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Electrochemical synthesis and characterization of poly(pyrrole-co-o-toluidine)
Progress in Organic Coatings 63 (2008) 424–433
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Electrochemical synthesis and characterization of poly(pyrrole-co-o-toluidine) Süleyman Yalc¸inkaya ∗ , Tunc¸ Tüken, Birgül Yazici, Mehmet Erbil C¸ukurova University, Science and Letters Faculty, Chemistry Department, 01330 Adana, Turkey
a r t i c l e
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Article history: Received 8 October 2007 Received in revised form 19 June 2008 Accepted 7 July 2008 Keywords: o-Toluidine Pyrrole Copolymer Corrosion
a b s t r a c t The electrochemical polymerization of o-toluidine has been investigated in oxalic acid solution. It was shown that the oxidation of monomer could be achieved but this process does not yield a stable, homogenous polymer film on either platinum or mild steel electrodes. Therefore the copolymerization between pyrrole and o-toluidine has been studied as an alternative method for obtaining good quality coating (low permeability and water mobility, high stability), which could also be easily synthesized on steel. For this aim, various monomer feed ratio solutions of pyrrole:o-toluidine 9:1, 8:2 and 7:3 have been examined, in aqueous oxalic acid solution. By using cyclic voltammetry technique, copolymer films were realized on platinum and steel, successfully. The temperature of synthesis solution was found to have a vital role on polymerization and film growth, as much as the monomer feed ratio. The synthesis of homogenous copolymer film could only be achieved under ≤25 ◦ C conditions with using the 9:1 ratio, while the 8:2 ratios could only produce stable films below 5 ◦ C. As the amount of o-toludine increased the required temperature value decreased further, 7:3 ratio could only give a stable copolymer film below 2 ◦ C. The characterization of deposited copolymer coating has been realized by using SEM micrographs, UV–vis and FT-IR spectroscopy techniques and cyclic voltammetry. The protective behaviour of these coatings was also investigated against mild steel corrosion in 3.5% NaCl solution, by means of electrochemical impedance spectroscopy (EIS) and anodic polarization curves. It was found that the monomer feed 8:2 ratio gave the most effective coating against the corrosion of mild steel. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The studies aiming to develop conducting polymer films for various applications (electrochromic devices [1], photoelectrochemical devices [2], rechargeable batteries [3], sensors [4,5] and corrosion protection [6–9]) frequently involve structural modification of the polymer backbone to enhance the properties, e.g. incorporation of various functional groups changes conductivity and porosity. Polyaniline, polypyrrole and their derivatives have been regarded as the most important conducting polymers, owing to their stability and synthesis advantages [10,11]. The electropolymerization of aniline (and its derivatives) brings about some difficulties like slow nucleation and film growth, but its high stability and interesting electrochemical properties have attracted much attention [10]. On the other hand, polypyrrole films generally exhibit better conductivity and are more easily synthesized by electropolymerization, when compared to polyaniline [12,13]. The role of anticorrosive polymer coatings on oxidizable metals is to hinder the attack of corrosive environment and reduce the corrosion rate [14,15]. The conducting polymer films gener-
∗ Corresponding author. Tel.: +90 322 338 60 81; fax: +90 322 338 60 70. E-mail address: [email protected] (S. Yalc¸inkaya). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.07.002
ally have nobler reduction potentials with respect to mild steel and this give rise to an interesting phenomena; the anodic protective effect of conducting polymer films. The protection comes from formation of more stable ferric compounds under oxidizing and stabilizing effect of polymer film [16–18]. On the other hand, the hydrophilic and porous nature of conducting polymer films lead to serious drawbacks for anticorrosive applications under severe conditions. The copolymerization has long been utilized to improve various properties (conductivity, stability, porosity, etc.) of polymer films [19]. The similar structural properties of pyrrole and aniline allow the copolymerization between these species. It was reported that this process undergoes exothermically. Li et al. have reported that copolymer film of pyrrole–toluidine (feed ratio of 3:7) could be realized at 2 ◦ C temperature, with chemical synthesis technique [20]. Poly(o-toluidine) homopolymer film exhibit good stability. Vandana et al. have reported that electrochemically synthesized poly(o-toluidine) could protect copper against corrosion in chloride media, successfully [21]. However, the attempts aiming the electrosynthesis of poly(o-toluidine) film on mild steel surface have failed. This was attributed to slow nucleation and low surface coverage. Therefore the anodic oxidation of steel continues at monomer oxidation region and inhibits the formation of stable-adherent film on the surface. In order to obtain a film which combines the
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advantages of pyrrole (ease of synthesis and conductivity) and o-toluidine (low permeability and high stability), poly(pyrroleco-o-toluidine) coating have been electrosynthesized on mild steel. The copolymer films were characterized by FT-IR, UV–vis spectroscopy, cyclic voltammetry measurement and SEM micrographs. The corrosion behaviour of poly(pyrrole-co-o-toluidine) copolymer was investigated in 3.5% NaCl solution. 2. Experimental All the electrochemical studies were carried out in a conventional three-electrode set up, open to the atmosphere, by using CHI604 model electrochemical analyzer. The counter electrode was a platinum foil with 2 cm2 surface area and Ag/AgCl electrode was used as the reference, all the potential values were referred to this electrode. Mild steel samples were cylindrical rods measuring 0.40 cm in the radius and with the following composition (W%) 0.082, C; 0.621, Mn; 0.181, Si; 0.0129, P; 0.0162, S; 99.0866, Fe, the working are 0.5024 cm2 while rest of electrode was isolated with thick polyester block. The copolymer films were electrochemically synthesized by using cyclic voltammetry technique. The synthesis solution (all chemicals were purchased from Merck) composition was 0.1 M oxalic acid +0.1 M monomer, varying the ratio pyrrole:o-toluidine (9:1, 8:2, 7:3) but keeping constant the total concentration of monomers. Each monomer feed ratio was studied for a set of temperatures, then the most appropriate values were determined and
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applied, these temperatures were 25, 5 and 2 ◦ C for 9:1, 8:2 and 7:3 monomer feed ratios, sequentially. These temperatures were yielding stable copolymer films on mild steel surface. The solution of 8:2 monomer feed ratio did not give a stable polymer film above 5 ◦ C, while the ratio of 7:3 involved lower temperature like 2 ◦ C. There was a monomer oxidation which was observed as current increase, but this process could not lead to a sufficiently thick and homogenous film. The thickness of coatings was approximately the same, by balancing the CV numbers and passing charges within monomer oxidation potential regions. The applied charge density value was 1.17 C/cm2 for each sample. UV–vis spectra of the copolymer solution in dimethyl sulfoxide (DMSO) were recorded on a PerkinElmer Lambda 25 UV–Vis spectrophotometer. FT-IR spectra measurements were conducted using a PerkinElmer spectrum RX1 FT-IR system instrument. For this aim, the polymer coatings electrosynthesized on mild steel were peeled off the surface and their pellets were prepared with bulk KBr. Electrochemical impedance spectroscopy (EIS) and anodic polarization curves were used to investigate the corrosion performance of these coatings. 3. Results and discussion 3.1. Synthesis Fig. 1 shows the cyclic voltammograms recorded for platinum (Pt) electrode in 0.3 M oxalic acid +0.1 M o-toluidine (at 25 ◦ C) by
Fig. 1. The voltammograms recorded for Pt electrode in 0.3 M oxalic acid +0.1 M o-toluidine scan rate, 10 mV/s (a), 20 mV/s (b) and 50 mV/s (c).
