- Email: [email protected]
Electrochemical synthesis of poly-2-aminothiazole on mild steel and its corrosion inhibition performance
Progress in Organic Coatings 70 (2011) 122–126
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Electrochemical synthesis of poly-2-aminothiazole on mild steel and its corrosion inhibition performance Ramazan Solmaz ∗ Bingöl University, Science and Letters Faculty, Chemistry Department, 12000 Bingöl, Turkey
a r t i c l e
i n f o
Article history: Received 13 May 2010 Received in revised form 13 September 2010 Accepted 8 November 2010 Keywords: Electropolymerization Conducting polymers Poly-2-aminothiazole Corrosion
a b s t r a c t 2-Aminothiazole (AT) was polymerized by electrochemical technique on a mild steel (MS) electrode from 0.01 M monomer containing 0.3 M ammonium oxalate solution. Cyclic voltammetry was used for the synthesis. Poly-2-aminothiazole (pAT) film with a light-brownish color was obtained on the MS surface. The effectiveness of polymer film in preventing corrosion of MS was tested in 0.5 M HCl solution. For corrosion tests, anodic polarization curves, electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR) techniques were utilized. The results obtained indicated that, the polymer film adherent to the steel surface. The polymer film gives a good corrosion protection against the attack of corrosive environment. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Conducting polymers have been interested both for fundamental research and technological applications during the last two decades. These materials present interesting properties in numerous domains such as electrocatalysis, electrochromic devices, microelectronic devices, biosensors,and rechargeable batteries. Recently, there has been an increasing interest in the use of these polymers as protective coatings against the corrosion [1–12]. It has been reported that conducting polymers provide both anodic protection and barrier effect to prevent attack by a corrosive environment [1,4,5]. The advantages of conducting polymers over the other coatings such as paints, chromium and heavy metals based inhibitors are that they do not contain toxic substances that are harmful to the environment [6–8]. Their production process is simple and economical. But, low processability, poor mechanical and thermal stability and protection life time are the greatest problems for their use in corrosion protection [8]. At present, there is a considerable interest not only in the synthesis of new types of polymers, but also in the modification of the polymers in order to improve their corrosion protection ability [6,9–12]. Aminothiazole and its derivatives are reported to have anticorrosive properties [13,14]. pAT may function as organic inhibitor due to its great number of bonds, reducing the number of active sites on the metal surface through adsorption and acting both as a barrier by decreasing the transport of corrosive agents and
∗ Tel.: +90 426 213 2550; fax: +90 426 213 2866. E-mail addresses: [email protected], [email protected] 0300-9440/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2010.11.003
anodic protection. In our previous study [15], the thermal analysis results were shown relatively good thermal stability of the pAT. This advantage makes possible using pAT for materials protection even at higher temperatures. It was also shown that pAT has good electrochemical stability [15]. The pAT is partially soluble in DMSO, but insoluble AC, THF, ACN, DCM, DMF, and in acidic solutions [15]. The pAT is insoluble in usual solvents and acidic solutions that can be of a great interest in the domain of the materials protection against the corrosion. In this work, we have synthesized pAT on MS surface using cyclic voltammetry technique. The protection ability of polymer film synthesized for the MS corrosion was tested in 0.5 M HCl solution using some electrochemical techniques. 2. Experimental The pAT was synthesized electrochemically on a MS specimen (MS/pAT) (C: 0.46%, Si: 0.40%, Mn: 0.65%, P: 0.045%, S: 0.045% and remainder Fe) from 0.01 M AT + 0.3 M ammonium oxalate solution. For this aim, cyclic voltammetry technique was used. Prior to polymer synthesis, the surface of MS electrode was pre-passivated in 0.3 M ammonium oxalate solution applying 5 whole cycles between −0.60 V and 1.35 V (Ag/AgCl) potential with 0.010 V s−1 scan rate. Then, the polymer film was synthesized on the pre-passivated steel surface by cycling the potential between 0.60 V and 1.30 V (Ag/AgCl) with 0.050 V s−1 scan rate. The chemical structure of resulting polymer was reported previously as shown in Fig. 1 [15]. The corrosion behaviour of MS and MS/pAT electrodes were investigated in 0.5 M HCl solution using EIS, LPR and anodic polarization curves. The working electrode was immersed in test
R. Solmaz / Progress in Organic Coatings 70 (2011) 122–126
H
H
N
S
H
N
N
N S
H
N
N
N S
123
N S
Fig. 1. Chemical structure of poly-2-aminothiazole.
