PPy coatings doped by phosphotungstate on mild steel and their corrosion resistances

Progress in Organic Coatings 88 (2015) 84–91

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Electrosynthesis of PAni/PPy coatings doped by phosphotungstate on mild steel and their corrosion resistances Jinqiu Xu a , Yanqing Zhang a , Dongqin Zhang a , Yongming Tang a,∗ , Hui Cang b a b

School of Science, Nanjing Tech University, Nanjing 211816, PR China College of Chemical Engineering and Biological, Yancheng Institute of Technology, Yancheng 224051, PR China

a r t i c l e

i n f o

Article history: Received 13 February 2015 Received in revised form 22 June 2015 Accepted 24 June 2015 Keywords: Conducting polymer Polyaniline/polypyrrole Phosphotungstate-doped Corrosion resistance Electrodeposition

a b s t r a c t Polyaniline/polypyrrole (PAni/PPy), polyaniline-phosphotungstate/polypyrrole (PAni-PW12 /PPy) and PAni/PPy-PW12 have been successfully electrodeposited on mild steel (MS) by cyclic voltammetry in aqueous oxalic acid solutions. It was found that the incorporation of PW12 enhanced the corrosion resistance of PAni/PPy coating. Moreover, in comparison to PAni-PW12 /PPy, PAni/PPy-PW12 coating exhibited better corrosion resistance for mild steel. After immersion of 36 h in 0.1 M HCl, for instance, the polarization resistance of PAni/PPy-PW12 coating reached 1695  cm2 , more than those of both PAni/PPy and PAni-PW12 /PPy. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Conducting polymers (CPs) have attracted considerable attention due to their technological applications in various fields such as battery materials [1,2], chemical capacitors [3], gas sensors [4] and corrosion protection [5,6]. Of various conducting polymers, polyaniline (PAni) [7,8] and polypyrrole (PPy) [9,10] were most widely studied for corrosion inhibition due to their environmentfriendly nature and ease of synthesis. Tsirimpis et al. reported that polyaniline and polypyrrole coatings could be electrodeposited on aluminum from oxalic acid medium via cyclic voltammetry technique [11]. They also demonstrated that the formed coatings have a dramatic improvement on electrochemical behavior compared to the uncoated substrate. In recent years, much attention has been paid to test electrochemical synthesis of PAni or PPy multilayer coatings on mild steel. There have been many studies on the utility of bilayer conducting polymer as corrosion-resistant coating in which two layers of conducting polymer films were successively deposited on the studied substrate. Those bilayer coatings exhibited better corrosion resistance compared to the single layer coatings [12,13]. Tan and Blackwood performed the galvanostatic electrodeposition of PPy/PAni coating on stainless steel from H2 SO4 and acetonitrile mediums, respectively, and investigated anti-corrosion behaviors of the coatings in 0.028 M NaCl solutions [14]. It was found

∗ Corresponding author. E-mail address: [email protected] (Y. Tang). http://dx.doi.org/10.1016/j.porgcoat.2015.06.024 0300-9440/© 2015 Elsevier B.V. All rights reserved.

that the PPy/PAni coating reduced the corrosion rate of stainless steel to 1.05 × 10−2 ␮m yr−1 with respect to 17.6 ␮m yr−1 of the bare stainless steel. In addition, they found that PPy/PAni coating showed significant improvement in protecting stainless steel against pitting corrosion over previously reported PAni coating. Zeybek et al. investigated the electrosynthesis of monolayer and bilayer coatings of poly(N-methylaniline) (PNMA), polypyrroledodecylsulfate (PPy-DS), PPy-DS/PNMA and PNMA/PPy-DS on mild steel surface by cyclic voltammetry in aqueous oxalic acid solutions [15]. Among these coatings, DS ions as dopant were incorporated in the inner or outer layers of coatings. Corrosion behaviors of these coatings in aggressive 0.5 M HCl revealed that the PNMA/PPy-DS bilayer exhibited the best corrosion resistance. Even after immersion of 240 h, PNMA/PPy-DS still showed a protection efficiency of 91.5%, far higher than other coatings. The aim of this work is to prepare composite PAni/PPy coatings doped by phosphotungstate (PW12 ) and evaluate their corrosion inhibition for mild steel substrate in aggressive medium. In the previous study, we had electrodeposited homogeneous and adherent PPy coating doped by PW12 on mild steel which exhibited high corrosion resistance [16]. To our knowledge, there has been no report on the syntheses of PAni-PW12 /PPy and PAni/PPy-PW12 coatings on mild steel and their corrosion performance. 2. Experimental Aniline (Ani), pyrrole (Py), phosphotungstic acid (PW12 ), oxalic acid dihydrate and hydrochloric acid (HCl) were of analytical grade and were used without further purification unless otherwise

