluminum [16] and zinc [17,18] also report that these coatings show good adherence on the surface and good performance in corrosive media. Despite the fact that Mg and Mg alloys are widely used metals, only few attempts have been reported related to conductive polymer coatings on these materials. Guo et al. [19] showed electropolymerization of aniline on AZ91D alloy surface using an electrochemical pulse method in alkaline solutions, and Jiang et al. [20] reported electropolymerization of pyrrole on a Cu–Ni plated Mg AZ91D surface. Such surface modifications of Mg with this type of coatings may be promising for many applications of Mg alloys, including their use as biodegradable metallic implant materials (for instance as self-degradable stents) or as degradable biosensors. For biomedical applications of Mg alloys, the corrosion rate needs to be controlled; one way to achieve this may be the use of coatings, which in addition should be biocompatible. In contrast to other type of engineering applications, the coatings should not completely stop corrosion (as biodegradation is desired). There-

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fore, coatings with weak barrier properties may be an optimum choice. Due to its fast dissolution rate and very negative corrosion potential, electrochemical surface passivation and functionalization of magnesium is a challenging task. The fast dissolution rate hinders a direct polymerization of conducting polymers on the surface. To overcome this problem, surface passivation is required [15,21] prior to initiation and attachment of conductive polymer coating. Different to this approach, Aeiyach et al. [22] recently showed electropolymerization of PPy coatings from sodium salicylate solutions on zinc without any pretreatment. Taking the advantage of sodium salicylate, film formation occurs in a one-step process, and does not need any preliminary treatment of the metal. In the present study, we report a direct electrochemical formation of a PPy coating on Mg alloy AZ91D from aqueous solutions of carboxylic acid salts. The aim of the study was to exploit the potentially easy electrodeposition route for Mg alloy surface functionalization, in order to reduce (but not completely stop) the corrosion rate. 2. Experimental Magnesium AZ91D coupons (2 cm × 2 cm × 0.5 cm) were ground with 1200 grit emery paper and then were cleaned in 1:1 ethanol:acetone mixture for 10 min in an ultrasonic bath. The AZ91D working electrode was sealed to the electrochemical cell by an O-ring, to expose a defined electrode surface to the electrolyte, by pressing a Cu plate, which acted as electrical contact, to the back of the electrode. A potentiostat/galvanostat Autolab PGSTAT 30 with three electrode system was used for cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analyses where a platinum foil served as the cathode and Ag/AgCl (3 mol dm−3 KCl) was used as reference electrode. The PPy films were synthesized from 0.1 mol/l pyrrole (Py) containing aqueous solutions of 0.5 mol/l carboxylic acid salts, sodium oxalate (Sigma), sodium malonate (Sigma) and sodium salicylate (Sigma) (pH 6.50), using cyclic voltammetry (CV) in the potential region between −0.5 V and +1.0 V at 20 mV s−1 scan rate over 20 cycles. Electrochemical characterization of thin PPy films was performed in monomer-free solutions of the same electrolyte via CV. EIS measurements were performed in 0.1 M NaSO4 with a sinusoidal perturbation of ±10 mV for 10 points in each decade over the frequency range from 100 kHz to 10 mHz. Functional groups were characterized with a Fourier-Transform Infrared Spectrophotometer (Bruker Instruments—Germany). The chemical composition was investigated by employing X-ray induced photoelectron spectroscopy (XPS)-Phi 5600, using Al K␣ radiation. Also, positive and negative static time-of-flight secondary-ion-mass-spectrometry (ToF-SIMS) measurements were performed on a ToF.SIMS V spectrometer (ION TOF, Münster). The samples were irradiated with a pulsed 25 keV Bi+ liquid–metal ion beam. Spectra were recorded in the high mass resolution mode (m/m > 8000 at 29 Si). A fieldemission scanning electron microscope (Hitachi FE-SEM S-4800) was used to investigate surface morphology. 3. Results and discussion 3.1. Electrochemical polymerization and characterization of pyrrole Polarisation of AZ91D samples was performed between −0.5 and 1 V with cyclic voltammetry to observe electrochemical changes in the system in more detail. It is also possible to use conventional methods for electropolymerization. Electrolytes of 0.5 M sodium oxalate, sodium salicylate and sodium malonate with addi-

