Comparative Study on the Electrodeposition and Corrosion

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Comparative study on electrodeposition and corrosion resistance of polypyrrole doped by phosphotungstate and benzalkonium chloride Jinqiu Xu, Yanqing Zhang, Yongming Tang, Hui Cang, and Wenheng Jing Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie502878a • Publication Date (Web): 20 Oct 2014 Downloaded from http://pubs.acs.org on November 8, 2014

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Industrial & Engineering Chemistry Research

Comparative study on electrodeposition and corrosion resistance of polypyrrole doped by phosphotungstate and benzalkonium chloride

Jinqiu Xua, Yanqing Zhanga, Yongming Tanga,*, Hui Cangb, Wenheng Jingc,d

a b

School of Science, Nanjing University of Technology, Nanjing 210009,PR China

College of chemical engineering and biological, Yancheng Institute of Technology, Yancheng 224051, PR China c

College of Materials Science and Engineering, Nanjing University of Technology, Nanjing 210009,PR China d

State Key Laboratory of Material-Oriented Chemical Engineering, Nanjing 210009,PR China

* Corresponding author. Tel.: +86 25 58139527; fax: +86 25 58139539. E-mail address: [email protected] (Y. M. Tang)

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ABSTRACT Polypyrrole (PPy) coating and polypyrrole composite coatings were successfully electrodeposited on mild steel in the electrolytes of oxalic acid in the absence and presence of dopants using cyclic voltammetry (CV). Two types of doping ions were employed: phosphotungstate (PW12O403-) and benzalkonium chloride (BAC). Corrosion-resistant performances of the coatings in 0.1 M HCl showed that PPy composite coatings in the presence of phosphotungstic acid and BAC exhibit more effective protection for iron in comparison with PPy formed in 0.3 M oxalic acid, and corrosion resistance of PPy-BAC is better than PPy-PW12 even though PPy-PW12 shows nobler open-circuit potential for longer period of time than PPy-BAC. Impedance measurements show that after immersion of 12 h in 0.1 M HCl, the polarization resistances of PPy-PW12 and PPy-BAC coatings reach 1860 and 3110 Ω cm2, respectively, compared to 640 Ω cm2 of PPy deposited from 0.3 M oxalic acid. Keywords: Mild steel; Conducting polymer; Polypyrrole; Doping; Corrosion resistance

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1. Introduction Among various corrosion inhibition strategies, conducting polymer coatings have shown great potential application due to their high electrical conductivity, good adhesion and non-toxic properties.1-3 There have been many reports about the electrosynthesis of polyaniline,4,5 polythiophene6,7 and polypyrrole8-10 on oxidizable metals. In recent years, several studies have demonstrated that polypyrrole (PPy) is a conductive polymer which is used as protective coatings to avoid corrosion for metals.11,12 PPy can be chemically or electrochemically synthesized through oxidation of the pyrrole (Py) monomer. Some researchers have demonstrated that PPy coatings can not only provide both anodic protection but also act as physical barrier to prevent metals from being attacked in corrosive media.13-15 The electrochemical synthesis of PPy conducting polymer on mild steel (MS) has attracted more and more attention. Oxalic acid is referred to as electrolyte for electrodeposition of PPy on MS in many studies.16-18 Beck et al. performed the electropolymerization of pyrrole on iron from oxalic medium and obtained a strongly adherent and smooth PPy coating.2 They also studied the protective effectiveness of PPy coatings in various media. Study of Iroh showed that both adhesion strength to the steel substrates and corrosion resistance of PPy were influenced by pH of aqueous oxalic acid and applied potential. The PPy coating deposited from oxalic acid solution with pH = 2.4 decreased the corrosion current of mild steel by approximately 3 orders of magnitude compared to the uncoated steel.19 Also, codeposition of PPy with other conducting polymers such as polyaniline to form bilayers has been reported to provide

