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Electrochimica Acta 52 (2007) 5958–5967

Corrosion protection aspects of electrochemically synthesizedpoly(o-anisidine-co-o-toluidine) coatings on copper

Pritee Pawar a, A.B. Gaikwad b, P.P. Patil a,∗a Department of Physics, North Maharashtra University, Jalgaon 425001, Maharashtra, India

b Center for Materials Characterization, National Chemical Laboratory, Pashan, Pune 411008, Maharashtra, India

Received 9 December 2006; received in revised form 14 March 2007; accepted 16 March 2007Available online 20 March 2007

bstract

The poly(o-anisidine-co-o-toluidine) coatings were synthesized on copper substrates by electrochemical copolymerization of o-anisidine with-toluidine using sodium salicylate as supporting electrolyte. These coatings were characterized by cyclic voltammetry, UV–vis absorption spec-roscopy, Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR) and scanning electron microscopySEM). The formation of the copolymer with the mixture of monomers in the aqueous sodium salicylate solution was ascertained by a criticalomparison of the results obtained with the polymerization of the individual monomers, o-anisidine and o-toluidine, respectively. The corro-ion protection aspects of poly(o-anisidine-co-o-toluidine) coatings to copper was investigated in aqueous 3% NaCl solution by potentiodynamic

olarization technique and electrochemical impedance spectroscopy (EIS). The results of the potentiodynamic polarization measurements andIS studies showed that the poly(o-anisidine-co-o-toluidine) coatings provided the effective corrosion protection to copper than that of respectiveomopolymers. The corrosion rate is observed to depend on the feed ratio of o-toluidine used for the synthesis of the copolymer coatings.

2007 Elsevier Ltd. All rights reserved.

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eywords: Copolymer coatings; Poly(o-anisidine-co-o-toluidine); Corrosion re

. Introduction

The use of conducting polymers for the protection of activeetals against corrosion has received focused attention in recent

ears [1–5]. It is now well established that the electrochemicalolymerization is a simple and most convenient method for theynthesizing novel conducting polymers on metallic surfaces6,7]. Mengoli et al. [8] was the first to examine the protec-ive behavior of polyaniline on stainless steel and then in 1985,eBerry [9] showed that the electrochemically synthesizedolyaniline acts as corrosion protective layer on stainless steel inM H2SO4. Since then, several research groups [10–13,5] have

ystematically investigated the electrochemical synthesis of var-ous conducting polymers on oxidizable metals for corrosion

rotection purposes. The common feature of these investiga-ions is that the electrochemical synthesis of conducting polymeroatings on oxidizable metals is preceded by the dissolution of

∗ Corresponding author.E-mail address: pnmu@yahoo.co.in (P.P. Patil).

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013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2007.03.043

t coatings; Cyclic voltammetry; Copper

he base metal at a potential lower than the oxidation poten-ial of monomer. Thus, the oxidation of the metal appears as aimultaneous and competitive oxidation process at the poten-ials adequate for the formation of polymer. Hence, a successfullectrochemical synthesis of conducting polymer coatings onxidizable metals demands a careful choice of the solvent and/orupporting electrolyte and the establishment of electrochemi-al parameters, which will strongly passivate the metal withoutmpeding the electropolymerization process.

Among the conducting polymers, polyaniline and polypyr-ole are the most promising conducting polymers for corrosionrotection of metals. The extent of using these conducting poly-ers is limited due to the exclusivity of the monomers that

re essential for their synthesis [14,6]. To overcome this lim-tation, different synthesis approaches have been attempted.he first approach involves the synthesis of substituted con-ucting polymer coatings on oxidizable metals, with a view

o explore the possibility of utilizing them as an alternativeo parent polymers for corrosion protection. We have devel-ped the appropriate electrochemical polymerization procedureso synthesize strongly adherent poly(o-anisidine) [15,16],

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P. Pawar et al. / Electrochim

oly(o-toluidine) [17,18], poly(2,5-dimethoxyaniline) [19] andoly(2,5-dimethylaniline) [20] coatings on low carbon steelLCS) and copper (Cu) substrates. It was shown that these coat-ngs have remarkable capability to protect LCS and Cu againstorrosion.

