Anticorrosive properties of PPy|Co3O4 composite films coated on Al-1050 in OXA-DBSA mix electrolyte

Synthetic Metals 254 (2019) 10–21

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Anticorrosive properties of PPy|Co3O4 composite films coated on Al-1050 in OXA-DBSA mix electrolyte Merve Menkuer, Hatice Ozkazanc

T

⁎

Department of Chemistry, Kocaeli University, 41380 Kocaeli, Turkey

A R T I C LE I N FO

A B S T R A C T

Keywords: Aluminum Galvanostatic EIS Acid corrosion Electrodeposited films Polymer coatings

In this study, Polypyrrole|cobalt oxide (PPy|Co3O4) composite films were obtained by the galvanostatic method in oxalic acid (OXA) and dodecyl benzene sulfonic acid (DBSA) mix electrolyte on aluminum-1050 (Al-1050). The corrosion protective effect of these films in a 0.1 M HCl corrosive medium was investigated by Tafel polarization method and impedance measurements. Characterization and surface morphology of the films were examined by Fourier transform infrared (FT-IR), Thermogravimetry (TG), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). The OXA-DBSA mix electrolyte significantly increased the homogeneity and adhesion of the films. Cobalt nanoparticles significantly reduced the thickness of the PPy film to about 13 μm from 50 μm. The composite film with the highest Co3O4 level showed protection efficiency close to 100%. Polarization resistance values determined from the Cole-Cole plots showed that PPy film increased the resistance of Al-1050 by about 15 times, while the composite films increased the film resistance of Al-1050 up to 124 times, depending on the amount of Co3O4 nanoparticles. It was observed that all of the films in the corrosive medium protected the surface of the Al-1050 quite well even at the end of the fifth day.

1. Introduction The use of the conducting polymers in various applications such as batteries [1], sensors [2], super capacitors [3], membranes [4] and electrochromic devices [5] has increased in recent years to protect metals and alloys from corrosion [6]. Conducting polymers which do not contain chromate and toxic substances like phosphate can be coated on the surfaces of the metals and their alloys to prevent the corrosive effects by electrical conduction and ionic transport [7]. Since Polypyrrole (PPy) has high electrical conductivity and chemical stability, and since metals can also be easily coated with it in aqueous solutions, it is one of the most important conducting polymers [8]. The main problem encountered in such organic coatings is that the coatings have a porous structure which allows the corrosive species to easily reach the metal. These pores in the coatings can be reduced by embedding various organic/inorganic materials electrochemically into the polymer matrix. It has been reported that PPy-formed composites of micronsized or submicron-sized particles such as ZnO [9] and SiO2 [10] improve the corrosion resistance and mechanical properties of such porous coatings. For example, Herrasti et al. [11] reported that large sized particles were the cause of heterogeneous dispersion in the polymers, and that the homogeneity of the coating could be increased when nano-scale particles were added into the polymer matrix. In ⁎

addition, Ladan et al. [12] showed that the increase in surface area of the polymers formed around the nanoparticles gave better protection by increasing the interaction between the ions released during the metal corrosion and the coating. Hosseini et al. [13] also found that nanoscale materials in the polymer matrix significantly increased the barrier properties and coating lifetime of the polymer films. The electrochemical method used to obtain the coatings in studies mentioned above is a particularly suitable synthesis method for organic-inorganic hybrid bilayers [14]. The use of the classical electrochemical methods such as potentiostatic, galvanostatic, potentiodynamic or pulse methods allows for the control not only of the amount of the added polymer/metal double layer, but also of the structure of metallic layer or doped metal particles on the polymer surface or in the polymer matrix [15]. The metal filling process and the properties of the metallic phase in the polymer matrix affect the structure and morphology of the polymeric layer. For this reason, the electrochemical behavior of conducting polymer-metal coatings depends on the size, quantity and location of the deposited metallic particles in the polymeric layer [16]. Another important factor to consider when applying an electrochemical method for PPy-metal film synthesis is the adhesion strength of the coating to the metal surface. The Co3O4, a p-type metal oxide doped into the polymer matrix to obtain composite film in this study, is widely used in the important

Corresponding author. E-mail addresses: [email protected] (M. Menkuer), [email protected] (H. Ozkazanc).

https://doi.org/10.1016/j.synthmet.2019.05.010 Received 3 January 2019; Received in revised form 22 April 2019; Accepted 17 May 2019 Available online 28 May 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.

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2. Experimental details

technological applications such as sensors, heterogenic catalysts, lithium batteries, magnetic materials and optoelectronic devices [17]. While trivalent cobalt oxide is unstable in the natural environment, the other forms of cobalt oxide such as Co3O4 and CoO are more stable and are therefore more suitable for industrial use [18]. Wei et al. [19] investigated the utility of PPy/Co3O4/CP ternary composite coatings obtained by potentiostatic electropolymerization method as electrode material for super capacitor applications and found that this composite exhibits excellent cycle stability with negligible capacitance loss after 1000 cycles. Guo et al. [20] reported that an electrochemically synthesized PPy-Co-O complex exhibited high performance in lithiumstorage materials while Co3O4/PPy composites were a novel lithiumstorage composite material. However, as far as we know, there is no literature on the corrosion behavior of composites obtained from PPy and Co3O4. The Co3O4 is thought to increase the barrier effect of the coating by filling the pores that may form on the surface of PPy, since it is both highly stable and nano-sized. This study presents a detailed corrosion protection behavior of PPy|Co3O4 composites for the first time. Another important aspect of this study is the use of OXA-DBSA mixture electrolyte for the first time in the synthesis of PPy|Co3O4 composites. Reghu et al. [21] reported that DBSA, a functionalized protonic acid, increased the homogeneity and solubility of conducting polymer films. In addition, DBSA provides better distribution of conducting polymer particles into the structure [22]. Herrasti et al. [23] and Hammache et al. [24] obtained adhesive and homogeneous PPy coatings on the surface of copper and iron electrodes in aqueous OXA solution, respectively. The anticorrosive properties of conductive polymer coatings obtained in different mixture electrolytes on the metals were seen to have been studied in the related literature. However, to the best of our knowledge, there are very few studies using OXA-DBSA mixture electrolyte, and only a few of these are related to corrosion. In some studies using OXA-DBSA blend electrolyte, it was reported that the mixture electrolytes obtained from large anions such as DBSA and small anions such as OXA make the conducting polymer based coatings both more adhesive and homogeneous [25,26]. Inaddition, Mahmoudian et al. reported that poly(pyrrole-co-phenol) [27] and poly(N-methylpyrrole)/TiO2 [28] coatings have good protective effect on steel. In our previous study [29], we found that the PPy coating we obtained in the OXA-DBSA mixture electrolyte on the copper electrode surface was very adhesive and homogeneous, and this PPy film showed a protection efficiency of almost 100% against corrosion even at the end of the fifth day. In this study, we examined the anticorrosive properties of the PPy|Co3O4 composite films on Al-1050 electrode in the OXADBSA mixture electrolyte for the first time. In addition, the Rct values determined from both equivalent circuit fit and Cole-Cole diagram were compared [30]. As Córdoba-Torres [31] reports, electrochemical impedance spectroscopy (EIS) method is used in a wide range of applications such as corrosion, electro-deposition, fuell cells and sensors. However, the equivalent circuit modeling method on which these applications are based cannot fully enlighten the mechanisms related to these applications. In fact, Holm and Harrington [32] stated that the researchers were able to fit the same experimental data into different equivalent circuits, but as a result, they obtained different impedance parameters for the same experimental data. Accordingly, the Cole-Cole plotting methods or similar ones are preferred rather than the equivalent circuit method in the analysis of the impedance curves of the complex systems [33]. In the first part of this study, the surface of Al-1050 was coated with films of PPy and PPy|Co3O4 composites by galvanostatic method in the OXA-DBSA mix electrolyte. In the second part, the protection behavior of the coatings in 0.1 M HCl was investigated by Tafel polarization method and impedance measurements. Finally, the characterization and surface morphologies of the coatings were studied by FT-IR, TGA, SEM and AFM.