J. Xu et al. / Progress in Organic Coatings 88 (2015) 84–91

specified. Ani and Py monomers were distilled under nitrogen and stored at low temperature (4 ◦ C) prior to use. All solutions were prepared with ultrapure water. All electrochemical experiments were performed on a Vertex electrochemical workstation (Ivium, Netherlands). Electrochemical measurements were carried out in a three-electrode cell with mild steel (MS) as the working electrode, platinum foil with an area of 3.10 cm2 as the auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode. The MS electrode was a cylindrical rod measuring 1.00 cm in diameter with the composition (wt%): 0.17 C, 0.20 Si, 0.37 Mn, 0.03 S, 0.01 P and the balance Fe. The working area of the MS electrode was 0.785 cm2 while the rest of electrode was embedded in a thick polyester block. Before electrodeposition, the working electrode was consecutively abraded with emery papers with grit sizes 180, 300, 1200 and 2000. Afterwards, the electrode was washed with ethanol and then rinsed with ultrapure water, and finally dried at room temperature. All of the polymerization solutions were deaerated with purified nitrogen for 10 min before electrochemical synthesis. The volume of the electrolytes for electropolymerization was 100 mL. To minimize the IR drop between the working electrode and the electrolyte, a Luggin capillary was used with the tip set at a distance of approximately 1 mm from the surface of working electrode. The PAni/PPy, PAni-PW12 /PPy and PAni/PPy-PW12 bilayer films were electrochemically synthesized by cyclic voltammetry technique in 0.3 M oxalic acid containing 0.1 M Ani or Py monomers. Electrodeposition of PAni film was first carried out by cyclic voltammetry of 15 cycles with the potential range from −0.5 to 1.5 V. Then PPy layer was electrodeposited on the MS/Pani electrode by cyclic voltammetry of 15 cycles with the potential range from −0.5 to 0.9 V. To prepare PAni-PW12 and PPy-PW12 composite coatings, 2 g L−1 PW12 was introduced into the polymerization solutions. After electrodeposition, the polymer-coated electrodes were removed from the polymerization mediums and washed with ultrapure water followed by drying in air. The morphologies of the samples were observed by field-emission scanning electron microscope (FESEM, S-4800) equipped with a QUANTAX 400 Energy Dispersive X-ray (EDX) detector to analyze the compositions. In addition, a Nicolet 870 attenuated total reflection Fourier transform infrared spectrometer (ATR-FTIR) was used to characterize the chemical groups of the films. Corrosion resistances of the coatings were evaluated by opencircuit potential (OCP), Tafel polarization and electrochemical impedance spectroscopy (EIS) in 0.1 M HCl solution. The OCP was recorded as a function of immersion time in the aggressive solution. Prior to Tafel tests, both bare and polymer-coated MS electrodes were held in 0.1 M HCl solution until a steady-state open circuit potential. Potentiodynamic polarization curves were recorded at a scanning rate of 0.5 mV s−1 . EIS was performed at open-circuit potential in the frequency range of 105 –0.01 Hz with an amplitude of 10 mV.

