Tunable electrochemical performance of polyaniline coating via facile ion exchanges

Progress in Organic Coatings 136 (2019) 105309

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Tunable electrochemical performance of polyaniline coating via facile ion exchanges

T

Amit Nautiyal, Jonathan E. Cook, Xinyu Zhang

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Department of Chemical Engineering, Auburn University, AL, 36849, USA

ARTICLE INFO

ABSTRACT

Keywords: Polyaniline Ion exchange Redox sites Accessibility coefficient Corrosion protection

Polyaniline (PAni) was electropolymerized on stainless steel (SS) surface using sulfates (sulfuric acid, SA and ptoluene sulfonic acid, p-TSA) as doping anions. These dopant anions were electrochemically exchanged to phosphates (phosphoric acid, PhAc and phytic acid, PA) to enhance the electrochemical performance of the polymer. The anion exchange was studied by studying the number of redox sites accessible in the polymer during the exchange. The dopants used for polyaniline was substantially affecting the anticorrosion performance of the coating depending on type of dopants, size and their alignment in polymeric chain. The protection efficiency was significantly improved when sulfates were exchanged to phosphates due to their inhibiting properties towards corrosion for stainless steel.

1. Introduction Conducting polymers are the most widely studied materials in plentiful applications due to their facile synthesis, environmental friendliness, no toxicity etc. [1–5]. Many researcher have explored advantages of these polymers in the areas of not limited to energy storage [6], catalyst [7], sensors [8] and membrane filters [9] etc. Polyaniline (PAni) is one these materials that has been used for more than a decade in variety of applications due to its reversible redox behavior and acid/base doping/de-doping chemistry. The performance of the polymer is influenced by various factors such as type and size of the dopant [10,11], mobility of ions available [12], porosity [13], interfacial surface area, thickness [14]. It is a unique conducting polymer that have three interchangeable oxidation states: leucoemeraldine (L), emeraldine (E) and pernigraniline (P). The emeraldine state is most useful for various applications due to its easy transformation from nonconducting, known as emeraldine base (EB) to conducting form, known as emeraldine salt (ES) by treating it with protonic acids. The unique property of reversible change between conducting and insulating form is known as doping and de-doping process. The transformation of these three different oxidation states (L, E, P) of the polymer can also be observed by applying potential in a cyclic manner in strong acidic electrolyte. During positive potential scan, the first transformation occurs from LH (protonated leucoemeraldine) to EH (protonated emeraldine) that involves release of proton and anion from polymer chain with no insertion of anion. This is followed by conversion of EH to P,

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which also involves release of protons and anions [15]. During negative potential scan, the reduction states were transformed by inserting anions and protons from the electrolyte, thus exchanging the anions. The level of doping is affected by pH of the solution and potential applied which depends on the degree of protonation. However, anion movement is of equal importance as degree of protonation, when polyaniline film is used in electrolytes. This feature leads to the changes in redox behavior that alters the electrical and optical properties of the polymer [16–18]. This property is used to develop conducting polymer-based membranes used in actuators, sensors, energy storage devices etc. [19,20]. Polyaniline has been explored in numerous application such as improved methane production from wastewater [21], Cr (VI) reduction and adsorption [22,23], magnetoresistor [24]. Its performance can be enhanced by making composite or hybrid with carbon related material. For instance, Liu et al. demonstrated the rate capability of carbon aerogel-PAni hybrid material was significantly improved by increasing the number of pores [25]. The performance of each device is influenced by the number of ions that can access the redox sites in the polymer. The polymer-electrolyte interface restricts these ions that can be inserted and removed from the polymer which affects the electrochemical storage capacity. This storage capacity can be studied by observing the accessibility of the redox sites at the polymer-electrolyte interface by electrochemically exchanging the anions from the polymer chains. This study showed the facile electrochemical anion exchange from electropolymerized polyaniline coating.

Corresponding author. E-mail address: [email protected] (X. Zhang).

https://doi.org/10.1016/j.porgcoat.2019.105309 Received 19 April 2019; Received in revised form 26 June 2019; Accepted 3 September 2019 Available online 08 September 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.

Progress in Organic Coatings 136 (2019) 105309

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Fig. 1. Chemical structure of doping agent for polyaniline used in this study.