2.1. Materials Pyrrole (97%), hydrochloric acid (HCl) and oxalic acid (H2C2O4) were obtained from Merck, Co3O4 (< 50 nm) and DBSA (70 wt% in isopropanol), from Aldrich. Solutions were prepared using distilled water. Pyrrole was used as the monomer, OXA-DBSA mixture was used as the electrolyte and the HCl solution was used as the corrosive medium. These materials were used without further purification. 2.2. Electrodes Al-1050 was used as a working electrode, saturation calomel electrode (SCE) was used as a reference electrode and platinum wire was used as a counter electrode. Al-1050 electrode has the elemental composition of 99.72, 0.155, 0.078, 0.00 and 0.001% of Al, Fe, Si, Mg and Cu, respectively. The cylindrical aluminum electrodes were covered with a polyester block so that only one of the base areas was exposed and electrodes with a surface area of 1.76 cm2 were obtained. For SEM analyses, 1 x 1 cm size square plate aluminum electrodes were used. Immediately prior to the synthesis, the surfaces of the working electrodes were polished with emery papers at different thicknesses (180, 400, 800 and 1200) by using a mechanical polishing machine (METKOM). Later, they were washed with tap water, distilled water and acetone, respectively and dried. 2.3. Measurements All electrochemical measurements were performed with the Reference 3000 Potentiostat/Galvanostat/ZRA instrument. The Echem analysis program was used in the fitting process of the obtained curves. 2.3.1. Electrochemical polymerization of pyrrole films The PPy film was coated on the surface of the Al-1050 electrode by a galvanostatic method in a 0.1 M OXA + 0.1 M DBSA mix electrolyte and 0.2 M pyrrole containing solution. The coating process was carried out for a period of one hour by applying a current of 2 mA/cm2. In the Al-1050 electrode coating process, different periods under 1 h were tried, but the electrode surface was not fully covered. Gaps were observed in several places and the coating did not adhere well on the surface. The coatings at the end of the 1 h period completely covered the surface and were highly adherent. In addition, optimum experimental parameters (such as electrolyte and monomer concentration) were determined and preliminary tests were performed to obtain the best coating. 2.3.2. Electrochemical polymerization of PPyǀCo3O4 composite films First, 2.52 g of OXA in 50 mL of distilled water was stirred in the magnetic stirrer for 15 min. 9.40 mL of DBSA was then added dropwise to this OXA solution and stirred for a further 15 min. The final blend was admixtured with 20 mg Co3O4 and stirred in a 30 min ultrasonic bath to prevent/minimize the agglomeration. 2886 μL of pyrrole monomer was added into the OXA + DBSA + Co3O4 mixture, and finally by adding distilled water, a total of 200 mL solution (amount of the pyrrole and OXA-DBSA in the solutioun is 0.2 and 0.1 M, respectively) was obtained. The final mixture was stirred for a further 30 min with a magnetic stirrer to ensure complete dispersion of the metal oxide in solution. On the Al-1050 electrode, a PPyǀCo3O4(I) composite film was obtained after 1 h in a solution containing 0.2 M pyrrole + 0.1 M oxalic acid + 0.1 M DBSA +20 mg Co3O4 by a galvanostatic method (2 mA/cm2). Another batch of PPyǀCo3O4(II) composite filmwas prepared in exactly the same way as above but with 50 mg of Co3O4. The PPyǀCo3O4(I) and PPyǀCo3O4(II) composite films obtained on 11

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the surface of the Al-1050 will be coined as composite-A and -B films, respectively for the remainder of this text. 2.3.3. Corrosion measurements The corrosion behaviors of uncoated Al-1050 electrode, coated with PPy, composite-A and -B films were investigated by Tafel polarization and electrochemical impedance techniques in 0.1 M HCl aqueous solution. Anodic and cathodic polarization curves were obtained at a constant scan rate of 5 mV/s and between -250 and +250 mV. The corrosion potential (Ecorr), the corrosion current density (Icorr), the anodic tafel slope (βa), the cathodic tafel slope (βc) and the corrosion rate (mpy) were determined using semi-logarithmic current-potential curves. Impedance measurements of the uncoated and coated Al-1050 electrodes were carried out at an open circuit potential in the frequency range of 105 to 0.01 Hz for 10 mV AC amplitude value at various immersion times (1, 24, 72 and 120 h). 2.3.4. SEM and AFM analysis The coating process on the square shaped (1 × 1 cm) Al-1050 electrode was carried out under the electrochemical conditions applied to the cylindrical electrode. The coatings were washed with distilled water and dried and there after their surfaces were analyzed by SEM (JEOL JSM 6060) measurements. The surface topography analysis of nanoparticles was investigated by AFM (Nanosurf-NaioAFM) images with tapping mode. For AFM analysis, the powder nanoparticles were dispersed in chloroform by an ultrasonic bath and then, coated on a glass surface by using a spin coater.

Fig. 2. FT-IR spectra of PPy, composite-A and -Bfilms.

