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Progress in Organic Coatings 88 (2015) 106–115

Contents lists available at ScienceDirect

Progress in Organic Coatings

j o ur nal ho me pag e: www.elsev ier .com/ locate /porgcoat

valuation of corrosion resistance of polypyrrole/functionalizedulti-walled carbon nanotubes composite coatings on 60Cu–40Zn

rass alloy

li Davoodia,∗, Saleheh Honarbakhshb, Gholam Ali Farzib

Materials and Metallurgical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad 91775-1111, IranHakim Sabzevari University, Materials and Polymer Engineering Department, Sabzevar, P.O. Box 391, Iran

r t i c l e i n f o

rticle history:eceived 11 June 2014eceived in revised form 7 April 2015ccepted 21 June 2015

eywords:

a b s t r a c t

Polypyrrole/multi-walled carbon nanotubes (PPy/MWCNT) and its carboxylic functionalized(PPy/MWCNT-COO−) composite films were successfully electropolymerized by cyclic voltammetry asprotective coating against corrosion on 60Cu–40Zn brass alloy surface. It yielded to strongly adherentand smooth nanocomposite films. Kinetics of the corrosion protection was investigated in 3.5 wt% NaClsolutions by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests.

unctionalized carbon nanotubeISolypyrrole coatingmart coatingrass 60Cu–40Zn alloy

The results showed that the presence of MWCNT in PPy coat considerably reduces the corrosion rateof 60Cu–40Zn brass alloy. The enhanced inhibition is most likely due to interaction between MWCNTand PPy. This in turn, improves the alloy passivation improvement and alters the permselectivity of thecoating from anionic selectivity to the cationic selectivity. Moreover, PPy/MWCNT-COO− functionalizednanocomposite provided higher corrosion resistance coating than PPy/MWCNT alone.

© 2015 Elsevier B.V. All rights reserved.

. Introduction

In recent years, application of conductive polymer coatings suchs polypyrrole (PPy) and polyaniline (PANi) on ferrous and non-errous alloys against corrosion has received much attention [1,2].n addition to applying the physical barrier, which is the protection

echanism of most coatings, the conductive polymers are capableo anodic protection of metal surface by the healing their oxidativeroperties and accelerating the formation of stable metal oxides onhe surface of the substrate [3,4]. In other words, the ability of theonducting polymer to oxidize the substrate metals allows poten-ial of metals to be shifted to the passive state, in which the metalsre protected by the passive oxide formed beneath the conduct-ng polymer. The application of the conducting polymer coating tohe corrosion protection of steels was reviewed by Tallman et al.4].

Various factors during synthesis process of conductive poly-ers coatings are effective on the conductivity of the final polymer.

n addition, the conductivity and oxidation level of the initialonomer plays an important role in causing the oxidative proper-

ies of these coatings. Consequently the researchers have assigned

∗ Corresponding author.E-mail address: a.davodi@um.ac.ir (A. Davoodi).

ttp://dx.doi.org/10.1016/j.porgcoat.2015.06.018300-9440/© 2015 Elsevier B.V. All rights reserved.

numerous studies on the effects of synthesis process parameterssuch as production method [5–7], the environment pH [8], appliedcurrent level [5,9–11], potential [9,12,13], the type and amount ofdopants [14–17], and the type of electrolyte [18–21]. It is knownthat in the doping process of conducting polymers, one positivecharge (cation) can be inserted in maximum three or four pyrroleunits. By adding more positive charge, the PPy changes to over-oxidation state and lose its conductivity [22]. These conditionsmay lead to a slight increase in resistance against corrosion of thesub-layer surface, but it weakened other properties of the coatingsuch as its adhesion and mechanical properties [9,23]. Therefore,researcher’s attempts were recently to strengthen the oxidativeproperties of conductive polymers in other ways and examinedadding a broad range of materials such as surfactant [21,24,25],filler [19,20], producing composites, [26,27], and copolymer [28].An interesting studies conducted by Heina et al. demonstratedthe application of bi-layer coating of polypyrrole on mild steel, inwhich the first and second layers had been sequentially doped bydodecyl sulfate and tetra oxalate anions [14]. They concluded thatthe presence of dodecyl sulfate anions in inner layer prevents thechloride ions attack and the corrosion expansion, since the pres-