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excellent protection for stainless steel.20 In addition, pyrrole-co-phenol coating reduced the corrosion rate of mild steel to 1.25 mm year-1 with respect to 7.2 mm year-1 of the uncoated steel.21 Several works reported that PPy films electrosynthesized in the presence of dopants exhibit good mechanical properties and high electrical conductivity due to the highly ordered polymeric chain attained with dopants. Hosseini and Sabouri demonstrated that adherent and homogeneous phosphate-doped PPy films can be achieved on mild steel in oxalic acid medium using cyclic voltammetry (CV) technique.22 They investigated anti-corrosion behavior of those coatings in 3.5% NaCl solutions and found that the incorporation of phosphate into PPy coatings significantly depresses the corrosion rate to 7.67 mpy with respect to 35.42 mpy of PPy deposited in oxalic acid solution without phosphate, indicating the PPy– phosphate coatings could provide much better protection than PPy. Su and Iroh reported electrodeposition of PPy composite films in aqueous oxalate solutions in the presence of triethylamine and allyamine in the galvanostatic modes.23 Smooth, uniform and strongly adherent coatings could be formed on the steel substrate under proper reaction parameters. Titanate nanotubes were also incorporated in PPy to prepare composite coatings, and it was found that the anticorrosive properties of the composite increased with respect to stainless steel by approximately 400 times and twice that obtained on the PPy coating without nanotitanate inclusions.24 In addition, it was reported that during electrodeposition of conducting coatings the incorporation of Keggin-type anions such as PMo12O403- and SiMo12O403- can not only enhance the 4


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corrosion resistance and self-healing capability of the coatings but stabilize the passive oxide film at the metal-polymer interface for the formation of conducting polymer.25-28 Therefore, dopants play an important role in enhancing the corrosion resistance of PPy for MS. In this study, we first electropolymerized homogeneous and adherent PPy on mild steel electrode from aqueous oxalic acid via cyclic voltammetry (CV) by a two-step procedure. Then we investigated the influence of phosphotungstate (PW12O403-) and benzalkonium chloride (BAC) on PPy electrosynthesis as well as their abilities to mitigate corrosion of mild steel. Phosphotungstate as heteropolyacid anion was wildly used as oxidation-type corrosion inhibitor in aqueous media whereas benzalkonium cation is typically adsorptive inhibitor. To our knowledge, no study focusing on the corrosion resistance of both PW12O403--doped and BAC-doped PPy coatings on mild steel was reported. Therefore, the study on the two types of dopants is expected to obtain significant information on fabricating highly corrosion-resisting PPy coatings. 2. Experimental All the chemicals were of analytical grade, and all the solutions were prepared by using ultrapure water. Phosphotungstic acid (PW12) and benzalkonium chloride (BAC) were used as received. Pyrrole monomer was distilled under nitrogen and stored at 4 °C prior to use. All the electrochemical studies were carried out on a Vertex electrochemical workstation (Ivium, Netherlands). The electrochemical cell was a three-electrode cell where the auxiliary electrode was a platinum sheet with the area of 3.10 cm2 and a saturated calomel electrode (SCE) was used as the reference. Mild 5



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steel containing 0.17 wt.% C, 0.20 wt.% Si, 0.37 wt.% Mn, 0.03 wt.% S, 0.01 wt.% P, and balance iron was used as the work electrode. The working area is 0.785 cm2 while the rest of electrode was embedded in a thick polyester block. Prior to electrosynthesis, the electrode was abraded with SiC paper (up to 2000 grit), cleaned with ethanol and washed with ultrapure water, and finally dried at room temperature. The volume of the electrolytes for electrodeposition is 100 ml. To minimize the IR drop between electrolyte and working electrode, the tip of Lugin capillary was set at a distance of approximate 1 mm from the surface of the working electrode. The PPy, PPy-PW12 and PPy-BAC composite films were electrochemically synthesized by using cyclic voltammetry technique in 0.3 M oxalic acid containing 0.1 M pyrrole. The potential range of the first scan was from -0.5 to 1.0 V and the subsequent potential scans were performed at between -0.5 and 0.9 V. After 15 scanning circles, polymer-coated electrodes were removed from the polymerization medium and rinsed with deionized water before being dried in air. Morphology of these films were examined by field emission scanning electron microscopy (FESEM, S-4800) equipped with a QUANTAX 400 Energy Dispersive X-ray (EDX) detector to analyze the compositions. The protective performance of these coatings was evaluated by open circuit potential (OCP), Tafel polarization and electrochemical impedance spectroscopy (EIS) in 0.1 M HCl solution. Before Tafel, the bare and polymer-coated MS electrodes were held in the test solution until a steady-state open circuit potential. Potentiodynamic polarization curves were recorded at a sweeping rate of 0.5 mV s-1. EIS was performed in the frequency range of 105- 0.01 Hz with an amplitude of 10 6


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mV at the EOCP. 3. Results and discussion 3.1. Electrochemical synthesis of PPy , PPy-PW12 and PPy-BAC on mild steel

Figure 1. Cyclic voltammograms of MS in 0.3 M oxalic acid + 0.1 M Py. Scan rate: first scan, 4 mV s-1; subsequent scans, 20 mV s-1.