OXA concentration and DBSA concentration for all coatings in this study were kept constant at 0.2, 0.1 and 0.1 M, respectively. When Al-1050 is exposed to a current from its active state potential, the electropolymerization process begins within a few seconds or at most in one minute (as soon as immersion is performed). This is because the spontaneously formed surface oxide film facilitates electropolymerization by inhibiting the dissolution of the substrate, and the monomer oxidation takes place immediately with the applied anodic current [36]. The solution was stirred during the polymerization to facilitate continued nanometal oxide distribution.

2.3.5. FT-IR measurements All films coated on Al-1050 electrode were washed with distilled water and dried, then removed from the electrode surface. FT-IR (Agilent Cary 630) spectra of the samples, for which KBr pellets were prepared, were recorded in the range 4000–400 cm−1. 2.3.6. Thermogravimetric analysis TG analysis of the films was examined by a Perkin Elmer TGA 4000 instrument at 35–900 °C and 10 °C/min scanning rate (under nitrogen atmosphere).

3.2. FT-IR spectrum

3. Results and discussion

Fig. 2 shows the FT-IR spectra of the films coated on the Al-1050 electrode. The characteristic peaks of PPy are clearly visible in the FTIR spectra. These are 3437 cm−1 for NeH stretching vibration, 2926 cm-1 for assymmetric CH2 stretching vibration, 2854 cm−1 for symmetric CH2 stretching vibration, 1633 cm−1 for CeN streching vibration, 1383 cm−1 for NeH vibration, 1265 cm−1 for CeH in plane deformation, 1036 cm−1 for CeH vibration and 615 cm−1 for ring bending [37–40]. The peaks at 1118 and 701 cm-1 are associated with the SO3 and S]O stretching, respectively, originated from DBSA [41]. The characteristic peaks belonging to both the PPy matrix and DBSA are also clearly observed in the FT-IR spectra of the composite-A and -Bfilms. In addition, the relatively weak band observed at about 633 cm−1 for both is due to CoeO metal oxygen stretching resulting from the filling process [42]. It appears that there is no noticeable shift in the spectrum of PPy depending on the filling process which suggests that Co3O4 nanoparticles do not form a remarkable coordination with the polymer chains, and that there is possibly a weak electrostatic interaction between them.

3.1. Coating of PPy and composite films on Al-1050 The potential-time curves of the films coated on the Al-1050 by the galvanostatic method are given in Fig. 1. Previous studies on the electrochemical method on aluminum have revealed that the pyrrole concentration should be higher than 0.1 M to allow PPy accumulation in the acid medium [34,35]. For this reason, the pyrrole concentration,

3.3. Potantiodynamic polarization analysis The current-potential variations for uncoated Al-1050, Al-1050 coated with the PPy and composite-A and -B films in a corrosive solution are seen in Fig. 3. The electrochemical parameters for these curves

Fig. 1. Potential variation with respect to time during galvanostatic synthesis of PPy, composite-A and -B films. 12

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diagrams associated with one time constant in Bode plots. Accordingly, the corrosion behavior of the uncoated Al-1050 can be explained by a single mechanism. The corrosion process here is usually due to the charge transfer at the electrode/solution interface [45,46]. An inductive loop seen for all immersion times at low frequency domain indicates the presence of various weak adherent corrosion products on the surface. This inductive region is generally associated with pitting corrosion and dissolving interface. Therefore, inductive elements must also be considered when determining the equivalent circuit. In order to fit the Nyquist curves of the uncoated Al-1050 electrode the equivalent circuit [(Rs (CPEt (Rct) (LRL))] which is given in Fig. 5a was used. In this equivalent circuit, Rs is the solution resistance; n is the number of electrons exchanged at the electrode/solution interface in the electrochemical reaction (0 < n < 1); CPEt is a constant phase element used in place of capacitance (C) to describe the inhomogeneity in the system and Rct is the charge transfer resistance. This equivalent circuit also includes inductive elements, RL and L as mentioned above [47]. Here, RL is the total resistance resulting from the accumulated species such as corrosion products, molecules and/or ions. Accordingly, the electrochemical impedance parameters obtained from the fit process to the equivalent circuit seen in Fig. 5a are given in Table 2 for uncoated Al1050. When the Nyquist and Bode diagrams of PPy coated Al-1050 are examined (Fig. 4b), three loops are observed depending on the frequency. The first two small loops, which can be seen slightly more in the Bode phase diagram, are due to the inductive resistance in the low frequency region and the film resistance in the mid-frequency region. The relatively more obvious loop observed in the high frequency region is related to the charge transfer resistance. Accordingly, the most suitable equivalent circuit for the impedance curves of PPy coated Al-1050 [(Rs(CPEfRf) (CPEtRct(LRL)))] is given in Fig. 5b [48]. Here, Rf represents the film resistance and CPEf is the constant phase element of the film, the rest of the circuit elements has defined above. Nyquist and Bode diagrams of the composite films vary according to their immersion times. At the end of the 1st hour, two loops can beseen in the Nyquist and Bode diagrams of the composite-A film (Fig. 4c). The first loop in the high-frequency region is associated with the film resistance, while the second loop in the mid-frequency region is related to the charge transfer resistance. In this case, the most suitable equivalent circuit for compsite-A film at the end of the 1st hour [(RS (CPEf(Rf(CPEtRct)))] is given in Fig. 5c. For the 1st hour, Rp is calculated from the sum of chargetransfer and film resitances. At other immersion times (24, 72 and 120 h) for composite-A film, an additional inductive loop originating from the corrosive species occurs.Therefore, the equivalent circuit given in Fig. 5b was used for these immersion times. At the end of the 1st hour for composite-B film, two loops were observed as in composite-A film, and accordingly the equivalent circuit in Fig. 5c was used. For the 1st hour, Rp is calculated from the sum of charge-transfer and film resitances. For other immersion times (24, 72 and 120 h) of the composite-B film, the loop in the mid-frequency region associated with the film resistance is not observed, but only the inductive loop which results from the corrosive species is observed (Fig. 4d). The equivalent circuit [(Rs (CPEt (Rct) (LRL))] given in Fig. 5a was used for these immersion times of the composite-B film. The electrochemical impedance parameters obtained from the fit processes for PPy, composite-A and -B coatings are given in Table 3. In the related literature, especially when the studies on the impedance curves with inductive loops are examined, it is seen that some researchers use fit circuits in determining Rp and some researchers do not use fit circuits at all. Metikoš-Huković et al. [49] reported that in such complex systems, it is difficult to calculate the Rp since it is a complex function including factors such as charge transfer, mass transfer and chemical reaction. Elkin et al. [50] reported that fit circuits should not be used for impedance curves exhibiting inductive loop. This is because, in electrolyte media with low magnetic susceptibility, it is