ence of this substance alters the permselectivity of the coating fromanionic selectivity to the cationic selectivity. Similar results werealso reported by Nguyen and Schafinghen [29]. In summary, theresearchers concluded that the anodic protection greatly depends

rganic Coatings 88 (2015) 106–115 107

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n the passivity capability of substrate and meanwhile the pre-ention aggressive anions penetration play two important role forhe protection. An ideal coating design combines the inner layers stabilizer of the passive oxide and the outer layer as barrier toggressive anions (Cl−) penetration through PPy to the substrate16].

In the present study, as an alternative strategy is used to increasehe conductivity of PPy and in order to improve its corrosion pre-ention capabilities, the multi-walled carbon nanotubes (MWCNT)ere added. The MWCNT is a conductive nanotube with high ther-al stability. The use of nano-composites consisting this nanotube

nd conductive polymers such as poly thiophene, polypyrrole, etc.reated a new approach for making capacitors, sensors and elec-rodes, etc. [29–35]. This nanotube can increase the conductivityf the polymeric substrate without direct effect on the polymerichains of matrix and its conductivity [31,33–35]. Therefore, theurpose of this research is to study the effect of MWCNT on thetability of the passive layer of the 60Cu–40Zn brass alloy surfacend corrosion resistance of the PPy–MWCNT nano-composite coat-ng. Moreover, in order to study MWCNT functionalization effect, itarboxylic functionalized film (PPy/MWCNT-COO−) was also suc-essfully electropolymerized on above mentioned brass surface andhe results were finally compared.

In brief, the innovation of the present work is the study of syner-ic effect of PPy and functionalized MWCNT on corrosion behaviorf coating which is not reported to the best of our knowledge pre-iously.

Regarding to the substrate it should be mentioned that brassCu/Zn alloy system) is alloy applicable for various outdoor andndoor applications including marine environments, but the corro-ion is a problem due to the presence of zinc in this alloy since itorroded preferentially based on the process called dezincificationrocess. Usually Ni-bronze (Cu70/30Ni alloy) is more promisinglloy system choice but it is much more expensive (because ofickel price) than brass. Our attempt in this research is to use brassith an advanced coating as alternative one.

. Experimental methods and materials

.1. Chemicals and samples

Pyrrole (≥97% pure, Merck) and oxalic acid (Merck) weresed. Pyrrole had a brownish coloring appearance indicating ofigher molecular weight impurities, such as dimmers. It washerefore purified before using by distillation at 100 ± 5 ◦C andtored in a dark bottle at a refrigerator. Sodium salt (>99%) pur-hased from Merck and MWCNT and carboxylic functionalizedWCNT (MWCNT-COOH) purchased from CheapTubes Inc., USA,ith 10–20 nm in diameter and 30 �m in length (>95% pure, 2 wt%OOH content). Solutions were prepared by appropriate dilutionf the material in distilled water. The coating systems were thenpplied on flat 60Cu–40Zn brass alloy electrode surface with 1 cm2

urface area.All the electropolymerization processes and electrochemical

xperiments were carried out with a potentiostat IviumStat, Ivium,he Netherlands. The electrochemical experiments were performedy using three-electrode configuration. The brass and platinumesh electrodes were used as working and counter electrodes,

espectively, and all the potentials were measured vs. saturatedalomel reference (SCE) electrode. To make the coating adherermly to the surface of the brass 60Cu–40Zn alloy, coupons

ere first cleaned mechanically with increasing grades of emeryaper having grit sizes of 100, 400, 600, and 1000. Subsequently,ater/acetone (1:1) solution was used to remove any oil or grease.