The second approach involves the formation of bilayer coat-ngs which either consists of a top coat of conducting polymer onhe layer of the other conducting polymer or a top layer of con-ucting polymer on the metallic coating such as nickel. Recently,uken and his research group [21–24] extensively studied thelectrochemical synthesis of polyaniline and polypyrrole coat-ngs on nickel plated mild steel and copper substrates fromqueous media. They investigated the corrosion behavior ofhese coatings through immersion tests performed in differentorrosive media and the recorded electrochemical impedancepectra at different exposure times. The third approach involveshe use of copolymerization to prepare new polymers withnbuilt tailor-made properties suitable for the application.

ore recently, Bereket et al. [25] reported the electrochemi-al synthesis of poly(aniline-co-2-anisidine) films on stainlessteel in tetrabutylammonium perchlorate/acetonitrile solutionontaining perchloric acid. They investigated the corrosionroperties of these copolymer coatings in 0.5 M hydrochlo-ic acid solution by potentiodynamic technique, open circuitotential measurements and electrochemical impedance spec-roscopy. They found that the polyaniline, poly(2-anisidine)nd poly(aniline-co-2-anisidine) films have corrosion protec-ion effect for 304-stainless steel in aggressive medium of 0.5Cl solution. However, the durability of the poly(aniline-2-

nisidine) films is limited to 3 h.In the work reported in this paper, we have made an attempt

o synthesize strongly adherent poly(o-anisidine-co-o-toluidine)oatings on Cu substrates from aqueous media by using cyclicoltammetry and evaluated the performance of these copolymeroatings against corrosion of Cu in aqueous 3% NaCl. To the bestf our knowledge, there are no reports in the literature dealingith the direct deposition of poly(o-anisidine-co-o-toluidine)

oatings on Cu from aqueous media.The objectives of the present study are—(a) to report the

ew findings on the electrochemical copolymerization of o-nisidine with o-toluidine on Cu from aqueous media; (b)o find potentially good, low cost and easily available elec-rolyte for the electrochemical synthesis of strongly adherentoly(o-anisidine-co-o-toluidine) coatings on Cu substrates; (c)o characterize these coatings by using the electrochemical andpectroscopic techniques; (d) to evaluate the performance ofhese copolymer coatings against corrosion of steel in aqueous% NaCl.

More recently, we have shown that the electrochemicalolymerization of o-anisidine in the aqueous salicylate solu-ion results in the deposition of uniform and strongly adherentoly(o-anisidine) coatings on Cu substrates [26]. These coat-ngs exhibit excellent corrosion protection properties and the

oly(o-anisidine) is found to be most promising coating mate-ial for corrosion protection of Cu in aqueous 3% NaCl. In viewf its ability of without passivating the Cu substrates and with-ut preventing the monomer oxidation, the sodium salicylate

ct

cta 52 (2007) 5958–5967 5959

as chosen as the supporting electrolyte for the electrochemicalopolymerization of o-anisidine with o-toluidine.

. Experimental

.1. Materials

The monomers, o-anisidine and o-toluidine were doubly dis-illed prior to being used for the synthesis of homopolymer andopolymer coatings. In the present study the aqueous sodiumalicylate solution was used as supporting electrolyte.

The Cu substrates (size ∼10 mm × 15 mm and 0.5 mm thick)ere polished with a series of emery papers of different grit size

180, 400, 600, 800 and 1200). After polishing, the substratesere cleaned with acetone and double distilled water and dried

n air. Before any experiment, the substrates were treated asescribed and freshly used with no further storage.

.2. Synthesis of poly(o-anisidine) (POA), poly(o-toluidine)POT) and poly(o-anisidine-co-o-toluidine) (OAOT)opolymer coatings

The POA, POT and OAOT copolymer coatings were syn-hesized on Cu substrates by using cyclic voltammetry. Toynthesize homopolymers, POA and POT coatings the con-entrations of respective monomer and sodium salicylate wereept constant at 0.1 M each. For the electrochemical copoly-erization, a mixture of o-anisidine and o-toluidine with total

oncentration of 0.1 M was used in 0.1 M aqueous salicylateolution. The copolymer synthesis experiments were carriedut by using feed ratio of o-toluidine as 0.1, 0.3, 0.5, 0.7 and.9 by taking the following ratios of monomers o-toluidine/(o-nisidine + o-toluidine) as 10:90, 30:70, 50:50, 70:30 and 90:10,espectively. The synthesized copolymer coatings were abbre-iated by taking into consideration the feed ratio o-toluidinesed for the synthesis. For example, the copolymer coating syn-hesized with a feed ratio o-toluidine 0.5 was abbreviated asAOT5.

The electrochemical polymerization was carried out in aingle compartment three electrode cell with Cu as workinglectrode (1.5 cm2), platinum as counter electrode and sat-rated calomel electrode (SCE) as reference electrode. Theyclic voltammetric conditions were maintained using a SI280B Solartron Electrochemical Measurement System (U.K.)ontrolled by corrosion software (CorrWare, Electrochem-stry/Corrosion Software, Scribner Associates Inc. supplied byolartron, U.K.). The synthesis was carried out by cycling con-

inuously the electrode potential between −1.0 and 1.8 V at aotential scan rate of 0.02 V/s. After 10 cycles, the working elec-rode was removed from the electrolyte and rinsed with doubleistilled water and dried in air.