Fig. 3. Semi-logarithmic plot of current-potential curves of uncoated aluminum electrodes and coated with PPy, composite-A and -B films. Table 1 Electrochemical polarization parameters of uncoated A-1050 and coated with PPy, composite-A and -B films. Samples

Ecorr (mV)

Icorr (μA/ cm2)

CaBeta (mV/dec)

AnBeta (mV/dec)

CoRate (mpy)

PE%

Al-1050 PPy Composite-A Composite-B

−745 −347 −194 −97

65.90 3.71 3.1 0.148

98.30 143.4 135.7 50.1

132.7 127 473.8 64.9

48.22 2.713 2.27 0.108

—— 94.3 95.2 99.77

are given in Table 1. The corrosion potential of the coated electrodes was observed to be more positive than that of the uncoated electrode (anodic protection). It is thought that this electrochemical protection is caused both by the increase of the corrosion potential and by the formation of a protective passive layer on the surface of Al-1050 electrode. The Icorr values of the uncoated aluminum electrodes, the ones coated with PPy, composite-A and -B films were estimated to be 65.90, 3.71, 3.10 and 0.148 μA, respectively. That Al-1050 coated with PPy, composite-A and -B films have low Icorr values compared to uncoated Al-1050 indicates that all films have a significant barrier effect against corrosive species. Protection efficiency of the coatings is calculated according to Eq. 1 below;

PE%= [(Icorr − Icorr(c) )/Icorr ] × 100

(1)

where Icorr and Icorr(c) are the corrosion current density values in the absence and presence of the coating, respectively [43]. The calculations show that the protection efficiency of all coatings is very high and the efficiency of the coating with composite-B is close to 100%. Accordingly, metal oxide nanoparticles in composite-A and -B films appear to significantly increase the anticorrosive activity of the polymer matrix. 3.4. AC impedance measurement results Electrochemical Impedance Spectroscopy (EIS) analyzes are typically evaluated using the Nyquist and Bode diagrams together. While the Nyquist plot gives an idea of the possible mechanism, it does not fully explain the impedance's dependence on the frequency. For this reason, dependence on frequency of the impedance and phase angle can be examined more conveniently via Bode plots [44]. Both Bode and Nyquist diagrams in the corrosive medium for various immersion times are shown in Fig. 4 for uncoated and for film coated Al-1050 electrodes. Fig. 4a shows a capacitive loop in the high frequency region at all immersion times for uncoated Al-1050 followed by an inductive loop at the low frequency region. There is only one semicircle in Nyquist 13

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Fig. 4. Nyquist (left side) and Bode (right side) plots of Al-1050 electrodes in 0.1 M HCl for various immersion times for (a) uncoated, and coated with (b) PPy, (c) composite-A, (d) composite-B films.

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Fig. 5. Equivalent circuit models for (a) uncoated Al-1050 and composite-B film for 24, 72 and 120 h immersion times; for (b) PPy (1, 24, 72, 120 h) and composite-A (24, 72, 120 h); and for (c) the 1st immersion hour for composite-A and -B films. Fig. 6. For the impedance curve obtained at the first hour of uncoated Al-1050: (a) the equivalent circuit fit curve of the experimental data, (b) the Cole-Cole diagram plotted using this fit curve.

Table 2 Electrochemical impedance parameters of uncoated Al-1050 electrode obtained at various immersion times in 0.1 M HCl. Immersion times (h) 1 24 72 120

RS (Ω) 6.92 7.37 7.15 7.71

CPEt (Ss−n) −5

8.31 × 10 4.76 × 10−5 7.13 × 10−5 8.25 × 10−5

n

Rct (Ω)

L (H)

RL (Ω)

0.9148 0.9726 0.9835 0.9919

178.8 14.11 12.2 12.05

5.567 36.74 22.62 30.92

27.6 57.1 39.7 41.8

meaning. In addition, there are differences between the methods used by researchers who calculate Rp by using resistors in the equivalent circuits. For example, in calculation of Rp, while some researchers sum the resistances they obtain from the fit process, depending on the parallel or series connection status of the resistors (such as: Rp = R ct + Rf + RL ) [52,53], some others subtract them (such as: Rp = R ct − RL ) [54]. At the end of our detailed literature review, we concluded that the most appropriate method for determining the Rp value for the impedance curves with inductive loops is the Cole-Cole plotting or Faradic impedance plotting methods, as Lorenz and

extremely difficult to correlate the inductance with the energy generated by the magnetic field accumulated on the metal electrode surface. In fact, as Córdoba-Torres et al. [51] point out some researchers have claimed that the inductive loop has not got a significant physical

Table 3 Electrochemical impedance parameters of PPy, composite-A and -B films obtained at various immersion times in 0.1 M HCl. Rs (Ω)

CPEf (Ss−n)

n

Rf (Ω)

CPEt (Ss−n)

n

Rct (Ω)

L (H)

RL (Ω)

PPy 1h 24 h 72 h 120 h

7.16 19.51 56.74 47.45

2.39 × 10−5 3.02 × 10−5 2.38 × 10−5 2.17 × 10−5

0.72 0.77 0.86 0.90

2831 3094 2691 2359

6.54 × 10−6 1.33 × 10−6 10−5 10−5

0.80 0.80 0.80 0.80

355.8 101.5 242 220.5

9.145 1.626 2.6 × 10−3 14.7 × 10−4

798.1 309.5 87.1 95.9

comp-A 1h 24 h 72 h 120 h

6.29 7.77 4.99 6.01

1.21 × 10−5 1,66 × 10−5 3.15 × 10−5 2.78 × 10−5

0.90 0.80 0.80 0.80

1023 4733 4715 3012

8.42 × 10−5 5.5 × 10−6 10−5 3.3 × 10−6

0.45 0.83 0.73 0.80

14490 1059 3773 423.1

– 10 36.94 × 10−3 55.72 × 10−3

– 1865 335.2 423.1

comp-B 1h 24 h 72 h 120 h

8.12 97.64 61.18 52.39

1.69 × 10−5 – – –

0.90 – – –

569.1 – – –

15.11 × 10−5 4.5 × 10−5 5.3 × 10−5 4.7 × 10−5

0.55 0.80 0.80 0.80

22720 2890 2891 3651

– 4867 285.8 936.8

– 840.9 1251 2385

15

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Fig. 7. For the impedance curve obtained at the end of the first hour of the Al1050 coated with the composite-B film; (a) the equivalent circuit fit curve of the experimental data; (b) the Cole-Cole diagram plotted using this fit curve. Table 4 The Rct, R'ct and Rp values obtained from Cole-Cole diagrams. Rct (Ω)