Conventionally, polymers have been used as an anticorrosive coating to protect the metal from degradation by creating a barrier between metal surface and surrounding environment. This prevents the ingress of corrosive ions through the coating system. For instance, uniform poly (methylhydrosiloxane) coating was prepared for aluminum corrosion protection [26,27]. The corrosion rate of prepared coating was lower by three orders of magnitude compared to bare metal due to its hydrophobic property. Many strategies has been adopted for corrosion protection such as developing a water rust converter that exhibits rust removal properties as well [28], waterproof conductive paper based sensor that possess self-cleaning and anticorrosion property [29], or simply using anticorrosive agent in conventional epoxy coatings [30]. However, common practice used to protect metals like stainless steel (SS) includes conversion films or to treat it with chromium to form chromate (Cr2O3) film on the surface. This chromate film prevents the metal surface from degradation in aggressive medium due to its high redox potential (+0.42 V) [31]. However, chromate is toxic, hazardous to living being and is considered as carcinogenic agent [32]. Therefore, other materials must replace the use of toxic chromates to prevent its harmful effect. Conducting polymers are believed to be the suitable replacement materials for corrosion protection due to their environmentally friendliness, ease in synthesis and higher redox potential (+0.3-1.2 V) than chromium. Polyaniline (PAni), widely used conducting polymer, showed great potential for corrosion protection of metals and alloys that was first reported to prevent stainless steel degradation [33]. Since then numerous work has been reported on corrosion protection performance of PAni on various metals such as Al, Cu, etc. [34–36]. There are several ways to use polyaniline for corrosion protection such as blended with paints, used as primer for conventional coatings or uses as inhibitors. However, direct deposition of polyaniline on metal surface yields in uniform and compact coating that provides barrier, anodic and inhibition protection in single layer. Besides, electropolymerization of polyaniline and its derivatives were widely studied on variety of surfaces such as steel [37], carbon electrodes [38], aluminum alloys [39] that can be used in numerous applications not limited to microbial fuel cell [40], supercapacitors [41] and biosensors [42]. The electropolymerization method is considered as an efficient way to synthesize polyaniline that results in superhydrophobic coating that can be used as anticorrosive coating [43]. Additionally, electropolymerization is an economical process that has minimum processability issues and offers control over thickness of the coating by controlling potential or current density. The main aim of this paper is to study the accessibility of redox sites in polyaniline by studying anion exchange and its application in corrosion protection of metals like stainless steel. The dopant anions used were exchanged from sulfates (SA and p-TSA) to phosphates (PhAc, PA) because it is easy to electropolymerize polyaniline from sulfates as compared to phosphates [44]. Besides, phosphates have mild corrosion inhibitive property towards stainless steel.

was obtained from Alfa Aesar, USA. Sulfuric acid (SA) was purchased from Anachemia. Phosphoric acid (PhAc) was purchased from EM Science, USA, Phytic acid (PA) was obtained from Tokyo Chemical Industry. All chemicals were used without further treatment or purification. Stainless steel sheet (SS316 L, 1 mm thick) was used for electrodeposition surface (Fig. 1).

2. Experimental section

The anion exchange of produced polyaniline coating was carried using cyclic voltammetry (CV) by sweeping potential between -0.3 to 1 V at 50 mV·s−1. The produced PAni coating (PAni/Sf) was rinsed with DI water and dried in air. The dried PAni/Sf coating was transferred to phosphates solution (PhAc or PA) and impedance spectra was obtained at open circuit potential. Then cyclic voltammogram was recorded for

