Electrochemical impedance spectroscopy investigation of ... · PDF fileElectrochemical impedance spectroscopy investigation of chlorinated rubber-based coatings containing polyaniline

790 DOI: 10.1002/maco.200905254 Materials and Corrosion 2010, 61 No. 9

Electrochemical impedance spectroscopy investigation ofchlorinated rubber-based coatings containing polyaniline

as anticorrosion agent

A. F. Baldissera, D. B. Freitas and C. A. Ferreira*

Corrosion protection of mild steel by a newly developed chlorinated rubber (CR)-

based coating system containing the inherently conductive polymer polyaniline

(PAni) as an anticorrosion agent was studied. The synthesis of PAni and

preparation of CR-based paint containing this polymer are described herein.The

corrosion behavior of mild steel samples coated with a CR resin, CR/PAni-EB

(emeraldine base), CR/PAni-ES (emeraldine salt), and CR/DBSA-doped PAni were

investigated in 3.5%NaCl solution. For this purpose, electrochemical impedance

spectroscopy and corrosion potential versus time measurements were utilized.

It was found that the addition of the two forms of PAni, doped and undoped, to

the CR resin increased its corrosion protection efficiency.

1 Introduction

Conducting polymers have been extensively studied in recent years

due to a great variety of possible applications in several fields, such

as energy storage systems [1–3], electrocatalysis [4–6], electrodialysis

membranes [7–10], sensors [11–13], and anticorrosive coatings

[14–27]. They exhibit different oxidation states and behave as elec-

tronic or mixed conductors [28]. A polymer coating is expected to

act as a surface modifier which can increase the adhesion of paints

to a metal surface [29] and reduce the corrosion rate of the

protection system. In some cases, the redox behavior of the coating

can provide anodic protection to the substrate [30]. The degree of

corrosion protection afforded by a conducting polymer coating

depends on both its structural and electronic properties [16, 30].

Among the well-known conducting polymers PAni and poly-

pyrrole drawn particular interest frommany researchers owing to

their electronic applications, and also because they have been

used as anticorrosive coatings for almost two decades [16, 31, 32].

PAni is one of most readily prepared conducting polymers [33],

and has controllable electrical conductivity, excellent environ-

mental stability, and easy processability [34, 35].

An organic coating protects a metal substrate from corrosion

primarily via twomechanisms: by forming a barrier against reactants

(water, oxygen, ions, etc.) and by acting as a reservoir for corrosion

inhibitors. The barrier properties of the coating can be improved by

the presence of a pigment (chromate, lead oxides, etc.) [36].

Rohwerder et al. [22–24] discussed that continuous coatings ofconducting redox polymer will fail to provide corrosion protection

in the presence of larger defects, which they cannot passivate, and

C. A. Ferreira, A. F. Baldissera, D. B. Freitas

LAPOL/PPGEM, Universidade Federal do Rio Grande do Sul, P.B. 15010,

CEP 91501-970, Porto Alegre (Brazil)

E-mail: [email protected]

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

will show a fast break-down of the whole coating by fast reduction.

This phenomenon is caused by high cation mobility in the

reduced polymer, which is due to the gradual transformation of

the polymer into an ‘‘autobahn’’ for fast cation transport with

increasing progress of the reduction front. It is proposed that this

is true for all kinds of redox polymer, regardless the kind of

dopant, polymerization conditions, etc. It is assumed that this fast

cation transport will also occur in composite coatings containing

pigments or filaments of conducting polymer in a non-conductive

matrix polymer where high conductivity is reached by extended

percolation networks of the conducting polymer.

PAni-containing paints offer high corrosion resistant coat-

ings for steel surfaces [37–40]. Wessling [41] established a

relationship between the corrosion protection offered by PAni

along with an increase in the corrosion potential and the redox

catalytic activity of the conducting polymer, which was attributed

to the formation of a passive layer of metal oxide. Riaz et al. [26]

discussed that the presence of the conducting polymeric

nanoparticles (PAni and (poly(1-naphthylamine) - PNA) dis-

persed in alkyd coatings) neither seems to alter the strength of the

passive oxide film nor the polymer undergoes deprotonation,

which results in the simultaneous release of the doping anion.