he stem of the coupon was insulated with mounting so as to

Fig. 1. Cyclic voltammetry curve of brass alloy Cu60–40Zn (3 cycles) in 0.3 M oxalicacid solution with scan rate of 4 mV/s used as prepassivation process of bare surface.The current peak decreases sequentially.

expose the desired deposition area of 1 cm2. Sonication was car-ried out using a Hieschler (Germany) ultrasound instrument (modelUP200s) to disperse carbon nanotubes in the precursor solution(the frequency was 24 kHz and the power was 100 W). Normal hotplate-stirrer was used for stirring and heating of the samples.

2.2. Preparation of coating

All films were synthesized at room temperature. To make thecoating adhered firmly to the surface, the samples were first prepas-sivited by applying cyclic voltammetry at a linear potential sweeprate of 4 mV/s between −0.200 V and 0.350 V vs. SCE during 3 cyclesin 0.3 M oxalic acid solution as shown in Fig. 1. Brass electrode,platinum mesh electrode (3.5 cm2) and saturated calomel electrode(SCE) were used as working electrode, counter electrode and refer-ence electrode, respectively.

In order to prepare pure PPy coating, cyclic voltammetry wasused to deposit PPy coatings from an electrolyte containing 0.2 Mpyrrole and 0.3 M oxalic acid at a linear potential sweep rate of13 mV/s between 0.3 V and 0.9 V vs. SCE during 40 cycles, Fig. 2a.After coating and before corrosion analysis, the samples wererinsed with distilled water and dried at 35 ◦C for 18 h.

To prepare PPy–MWCNT nanocomposite coating, the films wereprepared through the following procedure; the MWCNT in a cer-tain feeding mass ratio to pyrrole were dispersed in 0.3 M oxalicacid aqueous solution and sonicated for 1 h. Then, 20 mL pyrrolewas dissolved in this emulsion solution under ultrasonic stirringfor 15 min at room temperature and composite films were elec-trochemically synthesized in this medium (Fig. 2b). PPy–MWCNTfilms were washed repeatedly with distilled water and methanolto remove the extra electrolyte and the loose monomers and thendried at 35 ◦C temperature for 18 h in an oven and then sam-ples were kept under vacuum (∼1500 Pa). The feeding mass ratiosof MWCNT were controlled at 0.25, 0.75 and 1 wt%. To charac-terize the chemical structure of synthesized nanocomposites theFourier transform infrared (FTIR) was carried out. FTIR spectra werecollected in transmission mode by using a Bruker (EQUINOX 55,Ettlingen, Germany) FTIR spectrophotometer with DTGS detector(16 scans) in the range of 400–4000 cm−1 wave number at a reso-lution of 4 cm−1.

To provide PPy–MWCNT-COO− nanocomposite coating, the

nanocomposite coating of polypyrrole and MWCNT-COO was syn-thesized electrochemically via in situ electropolymerization from asolution containing both the functionalized-MWCNT and the pyr-role monomer. Here, the partially ionizable carboxylic acid groups

108 A. Davoodi et al. / Progress in Organic Coatings 88 (2015) 106–115

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Fig. 2. Cyclic voltammograms (a selective 6–8 cycles are shown) of brass alloyCu60–40Zn polarized in (a) 0.3 M oxalic acid plus 0.2 M pyrrole, (b) 0.3 M oxalic acidplus 0.2 M pyrrole plus 1 wt% MWCNT and (c) 0.3 M oxalic acid plus 0.2 M pyrroleplus 1 wt% MWCNT-COO− . The scan rate was 13 mV/s. The current peaks decreasesbM

cbeT(ptmb

Table 1Nomenclature of the samples.

