of anticorrosion protection methods. Among various techniques, using organic coatings in conjunction with cathodic protection is one of the most effective.
The 6th International Color & Coating Congress 10-12 November 2015 Institute for Color Science and Technology, Tehran, Iran
Electrochemical impedance spectroscopy evaluation on the anticorrosive performance of epoxy/polyaniline –ZnO nanocomposite coated mild steel under cathodic polarization M.G. Hosseini , P.Yardani Physical Chemistry Department, Tabriz University, Tabriz, Iran.
Abstract Polyaniline and polyaniline- ZnO nanocomposite were synthesized via in-situ polymerization method. The morphology of the synthesized nanocomposite was determined using of scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. The influence of nanosized ZnO particles on the corrosion protection of epoxy- PANI (EP/ PANI) coatings on mild steel substrate were investigated by electrochemical impedance spectroscopy (EIS), in 3.5 wt.% NaCl at 65˚C under different cathodic potentials (-0.5, -1 and -1.5 V). The results showed that corrosion resistance, adhesion and cathodic disbonding resistance of EP/PANI in the presence of ZnO increased under cathodic polarization and applying negative voltages.
Keywords: Epoxy- Polyaninline- ZnO, Nanocomposite- Cathodic polarization- EIS. 1-Introduction Corrosion of mild steel in aqueous environments is an important problem which is necessitated the use of anticorrosion protection methods. Among various techniques, using organic coatings in conjunction with cathodic protection is one of the most effective ways for providing synergistic protection of steel [1]. One of the most important criterion for cathodic protection is the applied cathodic potential. At more negative potentials, cathodic disbonding occurs due to hydrogen evolution and accumulation of OH- ions at the coating/substrate interface. This can be loosen the bond between the coating and the metallic substrate. Therefore, applied cathodic potential should be controlled efficiently[2]. Epoxy resins have been extensively used as coating materials because of their great processability and effective adhesion to metallic surfaces. The barrier performance of epoxy coatings can be increased by the incorporation of a second phase such as conducting polymers and nano sized anticorrosion pigments [3]. The scope of this work is studing the effect of ZnO nanoparticles addition on the corrosion resistance of epoxy-polyaniline (EPPANI) coating under cathodic polarization (- 0.5, -1 and -1.5 V) via EIS.
2-Experimental The 1×1 cm2 mild steel panels were mounted and pre-treated by degreasing in ethanol for 10 min. For preparation of the coatings, 0.306 g of pure PANI and PANI-ZnO nanocomposite powder were suspended in 3 ml TEPA, followed by proper mixing for 12h. Then 1g of EP resin was mixed with 0.2 g
of suspension. The obtained liquid paints were coated on substrate by brushing. Samples (EP-PANI, EP-PANI-ZnO) was exposed to the 3.5% NaCl solution at 65 Ԩ without and with applying potential (- 0.5, -1 and -1.5 V vs Ag/AgCl) for 150h at 65Ԩ. EIS measurements were performed on Potentiostat/Galvanostat (EG& G model Parstat 2263) over a frequency range of 100 kHz to 0.01 Hz, using ±5 mV amplitude of sinusoidal voltage as perturbing signal. Experimental data was fitted and analyzed by ZView(II) software. Corrosion cell equipped with graphite as counter electrode and Ag/AgCl as reference electrode.
3-Results and Discussion Bode plots for EP/PANI and EP/PANI-ZnO coated mild steel specimen in various immersion times are shown in Fig. 1. The obtained EIS parameters after 150 h immersion is presented in Table 1. Coating resistance (Rcoat), coating capacitance (Ccoat) and water uptake of coatings are the main criteria for studying the anticorrosive performance of organic coatings. It can be seen that for both coatings in the absence of negative voltages, Rcoat decreased initially due to gradual diffusion of electrolyte into the pores of the coating according to Fick’s law. Then, Rcoat values started to increase steadily because produced passive oxides (Fe2O3) occupy the coating pinholes and decrease the number of pores and increase the value of Rcoat. Due to the hydrophilicity of the epoxy coating, it can absorb water. Consequently, water uptake within the coating increase Ccoat. In the presence of CP, electrons are applied to the metal surface and reduce
The 6th International Color & Coating Congress 10-12 November 2015 Institute for Color Science and Technology, Tehran, Iran 6.5
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Table 1. The EIS results of EP/PANI and EP/PANI-ZnO under different voltages after 150 h immersion in NaCl 3.5 wt% at 65Ԩ. Rcoat (Ωcm-2)
5709 23524 14125 11803 16526 517800 13821 40260
Ccoat (Fcm-2) 5.6×10-8 7.3×10-8 9.6× 10-7 9.8×10-5 4.8× 10-10 8.4×10-10 3.2×10-9 1.5×10-8
Water uptake 65.9 12.8 30.9 45 48.9 10.8 21.4 35.7
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Corros. Sci, 55, 246–253, 2012. 2. Martinez S., Zulj L.V., Kapor F., Corros. Sci, 51, 2253–2258. 3. Hosseini M.G.,Raghibi-Boroujeni M., Ahadzadeh I., Najjar R., Seyed Dorraji,M.S., Prog. Org. Coat., 66, 321-327, 2009. 4. Hosseini M.G., Jafari M., Najjar R., Surf.Coat.Technol., 206, 280-286, 2011.
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Reference 1. Refait Ph., Fontaine S., Castillon F., et.al .,
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This study indicated that presence of ZnO nanoparticals and polyaniline in epoxy coating increased synergistically corrosion resistance and adhesion of coating to the substrate under cathodic polarization. The ability of ZnO nanoparticals was related to the role of them on delayment in diffution and achievement of the corrosive ions on substrate.
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the anodic reaction. Therefore, generation of corrosion products which have barrier properties decreased with immersion time. In this case, microscopic pores are permeatable against the electrolyte. So, Rcoat values decreased and Ccoat and water uptake increased with immersion time. Applying negative potentials lead to enhancement of cathodic reaction and production of OH-, resulting in an elevation of alkalinity under the coating and decrease the adherence of coating to substrate. It is recognized that EP/PANI-ZnO shows more resistance than EP/PANI in the presence of cathodic polarization. ZnO nanoparticles zigzagate the electrolyte penetration pathways and decrease the pores of coating. Therefore, incorporation of ZnO nanoparticles in the polymeric matrix retards the cathodic reaction and prevents the accumulation of OH- ions and increases the coating resistance against disbonding phenomena at more negative potentials (-1.5 V).
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Fig. 1. Typical Bode plots of (a) EP/PANI (b) EEP/PANIZnO coated mild steel under differents cathodic voltages after 150 h immersion in NaCl 3.5 wt% at 65 Ԩ.
