metal oxide nanocomposites for corrosion protection of mild steel—A comparative study

metal oxide nanocomposites for corrosion protection of mild steel—A comparative study

Synthetic Metals 247 (2019) 183–190 Contents lists available at ScienceDirect Synthetic Metals journal homepage: www.elsevier.com/locate/synmet Ele...

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Synthetic Metals 247 (2019) 183–190

Contents lists available at ScienceDirect

Synthetic Metals journal homepage: www.elsevier.com/locate/synmet

Electrodeposition of polypyrrole/ metal oxide nanocomposites for corrosion protection of mild steel—A comparative study

T

Rasoul Babaei-Satia, , Jalal Basiri Parsaa, Mojtaba Vakili-Azghandib ⁎

a b

Department of Applied Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan, 65174, Iran Department of Materials Engineering, Faculty of Engineering, University of Gonabad, Gonabad, Iran

ARTICLE INFO

ABSTRACT

Keywords: Conducting polymers Metal oxide Nanocomposite Electrodeposition Corrosion

Electrodeposition of polypyrrole (PPy) and PPy-metal oxide nanocomposites on mild steel (MS) was carried out in oxalic acid solution by constant potential technique. The protection properties of coatings were studied in 0.5 M H2SO4 solution by Tafel polarization and electrochemical impedance spectroscopy (EIS). The effect of different nanoparticles (Al2O3, ZnO, TiO2, CeO2 and SnO2) on the protection performance of the nanocomposite coatings was compared. The results reveal that PPy/Al2O3 nanocomposite provided the best performance for corrosion protection of the MS by reducing its corrosion current density by 18 times. Furthermore, corrosion protection behavior of PPy/nano-Al2O3 and PPy/micro-Al2O3 composites was compared.

1. Introduction

conducting polymers over other coatings, such as organic coatings, is that conducting polymers act as both a physical and an electronic barrier, improving the corrosion protection in comparative to other materials that act only as physical barriers. Additionally, CPs do not possess the toxicity of other corrosion inhibitors such as hexavalent chromates [12,13]. Among CP’s investigated for corrosion protection, polypyrrole (PPy) has been one of the most studied polymers owing to its easy synthesis, excellent physical and electrical properties and low cost [14–16]. However CPs have disadvantages such as porosity and ion exchange that allow the inward diffusion of electrolytes, resulting in problems with its application to coating [17,18]. Therefore, it is necessary to decrease the porosity of CPs for enhancing corrosion protection [19,20]. The use of PPy as copolymers, composites, nanocomposites or bilayers are the possible solutions to overcome the problems associated with PPy application to the coatings [21,22]. Nanocomposites are produced by usage of some component at nanoscale to obtain desired properties. Because of greater surface activity of nanoparticles (NPs), they can absorb more polymer compare to conventional fillers and thus reduce the free space among polymer matrix and thus enhance the protective properties and performance [23]. Most commonly used NPs in coatings are Al2O3 [24], SiO2 [25], TiO2 [24,26,27], ZrO2 [28], CeO2 [29] and ZnO [22,30] due to their inherent properties. The protective performance of these NPs has been investigated in separate studies, with different experimental conditions, and there are a few comprehensive comparison studies among different NPs under unique condition.

Mild steel (MS) is used extensively in structural work, transport, shipbuilding and many industrial installations due to the low installation costs and its high mechanical strength [1,2]. However, the main drawback of MS is its low resistance to corrosion in acidic environments [3,4]. Therefore, it must be protected against corrosion related degradation [5]. Traditionally, chromate based conversion coatings have been used for many years for the protection of MS. But Over the past several years, the use of chromium containing compounds have been increasingly limited due to their toxic and possibly carcinogenic natures [6]. As an alternative approach, organic coatings are widely used to protect steel substrates against corrosion because they are easy to apply and cost effective [7]. Over the last three decades, conducting polymers (CPs) have been receiving increasing attention as possible components of corrosion resistant coating systems [8]. They are usually deposited chemically or electrochemically in their pure form on the metal. This is an innovative technology. These polymers, when in their doped and conducting condition, are able to protect alloys of steel from corrosion by barrier effect and an anodic protection mechanism (i.e. production of an iron oxide layer with a high protection ability) and they are also able to regenerate the metal oxide layer if the coating is fractured [9]. The ecofriendly nature of CPs along with their simplistic synthesis procedure, good stability and redox properties make them promising coating materials for the corrosion protection of metals [10,11]. The advantage of



Corresponding author. Fax: +98 81 38257407. E-mail address: [email protected] (R. Babaei-Sati).

https://doi.org/10.1016/j.synthmet.2018.12.009 Received 29 August 2018; Received in revised form 30 November 2018; Accepted 11 December 2018 Available online 18 December 2018 0379-6779/ © 2018 Published by Elsevier B.V.

