Progress in Organic Coatings Polypyrrole and

Progress in Organic Coatings 64 (2009) 429–434

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Polypyrrole and polypyrrole–tungstate electropolymerization coatings on carbon steel and evaluating their corrosion protection performance via electrochemical impedance spectroscopy M. Sabouri a , T. Shahrabi a,∗ , H.R. Faridi b , M.G. Hosseini c a b c

Corrosion & Protection Research Laboratory, Department of Materials Science and Engineering, Tarbiat Modares University (TMU), Tehran, Iran Corrosion Department, Research Institute of Petroleum Industry (RIPI), Tehran, Iran Electrochemistry Research Laboratory, Department of Physical Chemistry, Chemistry Faculty, University of Tabriz, Iran

a r t i c l e

i n f o

Article history: Received 10 January 2007 Received in revised form 29 July 2008 Accepted 5 August 2008 Keywords: Polypyrrole Tungstate (WO4 2− ) Corrosion Carbon steel

a b s t r a c t Polypyrrole (PPy) and polypyrrole–tungstate (PPy–WO2− 4 ) coatings with excellent adherence properties were electropolymerized on carbon steel using oxalic acid solutions containing pyrrole and pyrrole–tungstate, respectively. The electropolymerizations were carried out using cyclic voltammetry at the scan rate of 20 mV s−1 . Electrochemical impedance spectroscopy (EIS) was also used to evaluate the performance of two kinds of polypyrrole coatings on carbon steel surfaces under immersion in a 3.5% sodium chloride (NaCl) solution. Corrosion potentials as well as impedance data were obtained for each coating after various immersion times. Obtained results revealed that PPy–WO2− 4 coating provided a noticeable protection enhancement against corrosion progression. © 2008 Elsevier B.V. All rights reserved.

1. Introduction For almost two decades, electrically conductive polymers have attracted significant interest since their discovery. Soon after the discovery, it became clear that their unique properties could be used to advance technological applications such as; sensing devices, polymer-based switching devices, light-emitting plastics and new class of superconductors. The volume of fundamental and applied research in this field makes it inevitable that conductive polymers will surely find an increasing range of applications [1]. Polypyrrole due to non-toxic properties, good environmentally friendly specification, stability and ease of synthesis has been studied for the development of the aforementioned technological applications [2,3]. Another application of PPy could be its usage for protecting metals and alloys from corrosion [2–21]. Application of PPy coatings for corrosion protection of metals and alloys is however subjected to some limitations. First, charges stored in the polymer layer can be irreversibly consumed during the system’s redox reactions. Consequently, protective quality of the polymer coatings may be lost with time. Also, porosity and ion exchanges of PPy coatings might be disadvantageous, particularly when it comes to localized corrosion caused by small and aggres-

∗ Corresponding author. Tel.: +98 21 82883378; fax: +98 21 82883381. E-mail address: [email protected] (T. Shahrabi). 0300-9440/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2008.08.003

sive anions. Therefore, interests have been focused on the usage of conducting polymers as copolymers [6–8], composites [2,9–17], or bilayers [6,18–20]. In this contribution, following our previous investigations [12–15], first, the possibility of electrochemical synthesis of PPy–WO2− on carbon steel electrode was studied. Then, its 4 protective performance against corrosion was evaluated using electrochemical impedance spectroscopy (EIS) in 3.5% NaCl solution. 2. Experimental Pyrrole and oxalic acid were purchased from Merck Chemical Company Inc. Aqueous solutions used in the experiments were prepared using deionized water. Commercial carbon steel samples, with elemental composition of Fe–0.1 C–0.4 Mn–0.035 S–0.06 P–0.01 Si–0.05 Al–0.2 Cu (wt.%), were used and embedded in resin with an exposed surface area of 0.25 cm2 as the working electrode. Before each experiment, the working electrodes were carefully polished with sequence emery papers of various grades (400–1200 grit) and finally rinsed with distilled water, then electro-polished at 4.0 V in a solution containing 50 g/L NaOH at 50 ◦ C followed by rinsing them again in distilled water and their surface was then activated by immersing in 0.1N HCl for 3 s. The carbon steel samples were rinsed twice prior to the electrochemical tests. A potentiostat/galvanostat (PAR EG&G Model 273A) was used for the electrochemical experiments. PPy coatings were electropolymerized