2.2. Sample preparation The steel samples of 1.5 cm × 0.5 cm were cut from 1 mm thick sheet and cold mounted into acrylic resin so that only one surface was exposed. The embedded sample was then polished by emery papers of different grit size starting from 800, 1000, and 1200. It was followed by ultrasonic cleaning in deionized (DI) water for 1 min (DI water with ∼18.2 Ω·cm resistivity). 2.3. Electropolymerization Polyaniline coating was electrochemically deposited using electrolyte containing 0.1 M aniline and 0.5 M dopant (SA, p-TSA). The electrochemical cell set-up used was a single compartment with three-electrode system, where SS (1.5 cm x 0.5 cm) was used as a working electrode, Pt as a counter electrode, Silver/Silver chloride/KCl (3 M) (Ag/Ag+, E° = +205 mV vs. SHE) was used as reference electrode. Electrodeposition was carried out on CH Instruments (CHI 760D) potentiostat using Electrochemical Analyzer software (version 17.06) by applying constant potential of 1 V for 5 min when dopants used were SA, p-TSA. For comparison PhAc and PA doped PAni was deposited using constant potential of 1.2 V for 60 min for phytic acid and cyclic voltammetry (CV) for phosphoric acid (swept between -0.4 to 1.3 V @10mV·s−1 for 20cycles). 2.4. Characterization The surface morphology of the coating was recorded under Scanning Electron Microscope (SEM) on JEOL JSM-7000 F at 20 keV with 10 mm working distance. Anion exchange was studied by Electrochemical Impedance Spectroscopy (EIS) and confirmed by Fourier Transform Infrared (FTIR) analysis that was conducted on a Thermo Nicolet 6700 FTIR. The corrosion tests were performed in 3.5% NaCl with linear sweep voltammetry (LSV) at a scan rate of 1 mV·s−1, scanning ± 150 mV from the open circuit potential (OCP). The corrosion test was performed on both as-produced and anion exchanged PAni coating. The EIS was performed at open circuit potential (0.4 V) in the respective electrolyte solution (mentioned wherever plotted). 2.5. Anion exchange

2.1. Materials Aniline (Mw=93.13 g·mol−1, density = 1.02 g·cm−3), platinum (Pt) gauze (100 mesh, 99.9% metal basis), p-toluenesulfonic acid (p-TSA) 2

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known to occur in three stages [46]: a) passivation and adsorption of monomer, b) monomer oxidation along with nucleation of the polymer and c) polymer growth over the surface. These three stages were represented in chronoamperometric curves of electropolymerization of polyaniline on stainless steel surface when doped with sulfuric acid and p-TSA, respectively (Fig. 2). There is a sharp drop in current density in stage I that corresponds to the passivation of the surface. This passivation is necessary prior to electrodeposition of polyaniline on steel surface as metal dissolution starts before aniline oxidation occurs at higher potential [47,48]. The stages II start with increase in current due to aniline oxidation and nucleation process occurring simultaneously. This gives us green color coating over the steel surface. This is followed by stage III where polymer film grows on the steel surface. The growth is more uniform when p-TSA was used as a dopant than when sulfuric acid was used. This uniform growth of polyaniline looks compact in SEM images (Fig. 8). Fig. 2. Chronoamperometric curves for electropolymerization of polyaniline coating on stainless steel using dopants: a) SA and b) p-TSA.

3.2. Anion exchange It is well known that redox behavior of polyaniline can be studied electrochemically by cyclic voltammetry technique. The typical cyclic voltammogram curve of PAni includes the oxidation and reduction peaks that explains the change in oxidation states of polyaniline in acidic electrolyte. In Fig. 3, the curve shows the behavior of polyaniline in phytic acid (Fig. 3a & c) and phosphoric acid (Fig. 3b & d) solution for anion exchange. During first cycle, there is an increase in current density reaches its maxima that shows the transformation of emeraldine to pernigraniline state of polyaniline (visually observed the change in color of the coating from green to dark blue). This indicates the release of anions and protons from the polymer chain leaving the polymer dedoped and leaves a vacant site in the coating. The peak current density decreases with increase in number of cycles due to degradation of polyaniline on continuing exchange of anion. Moreover, the oxidation peak potential shift positive with increase in number of scans indicates

20cycles for anion exchange followed by EIS. This procedure was repeated for another 20cycles and EIS was obtained. The full exchange cycle can be understood as: EIS-CV-EIS-CV-EIS. The anion exchanged PAni coating was then designated as PAni/Sf-Ph (Sf: sulfuric or p-toluene sulfonic acid and Ph: phosphoric or phytic acid). The size of anions of dopants used are arranged according to their ionic radii: phytic acid anion > tosylate (TsO-) > hydrogen sulfate, HSO4- (0.221 ± 0.019 nm) > dihydrogen phosphate, H2PO4- (0.213 ± 0.019 nm) [45]. 3. Results 3.1. Electropolymerization The electrodeposition of a conducting polymer on metal surface was

Fig. 3. Cyclic voltammogram of polyaniline on stainless steel for anion exchange from: a) p-TSA to PA, b) p-TSA to PhAc, c) SA to PA and d) SA to PhAc in their respective electrolyte solution. 3