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Fig. 1. Cyclic voltammograms of MS in 0.3 M oxalic acid + 0.1 M Ani. Scan rate: 20 mV s−1 .

about 0.35 V and 0.30 V, which is attributed to the oxidation and reduction of aniline monomer on the MS electrode. It should be noted that the intensities of the peaks in the following 5, 10, 15 cycle gradually increase, indicating the buildup of electroactive polymer polyaniline on the electrode surface. There is also a pair of redox peaks at 1.08 V and −0.05 V which is assigned to inter-conversion of polyaniline states [19,20]. Fig. 2 shows the cyclic voltammogram for the electrodeposition of PPy on top of PAni in 0.3 M oxalic acid + 0.1 M monomer. The oxidation of pyrrole monomer is observed at around 0.6 V. During the reverse scan, a broad cathodic current peak appears at around 0 V, corresponding to the reduction of PPy coatings [21]. From Fig. 2, the intensities of redox peaks increase with scanning cycle within the initial deposition. After 10 cycles, however, the oxidation current begins to decrease, which can be attributed to the gradual decrease in conductivity resulting from the outgrowth of PPy film on the surface [12,22]. Inspection of the electrode reveals that an intact black PPy film covers the PAni-coated mild steel surface fully. Fig. 3 depicts the electropolymerization of PAni in the presence of 2 g L−1 phosphotungstate acid. The passivation current density of MS (8.5 mA cm−2 ) at around −0.38 V is slightly higher than that in the PW12 -free solution (7.3 mA cm−2 , Fig. 1). In fact, due to its

3. Results and discussion 3.1. Electrochemical synthesis of PAni/PPy, PAni-PW12 /PPy and PAni/PPy-PW12 on MS Fig. 1 shows the cyclic voltammogram for the electrodeposition of PAni in 0.3 M oxalic acid + 0.1 M monomer. During the first positive scan, the peak potential for the formation of iron oxalate layer is observed at −0.36 V [17]. The formation of insoluble oxalate film underlies the electrodeposition of PAni. According to Hasanov’s study, the first monomer radical cation should appear at around 1.0 V at the first cycle of this voltammogram [18]. During subsequent potential scans, new anodic and cathodic peaks appear at

Fig. 2. Cyclic voltammograms of MS/PAni in 0.3 M oxalic acid + 0.1 M Py. Scan rate: 20 mV s−1 .

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Fig. 3. Cyclic voltammograms of MS in 0.3 M oxalic acid + 0.1 M Ani + 2 g L−1 PW12 . Scan rate: 20 mV s−1 .

Fig. 5. FTIR spectra of PAni deposited on MS (a) PAni film deposited without additives. (b) PAni film deposited in the presence of 2 g L−1 PW12 . Both of the coatings were electrodeposited by CV 15 scanning cycles.

strong oxidizability in acidic media PW12 may convert Fe (II) to Fe (III) species, hindering maintenance of the insoluble iron (II) oxalic passive film. In the presence of PW12 , furthermore, the oxidation current of aniline is much higher than that in the absence of PW12 . At the fifteenth cycle, for example, the peak current density with the introduction of PW12 is 4.9 mA cm−2 , approximately as five times as 0.9 mA cm−2 without PW12 . This result suggests that the presence of PW12 favors the electrodeposition of PAni and enhances the exchange of anions such as oxalate and phosphotungstate in the PAni film. Fig. 4 shows the cyclic voltammogram for the electrodeposition of PPy on top of PAni in 0.3 M oxalic acid + 0.1 M Py in the presence of 2 g L−1 PW12 . Compared to the electrodeposition of PPy in the absence of PW12 , the introduction of PW12 increases the oxidation current density of Py at 0.9 V from 10.2 mA cm−2 to 11.8 mA cm−2 at the first positive scan. Furthermore, as can be seen from Fig. 4, the oxidation current density increases with scanning cycle, indicating that the introduction of PW12 favors the electrodeposition of PPy. Additionally, the introduction of PW12 also results in the increase of both oxidation (0.4–0.5 V) and reduction (−0.2 V) currents of PPy

with scanning cycle, indicating that the doping of PW12 improves the conductivity of the PAni/PPy film [16].