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In this work, series of PPy/ metal oxide (Al2O3, ZnO, TiO2, CeO2 and SnO2 NPs) nanocomposites were electrochemically synthesized on MS. Then the protective performance of these coatings against corrosion was evaluated by electrochemical impedance spectroscopy (EIS) and Tafel polarization measurements in 0.5 M H2SO4 solution. 2. Experimental 2.1. Materials Pyrrole, oxalic acid dehydrate and sulfuric acid were obtained from Merck Company and were used as received. α- Al2O3, ZnO, CeO2 and TiO2 NPs were purchased from MK nano (Canada) and SnO2 NPs were supplied by Neutrino Corporation (Iran). All solutions in the study were prepared by deionized water with conductivity less than 0.08 μS cm−1. 2.2. Electrodeposition of PPy and PPy/ metal oxid nanocomposites on MS Pure PPy and PPy/ metal oxide nanocomposites were electrochemically synthesized by the potentiostatic method with an electrochemical workstation (electroanalyzer system SAMA 500, Iran) in a conventional three-electrode undivided cell. Firstly 5 wt% of metal oxide NPs (in relation to the pyrrole monomer) were dispersed in 50 mL of 0.3 M oxalic acid dehydrate by using ultrasonic bath for 60 min. Then pyrrole monomer was dissolved in electrolyte solution under ultrasonic stirring for 1 min. The prepared solutions were deaerated by bubbling purified nitrogen prior to electrodeposition. Mild steel (St37) electrode was utilized as the working electrode. The rod form of MS electrode was insulated with polyester resin and thus only its cross section (1 cm2) was allowed to contact the electrolyte. A graphite rod and saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. Before electrodeposition, the working electrode was polished with emery papers (120–2000 grade) and then washed with deionized water. Electrodeposition was carried out in two steps. First a cyclic potential from –0.5 to 1.2 V vs. SCE at a scan rate of 10 mVs−1 was applied, then electropolymerization continued at a constant potential of +0.95 V vs. SCE for 200 s. During the polymerization, electrolyte solution was kept under slow speed magnetic stirring (rpm = 80). After polymerization the coated MS was rinsed with deionized water and used directly for electrochemical measurements.

Fig. 1. Tafel polarization curves for bare MS and MS coated by pure PPy and PPy/metal oxide nanocomposites in 0.5 M H2SO4 solution. Table 1 Tafel parameters for uncoated and coated MS substrates. Samples

Ecorr (V)

icorr (A cm−2)

ba (mV)

bc (mV)

Rp (Ω cm2)

Bare MS MS/PPy MS/PPy-SnO2 MS/PPy-CeO2 MS/PPy-TiO2 MS/PPy-ZnO MS/PPy-Al2O3

−0.512 −0.532 −0.538 −0.505 −0.510 −0.533 −0.525

0.00148 0.00054 0.00024 0.000174 0.000144 0.000121 0.000082

116.3 109.8 103 96.3 98.8 95.7 105.9

94.3 94.1 113.7 114 109.1 114.3 122

12.9 33.6 78.6 100.7 122 158.4 220

Table 2 Reduction potentials of different metal oxides.

2.3. Electrochemical measurements

NPs

Al2O3

-ZnO

-TiO2

SnO2

Reduction Potentials (V)

