Electropolymerisation of TiO2/polyaniline nanocomposite and its use as a protective coating on mild steel

JOURNAL OF SCIENCE OF HNUE Interdisciplinary Science, 2013, Vol. 58, No. 5, pp. 22-29 This paper is available online at ELECTROPOLYMERISATION OF TiO2/POLYANILINE NANOCOMPOSITE AND ITS USE AS A PROTECTIVE COATING ONMILD STEEL Le Minh Duc1 and Vu Quoc Trung2 1Faculty of Chemistry, University of Technology, Da Nang University of Technology 2Faculty of Chemistry, Hanoi National University of Education Abstract. Nanocomposite material has been getting much more attention in recent years by scientists. Nanomaterials, which can be used to fabricate nanocomposite, come in the form of wire, tubes or particles. The matrix polymer can be thermoplastics or thermoset plastics such as phenolic resin, epoxy resin, and unsaturated polyester, or conducting polymers such as polyaniline, polypyrrole, polyethylene, polypropylene and polystyrene. In this study, TiO2 was used as an additive for polyaniline, considered to be a matrix polymer. The nanocomposite film was fabricated electrochemically on metal surfaces (mild steel and stainless steel). The electrochemical properties of polyaniline could be seen on the cyclic voltammogram. With mechanical stirring during electropolymerisation, TiO2 nano particles could be incorporated into the nanocomposite film. A TEM image showed that TiO2 particles were distributed homogenously in the coating. The presence of TiO2 also could be seen with X-Ray Diffraction (XRD) measurement. A Scanning Kelvin Probe (SKP) showed that the adhesion and the corrosion resistance of a nanocomposite coating could be improved with TiO2 in a solution of 3% NaCl. Keywords: Nanocomposite, polyaniline, electropolymerisation, corrosion protection, TiO2, Scanning Kelvin Probe. 1. Introduction Nanocomposite material has been getting a lot more attention from scientists in recent years. Due to its good properties, nanocomposite has been investigated more than other materials [1, 2]. Nanomaterial consists of at least one constituent that is in the nano meter scale. It can be in the form of particle, wire or tube. Because they are so small and on a large Received May 20, 2013. Accepted June 25, 2013. Contact Vu Quoc Trung, e-mail address: trungvq@hnue.edu.vn 22 Electropolymerisation of TiO2/polyaniline nanocomposite and its use as a protective coating... specific area, nanomaterials provide very special properties that traditional composites do not possess. This suggests that nanomaterial can be used as reinforcement filler in nanocomposite, especially to improve the properties of organic coatings. This approach can be an alternative in the production of organic or nanocomposite coatings. However, there has been little interest in nanocomposite in Vietnam. Nanocomposite can be fabricated based on organic or inorganic matrices. In recent years, carbon nanotube, Nanowire and TiO2 nanoparticles have served as fillers in the polymer matrix. Among them, TiO2 drew considerable interest in nanocomposite fabrication. TiO2 is a relatively inert, non-toxic and traditional semi-conducting material. TiO2 could enhance the mechanical and optical properties of nanocomposite [3-5]. In principle, polymer matrix can be thermoplastics or thermoset plastics such as polyethylene, polypropylene, polystyrene or conducting polymer. Depending on the application purposes, polymer matrix can be selected. Conducting polymer has been of interest in the work of Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa in 1976. The 2000 Nobel Prize in Chemistry was awarded jointly to them "for the discovery and development of conductive polymers”. Polyaniline (PANi) can be synthesised either by chemical or electrochemical oxidation of monomer aniline. PANi was largely studied because of its metallic behavior, good chemical and thermal stabilities, redox reversibility and its high electric conductivity when it is doped in an acid media. In addition, polyaniline has been used in some fields of applications such as batteries, protection of metals against corrosion and electrocatalysis [1]. In this study, TiO2/PANi nanocomposite was fabricated electrochemically. TiO2 nanoparticles (∼25 nm diameter) were incorporated into nanocomposite film with stirring. The properties of the film were characterized. The anticorrosive ability of the film was also tested. 