.3. Characterization of coatings

The POA, POT and OAOT coatings were characterized byyclic voltammetry, UV–vis absorption spectroscopy, Fourierransform infrared (FTIR) spectroscopy, nuclear magnetic

5 ica Acta 52 (2007) 5958–5967

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960 P. Pawar et al. / Electrochim

esonance (NMR) spectroscopy and scanning electronicroscopy (SEM). The FTIR transmission spectra of the

omopolymer and copolymer coatings in the powder formcompressed KBr pellets) was recorded in the spectral range000–400 cm−1 using a Perkin-Elmer spectrometer (1600eries II, U.S.A.). The UV–vis absorption spectroscopy studyas carried out ex situ in the wavelength range 300–1100 nmsing microprocessor controlled double beam UV–vis spec-rophotometer (Model U 2000, Hitachi, Japan). SEM wasmployed to characterize the surface morphology with a Leicaambridge 440 Microscope (Cambridge, England, U.K.). TheH NMR spectra were obtained on a Bruker DSX-300 solid stateMR spectrometer, operating at 300 MHz. The measurementsere carried out in DMSO-d6 solution at ambient temperature.

.4. Study of corrosion performance of the coatings

The corrosion protection performance studies were per-ormed at room temperature in aqueous solution of 3%aCl by using potentiodynamic polarization measurements and

lectrochemical impedance spectroscopy (EIS). For these mea-urements, a Teflon holder was used to encased the polymeroated Cu substrates so as to leave an area of ∼0.4 cm2 exposedo the solution. The potentiodynamic polarization measurementsere performed by sweeping the potential between −0.25 and.25 V from open circuit potential at the scan rate of 0.0002 V/s27]. Before polarization the substrates were immersed into theolution and the open circuit potential was monitored until aonstant value was reached. The potentiodynamic polarizationurves were analysed by using Corr-view software from Scrib-er Associates [28]. This software performs the Tafel fitting andalculates the values of the corrosion potential (Ecorr), corrosionurrent density (jcorr) and corrosion rate (CR) in mm per year.ll the measurements were repeated at least four times and good

eproducibility of the results was observed.The EIS measurements of the POA, POT and OAOT copoly-

er coated Cu were carried out at the open circuit potentialn aqueous 3% NaCl solution. The frequency was varied from.1 Hz to 20 kHz using peak to peak ac excitation potential of.010 V. The analysis of the impedance spectra was done bytting the experimental results to equivalent circuits using theon-linear least square fitting procedure. The quality of fitting toquivalent circuit was judged firstly by the chi-square value (χ2,.e. the sum of the square of the differences between theoreticalnd experimental points) and secondly by limiting the relativerror in the value of each element in the equivalent circuit to 5%.

. Result and discussion

.1. Electrochemical synthesis of homopolymers, POA andOT and OAOT copolymer coatings on Cu

The cyclic voltammogram of the first scan recorded dur-

ng the synthesis of homopolymers, i.e. POA and POT and theopolymer OAOT5 coatings on the Cu substrate from aqueousalicylate solution are shown in Fig. 1. The first positive cyclef these voltammograms is characterized by—(i) a small anodic

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ig. 1. Cyclic voltammograms of the first scan recorded during the synthesis ofa) POA, (b) POT and (c) OAOT5 copolymer coatings on Cu substrates.

eak A and (ii) the onset of oxidation wave followed by an oxida-ion peak B. During the reverse cycle, the anodic current densityecreases rapidly and a negligibly small current density is seenill 0.061 V versus SCE. The negative cycle terminates with aroad peak C at ∼−0.424 to 0.912 V versus SCE. These cyclicoltammograms are qualitatively similar to those reported byhaudhari et al. [24] who have investigated the electrochemicalolymerization of o-anisidine on Cu in the aqueous salicylateolution.

The small anodic peak A is attributed to the formation ofhe passive layer which consists of the copper oxide (Cu2O)nd/or to copper salicylate complex. This passive layer reduceshe further dissolution of the copper without preventing the elec-rochemical polymerization process.

The oxidation peak B corresponds to the oxidation of theonomer(s) and formation of radical cations, which is consid-

red to be the first step in the polymerization of the conductingolymers.

It is significant to note that the onset potentials for the oxida-ion of the monomer(s) present in the electrolyte depends on theype of the monomer(s) and it follows the order—o-anisidine0.386 V versus SCE) < (mixture of o-anisidine and o-toluidine)0.443 V versus SCE) < o-toluidine (0.672 V versus SCE). Thenset potential for the oxidation of a mixture of monomers shiftso lower potential than that for o-toluidine, indicating that the

ixture of o-anisidine and o-toluidine oxidizes more easily than-toluidine alone. It is indicative of the predominance of thenfluence of the o-anisidine on the initiation of the copolymer-zation.