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applying various scan rates 10 mV/s (a), 20 mV/s (b) and 50 mV/s (c). The monomer oxidation process was observed as current increase at ∼+0.70 V, in the first forward scan for all scan rates. Even though the oxidation current value increased with increasing scan rate (from 10 to 50 mV/s), it decreased at the second and following cycles. This oxidation process led to formation of poly(otoluidine), but a stable homogenous film could not be obtained on the surface. During the successive CVs applied for film growth (for increasing the thickness), the oxidation–reduction process of produced poly(o-toluidine) was observed as anodic and cathodic waves between approximately +0.2 and +0.4 V, in the forward and reverse scans. However, none of the scan rates gave sufficiently thick and adherent polymer film which could cover the surface successfully. Also, it should be noted that the hydrogen reduction process was observed beyond −0.3 V (toward negative direction) for all scan rates. Fig. 2 shows the voltammograms of MS electrode obtained in 0.3 M oxalic acid +0.1 M o-toluidine solution by applying various scan rates, at 25 ◦ C. The typical oxidation–passivation behaviour of MS appeared as broadened anodic peak starting at −0.6 V, during the first forward scan. This process occurs via oxidation of MS to give Fe(II) ions which give rise to Fe(II) oxalate complex formation and passivation of the surface. The increasing scan rate caused greater oxidation–passivation peak due to higher dissolution rate. The monomer oxidation process was also observed at around +0.70 V, during the first forward scan. In the case of MS substrate the amount of adhered polymer product was observed to be
higher than the platinum. This was also observed by naked eye with the presence of a polymeric film on the surface after a few cycles. However the surface coverage of this film was not sufficient so that the re-passivation peak was observed for each reverse scan. During this re-passivation process the passivity is broken due to reduction of Fe(III) compounds to yield Fe(II). Then the formation of Fe(II) oxalate provides the passivity, once again [14]. The appearance of this peak was indicating of an interaction between the solution and the substrate thus that the film was not homogenously covering the surface. Since it was not possible to obtain a stable and adherent poly(o-toluidine) film on metal surface (neither platinum nor steel) via electropolymerization, the copolymer of pyrrole and o-toluidine was studied. It was believed that this copolymer would be electrosynthesized easily and has the advantages of pyrrole and o-toluidine. Figs. 3 and 4 shows the CVs obtained on Pt electrode in various monomers feed ratio of pyrrole and o-toluidine. In Fig. 3, successive three cycles were given for a potential range of −0.6 and 1.2 V. During these measurements, a scan of 50 mV/s was applied. The CVs exhibited remarkably different pattern when compared to single o-toluidine solutions. The monomer oxidation potential value of pyrrole was also reported to be around +0.70 V, in literature [8]. Therefore, the oxidation of these two monomers could be achieved simultaneously. Then the formed radical cations could combine to yield a copolymer structure. However, the studied monomer feed ratios could only yield stable polymer film under specific temperature conditions. These temperature values were
Fig. 2. The voltammograms recorded for MS electrode in 0.3 M oxalic acid +0.1 M o-toluidine scan rate, 10 mV/s (a), 20 mV/s and (b) 50 mV/s (c).
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Fig. 3. The voltammograms recorded for Pt in (0.3 M oxalic acid +0.09 M pyrrole +0.01 M o-toluidine, at 25 ◦ C) (a), (0.3 M oxalic acid +0.08 M pyrrole +0.02 M o-toluidine, at 5 ◦ C) (b) and (0.3 M oxalic acid +0.07 M pyrrole +0.03 M o-toluidine, at 2 ◦ C) (c), 50 mV/s.
determined after many attempts with various temperatures for each monomer feed ratio. The ratio of 9:1 did not give a polymer film above the 25 ◦ C temperature, while the 8:2 ratios could only produce stable films below 5 ◦ C. As the ratio of o-toluidine increased the required temperature value was found to decrease further, therefore a stable copolymer film could only be realized below 2 ◦ C, with the ratio of 7:3. This case could be explained by the exothermic nature of polymerization mechanism suggested for aniline derivatives in literature [20]. It must be noted that the observed current values for monomer oxidation and polymerization increased with respect to single o-toluidine. This could simply be explained with significantly different mechanism of polymerization, in presence of pyrrole. As a matter of fact, the current decrease observed in this region could not be observed for single pyrrole polymerization [10]. This was also related to a decreasing conductivity with incorporation of otoluidine into the structure. The produced copolymer film was in oxidized state and its reduction was observed as cathodic wave during the reverse scan, also re-oxidation process was observed at the following forward scan. The surface coverage of the produced homogenous film could be considered as well, since the hydrogen reduction process could not be observed at the first reverse scan. The copolymer film inhibited the hydrogen gas evolution which was observed beyond −0.3 V (towards negative direction) previously for single o-toluidine conditions. The CVs recorded for the copolymer film growth on platinum electrode are given in Fig. 4, for each monomer feed ratio. It was apparent that the copolymer film thickness could be increased with successive cycles. The current
values corresponding to oxidation–reduction behaviour of polymer film increased gradually with the cycle numbers. The convenient temperature values determined for copolymerization of pyrrole and o-toluidine were also applied for film growth on mild steel substrate and Figs. 5 and 6 shows the CVs obtained. The typical oxidation–passivation behaviour of MS appeared as broadened anodic peak starting at −0.6 V, during the first forward scan for the all monomer feed ratios. However, it was clear that the monomer feed ratio altered the rate of film growth and the resulting copolymer in aspect of conductivity and surface coverage. Thus, the re-passivation peak could be observed as a little wave for the ratio 9:1 while this event was observed for the other ratios; however it was apparent that the increasing o-toluidine ratio decreased the surface coverage and polymerization rate. The re-passivation peak was observed even after third cycle, but it became smaller after each cycle due to film growth. The monomer oxidation process was also observed at around +0.7 V, during the first forward scan. It is apparent from Fig. 6 that adherent and homogeneous copolymer films could be realized on MS surface, at convenient temperature conditions. 3.2. Spectroscopic characterization of copolymer films FT-IR spectra of polypyrrole and copolymer samples are shown in Fig. 7. It was reported that the band centered at 3250 cm−1 was related to characteristic –NH– stretching vibration. This indicated the presence of –NH– groups in o-toluidine and pyrrole units that showed in all spectra [12,22]. Both polypyrrole and copolymer spec-
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Fig. 4. The voltammograms recorded during the poly(pyrrole-co-o-toluidine) film growth on Pt electrode 9:1 at 25 ◦ C (a), 8:2 at 5 ◦ C (b) and 7:3 at 2 ◦ C (c); scan rate, 50 mV/s.