2. Content 2.1. Experiment In this study, distilled water is used as solvent. The reagents (Aldrich products) were titanium (IV) oxide (anatase-type TiO2) 25 nm rutile powder as a doping semiconductor and aniline with 99.99% purity as a monomer. The supporting electrolyte used is oxalic acid (H2C2O4) for electropolymerisation of aniline. Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD) was done at the Polymer Laboratory, Ho Chi Minh University of Technology. Electrochemical tests were done in a 3-electrode cell connected to a potentiostat PGS-HH10 (Vietnam). Ag/Gal was a reference electrode and stainless steel was used as a counter electrode. A Scanning Kelvin Probe (SKP) was used at the Max-Planck Institute for Iron, Dusseldorf, Germany. The sample was prepared on mild steel. The delamination 23 Le Minh Duc and Vu Quoc Trung experiment was carried out in 98% relative humidity and at 25 0C. A solution of 3% NaCl was used as the medium of delamination. The SKP experiment was arranged as shown in Figure 1. Figure 1. The arrangement of SKP measurement: 1. NaCl solution; 2. Nanocomposite coating; 3. Substrate; 4. Kelvin probe (100 µm diameter) Nanocomposite film was fabricated electrochemically in the electrolyte containing 0.3 M H2C2O4, 0.3 M aniline monomer and 0.01 M TiO2. Before using, the solution was stirred thoroughly for 24 hours at the rate of 500 rpm. Purchased mild steel was polished with abrasive paper: P600 and then P1000. The sample was degreased in ethanol and air dried before use. 2.2. Results and discussions 2.2.1. Electropolymerisation of TiO2/PANi with cyclic voltammetry A mild steel sample was treated with the above procedure. Nanocomposite film was obtained in a solution containing 0.3 M H2C2O4, 0.1 M aniline monomer and 0.01 M TiO2 nanopowder. The cyclic voltammogram is shown in Figure 2. Figure 2. Cyclic voltamogram of TiO2/ PANi on (a) stainless steel, (b) mild steel in electrolyte 0.3 M H2C2O4, 0.1 M monomer aniline and 0.01 M TiO2, scan rate 10 mV/s It can be seen in Figure 2a that the electrode potential increased rapidly at +0.9V (vs. Ag/AgCl) in the first positive scan. This is the oxidation potential of monomer aniline. From that time, PNAi was formed on the steel electrode so that reduction peaks could be observed in the back scan. In the next scans, oxidation and reduction peaks of PANi 24 Electropolymerisation of TiO2/polyaniline nanocomposite and its use as a protective coating... appeared. Because there was a thicker layer of PANi, the currents were increased for the next scans. The same behaviour of PANi film could be seen when electropolymerisation occurred on mild steel (Figure 2b). After 9 scans, the thickness of nanocomposite fllm was about 150 µm. Figure 3. SEM image of PANi on mild steel halfway through the scan Oxalic acid played an important role in passivating the mild steel electrode during polymerisation. Iron oxalate was formed easily in the first half of the scan. Figure 3 shows the crystal of iron oxalate that precipitated. It can be seen that the presence of TiO2 during polymerisation had no effect on PANi formation. The film could be formed directly on low carbon steel in the oxalic solution [1, 6]. 2.2.2. TEM images TEM images of TiO2 nanopowder and TiO2/PANi nanocomposite were shown in Figures 4a and 4b, respectively. The TiO2 crystal could be seen clearly in the TEM picture (Figure 4a). The diameter was about 30 nm. The crystal structure of TiO2 could also be observed in the nanocomposite film. Mechanical stirring could disperse TiO2 nanoparticles well in PANi. Figure 4. TEM images of a) TiO2 nanoparticles b)TiO2/PANi nanocomposite 25 Le Minh Duc and Vu Quoc Trung 2.2.3. XRD results After formation, a TiO2/PANi sample was tested using XRD spectroscopy. The results are shown in Figure 5. It is clear that the characteristic peaks of TiO2 could be observed in the XRD spectroscopy graph of TiO2/PANi nanocomposite. The results showed that anatase-type TiO2 was incorporated in nanocomposite during electropolymerisation of PANi. Figure 5. XRD spectroscopy of a) TiO2 nano crystal; b)TiO2/PAni nanocomposite Besides, EDX results showed that the TiO2 level incorporated in the nanocomposite was about 0.4%. These results were not presented in the above picture. 