Journal of Electroanalytical Chemistry Journal of Electroanalytical Chemistry 583 (2005) 133–139 www.elsevier.com/locate/jelechem Corrosion protection by polyaniline-coated latex microspheres Yousuf Mohammad Abu, Koichi Aoki * Department of Applied Physics, University of Fukui, 3-9-1 Bunkyo, Fukui-shi 910-8507, Japan Received 18 April 2005; received in revised form 24 May 2005; accepted 24 May 2005 Available online 7 July 2005 Abstract The iron plate covered with ﬁlms of polyaniline-coated polystyrene latex (PANI–PS) microspheres were protected against corrosion in 0.01 mol dm3 HCl and 3% (w/v) NaCl aqueous solution. PANI–PS particles 1.85 ± 0.06 lm in diameter were synthesized by polymerizing chemically aniline on polystyrene (PS) latex in the suspension. The conducting state (emeraldine salt) of PANI ﬁlm shifted the corrosion potential of the underlying iron toward a positive from the insulating state of PANI (leucoemeraldine form). The ﬁlm from which core-polystyrene was removed by dissolution in tetrahydrofuran had the similar anti-corrosive properties. Tafel plots, open circuit potential–time diagrams were used to examine the corrosion properties of PANI–PS coated and uncoated electrodes. 2005 Published by Elsevier B.V. Keywords: Corrosion; Iron; Polyaniline; Latex; Microspheres; Tafel plot 1. Introduction Conducting polymers and their derivatives have shown to have protection of corrosion in the past two decades, expecting a replacement of chromium-containing materials [1–5]. Since conducting polymers make good adhesion on the metal surfaces, they work not only as physical barrier against oxygen but also as an electrochemical barrier against aggressive ions [6–11]. Polyaniline (PANI) has been the most widely studied of conductive polymers because of its ease of synthesis [12–14], redox and thermal stability and high corrosion resistance [15–20]. PANI based blends on the surface of stainless steel [19,20] and on iron [21,22] have shown corrosion inhibition and the mechanistic behavior has been reported. However, it is not easy to make large polyaniline ﬁlms for coating metal surfaces because polyaniline is brittle, insoluble in water, sparingly solu- * Corresponding author. Tel.: +81 776 27 8665; fax: +81 776 27 8494. E-mail address: [email protected] (K. Aoki). 0022-0728/$ - see front matter 2005 Published by Elsevier B.V. doi:10.1016/j.jelechem.2005.05.014 ble in organic solvents such as m-cresol and N-methylpyrrolidone (NMP) and non-fusible even by heating up to their decomposition temperature. The electropolymerization oﬀers some advantages over other coating techniques [23–32] in getting uniformly thin and adhesive ﬁlms on the metal surfaces with simple instruments. However, corrosion-susceptible metals are often oxidized or dissolved in the potential domain of the electropolymerization of polyaniline. A well-known technique of coating the large metal surface is to spread suspensions of microparticles and to dry them [33,34], like painting. If PANI–PS microspheres are spread on a substrate to form a ﬁlm, the problems of the poor processability and roughness of ﬁlm thickness on a large area will be solved. Synthetic techniques of nearly mono-dispersed PANI–PS are to polymerize chemically aniline on PS latex surface in the presence of surfactant, such as poly(vinyl alcohol), poly(N-vinylpyrrolidone) (PVP), poly(vinyl methyl ether), poly(ethylene oxide) or cellulose ethers [35–42]. Since the PANI–PS is dispersed in aqueous phase, no organic solvent is needed for the ﬁlm formation, in 134 Y.M. Abu, K. Aoki / Journal of Electroanalytical Chemistry 583 (2005) 133–139 contrast with using m-cresol or NMP for the organic solvents-aid coating. The PANI–PS has a core-shell structure, that is, a PS core and a PANI shell . This structure is ascribed to the immiscibility of the hydrophobic PS and the hydrophilic PANI. Since the PANI–PS is stable in a suspension form, it has been applied to voltammetry of colloidal suspensions [41–44]. The suspensions have shown diﬀusion-controlled voltammetric responses [41,42], and sometimes associated with periodic current [43,44]. Self-standing PANI ﬁlm of mono-particle-layer has been generated from the PANI–PS. The electrochemical properties are similar to those of the electrochemically polymerized ﬁlms [45,46] if the PANI–PS is puriﬁed at low temperature. This ﬁlm may be helpful for corrosion inhibition as for conventional polyaniline ﬁlms. In this article, we report the protection properties of the PANI–PS-coated iron electrode in chloride-containing solutions. 