Sample number System studied

1 Uncoated brass2 Pure PPy coated3 PPy–0.25% MWCNT4 PPy–0.75% MWCNT5 PPy–1% MWCNT6 PPy–0.25% MWCNT-COOH−

ites were obtained from the extracted powder scratch from the

y increases in the number of cycles and increases by adding both MWCNT andWCNT-COO− .

an act as both the charge carriers in solution and the charge-alancing dopant in the polymer [34–36]. Therefore, no supportinglectrolyte (oxalic acid) is necessary in the polymerization solution.he coating procedure was as follows; the functionalized-MWCNTin a certain feeding mass ratio to pyrrole) was ultrasonically dis-ersed in 150 mL water for 1 h at 28 ◦C and then 2 mL pyrrole (equal

o 0.2 M) was added and sonicated for 15 min. The composite poly-

er coating was directly deposited from this solution on the brassy applying cyclic voltammetry as shown in Fig. 2c. Then, samples

7 PPy–0.75% MWCNT-COO−

8 PPy–1% MWCNT-COO−

washed, dried and kept under vacuum. As can be seen in Fig. 2, thecurrent peaks increase with increases in the number of cycles andalso by adding Pyrrole and MWCNT. Nomenclature of all sampleswith various percentages of MWCNT and MWCNT-COO− are givenin Table 1.

2.3. Corrosion evaluation of coating by electrochemical analysis

All experiments concerning the corrosion investigations pre-sented in this paper were performed in the 3.5% NaCl solution.The corrosion behavior of brass coated by the electrodepositedPPy, PPy–MWCNT and PPy–MWCNT-COO− films was investigatedby the ac impedance (EIS) and polarization technique. Each mea-surement was carried out at a constant imposed potential equalto the stabilized OCP at the beginning of the experiment. The acimpedance measurements were carried out in the frequency regionof 100 kHz–10 mHz. The amplitude of the sinusoidal potential was±10 mV and nine points per frequency decade was recorded. Oncoating test, particularly EIS data acquisition, scattering of the datais a big challenge because of coating thickness variations and alsohigh sensitivity of the EIS technique. A reasonable data scatteringat least for three times measurements on three individual coatedbrass surfaces was observed. In other words, the experiments wererepeat three times to verify the repeatability, however, for datapresentation, the highest achieved impedance data for all measure-ments were chosen providing that the lowest data values were notless than 80% of highest value. Real (Z′) and imaginary (Z′′) compo-nents of the impedance spectra in the complex plane were analyzedusing the EIS analyzer fitting program to estimate the parametersof the equivalent electrical circuit. Polarization curves were car-ried out once the open-circuit potential (OCP) was stable. This isnecessary to allow the surface to reach the equilibrium and obtainaccurate values of the exchange current density and Tafel plotsat the employed conditions. Finally, the Tafel plots were recordedpotentiodynamically to over potential of ±600 mV from the OCP at4 mV/s linear potential sweep rate. The main focus on data analysiswas based on EIS data which is most promising electrochemi-cal techniques for coating evaluation. Therefore, its analysis andinterpretation was done in more details particularly on equivalentcircuit elements. OCP and PDP results was shown and describedbriefly as confirmation of EIS results.

3. Results and discussion

3.1. FTIR results

Fig. 3 stands for the FTIR spectra of PPy–0.75% MWCNT andPPy–0.75% MWCNT-COO− nanocomposite that have been preparedby electropolymerization (other samples with various MWCNTconcentrations show similar spectra). The spectra of nanocompos-

coated brass surface. As can be seen, the spectra of nanocompos-ites are similar to the pure PPy [37]. The chief distinctive bands seenin the spectra of nanocomposites are: the peak at ∼3415 cm−1 is