−2.33

−1.26

−0.502

0.15

bare MS and coated MS with pure PPy and PPy/metal oxide nanocomposites in 0.5 M H2SO4 solution. The corrosion resistance (Rp) is estimated from the linear relation of i - E curve at the potential region near corrosion potential (Ec) [31]. As can be seen, compared with the uncoated MS substrate, all the coated MS exhibited a drastic decrease in both anodic and cathodic current densities, thus revealing the improved corrosion resistance of the coated MS substrates. In particular, after addition of metal oxide NPs into PPy, the coated substrates show much lower icorr than those of pure PPy coated and bare MS, demonstrating a reduction in the corrosion rate of the nanocomposite coated substrates. It can be resulted that a dense and compact PPy/metal oxide nanocomposite provide an improved protective barrier layer to prevent the corrosion of MS surface, whereas porous PPy coating might not offer such adequate protection as more electrolytic species can penetrate through the coating to the underlying substrate [32]. According to Fig. 1, the nanocomposites exhibit different protective effects with an order of PPy-Al2O3 > PPyZnO > PPy- TiO2 > PPy-CeO2 > PPy-SnO2. The reduction potential of employed NPs were illustrated in Table 2. Whereas the average size of used different NPs is in the similar range (20–40 nm), the difference in protective performance probably can be related to their standard reduction potential values, so that Al2O3 with the lowest reduction potential (-2.33 V) has the highest protective effect. When a coating containing metal oxide NPs with higher reduction potential than MS (e.g. SnO2 with reduction potential of -0.117) is deposited on MS, NPs

The electrochemical studies were performed by Zahner/Zennium potentiostat/galvanostat (Zahner, Germany), in a conventional threeelectrode (working electrode, counter graphite rod and reference SCE electrodes) at room temperature. The protection properties of coatings were investigated in nondeaerated 0.5 M H2SO4 solution by Tafel polarization and EIS techniques after 3 h of immersion in electrolyte. The Tafel polarization curves were measured at the open circuit potential at the rate of 2 mVs−1 and the EIS measurements were made at open circuit potential with a perturbation voltage of 10 mV in a frequency range of 100 kHz to 10 mHz. Zview software was applied to fit EIS data to the proposed equivalent circuit. The samples morphology, after 3 h immersion in 0.5 M H2SO4 solution, was examined by a JSM-840 A scanning electron microscopy (SEM). 3. Results and discussion 3.1. Corrosion protection properties of PPy and PPy/metal oxide nanocomposites coatings 3.1.1. Tafel polarization test Polarization studies have been carried out for evaluation of the protective properties of coatings. Fig. 1 and Table 1 demonstrate the Tafel polarization curves and corresponding corrosion parameters of 184

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Fig. 2. SEM images of (A) pure PPy, (B) PPy/Al2O3, (C) PPy/CeO2, (D) PPy/SnO2, (E) PPy/TiO2 and (F) PPy/ZnO nanocomposites coatings on MS substrate.

Fig. 3. Nyquist plots for uncoated and coated MS substrates in 0.5 M H2SO4 solution.

that are in contact with MS can be reduced to highly soluble species or their metal state. During the corrosion tests in the high acidic medium, the reduced metal oxide NPs can be oxidized and dissolved into the solution, consequently coating porosity increases compared to PPy-

Al2O3 nanocomposite. Fig. 2 shows SEM images of pure PPy and PPy/ metal oxide nanocomposites after 3 h immersion in 0.5 M H2SO4 solution. As can be seen the pure PPy has the highest porous structure and PPy-Al2O3 nanocomposite show lower porosity than the other coatings. 185

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Fig. 4. Equivalent circuits for fitting the impedance data: (A) Bare MS; (B) PPy and PPy/metal oxide coatings.

3.1.2. Electrochemical impedance spectroscopy (EIS) The Nyquist diagrams of PPy and PPy/metal oxide nanocomposites coatings after 3 h immersion in 0.5 M H2SO4 solution and fitting of the Nyquist diagrams are shown in Fig. 3. Equivalent circuit models for simulating the process are illustrated in Fig. 4 and the fitting values are given in Table 3. To acquire a sufficient fitting data, the double layer capacitance was replaced by the constant phase element (CPE), which takes into account the non-ideal behavior of the interface. In Fig. 4a, uncoated MS exhibits one single semicircle, indicating the charge transfer at electrode/solution interface. The Nyquist plots of PPy and PPy/metal oxide nanocomposites exhibit two overlapped loops at high and low frequencies that are associated to corrosion process happening at the electrolyte/coating interface (associated parameters: Rcoat and CPEcoat) and the electrolyte/substrate interface (associated parameters: Rcorr and CPEdl) respectively. The capacitance can be calculated from CPE constant (Y) by using the following expression:

Fig. 5. Tafel polarization curves for pure PPy and micro- and nanocomposite of PPy/Al2O3 in 0.5 M H2SO4 solution.