R'ct (Ω)

Rp (Ω)

Al-1050 1h 24 h 72 h 120 h

202.44 84.33 64.79 66.57

211.09 88.07 68.27 70.17

185.84 18.57 14.37 13.59

PPy 1h 24 h 72 h 120 h

3147.4 3117.6 3002.6 2589.8

3648.8 3580.6 3125.8 2827.7

2742.7 2733.7 1957.4 1408.1

composite-A 1h 24 h 72 h 120 h

14946.6 5873.6 4980.6 3375.7

—— 6379.7 5251.5 3600.7

15890.2 2270.6 3329.1 1964.9

composite-B 1h 24 h 72 h 120 h

22401.1 3543.9 3039.2 3980.4

—— 3699.7 3181.5 4271.8

22983.4 2711.2 2427.1 2575.8

Fig. 8. Photo images of Al-1050 electrodes out of corrosive medium for (a) uncoated, (b) PPy coated, (c) composite-A coated, (d) composite-B film coated, and images of the Al-1050 electrodes after 120 h of immersion time for (e) uncoated, (f) PPy coated, (g) composite-A coated, and (h) coated with composite-B film.

are illustrations for the determination of Rp and Rct from the Cole-Cole plot. Here R 'ct , is the Cole-Cole correction value of Rct according to the semicircle plotting. Similarly, the Fig. 7a shows the Nyquist curve without inductive loop at the end of the 1 st hour for the composite-B film and the plottings for the determination of Rp and Rct from the ColeCole plot are given in Fig. 7b [57]. Accordingly, the values of Rp, Rct and R 'ct obtained from Cole-Cole plots for uncoated Al-1050 and coated with PPy, composite-A and -B films are given in Table 4. A difference between the Rct values obtained from the equivalent circuit fit of the EIS data containing the inductive loop (Tables 2 and 3) and the Rctvalues obtained from the Cole-Cole plot (Table 4) was observed. However, it is also seen that with respect to the EIS data having no inductive loop (impedance curves of composite-A and -B at the end of 1 st hour) the Rct values determined from both fitting process and Cole-Cole plotting are quite close. In addition, the Rp values obtained from the sum of Rf and Rct resistances determined from the fit process, and the Rp values obtained from the Cole-Cole plot are also very close. These results show that for impedance diagrams with inductive

Mansfeld [55] also pointed out. Harrington and Driessche [56] reported that Rct was greater than Rp when the inductor was dominant in the impedance. According to this, in this study, we used the Cole-Cole plot, where in Rp is larger than Rct for the impedance curves having inductive loop (Figs. 6 and 7). Fig. 6a shows an example of a uncoated Al-1050 with the Nyquist curve obtained at the end of the one hour immersion time and the fit curve determined according to the equivalent circuit. In Fig. 6b, there 16

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Fig. 9. Sellotape test of the composite-B film after 120 h of immersion time in the corrosive solution.

Fig. 10. (a) SEM and (b) AFM images of Co3O4 nanoparticles.

increase the homogeneity of organic coatings [58]. Zeybek et al. [59] reported that the dodecylsulphate (DS) used in PNMPy-DS coatings greatly improved the protective behavior of a polymer film. They have suggested that DS ions in the polymer matrix act as permselective membranes against corrosive anions which are in the corrosive solution and are carried to the metal surface. Since the DBSA that we used in our work contains large dopant anions, it is thought to inhibit the passage of the corrosive solution to the metal surface by repelling chloride ions in the solution. Along with the increase in the immersion time, the Rp values of the coatings decreased slightly, but even by the end of the fifth day, the coatings provided significant protection for Al-1050 against corrosion and the films were observed not to separate from the electrode surface. Photo images of all coatings obtained before and 5 days after immersion in a corrosive solution are given in Fig. 8. A distinct deformation (swelling, shrinkage, cracking, etc.) was not observed on the coatings even after 5 days in the corrosive environment. Moreover, Fig. 9 shows the sellotape test of the composite-B film, which provides the best protection. As it can be seen, the coating is well adhered to the Al-1050 surface which was marked with a white arrow in Fig. 9.

loops, Rct and Rp values are better determined by using plotting methods. As reported by Klotz [33], the frequency and time-dependent behaviors of the inductive loop-containing systems and the physical processes related to them have not yet been fully explained. This makes it difficult to determine the electrochemical impedance parameters using the equivalent circuit models. The main reasons for this are roughness of the coating surface, corrosion caused by corrosive species and/or adsorption resulting from a variety of the chemical reactions. As seen in Table 4, the polarization resistance at the end of the first hour for uncoated Al-1050 electrode is 185.84 Ω. Polarization resistivities were determined to be 2742.7, 15890.2 and 22983.4 Ω, respectively, when aluminum electrode was coated with PPy, compositeA and -B films, which points to an increase in the polarization resistance of Al-1050 by about 15, 86 and 124 times, respectively. This significant increase, obtained for composite-A and -B films, shows that the nanoparticles significantly improve the corrosion performance of the PPy. The Co3O4 nanoparticles in PPy can inhibit the entry of water and ions through the pores. Another reason for the significant increase in the Rp of all coatings is the use of an OXA-DBSA mixture containing both small and large dopant anions as the electrolyte solution. This is because the PPy film remains stable due to the fact that the large size anions cannot be separated from the polymer matrix. Films prepared using DBSA electrolyte have been also reported to 17

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Fig. 11. SEM images of (a) PPy, (b) composite-A and (c) composite-B films.

Fig. 12. Thickness measurements obtained from SEM images of Al-1050 electrodes coated with (a) PPy, (b) composite-A and (c) composite-B films.

