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Progress in Organic Coatings 67 (2010) 233–238

Contents lists available at ScienceDirect

Progress in Organic Coatings

journa l homepage: www.e lsev ier .com/ locate /porgcoat

reparation, characterization and anticorrosive properties of a novelolyaniline/clinoptilolite nanocomposite

li Olad ∗, Babak Naseriolymer Composite Research Laboratory, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 29 Bahman Street,abriz, East Azarbayjan 5166614776, Iran

r t i c l e i n f o

rticle history:eceived 3 June 2009eceived in revised form 5 November 2009ccepted 3 December 2009

a b s t r a c t

Nanocomposite of polyaniline (PANI) with natural clinoptilolite (Clino) was prepared. Formation ofnanocomposite and incorporation of polyaniline in the clinoptilolite channels was confirmed and char-acterized using FTIR spectroscopy studies, X-ray diffraction (XRD) pattern, scanning electron microscopy(SEM) and cyclic voltammetry techniques. The anticorrosive properties of a 20 �m thickness coating of

eywords:olyanilinelinoptiloliteeoliteanocompositeorrosiononducting polymers

PANI/Clino nanocomposite with various weight ratios (1, 3 and 5%, w/w) of clinoptilolite content oniron coupons was evaluated and compared with pure polyaniline coating. According to the results inacidic environments PANI/Clino nanocomposite has enhanced corrosion protection effect in compari-son to pure polyaniline coating. Comparative experiments revealed that PANI/Clino nanocomposite with3% (w/w) clinoptilolite content has the best protective properties. Further experiments showed thatthe PANI/Clino nanocomposite has considerably different corrosion protection efficiencies in variouscorrosive environments.

. Introduction

Corrosion is a natural process that has troubled human beingsver since the use of metals. Hence, efforts to develop more effi-ient and environmentally compliant methods to prevent corrosionave been ongoing throughout this century. Three approaches com-only are used to reduce the rate of corrosion: cathodic protection,

assivation (anodic protection), and barrier coatings [1].Conducting polymers have become one of the most attractive

ubjects of investigation in last decades [2]. Their unique prop-rties such as electrical conductivity, electrochemical properties,echanical strength and possibility of both chemical and electro-

hemical synthesis, make them useful in wide area of applicationsuch as rechargeable batteries [3,4], chemical and electrochemicalensors [5,6], electro-chromic devices [7], environmental abate-ent [8–10], switchable membranes [11] and corrosion protection

12,13]. The most popular among the conducting polymers isolyaniline due to its acceptable chemical stability combined withelatively high level of electrical conductivity, monomer availabil-

ty and ease of polymer synthesis. Polyaniline has been showno protect metals against corrosion through its catalytic abil-ty to form a passivating oxide layer. Earlier studies by DeBerry14] confirmed that stainless steel, in the presence of polyani-

∗ Corresponding author. Tel.: +98 411 3393164; fax: +98 411 3340191.E-mail address: a.olad@yahoo.com (A. Olad).

300-9440/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2009.12.003

© 2009 Elsevier B.V. All rights reserved.

line, was passivated considerably under highly acidic conditions.Several strategies have been used to increase the effectiveness ofpolyaniline as anticorrosive coating on metals. Layered materialssuch as montmorillonite have attracted much research interestfor the preparation of polymer/clay nanocomposite in the pastdecades [15,16]. The polyaniline/montmorillonite nanocompositewas found to have enhanced gas barrier, thermal stability, mechan-ical strength, fire retardant and anticorrosive properties [17,18].

Zeolites are crystalline aluminosilicates with a three-dimensional, open anion framework consisting of oxygen-sharingTO4 tetrahedra, where T is Si or Al. Their framework structurecontains interconnected voids that are filled with cations whichcan be exchanged with other cations (such as anilinium cations)[19]. Clinoptilolite (Clino) is the most common and widely dis-tributed zeolite mineral found in nature. Clinoptilolite has atwo-dimensional layer-like structure in which (Si,Al)O4 tetrahe-dral are linked through oxygen atoms in layers [20]. By attention tothe unique properties of layered silicates to enhance the anticor-rosive effect of polyaniline coatings, which has been investigatedin our previous work [21], it is conceivable that other layeredstructures like clinoptilolite can also be tailored to enhance theanticorrosive effect of polyaniline coatings because of the ability

to promote the barrier property of polyaniline against aggressivespecies.

In this work a nanocomposite of polyaniline with natural clinop-tilolite was prepared by chemical oxidative polymerization ofanilinium cations replaced by protons of acidic clinoptilolite. The

2 rganic Coatings 67 (2010) 233–238

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Table 1Chemical composition of the iron samples.