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M. Sabouri et al. / Progress in Organic Coatings 64 (2009) 429–434

in a solution containing 0.1 mol L−1 pyrrole and 0.1 mol L−1 oxalic acid. Similar solutions containing 0.001 mol L−1 sodium tungstate (Na2 WO4 ·2H2 O) was also used for the electropolymerization of PPy–WO2− 4 coatings. Experiments were carried out at room temperature (∼25 ◦ C). Solutions were deaerated with purified nitrogen ∼25 min before each experiment. The cells were then completely sealed to prevent oxygen dissolution. Electropolymerization experiments were performed using cyclic voltammetry over two different potential ranges in unstirred conditions. In the first experiment, the potential was scanned between −0.6 and 1.2 V for one cycle. In the second experiment, the potential was scanned from −0.6 to 1.0 V for 10 cycles. The restriction of upper potential was to prevent the overoxidation of the polymer coatings. In the study, all measurements were performed at a scan rate of 20 mV s−1 . All experiments were repeated for three times. For all cases, the cyclic voltammograms were reproducible within an experimental range of ±3 to 5%. After electropolymerization, the coated electrodes were rinsed with distilled water and finally with acetone. They were then held in nitrogen atmosphere for 24 h. The protective quality of PPy and PPy–WO2− 4 coatings were investigated at room temperature (∼25 ◦ C) in 3.5% NaCl solution under unstirred conditions using open circuit potential monitoring and electrochemical impedance spectroscopy. Impedance measurements were carried out at open circuit potentials with sufficiently small excitation signal amplitude to remain within the quasi-linear response region of the systems. A computer-controlled potentiostat/galvanostat (PAR EG&G Model 273A) coupled to a frequency response analyzer (EG&G Model 1025) was used. All impedance experiments were carried out at the frequencies from 100 kHz to around 10 mHz where the AC amplitude of the perturbation voltage was 10 mV (rms). Impedance values were reproducible over a range of ±4 to 5%. Real and imaginary components of the impedance spectra in a complex plane were analyzed using the ZView software (version: 2.1a, Scribner Associates Inc.) to estimate the parameters of the equivalent electrical circuits. In this study, the error percentage for fitting data was at maximum 10%.

Fig. 2. First cycles during (a) pyrrole and (b) pyrrole–tungstate electropolymerization on Carbon steel electrode; scan rate: 20 mV s−1 .

pyrrole monomers were not affected by the presence of WO2− 4 ions, but it seems the growth of PPy coating depends on the nature of the ions. This revealed by different amount of charges consumed during pyrrole oxidation which are estimable according to the surface area under peak 1 (Fig. 1). Trivedi [22] reported the same results during aniline electropolymerization and pointed out the influence of different dopants in electropolymerization processes for the following reasons: adsorption of the ions on the electrode surface, the redox potential of the ions, ionic charge and ionic size. The electropolymerizations of PPy and PPy–WO2− 4 coatings on carbon steel were performed using cyclic voltammetry at 20 mV s−1 in two steps (Figs. 2 and 3). First, a single cycle was chosen in a potential range between −0.6 and +1.2 V, which included active dissolution of carbon steel. It is clearly evident that this active dissolution was followed by the passivation processes (peak 1, Fig. 2). As it was observed, the maximum potential and the current related to this peak decreased when WO2− 4 ions are added in the electropolymerization solution. This behaviour may be accounted for

3. Results and discussion 3.1. Electropolymerization Fig. 1 shows the cyclic voltammograms of electropolymerization of PPy and PPy–WO2− 4 on platinum electrodes. During pyrrole electropolymerization, only a single peak appeared due to pyrrole oxidation (peak 1), and no significant electrochemical processes occurred on the platinum electrode in the studied region even by the presence of WO2− 4 ions. Also, it showed that the oxidation of

Fig. 1. First cycle during (a) pyrrole and (b) pyrrole–tungstate electropolymerization on platinum electrode; scan rate: 20 mV s−1 .

