Cyclic Voltammetric Synthesis of Poly(N-methyl pyrrole) - American

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Cyclic Voltammetric Synthesis of Poly(N-methyl pyrrole) on Copper and Effects of Polymerization Parameters on Corrosion Performance Berrin Duran* and Gözen Bereket Faculty of Science and Letters, Department of Chemistry, Eskişehir Osmangazi University, 26480, Eskişehir, Turkey. ABSTRACT: In the present work, we report the cyclic voltammetric synthesis of poly(N-methyl pyrrole) films onto copper surface from aqueous solutions of N-methyl pyrrole and oxalic acid. The effects of electropolymerization parameters (applied potential, scan rate, and cycle number) on the protective properties of poly(N-methyl pyrrole) films have been systematically investigated, and it was shown that protection efficiency strongly depends on the electrodeposition parameters. Nanoscale coatings electrodeposited at optimum electrochemical conditions were characterized by cyclic voltammetry, ex-situ attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and scanning electron microscopy (SEM) studies. The performance of poly(N-methyl pyrrole) as protective coating against corrosion of copper in 0.1 M H2SO4 solution was assessed by electrochemical impedance spectroscopy (EIS) and anodic polarization techniques. Corrosion test results demonstrated that the poly(N-methyl pyrrole) coating has ability to protect the copper in acid rain corrosive media during 12 days and the protective behavior of the polymer film results from self-healing effect of the coating against the attack of the corrosive environment.

1. INTRODUCTION Copper has been one of the important materials in industry owing to its high electrical and thermal conductivity, mechanical workability, and its relatively noble properties. It is widely used in many applications of electronic industries and communications as a conductor; also it is used in electrical power lines, pipelines for domestic and industrial water utilities, heat conductors, and heat exchangers.1,2 Thus, corrosion of copper and improving the corrosion resistance of this metal in a wide variety of media have attracted the attention of researchers.1−9 In order to protect metal surfaces, use of conducting polymers as advanced coating materials has become one of the most exciting research fields in recent decades.10−16 Therefore, the goals of synthesizing these coatings as in situ on iron, aluminum, and their alloys and evaluating their corrosion protection properties have led to growing interest.17−25 However, comparatively few studies have been carried out on the protection of copper by conducting polymer coatings.26−30 This case partly results from the difficulty in the electropolymerization of the monomer to generate the conducting polymer on copper. The main difficulty is based on classical problem of metal dissolution which competes with the first step of monomer polymerization,31 because thermodynamically the metal dissolves before the electropolymerization potential of the monomer is reached.32 This problem can be solved by selection of an appropriate dopant and/or pretreatment of the metal surface which lead to partial passivation of the metal and decrease its dissolution rate in order to achieve the deposition of a conducting polymer.33−35 Some researchers propose the use of oxalic acid,31,34 sodium oxalate,36−38 or sodium salicylate39,40 as the electrolyte, and in that case, it was demonstrated that the growth of polymer films is possible after the initial oxidation of the copper electrode that generated a copper oxalate/salicylate pseudopassive layer. This pseudolayer © 2012 American Chemical Society

has the capability of reduce the metal dissolution rate without hindering electron transfer process, which is necessary for polymer formation.41 In the literature, it was reported that modification of copper substrate via electrochemical treatment with aqueous solutions is required for polymer growth.34,41,42 Furthermore, it was stated that incorporation of dopant anions to polymer morphology as counterions may improve the coating properties.35,43 The majority of the reported studies on the corrosion protection of copper by conducting polymers are related with the electropolymerization of ring substituted anilines in neutral aqueous medium.10,28,29,44 Additionally, electropolymerization of N-substituted aniline derivatives onto the copper surface in aqueous acidic medium was also introduced to the literature by previous studies.45,46 Poly(pyrrole) is a potential material among conducting polymers, due to its high conductivity, stability, relatively ease of synthesis, and eco-friendly features.35,42 There have been many studies on the investigation of poly(pyrrole) as an anticorrosive coating for the protection of various metals, such as magnesium,47 iron,17 aluminum,19 steel,48,49 and copper,34,36,42,50−52 and it was demonstrated that the poly(pyrrole) enhances the resistance of these metals against corrosion in different aggressive media. In contrast to the works related with the corrosion protection properties of poly(pyrrole), little attention has been paid to the study of pyrrole derivatives. These derivatives may exhibit similar properties in films to poly(pyrrole), and they can also be electrochemically deposited on copper surface like poly(pyrrole). These polymers would be good candidates for corrosion protection.53 However, while Received: Revised: Accepted: Published: 5246

January 24, 2012 March 12, 2012 March 17, 2012 March 18, 2012 dx.doi.org/10.1021/ie300208c | Ind. Eng. Chem. Res. 2012, 51, 5246−5255