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chains. This makes doping efficiency variable for different anions, hence, different cyclic voltammogram for PAni/pTSA and PAni/SA. In Fig. 3c & d, the PAni/SA coating shows similar characteristics peaks in both phytic and phosphoric acid, respectively, like that of PAni/pTSA. In many reports the dopant anion for sulfuric acid in PAni chain was proved to be HSO4− instead of SO42- [52,53]. On comparing phosphoric acid and phytic acid anion, it was observed that peak current density at 0.6 V are highest in first cycle. This explains the easy release of hydrogen sulfate from the polymer in phosphate solutions. The reduction peaks at 0-0.1 V show the insertion of phosphates anion with intensity decreasing with increase in number of scans. In Fig. 3c, the reduction peaks diminish after 20th scan, indicating slow exchange and is not complete after 20th cycle in phytic acid. However, the exchange in phosphoric acid is noteworthy until 25th cycle after which reduction peak disappears. The explanation for this is the smallest size of dihydrogen phosphate (H2PO4−) allows more phosphates to replace sulfate anions more easily due to size memory effect in polyaniline.

Fig. 4. Equivalent circuit model for polyaniline coating on stainless steel for anion exchange.

the level of difficulty for dopant anion to enter the polymer chains. This also indicates the increase in acidity of the solution as sulfates anion releases into the phosphates electrolyte solution [1]. This positive peak shift was observed in all the voltammogram shown in Fig. 3. The reduction peak was seen at 0-0.2 V on negative scan transforming the reduction states of polyaniline by inserting anion from electrolyte. The inserting anions were phosphates occupies the pores created by sulfates due to their higher charge and adsorption tendency over the steel surface [49,50]. The phosphates compensate the charge neutrality in polyaniline chain and re-doped the polymer (color of the coating turns back to green). It was observed (visually) that the release of sulfate anion and insertion of phosphates anions attributes to de-doping and redoping process. In case of PAni/pTSA, oxidation and reduction peaks were present even after 40cycles this indicates the exchange was partial as shown in Fig. 3a & b. This is due to the size of phytic acid anion is much bigger than that of p-TSA (tosylate), therefore exchange is slow. However, in Fig. 3b, when TsO− anion was exchanged to dihydrogen phosphate, the peak intensity significantly decreased with number of CV scans and get stable after 25 cycles. This indicates tosylate anion were exchanged at a faster rate in phosphoric acid than that in phytic acid [51]. The smaller size of H2PO4− makes them easy to exchange bigger size TsO− anion in polyamer chain. This is due the existence of ‘size memory effect’ of PAni

3.3. Electrochemical impedance spectroscopy (EIS) EIS is a useful technique to characterize conducting polymers electrodes in various application such as energy storage, sensors, corrosion protection etc. Based on impedance spectra obtained during exchange the equivalent circuit model was proposed as shown in Fig. 4. The ideal case capacitor (C) was replaced by constant phase element (Q) to obtain best fitting for the spectra with error of 1 × 10−12. The Q1 is associated to electrical double layer forms at electrode/electrolyte interface, and Q2 is due to pseudocapacitive behavior of redox PAni. The relation between Q and C can be understood as: C = Qn where values of n varies between 0 and 1 [54]. For ideal capacitor n = 1 whereas for rough or porous electrodes such as conducting polymers, n < 1. The impedance spectra of polyaniline coating on stainless steel to study anion exchange is shown in Fig. 5. As shown in Fig. 5a–d, the

Fig. 5. Impedance spectra of PAni coating on SS for anion exchange from: a) p-TSA to PA, b) p-TSA to PhAc, c) SA to PA and d) SA to PhAc in their respective electrolyte solution. 4

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Table 1 Impedance parameters obtained after fitting of PAni/pTSA coating before and after exchange. PAni/pTSA-PA

Before exchange After 20cycles After 40cycles

PAni/pTSA-PhAc

Rs (Ω)

Q1 (μF)

n1

Rct (Ω)

Q2 (mF)

n2

Rs (Ω)

Q1 (μF)

n1

Rct (Ω)

Q2 (mF)

n2

11.65 11.75 11.68

30.40 35.19 31.24

0.92 0.92 0.91

560.5 3425 4352

9.90 8.44 8.21

0.92 0.91 0.92

16.81 17.23 17.16

33.73 53.51 34.88

0.90 0.89 0.90

226.2 2015 1993

11.20 10.12 11.66

0.87 0.89 0.91

Table 2 Impedance parameters obtained after fitting of PAni/SA before and after exchange. PAni/SA-PA