Fig. 4. Cyclic voltammograms of MS/PAni in 0.3 M oxalic acid + 0.1 M Py + 2 g L−1 PW12 . Scan rate: 20 mV s−1 .

3.2. Characterization of coatings FTIR spectra of both PAni and PAni-PW12 layers on mild steel electrosynthesized by cyclic voltammetric technique are shown in Fig. 5. The absorption bands at 1603 cm−1 and 1492 cm−1 are characteristic of the stretching vibrations of the quinoid (Q) ring and the benzenoid (B) ring, respectively [23]. The absorption bands at around 1361 cm−1 and 1156 cm−1 are assigned to the C H stretching vibration of aromatic conjugation [24]. The peak centered at 1700 cm−1 indicates the presence of carbonyl groups in the coating. Additionally, a band appearing at near 1456 cm−1 represents the C C of quinoid [25]. Fig. 6 shows the spectra of samples covered with the PPy and PPy-PW12 layers. The bands at 762 cm−1 , 910 cm−1 and 972 cm−1 are assigned to the out-of-plane and inplane C-H bending motions of the aromatic rings, respectively [26]. Other bands observed at 1156 cm−1 , 1450 cm−1 , 1494 cm−1 and

Fig. 6. FTIR spectra of PPy deposited on MS (a) PPy film deposited without additives. (b) PPy film deposited in the presence of 2 g L−1 PW12 . Both of the coatings were electrodeposited by CV 15 scanning cycles.

J. Xu et al. / Progress in Organic Coatings 88 (2015) 84–91

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Fig. 7. FESEM micrographs of (a) PAni from 0.3 M oxalic acid + 0.1 M Ani, (b) PAni-PW12 from 0.3 M oxalic acid + 0.1 M Ani + 2 g L−1 PW12 , (c and d) PPy on top of PAni from 0.3 M oxalic acid + 0.1 Py, (e and f) PPy on top of PAni-PW12 from 0.3 M oxalic acid + 0.1 Py, (g and h) PPy-PW12 on top of PAni from 0.3 M oxalic acid + 0.1 M Py + 2 g L−1 PW12 . All of the coatings were electrodeposited by CV with 15 scanning cycles.

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Fig. 9. OCP monitorings for MS, PAni/PPy, PAni-PW12 /PPy and PAni/PPy-PW12 electrodes in 0.1 M HCl.

the cauliflower-like structure was related to the intercalation difficulty of dopants in the disordered polymeric chain [31]. Actually, PPy-PW12 coating exhibits an aggregated globular structure with the size of the refined grains less than 1 ␮m (Fig. 7h). The EDX analysis in Fig. 8b also confirms the successful incorporation of PW12 into the PPy matrix of PAni/PPy-PW12 bilayer coating. Fig. 8. EDX of (a) PAni-PW12 /PPy and (b) PAni/PPy-PW12 .

2930 cm−1 can be attributed to the breathing vibration of the pyrrole ring and the C C, C C and C H stretching vibration of pyrrole ring, respectively [27,28]. In addition, it has been reported that the characteristic absorption band of W O W appears at around 820 cm−1 [5,29]. From Fig. 5, there is slight difference in absorption between the undoped PAni and PAni-PW12 in the range of 800 cm−1 . The similar results can also be obtained in the case of PPy coatings (Fig. 6). However, the difference is so small that it cannot be definitely concluded the presence of W O W band. In fact, the incorporation of PW12 into those coatings can be confirmed easily by EDX analysis as shown in Fig. 8a and b. Fig. 7 shows the surface morphologies of PAni, PAni-PW12 , PAni/PPy, PAni-PW12 /PPy and PAni/PPy-PW12 films deposited on MS. As it can be seen, the PAni film in Fig. 7a presents an inhomogeneous nodular structure associated with lots of rugged valleys, which is compatible with the characteristic morphology of the reported PAni [15,30]. Fig. 7b shows that the texture of PAni film is significantly improved by the doping of PW12 . It appears that the PPy-PW12 film is more intact than the PW12 -free PAni film. In addition, the EDX analysis (Fig. 8a) confirms the existences of both tungsten and phosphor in the PAni-PW12 /PPy bilayer coatings, demonstrating the incorporation of PW12 into the PAni layer. The micrographs for PW12 -free PPy on top of PAni (Fig. 7c and d) are markedly different from that of PAni. PPy has a globular texture constituted by spherical grains as previously reported [16]. It should be noted that the PPy grains are not uniform in size with the range from 2 ␮m to 10 ␮m. Fig. 7e and f shows the surface morphology of PPy deposited on the PW12 -doped PAni. Compared to the PAni/PPy film, it is clear in the case of PAni-PW12 /PPy that a more homogeneous and denser PPy layer is formed and the globular microstructures have a size of about 1 ␮m. Combined with the results of Fig. 7a and b, we speculate that the incorporation of PW12 into PAni film may be responsible for the formation of dense PPy film on PAni layer. Fig. 7g shows a cauliflower-like structure of PPy-PW12 layer consisting of micro-spherical grains on PAni. It has been reported that