During the polymerization, both monomers, o-anisidine and-toluidine are simultaneously oxidized to generate the respec-ive cation radicals. There can be two possibilities for theurther reactions. Firstly, these cation radicals undergo self-

olymerization, which results in the formation of respectiveomopolymers, i.e. POA and POT. In such a case, the subse-uent scans of the cyclic voltammogram recorded during thelectrochemical copolymerization of o-anisidine and o-toluidine

P. Pawar et al. / Electrochimica Acta 52 (2007) 5958–5967 5961

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asmtions as –OCH3 is more electron donating group than –CH3group. Therefore, the results of the cyclic voltammetry maybe attributed to the higher reactivity of o-anisidine than that

ig. 2. Cyclic voltammograms of the second scan recorded during the synthesisf (a) POA, (b) POT and (c) OAOT5 copolymer coatings on Cu substrates.

hould bear a resemblance with the voltammogram scans of theomopolymers, POA and/or POT. On the other hand, the reac-ivities of the generated cation radicals favor the cross-reactionetween them, which would result into a new intermediate con-isting of o-anisidine and o-toluidine units. In such a case, theyclic voltammograms of the subsequent scans recorded duringhe electrochemical copolymerization of o-anisidine and o-oluidine should be different than the voltammograms observedor the formation of homopolymers, i.e. POA and POT.

The cyclic voltammograms of the second scan recorded dur-ng the synthesis of homopolymers, i.e. POA and POT and theopolymer OAOT5 coatings on the Cu substrate from aqueousalicylate solution are shown in Fig. 2. These voltammogramsndicate that the anodic peak A is not observed. In additiono this, a new anodic peak A1 is observed and the currentensity corresponding to this peak is observed to depend onhe type of the monomer(s) present in the electrolyte in theollowing order—o-anisidine > (mixture of o-anisidine and o-oluidine) > o-toluidine. The presence of the broad reductioneak at about −0.424 to 0.912 V versus SCE in the reverse scannd the emergence of the anodic peak A1 at much lower potentialhan that of the monomer oxidation in the second scan provideshe evidence for the formation of polymer coatings on Cu.

The cyclic voltammogram of the second scan [Fig. 2(c)]ecorded during the electrochemical copolymerization of o-nisidine with o-toluidine is qualitatively similar to thosebtained in the case of the individual homopolymerization. Theoltammograms of the subsequent scans identical to that of sec-nd scan are obtained upto the third scan. After the third scan,he current density corresponding to the anodic peaks decreaseradually with the number of scans and the cyclic voltammo-rams do not show well-defined redox peaks. A typical cyclicoltammogram of the tenth scan recorded during the synthesis of

OA, POT and the copolymer OAOT5 coatings on the Cu sub-trate are shown in Fig. 3. These observations strongly suggesthat the electrochemical copolymerization of o-anisidine with-toluidine on the Cu in aqueous salicylate solution generates F

ig. 3. Cyclic voltammograms of the tenth scan recorded during the synthesisf (a) POA, (b) POT and (c) OAOT5 copolymer coatings on Cu substrates.

he mixture of the corresponding homopolymers rather than trueopolymer.

When the feed concentration of o-toluidine in the comonomereed is varied, the area of peak A1 shows the systematic varia-ion. The variation of the area of peak A1 as a function of theoncentration of o-toluidine in the comonomer feed is shown inig. 4. It is observed that the area of peak A1 decreases with the

ncrease in the concentration of o-toluidine in the comonomereed.

The o-anisidine and o-toluidine are substituted derivatives ofniline with methoxy (–OCH3) and methyl (–CH3) groups sub-tituted at the ortho-position, respectively. The o-anisidine isore reactive than o-toluidine in electrophilic substitution reac-

ig. 4. Variation of the area of peak A1 as a function of feed ratio of o-toluidine.

5962 P. Pawar et al. / Electrochimica A

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homopolymers and OAOT5 copolymer coatings are shown inFig. 6. As can be seen from Fig. 6, the spectral characteristics ofthe OAOT5 copolymer are observed to be remarkably differentfrom those of the individual homopolymers POA and POT.

ig. 5. FTIR spectra of the (a) POA, (b) POT and (c) OAOT5 copolymer coatingsynthesized on Cu substrate using cyclic voltammetry. Q and B denotes thetretching vibrations of the quinoid and benzoid rings.

f o-toluidine during the electrochemical copolymerization of-anisidine with o-toluidine [29].

The FTIR spectra of the homopolymers, POA and POT

nd copolymer OAOT5 coatings on are shown in Fig. 5. Theharacteristic bands in the FTIR spectrum [Fig. 5(a)] of POAoating synthesized on Cu from aqueous salicylate medium aressigned as follows [30–33]: a broad and weak band centered at

Fmr

cta 52 (2007) 5958–5967

3328 cm−1 due to the characteristic N–H stretching vibrations,he bands at ∼1513 and 1590 cm−1 are indicative of stretchingibrations in quinoid (Q) and benzoid (B) rings, respectively,he bands at ∼1250 and 1021 cm−1 are attributed to the pres-nce of an o-methoxy group in the POA and the bands around46 cm−1 and 800–700 cm−1 reveal the occurrence of 1,2- and,3-substitutions.