The nanoparticles seem to act as ‘‘effective binders’’ and enhance

the crosslinking of the alkyd matrix with the mild steel. The

uniform dispersion of conducting polymer cements the pores in

the alkyd matrix and helps in the formation of a well adherent,

dense and continuous network-like structure which impedes the

penetration of the corrosive ions until the metal substrate and

protects the mild steel from the attack of the corrosive species.

Sabouri et al. [25] have described that using PAni and PAni-Wcoatings, the dominant protection mechanism will be galvanic as

well as barrier mechanisms. Also, it was demonstrated that the

protection mechanism of the prepared PAni layer will depend on

the nature of the species present into the electropolymerization

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Materials and Corrosion 2010, 61 No. 9 ELS investigation of chlorinated rubber-based coatings 791

Table 1. Paint characteristics and thickness of the dry film

Sample PAnitype

PAniadded (%)

Dry filmthickness (mm)

Paint 1 – 0 95.6� 2

Paint 2 EB 5 108.7� 5

Paint 3 EB 10 86.4� 4

Paint 4 ES 5 114.0� 2

Paint 5 ES 10 105.2� 3

Paint 6 DBSA 5 122.1� 3

Paint 7 DBSA 10 92.5� 3

solution. Tungstate dopants have prolonged the duration of the

galvanic effects and have improved the barrier behavior of the

PAni coating.

Huang et al. [42] utilized electrochemically synthesized PAni

to protect stainless steel used as a bipolar substrate to proton

exchange membrane fuel cell (PEMFC). They described that

corrosion potential of steel coated with PAni film has increased

from �350 to 250mV when tested into the simulated solution

containing sulfate and chloride ions.

Andrew et al. [43] have described in their patent that the

emeraldine base (EB) form of PAni can perform better than the

emeraldine salt (ES), even though the latter is themost conductive

form of PAni. Wei et al. [44] have found a similar behavior for

PAni-EB and acid-doped forms of PAni as protective coating to

cold rolled steel in the aqueous NaCl medium. The PAni-EB was

found to offer good corrosion protection as evidence by the

increase in the corrosion potential and polarization resistance.

This phenomenon may not originate merely from the barrier

effect of the coatings because the nonconjugated polymers, such

as polystyrene and epoxy, did not show the same electrochemical

behavior. Chen et al. [45] reported that PAni-EB/epoxy resin

coating offered efficient corrosion protection ofmild steel in 3.5%

NaCl solution, especially when the EB content was 5–10%.

CR paint systems possess very good chemical and water

penetration resistance. They have therefore long been used as

corrosion protection coatings for steel, concrete, and other civil

engineering materials in marine applications and bridge

construction and as a protective coating for swimming pools.

They can be easily mixed with various pigments and materials,

such asmicaceous iron oxide, titanium dioxide, zinc powders, red

lead, and metallic lead, etc, for steel protection [46, 47].

Electrochemical impedance spectroscopy (EIS) is a powerful

tool providing important information regarding the electroche-

mical characteristics of a system, such as double layer capacitance,

charge transfer resistance, diffusion impedance, and solution

resistance [33]. Using EIS, a vast range of coatings has been tested

as effective barriers against corrosion of metal surfaces in the last

few decades. Paints and other organic and inorganic emulsions

deposited on a metal surface gradually break down creating pin

holes, craters, and other defects when corroded. Subsequently,

Figure 1. Infrared absorption spectra of (a) CR, (b) PAni-EB and (c) Paint

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water as well as other free ions present can penetrate into the

polymer. This so-called ionic attack alters the insulating structure

of the polymer, which modifies the impedance characteristics of

the overall metal/polymer element [48].

EIS technique displays a limitation related to the stability and

steady state of the system during the whole duration of the

experiments. For this reason the frequency response analysis

(FRA) is useless in the analysis of dynamic systems. To be useful,

no changes should be detected during the time system data are

been obtained [33].

The main objective of this study was to prepare adherent

films obtained from CR-based paints containing undoped and

HCl and dodecylbenzenesulfonic acid (DBSA)-doped PAni as

anticorrosion additive applied onto mild steel, and to investigate

the corrosion performance of these coatings in 3.5% NaCl solution.