Industrial & Engineering Chemistry Research

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2.3. Electropolymerization Process. Before electropolymerization, a copper electrode surface was pretreated in 0.3 M oxalic acid solution, between −0.5 and +1.4 V at a 20 mV/s scan rate by applying 3 cycles. In order to eliminate probable differences in copper electrode samples, the same copper electrode was used in optimization studies. Different electropolymerization parameters were tested to synthesize the best protective PNMPy film on Cu (from 0.1 M NMPy containing 0.3 M aqueous oxalic acid solution) by cyclic voltammetry, and the best corrosion performance was obtained with the following parameters. The PNMPy films were synthesized by cycling continuously the electrode potential, between +0.3 and +0.9 V at 20 mV/s scan rate by applying 30 cycles. After electrodeposition of the PNMPy film, polymer coated electrode (Cu/PNMPy) was removed from the polymerization medium and immersed in ultra pure water to remove adsorbed electrolytes, monomers, and the soluble oligomers and dried at room temperature before stability and corrosion tests. 2.4. Corrosion Tests. Corrosion tests of uncoated and PNMPy coated copper electrodes were carried out in aerated 0.1 M H2SO4 solution (pH = 1.11), at room temperature by electrochemical impedance spectroscopy (EIS) and anodic polarization techniques. Measurements were carried out three times and found to be reasonably reproducible. All impedance and polarization curves were recorded at open circuit potential, in an unstirred state, after 30 min of immersion of the electrode in the corrosive test solution, and the time to reach a stable open circuit potential was limited to 30 min due to the fact that surface conditions may be altered within a longer period. The frequency used for the impedance measurements was changed from 100 kHz to 10 mHz, and the signal amplitude was 10 mV. Anodic polarization curves were recorded by sweeping the potential from the open circuit potential toward +0.8 V in the anodic direction at a constant scan rate of 1 mV/s. From the obtained polarization curves, the anodic Tafel region was identified and extrapolated to corrosion potential (Ecorr) to get corrosion current (icorr).

poly(pyrrole) is readily formed by cyclic voltammetry in aqueous solutions, the electropolymerization of some of its derivatives such as N-methyl pyrrole (NMPy) is not simple in aqueous environment. 43 In this respect, even though information is available on direct electrochemical deposition of poly(N-methyl pyrrole) (PNMPy) on copper from 0.1 M oxalic acid solution, the applied technique was chronoamperometry, and it was reported that, when cyclic voltammetry applied for the synthesis of PNMPy on copper, results were unsatisfactory for corrosion protection, since the film was very thin and inhomogeneous.53 As far as authors are aware, no previous study on the synthesis of PNMPy onto copper by cyclic voltammetry technique has been stated so far, and in this study, we are demonstrating for the first time that regular repetitive cyclic voltammograms belong to electropolymerization of NMPy on copper.

2. EXPERIMENTAL SECTION 2.1. Materials. N-methyl pyrrole (99%), oxalic acid (98%), and all other chemicals used in this study were purchased from Aldrich. The NMPy monomer was distilled before use and resulted in a light yellow liquid stored in the dark at 4 °C. Other analytical grade chemicals were used without any further purification. All solutions were prepared with ultra pure water. Electrochemical synthesis experiments were performed in freshly prepared 0.1 M NMPy containing aqueous 0.3 M oxalic acid solutions, at room temperature without deaeration. Electropolymerization and subsequent electrochemical studies (stability and corrosion tests) were carried out in single compartment three electrode cell, consisting of Cu disk as a working electrode, platinum wire as the counter electrode (CHI Instruments), and Ag/AgCl (3 mol/dm3 KCl) as the reference electrode (Gamry). The working electrode was prepared by mounting cylindrical copper rod (99.98% purity) into a Teflon holder, and polyester was used the fill the space between Teflon and copper. The circular cross-sectional disk area exposed to the solution used in electrochemical measurements was 0.1548 cm2. Prior to each electropolymerization, the copper working electrode was mechanically ground with 1200 grit abrasive paper by a Forcipol 1 V grinder/polisher; subsequently, it was ultrasonically cleaned in 1:1 (v/v) acetone/ethanol mixture to remove residues, then rinsed with water, and finally dried in air and freshly used with no further storage. 2.2. Instrumentation. Gamry reference 600 potentiostat/ galvanostat/ZRA system was used for electropolymerization and corrosion studies. This system was interfaced to a personal computer to control the experiments, and the data were analyzed using Gamry Framework/Echem Analyst (Version 5.50) software. The structure of PNMPy film on the copper surface was analyzed by FTIR reflectance spectrophotometry (Perkin-Elmer, Spectrum One, with universal ATR attachment with a diamond and ZnSe crystal). Morphologies of PNMPy coated copper surfaces were investigated via a field-emission scanning electron microscope (Hitachi FE-SEM S4800). The analysis of the impedance spectra and fitting of the experimental results to equivalent circuits were performed by using ZSimpWin (Version 3.21) software from Princeton Applied Research. The quality of fitting to an equivalent circuit was judged first by the chi-square value (χ2, i.e. the sum of the square root of the differences between theoretical and experimental values) and second by comparing the experimental data with the simulated data.