PPy was electroploymerized on steel in 0.3 M oxalic acid containing 0.1 M pyrrole by cyclic oxidation. It was suggested by Otero et al. that with high anodic potential in aqueous solution, PPy oligomers may suffer irreversible opening of a ring due to a nucleophilic attack of nucleophiles such as H2O molecules.29 Based on that, a two-step procedure was applied to electrodeposition of PPy in this study. A single cycle was first carried out in a potential range from -0.5 to 1.0 V at 4 mV s-1 to form an inert layer and start the polymerization of Py on MS, and then new reverse potential of 0.9 V was applied during the next 14 successive scans at 20 mV s-1. Figure 1 shows the cyclic voltammogram recorded during the electrodeposition of PPy in 0.3 M oxalic acid + 0.1 M monomer. During the first scan, the peak potential 7



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(Ep) for the activation-passivation transition of MS is observed at -0.4 V where a sparely soluble film is formed on the electrode surface and the dissolution of iron is inhibited.30 O. Iroh et al.31 proposed that the following two reactions occurred during the passivation: Fe - 2e → Fe2+

E = -0.44 V vs. NHE

(1)

Fe2+ + C2O42- + 2H2O → FeC2O4· 2H2O

(2)

Iron (II) oxalate is insoluble and adhered to the electrode surface, preventing further dissolution of iron. As the potential reaches around 0.6 V, the oxidation of pyrrole monomer occurs and the oxidation current increases sharply with the appearance of a well-defined peak at around 0.9 V. At the same time, black PPy film can be observed on the surface of MS. During the reverse scan, a broad cathodic current peak appears at around -0.2 V, which is highly probably due to the reduction of PPy coatings.22 The reduction of PPy results in the extraction of counter anions (oxalate ions) that have been electrochemically incorporated in PPy during electrodeposition. During the subsequently successive scans, the oxidation process of pyrrole cannot be observed as a separate peak and the current density at 0.9 V gradually decreases with the increase in scanning cycles. That is to say, the oxidation of pyrrole monomers is reduced with scanning circles, which is attributed to the gradual decrease in conductivity resulting from the outgrowth of PPy film on the surface.17,32 In addition, Figure 1 shows a shoulder at 0.3 - 0.5 V and a reduction peak at around -0.2 V on the CV curves. These peaks are associated with exchange of oxalate anions.33,34 Oxidation of PPy corresponding to the anodic peak results in the incorporation of oxalate anion while 8


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reduction of PPy corresponding to the cathodic one results in the extraction of oxalate anion.34 It is worth to note that the relatively stable oxidation and reduction currents of PPy at 0.3 - 0.5 V and -0.2 V are reached after 10 successive scans, indicating that the film electroneutrality is reached by exchange of oxalate anion.33

Figure 2. Cyclic voltammograms of mild steel in 0.3 M oxalic acid + 0.1 M Py + 5 g L-1 PW12. Scan rate: first scan, 4 mV s-1; subsequent scans, 20 mV s-1.

Figure 2 depicts the electropolymerization of PPy in the presence of 5 g L-1 phosphotungstate acid. In the presence of PW12, the passivation current density of MS (6.5 mA cm-2) at around – 0.4 V is similar to that in the oxalic acid solution without additive (6.6 mA cm-2), impliying that PW12 has hardly any effect on the formation of Fe-oxalic complex film on mild steel. However, the oxidation current of pyrrole at 0.9 V is significantly reduced from 12.2 mA cm-2 to 9.2 mA cm-2 by the introduction of PW12, indicating the inhibitive effect of PW12 on the oxidation of pyrrole. Different from the electrodeposition in the oxalic acid solution without additive, furthermore, introduction of PW12 apparently modifies the CV curves of Py electroploymerzation. 9



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It can be observed from Figure 2 that in the presence of PW12 the oxidation current density of monomer at 0.9 V remains almost unchanged (ca. 17 mA cm-2) with scanning circle. In the oxalic acid solution without additive, on the contrary, the oxidation current density at 0.9 V decreases from 18 mA cm-2 of the second scan to 7 mA cm-2 of the fifteenth scan. The result indicates that the doping of PW12 improves the conductivity of PPy film. Compared to the PPy formed in 0.3 M oxalic acid, additionally, the doping of PW12 results in the increase of both oxidation (0.3 - 0.5 V) and reduction (-0.2 V) currents with scanning circle, indicating that PW12 enhances the exchange of anions such as oxalate and phosphotungstate.