The FTIR spectrum of POT [Fig. 5(b)] coating synthesized onu from aqueous salicylate medium is shown in Fig. 5(b). Theain characteristic bands of POT are assigned as follows—the

road band at ∼3365 cm−1 is due to N–H stretching mode, thetretching vibrations for the Q and B rings occur at 1594 and492 cm−1, the bands at ∼1314 and 1247 cm−1 are assigned tohe C–N stretching mode for B rings, the band at ∼1144 cm−1 isttributed to a plane bending vibration of C–H, which is formeduring protonation, while the bands between 800 and 700 cm−1

eveals the occurrence of the 1–3 substitutions.The FTIR spectrum of the OAOT5 copolymer coating on

u is presented in Fig. 5(c). The most striking feature of thispectrum is that it exhibits very similar characteristics to thosef POA. This indicates that in the resulting OAOT5 polymeroating there are more o-anisidine units.

The optical absorption spectra of the POA and POT

ig. 6. Optical absorption spectra of (a) POA, (b) POT and (c) OAOT5 copoly-er coatings synthesized on Cu using cyclic voltammetry. (The spectrum was

ecorded ex situ in DMSO solution.)

ica Acta 52 (2007) 5958–5967 5963

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P. Pawar et al. / Electrochim

The optical absorption spectrum of POA coating synthesizedn Cu [Fig. 6(a)] shows a high value of the absorbance between00 and 800 nm with a weak peak at ∼744 nm. This shouldereak at 744 nm is attributed to the formation of emeraldine saltES) form of POA in the coating [34]. Thus, the optical absorp-ion spectroscopy reveals the formation of ES form along withhe some other oxidation state species. The optical absorptionpectrum of POT coating [Fig. 6(b)] shows a broad well definedeak at about 562 nm is attributed to the presence of pernigrani-ine base (PB) form of POT. The PB is the fully oxidized formf POT and is insulating in nature [34].

The optical absorption spectrum of the OAOT5 copolymeroating [Fig. 6(c)] show a broad peaks at about 562 and 720 nm.he simultaneous appearance of the peaks at 562 and 720 nm

eveals the formation of mixed phase of PB and ES.The surface morphology of POA and POT homopolymers

nd OAOT5 copolymer coatings synthesized on Cu were char-cterized by SEM. The surface morphology of the POA coatingFig. 7(a)] is featureless. The surface morphology of POToating [Fig. 7(b)] is relatively uniform and featureless. TheEM image of the copolymer OAOT5 coating synthesizedn Cu is shown in Fig. 7(c). It reveals that the copolymerAOT5 coating is relatively uniform, featureless and pore

ree.In order to establish whether the OAOT5 polymer is a

opolymer or a mixture of homopolymers POA and POT, theseolymers were further characterized by 1H NMR spectroscopy.he 1H NMR spectra of POA, POT and OAOT5 copolymer

ecorded in DMSO-d6 are shown in Fig. 8.The 1H NMR spectrum of POA [Fig. 8(a)] is characterized

y the presence of three main signals [35,36]. The signals inhe region of 6.3–7.9 ppm are due to the protons of the aro-

atic rings. The resonances in the region of 3.5–4.3 ppm arettributed to the methoxy protons. The two strongest peaks at.39 and 3.28 ppm are due to protons of DMSO and water inMSO.The 1H NMR spectrum of POT [Fig. 8(b)] is also charac-

erized by the presence of three main signals. The signals inhe region of 6.3–7.6 ppm are due to the protons of the aro-

atic rings. The resonances in the region of 1.97–2.3 ppm arettributed to the methyl protons. The two strongest peaks at.5 and 3.6 ppm are due to protons of DMSO and water inMSO.The 1H NMR spectrum of OAOT5 copolymer has essentially

he characteristics as that of the POA and POT. This spectrums characterized by five main signals which exactly correspondo the five types of protons on the copolymer chains. As cane seen from [Fig. 8(c)], this spectrum exhibits the signals inwide range from 6.4 to 7.8 ppm, which are attributed to the

romatic protons on the o-anisidine and o-toluidine units. Theesonance peaks from 1.9 to 2.3 ppm are assigned to the pro-ons of the methyl groups. The two strongest peaks at 2.5 and.3 ppm are due to protons of DMSO and water in DMSO. Again

he resonance peaks from 3.1 to 4.1 ppm are assigned to therotons of the methoxy groups. Additionally, two negligiblymall signals between 5 and 6 ppm are assigned to the –NH–inkages. This indicates the formation of high molecular weight

strt

ig. 7. SEM images of (a) POA, (b) POT and (c) OAOT5 copolymer coated Cu.