3. RESULTS AND DISCUSSION 3.1. Pretreatment of Copper Surface. Prior to electropolymerization of NMPy, the copper electrode was pretreated in monomer-free 0.3 M oxalic acid solution by potential cycling according to the stated procedure in the Experimental section. This pretreatment process is based on our previous work45 and leads to partial passivation of the copper surface due to the formation of insoluble copper oxalate film on the copper electrode surface. The work explains the poly(N-ethyl aniline) synthesis on copper and only described pretreatment conditions allowed for meaningful, regular repetitive cyclic voltammograms belonging to the synthesis of a poly(N-ethyl aniline) coating on copper. According to our attempts, it is worthwhile to note that, when the pretreatment scan rate was 4 mV/s in the same potential range, no regular cyclic voltammograms obtained during N-ethyl aniline electropolymerization (for example, a gradual increase of polymer oxidation− reduction peaks, which indicates polymer growth, was not seen in the cyclic voltammogram). Additionally, we have compared the morphological properties of the partially passivated copper surfaces prepared at two different scan rates (4 and 20 mV/s) by SEM images (taken at the same magnification) and we have seen that irregular ordered, relatively large sized copper oxalate grains are formed at low 5247

dx.doi.org/10.1021/ie300208c | Ind. Eng. Chem. Res. 2012, 51, 5246−5255


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scan rate; while more compact, regular ordered, smaller grains are formed at a fast scan rate. So it is possible to infer that just strict conditions allowed regular cyclic voltammograms for polymer growth to be obtained.54 For this reason, the same pretreatment procedure was attempted to achieve the cyclic voltammetric synthesis of PNMPy on copper. Cyclic voltammograms for pretreatment of copper, in other words, electrochemical behavior of copper electrode in oxalic acid medium, have been thoroughly investigated in many studies, and peaks that appeared during potential cycling have been explained.34,39,55 In order to avoid repetition, the same voltammograms were not given here again. 3.2. Optimization of Electropolymerization Parameters. Once a copper electrode has been pretreated, NMPy monomer was added to the same solution, and electropolymerization was carried out using cyclic voltammetry technique and a broad study was made for the selection of electropolymerization parameters (upper potential limit, scan rate, and cycle number). These parameters affect deposition and optimization of each parameter provides to obtain coating, which has the best corrosion performance. It was seen that all the tested parameters allowed the electrodeposition of PNMPy and the copper surface was completely covered by the polymer film. In order to investigate the anticorrosive properties of the obtained deposits, electrochemical impedance spectroscopy was employed, due to its nondisturbing and informative properties. Electrochemical impedance spectra of the polymer coated electrodes were recorded during a 6 day immersion period in 0.1 M H2SO4 solution; to prevent complexity in the figures, just four of the recorded spectra were given for optimization studies. PNMPy films were electrodeposited by varying any of the electropolymerization conditions (potential range, scan rate, and cycle number) to find the optimum parameters of the film for its use as an anticorrosive layer. Collected impedance data for optimization are given in Figures 1−3, and these figures show upper potential, scan rate, and cycle number optimizations, respectively. First, the potential range was determined for the electrosynthesis of PNMPy. From our polarization studies of copper electrode in 0.3 M oxalic acid solution and from cyclic voltammograms of copper recorded by other researchers34,55 in the same solution, it can be seen that the copper surface is at passive state beyond +0.3 V vs Ag/AgCl. Hence, the copper electrode behaves like an inert metal above this potential and electropolymerization of NMPy is expected to occur more easily. For this reason, the potential scan was started from +0.3 V for the electropolymerization process. Using this initial potential, the optimum upper potential limit was examined by electrodepositing PNMPy at various upper potential limits (+0.9 and +1.0 V) while keeping the scan rate (20 mV/s) and cycle numbers (20 cycles) fixed. Meanwhile, it was reported that polymer films will be irreversibly overoxidized at high potentials resulting in loss of conductivity and electroactivity as well as degradation of mechanical properties.48,56 The overoxidation of polymer film also leads to an increase in its permeability, thus the coating becomes less protective48 and the applied potential is also expected to affect the corrosion performance of the polymer film. In order to prevent further oxidation of the polymer film, Tüken et al. limited the upper potential at +0.85 V during the cyclic voltammetric synthesis of PNMPy on a mild steel electrode in 0.3 M oxalic acid medium.23 For the same reason, upper potentials beyond +1.0

Figure 1. Determination of optimum upper potential limit via EIS measurements by changing upper potential limit, when the scan rate and cycle number are fixed.

V were not preferred for the electrodeposition of PNMPy on copper. Nyquist diagrams of PNMPy films electrosynthesized at varied upper potentials (Figure 1) show that, much protective results were obtained when the copper electrode was coated at a +0.9 V upper potential limit. Hereby, the potential region was determined to be from +0.3 to +0.9 V for the electropolymerization of NMPy. The second stage of optimization was scan rate determination and different scan rates (10, 20, 30 mV/s) were applied to synthesize PNMPy films in the potential region between +0.3 and +0.9 V, at fixed cycle numbers (20 cycles). Afterward, the synthesized PNMPy films were tested by EIS. It was deduced from the Nyquist diagrams in Figure 2 that PNMPy film synthesized at a 20 mV/s scan rate has the best corrosion resistance, and the optimum scan rate was selected as 20 mV/s. After these steps, we determined optimum cycle number by changing the cycle number for previously optimized potential range and scan rate. At first sight, impedance spectra in Figure 3 show that PNMPy films synthesized with 20 and 30 cycles gave very close corrosion performance. However, it can be seen from Figure 3 that deviations from the semicircle shape are less and imaginary impedance is slightly higher when the polymer film was electrodeposited with 30 cycles. This indicates that the polymer film electrodeposited with 30 cycles exhibits slightly better performance than the other coatings. Besides, high imaginary impedance values are known to be connected with the high phase angles in Bode format of impedance diagrams, and this has been attributed to the high barrier property of the coating.57 Consequently, electrodeposition parameters of 5248



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Figure 3. Determination of the optimum cycle number via EIS measurements by changing the cycle number with the potential range and scan rate fixed.