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70 nm to about 300 nm (Fig. 10b). Fig. 11 shows SEM micrographs of the films on the surface of the Al1050 electrode. It is seen that the PPy film is homogeneous and there are grains with sizes approximately between 10 and 40 μm (Fig. 11a). On these grains, relatively much smaller agglomerate particles can be seen. Despite the fact that the surface of the electrode is completely covered, it is also seen that there are some interfaces. In addition, the characteristic granular structure of the PPy matrix is clearly seen. It is seen that this granular structure is generally preserved in the composite-A film as well (Fig. 11b). However, with the metal oxide filling, this granular structure is slightly thinner. In addition, the interfaces that act as discontinuities in the structure and that cause anodic dissolution are reduced as compared to the PPy film. In the case of composite-B film, where the amount of metal oxide is higher, it is observed that the film is also in a granular structure, but with increasing cobalt content, the interfaces are completely refined and transformed into a more homogeneous structure (Fig. 11c). The thickness measurements given in Fig. 12 show more clearly that the metal oxide filling process makes the surface morphology of the PPy quite homogenous. In addition, it is seen that the thickness of the coating decreased significantly as the amount of filling increased. While the evaluated film thickness of the PPy is about 50 μm, the film thicknesses for the composite-A and -B films are about 18 and 13 μm, respectively. EDS analyzes (Fig. 13) taken from a randomly selected region of the films, the cobalt concentration in the composite-A film was found to be 0.103% and in the case of the composite-B film, this value increased to about 0.378% by about 3.5 times. EDS analyzes were used to find out whether cobalt entered into the structure of composites or not, rather than determining the amount of cobalt in the composite-A and -B films. 3.6. Thermogravimetric analysis The TG curves of the samples are shown in Fig. 14. The Co3O4 nanoparticles used for filling appear to be highly stable at the working temperature range [60]. In the TG analyses of the PPy, composite-A and -B films, the limited weight loss observed at around 100 °C is due to moisture. However, two sequential weight loss steps at about 200–450 °C and 450–650 °C are associated with decomposition and degradation processes, respectively [61]. Decomposition process is usually related to removel of the fragments with low molecular weight such as dopants, oligomers and unreacted monomers from the structure. The degradation process is caused by deterioration of the main chain segments of the polymer matrix. Table 5 gives weight losses calculated from TG and derivative TG curves (please see Supplementary file-1), initial (Ti) and final (Tf) temperature values of weight losses, and onset decomposition/degradation temperature values. As can be seen from Table 5, there is a total weight loss of about 9% for the cobalt oxide. The weight loss rates for PPy, composite-A and -B films are close to each other, but it can be said that PPy film is slightly more stable than composite films-A and -B. Depending on the increase in the amount of filler, the weight loss in the composites also increases. It is not surprising that the onset decomposition temperatures of the PPy, composite-A and -B films are particularly close to one another. Since all films are prepared under the same conditions, approximately the same amount of dopants in the films will be removed as a result of heating. In addition, there is no significant difference between the onset degradation temperature values of the films. This may be due to the fact that the interaction of nanometal oxide particles with the polymer chains is not strong enough to increase the thermal stability, and that it is merely a weak electrostatic interaction as mentioned in the FT-IR analyses.

Fig. 13. EDS analyses of composite-A and -B films.

Fig. 14. Weight loss variations of Co3O4 nanoparticles and coatings.

3.5. Surface morphology of PPy and composite films Fig. 10 shows SEM and AFM images of Co3O4 particles filled to PPy for composite film preparation. On the SEM images of the particles, there are different sizes (about 2 μm and smaller) of agglomerate structures and on top of these structures some extremely small particles are seen (Fig. 10a). In AFM topography images, the grain structure of Co3O4 particles is slightly clearer. The particle sizes range from about

4. Conclusion Corrosion behavior of PPy and its composite films prepared in the 19

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Table 5 Onset decomposition and degradation temperature values for weight loss steps. Decomposition Step

Degredation Step

Sample

Ti (°C)

Tf (°C)

Onset (°C)

Ti (°C)

Tf (°C)

Onset (°C)