AOT5 copolymer. The actual o-anisidine/o-toluidine ratio inhe copolymer can be roughly calculated based on the area ratiof the o-anisidine aromatic proton peak to the o-toluidine aro-atic proton peak. The actual o-anisidine/o-toluidine molar ratio

alculated by this method is ∼1.50 for the OAOT5 copolymer.his suggests that in the resulting copolymer there are more-anisidine units than that of o-toluidine units. This supports aigher reactivity of o-anisidine as compared to o-toluidine inhe copolymerization and reflected in the cyclic voltammetrynd FTIR spectroscopy results. Thus, 1H NMR spectroscopic

tudy confirms that o-anisidine and o-toluidine are indeed ableo copolymerize on Cu in aqueous salicylate medium and in theesulting OAOT5 polymer there are more o-anisidine units thanhat of o-toluidine.

5964 P. Pawar et al. / Electrochimica Acta 52 (2007) 5958–5967

Fig. 8. 1H NMR spectra of (a) POA, (b) POT and (c) OAOT5 copolymer recordedin DMSO-d6 at 300 MHz.

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ig. 9. Potentiodynamic polarization curves for (a) POA coated Cu, (b) POToated Cu, (c) OAOT5 coated Cu and (d) uncoated Cu recorded in aqueous 3%aCl solution.

.2. Corrosion protection performance of the OAOTopolymer coating

The porosity in the coating strongly governs the anticorro-ive behave(i)or of the coatings; therefore, the determination oforosity in the coating is essential in order to estimate the over-ll corrosion resistance of the coated substrate. The porosity inOA, POT and OAOT5 copolymer coatings on Cu substrates wasetermined from potentiodynamic polarization measurements.he potentiodynamic polarization curves for uncoated Cu, POAoated Cu, POT coated Cu and OAOT5 coated Cu recorded inqueous 3% NaCl are shown in Fig. 9. The values of the Ecorr,corr, Tafel constants (βa and βc), polarization resistance (Rpol)nd CR obtained from these curves are given in Table 1. Theorosity of the coatings was calculated with the following theelation [37]:

= Rps

Rpc10−(|�Ecorr|/βa) (1)

here P is the total porosity, Rps the polarization resistance of thencoated Cu, Rpc the measured polarization resistance of coatedu, �Ecorr the difference between corrosion potentials and βa is

he anodic Tafel slope for bare Cu substrate. The validity of Eq.1) to calculate porosity values is questionable as the conductingolymers are not electrochemically inert. However, it can onlye used to get an idea about the electrochemical activity of theolymers. The porosity in POA, POT and OAOT5 copolymeroating was found to be ∼2.42, 3.5 × 10−1 and 1.5 × 10−4%,espectively. The significantly lower values of the porosity in theAOT5 copolymer coatings as compared with the respectiveomopolymer coatings permit substantial improvement of theorrosion resistance by hindering the access of the electrolyte tohe Cu substrates.

An analysis of these potentiodynamic polarization curveshows the positive shift in the Ecorr and substantial reductionn the jcorr of the Cu due to the POA, POT and OAOT5 coatings,hich indicates the corrosion resistant character of the POA,

P. Pawar et al. / Electrochimica Acta 52 (2007) 5958–5967 5965

Table 1Results of the potentiodynamic polarization measurements

Sample Ecorr (V) Icorr (�A/cm2) βa (V/dec) βc (V/dec) Rpol (�/cm2) CR (mm/year) %P PE%

Uncoated Cu −0.201 9.370 0.06486 0.2390 2.34 × 103 0.108 – –POA −0.152 1.027 0.04608 0.2609 1.65 × 104 0.012 2.42 85.81POT −0.153 0.188 0.05567 0.6230 1.17 × 105 0.004 0.35 98.00OAOT1 −0.077 0.580 0.04102 0.3811 2.77 × 104 0.006 0.097 91.55OAOT3 −0.001 0.108 0.122 0.1860 2.96 × 105 0.001 0.0005 99.20O 0.172 5

O 0.117O 0.164

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The impedance plots of the POA coated Cu [Fig. 11(b)], POTcoated Cu [Fig. 11(c)] and OAOT5 coated Cu [Fig. 11(d)] are

AOT5 0.014 0.046 0.119AOT7 −0.036 0.0569 0.122AOT9 −0.079 0.084 0.425

OT and OAOT5. The shift in the Ecorr depends on the coatingnd decreases in the order OAOT5 > POA > POT. This implieshat the OAOT5 copolymer coating provides effective protectionor Cu against corrosion in aqueous 3% NaCl in comparison withhose of the corresponding homopolymers.