Figure 3. Cyclic voltammograms of mild steel in 0.3 M oxalic acid + 0.1 M Py + 5 g L-1 BAC. Scan rate: first scan, 4 mV s-1; subsequent scans, 20 mV s-1.

Electropolymerization of the cation-doped PPy-BAC in 0.3 M oxalic acid solution with 5 g L-1 BAC was carried out by sweeping the potential region between -0.5 V and 0.9 V as shown in Figure 3. During the first scan, the active activation-passivation peak of MS (0.7 mA cm-2) at around -0.35 V is much smaller 10


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than those in both Figures 1 and 2, revealing that BAC can effectively inhibit the dissolution of iron and promote the passivation of iron. In the study on the effect of phosphate on the dissolution of iron, MacDiarmid et al. proposed that the phosphate reacts with iron to form a FeHPO4 layer, thus inhibiting dissolution of iron.35 Similarly, the quaternary ammonium cations of BAC may absorb on the surface of iron and then take part in the formation of the insoluble Fe-oxalate complex film, promoting the passivation of iron and inhibiting dissolution of MS. From the subsequently successive scans, neither distinct shoulder at 0.3 - 0.5 V nor reduction peaks at around -0.2 V can be observed. This means that the introduction of BAC remarkably restricts the exchange of anions during electrodeposition of PPy. For both PPy-PW12 and PPy-BAC, adherent and homogenous polymer coatings are obtained on the mild steel surface after electropolymerization. 3.2. FESEM and EDX analysis The FESEM micrographs of the PPy and PPy composite electrodeposited on MS substrate are shown in Figure 4 where the magnifications of the left and right images are 20 μm and 5 μm, respectively. It should be noted that all of the coatings for FESEM examinations were deposited under the same conditions as shown in Figures 1, 2 and 3. As observed, the surface morphology of PPy coating deposited from 0.3 M oxalic acid (Figure 4a) is characterized by a globular texture constituted by spherical grains with the diameter of ca. 3 μm (Figure 4b), which is in agreement with the previous reports.25,36,37 Furthermore, it appears that, unlike in the inner layer of PPy, in the outer layer those aggregates of spherical grains do not form compact and 11



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coherent film, suggesting the poor corrosion resistance of PPy deposited in 0.3 M oxalic acid. Figure 4c reveals that the introduction of PW12 into the deposition solution results in a cauliflower-like structure of PPy-PW12 coating consisting of micro-spherical grains. It has been reported that the cauliflower-like structure is associated with the dopant intercalation difficulty in the disordered polymeric chain.38,39 Moreover, Figure 4c shows that the introduction of PW12 apparently make the PPy film more uniform than without the additive. Inspection of Figure 4d reveals that the size of those micro-spherical grains (ca. 0.5 μm) is smaller than that in the case of PPy without the additive. Figure 4e shows the formation of compact and homogeneous PPy-BAC film in the presence of BAC. Under higher magnification (Figure 4f) it is clear that the PPy aggregates, constituted by merging many spherical grains with the diameter of 0.5 -1μm, pack together so closely that it is difficult to distinguish the boundaries among those PPy aggregates. These results imply PPy-BAC has highly protective performance as will be shown in the following EIS measurements. EDX analysis confirms the successful incorporations of PstW12 and BAC into PPy matrix, as shown in Figure 5. Carbon, nitrogen and oxygen as well as Fe from substrates were detected in the PPy film formed in 0.3 M oxalic acid (Figure 5a). Oxygen element is from the incorporation of oxalate ions during electropolymerization. In addition, oxidation of PPy under relatively positive potential may be partly responsible for the existence of oxygen. In the PW12-doped sample, the presence of both tungsten and phosphor confirms the incorporation of PW12 (Figure 5b). Figure 5c reveals the existence of Cl in the PPy-BAC film, demonstrating the 12


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incorporation of BAC.