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tion of 0.1 M Py was used. Formation of a black film (pyrrole black) was obtained only from 0.5 M sodium salicylate +0.1 M Py solution. With the other electrolytes used, either corrosion of AZ91D resulted or no surface coverage was obtained. Electropolymerization of pyrrole in presence of salicylate ions has been studied previously. Cascalheira et al. [23] determined the oxidation peak of salicylate at around 1 V and showed the role of salicylate ions in the anodic behavior of copper where formation of a Cu(II)-salicylate film is favoured at the electrode surface and contributes to its passivation. In references [24,25] it is claimed that in aqueous media, sodium salicylate is the most suitable candidate that allows the formation of adherent and homogeneous PPy films without pre-treatment on active metals. Fig. 1(a) shows the electrochemical behaviour of magnesium AZ91D electrode in 0.5 M sodium salicylate solution. It is clearly seen that the surface of Mg AZ91D alloy cannot be passivated easily due to anodic dissolution of Mg ranging from −0.5 to 0 V (inset figure of Fig. 1(a)). However, a drop of current density around 0 V is observed suggesting lowering of the anodic dissolution rate. At the end of the forward scan, a decrease of the current drop is obtained with a corresponding peak in the reverse scan direction at around 0.9 V. This reversible redox couple is attributed to oxidation–reduction of sodium salicylate. At the beginning of the 2nd cycle, the current density value does not reach as high values in the potential range of −0.5–0 V as in the 1st cycle, probably indicating partial passivation of the surface, but the current density values are still high. Repetitive 20 cycles in Fig. 1(a) show a shift of current density to more anodic values, suggesting that reversible redox behaviour of salicylate takes place, and dissolution of AZ91D surface is dominant. Fig. 1(b) shows electrochemical polymerization of Py on AZ91D surface from sodium salicylate solution. Oxidation potential of Py – that is reported around 0.8 V – cannot be detected due to overlapping with the oxidation peak of sodium salicylate. However, in contrast to the first cycle (inset figure of Fig. 1(a)) of monomer free solution, the intensity of the oxidation peak around ∼0.8 V sharply decreases at the end of the first cycle in Py containing electrolyte (inset figure of Fig. 1(b)). This strong drop of the peak intensity upon addition of pyrrole is a result of pyrrole being involved in the passivation process and the current decrease continues in repetitive cycles in Fig. 1(b). A slight current increase was observed at the end of the each cycle that is attributed to monomer oxidation process by a step by step polymer film growth. Fig. 1(c) shows the scan rate dependence of the PPy films in monomer-free solution. One broad oxidation peak is observed during anodic scanning that is associated with overlapping of the PPy and salicylate peaks. On the cathodic part, a wide reduction peak is observed. Anodic and cathodic peak current densities are related to the oxidation and reduction of PPy. The electrochemical process is diffusion controlled, as both the anodic and cathodic peak intensities are increased with increasing scan rate. Fig. 1(d) compares the electrochemical impedance of bare AZ91D alloy with fresh PPy coatings. Electrochemical impedance spectroscopy of pure magnesium in sodium sulfate solutions has been previously studied [26,27]. The Nyquist plots were characterized by two well-defined capacitive loops followed by an inductive loop. Detailed information about these loops can be found elsewhere [28,29]. The PPy coated AZ91D clearly shows a significant increase of the polarization resistance (Rp ) as compared to the bare sample, indicating the barrier effect of PPy [29]. Moreover, the inset figure of Fig. 1(d) shows the open circuit potential (OCP) recorded during 1 day in the same electrolyte. PPy coated samples did not show noisy oscillations as compared to bare AZ91D, also suggesting that the PPy coating decreases the dissolution of Mg in Na2 SO4 electrolyte.

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Fig. 1. (a) Cyclic voltammogram of AZ91D in 0.5 M sodium salicylate solution, scan rate 20 mV s−1 . Inset: First cycle (b) Cyclic voltammogram of AZ91D in 0.5 M sodiumsalicylate +0.1 M Pyrrole, scan rate 20 mV s−1 . Total Charge; Qdep = 4.995 C Inset: First cycle (c) Cyclic voltammogram of PPy film (deposited from 0.1 M pyrrole in 0.5 M sodium salicylate, at a scan rate of 20 mV s−1 ; Qdep = 4.995 C) in a monomer free solution of 0.5 M sodium salicylate at scan rates of (a) 10, (b) 20, (c) 30, (d) 40, (e) 50 mV s−1 . (d) Nyquist plots of bare and PPy coated AZ91D alloy samples in 0.1 M Na2 SO4 solution. Inset: Open circuit potentials of bare and PPy coated AZ91D alloy samples recorded in 0.1 M Na2 SO4 solution during 24 h.