2 Experimental

2.1 Preparation of PAni

The PAni-ES used in this study (PAni doped with HCl) was

prepared according to the classical procedure, with the amounts

adjusted to allow the polymerization in a 10 liter-capacity double-

walled reactor.

The polymerization reaction of aniline was carried out at

temperatures between �4 and 0 8C, for 8 h. A solution consisting

of an oxidizing agent [(NH4)2S2O8] in 6M HCl was added slowly

2 (CR/PAni-EB)


792 Baldissera, Freitas and Ferreira Materials and Corrosion 2010, 61 No. 9

Figure 2. TGA thermograms of CR, PAni/DBSA and Paint 7 (CR/PAni/

DBSA)

Figure 3. Ecorr versus time measured in 3.5% NaCl solution

with constant agitation to a second 6M HCl solution containing

the monomer.

The PAni-ES obtained was filtered through porous glass funnel

#G5, under low pressure to speed up the process. The green powder

was exhaustively rinsed with distilled water in order to eliminate

the excess of HCl and finally dried in an oven at 60 8C for 24 h.

2.2 Undoping of PAni

PAni-EB was obtained after treatment of PAni-ES with a 0.1mol/L

NH4OH solution in water. After 24 h under stirring and constant

Figure 4. Nyquist plots recorded for Paint 1 after various times of expos


heating, the solution was filtered and the dark blue powder was

rinsed with distilled water and cold acetone (T¼ 5 8C). The

product was dried in an oven at 60 8C for 24 h.

2.3 Doping of PAni with DBSA

Doping of PAni-EB was achieved in a 0.16mol/L aqueous

solution of a commercial DBSA, NACURE 5076 (Kings Indus-

tries Ltd, GB), maintained at 40 8C under vigorous stirring for

24 h. The PAni/DBSA was rinsed with distilled water and cold

acetone (T¼ 5 8C). The polymer was dried in an oven at 60 8C for 24h.

2.4 Preparation of paints containing PAni

A solution of the powdered CR resin Pergut S201 (Bayer) was

prepared by dissolving CR in toluene in a 1:4 weight ratio. The

pigment PAni was completely dispersed in the resin using a

ure to 3.5% NaCl

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GARDNER disperser model DISPERMAT N1, with a Cowles diskoperated at 5000 rpm. The size of particles dispersion, evaluated

with a Fineness of Grind Gage (BYKGardner), was between 5 and

6 Hegman (40 and 30mm). The PAni content in the paint was 5

and 10wt% in relation to the dry resin.

The SAE 1020 mild steel panels measuring (125� 75�0.8) mm were degreased with xylene and the PAni pigmented

paints were applied using a brush. The coating’s thicknesses were

evaluated after solvent evaporation at room temperature.

2.5 Instruments

2.5.1 Electrical conductivity

The electrical conductivity of conductive polymers was measured

using the standard method of four points in an equipment

Signatone model S-301-6, associated with a source Keithley 2400.


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2.5.2 Coating thickness measurements

The thickness of each coating was measured at eight different

locations on the surface using a Byko-test 7500 (BYK Gardner)

thickness meter.

2.5.3 FTIR analysis

Characterization of the polymer and paint films was carried out

by FTIR analysis performed on a FTIR Spectrometer Perkin

Elmer model Spectrum 1000. The samples were pressed into KBr

pellets and analyzed. The FTIR spectra were recorded in the

wavenumber range 4000–400 cm�1 with a spectral resolution of

4 cm�1.

2.5.4 Thermal analysis

The thermal stability of paint coatings was determined with a TA

Instruments model TGA 2050 – Thermogravimetric Analyzer at a

ure to 3.5% NaCl



heating rate of 20 8C/min, from 0 to 1000 8C and under nitrogen

atmosphere.

2.5.5 Electrochemical measurements

All electrochemical experiments were performed in a single com-

partment cell with three electrode configurations: the working

electrode consisted of a steel plate coated with a dry film of the

paint, with an exposed surface area of 0.636 cm2. The reference

electrode was a saturated calomel electrode (SCE) and a platinum

plate was used as counter electrode. Corrosion potential Ecorr as afunction of time of immersion into a 3.5%NaCl solution has been

measured using an ECOCHIMIE model Autolab 30 potentiostat.