Fig. 3. (A) PPy and (B) PPy–WO2− growth on carbon steel (1st, 5th, 8th and 10th 4 cycles shown): scan rate: 20 mV s−1 .


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Fig. 4. Nyquist plots in 3.5% NaCl solution for PPy and PPy–WO2− coated electrodes after (a) 60 min, (b) 180 min, (c) 240 min, (d) 360 min, (e) 420 min and (f) 480 min of 4 exposure. 2− the inhibitory effects of WO2− 4 ions. Also, it seems that WO4 ions participated in the passivation processes. However, electropolymerization process and its rate may be affected by the composition changes on the surface of the electrode. By following up the positive scan rate, peak 2 appeared at around 0.7 V and the current increased sharply. This is due to pyrrole oxidation, as it was also observed in the case of platinum electrode. In the reverse scan (cathodic branch), two peaks were observed; one peak appeared around zero potential (peak 3) and the second around −0.4 V (peak 4). The third peak in both cases is related to the passivation and re-passivation of the surface. These peaks are very weak, meaning the polymer layers are low in porosity and high in adhesion to the carbon steel surfaces. The fourth peak is formed most likely due to the reduction of PPy and PPy–WO2− 4 coatings. During the reduction reactions, some anions which have been entrapped in polymer are released. There-

fore, the weakness of the fourth peak in the case of PPy–WO2− 4 coating may be attributed to the lower anion exchange proper2− ties of PPy–WO4 compared to standard PPy coating. However, this behaviour may also be due to the formation of new and thinner layers in the case of PPy–WO2− 4 . Fig. 3 illustrates the second step of the electropolymerization of PPy and PPy–WO2− 4 coatings on carbon steel electrodes (as shown in 1st, 5th, 8th and 10th cycles). The 10 following cycles have limited dimensions and the ranges are between −0.2 and +1.0 V. These specific potential ranges corresponding to the polymer growth were determined empirically based on the characteristics of the polymer coatings obtained. It is reported [23] that at lower applied potentials (1.0 V vs. SCE), undesirable side reactions such as ring opening and breaking of conjugated bonds may take place, resulting in the formation of defects and films with low

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Fig. 5. Proposed equivalent circuits for (a) uncoated, (b) CS/PPy, and (c and d) CS/PPy–WO2− ectrodes. 4

called CPE is an element in which its admittance or impedance value is a function of frequency but the phase is independent. The admittance and impedance are defined, respectively as

conductivities. The resultant polymer coatings were black in color, compact, and very adherent to carbon steel sample surfaces. Visually, the surfaces of the coated electrodes were uniform and smooth.

YQ = Y0 (jω)

3.2. Electrochemical impedance characteristics of PPy and PPy–WO2− 4 coatings

n

(1)

and ZQ =

Fig. 4 shows recorded Nyquist diagrams for coated carbon steel electrodes of various exposure times in 3.5% NaCl solution. All impedance plots were analyzed with the help of equivalent circuits in Fig. 5. In order to give more accurate results, constant phase element (CPE) was used to substitute the capacitive effect. The so-

1 −n (jω) Y0

(2)

where Q represents a CPE, Y0 is the modulus, ω and n are angular frequency and phase, respectively [24]. The reason why CPE is conceptually substituted for capacitor in analysis of impedance spectra

Table 1 Values for parameters of equivalent circuits obtained from best fit to impedance data Electrode

t (min)

EOCP (mV)

Rs ( cm2

Rct ( cm2 )

Y01 (−1 cm−2 sn ) −5

Rf ( cm2 )

n1

CS

240

−697

1.34

50.1

9.32 × 10

0.93

CS/PPy

60 120 180 240 300 360 420 480

−336 −454 −563 −620 −600 −613 −612 −612

5.84 4.86 4.75 4.59 4.39 3.91 3.63 3.44

122.5 266.75 350 377.5 372 358.25 367.5 369.75

34 × 10−5 41.2 × 10−5 34.02 × 10−5 32.8 × 10−5 34.8 × 10−5 32.4 × 10−5 31.2 × 10−5 30.4 × 10−5