Figure 4. FESEM micrographs of (a, b) PPy from 0.3 M oxalic acid + 0.1 M Py, (c, d) PPy-PW12 from 0.3 M oxalic acid + 0.1 M Py + 5 g L-1 PW12, (e, f) PPy-BAC from 0.3 M oxalic acid + 0.1 M Py + 5 g L-1 BAC. Left: magnification in 20 μm, right: magnification in 5 μm. All of the coatings were electrodeposited by CV with 15 scanning circles.

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Figure 5. EDX of (a) PPy , (b) PPy-PW12 and (c) PPy-BAC.

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3.3. Evaluation of corrosion performance 3.3.1. Measurements of OCP

Figure 6. OCP monitoring for ■ MS, ◆ MS/PPy, ▲ MS/PPy-PW12 and ▼ MS/PPy-BAC electrodes in 0.1 M HCl solution.

It must be noted that all of the coatings used in corrosion evaluation were electrodeposited by CV with 15 scanning circles. The OCP was measured for PPy and PPy composite coatings exposed to 0.1 M HCl solution as shown in Figure 6. At the beginning of exposure, EOCP values for MS/PPy, MS/PPy-PW12, MS/PPy-BAC coatings are 243, 232 and 188 mV, respectively, more positive than that of bare MS at the outset (-503 mV). The nobler EOCP values demonstrate that PPy and PPy composite coatings have protective effects for iron in the corrosive solution. After immersion of 35 min, the EOCP for MS/PPy begins to decrease dramatically until around 100 mV, and then a relatively gentle decrease can be observed within about 130 min. After immersion of 165 min, EOCP decreases drastically again reaching the corrosion potential of uncoated MS. This sharp decrease in OCP reveals PPy coating 15



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is permeable enough for uptake of corrosive solution. However, more ennobling effects are observed in the PPy-PW12 and PPy-BAC coatings in comparison to PPy coating during the general immersion time. Especially for the PPy-PW12, the EOCP curve exhibits a plateau in the whole immersion. This is associated with the initiating passivation performance of PW12 for MS due to the strong oxidizability of PW12 in acid media. 3.3.2. Tafel polarization

Figure 7. Potentiodynamic polarization of ■ blank, ◆ PPy, ▲ PPy-PW12 and ▼ PPy-BAC electrodes in 0.1 M HCl solution. Table 1 Potentiodynamic polarization parameters of MS, MS/PPy, MS/PPy-PW12, MS/PPy-BAC electrodes in 0.1 M HCl solution. Electrodes

Ecorr (V)

icorr (A cm-2)

ba (V dec-1)

bc (V dec-1)

MS

-0.572

8.67×10

-4

0.258

0.188

MS/PPy

-0.556

5.33×10

-5

0.140

0.333

93.85

MS/PPy-PW12

-0.483

1.43×10

-5

0.362

0.240

98.35

MS/PPy-BAC

-0.475

1.70×10

-5

0.255

0.281

98.04

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PE (%)

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The potentiodynamic polarization curves recorded in 0.1 M HCl for PPy, PPyPW12 and PPy-BAC coated steels are given in Figure 7. Electrochemical parameters including corrosion potential (Ecorr), corrosion current density (icorr) and Tafel slopes (ba and bc) are listed in Table 1. The protection efficiency (PE) was calculated as follows:40 = PE %

0 icorr − icorr ×100% 0 icorr

(3)

Where i0corr and icorr are the corrosion current density values for uncoated and coated MS electrodes, respectively. As can be seen from Figure 7 and Table 1, the corrosion potentials (Ecorr) of coated electrodes are shifted towards positive compared to that of the uncoated MS electrode. When MS was coated by PPy or PPy composite films, both cathodic and anodic reactions are depressed, especially for the anodic one. The icorr values decreased from 8.67×10-4 A cm-2 of the uncoated steel to 1.43×10-5 A cm-2 and 1.70×10-5 A cm-2 of PPy-PW12 and PPy-BAC coated steels, respectively. In further comparison with the PPy film formed in 0.3 M oxalic acid, the doped PPy films exhibit lower corrosion currents, indicating that the incorporations of both BAC and PW12 enhance the corrosion resistance of PPy films. It should be noted that even though PPy-PW12 coated MS remarkably exhibits nobler OCP than PPy-BAC (Figure 6), only very slight difference in protection efficiency between them can be obtained (Table 1). This result suggests that for corrosion-resistant dopants the performance of initiating passivation cannot be directly connected with the increase in the protection capability of PPy-coatings. It is worth to note that the current measured by polarization curves contains both corrosion current of mild steel and current 17