It is observed that the jcorr decreases from 9.37 �A/cm2 forncoated Cu to 1.027, 0.188 and 0.046 �A/cm2 for POA, POTnd OAOT5 coated Cu, respectively. The corrosion rates of POA,OT and OAOT5 coated Cu are found to be ∼0.012, 0.004 and.00015 mm/year, which are ∼9, 27 and 720 times lower thanhat observed for uncoated Cu. The protection efficiency (PE)as calculated by using the expression:

E% =[Rpc − Rps

Rpc

]× 100 (2)

he PEs for POA, POT and OAOT5 copolymer calculated fromotentiodynamic polarization data are ∼85.81, 98 and 99.66%,espectively. These results reveal that the OAOT5 copolymerffectively protects the Cu and improves the corrosion PE withespect to that offered by the corresponding homopolymers.

To investigate the influence of the o-toluidine concentration inhe comonomer feed on the corrosion protection properties of theAOT copolymer coatings, we have synthesized the coatings bysing feed ratio of o-toluidine as 0.1, 0.3, 0.5, 0.7 and 0.9 and theotentiodynamic polarization measurements were performed inqueous 3% NaCl solution. The dependence of the positive shiftn Ecorr (�Ecorr) on the feed ratio of o-toluidine is shown inig. 10. It is found that the �Ecorr increases with the feed ratiof o-toluidine and it attains a maximum value of 0.215 V at 0.5eed ratio. With further increase in the feed ratio of o-toluidine,he �Ecorr rapidly decreases.

The variation of the jcorr as a function of the feed ratio of-toluidine is also shown in Fig. 10. It is observed that initially

corr decreases significantly when the feed ratio of o-toluidinencreases and is lowest (∼204 times lower than uncoated Cu)or the feed ratio of 0.5. With further increase in the feed ratio of-toluidine, the jcorr increases slowly and with the feed ratio of 1he corrosion rate is found to be 0.188 �A/cm2. Thus, the moreffective protection of the Cu against the corrosion in 3% NaClan be achieved by the copolymer coatings synthesized with aeed ratio of o-toluidine as 0.5.

The Nyquist impedance plot of uncoated Cu, POA coatedu, POT coated Cu and OAOT5 coated Cu recorded in aqueous% NaCl is shown in Fig. 11. These impedance plots were mod-led by the equivalent circuit depicted in Fig. 12. The equivalent

Ft

0 6.64 × 10 0.0005 0.00015 99.660 4.56 × 105 0.0006 0.0013 99.480 6.12 × 105 0.0009 0.004 99.61

ircuit consists of the electrolyte resistance (Rs), pore resistanceRpore), coating capacitance (Cc), charge-transfer resistance (Rct)nd double layer capacitance (Cdl). Instead of capacitance, con-tant phase element (CPE) was used. The CPE represents theeviation from the true capacitance behavior. Although there isvery good agreement between the experimental and simulatedata generated using the same equivalence circuit, there is aonsiderable difference between the uncoated and POA coatedu. Table 2 gives the values of the representative parameters of

he best fit to the experimental data using circuit of Fig. 12 forncoated Cu and polymer coated Cu. The considerable varia-ion in the Rs values is observed, which may be attributed to thelectrolyte trapped inside the polymer. The electrolyte presentnside the pores is likely to have higher resistance due to entryf anions into the polymer layer [38]. However, further investi-ations are required to be carried out to understand the variationn the Rs values.

The impedance plot of uncoated Cu [Fig. 11(a)] can be fittedith two semicircles, a smaller one at high frequency range

ollowed by a larger one at lower frequencies. The first semicircles attributed to the formation of the corrosion film and the secondne to processes occurring underneath the corrosion film. Thus,he impedance plot of uncoated Cu is a characteristic of a systemndergoing a dissolution with the precipitation of a corrosion

ig. 10. Dependence of �Ecorr and the corrosion rate on the feed ratio of o-oluidine.

5966 P. Pawar et al. / Electrochimica Acta 52 (2007) 5958–5967

F OT cp

aFiowt

t4ttbft

vt

P

wwor

TI

S

UPPOOOOO

ig. 11. Nyquist impedance plots for (a) uncoated Cu, (b) POA coated Cu, (c) Plots were recorded at open circuit potential in aqueous solution of 3% NaCl.