0.59 0.69 0.72 0.73 0.73 0.74 0.75 0.75

81.66 53.9 47.75 45.41 37.61 44.08 37.62 33.1

Y02 (−1 cm−2 sn )

3.76 × 10−5 2.4 × 10−5 2.47 × 10−5 1.73 × 10−5 1.2 × 10−5 1.9 × 10−5 l.0 × 10−5 0.82 × 10−5

n2

Ws-R ( cm2 )

Ws-T (s)

Ws-P

512

0.7

0.43

0.43 0.50 0.51 0.52 0.55 0.48 0.55 0.57

Electrode

t (min)

EOCP (mV)

Rs ( cm2

R1 ( cm2 )

Y01 (−1 cm−2 sn )

n1

Ws-R ( cm2 )

Ws-T (s)

Ws-P

CS/PpyW

60 120 180 240 300

54 53 36 18 −72

2.23 2.4 2.57 3.22 4.65

47 48 55 60 86

2.8 × 10−5 1.6 × 10−5 2.35 × 10−5 6.61 × 10−5 28.6 × 10−5

0.62 0.67 0.66 0.59 0.50

2682 3176 3075 3051 3027

9.7 37 38 41 41

0.72 0.68 0.71 0.71 0.73

Y02 (−1 cm−2 sn )

n2

Ws-R ( cm2 )

Ws-T (s)

Ws-P

0.54 0.58 0.56

3005 2670 2475

43 45 51

0.69 0.65 0.61

Electrode CS/PpyW

t (min) 360 420 480

EOCP (mV) −254 −363 −418

Rs ( cm2 4.7 4.7 5.22

Rct ( cm2 ) 141.4 200 475

Y01 (−1 cm−2 sn ) −5

10.08 × 10 1.2 × 10−5 1.88 × 10−5

n1 0.53 0.67 0.65

Rf ( cm2 ) 2.7 2.66 1.39

−5

34.8 × 10 27.3 × 10−5 27.6 × 10−5

According to ZView, at very low frequencies, Z approaches Ws-R and Z goes to zero. Ws-T = L2 /Deff (L is the effective diffusion thickness, and D is the effective diffusion coefficient of the particle), 0 < Ws-P < 1.


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Table 2 Values of capacitances calculated based on Eq. (5) Cidl (␮F cm−2 )

Electrode

Immersion time (min)

CS

240

74

CS/PPy

60 120 180 240 300 360 420 480

37.4 152.8 148.7 151.5 163.4 152 151.6 146.7

Cif (nF cm−2 )

17.5 31 37.9 23.5 21.6 84.3 16.1 16.7

Electrode


Ci1 (␮F cm−2 )

CS/PPy–WO2− 4

60 120 180 240 300

0.48 0.47 0.76 1.4 7.03

Electrode


Cidl (␮F cm−2 )

Cif (nF cm−2 )

CS/PPy–WO2− 4

360 420 480

26.8 33.3 55.9

939 580 521

is that most experimentally measured impedance loops are similar to depressed semi-circles [25,26]. The amount of depression depends on the phase of CPE [24]. The required circuit element values for fitting the impedance plots are given in Table 1. Moreover, a CPE can be treated as a parallel combination of pure capacitor and resistor and it is inversely proportional to the angular frequency [27]. For a parallel circuit composed of a CPE (Qi ) and a resistor (Ri ), (Qi Ri ), the relaxation time constant of the circuit, i , is given as: i = Ci Ri

of the existence of an excellent barrier for the PPy–WO2− 4 coating. Furthermore, the Warburg impedance that can be evaluated from linear parts of graphs support the idea of coating system acting like an almost perfect cover since no significant corrosion occurred on the carbon steel surface. The relevant –f diagram also supports this idea when the value of phase angle () is kept around 50◦ at the minimum frequencies (around 10 mHz) (Fig. 6(1c)). The values

(3)

where Ci is capacitance of pure capacitor comparing to CPE, Qi . There exists a relation amongst Y0i , ni , i , and Ri : Y0i =

ini Ri

(4)

where Y0i is the modulus and ni is the phase of Qi . Combining Eqs. (3) and (4) leads to Ci = (Y0i Ri1−ni )1/ni