EIS measurements were performed with the same potentio-

stat that was equipped with a frequency response analyzer.

Impedance data were measured periodically at the open circuit



potential in 3.5% NaCl solution in the frequency range of 106 to

1Hz using amplitude of sinusoidal perturbation as 10mV AC. A

Faraday cage has been used during EIS experiments. The

resistances Rt were extracted directly from the Nyquist plots and

the capacitance was calculated using the equation

C ¼ 1

2pfRt(1)

where f is frequency at the higher imaginary value; Rt is arc’s

resistance at the intersection with real axis.

No simulation has been done for the system and we have

considered that the equivalent circuit to model the impedance

data is a classical one for polymer coatings on metal substrate.

ure to 3.5% NaCl

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3 Results and discussion

3.1 Electrical conductivity

Electrical conductivity was measured by four point’s method [49].

The conductivity of the PAni-ES was 20 S/cm and of the PAni/

DBSA was 0.3 S/cm. It was not possible to measure PAni-EB

conductivity as well as conductivity of all coating films, because

their conductivity was too low to be adequately measured. Even

films obtained from paints containing 5 or 10% PAni did not

display this propriety. It could be attributable to the other

components present in the films as the pigment and fillers that

are electrical insulators and contributed to the low conductivity of

the samples.


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3.2 Paint and film characteristics

Paint samples with the same basic formulation were prepared,

differing only in the amount and type of PAni added. The

characteristics of the seven samples and the thickness of the

applied films are given in Table 1.

The mean value of the thickness was 103.5mm and the

coatings were very homogeneous on the steel surface.

3.3 FTIR analysis

The pure resin, the polymers used as pigments, and the paints

prepared were subjected to analysis by infrared spectroscopy.

ure to 3.5% NaCl



Figure 1 shows the infrared spectra for CR, PAni-EB, and

Paint 2.

In the CR spectrum, a band of absorption is present at

731 cm�1 assigned to C–Cl stretching. The band at 1427 cm�1 is

attributed to the bending of the methyl group and the band at

2980 cm�1 assigned to C–H stretching.

The PAni-EB spectrum showed two strong absorption bands

at 1586 and 1495 cm�1 assigned to the stretching of the C––C

bonds of the aromatic ring of the quinoid-type (Q) and benzonoid-

type (B) structures, respectively. The band at 1309 cm�1 is

assigned to the stretching of C–N–H bonds and that at 1144 cm�1

to NH–Q–NH bonds [50]. These values are similar to those

described in the literature by many other authors working in the

field of conducting polymers [51, 52].

In the spectrum of Paint 2, an overlapping with the bands

of the CR and PAni-EB spectra is observed. Also, there is a

new band at 1723 cm�1 assigned to the additive (Disperbyk 2070 –

BYKChemie) used to improve the dispersion of PAni in the resin.



As expected, no reaction occurs between the two polymers, as

the FTIR spectra show the band characteristics of both polymers.

3.4 Thermal analysis

Figure 2 shows the thermograms of CR, PAni/DBSA, and Paint 7.

For CR and Paint 7, the weight loss attributed to the chain

polymer degradation occurs between mainly 200 and 600 8C. ThePAni/DBSA thermogram has two mass losses; the first occurs

until around 250 8C and is attributed to water and low boiling

point compounds present in the material, which comprise

between 5 to 15% of the total mass. The second weight loss

starting at 250 8C and can be attributed to the degradation of the

polymer chain. As the amount of PAni/DBSA is small (10wt%)

compared to that of CR, the thermogravimetric curve shows a

trend similar to the curve of CR. The TGA thermograms indicated

that a paint containing PAni can be used in applications where the

ure to 3.5% NaCl

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temperatures are under 175 8C. A similar behavior was observed

for the other polymers and paints evaluated.

3.5 Ecorr versus time test

Corrosion performance of the seven samples of steel coated by a film

of paint was investigated in a 3.5% NaCl solution at room

temperature. The Ecorr versus time plot is shown in Fig. 3. Initial

Ecorr values measured were between 0.13V for Paint 1 without PAni

and�0.61V for Paint 2with 5%of undopedPAni.However, theEcorrshifted towards more negative values over time, as a consequence of

an electrolyte-solution uptake process. For example, Ecorr was

found to be �0.55V for the Paint 1 sample after 840h of immer-

sion. This value is similar to that of bare steel in the same solution.