lso modeled by using the same equivalent circuit depicted inig. 12. However, the parameter values of the best fit to the

mpedance plots are significantly different as compared to thosebtained for uncoated Cu. In this case, the first capacitive loopas attributed to the polymer coating itself and the second one

o processes occurring underneath the coating.The Rct values for POA, POT and OAOT5 coated Cu are found

o be of the order of 1.77 × 104 �/cm2, 9.28 × 104 �/cm2 and.38 × 106 �/cm2 which is about 8, 44 and 2061 times higherhan that of the uncoated Cu. The highest value of the Rct for

he OAOT5 coated Cu is attributed to the more effective barrierehavior of the OAOT5 coating. The lower values of Cc and Cdlor the OAOT5 coated Cu provide further support for the protec-ion of Cu by the OAOT5 copolymer coating. Thus, the higher

nTvT

able 2mpedance parameter values extracted from the fit to the equivalent circuit for the im

ample Rs (�/cm2) Rpore (�/cm2) Cc (F/m)

ncoated Cu 2.75 30.00 3.63 × 10−5

OA 10.13 8.06 × 102 3.17 × 10−7

OT 86.00 2.67 × 102 5.17 × 10−7

AOT1 1.00 9.04 × 102 7.39 × 10−9

AOT3 391.70 1.32 × 105 2.30 × 10−8

AOT5 267.00 1.21 9.95 × 10−9

AOT7 1.00 2.54 × 105 1.77 × 10−9

AOT9 1.05 × 10−5 1.83 × 104 5.10 × 10−7

oated Cu and (d) OAOT5 coated Cu synthesized using cyclic voltammetry. The

alues of Rct and Rpore and lower values of Cc and Cdl indicatehe excellent corrosion performance of the OAOT5 coating.

The PE was calculated by using the expression:

E% =[Rctp − Rct

Rctp

]× 100 (3)

here Rct and Rctp denote the charge-transfer resistance of Cuithout and with coating. The PEs calculated from EIS dataf POA, POT and OAOT5 are ∼88.35%, 97.68%, 99.95%,espectively, which is in agreement with the potentiody-

amic polarization measurement results. As can be seen fromables 1 and 2, the Rct values obtained from EIS data and the Rpalues are observed to depend on the feed ratio of o-toluidine.he Rct and Rp values are found to be maximum at 0.5 feed ratio,

pedance spectra recorded in aqueous 3% NaCl solution

n Rct (�/cm2) Cdl (F/m) n PE%

0.69 2125 7.274 × 10−4 0.72 –0.73 1.77 × 104 2.83 × 10−6 0.65 88.350.89 9.28 × 104 2.99 × 10−6 0.62 97.680.85 4.13 × 104 7.91 × 10−8 0.69 94.890.86 1.21 × 106 2.39 × 10−7 0.64 99.830.92 4.38 × 106 2.42 × 10−7 0.60 99.950.87 1.01 × 106 2.22 × 10−8 0.62 99.820.62 1.37 × 105 2.05 × 10−8 0.84 98.61

P. Pawar et al. / Electrochimica A

wcvi

4

fstwepaoNolaiiacppchoCbw

A

(tIUaIr

R

[[

[

[[

[

[[[

[[

[

[

[

[[[

[

[

[[[

[

[[

Fig. 12. Equivalent circuit used for modeling the impedance plots.

hich further supports the result that the OAOT5 (copolymeroatings synthesized with a feed ratio of o-toluidine as 0.5) pro-ides more effective protection of the Cu against the corrosionn 3% NaCl.

. Conclusions

The poly(o-anisidine-co-o-toluidine) coatings were success-ully synthesized on Cu substrates from aqueous salicylateolution by using cyclic voltammetry and the corrosion pro-ection aspects of these coatings in aqueous 3% NaCl solutionas investigated by potentiodynamic polarization technique and

lectrochemical impedance spectroscopy (EIS). The results ofresent study show that aqueous sodium salicylate solution issuitable medium for the electrochemical copolymerization

f o-anisidine with o-toluidine on the Cu. The FTIR and 1HMR spectroscopic studies reveal that the copolymerization of-anisidine and o-toluidine takes place on Cu in aqueous salicy-ate solution and it is observed that in the resulting polymer therere more o-anisidine units than that of o-toluidine. The porosityn the coating was estimated by using the potentiodynamic polar-zation measurements and it was found that the porosity valuesre significantly lower for the poly(o-anisidine-co-o-toluidine)oatings as compared to the respective homopolymers, i.e.oly(o-anisidine) and poly(o-toluidine). The potentiodynamicolarization measurements reveal that the poly(o-anisidine-o-o-toluidine) coatings effectively protect the Cu and theyave higher corrosion protection efficiency as compared to thatffered by the corresponding homopolymers. The protection ofu against corrosion in 3% NaCl by the copolymer coatings cane achieved more effectively when the synthesis was carried outith a feed ratio of o-toluidine as 0.5.

cknowledgements

The financial support from University Grants CommissionUGC), New Delhi, India under SAP-DRS programme andhe Defence Research and Development Organization (DRDO),ndia through DRDO/ISRO-PUNE University Interaction Cells,

niversity of Pune, India is gratefully acknowledged. The

uthors gratefully acknowledge the NMR Research Center,ndian Institute of Science, Bangalore, India for their help inecording the NMR spectra.

[[[[

cta 52 (2007) 5958–5967 5967

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