Figure 2. Determination of the optimum scan rate via EIS measurements by changing the scan rate with the potential range and cycle number fixed.

formed PNMPy film. In the following forward scan, the oxidation of the PNMPy film was also observed as a broad anodic peak at +0.570 V potential. During consecutive scans, continuous current increase of these broad redox waves is associated with the oxidation−reduction of the polymer film. This demonstrates gradual deposition of an electroactive polymer film on the copper surface with repeated scans and thickening of the film.58−60 On the other hand, decrease of monomer oxidation current values in the second and following cycles indicates that the freshly formed polymer has low conductivity.22 At the end of 30 cycles, a dark brown colored, uniform PNMPy film was synthesized on the copper surface. According to voltammograms given in the inset of Figure 4, it can be said that the synthesis of PNMPy on the copper surface

PNMPy were determined to be in the +0.3−+0.9 V potential region, 20 mV/s scan rate, and 30 cycle numbers, since these conditions show promising corrosion protection. 3.3. Electropolymerization of NMPy on Copper at Optimized Conditions. Cyclic voltammograms for electropolymerization of NMPy on copper are presented in Figure 4. In the first anodic potential scan, the irreversible oxidation peak starting from +0.690 V corresponds to NMPy oxidation and this oxidation process gives a start to the formation of the PNMPy film on the copper surface. After the initial oxidation of the monomer, a broad peak appeared at +0.420 V in the first reverse scan, and this peak indicates the reduction of the freshly 5249



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stability of PNMPy coating is higher than that of the PNMA coating. 3.5. Spectroscopic and Morphological Characterization of PNMPy Films. The FTIR spectrum of PNMPy coating was obtained by reflectance measurements of the polymercoated copper surface and presented in Figure 6. The deposited

Figure 4. Cyclic voltammograms recorded during PNMPy film growth on pretreated Cu electrode: (inset) First and thirtieth cycles.

in described conditions is similar to the growth of an electroactive polymer on an inert metal. 3.4. Electrochemical Stability of PNMPy Films. PNMPy films were characterized in monomer free solution of the same electrolyte via cyclic voltammetry in order to investigate stability of the film. The stability of any conducting polymer in reduced and oxidized states is an important parameter for technological applications. The main factor that determines the lifetime of a conducting polymer is the chemical stability of the matrix itself. The stability of PNMPy film was measured by application of 50 cycles in a monomer-free electrolyte, and related cyclic voltammograms are given in Figure 5. When

Figure 6. ATR-FTIR spectrum of PNMPy film deposited onto the copper electrode surface.

poly(N-methyl pyrrole) coating shows the characteristic infrared bands associated with N-methyl pyrrole units and the oxalate counterions. The broad peak located at around 3440 cm−1 is assigned to the OH stretching vibrations of the counterions. The weak peak at 2924 cm−1 characterizes the  CH3 stretching of the NMPy units. The strong peak at 1633 cm−1 corresponds to CO stretch of the counterions. Three weak peaks at 1438, 1362, and 1318 cm−1 result from the ring stretch of the NMPy units. The peak at 1058 cm−1 is attributed to the CH out of plane deformation of the NMPy units. The peak at 818 cm−1 is related to the OCO vibrations of the oxalate ions. The presence of a strong carbonyl band in the infrared spectrum of PNMPy film electrosynthesized from aqueous oxalic acid solution has also been previously reported.53,62 Thus, the FTIR spectrum confirms the formation of poly(N-methyl pyrrole) coating on the copper surface and incorporation of the electrolyte into the coating. Figure 7a−d corresponds to the uncoated copper, PNMPycoated copper (top-view and cross-section), and corroded PNMPy-coated copper surfaces, respectively. By comparing Figure 7a and b, it is clearly seen that the PNMPy-coated copper surface is completely different from the uncoated copper surface and the polymer film mainly consists of two different kinds of grains. A characteristic cauliflower-like structure constructed by microspherical grains of polypyrrole derivatives can be observed in the PNMPy-coated surface. On the other hand, flat, elliptical, smaller, and bright-looking granules belong to copper oxalate grains. The morphology of the pretreated copper surface was also investigated at the same magnification (20 000×) in our previous study, and the shape of the copper oxalate grains was described in that study.45 In this regard, the SEM micrograph of the PNMPy film shows the entrance of oxalate grains into the polymer matrix as dopant. On the basis of the SEM observations, it is deduced that PNMPy and copper oxalate grains coexist on the metal surface. For this reason, the formed layer may be called a nanocomposite coating. Additionally, the existence of some fissures on the PNMPy-coated surface can be seen as well, and this might be explanation of low corrosion resistance of the coating

Figure 5. Stability test potentiodynamically for PNMPy film using multiple cycles in monomer-free solution at 20 mV/s.

cycling proceeds, the current density decreases with each cycle and finally reaches a stable value. ΔIpa is the difference between peak currents of initial and final cycle. The polymer that exhibits minimum decrease in Ipa on repetitive cycling is electrochemically more stable.61 Peak current densities of initial and final cycles relating to oxidation of PNMPy polymer are 4.504 and 0.697 mA/cm2, respectively. From here, the ΔIpa value for PNMPy coating was found as 3.807 mA/cm2. This value is smaller than the ΔIpa value (18.44 mA/cm2) calculated by the application of 50 cycles in monomer free 0.3 M oxalic acid solution to the poly(N-methyl aniline) (PNMA) film, which was electrodeposited from 0.1 M N-methyl aniline containing 0.3 M oxalic acid solution onto the copper surface under optimized electropolymerization conditions.46 Comparison of ΔIpa values of PNMPy and PNMA films reveals that the 5250