solution for 1 h to establish steady state open circuit potential. After reaching the steady state open circuit potential (1 h), the electrochemical measurements were performed. The EIS experiments were conducted in the frequency range of 100 kHz to 0.01 Hz at open circuit potential. The amplitude was 0.005 V. The LPR measurements were carried out by recording the electrode potential ±0.010 V around open circuit potential with 0.001 V s−1 scan rate. The polarization resistance (Rp ) was determined from the slope of current–potential curves obtained. The anodic polarization curves were performed starting from open circuit potential after reaching a steady state open circuit potential. Electrochemical measurements and polymer synthesis were carried out using a CHI 604 electrochemical analyzer under computer control. A platinum sheet with 1 cm2 surface area was used as a counter electrode and Ag/AgCl (3 M KCl) was used as a reference electrode. The potential values given in this study are with respect to this reference electrode. All the test solutions were prepared from analytical grade chemical reagents in distilled water without further purification. The tests were performed at room temperature. 3. Results and discussion 3.1. Synthesis of polymer film During the synthesis of conducting polymers on metal surfaces at low potentials or currents, there is an induction time during which the system is not stable and this gives rise to continuous dissolution of metal without the formation of polymer layer [8]. It is also well known that the passivation process and the quality of passive layer on the surface are very effective on physicochemical properties and adhesion behaviour of coatings synthesized [10]. In order to overcome this disadvantage, the synthesis of pAT film on the MS surface was achieved by two consecutive steps. Firstly, the surface of electrode was passivated in monomer free 0.3 M ammonium oxalate solution at a low scan rate. This low scan rate could provide a better passivity and this passive layer has much importance for the next coming polymerization stage [10,16]. Then, the polymer film was achieved over the passivated surface. The passivation stage of MS electrode in monomer free ammonium oxalate solution is graphically presented in Fig. 2. The anodic peak (A1) in the forward sweep centered at −0.2 V (Ag/AgCl) was attributed to dissolution of iron and formation of low soluble ferrous oxalate layer [1,6,17]. In the reverse scan re-passivation peak (C1) of iron was observed around 0.2 V (Ag/AgCl) due to the decomposition of passive layer. After consecutive scans, the oxidation–passivation peaks disappeared and a wide passive plateau which can facilitate the synthesis of more protective polymer film against corrosion was formed. Oxygen evolution on Pt electrode was found to start above +0.60 V (Ag/AgCl) [17]. However, the same event starts at 1.35 V (Ag/AgCl) on MS electrode. This behaviour was explained by passivated ferrous oxalate layer existing on the surface [10]. The cyclic voltammograms of pre-passivated MS electrode in 0.3 M ammonium oxalate solution with the addition of 0.01 M AT are shown in Fig. 3. For comparison the same diagram for Pt electrode was also obtained and given in the same figure as inset. As it
Fig. 2. The cyclic voltammograms of MS electrode recorded in 0.3 M ammonium oxalate (5 cycles), scan rate: 0.010 V s−1 .
is seen from Fig. 3, MS electrode shows the similar profile with that of Pt electrode. A broad anodic peak centered at 1.01 V (Ag/AgCl) in the first cycle during the forward scan was appeared caused by the oxidation of monomer and formation of 2-aminothiazole cation radical. The coupling of cation radicals onto the clean electrode surface takes places following. During the further cycles, monomer oxidation is accompanied by the formation of the polymer film on the electrode surface. As the sweep segments increased, a decrease in oxidation process was observed indicating the formation of pAT with low conductivity. After 50 whole cycles, a light-brownish colored film was synthesized on MS surface. 3.2. Corrosion tests Representative Nyquist and log(freq)–log Z plots of uncoated MS electrode are given in Fig. 4. It was apparent from Fig. 4, the Nyquist plot of MS electrode yields a slightly depressed semi circular shape and only one time constant was observed in log(freq)–log Z plot. This behaviour indicates that the corrosion of bare MS is mainly controlled by a charge transfer process in 0.5 M HCl solution [18,19]. Fig. 5 shows the Nyquist and the log(freq)–log Z plots of MS/pAT
Fig. 3. The cyclic voltammograms recorded during the film growth on the prepassivated MS electrode in 0.3 M ammonium oxalate + 0.01 M AT (50 cycles), scan rate: 0.100 V s−1 .