2. Experimental 2.1. Chemicals Styrene (Wako) was puriﬁed by distillation under vacuum at 60 C. Poly(n-vinyl pyrrolidone) (PVP) (Wako) of 360 kg mol1 was used as a stabilizer or a surfactant during the dispersion polymerization of styrene. Ammonium peroxydisulfate (Wako), aniline hydrochloride (Kanto), a-azoisobutyronitrile (AIBN) (Kanto), 2-propanol (Wako), tetrahydrofuran (THF) (Wako), hydrazine monohydrate (Kanto) and sulfuric acid and PVP were used as received. All aqueous solutions were prepared with deionized water made by an Advantec ultra pure water system (CPW – 100). 2.2. Synthesis of PVP-stabilized polystyrene latex A mixture of 120 cm3 2-propanol and 2.1 g PVP was heated to 70 C with bubbling nitrogen during mechanical stirring for 24 h in a three-necked round-bottomed ﬂask to remove trace of oxygen. To this solution was added styrene (15 g) containing AIBN (0.15 g) drop-wise for 40 min. The solution was maintained at 70 C for 24 h, during which the polymerization occurred. The resulting milky-white mixture was centrifuged with a centrifuge, SRX – 201 (TOMY, Tokyo) equipped with a cooling system. The supernatant was decanted and replaced by deionized water to yield white sediment of PS latex particles. This centrifugation–redispersion cycle was repeated several times in order to remove residual reactants. Finally the particles were kept in aqueous medium with 3% (w/v) PVP. Then this latex suspension was used for the next coating process. Features of the suspension of the PS latex particles were observed by using a video microscope, VH-Z450 (Keyence, Osaka). 2.3. Coating the latex particle with PANI About 1.11 g aniline hydrochloride was dissolved in 100 cm3 of the PVP-stabilized 3% (w/v) PS latex suspension. The mixture was stirred and cooled to 0 C in an ice bath for 2 h in order to adsorb aniline on the surface of latex prior to polymerization. The polymerization was made at 0 C for 12 h after adding 2.45 g (NH4)2S2O8 as the oxidant. The solution was kept at room temperature for further 24 h during stirring the mixture to complete the polymerization. Centrifugation of the suspension at 4 C yielded three layers. The second layer contained fragments of PANI. The bottom layer, which contained a high portion of the PANIcoated latex, was redispersed in 0.l M H2SO4 solution. This suspension was centrifuged several times until the H2SO4 solution layer became transparent. Then we obtained the puriﬁed PANI–PS latexes and stored in 0.1 M H2SO4 and 1% PVP solution. The particle under the reduced state was made by adding hydrazine monohydrate to the suspension from which H2SO4 and PVP had been removed by centrifugation and redispersion. After being kept for few hours, the particle was washed by distilled water and stored in 1% PVP solution. Both oxidized and reduced PANI–PS particles were used for making ﬁlms on electrodes. 2.4. Coating of iron electrode surface A certain amount of 0.5% (w/v) PANI–PS suspension including 0.05% (w/v) PVP was spread on the iron electrode surface. Upon drying it at slow evaporation rate in an open atmosphere with 70% humidity at 25 C, a uniform multi-particle ﬁlm was obtained on the electrode surface. The PANI–PS coated-iron (PANI–PS–Fe) electrode was inserted into THF for 45 min to remove the soluble PS-core. It was washed carefully by distilled water for several times and was dried. Then a thin PANI ﬁlm without PS was obtained on the electrode surface. The electropolymerization of aniline on the Fe electrodes was carried out in 0.1 M aniline including 0.3 M oxalic acid, as described by Tüken et al. . Firstly, the electrode surface was passivated by applying a single forward scan from 0.5 to 0.3 V at scan rate of 4 mV s1. Then one potential cycle was taken in the range from 0.0 to 1.6 V at a scan rate of 10 mV s1 and then 150 cycles were applied between 0.2 and 0.95 V at 50 mV s1. The coated electrodes were used as the working electrode for