Before exchange After 20cycles After 40cycles

PAni/SA-PhAc

Rs (Ω)

Q1 (μF)

n1

Rct (Ω)

Q2 (mF)

n2

Rs (Ω)

Q1 (μF)

n1

Rct (Ω)

Q2 (mF)

n2

12.67 12.87 12.85

45.65 48.82 37.59

0.88 0.91 0.91

72.23 2812 4542

56.57 29.46 37.52

0.89 0.72 0.94

19.44 20.62 20.53

104.9 55.31 36.89

0.78 0.89 0.90

16.58 799.1 980.8

52.26 30.73 37.07

0.94 0.81 0.91

charge transfer resistance is the smallest before anion exchange test, this shows PAni coating is in conductive state at the start and charge transfer is easier in phosphates solution. Further, this charge transfer resistance changes with CV scan indicating the different anion in PAni coating. This confirms the presence of phosphates in PAni. The large impedance value after exchange shows the increase in coating resistance. This indicates that exchange become difficult after every scan. Ideally, the vertical line at low frequency region indicates the capacitive behavior of the coating where it can store charge. However, small deviation from ideal capacitive line indicates the faradaic pseudocapacitive behavior of PAni. In Fig. 5a & b, semi-circle is smaller in phosphoric acid electrolyte than in phytic acid. This is due to smaller size of dihydrogen phosphate (H2PO4−) than phytic acid anion that can easily get transferred and exchange tosylate anions from PAni. Further, in Fig. 5a, after 40cycles charge transfer resistance is larger than that after 20cycles. This shows that anion exchange was slow in phytic acid due to its bigger anion size. However, tosylate anions were easily exchanged to H2PO4− due to its smaller anion size. The small vertical line at low frequency region indicated the storage of phosphates anion in PAni coating. The parameters obtained after simulation of PAni/pTSA coating are tabulated in Table 1. Similar behavior of anion exchange was observed when PAni/SA coating were used to exchange HSO4− to phytic acid and H2PO4− anion (Fig. 5c & d). The bigger semi-circle after exchange indicates the large coating resistance provided by PAni coating. This shows the exchange is

difficult due to larger anion size of phytic acid, which is why there is very little capacitive behavior in low frequency region. The large coating resistance, phytic acid anion size and little or no capacitive behavior of the coating indicates the partial exchange of hydrogen sulfate to phytic acid. The EIS results agrees with our cyclic voltammetry data where exchange is getting slower after each cycle. The parameters obtained after fitting of PAni/SA coating are tabulated in Table 2. 3.4. FT-Infrared characterization The FT-infrared spectra of p-TSA and SA doped polyaniline was recorded within frequency range of 4000-400 cm−1 is shown in Fig. 6. The spectra showed all signature peaks for polyaniline with quinoid (Q), benzene (B) ring stretching and their combinations were observed (Table 3). The double peaks at 1225 and 1280 cm−1 corresponds to -NH+ and C-N+· stretching that attributes to conductive polyaniline [55]. This was further corroborated by the presence of S]O stretching peaks around 1000-1100cm−1 that corresponds to tosylate and hydrogen sulfate (HSO4-) dopant anion in the polymer. Further, the broad peak at 1070 cm−1 was observed that corresponds to C–H stretching of quinoid ring and stretching vibration of dopant anion. This peak is broader in PAni/p-TSA than that in PAni/SA this is due to the high density of delocalized electrons in p-TSA [56]. Fig. 7 shows comparison of FTIR spectra of polyaniline coating on stainless steel after anion exchanged from sulfates to phosphates. In Fig. 7a & b, all the peaks associated to polyaniline are present. The only peak differences were observed at 1000-1100cm−1 due to the presence of different dopant anion; phytic acid (Fig. 7a) and dihydrogen phosphate (Fig. 7b). The intensity and shape of the peak corroborates the exchange of anion from sulfates to phosphates. In Fig. 7a, the strong peak at 1150 cm−1 with a shoulder at Table 3 FT-IR band assignment of PAni doped with p-TSA and SA. Peak assignment

CeH stretching CeH bending (aromatic from dopant) C]C stretching from quinoid-ring C]N stretching with C-H bending of benzene ring S]O from sulfate dopants

Fig. 6. Comparison of FT-IR spectra of polyaniline coating on stainless steel doped with: a) p-TSA and b) SA. 5

Peak positions (cm−1) PAni/pTSA

PAni/SA

2916 2846 1723 1561 1489 1074 1013

– – – 1567 1488 1072 1009

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Fig. 7. Comparison of FT-IR spectra of polyaniline coating when dopants (p-TSA and SA) were exchanged to: a) PA and b) PhAc.