3.3. Evaluation of the corrosion resistance 3.3.1. Measurements of OCP The variation in OCP with time for multilayered polymer coatings exposed to 0.1 M HCl solution is shown in Fig. 9. The initial EOCP values of PAni-PW12 /PPy and PAni/PPy-PW12 are 210 and 330 mV, respectively, more positive than that of PAni/PPy (−56 mV). It should be noted that the EOCP value for bare MS was −503 mV at the beginning of immersion, dropped to −570 mV after 20 min and then remained almost invariable for the rest of time. The nobler EOCP values demonstrate that both PAni-PW12 /PPy and PAni/PPy-PW12 composite coatings have more protective effects for iron in this corrosive solution than PAni/PPy coating. Within the initial immersion of 50 min, all of the bilayer coatings exhibit a gradual shift of potential toward negative direction. It is worth noting that the OCP value of PAni/PPy coating drop more dramatically compared to the PW12 -doped ones and attains a near-steadystate value of about −530 mV, which may be attributed to the fact that PAni/PPy coating is permeable enough to suck the corrosive solution. For the PW12 -doped coatings, however, the doping of PW12 makes the steady-state potential value more positive than the dopant-free coating, indicating that the incorporation of PW12 enhance the protective capacity of those conducting polymer coatings. In comparison to PAni-PW12 /PPy coatings, during the whole immersion the OCP of PAni/PPy-PW12 decays more gently. And after the immersion of 150 min the steady-state potential of PAni/PPy-PW12 coating is approximate 270 mV higher than that of PAni-PW12 /PPy coating. Therefore, to enhance the corrosion resistance, the dopant PW12 should be incorporated into the outer layer of those bilayer coatings. 3.3.2. Tafel polarization The potentiodynamic polarization curves were recorded to study the corrosion inhibition effects of bilayer coatings for MS in 0.1 M HCl. The results are displayed as Tafel plots in Fig. 10. The electrochemical parameters, including corrosion potential (Ecorr ), anodic and cathodic Tafel slopes (ba and bc ) and corrosion current

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Table 1 Potentiodynamic polarization parameters of PAni/PPy, PAni-PW12 /PPy, PAni/PPy-PW12 electrodes in 0.1 M HCl solution. Electrodes

Ecorr (V) −0.572 −0.517 −0.520 −0.508

MS PAni/PPy PAni-PW12 /PPy PAni/PPy-PW12

icorr (A cm−2 ) −4

8.67 × 10 4.34 × 10−5 3.60 × 10−5 1.35 × 10−5

Fig. 10. Potentiodynamic polarizations of MS, PAni/PPy, PAni-PW12 /PPy, PAni/PPyPW12 electrodes in 0.1 M HCl.

density (icorr ) derived from polarization curves are listed in Table 1. The protection efficiency (PE) was calculated as follows [32]:



PE(%) =

1−

icorr 0 icorr



× 100%

(1)