124
R. Solmaz / Progress in Organic Coatings 70 (2011) 122–126
Fig. 4. Nyquist and log(freq) − log(Z) (inset) plots of MS electrode.
Fig. 5. Nyquist and log(freq) Z − log(Z) (inset) plots of MS/pAT electrode.
Fig. 6. Electrical equivalent circuit diagrams used for modeling MS/solution interface (a), MS/pAT/solution interface (b) and schematic representation of metal/polymer/solution interface (c).
R. Solmaz / Progress in Organic Coatings 70 (2011) 122–126
125
Table 1 The electrochemical data determined from electrochemical measurements. Working electrodes
CPE1 (10−5 ) (F cm−2 )
R1 ()
CPE2 (10−5 ) (F cm−2 )
R2 ()
Rp ()
*
MS MS/pAT
159.47 1.91
305
4.36
1662
45 1967
44 1874
Rp ()
IE%
*
97.7%.
97.7%.
IE%
Rp —determined from EIS. * Rp —determined from LPR.
electrode. As it is seen from Fig. 5, the diameter of Nyquist plots increased considerably after the polymerization. The obtained data were fitted according to the electrical equivalent circuit diagram suggested in Fig. 6a and b to model the MS/solution and MS/pAT/solution interfaces, respectively. The fitting data are given in Table 1. We used constant phase element (CPE) in place of a double layer capacitance in order to give a more accurate fit to the experimental results [20]. Generally, the use of a CPE is required due to the distribution of relaxation times as a result of inhomogeneities present at a micro- or nano-level [21]. The polarization resistance of bare MS was found to be 45 . In the case of un-coated steel, the polarization resistance includes the charge transfer resistance, the diffuse layer resistance and all the other accumulated kinds of corrosion products at the metal/solution interface. The impedance data of polymer coated steel were successfully fitted according to the electrical equivalent circuit diagram given in Fig. 6b. A schematic representation of the MS/polymer/solution interfaces is included in Fig. 6c. The similar diagram was proposed for MS/PANI electrode [16]. The first time-constant was related to the pores of polymer film, whereas the second one was related to the polymer film. In this case, the MS corrosion could only take place on the free surface of metal and/or within the pores; if the metal surface is fully covered, the corrosive species must diffuse along these pores to interact with the metal surface [16]. The polarization resistance of polymer coated steel, 1926 (R1 + R2 ) considerably higher than that of un-coated MS (45 ) and CPE decreased considerably indicate good corrosion protection ability of the pAT film. The decrease in the constant phase element resulting from a decrease in local dielectric constant and/or increase in the thickness of double layer suggests that the polymer film functions by strong adhesion at the metal/solution interface and exhibits a barrier effect against the attack of corrosive environment [1,4]. The polarization resistance values were also determined from the LPR technique. The calculated values for MS and MS/pAT electrodes are added to Table 1. The inhibition efficiency (IE%) was calculated from the polarization resistance using the following formula:
IE% =
Rp − Rp Rp
× 100
(1)
where Rp and Rp are the polarization resistances of bare and polymer coated MS electrodes, respectively. The IE% values determined from impedance and LPR measurements are 97.7%. The anodic polarization curves recorded for MS and MS/pAT electrodes in 0.5 M HCl solution are given in Fig. 7. It can be seen from Fig. 7, anodic current was decreased after polymerization. The open circuit potential value shifted to more negative value about 41 mV with respect to the uncoated electrode. This observation indicates that the corrosion protection ability of pAT could not only be attributed to the passivation of mild steel surface. The corrosion protection of conducting polymers is generally attributed to the physical or electronic barrier effect [1,4,5]. It has also been proposed that the conductive polymers serve to mediate the current between the passivated surface and H+ or oxygen reduction on the polymer film [1,22]. At more positive potentials than −0.320 V (Ag/AgCl), the current value increased due to the dissolution of
Fig. 7. Anodic current–potential curves of bare MS (䊉) and MS/pAT () electrodes.