Ccoat=ԐԐ0S/d where d is the thickness of the coating, S stands for the surface area of the electrode, ε0 represents the permittivity of the vacuum, and ε is the local dielectric constant, respectively. The excellent anticorrosion property of nanocomposites in comparison to pure PPy can be related to the size and shape of NPs, which have the large aspect ratio, and their more compact structure. The origin of CPE behavior has been attributed to porosity, surface roughness, fractal geometry, non-uniform current distributions, and the presence of grain boundaries. Nyikos and Pajkossy showed that the CPE parameter could be expressed as a function of fractal dimension, and interpreted as a measure of surface irregularity regardless of the shape and structure of the irregularities. Accordingly the CPE behavior studies were used to coating porosity interpretation [35]. In the present work, the value of n for coatings has a tendency to decrease in coatings containing NPs with higher reduction potential than that of MS. As a result the metal oxide NPs are reduced in acidic media and dissolved in the solution resulting in the increase of the coating porosity.

C = Y (ωc)n−1 where ωc is the critical angular frequency (rad/s) at which the maximum in the imaginary component of the impedance occurs and n indicates CPE exponent representing the physical meaning of the CPE that varies in a range from 0 to 1. A value of zero for n implies pure resistance, and a value of 1 indicates pure capacitance [32,33]. Rcoat can be attributed to the electric resistance of ionic transfer through coating pores. In order to have clear comparison, the solution resistance, Rs, has been ignored in the curves. As can be seen in Table 3, the Rcorr values for pure PPy and PPy/metal oxide nanocomposites are significantly higher than the bare MS. The presence of a coating, in general, will cause a decrease in the rate of charge transfer between the metal and the solution interface. Also the higher Rcoat values for PPy and nanocomposites can be attributed to the effective barrier behavior of the coatings [18]. On the other hand, the capacity of the coating (Ccoat) decreased as its corrosion resistance increased. As can be observed in below equation, the coating capacity of different coatings decreases with the local dielectric constant reduction and/or the coating thickness increasing [34].

3.2. Anticorrosive behavior of micro- and nano- composite coatings To compare the effect of nano- and micro- sized particles on anticorrosive property of coating, micro- and nanocomposites of PPy/Al2O3 with same filler content (5 wt%), have been deposited on MS. Fig. 5 and Table 4 show the Tafel polarization curves and corresponding corrosion parameters of pure PPy and micro- and nanocomposite of PPy/Al2O3 in 0.5 M H2SO4 solution. As can be seen, PPy/micro-Al2O3 composite coating shows higher and lower corrosion protection compared to pure PPy and PPy/nano-Al2O3 composite coatings, respectively. Al2O3 microparticles can be placed into free spaces of the polymer matrix and prevent corrosive species from reaching the MS surface, hence PPy/ micro-Al2O3 composite showed better anti-corrosive behavior

Table 3 Fitting results of impedance spectra for uncoated and coated MS electrodes in 0.5 M H2SO4 solution. Samples

Rcoat (Ω. cm2)

ϒcoat × 10−4 (Sn.Ω−1. cm-2)

n1 Coating

Ccoat (μF. cm−2)

Rcorr (Ω. cm2)

ϒdl × 10−4 (Sn.Ω−1. cm-2)

n2 Metal

Cdl (μF. cm−2)

Bare MS MS/PPy MS/PPy-SnO2 MS/PPy-CeO2 MS/PPy-TiO2 MS/PPy-ZnO MS/PPy-Al2O3

– 2.4 30 34.9 34.4 59.9 154.3

– 2.11 4.2 1.4 1.5 3.1 2.8

– 0.79 0.71 0.62 0.58 0.65 0.48

– 0.59 0.52 0.04 0.06 0.34 0.07

0.8 26.9 35.7 57.8 95.5 119.3 277.4

3.32 5 2.6 6 4.2 1.4 2.52

48 0.78 0.75 0.68 0.53 0.73 0.85

17.07 1.73 0.78 1.21 0.43 0.53 1.44

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Table 4 Tafel parameters for micro- and nanocomposite of PPy/Al2O3 coated on MS substrate. Samples

Ecorr (V)

icorr (A cm−2)

ba (mV)

bc (mV)

Rp (Ω cm2)

MS/PPy MS/PPy-micro Al2O3 MS/PPy-nano Al2O3

−0.532 −0.550 −0.525

0.00054 0.00030 0.000082

304 191 251

157 167 217

33.6 90 220

Fig. 6. SEM images of (A) PPy/ micro-Al2O3 and (B) PPy/nano-Al2O3 coatings on MS substrate.