0 and icorr are the corrosion current density values for where icorr uncoated and coated MS electrodes, respectively. From Fig. 10, it is clear that both cathodic and anodic reactions of corrosion are significantly suppressed after electrodeposition of PAni/PPy coating, and the introduction of PW12 in these double layers further retards the corrosion of mild steel, in particular for the anodic reaction. Inspection of Table 1 shows that for PAni/PPy coatings the incorporation of PW12 into the PAni layer results in a slight decrease in corrosion current density merely from 4.34 × 10−5 A cm−2 to 3.60 × 10−5 A cm−2 . For PAni/PPy-PW12 coating, however, the corrosion current density is reduced to 1.35 × 10−5 A cm−2 . That is to say, the corrosion resistance of the bilayer coatings can be significantly enhanced by the incorporation of PW12 into the outer PPy layer, which is in accordance with the result of OCP measurements. The similar conclusion can also be drawn from the calculations for protective efficiency as shown in Table 1. It should be noted that for the bilayer coatings the incorporations of PW12 give rise to a remarkable change in both cathodic and anodic Tafel slopes. In consideration of the strong oxidizability of PW12 in acidic media, the changes may be related to the redox process of PW12 ions inside the coating. The detail on the corrosion inhibition and mechanism of those coatings will be presented in the following investigation.

3.3.3. EIS measurements The protective performances of the studied PAni/PPy, PAniPW12 /PPy and PAni/PPy-PW12 coatings were evaluated in 0.1 M HCl solution by electrochemical impedance spectroscopy. Figs. 11–13 show Nyquist plots for those coatings immersed in 0.1 M HCl. The equivalent circuit R(QR)(QR) shown in Fig. 14 was used to consider all the processes involved in the electrical responses of these coatings [15,21]. In the circuit, Rs represents solution resistance of

ba (V dec−1 )

bc (V dec−1 )

PE (%)

0.258 0.046 0.110 0.053

0.188 0.387 0.265 0.106

– 94.99 95.85 98.44

Fig. 11. Nyquist plots of PAni/PPy electrode after immersion in 0.1 M HCl.

electrolyte, Rpore is pore resistance of coating, Rct corresponds to the charge-transfer resistance, and the constant phase elements CPEc and CPEdl represent the capacitance of the coating and the double layer capacitance, respectively. The CPE impedance is expressed as follows: ZCPE = Y0−1 (jω)

−n

(2)

where Y0 is a proportional factor and n has the meaning of a phase shift. For n = 0, CPE represents a resistance and for n = 1 a capacitance. In the equivalent circuit, the CPE instead of the ideal capacitance is used to define the inhomogeneity in the system [33,34]. From Figs. 11–13, all Nyquist diagrams show two depressed semicircles within the whole immersion. The semicircle at high frequency region is assigned to the process at polymer/electrolyte and the one at low frequency region corresponds to the response at

Fig. 12. Nyquist plots of PAni-PW12 /PPy electrode after immersion in 0.1 M HCl.

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Table 2 EIS parameters fitted by the equivalent circuits presented in Fig. 14. Electrodes

Time (h)

Cc (F cm−2 )

Rpore ( cm2 )

−4

Rct ( cm2 )

Rp ( cm2 )