underlying metal through the pores of the polymer film. The further increase in current values at high potentials may caused by the degradation and/or removing polymer film from the surface which reduces protection ability. The anodic dissolution of metal occurring at the metal/polymer interface and the oxidation of polymer film are strictly related to permeability of coating [23]. 4. Conclusions pAT was synthesized electrochemically on MS surface by cyclic voltammetry technique. An adherent polymer film was obtained on the steel surface applying a procedure including two stages. The corrosion protection ability of synthesized polymer film was investigated in 0.5 M HCl solution. Corrosion tests showed that, the pAT film performs good protection ability for MS corrosion. Acknowledgements The author is greatly thankful to C¸ukurova University, Science and Letters Faculty, Chemistry Department, Physicochemical Research Laboratory for electrochemical measurements. References [1] K. Kamaraj, V. Karpakam, S. Sathiyanarayanan, G. Venkatachari, Mater. Chem. Phys. Mater. Chem. Phys. 122 (2010) 123–128. [2] E. Altunbas, R. Solmaz, G. Kardas, Mater. Chem. Phys. 121 (2010) 354–358. [3] G. Bereket, E. Hür, Prog. Org. Coat. 65 (2009) 116–124. [4] A.P. Srikanth, T.G. Sunitha, S. Nanjundan, N. Rajendran, Prog. Org. Coat. 56 (2006) 120–125. [5] P.A. Kilmartin, L. Trier, G.A. Wright, Synth. Met. 131 (2002) 99–109. [6] S.A. Kumar, K.S. Meenakshi, T.S.N. Sankaranarayanan, S. Srikanth, Prog. Org. Coat. 62 (2008) 285–292. [7] S. Sathiyanarayanan, S.S. Azim, G. Venkatachari, Electrochim. Acta 52 (2007) 2068–2074. [8] P. Herrasti, F.J. Recio, P. Ocon, E. Fatas, Prog. Org. Coat. 54 (2005) 285–291. [9] D. Kowalski, M. Ueda, T. Ohtsuka, Corros. Sci. 49 (2007) 1635–1644. [10] T. Tuken, B. Yazıcı, M. Erbil, Prog. Org. Coat. 50 (2004) 115–122.
126
R. Solmaz / Progress in Organic Coatings 70 (2011) 122–126
[11] H. Hammache, L. Makhloufi, B. Saidani, Corros. Sci. 45 (2003) 2031–2042. [12] O. Zubillaga, F.J. Cano, I. Azkarate, I.S. Molchan, G.E. Thompson, P. Skeldon, Surf. Coat. Technol. 203 (2009) 1494–1501. [13] J. Cruz, E. Garcia-Ochao, M. Castro, J. Electrochem. Soc. 150 (2003) B26. [14] R. Solmaz, G. Kardas¸, B. Yazıcı, M. Erbil, KORSEM’08 XI, in: International Corrosion Symposium, I˙ zmir, Turkey, 2008, pp. 466–475. [15] R. Solmaz, G. Kardas, Prog. Org. Coat. 64 (2009) 81–88. [16] T. Tuken, B. Yazıcı, M. Erbil, Mater. Chem. Phys. 99 (2006) 459–464. [17] H. Keles¸, R. Solmaz, M. Özcan, G. Kardas¸, I˙ . Dehri, Surf. Coat. Technol. 203 (2009) 1469–1473.
[18] M. El Achouri, S. Kertit, H.M. Gouttaya, B. Nciri, Y. Bensouda, L. Perez, M.R. Infante, K. Elkacemi, Prog. Org. Coat. 43 (2001) 267–273. [19] A. Chetouani, A. Aouniti, B. Hammouti, N. Benchat, T. Benhadda, S. Kertit, Corros. Sci. 45 (2003) 1675–1684. [20] M. Behpour, S.M. Ghoreishi, N. Soltani, M. Salavati-Niasari, Corros. Sci. 51 (2009) 1073–1082. [21] E. Navarro-Flores, Z. Chong, S. Omanovic, J. Mol. Catal. A: Chem. 226 (2005) 179–197. [22] U. Rammelt, P.T. Nguyen, W. Plieth, Electrochim. Acta 46 (2001) 4251–4257. [23] T. Tüken, B. Yazıcı, M. Erbil, Appl. Surf. Sci. 252 (2006) 2311–2318.