(5)

Based on Eq. (5), the capacitances (C1 , Cdl , and Cf ) calculated according to the concept of CPE from the equivalent circuits (see Table 2). As it can be seen in Fig. 5b, correspondent circuit model of PPy coated electrode consisted of two time constants. This is the most frequent circuit model used to describe corrosion mechanism of metal/polymer electrodes [28,29]. According to this configuration, a metal is protected against corrosion by establishing a barrier layer on its surface. The barrier performance can also be extracted from the values of equivalent circuit model parameters while considering very small variations in both charge transfer resistance (Rct ) and double layer capacitance (Cdl ) as well as reduction in coating resistance (Rf ) and an increase in coating capacitance (Cf ) through whole period of immersion time (Tables 1 and 2). coated electrode, however, the situation For the PPy–WO2− 4 is quite different. Here, up to 300 min of immersion, the Nyquist plots are depressed semi-circles at high frequencies followed by a linear part extended to lower frequencies region. The value of semi-circle’s diameter was equal to R1 , which is a summation of coating and charge transfer resistances [29,30]. In addition, C1 is considered to be the sum of coating and double layer capacitances [31]. During this period, the high noble values of Eocp give evidence

Fig. 6. Comparison between experimental and fitting data; (1) after 120 min of electrodes; (2) exposure times for (a) uncoated, (b) CS/PPy, and (c) CS/PPy–WO2− 4

CS/PPy–WO2− electrode after 240 min of exposure times. Solid lines present fitted 4 curves.

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of R1 and Eocp increased and decreased, respectively, with time, explaining additional solutions held by and within the coating. It is also confirmed by reduction in Ws-R as well as an increase in Ws-T values (Table 1). After 300 min of immersion, a weak semi-circle (or a time constant) is appeared on Nyquist plots that is only related to the polymer coating itself (Figs. 4(d)–(f) and 6(2)). Furthermore, the best fitting for the experimental data will be based on a new equivalent circuit model represented in Fig. 5d that is also a common model used to interpret a metal/polymer corrosion system [5]. Passing 300 min of immersion also helps to distinct the coating and the metal time constants from each other. In Tables 1 and 2, the higher values of Cf and the lower values for Rf of PPy–WO2− 4 coating in comparison to the standard PPy coating are demonstrated. Indeed, it is resulted from higher stored charges of new PPy coating which in turn originates from the incorporation of WO2− 4 dopants into the polymer chains during electropolymerization processes. In addition, the lower values of Cdl revealed the higher quality of primary passive layer resulting from the participation of WO2− 4 ions in primary passivation processes. After 480 min of immersion, value of the semi-circle’s diameter (Rct ) reaches its highest value while the Eocp value is lowest. Furthermore, linear region of the Nyquist plots shifting towards the real axis, means more deterioration of the protective PPy–WO2− 4 coating. This event was also confirmed by the decrease of the value at minimum frequencies (Fig. 6(2)). However, after 480 min of immersion, the PPy–WO2− 4 coating is still more protective than the PPy coating as the polarization resistance (equals to the sum of the Rs , Rct and Rf [29]) of the PPy–WO2− 4 is about two times that of PPy coating. 4. Conclusions Forming PPy–WO2− 4 composite coatings on carbon steel substrates were successfully achieved using cyclic voltammetry in oxalic acid medium. The results revealed that the electropolymerization processes were influenced by the WO42− ions. EIS studies clearly showed that WO2− 4 ions have increased the stored charge of the polymer coating, resulting in the fabrication of a new PPy coating with excellent corrosion protection performance. EIS studies showed the participation of WO2− 4 ions in the passivation pro-