1050 cm−1 corresponds to asymmetric PeO stretching present in phytic acid [57]. The presence of phytic acid in PAni coating was further substantiated by doublet of C–H stretching from alkanes (Table 4). In Fig. 7a & b, the peak for sulfate in PAni/pTSA spectra changes from broad to sharp when anions were exchanged to phosphates indicating the change in anion due to different stretching vibration in IR signals for different anion present in PAni chain. The absence of C–H stretching peak in PAni/pTSA-PhAc (Fig. 7b) showed the exchange of tosylate by dihydrogen phosphate (H2PO4-) anion. Similarly, in Fig. 7a & b, PAni/ SA has broad sulfate peak ∼1000cm−1, this peak split into three peaks at 1100, 1050, 922 cm−1 that corresponds to PeO stretching vibration from the phosphates indicating the exchange of sulfates in PAni. Further, peaks in 400-600 cm−1 region corresponding to OePeO bending vibrations corroborated the presence of phosphate anion in the PAni coating.

after anion exchange. The sulfuric acid doped PAni have fibrillar and porous morphology (Fig. 8a) whereas p-TSA doped polyaniline has more uniform and compact coating with little granules present (Fig. 8d). This is due to benzene ring present in p-TSA that creates a lamellar structure by π-π stacking within the molecule. In Fig. 8b & c, when hydrogen sulfate anion was exchanged to phosphates anion, there is little effect on morphology of the coating. The porous structure of PAni/SA was slightly reduced when phytic acid was used to exchange sulfate (Fig. 8b). This is due to bigger size of phytic acid anion and its tendency to adsorb on steel surface. However, coating looks more porous when HSO4− was exchanged to H2PO4− due to smaller anion size of phosphoric acid anions. This porous coating does not affect anticorrosion performance of PAni because of inhibiting property of phosphoric acid. In case of PAni/p-TSA, coating morphology changed from uniform and compact to non-uniform and less compact. This is due to absence of benzene ring in phosphate dopants that result in more irregular morphology (Fig. 8e & f).

3.5. Coating morphology Fig. 8 showed the SEM images of polyaniline coating before and

Fig. 8. SEM images of PAni coating on stainless steel: a) PAni/SA, b) PAni/SA-PA, c) PAni/SA-PhAc, d) PAni/pTSA, e) PAni/pTSA-PA and f) PAni/pTSA-PhAc. 6

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(than TsO-) makes coating more compact thus reducing the accessible sites. This can be observed in Fig. 10 that shows the accessibility coefficient (A) is much smaller for PAni/SA than that of PAni/pTSA and decreases sharply with number of scans.

Table 4 FT-IR band assignment of PAni with anion exchanged to phosphates. Peak assignment

Peak position (cm−1) PAni/PA

CeH stretching C]C stretching (Q) C]N stretching (Q) with C-H bending (B) PeO stretching OePeO bending