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Electrochemical synthesis of poly-2-aminothiazole on mild steel and its corrosion inhibition performance Ramazan Solmaz ∗ Bingöl University, Science and Letters Faculty, Chemistry Department, 12000 Bingöl, Turkey
a r t i c l e
i n f o
Article history: Received 13 May 2010 Received in revised form 13 September 2010 Accepted 8 November 2010 Keywords: Electropolymerization Conducting polymers Poly-2-aminothiazole Corrosion
a b s t r a c t 2-Aminothiazole (AT) was polymerized by electrochemical technique on a mild steel (MS) electrode from 0.01 M monomer containing 0.3 M ammonium oxalate solution. Cyclic voltammetry was used for the synthesis. Poly-2-aminothiazole (pAT) film with a light-brownish color was obtained on the MS surface. The effectiveness of polymer film in preventing corrosion of MS was tested in 0.5 M HCl solution. For corrosion tests, anodic polarization curves, electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR) techniques were utilized. The results obtained indicated that, the polymer film adherent to the steel surface. The polymer film gives a good corrosion protection against the attack of corrosive environment. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Conducting polymers have been interested both for fundamental research and technological applications during the last two decades. These materials present interesting properties in numerous domains such as electrocatalysis, electrochromic devices, microelectronic devices, biosensors,and rechargeable batteries. Recently, there has been an increasing interest in the use of these polymers as protective coatings against the corrosion [1–12]. It has been reported that conducting polymers provide both anodic protection and barrier effect to prevent attack by a corrosive environment [1,4,5]. The advantages of conducting polymers over the other coatings such as paints, chromium and heavy metals based inhibitors are that they do not contain toxic substances that are harmful to the environment [6–8]. Their production process is simple and economical. But, low processability, poor mechanical and thermal stability and protection life time are the greatest problems for their use in corrosion protection [8]. At present, there is a considerable interest not only in the synthesis of new types of polymers, but also in the modification of the polymers in order to improve their corrosion protection ability [6,9–12]. Aminothiazole and its derivatives are reported to have anticorrosive properties [13,14]. pAT may function as organic inhibitor due to its great number of bonds, reducing the number of active sites on the metal surface through adsorption and acting both as a barrier by decreasing the transport of corrosive agents and
∗ Tel.: +90 426 213 2550; fax: +90 426 213 2866. E-mail addresses: [email protected], [email protected] 0300-9440/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2010.11.003
anodic protection. In our previous study [15], the thermal analysis results were shown relatively good thermal stability of the pAT. This advantage makes possible using pAT for materials protection even at higher temperatures. It was also shown that pAT has good electrochemical stability [15]. The pAT is partially soluble in DMSO, but insoluble AC, THF, ACN, DCM, DMF, and in acidic solutions [15]. The pAT is insoluble in usual solvents and acidic solutions that can be of a great interest in the domain of the materials protection against the corrosion. In this work, we have synthesized pAT on MS surface using cyclic voltammetry technique. The protection ability of polymer film synthesized for the MS corrosion was tested in 0.5 M HCl solution using some electrochemical techniques. 2. Experimental The pAT was synthesized electrochemically on a MS specimen (MS/pAT) (C: 0.46%, Si: 0.40%, Mn: 0.65%, P: 0.045%, S: 0.045% and remainder Fe) from 0.01 M AT + 0.3 M ammonium oxalate solution. For this aim, cyclic voltammetry technique was used. Prior to polymer synthesis, the surface of MS electrode was pre-passivated in 0.3 M ammonium oxalate solution applying 5 whole cycles between −0.60 V and 1.35 V (Ag/AgCl) potential with 0.010 V s−1 scan rate. Then, the polymer film was synthesized on the pre-passivated steel surface by cycling the potential between 0.60 V and 1.30 V (Ag/AgCl) with 0.050 V s−1 scan rate. The chemical structure of resulting polymer was reported previously as shown in Fig. 1 [15]. The corrosion behaviour of MS and MS/pAT electrodes were investigated in 0.5 M HCl solution using EIS, LPR and anodic polarization curves. The working electrode was immersed in test
R. Solmaz / Progress in Organic Coatings 70 (2011) 122–126
H
H
N
S
H
N
N
N S
H
N
N
N S
123
N S
Fig. 1. Chemical structure of poly-2-aminothiazole.