A. Davoodi et al. / Progress in Organic Coatings 88 (2015) 106–115 109

Fc

aTCtdtss

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Fig. 5. Two proposed equivalent circuits suitable for fitting the EIS results of brasselectrode coated by (a) PPy–MWCNT film and (b) coated by PPy–MWCNT-COO− in3.5 wt% NaCl solutions. (a) RS, solution resistance; R1, coating resistance; R2, sumof charge transfer resistance in metal–polymer interface and oxide film resistancein metal–polymer interface. Q1, coating capacitance; Q2, metal–polymer interface

ig. 3. FTIR spectra of PPy–0.75% MWCNT and PPy–0.75% MWCNT-COO− compositeoats.

ttributable to the hydrogen bonded N H stretching vibration [37].he peaks at ∼1617 cm−1 and ∼1548 cm−1 could be attributed to

N and C C asymmetric and symmetric ring-stretching, respec-ively [32,38]. Additionally the peak near ∼1167 cm−1 presents theoping state of PPy [39,40] and the peak at ∼1041 cm−1 is attributed

o C H in plane deformation and N H stretching vibration. Inclu-ion of MWCNT and COOH− functional group in PPy results inhifting of the peaks [41]. It therefore reveals the interaction of

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No.1 No.2 No.6 No.7 No.8

ig. 4. Open circuit potential–time curves for brass electrode in 3.5% NaCl solutiona) coated by PPy–MWCNT film and (b) coated by PPy–MWCNT-COO− .

capacitance. (b) RS, solution resistance; R1, coating resistance; Q1, coating capac-itance. Suitable equivalent circuit was used on different coating systems withinexposure time.

MWCNT and MWCNT-COOH− with different reaction sites of PPyand shows the presence of MWCNT and MWCNT-COOH− in the PPymatrix.

3.2. Open circuit potential

The OCP changes over the time are shown in Fig. 4. As can beseen, Eocp of the nano-composite coatings are shifting toward morepositive values, which indicates increased in corrosion resistance.Also, PPy–MWCNT-COO− coatings reach to the stable potential inless time compared to other coatings. Eocp values change over thetime for all samples and the lowest change is related to samples4, 5 and 7, 8 as seen in Table 1. These samples with high contentof MWCNT have a relatively stable and adequately noble potentialvalues and, compared to other samples, expect to represent thebetter corrosion performance in the following sections.

3.3. Electrochemical impedance spectroscopy

The Nyquist plots at open circuit potential (the analysis of fre-quency response of samples was performed by selected equivalent

Fig. 6. Nyquist plots of bare brass (left) and PPy coated brass (right) as reference.

110 A. Davoodi et al. / Progress in Organic Coatings 88 (2015) 106–115

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eartliic[

Z

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Z' (Ω.cm )

Fig. 7. Nyquist plots of PPy–MWCNT and PPy–MW

lectrical circuits (shown in Fig. 4)) are shown in Figs. 5 and 7 forll samples. The equivalent circuits of (a) and (b) in Fig. 4 wereespectively considered for data showing one time constant andwo time constants whenever the existence of two distinguishableayers were verified. All equivalent circuit parameters were definedn Fig. 5 caption description. Due to the non-ideal capacitive behav-or at the metal/polymer/electrolyte interfaces, the Q parameter asonstant phase element (CPE) was used instead of a pure capacitor2,11]. The impedance of CPE is as follows:

CPE = [Q (jω)n]−1

(1)

here Q is the CPE element constant (representing capacitive char-cter of CPE element), ω is the angular frequency, and n is thePE element power (representing the surface non-uniformity asoughness). Figs. 8 and 9 represent variations of equivalent cir-

uit elements within the exposure time. Suitable equivalent circuitas used on different coating systems. According to extracted data

hown in Figs. 8 and 9 plots, we can see that the resistance ofhe nanocomposite coated containing MWCNT samples increased

Z' (Ω.cm2)

-COO− coated samples obtained from EIS results.

almost up to 8 days (192 h), indicating the greater resistance againstthe charge transfer. However, the resistance of PPy coating alonehas decreased since the seventh day (168 h). Also the “n” value,which is an indication of surface roughness, is lower in sample no.2 in comparing with nanocomposite coated samples no. 3–8. More-over, the charge transfer resistance of samples 5–8 did not decreasefrom the seventh day up to the tenth day. The improved corrosionresistance of these samples can also recognized also from to thesmaller value of Q [13]. In summary, after 216 h exposure to 3.5%NaCl, the sample coated by PPy–MWCNT-COO− gave the highestR1, n1, and n2 values with low C1 value.