cesses. Formation of a primary passive layer with higher quality was achieved. The existence of this stable passive layer under the coating can improve the protection of carbon steel PPy–WO2− 4 against corrosion. These investigations also indicated that passivator ions are promising dopants for fabricating new conducting polymer coatings with high corrosion protection performance. References [1] S. Haddlington, Chem. Ind. 1 (1995) 11. [2] H. Hammache, L. Makhloufi, B. Saidani, Corros. Sci. 4 (2003) 2031. [3] D.E. Tallman, G. Spinks, A. Dominis, G.G. Wallace, J. Solid State Electrochem. 6 (2002) 73. ´ M.V. Vojnovic, ´ C. Lacnjevac, ´ ´ Prog. [4] B.N. Grgur, N.V. Krstajic, L.J. Gajic-Krstajic, Org. Coat. 33 (1998) 1. [5] T.K. Rout, G. Jha, A.K. Singh, N. Bandyopadhyay, O.N. Mohanty, Surf. Coat. Technol. 167 (2003) 16. [6] C.K. Tan, D.J. Blackwood, Corros. Sci. 45 (2003) 545. [7] P. Herrasti, P. Öcón, A. Ibanez, E. Fatas, J. Appl. Electrochem. 33 (2003) 533. [8] G.S. Akundy, R. Rajagopalan, J.O. Iroh, J. Appl. Polym. Sci. 83 (2002) 1970. [9] M.G. Galkowski, M.A. Malik, J. Electrochem. Soc. 150 (2003) 249. [10] M.A. Malik, M.G. Galkowski, H. Bala, B. Grzybowska, P.J. Kulesza, Electrochim. Acta 44 (1999) 2157. [11] J. Bonastre, P. Garcés, F. Huerta, C. Quijada, L.G. Andión, F. Cases, Corros. Sci. 48 (2006) 1122. [12] M.G. Hosseini, M. Sabouri, T. Shahrabi, Mater. Corros. 57 (2006) 407. [13] M. Sabouri, T. Shahrabi, H.R. Faridi, M. Salasi, Corros. Eng. Sci. Technol. (2008), doi:10.1179/1743278o8X286257. [14] M. Sabouri, T. Shahrabi, M.G. Hosseini, Russ. J. Electrochem. 43 (2007) 1390–1397. [15] M. Sabouri, T. Shahrabi, M.G. Hosseini, Mater. Corros 59 (10) (2008) 814–818. [16] U. Riaz, S.M. Ashraf, S. Ahmad, Prog. Org. Coat. 59 (2007) 138. [17] I.L. Lehr, S.B. Saidman, Corros. Sci. 49 (2007) 2210. [18] A. Ya˘gan, N. Özçiçek Pekmez, A. Yıldız, Prog. Org. Coat. 59 (2007) 297. [19] D. Kowalski, M. Ueda, T. Ohtsuka, Corros. Sci. 49 (2007) 3442. [20] D. Kowalski, M. Ueda, T. Ohtsuka, Corros. Sci. 49 (2007) 1635. [21] T. Tüken, G. Tansu˘g, B. Yazıcı, M. Erbil, Surf. Coat. Tech. 202 (2007) 146–154. [22] D.C. Trivedi, J. Solid State Electrochem. 2 (1998) 85. [23] M. Satoh, K. Kaneto, K. Yoshino, J. Appl. Phys. 24 (1985) 423. [24] J.E. Ferrer, L. Victori, Electrochim. Acta 39 (1994) 581. [25] M. Leibig, T.C. Halsey, Electrochim. Acta 38 (1993) 1985. [26] A.V. Benedetti, P.T.A. Sumodjo, K. Nobe, P.L. Cabot, W.G. Proud, Electrochim. Acta 40 (1995) 2657. [27] X.J. Wu, H.Y. Ma, S.H. Chen, Z.Y. Xu, A.F. Sui, J. Electrochem. Soc. 146 (1999) 1847. [28] G.W. Walter, Corros. Sci. 41 (1986) 681. [29] W.S. Tait, An Introduction to Electrochemical Corrosion Testing for Practicing Engineers and Scientists, Wisconsin, Pair ODocs, 1994. [30] P.L. Bonora, F. Deflorian, L. Fedrizzi, Electrochim. Acta 41 (1996) 1073. [31] T. Hong, H. Shi, H. Wang, M. Gopal, P. Jepson, Corrosion, Paper No. 00044, 2000.