the measurement of corrosion. Y.M. Abu, K. Aoki / Journal of Electroanalytical Chemistry 583 (2005) 133–139 2.5. Electrodes The disk electrodes were made of an iron rod 1.2 mm in diameter. The cylindrical wall of the rod was covered with glass tube. A space between the inner wall of the glass tube and the iron rod was insulated by epoxy resin. After curing the resin the surface of the electrode was polished with 500, 1000, 1500 and 2000 mesh sand papers successively and washed with distilled water several times in an ultrasonic bath before each use. A platinum coil and saturated calomel electrode (SCE) were employed as a counter and a reference electrode, respectively. The potentiodynamic experiments were carried out with the coated and uncoated electrodes using the AutoLab potentiostat in 3% (w/v) NaCl solutions applying a sweep rate of 1 mV s1 at room temperature. 3. Results and discussion The PANI–PS was dark green, very similar to the color of electropolymerized PANI ﬁlms. The aqueous suspension through a video microscope showed that most particles were so well isolated each other that no lump of particles was generated and that each was subject to the Brownian motion. The acidic suspension was stable for a few days without gravitational sedimentation. The sedimented particles could be easily redispersed by simply mixing. The PANI–PS was nearly mono-dispersed and spherical with 1.85 ± 0.06 lm in diameter. Since the PS latex particle before loading PANI had 1.80 ± 0.05 lm, the thickness of PANI layer is ca. 0.025 lm [=(1.85 1.80)/2]. Fig. 1(a) and (b) are optical microscope photographs of the bare iron and PANI–PS–Fe electrodes after 7 days immersion in 3% (w/v) NaCl solutions, respectively. The coated electrode (b) was more anti-corrosive than the bare iron (a). The color of the PANI–PS ﬁlm on the iron changed from dark green of the emeraldine salt to pale green of leucoemeraldine, whereas no color change was 135 observed for the ﬁlm on the epoxy resin and glass coated area. PANI–PS particles coated at the iron electrode were found to be reduced by the corrosion as evidenced from the color change. The corrosion started near at the interface of Fe and the shielding material, as shown as a white product in Fig. 1(b), and the whole ﬁlm turned redbrown ﬁnally. This behavior was observed also for the electropolymerized PANI coated iron (PANI–Fe) electrode, as shown in Fig. 1(c). Open circuit potential, Eoc of coated and uncoated samples were measured, and plotted against the immersion time in 0.01 M HCl solution in Fig. 2. The PANIcoated electrodes (b)–(d) showed more positive potentials than the potential at the bare electrode (a), where the coating materials are (b) the reduced PANI–PS, (c) the electrochemically polymerized PANI, and (d) the oxidized PANI–PS. Consequently, the PANI ﬁlm works as protection of the corrosion [48–53]. The oxidized PANI is more eﬀective for the corrosion protection than the reduced form. Fig. 3 shows the Tafel lines (a), (b), (c) and (d) for uncoated-Fe, reduced PANI–PS–Fe, PANI–Fe and oxidized PANI–PS–Fe electrodes, respectively, in 3% (w/ v) NaCl medium at the scan rate of 1 mV s1. The ﬁlms were not removed from the Fe surface even at 1.2 V by hydrogen gas evolution. Indeed no gas evolution was observed. The corrosion potentials, Ecorr for all the three coated electrodes are more positive than that observed in the uncoated-Fe. The most anodic value of Ecorr indicates that the oxidized PANI–PS–Fe electrode should have the highest corrosion protection. The polarization resistances, Rp were evaluated from the Tafel plots, according to the Stearn–Geary equation , Rp ¼ ba bc =2.303ðba þ bc ÞI corr . ð1Þ Here, Icorr is the corrosion current determined by an intersection of the linear portions of the anodic and cathodic curves, and ba and bc are anodic and cathodic Tafel slopes (DE/Dlog I), respectively. The protection Fig. 1. Photographs of: (a) Fe electrode, (b) PANI–PS–Fe electrode, (c) PANI–Fe electrode, after 7 days immersed in 3% (w/v) NaCl aqueous solutions. The curve in (b) was drawn in order to display the location of the interface between the iron electrode and the shielding material (epoxy resin). White lumps in (b) and (c) are corroded products after 7 days of immersion. 