tra showed the peaks at 1500–1600 cm−1 which was attributed to –C C– stretching of reduced and oxidized quinoid form of benzene and pyrrole ring, respectively [23]. In the FT-IR spectrum of polypyrrole, the peak at 1033 cm−1 is due to –C–H in-plane deformation of the pyrrole unit [12]. It was reported that in the spectrum of the copolymer sample the peak at 3040 cm−1 is due to characteristic aromatic –C–H– stretching [24]. Also, the peak at 1380 cm−1 is attributed to methyl group in the structure of o-toluidine unit [25]. The peaks observed in the FT-IR spectrum of poly(pyrrole-coo-toluidine) were indicating to presence of o-toluidine unit in the obtained copolymer structure. The UV–vis spectra of the polypyrrole and copolymer samples solution in DMSO recorded at 25 ◦ C are shown in Fig. 8. The spectra representing homopolymer of polypyrrole exhibites the characteristic peak located at the 260 nm, which is attributed to –* transition (K band) of the pyrrole ring [26]. It was found that this peak shifted to 245 nm (K band) in the spectrum of the copolymer. The second band in the spectrum of copolymer at 279 nm is assigned to the –* transition (B band) of the benzene ring [26]. This band appearing in the spectrum of poly(pyrrole-co-otoluidine) indicates that the o-toluidine unit is in the copolymer chain. The structure of synthesized copolymer films were given in Fig. 9. The SEM micrographs of polypyrrole film and the copolymer films deposited on mild steel from 9:1 and 8:2 monomer feed ratio solutions are given in Fig. 10. It was apparent that the synthesized copolymer films had a structure significantly different from cauliflower like structure of polypyrrole. The particle size decreased with increasing o-toludine ratio and the appearance became more likely a globular structure of aniline derivatives.
3.3. Corrosion tests The EIS measurement results are given in Figs. 11–13 as a function of immersion time in corrosive solution. After 4 h of exposure time, the obtained Nyquist plot for the coating deposited from 9:1 monomer feed ratio solution was made up of two distinctive regions which were in appearance of semi-ellipses or arcs. The uncompensated ohmic resistance (Ru ) is always a small constant for given electrolyte solution and appeared as a shift on the real axis. The first region (from 100 kHz down to 10 Hz) included information about the electrochemical process occurring within the active pores of polymer film reaching the substrate surface [27–30]. Therefore the resistance depicted by this region was handled as pore resistance (Rpo ), which included the charge transfer resistance (Rct ) arising from kinetically controlled metal dissolution at the bottom of the pores, any species giving rise to resistive effect and diffuse layer resistance (Rd ) along the pore. In the case of extremely thin and porous conducting polymer coating, the corrosion process could only occur within the pores. Moreover, the said Rpo values were observed to decrease with time due to increase of water held by coating. This phenomenon increases the mobility of corrosive species towards the metal and increasing corrosion rate. The second region at lower frequency was related to polymer coating resistance; this Rf value includes both the corrosion products accumulating within the pores of polymer coating and the intact film itself. The sum of Rpo and Rf values were equal to polarization resistance Rp [31–33]. It must be noticed that the measured open circuit potential value (Ecorr ) of sample was −0.54 V, which indicated a metal/solution interface at the bottom of the pores. Once the solution reached
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Fig. 5. The voltammograms recorded for MS in (0.3 M oxalic acid +0.09 M pyrrole +0.01 M o-toluidine, at 25 ◦ C) (a), (0.3 M oxalic acid +0.08 M pyrrole +0.02 M o-toluidine, at 5 ◦ C) (b) and (0.3 M oxalic acid +0.07 M pyrrole +0.03 M o-toluidine, at 2 ◦ C) (c), 50 mV/s.
Fig. 6. The voltammograms recorded during the poly(pyrrole-co-o-toluidine) 9:1 at 25 ◦ C (a), 8:2 at 5 ◦ C (b) and 7:3 at 2 ◦ C (c) film growth on MS electrode; scan rate, 50 mV/s.
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Fig. 9. The structural representation of polypyrrole (a) and the copolymer (b).
Table 1 The Rp (ohm), Ecor (V), E% and P values after various exposure time (t) in 3.5% NaCl solution Fig. 7. FT-IR spectra of the polypyrrole (a) and poly(pyrrole-co-o-toluidine) (9:1) (b).
Electrode
t (h)
Rp (ohm)
Ecor (V)
E%
P
Uncoated MS MS/9:1
24 4 24 48 4 24 48 4 24 48
58 2511 1585 1259 2487 1995 1585 708 708 1122
−0.661 −0.540 −0.598 −0.603 −0.568 −0.585 −0.593 −0.569 −0.602 −0.587
– 98 96 95 98 97 96 92 92 95
– 0.23 0.12 0.14 0.14 0.12 0.13 0.48 0.24 0.21
MS/8:2
MS/7:3
Fig. 8. UV–vis absorption spectra of polypyrrole (- -) and 9:1 copolymer (—).
the metal surface, the corrosion process starts at the bottom of the pores and the measured potential of the electrode becomes a mixed potential value. Then this region should be considered as the pore resistance, since it is related to charge transfer and diffusion pro-
cess taking place within the pores. Considering the polymer film and the substrate as two separated metals connected in an electrolyte, the mixed potential reached by the couple depends on the surface ratio of noble and less noble metals in contact with the solution. Here, the polymer film behaves like the nobler synthetic metal, since it has the ability to oxidize the metal when it is found in its oxidized state. Then, electron transfer could take place between the metal and coating, at metal/coating interface. As the amount of corrosive solution interacting with the substrate metal increased, the Emix value approached the Ecorr value of the base metal (Table 1). The second region of Nyquist plot was related to the coating system which consisted of polymer film and the passive oxide layer formed at metal/polymer interface. The reduction of polymer film and the accumulation of corrosion products (within the pores of coating) would increase the film resistance with time; it was
Fig. 10. The SEM micrographs of polypyrrole (A), and copolymer coatings 9:1 (B) and 8:2 (C).
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Fig. 11. EIS results of 9:1 copolymer for 4 h (); 24 h (); 48 h () exposure times in % 3.5 NaCl.
Fig. 12. EIS results of 8:2 copolymer for 4 h (); 24 h (); 48 h () exposure times in % 3.5 NaCl.
observed that the total resistance decreased with time. This was related to water up taking process, during immersion period in corrosive solution. Also the pore and coating resistance regions could not be identified neither in Nyquist plot nor log f–log Z diagram. This could be explained with the ratio of these resistance values, with respect to each other. Therefore, the total resistance was handled as the polarization resistance value (Rp ) and given in Table 1, with respect to exposure time. These EIS results could be handled with an equivalent circuit including the resistive elements for the
coating (Rf ), the resistance against corrosion (Rpo ) and the uncompensated ohmic resistance (Ru ); the capacitive elements of intact coating Cf and the double layer capacitance (Cdl ), as it frequently appeared in the literature [27]. It was clearly seen that the other coating systems also exhibited the same feature with that deposited from 9:1 monomer feed ratio solution. However the highest resistance values were obtained with 8:2 ratio. Also this coating gave slightly nobler corrosion potential values with respect to others.