3.2. FTIR-ATR, XPS, ToF-SIMS measurements and SEM analyses FT-IR spectra of a salicylate copper complex have been reported previously [30]. Santos et al. [31] also showed FT-IR spectra of PPy molecules in sodium salicylate medium and determined the characteristic peaks with a shift of wave numbers due to a copper–salicylate complex. In addition, C O stretching of aromatic carboxylic acid was not detected in the 1700–1650 cm−1 region as a strong band. Instead, characteristic salicylate peaks were detected as a (s (COO− )) band around 1377 cm−1 and shifted to 1392 cm−1 in the case of copper salicylate by splitting into two components. Similarly, in our work, these characteristic (s (COO− )) bands were detected around 1597 and 1556 cm−1 (Fig. 2(a)), probably due to formation of a Mg salicylate complex. In addition, characteristic bands of the ring stretching are found between 1400 and 1500 cm−1 . The characteristic peaks of PPy were detected around 1300 cm−1 , 1185 cm−1 and 1035 cm−1 , according to references [32–35]. Fig. 2(b) shows an XPS survey spectrum of PPy/AZ91D surface containing mostly C (63.43%), O (28.00%) and N (3.42%). The presence of nitrogen atoms is a further indication of PPy coating.

Additionally, the surface coverage of PPy is relatively high as the signals from the substrate (Mg, Al, Zn) are very low. Fig. 2(c) and (d) show SEM pictures of PPy coated Mg AZ91D alloys with a typical cauliflower type morphology of PPy grains [36] with a diameter ranging between 500 nm to 800 nm. As the grind striations are still visible on the surface, the passivation process leads to a very thin film formation in sodium salicylate medium. ToF-SIMS investigation revealed the successful formation of a PPy layer. Fig. 2(e–h) shows characteristic fragments after electrochemical treatment in sodium salicylate (top) and a solution of PPy in sodium salicylate (bottom). Fig. 2(e) shows a decrease of the Mg+ signal, indicating a larger film thickness and better coverage of the coating containing PPy. Fig. 2(g) shows the fragment of C3 H3 O2 − , which is related to salicylate. A relative decrease of the signal upon addition of PPy is observed. The ratio of C3 H3 O2 − to Mg+ stays approximately constant. This implies that salicylate is also incorporated into the PPy layer. Furthermore, an increase in the appearance of PPy specific fragments (CN− , C6 H8 N+ ) is shown in Fig. 2(f) and (h). Also, the signal of C5 H4 NO+ (Fig. 2(h)) appears only in the PPy containing sample, indicating a possible reaction of salicylate and PPy.

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Fig. 2. (a) FTIR-ATR spectra and (b) XPS survey spectra of PPy polymer. SEM pictures of fresh PPy surface; (c) lower magnification image with grinding striations are visible. Inset: Higher magnification image showing usual PPy morphology. (d) High magnification of cauliflower like PPy structures. ToF-SIMS spectra of Mg AZ91D alloy before (top) and after (bottom) PPy coating. (e) Mg+ , (f) CN− ions, (g) C3 H3 O2 − and (h) C5 H4 NO+ , C6 H8 N+ .

4. Conclusions We demonstrate, for the first time, direct growth of a conductive polymer (polypyrrole) thin film on a magnesium alloy (AZ91D) without any pretreatment. Fourier-Transform Infrared Spectroscopy (FT-IR), X-Ray photoelectron spectroscopy (XPS), time of flight secondary ion mass spectrometry (ToF-SIMS) and scanning electron microscopy results showed that the magnesium AZ91D surface is covered by a thin PPy film. Electrochemical impedance spectroscopy results (EIS) demonstrate that sodium salicylate doped PPy layer on Mg alloy A/91D surface shows good corrosion protection property in Na2 SO4 solutions. In view of practical applications, for instance in biomedical applications of Mg and its alloys,

such salicylate doped PPy layers on Mg alloys have a high potential for tailoring the degradation rate, since all components (substrate, passivating agent, coating) of the resulting system are non-toxic. Acknowledgements The authors would like to thank Prof. Dr. Dirk M. Guldi and Dr. Christian Ehli (Friedrich-Alexander-University Erlangen/Nurnberg) for their valuable help during FT-IR analyses. We extend our sincere thanks to Helga Hildebrand and Anja Friedrich for XPS and SEM measurements and also to Martin Kolacyak for his valuable technical help. Financial support from DFG (German Research Foundation) is also acknowledged.

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