solution for 1 h to establish steady state open circuit potential. After reaching the steady state open circuit potential (1 h), the electrochemical measurements were performed. The EIS experiments were conducted in the frequency range of 100 kHz to 0.01 Hz at open circuit potential. The amplitude was 0.005 V. The LPR measurements were carried out by recording the electrode potential ±0.010 V around open circuit potential with 0.001 V s−1 scan rate. The polarization resistance (Rp ) was determined from the slope of current–potential curves obtained. The anodic polarization curves were performed starting from open circuit potential after reaching a steady state open circuit potential. Electrochemical measurements and polymer synthesis were carried out using a CHI 604 electrochemical analyzer under computer control. A platinum sheet with 1 cm2 surface area was used as a counter electrode and Ag/AgCl (3 M KCl) was used as a reference electrode. The potential values given in this study are with respect to this reference electrode. All the test solutions were prepared from analytical grade chemical reagents in distilled water without further purification. The tests were performed at room temperature. 3. Results and discussion 3.1. Synthesis of polymer film During the synthesis of conducting polymers on metal surfaces at low potentials or currents, there is an induction time during which the system is not stable and this gives rise to continuous dissolution of metal without the formation of polymer layer [8]. It is also well known that the passivation process and the quality of passive layer on the surface are very effective on physicochemical properties and adhesion behaviour of coatings synthesized [10]. In order to overcome this disadvantage, the synthesis of pAT film on the MS surface was achieved by two consecutive steps. Firstly, the surface of electrode was passivated in monomer free 0.3 M ammonium oxalate solution at a low scan rate. This low scan rate could provide a better passivity and this passive layer has much importance for the next coming polymerization stage [10,16]. Then, the polymer film was achieved over the passivated surface. The passivation stage of MS electrode in monomer free ammonium oxalate solution is graphically presented in Fig. 2. The anodic peak (A1) in the forward sweep centered at −0.2 V (Ag/AgCl) was attributed to dissolution of iron and formation of low soluble ferrous oxalate layer [1,6,17]. In the reverse scan re-passivation peak (C1) of iron was observed around 0.2 V (Ag/AgCl) due to the decomposition of passive layer. After consecutive scans, the oxidation–passivation peaks disappeared and a wide passive plateau which can facilitate the synthesis of more protective polymer film against corrosion was formed. Oxygen evolution on Pt electrode was found to start above +0.60 V (Ag/AgCl) [17]. However, the same event starts at 1.35 V (Ag/AgCl) on MS electrode. This behaviour was explained by passivated ferrous oxalate layer existing on the surface [10]. The cyclic voltammograms of pre-passivated MS electrode in 0.3 M ammonium oxalate solution with the addition of 0.01 M AT are shown in Fig. 3. For comparison the same diagram for Pt electrode was also obtained and given in the same figure as inset. As it
Fig. 2. The cyclic voltammograms of MS electrode recorded in 0.3 M ammonium oxalate (5 cycles), scan rate: 0.010 V s−1 .
is seen from Fig. 3, MS electrode shows the similar profile with that of Pt electrode. A broad anodic peak centered at 1.01 V (Ag/AgCl) in the first cycle during the forward scan was appeared caused by the oxidation of monomer and formation of 2-aminothiazole cation radical. The coupling of cation radicals onto the clean electrode surface takes places following. During the further cycles, monomer oxidation is accompanied by the formation of the polymer film on the electrode surface. As the sweep segments increased, a decrease in oxidation process was observed indicating the formation of pAT with low conductivity. After 50 whole cycles, a light-brownish colored film was synthesized on MS surface. 3.2. Corrosion tests Representative Nyquist and log(freq)–log Z plots of uncoated MS electrode are given in Fig. 4. It was apparent from Fig. 4, the Nyquist plot of MS electrode yields a slightly depressed semi circular shape and only one time constant was observed in log(freq)–log Z plot. This behaviour indicates that the corrosion of bare MS is mainly controlled by a charge transfer process in 0.5 M HCl solution [18,19]. Fig. 5 shows the Nyquist and the log(freq)–log Z plots of MS/pAT
Fig. 3. The cyclic voltammograms recorded during the film growth on the prepassivated MS electrode in 0.3 M ammonium oxalate + 0.01 M AT (50 cycles), scan rate: 0.100 V s−1 .