Fig. 13. EIS results of 7:3 copolymer for 4 h (); 24 h (); 48 h () exposure times in % 3.5 NaCl.
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The approximate values for porosity of coatings were also calculated for the aim of comparison, by using the EIS results and given in Table 1 [34]. The relevant equation is derived from Tafel equations written for coated and uncoated samples under the same over potential value, by doing so it is assumed that the coating blocks the anodic surface and decreases the metal dissolution. Thus the protective efficiency of coating is strictly related to surface coverage, . P =1− =
Fig. 14. Anodic polarization curves for coated and uncoated samples, MS (䊉), 9:1 ( ); 8:2 (); 7:3 () copolymer coated samples in % 3.5 NaCl.
By the help of Rp values the percent protection efficiency values (E%) were calculated for coated electrodes according to following E% = ((Rp−1(uncoated) − Rp−1(coated) )/Rp−1(uncoated) ) × 100 equation and given in Table 1. Even though, the amount of electrolyte solution increased at the metal/polymer interface and the Ecorr value decreased with increasing exposure time. The %E values were found not to decrease with increasing exposure time; this case provided additional evidence for the anodic protective behaviour of the polymer film.
Rp (uncoated) Rp (coated)
× 10−(E/ˇA )
However, the anodic protective effect of conducting polymer film is known to lead a passive layer (ferric compounds) on corroded area, therefore the corrosion process at the bottom of the pores should be better considered to take place under oxygen diffusion control. Under these circumstances, the value of ˇA should be used as 0.120 V/dec which could be derived from B value. Then the calculated porosity (P) values became more reliable in aspect of comparison. However, it was seen that he calculated P values were slightly high, considering the E% values. This case is explained with the charge transfer process occurring between metal and polymer film, which increased the total current value measured all over the surface. It was seen from Table 1 that the porosity values exhibited a tendency to decrease with immersion time. This was also indicative for anodic protective behaviour of coating systems. The porosity is one of the important parameters in aspect of anticorrosive behaviour of coatings and showed that the polymer coatings were able to hinder the access of corrosive solution to metal substrate. However it must be noted that the results summarized in Table 1 were indicating to better protective behaviour of the coating obtained in 8:2 monomers feed ratio solution.
Fig. 15. Successive CVs recorded for 9:1 (a), 8:2 (b) and 7:3 (c) copolymer coated samples on the Pt electrode in 3.5% NaCl.
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The anodic polarization curves recorded for coated samples and bare sample are given in Fig. 14. It was apparent that the coatings could effectively lower the anodic dissolution rate of substrate that the current values decreased remarkably. Also, the over oxidation of polymer coating becomes an important phenomena at high anodically polarized conditions, but the current values still remained lower than bare sample. The corrosion process could only take place through the pores of coating and once the coating prevented the mass transfer between the corrosive environment and substrate, the corrosion rate will decrease. The lowest dissolution rate was observed for the coating obtained from 8:2 monomer feed ratio. This also showed that this coating exhibited lower permeability and higher stability than the other coatings. In order to examine the stability of copolymer films obtained from different monomer feed ratio solutions, they have been synthesized on Pt electrode (applying the same conditions as before) and the successive cycles were recorded in 3.5% solution (Fig. 15). The oxidation of polymer film was observed in the forward scan and the reduction was realized at the reverse scan. The current density values involved during these successive cycles were informative in aspect of polymer film’s electroactivity and degradability. It was clearly seen that the current values increased with increasing cycle numbers for the coating deposited from 9:1 ratio. Also the highest and most stable current values were observed for the coating of 8:2 ratio. 4. Conclusions The synthesis of o-toluidine could not be achieved on mild steel, by direct oxidation of monomer from the solution. However, the synthesis of copolymer between pyrrole and o-toluidine could be realized electrochemically on mild steel. The increasing ratio of o-toluidine requires lower synthesis temperature; this could be explained with exothermic nature of polymerization. The deposited coatings were compact and adherent on the surface that very thin films could provide significant protection behaviour against the corrosion of steel. The ratio of 8:2 gave the coating with highest efficiency when compared to other coatings examined. Also this coating was shown to have high stability and low permeability in aggressive solution. The porosity value of this film was 0.13 and its protection efficiency was 96%, after 48 h immersion period in 3.5% NaCl solution.
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Acknowledgment This study has been financially supported by C¸ukurova University research fund. The authors are grateful to the C¸ukurova University research fund (Project No: FEF2006D21). References [1] V. Prevost, A. Petit, F. Pla, Syn. Met. 104 (1999) 79–87. [2] C. Özdemir, H. Kaplan Can, N. C¸olak, A. Güner, J. Appl. Polym. Sci. 99 (2006) 2182–2192. [3] T. Jeevananda, Siddaramaiah, S. Seetharamu, S. Saravanan, L. D’Souza, Syn. Met. 140 (2004) 247–260. [4] S.-M. Chen, Y.H. Fa, J. Electroanal. Chem. 553 (2003) 63–75. [5] D.D. Borole, U.R. Kapadi, P.P. Mahulikar, D.G. Hundivale, Mater. Lett. 60 (2006) 2447–2452. [6] P. Herrasti, F.J. Recio, P. Ocon, E. Fatas, Prog. Org. Coat. 54 (2005) 285–291. [7] S.U. Rahman, M.A. Abul-Hamaley, B.J. Abul Alem, Surf. Coat. Technol. 200 (2005) 2948–2954. [8] T. Tuken, B. Yazıcı, M. Erbil, Mater. Des. 28 (2007) 208–216. [9] T. Tuken, G. Tansu˘g, B. Yazıcı, M. Erbil, Prog. Org. Coat. 59 (2007) 88–94. [10] D.D. Borole, U.R. kapadi, P.P. Mahulikar, D.G. Hundivale, Mater. Lett. 58 (2004) 3816–3822. [11] B.C. Roy, M.D. Gupta, J.K. Ray, Macromolecules 28 (1995) 1727–1732. [12] X. Li, M. Huang, L. Wang, M. Zhu, A. Menner, J. Springer, Syn. Met. 123 (2001) 435–441. [13] J.W. Kim, C.H. Cho, F. Liu, H.J. Choi, J. Joo, Syn. Met. 135–136 (2003) 17–18. [14] P. Herrasti, P. Ocon, Appl. Surf. Sci. 172 (2001) 276–284. [15] S. Patil, S.R. Sainkar, P.P. Patil, Appl. Surf. Sci. 225 (2004) 204–216. [16] M. Kumar Ram, E. Maccioni, C. Nicolini, Thin Solid Films 303 (1997) 27–33. [17] B.C. Roy, M.D. Gupta, L. Bhoumik, J.K. Ray, Syn. Met. 130 (2002) 27–33. [18] P. Pawar, A.B. Gaikwad, P.P. Patil, Electrochim. Acta 52 (2007) 5958–5967. [19] A.J. Motheo, M.F. Pantoja, E.C. Venancio, Solid State Ionics 171 (2004) 91–98. [20] X.G. Li, L.X. Wang, Y. Jin, Z.L. Zhu, Y.L. Yang, J. Appl. Polym. Sci. 82 (2001) 510–518. [21] V. Shinde, S.R. Sainkar, P.P. Patil, Corros. Sci. 47 (2005) 1352–1369. [22] V. Shinde, A.B. Mandele, K.R. Patil, A.B. Gaikwad, P.P. Patil, Surf. Coat. Technol. 