All steel samples coated with a film of paint containing PAni

had roughly the same behavior in the test except for Paint 3, for

which the Ecorr values were always more positive than those for


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the other samples. The paint used in this case was prepared using

10% of PAni-EB in the formulation.

As the optical microscopy shows (see section 3.7, Figs. 11

and 12), films display some porosity, but even at that points

electrical resistance is high enough to maintain the substrate not

exposed to the corrosive medium and, in consequence, corrosion

potential higher than bare metal corrosion potential had been

detected. This fact has been confirmed by impedance measure-

ments.

From our results, it is possible to affirm that coatings

obtained from paints containing CR and PAni exhibited a more

stable and protective behavior than paint made with only CR

resin. This result confirms the controversy concerning the

efficiency and mechanism of protection of metals by conducting

polymers. Indeed, McAndrew [43, 53] reported a good perfor-

mance of PAni-EB alone and blended with classical polymers as

polyimide, epoxy, and urethane in the protection of mild steel in

ure to 3.5% NaCl



NaCl solution and salt spray test. Similar results were obtained by

Fahlmann et al. [54] using PAni-EB to protect iron and steel in

humidity air. However, Araujo et al. [55] showed that PAni-EB did

not protect steel in 0.01M Na2SO4, even with an epoxy topcoat.

Chen et al. [45] attributed the divergences in the results to the wide

variations in the methodology used in the different experiments.

Conditions as PAni-EB preparation, PAni-EB mixing, and use or

not of a topcoat contribute to the lack of homogeneity between

results described by different authors that work in the field of

corrosion protection of metals by conducting polymers.

3.6 EIS experiments

The Nyquist diagrams are given in Figs. 4 to 10 for different times

of exposure to 3.5% NaCl solution.

Figure 4 shows that for Paint 1 the Nyquist diagram obtained

after 96 h of immersion has a capacitive arc with a capacitance of

Figure 10. Nyquist plots recorded for Paint 7 after various times of expo


1.2� 10�10F/cm2 and the film resistance is around 6.8� 104V � cm2.

After 840 h of immersion the capacitive arc is deformed as a

consequence of the progressive degradation of the coating due to

the diffusion of water through the pore and the film resistance

decreases to 6.9� 103V � cm2.

In contrast with the other samples, Paints 2 and 3 (Figs. 5

and 6), prepared with PAni-EB, showed high film resistance

(2.0� 105V � cm2 or higher) even after 1080 h of the experiment,

characterizing a purely capacitive profile of the coating. This

behavior may be associated with the different sample porosities,

as indicated later in section 3.7.

After penetration of the electrolyte into the coating, all

samples have a minimal polarization resistance, except for

Paint 3. After the corrosive attack, the film resistance

increases again, probably due to the formation of corrosion

products that block the pores and ion transport becomes more

difficult.

sure to 3.5% NaCl

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Themaximum resistance of the coatings is dependent on the

type of sample. Paint 1 had a resistance of 6.8� 104V � cm2 after

96h of immersion in 3.5% NaCl solution while the coating

containing 5% of PAni-EB (Paint 2, Fig. 5) showed a resistance of

8.5� 105V � cm2 after 744h of immersion. Paints containing PAni-

ES (Paints 4 and 5, Figs. 7 and 8) had a resistance of 2.5� 106 and

7.5� 105V � cm2, respectively, after only 72h of the test, and their

resistance dropped to 6.0� 104 and 9.0� 104V � cm2, respectively,

after 1080h. Samples containing PAni/DBSA in their formulation

(Paints 6 and 7, Figs. 9 and 10) showed the lowest resistance value of

all samples: 4.5� 105 and 3.5� 103V � cm2, respectively, after

408 h and 3.5� 104 and 8.5� 102V � cm2, after 1080 h.

From these results, it is possible to note that Paint 3 has the

best performance when analyzed using the corrosion techniques

of this study, even for experiments maintained for a long duration

(more than 1000 h).