PAni/ pTSAPA

PAni/ SA-PA

PAni/ PhAc

PAni/ pTSAPhAc

PAni/ SAPhAc

2929 2840 1569 1483

2959 2900 1563 1472

– 2915 1569 1483

– – 1586 1495

– – 1570 1490

– – 1576 1490

1127 1057

1143 1008 922 583 503

1127 1041 922 583 502

1153 1089 1008 596 511

1110 1046 938 591 511

1115 1051 938 602 506

583 513

3.7. Anticorrosive properties The corrosion protection performance of PAni coating depends on the type of dopants used and its size because large size anions along with polymer chain provide barrier protection to the metal. Fig. 11 showed the anticorrosion performance of polyaniline coating when sulfate anions were exchanged to phytic acid (Fig. 11a) and phosphoric acid (Fig. 11b). The performance of the coating was measured as corrosion potential that represent the spontaneity of the corrosion reaction to occur. On comparing sulfates, the corrosion potential of polyaniline coating has increased by 100 mV when doped with p-TSA than that of SA. This is due to compact and uniform coating was produced when pTSA was used as a dopant for polyaniline. Additionally, lamellar structure was formed by π-π stacking of benzene ring in p-TSA molecule preventing ingress of chloride ions in the coating. The presence of benzene ring also increases the electron density in coating thus repelling negatively charged chloride ion. However, the corrosion potential of polyaniline coating showed significant increase when sulfate anion from PAni were exchanged to phosphates. In Fig. 11a, both sulfates were exchanged to phytic acid anion, both coatings (PAni/pTSA-PA and PAni/SA-PA) showed equal corrosion potential of 0.23 V. This corrosion potential after exchange is 80 mV and 200 mV more positive than PAni/pTSA and PAni/SA, respectively. Nevertheless, the corrosion potential of the coating after exchanging sulfate anion to phytic acid anion is lower than PAni/PA by 70 mV. The possible explanation is phytic acid anion is bigger than tosylate and HSO4−, the anion exchange in this case is considered as partial. Even though the exchange is partial, it is restricting the access of aggressive chloride ions at the interface. Fig. 11b shows corrosion test performed on the coating when both sulfates anions were exchange to phosphoric acid. The corrosion potential of coating after exchange to H2PO4− is 0.19 V. The Ecorr after exchange equals the corrosion potential of PAni/ PhAc this is because of smallest anion size of H2PO4− of all the dopants used. This showed the complete exchange of both the sulfates to dihydrogen phosphates. The corrosion potential showed significant increase when sulfate anions were exchanged because phosphates have tendency to inhibiting corrosion by forming chemical conversion film on steel. This film comprises of various iron oxide and phosphate that prevent corrosive attack on steel.

3.6. Accessibility of redox sites The charge accumulated in the polymer during the electropolymerization can be calculated from chronoamperometric curves. This charge can be used to calculate the number of redox sites in polyaniline coating using Eq. (1) [58]:

Nr =

Qt nF

(1)

Where Nr is number of redox sites (mol) in the polymer, Qt is total charge consumed during electropolymerization of the polymer (510mC for PAni/SA and 114mC for PAni/pTSA), n ( = 2.3) is number of electron transferred per aniline monomer [59,60] and F is faraday constant. The maximum charge (Qmax) that was produced during anion exchange can be calculated by:

Q max =

Qt (z ) n

(2)

Where z ( = 1) is the number of electrons produced during anion exchange. This maximum charge can be used to calculate the accessibility coefficient (A) that tells the number redox sites that were accessible during the exchange, given by:

A=

Qc Q max

(3)

Qc is the maximum charge accumulated in the polymer during process of anion exchange. In Fig. 9, the maximum charge accumulated during the anion exchange is more in PAni/SA than that of PAni/pTSA and decreases with increase in number of scans. This is the indication of slow exchange in PAni/pTSA with increasing scans, hence more redox sites are accessible for phosphates anion. On the other hand, the smaller HSO4− anion

4. Conclusion In conclusion, we reported a facile way to study the accessibility of redox sites in electropolymerized polyaniline. The study showed that accessibility of redox sites in the polymer is influenced by the size of a

Fig. 9. Maximum charge accumulated in PAni coating during the process of anion exchange in their respective electrolyte solutions. 7

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Fig. 10. Number of redox sites accessible in PAni coating during the exchange in their respective electrolyte solution.

Fig. 11. Potentiodynamic polarization curves of polyaniline coating in 3.5% NaCl before and after exchange of anions to: a) phytic acid, b) phosphoric acid and c) comparison of PAni coating doped with phosphoric and phytic acid.

dopant anion in the polymer. The bigger anions can be exchanged by smaller anions at faster rate due to size memory effect in the polymer chain thus can be easily accessed by the ions. On the other hand, smaller anion can still be exchanged by bigger anions at a slower rate. This is due to the compact coating was developed using smaller anion that restricts the number ions to access the redox sites at polymerelectrolyte interface. Besides, size of the dopant also affects corrosion performance of the coating. The bigger size dopants create a barrier at the interface preventing the ingress of negatively charged chloride ions. In addition, exchanged phosphates synergistically with polyaniline provided better protection than sulfates due to their inhibitive property towards steel thus providing anodic, barrier and inhibition protection all together with negligible influence on coating morphology because of change of dopant anions from the polymer.

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