16–17 (2006) 5094–5101. [23] A.L. Sharma, V. Saxena, S. Annapoorni, B.D. Malhotra, J. Appl. Polym. Sci. 81 (2001) 1460–1466. [24] X.G. Li, M.-R. Huang, Y. Jin, Y.L. Yang, Polymer 42 (2001) 3427–3435. [25] E.C. Zampronio, H.P. Oliveira, Meter. Res. Bull. 39 (2004) 1525–1538. [26] A.B.B. Cruz, J.L. Wardel, V.D. Rangel, R.A. Simao, A.M. Rocco, Syn. Met. 157 (2007) 80–90. [27] F. Mansfeld, J. Applied, Electrochemistry 25 (1995) 187. [28] G.W. Walter, Corrosion 26–9 (1986) 681–703. [29] T. Tüken, G. Arslan, B. Yazıcı, M. Erbil, Corros. Sci. 46 (2004) 2743. [30] G. Kousik, S. Pitchumani, N.G. Renganathan, Prog. Org. Coat. 43 (2001) 286–291. [31] T. Tuken, G. Tansu˘g, B. Yazıcı, M. Erbil, Surf. Coat. Technol. 202 (2007) 146–154. [32] S. Yalc¸ınkaya, T. Tuken, B. Yazıcı, M. Erbil, Prog. Org. Coat. 62 (2008) 236–244. [33] T. Tuken, A.T. Özyılmaz, B. Yazıcı, G. Kardas¸, M. Erbil, Prog. Org. Coat. 51 (2004) 27–35. [34] J. Creus, H. Mazille, H. Idrissi, Surf. Coat. Technol. 130 (2000) 224–232.
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Electrochemical synthesis and characterization of poly(pyrrole-co-o-toluidine) Süleyman Yalc¸inkaya ∗ , Tunc¸ Tüken, Birgül Yazici, Mehmet Erbil C¸ukurova University, Science and Letters Faculty, Chemistry Department, 01330 Adana, Turkey
a r t i c l e
i n f o
Article history: Received 8 October 2007 Received in revised form 19 June 2008 Accepted 7 July 2008 Keywords: o-Toluidine Pyrrole Copolymer Corrosion
a b s t r a c t The electrochemical polymerization of o-toluidine has been investigated in oxalic acid solution. It was shown that the oxidation of monomer could be achieved but this process does not yield a stable, homogenous polymer film on either platinum or mild steel electrodes. Therefore the copolymerization between pyrrole and o-toluidine has been studied as an alternative method for obtaining good quality coating (low permeability and water mobility, high stability), which could also be easily synthesized on steel. For this aim, various monomer feed ratio solutions of pyrrole:o-toluidine 9:1, 8:2 and 7:3 have been examined, in aqueous oxalic acid solution. By using cyclic voltammetry technique, copolymer films were realized on platinum and steel, successfully. The temperature of synthesis solution was found to have a vital role on polymerization and film growth, as much as the monomer feed ratio. The synthesis of homogenous copolymer film could only be achieved under ≤25 ◦ C conditions with using the 9:1 ratio, while the 8:2 ratios could only produce stable films below 5 ◦ C. As the amount of o-toludine increased the required temperature value decreased further, 7:3 ratio could only give a stable copolymer film below 2 ◦ C. The characterization of deposited copolymer coating has been realized by using SEM micrographs, UV–vis and FT-IR spectroscopy techniques and cyclic voltammetry. The protective behaviour of these coatings was also investigated against mild steel corrosion in 3.5% NaCl solution, by means of electrochemical impedance spectroscopy (EIS) and anodic polarization curves. It was found that the monomer feed 8:2 ratio gave the most effective coating against the corrosion of mild steel. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The studies aiming to develop conducting polymer films for various applications (electrochromic devices [1], photoelectrochemical devices [2], rechargeable batteries [3], sensors [4,5] and corrosion protection [6–9]) frequently involve structural modification of the polymer backbone to enhance the properties, e.g. incorporation of various functional groups changes conductivity and porosity. Polyaniline, polypyrrole and their derivatives have been regarded as the most important conducting polymers, owing to their stability and synthesis advantages [10,11]. The electropolymerization of aniline (and its derivatives) brings about some difficulties like slow nucleation and film growth, but its high stability and interesting electrochemical properties have attracted much attention [10]. On the other hand, polypyrrole films generally exhibit better conductivity and are more easily synthesized by electropolymerization, when compared to polyaniline [12,13]. The role of anticorrosive polymer coatings on oxidizable metals is to hinder the attack of corrosive environment and reduce the corrosion rate [14,15]. The conducting polymer films gener-
∗ Corresponding author. Tel.: +90 322 338 60 81; fax: +90 322 338 60 70. E-mail address: [email protected] (S. Yalc¸inkaya). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.07.002
ally have nobler reduction potentials with respect to mild steel and this give rise to an interesting phenomena; the anodic protective effect of conducting polymer films. The protection comes from formation of more stable ferric compounds under oxidizing and stabilizing effect of polymer film [16–18]. On the other hand, the hydrophilic and porous nature of conducting polymer films lead to serious drawbacks for anticorrosive applications under severe conditions. The copolymerization has long been utilized to improve various properties (conductivity, stability, porosity, etc.) of polymer films [19]. The similar structural properties of pyrrole and aniline allow the copolymerization between these species. It was reported that this process undergoes exothermically. Li et al. have reported that copolymer film of pyrrole–toluidine (feed ratio of 3:7) could be realized at 2 ◦ C temperature, with chemical synthesis technique [20]. Poly(o-toluidine) homopolymer film exhibit good stability. Vandana et al. have reported that electrochemically synthesized poly(o-toluidine) could protect copper against corrosion in chloride media, successfully [21]. However, the attempts aiming the electrosynthesis of poly(o-toluidine) film on mild steel surface have failed. This was attributed to slow nucleation and low surface coverage. Therefore the anodic oxidation of steel continues at monomer oxidation region and inhibits the formation of stable-adherent film on the surface. In order to obtain a film which combines the