Element Percent (%, w/w) Element Percent (%, w/w)

Fe 99.521 Co 0.0131C 0.0351 S 0.0158Si 0.0133 Cu 0.0481P 0.0145 W 0.0022

34 A. Olad, B. Naseri / Progress in O

ispersion of acidic clinoptilolite at different weight ratios (1, 3, and%, w/w) versus aniline monomer (approximate to the percentagef polyaniline) has been used. The prepared PANI/Clino nanocom-osite was investigated as anticorrosive coating on iron samplesnd compared with pure polyaniline coating in various corrosivenvironments.

. Experimental

.1. Reagents and materials

The natural clinoptilolite used in this study was obtained fromeianah mine in East Azerbaijan, Iran. The aniline (Merck) was

urified under vacuum distillation prior to use. Hydrochloric acid,ethanol, ammonium hydroxide, sulphuric acid, N-methyl pyrroli-

one (NMP) and ammonium persulphate were all purchased fromerck (Germany) and were used as received with out any further

urification.

.2. Instrumentation

A galvanostat/potentiostat SAMA 500 (Iran) and three-electrodelectrochemical cell system consisted of an iron sample (coatedith nanocomposite or pure polyaniline) as working electrode, alatinum gauze as counter electrode and an Ag/AgCl as referencelectrode were used for corrosion tests. A gold electrode coatedith nanocomposite was used as working electrode in electro-

hemical experiments. A micrometer SM 1201 Teclock CorporationJapan) was used to measure the thicknesses of polyaniline andanocomposite films and coatings. A scanning electron microscopySEM) LEO440i (England) was used to investigate the surface

orphology of polyaniline and prepared nanocomposite coating.Fourier transform infrared (FTIR) spectroscopy, Bruker Ten-

or 27 (Germany) was used to investigate the physicochemicalnteractions between organic and inorganic phases. An X-rayiffractometer (XRD) D500 Siemense (Germany) was used to studyhe crystallinity of nanocomposite.

.3. Preparation of acidic clinoptilolite

The natural zeolite rocks were first hammered to break downnto smaller particles. The smaller particles were grinded and mag-etically stirred for 48 h in HCl (0.1 M) solution. The slurry wasltered and washed with excess deionized water until the underashing solution became neutral, followed by drying at 150 ◦C forperiod of 2 h. The acidity of the zeolite can increase the adhesionf polymer to the zeolite.

.4. Synthesis of polyaniline

Chemical polymerization of aniline was carried out in an aque-us acidic solution. 8.09 g of ammonium persulphate was dissolvedn 200 ml of deionized water. This solution was added dropwise to

solution of 4 ml aniline dissolved in 200 ml of HCl (1 M) aque-us solution, while the reaction mixture was vigorously stirred at2 to −5 ◦C for a period of 6 h. The green emeraldine hydrochlo-

ide precipitate was collected and washed repeatedly with 80/20ater/methanol solution until the under washing solution became

olorless.

.5. Preparation of PANI/Clino nanocomposite

It is proposed that the weakly polar aniline finds it more difficulto penetrate into the clinoptilolite channels than a polar aniliniumation. Aniline which exists as anilinium cation in strong acidiconditions of HCl (1 M) solution can be exchanged by H+ cations

Mn 0.02408 Al 0.0347Ni 0.0325 Sn 0.0005Cr 0.0284 – –

of acidic clinoptilolite. Clinoptilolite at weight ratios of 1, 3, and 5%(w/w) versus aniline monomer was dispersed in 200 ml of HCl (1 M)solution containing 4 ml aniline. The mixture was magneticallystirred for 48 h at room temperature. 8.09 g of ammonium persul-phate dissolved in 200 ml of deionized water, was added dropwisefor a period of 6 h to the mixture of anilinium cation and dispersionof clinoptilolite, while the reaction mixture vigorously stirred at −2to −5 ◦C. The polymerization of anilinium cations in the clinoptilo-lite channels was carried out and the nanocomposite of PANI/Clinowas collected as precipitate. The precipitated nanocomposite waswashed repeatedly with 80/20 water/methanol solutions until theunder washing solution became colorless followed by drying at50 ◦C for 48 h.

2.6. Pretreatment and coating of iron samples

Iron coupons with 1 cm × 1 cm × 0.05 cm dimensions were usedin corrosion studies. The chemical composition of the iron samplesis given in Table 1. In order to remove any existing passive film, theiron coupons were mechanically polished using 100 and 400 gradeemery papers followed by rinsing with distilled water and acetoneprior to coating and corrosion experiments.