124
R. Solmaz / Progress in Organic Coatings 70 (2011) 122–126
Fig. 4. Nyquist and log(freq) − log(Z) (inset) plots of MS electrode.
Fig. 5. Nyquist and log(freq) Z − log(Z) (inset) plots of MS/pAT electrode.
Fig. 6. Electrical equivalent circuit diagrams used for modeling MS/solution interface (a), MS/pAT/solution interface (b) and schematic representation of metal/polymer/solution interface (c).
R. Solmaz / Progress in Organic Coatings 70 (2011) 122–126
125
Table 1 The electrochemical data determined from electrochemical measurements. Working electrodes
CPE1 (10−5 ) (F cm−2 )
R1 ()
CPE2 (10−5 ) (F cm−2 )
R2 ()
Rp ()
*
MS MS/pAT
159.47 1.91
305
4.36
1662
45 1967
44 1874
Rp ()
IE%
*
97.7%.
97.7%.
IE%
Rp —determined from EIS. * Rp —determined from LPR.
electrode. As it is seen from Fig. 5, the diameter of Nyquist plots increased considerably after the polymerization. The obtained data were fitted according to the electrical equivalent circuit diagram suggested in Fig. 6a and b to model the MS/solution and MS/pAT/solution interfaces, respectively. The fitting data are given in Table 1. We used constant phase element (CPE) in place of a double layer capacitance in order to give a more accurate fit to the experimental results [20]. Generally, the use of a CPE is required due to the distribution of relaxation times as a result of inhomogeneities present at a micro- or nano-level [21]. The polarization resistance of bare MS was found to be 45 . In the case of un-coated steel, the polarization resistance includes the charge transfer resistance, the diffuse layer resistance and all the other accumulated kinds of corrosion products at the metal/solution interface. The impedance data of polymer coated steel were successfully fitted according to the electrical equivalent circuit diagram given in Fig. 6b. A schematic representation of the MS/polymer/solution interfaces is included in Fig. 6c. The similar diagram was proposed for MS/PANI electrode [16]. The first time-constant was related to the pores of polymer film, whereas the second one was related to the polymer film. In this case, the MS corrosion could only take place on the free surface of metal and/or within the pores; if the metal surface is fully covered, the corrosive species must diffuse along these pores to interact with the metal surface [16]. The polarization resistance of polymer coated steel, 1926 (R1 + R2 ) considerably higher than that of un-coated MS (45 ) and CPE decreased considerably indicate good corrosion protection ability of the pAT film. The decrease in the constant phase element resulting from a decrease in local dielectric constant and/or increase in the thickness of double layer suggests that the polymer film functions by strong adhesion at the metal/solution interface and exhibits a barrier effect against the attack of corrosive environment [1,4]. The polarization resistance values were also determined from the LPR technique. The calculated values for MS and MS/pAT electrodes are added to Table 1. The inhibition efficiency (IE%) was calculated from the polarization resistance using the following formula:
IE% =
Rp − Rp Rp
× 100
(1)
where Rp and Rp are the polarization resistances of bare and polymer coated MS electrodes, respectively. The IE% values determined from impedance and LPR measurements are 97.7%. The anodic polarization curves recorded for MS and MS/pAT electrodes in 0.5 M HCl solution are given in Fig. 7. It can be seen from Fig. 7, anodic current was decreased after polymerization. The open circuit potential value shifted to more negative value about 41 mV with respect to the uncoated electrode. This observation indicates that the corrosion protection ability of pAT could not only be attributed to the passivation of mild steel surface. The corrosion protection of conducting polymers is generally attributed to the physical or electronic barrier effect [1,4,5]. It has also been proposed that the conductive polymers serve to mediate the current between the passivated surface and H+ or oxygen reduction on the polymer film [1,22]. At more positive potentials than −0.320 V (Ag/AgCl), the current value increased due to the dissolution of
Fig. 7. Anodic current–potential curves of bare MS (䊉) and MS/pAT () electrodes.