AC measurements follow the change in capacitance and resis-tance of a coated metal in an aqueous solution. As time progressed,the capacitance of coating increased and the resistance decreaseddue to the take-up of water and ions into the coating. The basic idea

is that the uptake of water modifies the dielectric constant, ε, of thecoating and hence the capacitance [10,12,15,19–21,26,30]:

C = εoεA

L(2)

A. Davoodi et al. / Progress in Organic Coatings 88 (2015) 106–115 111

0

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3.4. Polarization curves

ig. 8. Variations of R1, n1 and C1 with exposure time for various coated samples.

where εo is the dielectric constant of free space (vacuum), A ishe area, and L the thickness of the coating. On the other hand, theow-frequency values of |Z| can be used as an indicator of coating

erformance [3,10,11,14–18,20,21,28,30]. Therefore, the absolutealue of the impedance |Z| at ca. 0.01 Hz was measured for vari-us coating systems after 216 h immersion time. The results are

Time (hou r)

Fig. 9. Variations of R2, n2 and C2 with exposure time for various coated samples.

shown in Fig. 10. Coating no. 8 which gives more protective layerhad highest values of |Z| at 0.01 Hz over period of 216 weeks. Morediscussions on proposed corrosion protection mechanism are pre-sented in Section 3.5, accordingly.

Polarization results were shown in Fig. 11 (to exclude theerror, suitable IR drop compensation has been performed on raw

112 A. Davoodi et al. / Progress in Organic Coatings 88 (2015) 106–115

0

5

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20

25

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35

No.1 No.2 No.3 No.4 No.5 No.6 No.7 No.8

Tota

l Im

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Sam ple number

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Table 2Electrochemical parameters of PPy–MWCNT nanocomposite coatings corrosionbehavior obtained from potentiodynamic polarization measurements and Tafelmethod extrapolation in 3.5% NaCl.

Sample no. Ecorr (mV vs. SCE) icorr (mA/cm2) ipassivity (mA/cm2)

1 −420 4.27×10−2 22.912 −210 1.02×10−4 1.62×10−3

3 −220 1.20×10−2 1.15×10−3

4 −160 1.91×10−6 4.47×10−5

5 −100 5.50×10−7 1.62×10−5

6 2 1.20×10−6 1.38×10−6

ig. 10. Total impedance values of various coating systems at 0.01 Hz after 216 hmmersion time.

olarization data). The summarized results implies (in Table 2) theecreased corrosion current density in nano-composite coatings asell as shifted corrosion potential toward the noble values. Also,

he average of passivity current in PPy–MWCNT-COO− coating hasecreased, which indicates the positive effect of “COO−′′

functionalroup on stability of the passive layer. Another point understoodrom the chart is that increased potential of the anodic electrode

nd over-oxidation of the coating leads to the break-down ofome of polymeric bonds and increased permeability of the coat-ng as well as its reduced protective power. Yet, the current rates

ig. 11. Polarization curves of samples in 3.5% NaCl solution, scan rate 4 mV/s (a)Py–MWCNT and (b) PPy–MWCNT-COO− coated brass. No. 1 (uncoated brass) and

(PPy coated) samples are shown as a reference.

7 8 1.07×10−6 8.71×10−7

8 11 4.79×10−8 3.55×10−8

in nanocomposite coatings are still much less than the uncoatedsamples and the samples with PPy coating, which suggests the highstability of the nano-composite coating [42].