The green emeraldine hydrochloride as pure or nanocompositewas converted to the blue emeraldine base form by deprotonation(dedoping) in an aqueous ammonia solution (1 M) under vigorousstirring for 4 h. The blue precipitate was filtrated and washed with80/20 water/methanol solution until the under washing solutionbecame neutral. Emeraldine base powder was finally obtained afterthe filtrate was dried in an oven at 50 ◦C for 48 h.

One gram of the emeraldine base form of polyaniline or itsnanocomposite was added (over 5 h) to 40 ml of NMP solvent undermagnetically stirred condition at room temperature for 7 h. It hasbeen found that if the polyaniline powder was added too rapidlyto the NMP, it tended to aggregate. The resulting viscous solutionwas filtered to remove any undissolved particles. Coating of ironsamples by polyaniline or nanocomposite was carried out by cast-ing the viscous solution of polyaniline or nanocomposite in NMP(1 ml) on the surface of iron plates followed by drying in an oven at50 ◦C for 48 h.

3. Results and discussion

3.1. FTIR spectroscopy of PANI/Clino nanocomposite

The PANI/Clino nanocomposite was characterized using FTIRtechnique. The characteristic peaks observed in the FTIR spectraof PANI/Clino nanocomposite give valuable information regardingto the conformation of polyaniline in the clinoptilolite channelsand possible interaction between clinoptilolite and polyanilinechains. Fig. 1 shows the FTIR spectra of clinoptilolite, PANI/Clino

nanocomposite base with 5% (w/w) clinoptilolite content andpure polyaniline as emeraldine base. FTIR spectra of PANI/Clinonanocomposite exhibits bands characteristic of polyaniline as wellas of clinoptilolite which confirms the presence of both componentsin the PANI/Clino nanocomposite.

A. Olad, B. Naseri / Progress in Organic Coatings 67 (2010) 233–238 235

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of clinoptilolite with layer thicknesses in nanometer range (lessthan 100 nanometer). Also the SEM image of nanocomposite (Fig. 5)reveals that the size of polyaniline chains grown in clinoptilolitechannels is in nanometer rage.

ig. 1. FTIR spectra of clinoptilolite, pure polyaniline base and PANI/Clino nanocom-osite base with 5% (w/w) clinoptilolite content.

In the PANI/Clino nanocomposite, the observed peaks forolyaniline were shifted to lower wave numbers which indi-ates complete interaction between polyaniline and clinoptilolitend also reveals the presence of physicochemical interactions,uch as hydrogen bonding between clinoptilolite and polyaniline.oticeable changes were observed for bands which are related

o the structural OH vibrations in the region between 3500 and700 cm−1 related to the bridging OH groups in Al–OH–Si andther hydrogen atoms on different oxygen atoms in the frameworkf clinoptilolite [22]. Changes in these regions indicate that theres complete coverage between polyaniline and OH groups existingn the intrachannels and extrachannels surfaces of clinoptilolite. Inther word, because of the presence of OH groups in and outsidef the channels in clinoptilolite structure, and regarding the com-lete disappearance of OH characteristic absorption peak in FTIRpectra of nanocomposite (Fig. 1), it can be concluded that therere complete interactions between all OH groups (in and outsidef the clinoptilolite channels) and polyaniline chains and thereforeolyaniline chains have grown in and outside of the clinoptilolitehannels.

.2. X-ray diffraction pattern of PANI/Clino nanocomposite

X-ray diffraction is a versatile and non-destructive techniquesed for identification of the crystalline phases present in solidaterials and for analyzing structural properties of the phases

uch as stress, grain size, phase composition, crystal orientationnd defects. Therefore, X-ray diffraction patterns were recordedor clinoptilolite, polyaniline base and PANI/Clino nanocompositease with 5% (w/w) clinoptilolite content (Fig. 2).

For clinoptilolite the characteristic peaks were observed at� = 9.85◦, 22.4◦ and 30.0◦. Presence of these peaks in the XRD pat-ern recorded for PANI/Clino nanocomposite, confirms the presencef clinoptilolite in the nanocomposite composition. The intensityf the XRD pattern peaks can be influenced by crystallinity or byolyaniline chains order in nanocomposite structure. According toig. 2, XRD pattern of polyaniline suggests that it has relativelymorphous structure, but by encapsulation of polyaniline in thelinoptilolite channels, the alignment and arrangements of polyani-

ine chains were significantly improved and as a result, the intensityf the peaks related to the nanocomposite were increased (Fig. 2).