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advantages of pyrrole (ease of synthesis and conductivity) and o-toluidine (low permeability and high stability), poly(pyrroleco-o-toluidine) coating have been electrosynthesized on mild steel. The copolymer films were characterized by FT-IR, UV–vis spectroscopy, cyclic voltammetry measurement and SEM micrographs. The corrosion behaviour of poly(pyrrole-co-o-toluidine) copolymer was investigated in 3.5% NaCl solution. 2. Experimental All the electrochemical studies were carried out in a conventional three-electrode set up, open to the atmosphere, by using CHI604 model electrochemical analyzer. The counter electrode was a platinum foil with 2 cm2 surface area and Ag/AgCl electrode was used as the reference, all the potential values were referred to this electrode. Mild steel samples were cylindrical rods measuring 0.40 cm in the radius and with the following composition (W%) 0.082, C; 0.621, Mn; 0.181, Si; 0.0129, P; 0.0162, S; 99.0866, Fe, the working are 0.5024 cm2 while rest of electrode was isolated with thick polyester block. The copolymer films were electrochemically synthesized by using cyclic voltammetry technique. The synthesis solution (all chemicals were purchased from Merck) composition was 0.1 M oxalic acid +0.1 M monomer, varying the ratio pyrrole:o-toluidine (9:1, 8:2, 7:3) but keeping constant the total concentration of monomers. Each monomer feed ratio was studied for a set of temperatures, then the most appropriate values were determined and
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applied, these temperatures were 25, 5 and 2 ◦ C for 9:1, 8:2 and 7:3 monomer feed ratios, sequentially. These temperatures were yielding stable copolymer films on mild steel surface. The solution of 8:2 monomer feed ratio did not give a stable polymer film above 5 ◦ C, while the ratio of 7:3 involved lower temperature like 2 ◦ C. There was a monomer oxidation which was observed as current increase, but this process could not lead to a sufficiently thick and homogenous film. The thickness of coatings was approximately the same, by balancing the CV numbers and passing charges within monomer oxidation potential regions. The applied charge density value was 1.17 C/cm2 for each sample. UV–vis spectra of the copolymer solution in dimethyl sulfoxide (DMSO) were recorded on a PerkinElmer Lambda 25 UV–Vis spectrophotometer. FT-IR spectra measurements were conducted using a PerkinElmer spectrum RX1 FT-IR system instrument. For this aim, the polymer coatings electrosynthesized on mild steel were peeled off the surface and their pellets were prepared with bulk KBr. Electrochemical impedance spectroscopy (EIS) and anodic polarization curves were used to investigate the corrosion performance of these coatings. 3. Results and discussion 3.1. Synthesis Fig. 1 shows the cyclic voltammograms recorded for platinum (Pt) electrode in 0.3 M oxalic acid +0.1 M o-toluidine (at 25 ◦ C) by
Fig. 1. The voltammograms recorded for Pt electrode in 0.3 M oxalic acid +0.1 M o-toluidine scan rate, 10 mV/s (a), 20 mV/s (b) and 50 mV/s (c).
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applying various scan rates 10 mV/s (a), 20 mV/s (b) and 50 mV/s (c). The monomer oxidation process was observed as current increase at ∼+0.70 V, in the first forward scan for all scan rates. Even though the oxidation current value increased with increasing scan rate (from 10 to 50 mV/s), it decreased at the second and following cycles. This oxidation process led to formation of poly(otoluidine), but a stable homogenous film could not be obtained on the surface. During the successive CVs applied for film growth (for increasing the thickness), the oxidation–reduction process of produced poly(o-toluidine) was observed as anodic and cathodic waves between approximately +0.2 and +0.4 V, in the forward and reverse scans. However, none of the scan rates gave sufficiently thick and adherent polymer film which could cover the surface successfully. Also, it should be noted that the hydrogen reduction process was observed beyond −0.3 V (toward negative direction) for all scan rates. Fig. 2 shows the voltammograms of MS electrode obtained in 0.3 M oxalic acid +0.1 M o-toluidine solution by applying various scan rates, at 25 ◦ C. The typical oxidation–passivation behaviour of MS appeared as broadened anodic peak starting at −0.6 V, during the first forward scan. This process occurs via oxidation of MS to give Fe(II) ions which give rise to Fe(II) oxalate complex formation and passivation of the surface. The increasing scan rate caused greater oxidation–passivation peak due to higher dissolution rate. The monomer oxidation process was also observed at around +0.70 V, during the first forward scan. In the case of MS substrate the amount of adhered polymer product was observed to be
higher than the platinum. This was also observed by naked eye with the presence of a polymeric film on the surface after a few cycles. However the surface coverage of this film was not sufficient so that the re-passivation peak was observed for each reverse scan. During this re-passivation process the passivity is broken due to reduction of Fe(III) compounds to yield Fe(II). Then the formation of Fe(II) oxalate provides the passivity, once again [14]. The appearance of this peak was indicating of an interaction between the solution and the substrate thus that the film was not homogenously covering the surface. Since it was not possible to obtain a stable and adherent poly(o-toluidine) film on metal surface (neither platinum nor steel) via electropolymerization, the copolymer of pyrrole and o-toluidine was studied. It was believed that this copolymer would be electrosynthesized easily and has the advantages of pyrrole and o-toluidine. Figs. 3 and 4 shows the CVs obtained on Pt electrode in various monomers feed ratio of pyrrole and o-toluidine. In Fig. 3, successive three cycles were given for a potential range of −0.6 and 1.2 V. During these measurements, a scan of 50 mV/s was applied. The CVs exhibited remarkably different pattern when compared to single o-toluidine solutions. The monomer oxidation potential value of pyrrole was also reported to be around +0.70 V, in literature [8]. Therefore, the oxidation of these two monomers could be achieved simultaneously. Then the formed radical cations could combine to yield a copolymer structure. However, the studied monomer feed ratios could only yield stable polymer film under specific temperature conditions. These temperature values were
Fig. 2. The voltammograms recorded for MS electrode in 0.3 M oxalic acid +0.1 M o-toluidine scan rate, 10 mV/s (a), 20 mV/s and (b) 50 mV/s (c).
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Fig. 3. The voltammograms recorded for Pt in (0.3 M oxalic acid +0.09 M pyrrole +0.01 M o-toluidine, at 25 ◦ C) (a), (0.3 M oxalic acid +0.08 M pyrrole +0.02 M o-toluidine, at 5 ◦ C) (b) and (0.3 M oxalic acid +0.07 M pyrrole +0.03 M o-toluidine, at 2 ◦ C) (c), 50 mV/s.