3.5. Discussion on the effect of MWCNT and its functionalizing byCOO− on corrosion behavior

Considering the EIS results in Figs. 6–9, it can be seen that onlyone capacitive loop is found during the short periods of immersionand no sign was observed implying an attack on the metal sub-strate. The presence of a capacity loop (one time constant) indicatesthat the corrosion process is performed under control of chargetransfer [13,42]. But then, with increasing the immersion time, twocorresponding semicircles with two capacitive time constants areobserved, which indicate water penetration and the corrosion startof the base metal. These two semi-circle actually show the presenceof a surface layer and a charge transfer in the interface of the coat-ing and metal [10,11]. The high frequency capacitive loop is relatedto the coating and the low frequency loop is related to the doublelayer, since the thickness of the double layer is smaller than thecoating thickness [2]. This issue can be explained due to the anodicprotective properties of the polypyrrole coating, since the coatingacts as a physical barrier during the short times of dipping and onlyone time constant related to the coating is being faced, while inmore period times and by influence of the corrosive agents, the cor-rosion reactions at the interface of metal/coating will also begin andthere will be two time constants. The n2 parameter represents allthe effective factors on the surface non-uniformity and its reductionindicates the increased surface roughness, which is due to increaseddissolution of metal in the solution and a measure of increased cor-rosion rate [42]. Considering the above descriptions and the valuesin Figs. 8 and 9 as well as the values of passivity current in Table 2,among the PPy/MWCNT samples, samples of 2, 3 and 4 have themost resistance to the corrosion. Comparing with these samples,in the polypyrrole coating, after passing only 7 (168 h) days fromthe immersion time and due to the penetration of corrosive solu-tion to the surface of the sub-layer, the charge transfer resistanceis reduced and the capacity of the double layer increases, whichindicates the reduced corrosion resistance. In fact, with increasingthe immersion time, a greater amount of water penetrates into thecoating that creates many paths and channels to aggressive ionsand more corrosion reactions occur on the metal surface and willdestruct the coating. The values of parameter n1 will also confirmthis task so that the lowest values of ‘n’ belong to the polypyrrolecoating, indicating higher levels of non-uniformity of the film sur-face due to corrosion and can be a sign of local corrosion beginning,and thus the more heterogeneity of the coating; the maximumof “n” values is related to the PPy–1% MWCNT coating, which

represents the uniformity of the coatings surface in this sample.According to research conducted on conductive nano-compositepolymers in the presence of MWCNT and their conductance rateon the proposed mechanism of the nanotube impact on increased

A. Davoodi et al. / Progress in Organic Coatings 88 (2015) 106–115 113

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Fig. 12. Schematic illustration of PPy d

esistance against the coatings corrosion, the effects of this mate-ial on the conductivity of polypyrrole coating and improvement ofhe anodic protection mechanism of the coating can be mentioned.he cause of this process can be considered the faster formation ofassive films in PPy–MWCNT nano-composite coatings comparedo the polypyrrole coating. Also, the nanotubes cause the increasedesistance to the corrosion of nano-composite coating by creatingn obstacle against the path of corrosive ions and increasing theons’ paths [26].

Concerning the effect of functionalized MWCNT-COO− on cor-osion behavior, two issues can be mentioned: anodic protectionnd ion exchange phenomena. In the solution at neutral pH, theorrosion potential (or open circuit potential in corrosion) of bareetal is located in the active potential region and the corrosion

ate of the metal is usually relatively high. Owing to the coating ofonducting polymer the maximum current in the active–passiveransition region was limited by the barrier effect and then theotential can be easily shifted to the higher potential in the pas-ive state by strongly oxidative property of the conducting polymer

hich is reported previously [33]. In the passive state, the corro-

ion rate of metal becomes much lower. It is assumed that both thearrier effect and oxidative property induces the anodic protection.inally the potential of the substrate steel may be in agreement with

Fig. 13. Proposed schemes for the functionalization of MW

reaction mechanism with oxalic acid.