underlying metal through the pores of the polymer film. The further increase in current values at high potentials may caused by the degradation and/or removing polymer film from the surface which reduces protection ability. The anodic dissolution of metal occurring at the metal/polymer interface and the oxidation of polymer film are strictly related to permeability of coating [23]. 4. Conclusions pAT was synthesized electrochemically on MS surface by cyclic voltammetry technique. An adherent polymer film was obtained on the steel surface applying a procedure including two stages. The corrosion protection ability of synthesized polymer film was investigated in 0.5 M HCl solution. Corrosion tests showed that, the pAT film performs good protection ability for MS corrosion. Acknowledgements The author is greatly thankful to C¸ukurova University, Science and Letters Faculty, Chemistry Department, Physicochemical Research Laboratory for electrochemical measurements. References [1] K. Kamaraj, V. Karpakam, S. Sathiyanarayanan, G. Venkatachari, Mater. Chem. Phys. Mater. Chem. Phys. 122 (2010) 123–128. [2] E. Altunbas, R. Solmaz, G. Kardas, Mater. Chem. Phys. 121 (2010) 354–358. [3] G. Bereket, E. Hür, Prog. Org. Coat. 65 (2009) 116–124. [4] A.P. Srikanth, T.G. Sunitha, S. Nanjundan, N. Rajendran, Prog. Org. Coat. 56 (2006) 120–125. [5] P.A. Kilmartin, L. Trier, G.A. Wright, Synth. Met. 131 (2002) 99–109. [6] S.A. Kumar, K.S. Meenakshi, T.S.N. Sankaranarayanan, S. Srikanth, Prog. Org. Coat. 62 (2008) 285–292. [7] S. Sathiyanarayanan, S.S. Azim, G. Venkatachari, Electrochim. Acta 52 (2007) 2068–2074. [8] P. Herrasti, F.J. Recio, P. Ocon, E. Fatas, Prog. Org. Coat. 54 (2005) 285–291. [9] D. Kowalski, M. Ueda, T. Ohtsuka, Corros. Sci. 49 (2007) 1635–1644. [10] T. Tuken, B. Yazıcı, M. Erbil, Prog. Org. Coat. 50 (2004) 115–122.
126
R. Solmaz / Progress in Organic Coatings 70 (2011) 122–126
[11] H. Hammache, L. Makhloufi, B. Saidani, Corros. Sci. 45 (2003) 2031–2042. [12] O. Zubillaga, F.J. Cano, I. Azkarate, I.S. Molchan, G.E. Thompson, P. Skeldon, Surf. Coat. Technol. 203 (2009) 1494–1501. [13] J. Cruz, E. Garcia-Ochao, M. Castro, J. Electrochem. Soc. 150 (2003) B26. [14] R. Solmaz, G. Kardas¸, B. Yazıcı, M. Erbil, KORSEM’08 XI, in: International Corrosion Symposium, I˙ zmir, Turkey, 2008, pp. 466–475. [15] R. Solmaz, G. Kardas, Prog. Org. Coat. 64 (2009) 81–88. [16] T. Tuken, B. Yazıcı, M. Erbil, Mater. Chem. Phys. 99 (2006) 459–464. [17] H. Keles¸, R. Solmaz, M. Özcan, G. Kardas¸, I˙ . Dehri, Surf. Coat. Technol. 203 (2009) 1469–1473.
[18] M. El Achouri, S. Kertit, H.M. Gouttaya, B. Nciri, Y. Bensouda, L. Perez, M.R. Infante, K. Elkacemi, Prog. Org. Coat. 43 (2001) 267–273. [19] A. Chetouani, A. Aouniti, B. Hammouti, N. Benchat, T. Benhadda, S. Kertit, Corros. Sci. 45 (2003) 1675–1684. [20] M. Behpour, S.M. Ghoreishi, N. Soltani, M. Salavati-Niasari, Corros. Sci. 51 (2009) 1073–1082. [21] E. Navarro-Flores, Z. Chong, S. Omanovic, J. Mol. Catal. A: Chem. 226 (2005) 179–197. [22] U. Rammelt, P.T. Nguyen, W. Plieth, Electrochim. Acta 46 (2001) 4251–4257. [23] T. Tüken, B. Yazıcı, M. Erbil, Appl. Surf. Sci. 252 (2006) 2311–2318.