136 Y.M. Abu, K. Aoki / Journal of Electroanalytical Chemistry 583 (2005) 133–139 -0.5 -0.6 (d) Ecorr / V vs. SCE EOC / V vs.SCE -0.5 (c) -0.6 (b) (e) -0.7 -0.7 -0.8 (a) 0 1000 2000 t/s -0.9 -11 -10 -9 -8 -7 -6 ln(W / g) Fig. 2. Time-variations of open circuit potential, Eoc at: (a) Fe, (b) reduced PANI–PS–Fe, (c) PANI–Fe, (d) oxidized PANI–PS–Fe, and (e) Fe connected with PANI-coated Pt electrodes (Fig. 8) in 0.01 M HCl. Fig. 4. Plot of corrosion potential, Ecorr against the logarithm of the amount of PANI loaded on the electrodes for (circles) PANI–Fe (squares) oxidized PANI–PS–Fe and (triangles) PANI–PS–Fe after THF treatment. In all cases corrosion potentials were measured in 3% (w/v) NaCl aqueous solutions. eﬃciency (PEF%) values were estimated using the following equation : These corrosion parameters were calculated from the Tafel plots for several ratios of the amount the reduced PANI–PS and the oxidized PANI–PS, and were listed in Table 1. The polarization resistance as well as the protection eﬃciency increased with an increase in the fraction of the emeraldine form of PANI. The corrosion currents for the coated-Fe electrodes are smaller than the current at the uncoated-Fe. Fig. 4 shows the dependence of the corrosion potential, Ecorr on the logarithm of the amount of PANI, W, loaded on the Fe surfaces. The positive shift of Ecorr with an increase in W indicates the strong protection of the corrosion at a thicker ﬁlm. Since thicker ﬁlms can be fabricated by use of PANI–PS more easily than by electrochemical polymerization, the protection by PANI–PS is more eﬃcient than electrochemical fabrication. The oxidized PANI–PS can react with ferrous or ferric ion to yield corrosion-protective Fe–PANI complex . According to the reaction proposed , 1 1 P EF % ¼ 100½R1 p ðuncoatedÞ Rp ðcoatedÞ=Rp ðcoatedÞ. ðn=mÞFe þ ½PANInþ ! ðn=mÞFemþ þ ½PANI0 (a) (b) (c) log(|I| / A) -6 (d) -8 -10 -1.2 -1 -0.8 -0.6 -0.4 E /V vs.SCE Fig. 3. Tafel plots at: (a) Fe, (b) reduced PANI–PS–Fe, (c) PANI–Fe, (d) oxidized PANI–PS–Fe electrodes in 3% (w/v) NaCl aqueous solutions at a potential scan rate of 1 mV s1. ð2Þ ! ðFeÞn=m ½PANI; ð3Þ Table 1 Some corrosion parameters obtained in 3% (w/v) NaCl solutions, varying with the amount ratio of the oxidized PANI–PS to the reduced PANI–PS at a common value of the total loaded amount of PANI–PS Samples W(ox):W(rd) Ecorr/V vs. SCE Icorr/lA cm2 Rp/kX PEF % Uncoated-Fe PANI–PS/Fe PANI–PS/Fe PANI–PS/Fe PANI–PS/Fe PANI–PS/Fe PANI–PS/Fe PANI–PS/Fe – 0:1 1:4 2:3 1:1 3:2 4:1 1:0 0.880 0.865 0.812 0.755 0.723 0.698 0.645 0.614 15.3 17.0 8.2 7.4 6.3 5.3 3.4 1.9 22 76 180 372 552 694 833 1120 – 70 87 94 96 96 97 98 Y.M. Abu, K. Aoki / Journal of Electroanalytical Chemistry 583 (2005) 133–139 where m = 2 or 3. Letting jFe and jPANI are the current densities for the oxidation and reduction of Fe and PANI, respectively, we can write: ð4Þ þ jPANI ¼ k PANI ½PANI exp½bnFE=RT ; ð5Þ where a and b are transfer coeﬃcients. Since the anodic current is compensated with cathodic one in corrosion, summation of cathodic and anodic current densities yield, jFe þ jPANI ¼ 0. -0.7 -0.8 ð6Þ -0.9 -13 From Eqs. (4)–(6) we can write, Ecorr -0.6 Ecorr / V vs.SCE jFe ¼ k Fe expðamFE=RT Þ; 137 RT fln½PANIþ þ const.g ¼ F ðam þ bnÞ ð7Þ Values of am and bn were obtained from the Tafel slopes in Fig. 3(a). The value of RT/F(am+bn) is 0.052 V, which is close to the value of the slope in Fig. 4. Therefore the linearity in Fig. 4 is based on the simple combination of Eqs. (4)–(6). Fig. 5 shows the variation of Ecorr against the logarithm of the ratio of the amount of the oxidized PANI–PS to the reduced one, where the amounts of loaded PANI–PS were common. A Nernst-like variation is found, showing the slope of 0.060 V. If the linear relation were to be regarded as the Nernst equation, the value of n is 0.4. This is within the conventional domain of n from 0.2 to 0.5 [57,58]. Therefore, Ecorr at the PANI– PS–Fe electrode is strongly controlled by the redox reaction of PANI. Values of Ecorr for several values of W(ox)/W(rd) showed a linear variation of the logarithm of the corrosion current, as shown in Fig. 6. The slope was 0.13 V, which is equivalent to bn = 0.21. Since n = 0.4, we got b = 