Fig. 7. Nyquist plots for pure PPy and micro- and nanocomposite of PPy/Al2O3 in 0.5 M H2SO4 solution. Table 5 Fitting results of impedance spectra for micro- and nanocomposite of PPy/Al2O3 coated on MS substrate. Samples

Rcoat (Ω. cm2)

ϒcoat × 10−4 (Sn.Ω−1. cm-2)

n1 Coating

Ccoat (μF. cm−2)

Rcorr (Ω. cm2)

ϒdl × 10−4 (Sn.Ω−1. cm-2)

n2 Metal

Cdl (μF. cm−2)

MS/PPy-micro Al2O3 MS/PPy-nano Al2O3

12 154.3

1.4 2.8

0.63 0.48

0.14 0.07

81 277.4

5.9 2.52

0.72 0.85

1.60 1.44

Tafel polarization studies. The Nyquist plots of pure PPy and micro and nano composite of PPy/Al2O3 coatings in 0.5 M H2SO4 solution and fitting of the Nyquist plots are shown in Fig. 7. The fitting values are listed in Table 5, where PPy/nano-Al2O3 composite coating has much higher Rcorr and Rcoat values than that of PPy/micro-Al2O3 composite coating.

compared to pure PPy. On the other hand the smaller size and larger aspect ratio of NPs in comparative to microparticles lead to better distribution of NPs in PPy matrix and lower porosity of coating, and thus, better anti-corrosive property. Fig. 6 shows SEM images of microand nano composites of PPy/Al2O3. The more compact morphology of nanocomposite in compared to PPy/micro-Al2O3 composite is clearly visible from the images. EIS measurements confirm the results of the 187

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Fig. 8. Nyquist plots for the bare MS after various exposure times in 0.5 M H2SO4 solution.

Fig. 9. Nyquist plots for the PPy/Al2O3 nanocomposite coated MS after various exposure times in 0.5 M H2SO4 solution.

3.3. Effect of immersion time on corrosion behaviors of bare MS and coated MS

resistances decrease with increasing immersion time. For MS coated with PPy/Al2O3 nanocomposite, by increasing immersion time from 3 h to 48 h, the electrolyte can find more paths through the nanocomposite film to MS surface and the rate of charge transfer between the MS and the solution interface increases, and hence the corrosion resistance decreases. After 48 h, as corrosion products lead to a closing of the pores, the corrosion resistance does not show significant change. Fig. 10 and Table 7 show the Tafel polarization curves and corresponding corrosion parameters of bare MS and nanocomposite coated MS after

The effect of immersion time on protective performance of nanocomposite coating has been investigated in 0.5 M H2SO4 solution. The Nyquist diagrams obtained for bare MS and MS coated with PPy/Al2O3 nanocomposite at different immersion times (3, 24, 48 and 96 h) are presented in Figs. 8 and 9 respectively and the fitting values are given in Table 6. It can be seen clearly that for both uncoated and coated MS, 188

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Table 6 Fitting results of impedance spectra for bare MS and PPy/Al2O3 nanocomposite coated MS after various exposure times in 0.5 M H2SO4 solution. Samples

Time (h)

Rcoat (Ω. cm2)

ϒcoat × 10−4 (Sn.Ω−1. cm-2)

n1 Coating

Ccoat (μF. cm−2)

Rcorr (Ω. cm2)

ϒdl × 10−4 (Sn.Ω−1. cm-2)

n2 Metal

Cdl (μF. cm−2)

Bare MS

3 24 48 96 3 24 48 96

– – – – 154.3 122.8 31 40

– – – – 2.8 6.4 6.2 22.2

– – – – 0.48 0.48 0.53 0.37

– – – – 0.07 0.20 0.38 0.36

0.8 0.56 0.6 0.61 277.4 104.2 122 97

3.32 1.01 2.6 1.7 2.52 7.4 18.3 41.2

48 277 254 285 0.85 0.87 0.76 0.83

17.07 24.73 39.40 46.32 1.44 5.40 11.86 33.92

MS/PPy-Al2O3

Fig. 10. Tafel polarization curves for uncoated and coated MS substrates after 3 and 96 h exposure times.