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Figure 7. SEM images of (a) bare copper, (b) freshly produced PNMPy-coated copper, (c) cross-sectional view of PNMPy, (d) PNMPy-coated copper after 168 h immersion in 0.1 M H2SO4 solution.

at the beginning of immersion time in the corrosive solution. Coating thickness was measured by a cross-sectional SEM observation, and average values of the film thickness were obtained. As can be seen from Figure 7c, the average coating thickness varies between 500 and 700 nm. However, crosssectional visualization of the pretreated copper did not allow measurement of the thickness of the passive layer because the layer was extremely thin. SEM micrograph of the corroded PNMPy film-coated copper surface was recorded at the end of the best corrosion performance time (168 h) of the coating in 0.1 M H2SO4 solution, and Figure 7d shows the change of polymer morphology at the end of this immersion time. It can be said that even though the polymer coating reaches the maximum corrosion resistance after 168 h of immersion, partial peeling of the film was also observed unfortunately, due to penetration of corrosive species into the coating. 3.6. Corrosion Protection Performance of the PNMPy Coating. The corrosion protection performance of the PNMPy film synthesized at optimum parameters was examined in 0.1 M H2SO4 solution, which was described as acid rain corrosive media,33 against immersion time by electrochemical impedance measurements. The Nyquist plots recorded at different immersion times for Cu and Cu/PNMPy electrodes were given in Figure 8. It can be seen at first sight that the resistance of PNMPy-coated copper is higher than that of the uncoated copper. Additionally, the Nyquist diagrams of bare Cu consist of a capacitive loop at high frequency and a straight line at lower frequencies. The observed linear portion indicates that diffusion process controls the metal dissolution at the metal surface.36 Conversely, in Nyquist diagrams of the Cu/PNMPy electrode, a semicircle shape is more dominant, especially at longer immersion times. Moreover, diameters of the semicircles for the Cu electrode decrease with time, and this case is attributed to increasing corrosion of copper, while the diameters of the semicircles for the Cu/PNMPy electrode are increasing with time up to 168 h and then start to decrease. It was reported in the literature that corrosion of copper in aerated H2SO4 solution proceeds via two partial reactions.63 The first anodic step is the formation of a Cu2O film and diffusion of H+ ions, as given by reaction a. The second anodic step in the corrosion process is the dissolution of Cu2O to form

Figure 8. Nyquist plots obtained for Cu and Cu/PNMPy electrodes in 0.1 M H2SO4 solution after various exposure times.

Cu2+ ions as given by reaction b. The cathodic reaction is the reduction of oxygen according to reaction c.

Anodic reactions 2Cu + H2O → Cu2O + 2H+ + 2e−

(a)

Cu2O + 2H+ → 2Cu 2 + + H2O + 2e−

(b)

Cathodic reaction O2 + 4H+ + 4e− → 2H2O

(c)

When the coated electrode is immersed in corrosive solution, the redox process occurs between the coating and solution as a second cathodic reaction which results in ejection of oxalate ions as given by reaction d. (PNMPy y +y/2C2O4 2 −)n + nye− → (PNMPy)n + ny/2C2O4 2 −

(d)

The increase in diameter of semicircles for the Cu/PNMPy electrode can be explained by the result of redox chemistry of the deposited conducting PNMPy film. So, the increase in the amount of the reduced form of PNMPy by the released oxalate ions can facilitate stabilization and thickening of the passive copper oxalate layer (occurs between Cu2+ and oxalate ions, produced from reactions b and d) at the metal/polymer interface and cause self-healing of the passive layer.64 The small diffusion tail seen at low frequency in all the impedance responses indicates the movement of counterions of PNMPy through solution and, thereby, provides additional evidence for 5251



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of Cu2O film and diffusion of H+ ion) and elements in the lower frequency region represent the second step of the corrosion process (e.g., dissolution of Cu2O film). In this manner, Rs represents solution resistance, R1 corresponds to the charge transfer resistance connected with dissolution of metal, W reflects Warburg impedance of diffused species, R2 is the resistance of Cu2O film formed together with corrosion of copper, and Q1 and Q2 denote the constant phase element (CPE) of metal/solution and Cu2O film/solution interfaces, respectively. Constant phase elements are used to describe nonideal behavior of capacitors, and the impedance of CPE is described as ZCPE = [Q(jω)n]−1 where Q is the CPE constant, ω is the angular frequency, j is the imaginary number, and n is the exponent of Q.53 The evolution of these terms as a function of immersion time can be seen in Table 1. The equivalent circuit model elucidating the polymer/electrolyte interface is similar to the circuit model describing the metal/electrolyte interface, except the constant phase element was replaced with an ideal capacitor. The same equivalent circuit was used to clarify the poly(pyrrole)-coated copper surface in 3.5% NaCl solution.55 In the R(Q(RW))(CR) equivalent circuit model used to interpret the polymer/solution interface, Rs denotes solution resistance, R1 represents charge transfer resistance of metal, W defines Warburg impedance, R2 reflects PNMPy film resistance together with passive layer, and Q1 and C1 correspond to constant phase element of metal/solution and capacitance of polymer/solution interface, respectively. The variations of impedance parameters with immersion time are given in Table 2. Simulation results of fitted curves to experimental Bode curves for Cu and Cu/PNMPy electrodes (Figure 9) and chi-squared (χ2) values in Tables 1 and 2 revealed that, proposed equivalent circuits were successfully applied to experimental data, to explain the interfaces between the electrodes and electrolyte. In Tables 1 and 2, Rs solution resistances remain almost constant over long-term immersion for both electrodes (Cu and Cu/PNMPy). Rp values seen in Tables 1 and 2 are polarization resistance of the system and can be taken as Rp = R1 + R2.66 In Table 1, R1, R2, and Rp values of uncoated copper decreased, while Q1 values related to metal/solution interface increased during 0−168 h immersion. Decrease of Rp and increase of Q1 devoted to the corrosion process occurred at the copper/solution interface. It should be noted that variation of Rp values in Table 1 is in agreement with the diameter of semicircles recorded for uncoated Cu. As shown in Table 2, two different trends were observed in the variation of R1, R2, Rp, Q1, and C1 values with the immersion time. Until 168 h of immersion, the charge transfer resistance of metal (R1) increased while the constant phase element of metal/solution interface Q1 decreased; this means that the