0.5. This belongs to the conventional charge transfer process. -12 -11 ln( Io / A) Fig. 6. Variation of corrosion potential with logarithm of the corrosion current at PANI–PS–Fe electrodes. When the PS-core was dissolved in THF from the PANI–PS–Fe electrode [41,42] the resulting ﬁlm exhibited the linear variation of Ecorr with ln W, as shown in Fig. 4 (triangles). We succeeded in fabricating a large PANI ﬁlm (5 · 4 cm2) by removing PS-core from the PANI–PS ﬁlm in THF. This ﬁlm was uniform on the scale of 0.5 mm, as is shown in Fig. 7. However, it Ecorr / V vs.SCE -0.6 -0.7 -0.8 -1 0 1 ln[ W(ox) / W(rd) ] Fig. 5. Dependence of corrosion potential on the logarithm of the amount ratio of the oxidized PANI–PS to reduced PANI–PS at a common value of the total loaded amount of PANI–PS. Fig. 7. Photograph of large (5 · 4 cm2) PANI–PS ﬁlm on Fe-plate after removing the PS-core by THF. White part is the uncovered Feplate. The inset is a magniﬁed photograph. 138 Y.M. Abu, K. Aoki / Journal of Electroanalytical Chemistry 583 (2005) 133–139 a b bare bare Electrode Electrode Fig. 8. Illustration of: (a) the presence of the bare part of Fe surface and (b) local covering of the bare part with PANI. had a pattern of the PANI-shell on the scale of 10 lm, as is shown in the inset of Fig. 7. Since the domain among the grains in the inset was still green, it should be ﬁlled with PANI. Although the ﬁlm is heterogeneous in the grain form on the micrometer scale, it covers the electrode surface completely. A question may arise about whether the PANI–PS particles can cover uniformly the iron plate coated with spherical PANI–PS and whether the corrosion may occur at the uncoated part, as is illustrated as ‘‘bare’’ in Fig. 8(a). Since a sphere comes in contact with a plane at a point geometrically, most area of the iron should not be coated with PANI. However, we observed that the PANI–PS prevented hydrogen evolution at 0.0 V vs. SCE from the platinum electrode in the PANI–PS suspension , indicating that the bare part has been coated with invisible PANI ﬁlm automatically. This property is due to strong adsorption of PANI on surfaces. A PANI ﬁlm has bridged two separated electrodes electrically over the insulating wall [59,60]. PANI may transfer from the surface of adsorbed PANI–PS particle onto the iron surface to make an anti-corrosive PANI– iron interface, as illustrated in Fig. 8(b). In order to elucidate the corrosion mechanism we consider the two systems in the solution: (A) an iron plate of which surface is covered with a PANI ﬁlm, and (B) an iron block connected with a PANI block through a platinum wire, as illustrated in Fig. 9. The two systems are diﬀerent in the presence (A) and the absence (B) of iron–PANI interface. We constructed a model (B) by connecting electrically a PANI-coated Pt wire and an iron plate. The model immersed in chloride solution exhibited the white precipitate from the iron Pt Fe PANI Chloride solution Fig. 9. Illustration of a model electrode or the corrosion. The potential is measured against a reference electrode, e.g., SCE. plate. The open circuit potential was 0.52 V (Fig. 2(e)), being between Eoc at only the iron 0.65 V and Eoc at only the PANI ﬁlm 0.51 V. The redox state of the iron for (B) may be the same as the state at which 0.52 V is applied to the iron by a potentiostat. Thus, the protection of the corrosion can be ascribed the interface between Fe and PANI. PANI forms a complex with Fe at the interface , as is shown by reaction (3). The complex may make an anti-corrosive ﬁlm. 4. Conclusion The PANI–PS particles on the iron plate were useful to protect iron against corrosion. The ﬁlms were stable at the iron surface at potential region where gas evolution occurred. The oxidized form of PANI–PS ﬁlm showed better corrosion protection than its reduced form. The corrosion protection was demonstrated with the positive shift of the open circuit potential and the corrosion potential and by the decrease in the corrosion current. The corrosion potential had the linear relation with the logarithm of the amount ratio of the oxidized PANI to the reduced PANI. Pure polyaniline ﬁlm on iron was obtained by removing PS latex by THF. 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