determined after many attempts with various temperatures for each monomer feed ratio. The ratio of 9:1 did not give a polymer film above the 25 ◦ C temperature, while the 8:2 ratios could only produce stable films below 5 ◦ C. As the ratio of o-toluidine increased the required temperature value was found to decrease further, therefore a stable copolymer film could only be realized below 2 ◦ C, with the ratio of 7:3. This case could be explained by the exothermic nature of polymerization mechanism suggested for aniline derivatives in literature [20]. It must be noted that the observed current values for monomer oxidation and polymerization increased with respect to single o-toluidine. This could simply be explained with significantly different mechanism of polymerization, in presence of pyrrole. As a matter of fact, the current decrease observed in this region could not be observed for single pyrrole polymerization [10]. This was also related to a decreasing conductivity with incorporation of otoluidine into the structure. The produced copolymer film was in oxidized state and its reduction was observed as cathodic wave during the reverse scan, also re-oxidation process was observed at the following forward scan. The surface coverage of the produced homogenous film could be considered as well, since the hydrogen reduction process could not be observed at the first reverse scan. The copolymer film inhibited the hydrogen gas evolution which was observed beyond −0.3 V (towards negative direction) previously for single o-toluidine conditions. The CVs recorded for the copolymer film growth on platinum electrode are given in Fig. 4, for each monomer feed ratio. It was apparent that the copolymer film thickness could be increased with successive cycles. The current
values corresponding to oxidation–reduction behaviour of polymer film increased gradually with the cycle numbers. The convenient temperature values determined for copolymerization of pyrrole and o-toluidine were also applied for film growth on mild steel substrate and Figs. 5 and 6 shows the CVs obtained. The typical oxidation–passivation behaviour of MS appeared as broadened anodic peak starting at −0.6 V, during the first forward scan for the all monomer feed ratios. However, it was clear that the monomer feed ratio altered the rate of film growth and the resulting copolymer in aspect of conductivity and surface coverage. Thus, the re-passivation peak could be observed as a little wave for the ratio 9:1 while this event was observed for the other ratios; however it was apparent that the increasing o-toluidine ratio decreased the surface coverage and polymerization rate. The re-passivation peak was observed even after third cycle, but it became smaller after each cycle due to film growth. The monomer oxidation process was also observed at around +0.7 V, during the first forward scan. It is apparent from Fig. 6 that adherent and homogeneous copolymer films could be realized on MS surface, at convenient temperature conditions. 3.2. Spectroscopic characterization of copolymer films FT-IR spectra of polypyrrole and copolymer samples are shown in Fig. 7. It was reported that the band centered at 3250 cm−1 was related to characteristic –NH– stretching vibration. This indicated the presence of –NH– groups in o-toluidine and pyrrole units that showed in all spectra [12,22]. Both polypyrrole and copolymer spec-
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Fig. 4. The voltammograms recorded during the poly(pyrrole-co-o-toluidine) film growth on Pt electrode 9:1 at 25 ◦ C (a), 8:2 at 5 ◦ C (b) and 7:3 at 2 ◦ C (c); scan rate, 50 mV/s.
tra showed the peaks at 1500–1600 cm−1 which was attributed to –C C– stretching of reduced and oxidized quinoid form of benzene and pyrrole ring, respectively [23]. In the FT-IR spectrum of polypyrrole, the peak at 1033 cm−1 is due to –C–H in-plane deformation of the pyrrole unit [12]. It was reported that in the spectrum of the copolymer sample the peak at 3040 cm−1 is due to characteristic aromatic –C–H– stretching [24]. Also, the peak at 1380 cm−1 is attributed to methyl group in the structure of o-toluidine unit [25]. The peaks observed in the FT-IR spectrum of poly(pyrrole-coo-toluidine) were indicating to presence of o-toluidine unit in the obtained copolymer structure. The UV–vis spectra of the polypyrrole and copolymer samples solution in DMSO recorded at 25 ◦ C are shown in Fig. 8. The spectra representing homopolymer of polypyrrole exhibites the characteristic peak located at the 260 nm, which is attributed to –* transition (K band) of the pyrrole ring [26]. It was found that this peak shifted to 245 nm (K band) in the spectrum of the copolymer. The second band in the spectrum of copolymer at 279 nm is assigned to the –* transition (B band) of the benzene ring [26]. This band appearing in the spectrum of poly(pyrrole-co-otoluidine) indicates that the o-toluidine unit is in the copolymer chain. The structure of synthesized copolymer films were given in Fig. 9. The SEM micrographs of polypyrrole film and the copolymer films deposited on mild steel from 9:1 and 8:2 monomer feed ratio solutions are given in Fig. 10. It was apparent that the synthesized copolymer films had a structure significantly different from cauliflower like structure of polypyrrole. The particle size decreased with increasing o-toludine ratio and the appearance became more likely a globular structure of aniline derivatives.
3.3. Corrosion tests The EIS measurement results are given in Figs. 11–13 as a function of immersion time in corrosive solution. After 4 h of exposure time, the obtained Nyquist plot for the coating deposited from 9:1 monomer feed ratio solution was made up of two distinctive regions which were in appearance of semi-ellipses or arcs. The uncompensated ohmic resistance (Ru ) is always a small constant for given electrolyte solution and appeared as a shift on the real axis. The first region (from 100 kHz down to 10 Hz) included information about the electrochemical process occurring within the active pores of polymer film reaching the substrate surface [27–30]. Therefore the resistance depicted by this region was handled as pore resistance (Rpo ), which included the charge transfer resistance (Rct ) arising from kinetically controlled metal dissolution at the bottom of the pores, any species giving rise to resistive effect and diffuse layer resistance (Rd ) along the pore. In the case of extremely thin and porous conducting polymer coating, the corrosion process could only occur within the pores. Moreover, the said Rpo values were observed to decrease with time due to increase of water held by coating. This phenomenon increases the mobility of corrosive species towards the metal and increasing corrosion rate. The second region at lower frequency was related to polymer coating resistance; this Rf value includes both the corrosion products accumulating within the pores of polymer coating and the intact film itself. The sum of Rpo and Rf values were equal to polarization resistance Rp [31–33]. It must be noticed that the measured open circuit potential value (Ecorr ) of sample was −0.54 V, which indicated a metal/solution interface at the bottom of the pores. Once the solution reached
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Fig. 5. The voltammograms recorded for MS in (0.3 M oxalic acid +0.09 M pyrrole +0.01 M o-toluidine, at 25 ◦ C) (a), (0.3 M oxalic acid +0.08 M pyrrole +0.02 M o-toluidine, at 5 ◦ C) (b) and (0.3 M oxalic acid +0.07 M pyrrole +0.03 M o-toluidine, at 2 ◦ C) (c), 50 mV/s.
Fig. 6. The voltammograms recorded during the poly(pyrrole-co-o-toluidine) 9:1 at 25 ◦ C (a), 8:2 at 5 ◦ C (b) and 7:3 at 2 ◦ C (c) film growth on MS electrode; scan rate, 50 mV/s.
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Fig. 9. The structural representation of polypyrrole (a) and the copolymer (b).
Table 1 The Rp (ohm), Ecor (V), E% and P values after various exposure time (t) in 3.5% NaCl solution Fig. 7. FT-IR spectra of the polypyrrole (a) and poly(pyrrole-co-o-toluidine) (9:1) (b).