a redox potential of the PPy layer in the following reaction and thusdepends on the degree of oxidation state of the PPy layer [30].

ppyn+ ·(

n

x

)Ax− + me ↔ ppy(n−m)+ ·

(n − m

x

)Ax−aq. (3)

The oxidation degree and the conductivity are assumed to declinewith the longer exposure to environment. If oxidants in the envi-ronment re-oxidize the degraded PPy layer, the oxidation degreeand conductivity can be recovered. When the oxidant in the envi-ronment, typically oxygen gas in air, can recover the PPy layer, theduration to maintain the oxidative power of the PPy layer can beprolonged and the passive state of steel underneath the PPy layercan be kept for longer time period [4,16].

The other subject to the effectiveness functionalized MWCNT ision exchange issue. In the anodic protection, the largest problemis breakdown of passive oxide due to attack of aggressive anionssuch as chloride and bromide ions in solution and the breakdownwhich is followed by a large damage of localized corrosion of pittingand crevice corrosion. As contrasted with the cathodic protection,

there is a large risk of the localized corrosion connected with theanodic protection. When we control the doping ions in the PPylayer, we possibly prevent penetration of the aggressive anionsinto the PPy layer. When the metals covered with the conducting

CNT by COO− and doping effect of MWCNT by PPy.

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14 A. Davoodi et al. / Progress in O

Py are immersed in the sodium chloride solution, the anionsoped in the PPy can be exchanged with the chloride anions in thequeous solution. The chloride anions penetrate to the PPy and con-equently to the substrate metals, and then induce the breakdownf the surface passive oxide film, followed by the pitting corrosion.or the protection, the passivity and passive oxide must be kepttable [16,17]. Further, the prevention from penetration of aggres-ive anions plays an important role for the protection. In PPy andPy–MWCNT coating, polymer was dopped with oxalate in ions.xalic acid is an organic compound with the formula H2C2O4 andxalate ions (C2O4

2−) have two COO− group with cationic perms-lectivity (Fig. 12). However in PPy–MWCNT-COO− coating, theoppant ions were carboxyl groups that linked to carbon nano-ubes, so their mobility are very limited (Fig. 13). The PPy dopedith these anions thus is considered as a coating with negatively

harged fixed sites with cationic permselectivity. This film there-ore, excludes the insertion of aggressive anions such as chlorideons much more than coating with only MWCNT.

It should be noted that in samples containing more than 1 wt%f nano-tubes, due to the formation of particles combination andheir lack of proper distribution, a slight decrease was observed indhesion; so that, the PPy–2% MWCNT coating detached the sur-ace immediately after drying and the PPy–1.5% MWCNT coatingnvolved shrinkage after only 1 day of immersion, which indi-ates the local separation of coating from the sub-layer surfacend water accumulation at the interface of metal/coating. This isue to the existence of aggregations in the coating as the defectoints in the system, which adversely affects the coating properties33,36].

. Conclusion

The results can be summarizes as follows:

. Polypyrrole/multi-walled carbon nanotubes (PPy/MWCNT) andits carboxylic functionalized (PPy/MWCNT-COO−) compositefilms was successfully electropolymerized by cyclic voltam-metry as protective coating against corrosion on 60Cu–40Znbrass alloy surface.

. A strongly adherent and smooth nanocomposite film wasobtained.

. Corrosion performance studies in 3.5 wt% NaCl electrolyte by EISand potentiodynamic polarization tests showed that MWCNT inPPy coat considerably reduces the corrosion rate.

. MWCNT interacts with PPy and improves alloy passivation andaltering the permselectivity of the coating to the cationic selec-tivity.

. Interestingly, PPy/MWCNT-COO− functionalized nanocompos-ite provided higher corrosion resistance coating than PPy/MWCNT alone.

cknowledgement

The authors would like to thank Hakim Sabzebari University forroviding experimental facilities to perform the experiments andnancial support.

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