PPy comp-A comp-B Co3O4

210.67 208.91 209.03 –

421.20 430.19 418.30 –

365.61 367.69 365.11 –

476.19 461.58 470.44 705.91

611.30 605.96 604.08 883.15

512.27 509.82 506.14 848.49

[9] M.G. Hosseini, R. Bagheri, R. Najjar, Electropolymerization of polypyrrole and polypyrrole-ZnO nanocomposites on mild steel and its corrosion protection performance, J. Appl. Polym. Sci. 121 (2011) 3159–3166. [10] G. Ruhi, H. Bhandari, S.K. Dhawan, Designing of corrosion resistant epoxy coatings embedded with polypyrrole/SiO2 composite, Prog. Org. Coat. 77 (2014) 1484–1498. [11] P. Herrasti, A.N. Kulak, D.V. Bavykin, C.P. De Léon, J. Zekonyte, F.C. Walsh, Electrodeposition of polypyrrole-titanate nanotube composites coatings and their corrosion resistance, Electrochim. Acta 56 (2011) 1323–1328. [12] M. Ladan, W.J. Basirun, S.N. Kazi, F.A. Rahman, Corrosion protection of AISI 1018 steel using Co-doped TiO2/polypyrrole nanocomposites in 3.5% NaCl solution, Mater. Chem. Phys. 192 (2017) 361–373. [13] M. Hosseini, L. Fotouhi, A. Ehsani, M. Naseri, Enhancement of corrosion resistance of polypyrrole using metal oxide nanoparticles: potentiodynamic and electrochemical impedance spectroscopy study, J. Colloid Interface Sci. 505 (2017) 213–219. [14] M.A. Domínguez-Crespo, A.M. Torres-Huerta, E. Onofre-Bustamante, A. AlanisValdelamar, M.L. Escudero-Rincón, S.B. Brachetti-Sibaja, Corrosion studies of PPy/ Ni organic–inorganic hybrid bilayer coatings on commercial carbon steel, J. Solid State Electrochem. 19 (2015) 1073–1089. [15] Y. Haseko, N.K. Shrestha, S. Teruyama, T. Saji, Reversal pulsing electrodeposition of Ni/polypyrrole composite film, Electrochim. Acta 51 (2006) 3652–3657. [16] M. Wysocka-Zołopa, E. Gra̧dzka, K. Szymański, K. Winkler, Electrodeposition of nickel, cobalt, and iron on polypyrrole films, Thin Solid Films 548 (2013) 44–51. [17] C. Ramamoorthy, V. Rajendran, Effect of surfactants assisted Co3O4 nanoparticles and its structural, optical, magnetic and electrochemical properties, Optik 145 (2017) 330–335. [18] S.A. Makhlouf, Z.H. Bakr, K.I. Aly, M.S. Moustafa, Structural, electrical and optical properties of Co3O4 nanoparticles, SuperlatticesMicrostruct. 64 (2013) 107–117. [19] H. Wei, C. He, J. Liu, H. Gu, Y. Wang, X. Yan, J. Guo, D. Ding, N.Z. Shen, X. Wang, S. Wei, Z. Guo, Electropolymerized polypyrrole nanocomposites with cobalt oxide coated on carbon paper for electrochemical energy storage, Polymer 67 (2015) 192–199. [20] B. Guo, Q. Kong, Y. Zhu, Y. Mao, Z. Wang, M. Wan, L. Chen, Electrochemically fabricated polypyrrole-cobalt-oxygen coordination complex as high-performance lithium-storage materials, Chem. A Eur. J. 17 (2011) 14878–14884. [21] M. Reghu, Y. Cao, D. Moses, A.J. Heeger, Counterion-induced processibility of polyaniline: transport at the metal-insulatorboundary, Phys. Rev. B 47 (1993) 1758–1764. [22] C. Compère, É. Fréchette, E. Ghali, The corrosion evaluation of painted and artificially damaged painted steel panels by AC impedance measurements, Corros. Sci. 34 (1993) 1259–1274. [23] P. Herrasti, A.I. del Rio, J. Recio, Electrodeposition of homogeneous and adherent polypyrrole on copper for corrosion protection, Electrochim. Acta 52 (2007) 6496–6501. [24] H. Hammache, L. Makhloufi, B. Saidani, Electrocatalytic oxidation of methanol on PPy electrode modified by gold using the cementation process, Synth. Met. 123 (2001) 515–522. [25] R.A. Flores-Estrella, D.E. Pacheco, M.J. Aguilar Vega, M.A. Smit, Study of the influence of a mixed electrolyte of oxalic acid and dbsa on the properties of co-deposited poly(aniline-co-pyrrole), Int. J. Electrochem. Sci. 3 (2008) 1065–1080. [26] M.R. Mahmoudian, Y. Alias, W.J. Basirum, M. Ebadi, Poly (N-methyl pyrrole) and its copolymer with o-toluidine electrodeposited on steel in mixture of DBSA and oxalic acid electrolytes, Curr. Appl. Phys. 11 (2011) 368–375. [27] M.R. Mahmoudian, Y. Alias, W.J. Basirun, Electrodeposition of (pyrrole-co-phenol) on steel surfaces in mixed electrolytes of oxalic acid and DBSA, Mater. Chem. Phys. 124 (2010) 1022–1028. [28] M.R. Mahmoudian, W.J. Basirun, Y. Alias, Synthesis and characterization of poly(Nmethylpyrrole)/TiO2 composites on steel, Appl. Surf. Sci. 257 (2011) 3702–3708. [29] M. Menkuer, H. Ozkazanc, Electrodeposition of polypyrrole on copper surfaces in OXA-DBSA mix electrolyte and their corrosion behaviour, Prog. Org. Coat. 130 (2019) 149–157. [30] J. Fleig, J. Jamnik, J. Maier, Inductive loops in impedance spectroscopy caused by electrical shielding, J. Electrochem. Soc. 143 (1996) 3636–3641. [31] P. Córdoba-Torres, A general theory for the impedance response of dielectric films with a distribution of relaxation times, Electrochim. Acta 282 (2018) 892–904. [32] T. Holm, D.A. Harrington, Understanding reaction mechanisms using dynamic electrochemical impedance spectroscopy: modeling of cyclic voltammetry and impedance spectra, ECS Trans. 85 (2018) 167–176. [33] D. Klotz, Negative capacitance or inductive loop? – A general assessment of a common low frequency impedance feature, Electrochem. commun. 98 (2019) 58–62. [34] J.I. Martins, S.C. Costa, M. Bazzaoui, G. Gonçalves, E. Fortunato, R. Martins, Conditions to prepare PPy/Al2O3/Al used as a solid-state capacitor from aqueous malic solutions, J. Power Sources 160 (2006) 1471–1479. [35] D. Huerta-Vilca, S.R. De Moraes, A. De Jesus Motheo, Aspects of polyaniline electrodeposition on aluminium, J. Solid State Electrochem. 9 (2005) 416–420. [36] A.A. Hermas, M. Nakayama, K. Ogura, Formation of stable passive film on stainless steel by electrochemical deposition of polypyrrole, Electrochim. Acta 50 (2005) 3640–3647. [37] B.D. Mert, R. Solmaz, G. Kardaş, B. Yazıcı, Copper/polypyrrole multilayer coating for 7075 aluminum alloy protection, Prog. Org. Coat. 72 (2011) 748–754. [38] S. Bose, T. Kuila, M.E. Uddin, N.H. Kim, A.K.T. Lau, J.H. Lee, In-situ synthesis and characterization of electrically conductive polypyrrole/graphene nanocomposites, Polymer 51 (2010) 5921–5928. [39] N.V. Blinova, J. Stejskal, M. Trchová, J. Prokeš, M. Omastová, Polyaniline and

Total weight loss (%)

57.906 58.704 60.242 8.902

OXA-DBSA mix electrolyte on the surface of Al-1050 electrode by galvanostatic method in 0.1 M HCl were investigated by potentiodynamic and impedance measurement techniques. Structural, thermal and morphological studies were performed by FT-IR, TG, SEM and AFM analyzes. The filling process increased the homogeneity of PPy film and reduced the thickness of PPy film on the Al-1050 electrode. It was also observed that all coatings were very adhesive.Tafel polarization curves showed that the corrosion current densities of all coatings were significantly lower than uncoated Al-1050 electrode. It was determined that the composite-B film with the highest amount of fillers provides protection efficiency close to 100%. Especially for the inductive loop impedance diagrams, it is determined that the use of Cole-Cole plot is more reliable than the equivalent circuit fit method in the calculation of the polarization resistance. Impedance analyses showed that the polarization resistance of PPy film (˜2743Ω) is much higher than that of the uncoated Al-1050 electrode (˜185Ω). Polarization resistance of PPy film significantly increased from approximately 2750 to 23,000 Ω depending on the amount of cobalt oxide being increased. It was found that the filling process reduces the interfacial surface and it makes thinner the granular structures in the PPy film, thus providing better protection. All coatings provided very good protection on the Al-1050 surface even at the end of the fifth day, while the best protection is provided by the composite-B film, having the highest amount of Co3O4 nanoparticles. Funding This study was supported by the Research Fund of the Kocaeli University (Project No. 2013/21). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2019.05. 010. References [1] G.S. Akundy, J.O. Iroh, Polypyrrole coatings on aluminum-synthesis and characterization, Polymer 42 (2001) 9665–9669. [2] J.D. Stenger-Smith, Intrinsically electrically conducting polymers. Synthesis, characterization, and their applications, Prog. Polym. Sci. 23 (1998) 57–79. [3] W. Li, J. Chen, J. Zhao, J. Zhang, J. Zhu, Application of ultrasonic irradiation in preparing conducting polymer as active materials for supercapacitor, Mater. Lett. 59 (2005) 800–803. [4] D.L. Pile, A.C. Hillier, Electrochemically modulated transport through a conducting polymer membrane, J. Membr. Sci. 208 (2002) 119–131. [5] J.L. Alonso, J.C. Ferrer, M.A. Cotarelo, F. Montilla, S.F. de Ávila, Influence of the thickness of electrochemically deposited polyaniline used as hole transporting layer on the behaviour of polymer light-emitting diodes, Thin Solid Films 517 (2009) 2729–2735. [6] J.I. Martins, M. Bazzaoui, T.C. Reis, E.A. Bazzaoui, L. Martins, Electrosynthesis of homogeneous and adherent polypyrrole coatings on iron and steel electrodes by using a new electrochemical procedure, Synth. Met. 129 (2002) 221–228. [7] N.A. Ogurtsov, A.A. Pud, P. Kamarchik, G.S. Shapoval, Corrosion inhibition of aluminum alloy in chloride mediums by undoped and doped forms of polyaniline, Synth. Met. 143 (2004) 43–47. [8] D. Lesueur, N.D. Albérola, Dynamic mechanical behaviour of electrochemically synthesized polypyrrole films, Synth. Met. 88 (1997) 133–138.