Electrode
t (h)
Rp (ohm)
Ecor (V)
E%
P
Uncoated MS MS/9:1
24 4 24 48 4 24 48 4 24 48
58 2511 1585 1259 2487 1995 1585 708 708 1122
−0.661 −0.540 −0.598 −0.603 −0.568 −0.585 −0.593 −0.569 −0.602 −0.587
– 98 96 95 98 97 96 92 92 95
– 0.23 0.12 0.14 0.14 0.12 0.13 0.48 0.24 0.21
MS/8:2
MS/7:3
Fig. 8. UV–vis absorption spectra of polypyrrole (- -) and 9:1 copolymer (—).
the metal surface, the corrosion process starts at the bottom of the pores and the measured potential of the electrode becomes a mixed potential value. Then this region should be considered as the pore resistance, since it is related to charge transfer and diffusion pro-
cess taking place within the pores. Considering the polymer film and the substrate as two separated metals connected in an electrolyte, the mixed potential reached by the couple depends on the surface ratio of noble and less noble metals in contact with the solution. Here, the polymer film behaves like the nobler synthetic metal, since it has the ability to oxidize the metal when it is found in its oxidized state. Then, electron transfer could take place between the metal and coating, at metal/coating interface. As the amount of corrosive solution interacting with the substrate metal increased, the Emix value approached the Ecorr value of the base metal (Table 1). The second region of Nyquist plot was related to the coating system which consisted of polymer film and the passive oxide layer formed at metal/polymer interface. The reduction of polymer film and the accumulation of corrosion products (within the pores of coating) would increase the film resistance with time; it was
Fig. 10. The SEM micrographs of polypyrrole (A), and copolymer coatings 9:1 (B) and 8:2 (C).
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Fig. 11. EIS results of 9:1 copolymer for 4 h (); 24 h (); 48 h () exposure times in % 3.5 NaCl.
Fig. 12. EIS results of 8:2 copolymer for 4 h (); 24 h (); 48 h () exposure times in % 3.5 NaCl.
observed that the total resistance decreased with time. This was related to water up taking process, during immersion period in corrosive solution. Also the pore and coating resistance regions could not be identified neither in Nyquist plot nor log f–log Z diagram. This could be explained with the ratio of these resistance values, with respect to each other. Therefore, the total resistance was handled as the polarization resistance value (Rp ) and given in Table 1, with respect to exposure time. These EIS results could be handled with an equivalent circuit including the resistive elements for the
coating (Rf ), the resistance against corrosion (Rpo ) and the uncompensated ohmic resistance (Ru ); the capacitive elements of intact coating Cf and the double layer capacitance (Cdl ), as it frequently appeared in the literature [27]. It was clearly seen that the other coating systems also exhibited the same feature with that deposited from 9:1 monomer feed ratio solution. However the highest resistance values were obtained with 8:2 ratio. Also this coating gave slightly nobler corrosion potential values with respect to others.
Fig. 13. EIS results of 7:3 copolymer for 4 h (); 24 h (); 48 h () exposure times in % 3.5 NaCl.
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The approximate values for porosity of coatings were also calculated for the aim of comparison, by using the EIS results and given in Table 1 [34]. The relevant equation is derived from Tafel equations written for coated and uncoated samples under the same over potential value, by doing so it is assumed that the coating blocks the anodic surface and decreases the metal dissolution. Thus the protective efficiency of coating is strictly related to surface coverage, . P =1− =
Fig. 14. Anodic polarization curves for coated and uncoated samples, MS (䊉), 9:1 ( ); 8:2 (); 7:3 () copolymer coated samples in % 3.5 NaCl.
By the help of Rp values the percent protection efficiency values (E%) were calculated for coated electrodes according to following E% = ((Rp−1(uncoated) − Rp−1(coated) )/Rp−1(uncoated) ) × 100 equation and given in Table 1. Even though, the amount of electrolyte solution increased at the metal/polymer interface and the Ecorr value decreased with increasing exposure time. The %E values were found not to decrease with increasing exposure time; this case provided additional evidence for the anodic protective behaviour of the polymer film.
Rp (uncoated) Rp (coated)
× 10−(E/ˇA )
However, the anodic protective effect of conducting polymer film is known to lead a passive layer (ferric compounds) on corroded area, therefore the corrosion process at the bottom of the pores should be better considered to take place under oxygen diffusion control. Under these circumstances, the value of ˇA should be used as 0.120 V/dec which could be derived from B value. Then the calculated porosity (P) values became more reliable in aspect of comparison. However, it was seen that he calculated P values were slightly high, considering the E% values. This case is explained with the charge transfer process occurring between metal and polymer film, which increased the total current value measured all over the surface. It was seen from Table 1 that the porosity values exhibited a tendency to decrease with immersion time. This was also indicative for anodic protective behaviour of coating systems. The porosity is one of the important parameters in aspect of anticorrosive behaviour of coatings and showed that the polymer coatings were able to hinder the access of corrosive solution to metal substrate. However it must be noted that the results summarized in Table 1 were indicating to better protective behaviour of the coating obtained in 8:2 monomers feed ratio solution.
Fig. 15. Successive CVs recorded for 9:1 (a), 8:2 (b) and 7:3 (c) copolymer coated samples on the Pt electrode in 3.5% NaCl.
S. Yalc¸inkaya et al. / Progress in Organic Coatings 63 (2008) 424–433
The anodic polarization curves recorded for coated samples and bare sample are given in Fig. 14. It was apparent that the coatings could effectively lower the anodic dissolution rate of substrate that the current values decreased remarkably. Also, the over oxidation of polymer coating becomes an important phenomena at high anodically polarized conditions, but the current values still remained lower than bare sample. The corrosion process could only take place through the pores of coating and once the coating prevented the mass transfer between the corrosive environment and substrate, the corrosion rate will decrease. The lowest dissolution rate was observed for the coating obtained from 8:2 monomer feed ratio. This also showed that this coating exhibited lower permeability and higher stability than the other coatings. In order to examine the stability of copolymer films obtained from different monomer feed ratio solutions, they have been synthesized on Pt electrode (applying the same conditions as before) and the successive cycles were recorded in 3.5% solution (Fig. 15). The oxidation of polymer film was observed in the forward scan and the reduction was realized at the reverse scan. The current density values involved during these successive cycles were informative in aspect of polymer film’s electroactivity and degradability. It was clearly seen that the current values increased with increasing cycle numbers for the coating deposited from 9:1 ratio. Also the highest and most stable current values were observed for the coating of 8:2 ratio. 4. Conclusions The synthesis of o-toluidine could not be achieved on mild steel, by direct oxidation of monomer from the solution. However, the synthesis of copolymer between pyrrole and o-toluidine could be realized electrochemically on mild steel. The increasing ratio of o-toluidine requires lower synthesis temperature; this could be explained with exothermic nature of polymerization. The deposited coatings were compact and adherent on the surface that very thin films could provide significant protection behaviour against the corrosion of steel. The ratio of 8:2 gave the coating with highest efficiency when compared to other coatings examined. Also this coating was shown to have high stability and low permeability in aggressive solution. The porosity value of this film was 0.13 and its protection efficiency was 96%, after 48 h immersion period in 3.5% NaCl solution.
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