Table 7 Tafel parameters for uncoated and coated MS substrates after 3 and 96 h exposure times. Samples

Time (h)

Ecorr (V)

icorr (A cm−2)

Rp (Ω cm2)

Bare MS

3 96 3 96

−0.51 −0.507 −0.525 −0.54

0.00148 0.0028 0.000082 0.0001

12.9 8 220 180.2

MS/PPy- Al2O3

Fig. 11. The appearance of (A) bare MS, (B) PPy/Al2O3 nanocomposite coated MS, and (C) the surface of steel after removal of the PPy/Al2O3 nanocomposite coating, after 96 h of exposure to 0.5 M H2SO4 solution.

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immersion in 0.5 M H2SO4 solution for 96 h. As can be seen, icorr for the bare MS has been increased dramatically (from 0.00148 to 0.00280 A cm−2), whereas for coated MS, the change in icorr is negligible (from 0.000082 to 0.0001 A cm−2). The appearance of the bare MS and PPy/ Al2O3 nanocomposite coated MS samples after exposure to 0.5 M H2SO4 solution for 96 h is shown in Fig. 11. It can be seen that the bare MS surface is highly corroded and damaged. However the surface of steel after removal of the PPy/Al2O3 nanocomposite coating shows no corrosion and is free from rust.

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4. Conclusions In this study, pure PPy and PPy/metal oxide nanocomposites were successfully electrodeposited on mild steel in 0.3 M oxalic acid solution through constant potential technique. Tafel polarization and EIS measurements in 0.5 M H2SO4 solution proved that all PPy/metal oxide nanocomposites coatings exhibited significantly improved corrosion protection on MS substrate. Among nanocomposites, PPy/Al2O3 nanocomposite showed highest protection performance, so that the corrosion rate of this nanocomposite film coated on MS were found to be 18 times lower than those of bare MS. SEM studies showed that the PPy/ Al2O3 nanocomposite after 3 h immersion in 0.5 M H2SO4 solution exhibited more stable, uniform and compact surface morphology. In order to investigate the effect of micro-and nanoparticles on anticorrosive performance of PPy composite coatings, the protection performance of PPy/ nano-Al2O3 and PPy/ micro- Al2O3 composites was compared. Finally, the protection life-time of PPy/ Al2O3 nanocomposite was investigated. The results indicated that its anticorrosive performance showed no significant change after 96 h immersion in 0.5 M H2SO4 solution. It can be concluded that the PPy/Al2O3 nanocomposite is a promising coating for corrosion protection of MS. Acknowledgements The authors wish to acknowledge the Bu-Ali Sina University authorities for providing the financial support to carry out this work. References [1] P.F. Lye, Metalwork Theory - Book 1 Metric Edition, Oxford University Press, 2014. [2] I. Obot, A. Madhankumar, S. Umoren, Z. Gasem, Surface protection of mild steel using benzimidazole derivatives: experimental and theoretical approach, J. Adhes. Sci. Technol. 29 (2015) 2130–2152. [3] B. Priyadarshini, S.M. Stella, A.X. Stango, B. Subramanian, U. Vijayalakshmi, Corrosion resistance of mild steel in acidic environment; effect of Peltophorum ptetocarpum extract, Int. J. Chem. Technol. Res. 7 (2015) 518–525. [4] A. Madhankumar, N. Rajendran, A promising copolymer of p-phenylendiamine and o-aminophenol: chemical and electrochemical synthesis, characterization and its corrosion protection aspect on mild steel, Synth. Met. 162 (2012) 176–185. [5] Y. Ongun, M. Doğru, G. Kardaş, Electrochemical and quantum chemical studies of 2-amino-4-methyl-thiazole as corrosion inhibitor for mild steel in HCl solution, Corros. Sci. 83 (2014) 310–316. [6] A. Khramov, V. Balbyshev, N. Voevodin, M. Donley, Nanostructured sol–gel derived conversion coatings based on epoxy-and amino-silanes, Prog. Org. Coat. 47 (2003) 207–213. [7] T.T.X. Hang, T.A. Truc, N.T. Duong, N. Pébère, M.-G. Olivier, Layered double hydroxides as containers of inhibitors in organic coatings for corrosion protection of carbon steel, Prog. Org. Coat. 74 (2012) 343–348. [8] K. Shah, Y. Zhu, J. Iroh, O. Popoola, Corrosion protection properties of Polyaniline–polypyrrole composite coatings on Al 2024, Surf. Eng. 17 (2001) 405–412. [9] A. Baldissera, C. Ferreira, Coatings based on electronic conducting polymers for

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