2.2.4. Corrosion protection test of TiO2/PANi nanocomposite coating In corrosion protection experiments, a nanocomposite coating was applied to stainless steel and Tafel plots were obtained in a solution of 3% NaCl (Figure 6). It can be seen that the corrosion potential of the TiO2/PANi nanocomposite coating shifted in a positive direction and the corrosion current decreased when comparing with the steel and PANi/steel samples (Figure 6, curve 3). The presence of TiO2 in the coating may provide these effects. The semiconductive property of TiO2 could enhance the protective ability of steel. 26 Electropolymerisation of TiO2/polyaniline nanocomposite and its use as a protective coating... Figure 6. Tafel plots of a) Stainless steel; b)PANi on steel; c)TiO2/PANi nanocomposite on steel 2.2.5. Scanning Kelvin Probe experiments A nanocomposite coating was applied on steel and SKP experiments were carried out at 98% relative humidity and at 25 0C. To avoid difficulties resulting from a significant folding of the coatings during the delamination experiments, an additional polyvinyl butyral resin top-coat (Aldrich-Sigma) was applied to all PANi films studied with the SKP. A solution of 3%NaCl was used to stimulate the corrosion at the beginning. Delamination of the nanocomposite coating was obtained in Figure 7. Figure 7. Delamination of TiO2/PANi nanocomposite on steel each scan was obtained after 10 minutes It can be found that delamination of the nanocomposite coating took place at about 1,500 µm in 40 minutes. The coating delaminated quickly with corrosive species at the interface polymer/metal. However, the delamination slowed down in the first 20 minutes. The curves seem to overlap each other. TiO2 in the coating could play important role in inhibiting the delamination. It could inhibite water reduction, reducing pH at interface [7]. 27 Le Minh Duc and Vu Quoc Trung In the TiO2/PANi nanocomposite, TiO2 and PANi could form a special semiconductor. TiO2 was the n-conductor (the bangap 3.13 eV). PANi was the p-conductor with a low bandgap 2.1 eV. They formed a barrier to prevent an electron transfer through the coating. Inhibiting the electron transfer, the iron dissolution reaction would be inhibited (reation 1). The corrosion reaction would occur via many steps. Fe→ Fe2+ + 2e− (2.1) Fe2+ → Fe3+ + e− (2.2) O2 + 2H2O + 4e − → 4OH− (2.3) 2Fe2+ +O2 + 2H2O → 2FeOOH + 2H+ (2.4) During corrosion, oxidants would be necessary. They transfer electrons and form corrosion products. If one of these reactions was slowed down (reaction 2.1 - 2.4), delamination would decrease. The anti-corrosion ability of the coating was improved [8, 9]. 3. Conclusion TiO2/PANi nanocomposite was fabricated electrochemically on mild steel using a cyclic voltammatry technique. Electropolymerisation occurred in the solution with 0.3 M H2C2O4, 0.1 M monomer aniline and 0.01 M TiO2 nanoparticles. The electrolyte was stirring mechanically. Mild steel required no special treatments. The oxalic acid solution could passivate steel and the aniline monomer was oxidised without any difficulty. TEM images and XRD spectroscopy showed that TiO2 could be incorporated into the nanocomposite film during polymerisation. The amount of TiO2 in the film was about 0.4%. TiO2 played an important role in slowing down the delamination and enhancing the corrosion protection of the TiO2/PANi nanocomposite coating. REFERENCES [1] Feng Lin, 2006. Dissertation of Master Degree in Chemistry. University of Waterloo, Cananda. [2] S. Sathiyanarayanan, S. Syed Azim, G. Venkatachari, 2007. A new corrosion protection coating with polyaniline-TiO2 composite for steel. 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