20

Synthetic Metals 254 (2019) 10–21

M. Menkuer and H. Ozkazanc

[40]

[41]

[42]

[43]

[44] [45] [46]

[47]

[48]

[49] [50]

[51] P. Córdoba-Torres, M. Keddam, R.P. Nogueira, On the intrinsic electrochemical nature of the inductance in EIS–a Monte Carlo simulation of the two-consecutive steps mechanism: the rough 3D case and the surface relaxation effect, Electrochim. Acta 54 (2009) 6779–6787. [52] M. Březina, J. Minda, P. Doležal, M. Krystýnová, S. Fintová, J. Zapletal, J. Wasserbauer, P. Ptáček, Characterization of powder metallurgy processed pure magnesium materials for biomedical applications, Metals 7 (2017) 461–483. [53] M. Kissi, M. Bouklah, B. Hammouti, M. Benkaddour, Establishment of equivalent circuits from electrochemical impedance spectroscopy study of corrosion inhibition of steel by pyrazine in sulphuric acidic solution, Appl. Surf. Sci. 252 (2006) 4190–4197. [54] N. Murer, Impedance II : Characterization of a Corroding Metal, (2019) (17.04.2019), https://www.bio-logic.net/wp-content/uploads/Impedance_II_ corrosion.pdf. [55] W.J. Lorenz, F. Mansfeld, Determination of corrosion rates by electrochemical DC and AC methods, Corros. Sci. 21 (1981) 647–672. [56] D.A. Harrington, P. van den Driessche, Mechanism and equivalent circuits in electrochemical impedance spectroscopy, Electrochim. Acta 56 (2011) 8005–8013. [57] J.H. Wang, F.I. Wei, H.C. Shih, Modeling of atmospheric corrosion behavior of weathering steel in sulfur dioxide-polluted atmospheres, Corrosion 52 (1996) 900–909. [58] P.A. Savale, K. Datta, P. Ghosh, M.D. Shirsat, Synthesis and characterization of POA-PVS-DBS, POA-PVS-pTS, POA-pTS-DBS Co-dopants composite films: a comparative study, Int. J. Polym. Mater. 59 (2010) 87–97. [59] B. Zeybek, E. Aksun, A. Üğe, Investigation of corrosion protection performance of poly(N-methylpyrrole)-dodecylsulfate/multi-walled carbon nanotubes composite coatings on the stainless steel, Mater. Chem. Phys. 163 (2015) 11–23. [60] S.X. Wang, L.X. Sun, Z.C. Tan, F. Xu, Y.S. Li, Synthesis, characterization and thermal analysis of polyaniline (PANI)/Co3O4 composites, J. Therm. Anal. Calorim. 89 (2007) 609–612. [61] P.S. Abthagir, R. Saraswathi, Thermal stability of polypyrrole prepared from a ternary eutectic melt, Mater. Chem. Phys. 92 (2005) 21–26.

polypyrrole: a comparative study of the preparation, Eur. Polym. J. 43 (2007) 2331–2341. M. Porcher, C. Esnault, F. Tran-Van, F. Ghamouss, Electrochemical deposition and characterization of polypyrrole in electrolyte based on pyrrolidinium hydrogenosulfate protic ionic liquid, J. Appl. Electrochem. 46 (2016) 1133–1145. M.R. Mahmoudian, W.J. Basirun, Y. Alias, A.K. Zak, Electrochemical characteristics of coated steel with poly(N-methyl pyrrole) synthesized in presence of ZnO nanoparticles, Thin Solid Films 520 (2011) 258–265. N. Parvatikar, S. Jain, C.M. Kanamadi, B.K. Chougule, S.V. Bhoraskar, M.V.N.A. Prasad, Humidity sensing and electrical properties of polyaniline/cobalt oxide composites, J. Appl. Polym. Sci. 103 (2007) 653–658. M. Shabani-Nooshabadi, S.M. Ghoreishi, Y. Jafari, Electrosynthesis of polyanilineTiO2 nanocomposite films on aluminum alloy 3004 surface and its corrosion protection performance, J. Nanostruct. 3 (2013) 65–77. G.W. Walter, A review of impedance plot method sused for corrosion performance analysis of painted metals, Corros. Sci. 26 (1986) 681–703. B.D. Mert, Corrosion protection of aluminum by electrochemically synthesized composite organic coating, Corros. Sci. 103 (2016) 88–94. O. Grari, A.E. Taouil, L. Dhouibi, C.C. Buron, F. Lallemand, Multilayered polypyrrole-SiO2 composite coatings for functionalization of stainless steel: characterization and corrosion protection behavior, Prog. Org. Coat. 88 (2015) 48–53. M. Behpour, S.M. Ghoreishi, N. Soltani, M. Salavati-Niasari, M. Hamadanian, A. Gandomi, Electrochemical and theoretical investigation on the corrosion inhibition of mild steel by thiosalicylaldehyde derivatives in hydrochloric acid solution, Corros. Sci. 50 (2008) 2172–2181. M. Hatami, M. Saremi, R. Naderi, Improvement in the protective performance and adhesion of polypyrrole coating on AZ31 Mg alloys, Prog. Nat. Sci. Mater. Int. 25 (2015) 478–485. M. Metıkoš-Hukovıć, R. Babıć, Z. Grubač, The study of aluminium corrosion in acidic solution with nontoxic inhibitors, J. Appl. Electrochem. 32 (2002) 35–41. V.V. Elkin, A.I. Marshakov, A.A. Rybkina, M.A. Maleeva, Interpretation of the impedance comprising negative capacitance and constant phase elements on Iron electrode in weakly acidic media, Russ. J. Electrochem. 47 (2011) 136–146.

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