the role of redox behavior of PNMPy in protection of the copper surface through the passive oxalate layer formation. Similar findings were also documented by Kousik et al. for the corrosion behavior of poly(thiophene) coated steel.65 The impedance plots of Cu and Cu/PNMPy electrodes were modeled by the equivalent circuits depicted in the inset of Figure 9. Figure 9 shows agreement of fitted curves to

Figure 9. Agreement of fitted equivalent circuit to experimental Bode plots of Cu and Cu/PNMPy electrodes for various exposure times.

experimental Bode curves when the proposed equivalent circuits are used. The results obtained from analyzing of impedance spectra by given equivalent circuits for Cu and Cu/ PNMPy electrodes are listed in Tables 1 and 2, respectively. In the R(Q(RW))(QR) equivalent circuit used to describe copper/solution interface, elements at high frequency are related with the initial step of corrosion process (e.g., formation

Table 1. Impedance Parameters Obtained by Fitting the Measured Impedance Spectra of the Cu Electrode in 0.1 M H2SO4 Solution to the R(Q(RW))(QR) Equivalent Circuit t (h)

Rs (Ω)

0 24 48 72 96 120 168

7.33 7.24 7.83 6.80 6.99 6.85 7.40

Q1 (Ss−n) 4.92 3.95 4.47 5.11 6.24 8.17 7.65

× × × × × × ×

10−5 10−5 10−5 10−3 10−3 10−3 10−3

n

R1 (Ω)

W (Ss−1/2)

0.88 0.97 0.95 0.39 0.39 0.37 0.36

151.00 45.63 31.71 101.90 81.71 41.71 51.50

1.21 9.99 1.21 4.82 5.46 9.07 1.02

× × × × × × ×

10−2 10−3 10−2 10−3 10−3 10−3 10−2 5252

Q2 (Ss−n) 3.62 2.24 3.31 7.25 9.18 1.73 1.72

× × × × × × ×

10−3 10−3 10−3 10−5 10−5 10−4 10−4

n

R2 (Ω)

Rp (Ω)

0.40 0.45 0.42 0.96 0.93 0.85 0.86

108.30 184.30 156.70 15.84 17.88 13.85 14.42

259.30 229.93 188.41 117.74 99.59 55.56 65.92

χ2 2.66 4.92 1.17 8.63 8.01 6.28 7.48

× × × × × × ×

10−4 10−4 10−3 10−4 10−4 10−4 10−4



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Table 2. Impedance Parameters Obtained by Fitting the Measured Impedance Spectra of Cu/PNMPy Electrode in 0.1 M H2SO4 Solution to the R(Q(RW))(CR) Equivalent Circuit t (h) 0 24 48 72 96 120 144 168 192 216 240 264 288