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associated with redox property of PPy, and we can by no means distinguish them at present. This means Tafel polarization is not an ideal method to examine the corrosion resistance of conducting polymers. In the present study, in fact, the incorporation of BAC results in higher protection efficiency than that of PW12 in a long immersion time. The fact will be presented and discussed in more detail in the following EIS measurements. 3.3.3. EIS

Figure 8. Nyquist plots of uncoated MS electrode for 0.5 (■) and 12 h (●) in 0.1 M HCl solution.

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Figure 9. Equvivalent circuit models (a) for bare MS electrode, (b) and (c) for MS/PPy, MS/PPy-PW12 and MS/PPy-BAC electrodes.

EIS measurements were carried out to evaluate the protection of the polymer coatings against corrosion of MS in 0.1 M HCl. Figure 8 shows the Nyquist plots of uncoated electrode obtained in HCl for 0.5 h and 12 h. The obtained Nyquist plots have the shape of a slightly depressed semicircle. The diameter of the semicircle decreases with time. The equivalent circuit given in Figure 9a is used to describe this kind of uncoated metal/solution system.41 In this circuit, Rs is the solution resistance of electrolyte, Rct is the charge transfer resistance, and CPEdl is the electrical double layer capacitance formed at metal/solution interface. Here, to describe the non-ideal behavior of capacitance, constant phase element (CPE) is used instead of the capacitance (C) to define the inhomogeneities in the system.42,43

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Figure 10. Nyquist plots of MS/PPy electrode after 3 h (▲) and 12 h (●) in 0.1 M HCl. Inset: Nyquist plot of MS/PPy electrode after 0.5 h (■).

Figure 11. Nyquist plots of MS/PPy-PW12 electrode after 12 h (▲) and 24 h (●) in 0.1 M HCl. Inset: Nyquist plot of MS/PPy-PW12 electrode after 3 h (■).

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Figure 12. Nyquist plots of MS/PPy-BAC electrode after 12 h (▲) and 24 h (●) in 0.1 M HCl. Inset: Nyquist plot of MS/PPy-BAC electrode after 3 h (■).

Figures 10, 11 and 12 show Nyquist plots for PPy, PPy-PW12, PPy-BAC, respectively, after different exposure times. At the beginning of immersion, the Nyquist plots of all PPy coatings exhibit depressed semi-circles at high frequencies followed by almost a straight line extended to lower frequencies region. The type of response was qualitatively interpreted as a result of good barrier properties of the PPy coatings on oxidizable substrates.44,45 The semi-circles at high frequencies result from the resistances of the coatings themselves and of the passive layer, and the straight line is attributed to a fine length Warburg behavior which indicates the resistance of coating against the diffusion of corrosive species.46 For PPy-PW12 and PPy-BAC coatings, additionally, semi-circles at middle frequencies can be observed. This response corresponds to the electrochemical process of metal/solution interface resulting from the water uptake through the pores of polymer coatings during the initial immersion of 3 h. After 12 h or more immersed in 0.1 M HCl, all PPy coatings 21



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merely show two time-constants on their Nyquist plots, corresponding to barrier property of coating and corrosion process of metal/solution interface, respectively. At the same time, the coatings capacitance (Cc, see Table 2) increases with immersion time due to the gradual uptake of solution into coatings. Furthermore, the diffusion response disappears, indicating that the coatings are already saturated by water after immersion of 12 h or more. We carried out the fitting of the plots to more directly understand the corrosion-resistant effect of PPy coatings. The Nyquist plots of PPy composite coating immersed in 0.1 M HCl for 3h were fitted by the circuit of Figure 9b and other ones by the circuits of Figure 9c. In these circuits, CPEc is the constant phase element connected with the capacitance of coating, and W represents Warburg impedance, and other elements have their previous meanings. Those parameters we are interested in are presented in Table 2. 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.42,47,48 As seen from the Nyquist plots and Table 2, both total impedance and Rp for whether PPy or PPy composite coatings are relatively high with respect to those for uncoated mild steel due to the effective barrier behavior of the polymer films. Moreover, the comparison between PPy and PPy composite coatings reveals that total impedances are higher for PPy composite coatings, verifying that the incorporation of PW12 and BAC enhances the corrosion resistance of PPy coatings. From Figures 10 – 12 and Table 2, it is clear that both total impedance and Rp of the corrosion process increase with immersion time in all cases of PPy coatings. The increase in total impedance and Rp can be 22