It should be mentioned that the treatment of clinoptilolite incidic medium (HCl 0.1 M) which was described in Section 2.3, notaused to the structural distortion of clinoptilolite because of the

Fig. 2. X-ray diffraction patterns of clinoptilolite, polyaniline base and PANI/Clinonanocomposite base with 5% (w/w) clinoptilolite content.

low concentration of acid solution (0.1 M). Also the comparison ofXRD patterns of acid treated clinoptilolite (Fig. 2) and clinoptilo-lite which not treated with acid (Ref. [22]) shows no difference,and this confirms the non-destructive effect of acid treatment onclinoptilolite structure.

3.3. Investigation of surface morphology

Scanning electron microscopy (SEM) was used to investigate thesurface morphology of polyaniline, clinoptilolite and PANI/Clinonanocomposite and evaluate the effect of presence of both materi-als and the structure of each other. Figs. 3–5 respectively show theSEM images of polyaniline, clinoptilolite and PANI/Clino nanocom-posite. The SEM image of PANI/Clino nanocomposite (Fig. 5) showsthat the orientation of polyaniline chains in the nanocompositestructure has increased in compare to the pure polyaniline (Fig. 3).The SEM image of clinoptilolite (Fig. 4) shows the layered structure

Fig. 3. SEM image of polyaniline base.

236 A. Olad, B. Naseri / Progress in Organic Coatings 67 (2010) 233–238

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Fig. 6. Cyclic voltammetry of PANI/Clino nanocomposite with 5% w/w clinoptilolitecontent coated on a gold electrode (0.5 cm × 0.5 cm and 20 �m thickness) in HCl(1 M) solution with consecutively forty times scan at 25 mV/s scan rate.

Fig. 4. SEM image of clinoptilolite.

.4. Cyclic voltammetry

Cyclic voltammetry studies were carried out to evaluatehe electrochemical behavior and electrochemical stability ofANI/Clino nanocomposite. A gold electrode (0.5 cm × 0.5 cm)oated with a thin layer (20 �m thickness) of nanocomposite wassed as working electrode. Various solutions including HCl (1 M)nd H2SO4 (1 M) were used as electrolyte. Figs. 6 and 7 showhe cyclic voltammograms of PANI/Clino (5%, w/w) nanocompos-te films in HCl (1 M) and in H2SO4 (1 M) electrolytes respectively

ith consecutively forty times scans at 25 mV/s scan rate. Regard-ng the results, it was found that PANI/Clino nanocomposite islectroactive similar to pure polyaniline and present two pairs ofxidation/reduction peaks in cyclic voltammogram. Also resultshowed that the electrochemical behavior of the nanocomposites completely reversible and the nanocomposite has good electro-hemical stability.

.5. Corrosion studies

The electrochemical Tafel slope analysis was used to evaluatehe anticorrosive performance of PANI/Clino nanocomposite coat-ng on iron samples. Tafel plots for pure polyaniline coated and

ig. 5. SEM image of PANI/Clino nanocomposite base with 5% (w/w) clinoptiloliteontent.

Fig. 7. Cyclic voltammetry of PANI/Clino nanocomposite with 5% (w/w) clinoptilo-lite content coated on a gold electrode (0.5 cm × 0.5 cm and 20 �m thickness) inH2SO4 (1 M) solution with consecutively forty times scan at 25 mV/s scan rate.

nanocomposite coated iron samples were recorded by sweepingthe potential from equilibrium potential toward negative and pos-itive potentials against Ag/AgCl reference electrode in H2SO4 (1 M),HCl (1 M) and NaCl (3.5%, w/w) electrolytes. The iron couponswere coated with 20 �m thickness coatings of pure polyanilineand PANI/Clino nanocomposite. Figs. 8–10 show the Tafel plotsfor polyaniline and PANI/Clino nanocomposite with 1, 3 and 5%(w/w) clinoptilolite content coated iron samples respectively inH2SO4 (1 M), HCl (1 M) and NaCl (3.5%, w/w) solutions. In Tables 2–4the values related to the corrosion current (Icorr), corrosion poten-

tial (Ecorr) and corrosion rate (CR) calculated from Tafel plotsfor polyaniline and PANI/Clino nanocomposite with 1, 3 and 5%(w/w) clinoptilolite content coated iron samples respectively inH2SO4 (1 M), HCl (1 M) and NaCl (3.5%, w/w) solutions have been

Table 2Corrosion current (Icorr), corrosion potential (Ecorr) and corrosion rate (CR) valuescalculated from Tafel plots for polyaniline and PANI/Clino nanocomposite with 1, 3and 5% (w/w) clinoptilolite content coated iron samples in H2SO4 (1 M) solution.