Rs (Ω) 10.61 9.71 9.70 9.73 9.94 10.40 10.30 10.52 9.90 9.85 9.92 9.83 9.75

Q1 (Ss−n) 8.82 6.64 4.83 3.93 2.85 2.59 2.41 2.22 2.74 3.43 4.39 5.83 9.98

× × × × × × × × × × × × ×

−4

10 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4

n 0.52 0.53 0.56 0.58 0.61 0.63 0.64 0.64 0.61 0.58 0.57 0.55 0.51

R1 (Ω) 149 319 418 497 776 905 1149 1474 818 925 790 669 369

W (Ss−1/2) 1.63 7.76 7.84 1.04 1.14 1.58 1.89 3.24 1.61 2.27 3.15 3.13 4.96

× × × × × × × × × × × × ×

protection performance of the coating increased up to 168 h. It was also observed that film resistance (R2) decreased, while the capacitance value of polymer/solution interface increased during this immersion period. Increasing the polarization resistance (Rp) of the coated copper implies that the protective performance of the coating arises from the self-healing effect of the coating within this period. After 168 h, with the degradation of the coating, the film resistance increases and the capacitance value of the coating increases. The increase of film resistance can be attributed to plugging of the film with corrosion products. Increasing capacitance values are also an indication of increased electrolyte uptake and saturation of the coating. Decrease in charge transfer resistance of the metal explains the increase in the corrosion of copper. In parallel, the constant phase element of metal/solution increases. Accordingly, it is acceptable that, the variation of Q1 is another way to follow the corrosion process of copper. Polarization resistance of the polymer-coated electrode is consistent with the diameter of semicircles in Nyquist diagrams. It is concluded from the Table 2 that, the polymer film gives a good corrosion protection over long time immersion (168 h). As immersion goes on, PNMPy film will loose its electroactivity, thus the ability to reduce and oxidize successively and reversibly, vanishes. The successive and reversible conversion of doped form to undoped form may induce fatigue-like stress that could be responsible for cracking.48 In order to further reveal the corrosion protection ability of PNMPy coating, anodic polarization curves were recorded for Cu and Cu/PNMPy electrodes in 0.1 M H2SO4 solution, and the polarization curves were shown in Figure 10. The selection of a 168 h period to perform anodic polarization scan for Cu/ PNMPy is related with the best corrosion performance of the polymer film, and this period was determined from the impedance measurements. It is seen from the anodic polarization curves that PNMPy-coated copper exhibited lower current density values than the uncoated copper and the corrosion potential value of the Cu/PNMPy electrode is slightly shifted toward a more positive direction. The positive shift of corrosion potential and lower current density values indicate the protection of copper by the PNMPy coating. The anodic polarization curves were analyzed to estimate corrosion current density, and obtained corrosion parameters (Ecorr and icorr) were given in Table 3. It can be seen from Table 3 that the corrosion current density of PNMPy coated copper is 7.5 times lower than that of the bare copper.

−2

10 10−3 10−3 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2

C1 (F) 1.24 1.61 2.15 3.07 3.63 4.84 5.66 4.75 4.46 5.08 4.92 4.50 4.47

× × × × × × × × × × × × ×

−5

10 10−5 10−5 10−5 10−5 10−5 10−5 10−5 10−4 10−4 10−4 10−4 10−4

R2 (Ω)

Rp (Ω)

70 58 58 45 35 27 19 15 485 368 294 235 148

219 377 476 542 811 932 1168 1489 1303 1293 1084 904 517

χ2 3.76 1.74 1.46 1.14 8.05 5.56 3.41 8.06 5.84 3.12 2.64 3.57 4.89

× × × × × × × × × × × × ×

10−4 10−3 10−3 10−3 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4

Figure 10. Anodic polarization curves recorded for Cu and Cu/ PNMPy electrodes in 0.1 M H2SO4 solution.

Table 3. Corrosion Parameters Calculated from the Anodic Polarization Curves of Uncoated and PNMPy-Coated Copper Samples electrode

Ecorr (V)

icorr (A/cm2)

Cu Cu/PNMPy

0.019 0.033

3.00 × 10−4 4.00 × 10−5

For electrochemical impedance and potentiodynamic polarization measurements, protection efficiency (η) of the PNMPy coating can be calculated by using eqs 1 and 2. ηEIS(%) = [R pc − R p/R pc] × 100

(1)

o −i o ηPol (%) = [icorr corr /icorr] × 100

(2)

In these equations, Rp and Rpc denote polarization resistance of o and icorr refer uncoated and polymer coated copper, while icorr to corrosion current densities of the uncoated and polymer coated copper electrodes, respectively. For impedance and polarization data, if the protection efficiency of the PNMPy coating is calculated at the end of the 168 h immersion, it can be found that the PNMPy coating provides 82.6% protection to copper according to EIS results, whereas the 86.7% protection efficiency is obtained from the potentiodynamic polarization method. These results showed that there is an agreement between the protection efficiency values calculated from two 5253