PAni/PPy

6 12 36

184 268 504

1.45 × 10 3.40 × 10−4 6.46 × 10−4

310 427 512

494 695 1016

PAni-PW12 /PPy

6 12 36

60 128 482

3.33 × 10−5 7.15 × 10−5 5.80 × 10−4

450 604 680

510 732 1162

PAni/PPy-PW12

6 12 36

262 310 343

1.39 × 10−4 2.97 × 10−4 8.28 × 10−4

290 972 1352

552 1282 1695

metal/polymer interface [35]. The polarization resistance (Rp ) of polymer-coated electrode, reflecting the corrosion-resistant ability of those coatings, is considered to be equal to the sum of Rpore and Rct values [1,36]. The EIS parameters that we are interested in are listed in Table 2. The PW12 -doped coatings show higher Rct values than PW12 -free coating. The higher Rct values of PAniPW12 /PPy and Pani/PPy-PW12 can be explained by the effective barrier behavior of the polymer coatings, verifying that the incorporation of PW12 enhances the corrosion resistance of the conducting polymer coatings. In addition, the pore impedance of coating Rpore increases with the immersion time, indicating that the pores and/or defects may be blocked by the corrosion products, which makes diffusion of corrosive electrolyte difficult [15,37]. As seen from the Nyquist plots and Table 2, both total impedance and Rp of the corrosion process increase with immersion time in all cases of these double coatings. The increase in total impedance may be related to the auto-undoping properties of polymer in reduction processes [38,39]. It was reported that within the immersion of the conducting polymer-coated mild steel, the corrosion process could be described as follows: (1) Anodic dissolution of iron Fe − 2e → Fe2+ E ␪ = −0.44 VvsNHE

(3)

(2) Cathodic reduction of coatings that cause release of counter anions (i.e. C2 O4 2− ) (PAnix+ x/2C2 O4 2− )n + nxe− → (PAni)n + nx/2C2 O4 2−

(4)

(PPyy+ y/2C2 O4 2− )n + nye− → (PPy)n + ny/2C2 O4 2−

(5)

As the double coatings are gradually reduced, coating conductivity decreases and the barrier property of coatings increases. Moreover, the released C2 O4 2− anions favor maintenance of passive FeC2 O4 ·2H2 O layer on MS surface. Thus increase in total impedance and Rp with exposure time is related to both the reduction of the polymer coatings and healing of passive layers by the released C2 O4 2− anions [16]. It is worth to mention that compared to PAniPW12 /PPy, PAni/PPy-PW12 shows higher Rp value, especially for the long exposure time. After immersion of 36 h, for instance, the Rp values of PAni-PW12 /PPy and Pani/PPy-PW12 coatings reach 1162  cm2 and 1695  cm2 , respectively. Considering the strong oxidizability of PW12 in acidic media, it is reasonable to infer that Fe(II) species from anodic dissolution of iron would be rapidly oxidized to Fe(III) species by PW12 in the coating. Unlike sparsely soluble Fe(II)-C2 O4 complex, however, solubility of Fe(III)-C2 O4 compound in aqueous solution is pretty high [16]. Therefore, the presence of PW12 in the PAni-PW12 /PPy inner layer may hinder the maintenance of passive layer, reducing the corrosion resistance of PAni-PW12 /PPy. On the contrary, for PAni/PPy-PW12 in which PW12 ions are doped into the outer layer of the coating, it is difficult for PW12 to take part in the oxidation of Fe(II) species occurring at the MS substrate. On the other hand, SEM examination (Fig. 7) shows that PPy-PW12 film is more compact and denser than PAniPW12 , which may be in part responsible for the better protection performance of PAni/PPy-PW12 . 4. Conclusion

Fig. 13. Nyquist plots of PAni/PPy-PW12 electrode after immersion in 0.1 M HCl.

Fig. 14. Equivalent circuit for PAni/PPy, PAni-PW12 /PPy and PAni/PPy-PW12 electrodes.

The electrochemical depositions of the PAni/PPy, PAniPW12 /PPy and PAni-PPy-PW12 bilayer coatings on MS were performed using the cyclic voltammetry technique. The dopant PW12 modifies the electrodeposition and the morphology of PAni/PPy in aqueous oxalic acid. Their corrosion protection performances were investigated in 0.1 M HCl solution. The incorporation of PW12 in PAni/PPy enhances corrosion resistances of the coatings. In addition, PAni/PPy-PW12 exhibits better protection than PAni-PW12 /PPy for mild steel in 0.1 M HCl. The presence of PW12 in the inner layer of PAni/PPy may hinder the maintenance of passive layer, reducing the corrosion resistance of the bilayer. Moreover, the structure of PPy-PW12 film is more compact and denser than PAni-PW12 , which may also account for the stronger corrosion resistance of PAni/PPy-PW12 . Furthermore, it should be interesting to examine the influence of doping level on corrosion resistance of those coatings. This work would be performed in our future studies.

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