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explained by the auto-undoping properties of polymer in reduction processes.34,49 It was reported that during the immersion of the PPy-coated mild steels, the corrosion process could be described as follows: (1) Anodic dissolution of iron referring to Equation (1); (2) Cathodic reduction of PPy and PPy composite coatings that cause release of counter anions (i.e. C2O42-) [PPyy+(C2O42-)y]n + nye- → (PPy)n + nyC2O42-

(4)

As the PPy coatings are gradually reduced, coating conductivity decreases and the barrier properties of coatings increase. Moreover, the released C2O42- anions favor maintenance of passive FeC2O4·2H2O layer formed on MS before electrodeposition. Thus increase in total impedance and Rp with exposure time is related with the increase amount of reduced form of the polymer and healing of passive layers by the released C2O42- anions. It is worth to mention that compared to PPy-PW12, PPy-BAC shows higher Rp value, especially in the long time immersion. 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 coatings. Unlike sparsely soluble Fe(II)-C2O4 complex, however, solubility of Fe(III)-C2O4 compound in aqueous solution is pretty high. Therefore, the presence of PW12 hinders the maintenance of passive layer, reducing the corrosion resistance of PPy-PW12. On the other hand, SEM examination (Figure 4) shows that PPy-BAC film is more compact and denser than PPy-PW12, which may be in part responsible for the better protection performance of PPy-BAC. 23



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Table 2 EIS parameters fitted by the equivalent circuits presented in Figure 9. Rpore (Ω cm2)

Cc (F cm-2)

Rct (Ω cm2)

Electrodes

Time (h)

MS

0.5

21.5

12

7.38

MS/PPy

MS/PPy-PW12

MS/PPy-BAC

Rp (Ω cm2)

0.5

5.8

2.71×10-4

7.5

13.3

3

280

1.95×10-4

240

520

12

300

2.00×10-3

340

640

3

5.6

3.92×10-5

34

39.6

12

1020

5.31×10-4

840

1860

24

1460

8.46×10-4

780

2240

3

108

5.10×10-5

370

478

12

960

5.17×10-4

2150

3110

24

1490

6.00×10-4

2950

4440

4. Conclusion Both phosphotungstate (PW12) and benzalkonium chloride modified the electrodeposition of PPy in aqueous oxalic acid. Introduction of phosphotungstate enhances the exchange of oxalate anion of PPy whereas BAC exhibits a restriction effect. Corrosion resistance of PPy-coated mild steel is greatly improved by the incorporations of PW12 and BAC. After immersion of 12 h in 0.1 M HCl, the polarization resistances of PPy-PW12 and PPy-BAC coatings are 1860 and 3110 Ω cm2, respectively, with respect to 640 Ω cm2 of PPy deposited from 0.3 M oxalic acid without any additive. Both composite coatings show high protection efficiency of over 98%. Comparison between PPy-PW12 and PPy-BAC shows that the latter exhibits better corrosion resistance than the former in a long time test in 0.1 M HCl. Due to its strong oxidizability in acidic media, PW12 may hinder the maintenance of 24


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the insoluble Fe (II)-oxalic passive film. In addition, PPy coating in the presence of BAC is visually more compact and more uniform than that in the presence of PW12, which can account for the stronger corrosion resistance of PPy-BAC. Generally, we should pay more attention on the investigation on quaternary ammonium compounds as PPy dopant since they have a very positive effect on corrosion resistance of PPy and have hardly been investigated. In addition, it will be significant to quantify the doping levels of dopants and examine the effect of the doping level on corrosion resistance of PPy composite coatings. These works would be performed in our future studies. Acknowledgements The authors are thankful to financial support from State Key Laboratory of Material-Oriented Chemical Engineering (KL12-10), National Natural Science Foundation (NSFC 21303155, NSFC 21176116) and Natural Science Foundation of Jiangsu Province (BK20130427, BK 2011800).

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Comparative Study on the Electrodeposition and Corrosion

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