Article

Chloride Contents, Aeration and Humidity. J. Solid State Electrochem. 2009, 13, 1757. (8) Tang, Y. M.; Yang, W. Z.; Yin, X. S.; Liu, Y.; Wan, R.; Wang, J. T. Phenyl-substituted Amino Thiadiazoles as Corrosion Inhibitors for Copper in 0.5 M H2SO4. Mater. Chem. Phys. 2009, 116, 479. (9) Khiati, Z.; Othman, A. A.; Sanchez-Moreno, M.; Bernard, M. C.; Joiret, S.; Sutter, E. M. M.; Vivier, V. Corrosion Inhibition of Copper in Neutral Chloride Media by a Novel Derivative of 1,2,4-Triazole. Corros. Sci. 2011, 53, 3092. (10) Chaudhari, S.; Sainkar, S. R.; Patil, P. P. Anticorrosive Properties of Electrosynthesized Poly(o-Anisidine) Coatings on Copper from Aqueous Salicylate Medium. J. Phys. D:Appl. Phys. 2007, 40, 520. (11) Ö zyılmaz, A. T. The Corrosion Performance of Polyaniline Film Modified on Nickel Plated Copper in Aqueous p-Toluenesulfonic Acid Solution. Surf. Coat. Technol. 2006, 200, 3918. (12) Sazou, D.; Kourouzidou, M.; Pavlidou, E. Potentiodynamic and Potentiostatic Deposition of Polyaniline on Stainless Steel: Electrochemical and Structural Studies for a Potential Application to Corrosion Control. Electrochim. Acta 2007, 52, 4385. (13) Herrasti, P.; Recio, F. J.; Ocón, P.; Fatás, E. Effect of Polymer Layers and Bilayers on the Corrosion Behaviour of Mild Steel: Comparison with Polymers Containing Zn Microparticles. Prog. Org. Coat. 2005, 54, 285. (14) Gelling, V. J.; Wiest, M. M.; Tallman, D. E.; Bierwagen, G. P.; Wallace, G. G. Electroactive-Conducting Polymers for Corrosion Control: 4. Studies of Poly(3-Octyl Pyrrole) and Poly(3-Octadecyl Pyrrole) on Aluminum 2024-T3 Alloy. Prog. Org. Coat. 2001, 43, 149. (15) Nguyen Thi Le, H.; Garcia, B.; Deslouis, C.; Le Xuan, Q. Corrosion Protection and Conducting Polymers: Polypyrrole Films on Iron. Electrochim. Acta 2001, 46, 4259. (16) Shah, K.; Iroh, J. Electrochemical Synthesis and Corrosion Behavior of Poly(N-Ethyl Aniline) Coatings on Al-2024 Alloy. Synth. Met. 2002, 132, 35. (17) Lehr, I. L.; Saidman, S. B. Corrosion Protection of Iron by Polypyrrole Coatings Electrosynthesized from a Surfactant Solution. Corros. Sci. 2007, 49, 2210. (18) Yano, J.; Nakatani, K.; Harima, Y.; Kitani, A. Bilayer Polymer Coating Containing a Polyaniline for Corrosion Protection of Iron. Mater. Lett. 2007, 61, 1500. (19) Martins, J. L.; Costa, S. C.; Bazzaoui, M.; Gonçalves, G.; Fortunato, E.; Martins, R. Electrodeposition of Polypyrrole on Aluminium in Aqueous Tartaric Solution. Electrochim. Acta 2006, 51, 5802. (20) Karpagam, V.; Sathiyanarayanan, S.; Venkatachari, G. Studies on Corrosion Protection of Al2024 T6 Alloy by Electropolymerized Polyaniline Coating. Curr. Appl. Phys. 2008, 8, 93. (21) Su, W.; Iroh, J. O. Electrodeposition Mechanism, Adhesion and Corrosion Performance of Polypyrrole and Poly(N-methylpyrrole) Coatings on Steel Substrates. Synth. Met. 2000, 114, 225. (22) Keleş, H.; Solmaz, R.; Ö zcan, M.; Kardaş, G.; Dehri, I.̇ Copper Modified Poly-6-Amino-m-Cresol (Poly-AmC/Cu) Coating for Mild Steel Protection. Surf. Coat. Technol. 2009, 203, 1469. (23) Tüken, T.; Tansuğ, G.; Yazıcı, B.; Erbil, M. Poly(N-methyl pyrrole) and its Copolymer with Pyrrole for Mild Steel Protection. Surf. Coat. Technol. 2007, 202, 146. (24) Kilmartin, P. A.; Trier, L.; Wright, G. A. Corrosion Inhibition of Polyaniline and Poly(o-methoxyaniline) on Stainless Steels. Synth. Met. 2002, 131, 99. (25) Shukla, S. K.; Quraishi, M. A.; Prakash, R. A Self-Doped Conducting Polymer “Polyanthranilic Acid”: An Efficient Corrosion Inhibitor for Mild Steel in Acidic Solution. Corros. Sci. 2008, 50, 2867. (26) Patil, S.; Sainkar, S. R.; Patil, P. P. Poly(o-anisidine) Coatings on Copper: Synthesis, Characterization and Evaluation of Corrosion Protection Performance. Appl. Surf. Sci. 2004, 225, 204. (27) Ö zyılmaz, A. T.; Tüken, T.; Yazıcı, B.; Erbil, M. The Electrochemical Synthesis and Corrosion Performance of Polyaniline on Copper. Prog. Org. Coat. 2005, 52, 92.

different experimental methods, and this agreement provides further support to the obtained results. Comparison of corrosion and stability test results of PNMPy and previously synthesized PNMA coatings showed that the PNMPy coating has a ΔIpa value of 3.8 mA and provides protection against corrosion of copper in acid rain corrosive media up to 12 days, while the PNMA coating has a ΔIpa value of 18.4 mA and provides protection to copper in the same corrosive media up to just 2 days. It was possible to conclude that electrochemical stability is an important parameter to be taken into consideration for determination of corrosion performance of such coatings.

■

CONCLUSION The work described in this paper has been planned in order to check the possibility of preparing the poly(N-methyl pyrrole) coating on copper by cyclic voltammetry, and it was shown that it is possible to obtain poly(N-methyl pyrrole) deposits onto a pretreated copper surface using the cyclic voltammetry technique. Prepassivation of the electrode has fundamental importance to generate good deposits. The corrosion performance of the deposited film depends greatly on the choice of electrodeposition parameters, such as upper potential limit, scan rate, and cycle number. The PNMPy coating electrosynthesized under optimized conditions protects the copper in acid rain corrosive media for almost 12 days.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +90 (222) 239 3750/2868. Fax: +90 (222) 239 35 78. Email: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the Eskişehir Osmangazi University is gratefully acknowledged. The authors also would like to thank to Prof. A. Sezai Saraç and Prof. Sannakaisa Virtanen for the use of their technical facilities. We extend our thanks to Aslı Gençtürk and Can Metehan Turhan for taking the ATR-FTIR spectrum and SEM images, respectively.

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Cyclic Voltammetric Synthesis of Poly(N-methyl pyrrole) - American

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