CR (mm/year) ICorr (�A cm−2) ECorr (V) Coating

0.0653 5.62 0.015 Polyaniline0.146 12.59 0.02 Nanocomposite 1%0.0164 1.41 0.075 Nanocomposite 3%0.0367 3.16 0.055 Nanocomposite 5%

A. Olad, B. Naseri / Progress in Organic Coatings 67 (2010) 233–238 237

Fig. 8. Tafel plots for polyaniline and PANI/Clino nanocomposite with 1, 3 and 5%(w/w) clinoptilolite content coated iron samples in H2SO4 (1 M) solution.

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Table 3Corrosion current (Icorr), corrosion potential (Ecorr) and corrosion rate (CR) valuescalculated from Tafel plots for polyaniline and PANI/Clino nanocomposite with 1, 3and 5% (w/w) clinoptilolite content coated iron samples in HCl (1 M) solution.

CR (mm/year) ICorr (�A cm−2) ECorr (V) Coating

0.871 75 0.01 Polyaniline0.489 42.17 0.055 Nanocomposite 1%0.0367 3.16 0.065 Nanocomposite 3%0.0923 7.94 0.06 Nanocomposite 5%

Table 4Corrosion current (Icorr), corrosion potential (Ecorr) and corrosion rate (CR) valuescalculated from Tafel plots for polyaniline and PANI/Clino nanocomposite with 1, 3and 5% (w/w) clinoptilolite content coated iron samples in NaCl (3.5%, w/w) solution.

CR (mm/year) ICorr (�A cm−2) ECorr (V) Coating

0.00871 0.75 −0.074 Polyaniline

ig. 9. Tafel plots for polyaniline and PANI/Clino nanocomposite with 1, 3 and 5%w/w) clinoptilolite content coated iron samples in HCl (1 M) solution.

hown. It can be seen that the corrosion potential of PANI/Clino3%, w/w) nanocomposite coated sample is more positively shiftedhan PANI/Clino nanocomposites with 1 and 5% (w/w) clinoptilo-ite content coated samples specially in NaCl (3.5%, w/w) solution

ig. 10. Tafel plots for polyaniline and PANI/Clino nanocomposite with 1, 3 and 5%w/w) clinoptilolite content coated iron samples in NaCl (3.5%, w/w) solution.

0.3673 31.6 0.1 Nanocomposite 1%0.0121 1.04 0.188 Nanocomposite 3%0.0155 1.33 0.149 Nanocomposite 5%

(�E ≈ 0.262 V vs. polyaniline coated sample). Also it was found thatin acidic environments, corrosion current of PANI/Clino (3%, w/w)nanocomposite coated sample is much lower than that of purepolyaniline coated sample and PANI/Clino nanocomposite coatedsamples with 1 and 5% (w/w) clinoptilolite content.

An important point that exists here is encapsulation of polyani-line in the clinoptilolite channels. Clinoptilolite channels wouldincrease the corrosion rate of the iron substrate if the polyani-line were not in the clinoptilolite channels. In fact clinoptilolitechannels could act as a pathway for diffusion of corrosive agents.Therefore, it was found that the encapsulation of polyaniline inthe clinoptilolite channels and dispersion of clinoptilolite lay-ers in polyaniline matrix, promotes the anticorrosive efficiencyof PANI/Clino nanocomposite coating on iron samples. Howeverenhanced corrosion protection of PANI/Clino nanocomposite com-pared to pure polyaniline coated samples might result from layersof clinoptilolite dispersed in polyaniline matrix which increases thetortuosity of diffusion pathway of corrosive agents.

4. Conclusion

The preparation of PANI/Clino nanocomposite was successfullyperformed by in situ polymerization method and the incorpora-tion of polyaniline in the clinoptilolite channels was confirmed byFTIR and XRD studies. The SEM image of PANI/Clino nanocompos-ite showed that the alignment of polyaniline chains was increasedand also confirmed the nanometer size range of polyaniline chainsin clinoptilolite channels. The reversible electroactive behavior ofPANI/Clino nanocomposite and its electrochemical stability werefound by cyclic voltammetry technique. Results of the corrosionstudies showed that in acidic environments corrosion current ofPANI/Clino (3%, w/w) nanocomposite coated iron samples is muchlower than the pure polyaniline coated and PANI/Clino nanocom-posite with 1 and 5% (w/w) clinoptilolite content coated ironsamples.

Acknowledgement

The financial support of this research by the University of Tabrizis gratefully acknowledged.

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