Since the films obtained from all paints prepared with PAni

in this study remain longer time with a higher electrical resistance

than filmswhich did not contain PAni in its formulation (Paint 1), it

can be affirmed that PAni had a positive effect in the protection of

the substrate against corrosion in 3.5% NaCl solution.

It is also worth noting that both forms of PAni (undoped and

doped with two different acids) are capable of improving the

performance of CR resin in the protection of steel. Other authors

have previously described the better performance of coatings obtained

Figure 11. Surface of the coating before (left) and after (right) the

corrosion experiments: Paint 2 (a) and (b); Paint 3 (c) and (d); Paint 4 (e)

and (f)

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from paints containing PAni-EB compared to coatings from paints

containing PAni-ES [43, 45, 53, 56]. They attributed this behavior to

the formation of a dense and well adherent polymer film.

In a previous study we demonstrated that paints prepared

with alkyd resin and several forms of PAni were able to protect

steel against corrosion and that coatings containing an undoped

form of PAni offered the best performance compared with several

forms of PAni [50] and with a coating containing chromate

anticorrosion based pigment. McAndrew et al. [43] described a

modification in the behavior of coatings containing undoped

PAni when the samples were subjected to an air oxidation and

heating previous to corrosion evaluation. In our case, even if no

experimental evidence (conductivity or color change) has been

remarked during paint preparation, it could be possible that an

oxidation occurred during dispersion of PAni into the CR.

Dispersion is made in open air and it generates heat because of

high shear of the process.

These results confirm the controversy between authors

concerning the mechanism of protection of metal in the presence

of conducting polymers in their doped or undoped state.

3.7 Film morphology

Figures 11 and 12 show the aspects of the paint films obtained

by optical microscopy before and after being submitted to the

Figure 12. Surface of the coating before (left) and after (right) the

corrosion experiments: Paint 5 (a) and (b); Paint 6 (c) and (d); Paint 7 (e)

and (f)



immersion experiments. As all paints displayed the same

fineness, their morphology was similar before samples were

exposed to the corrosive solution (Figs. 11a,c,e and 12a,c,e). Only

the paints prepared using PAni/DBSA (Figure 12c and e)

presented a different morphology, with holes whose diameters

are higher than the other samples. Some of these experiments last

for more than 40 days, and almost all coatings have disappeared

from the surface.

It should be noted that Paint 3 (Figure 11c and d), which

had the best performance in the corrosion tests, is the sample

that underwent less degradation after these tests. After the

experiment this sample remained on the steel surface in a higher

amount than the other samples. The notable behavior of this paint

in the tests can be explained by the formation of a dense and well

adherent film of polymer on the steel surface, as reported

previously [53, 56].

4 Conclusions

The aim of mixing a conducting polymer with a classical polymer

that can be used to produce paints and latter to form a protective

coating is to take advantages of the electrochemical and electrical

proprieties of this polymer material in the protection of metals

against corrosion. Many other polymer systems had been tested

namely epoxy, acrylics, etc. Chlorinated rubber (CR) has been

chosen because paints formulated with this resin are largely

employed in the industrial maintenance. Many of these formu-

lations include conventional protective pigments that are presently

subject to environmental restrictive rules and their utilization in the

future may not be allowed. Conducting polymers are candidates

to replace the lack open by these restrictive laws.

Coatings prepared from paints containing CR and both

forms of polyaniline (PAni) and subject to an aggressive

accelerated assay were able to offer a better protection of mild

steel than CR resin. The coating containing 10% of the undoped

PAni-EB was found to have the best performance in the

protection of the metal among all the other samples. Only Paint

7 containing 10% of PAni/DBSA showed an anticorrosion

protection lower than the other coatings obtained from paints

containing PAni and the coatings obtained from the paint

containing only CR. This poor performance could be ascribed to

the nature and amount of the surfactant used as a dopant to the

PAni, because this sample has presented the worst adherence to

the metal substrate and the bigger holes in the surface even

before been dipped into the corrosive medium.

Acknowledgements: The authors would like to thank CNPq for

financial support of the project (Edital Universal 2006) and Bayer

S. A. for providing the chlorinated rubber resin.

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(Received: January 25, 2009)

(Accepted: August 12, 2009)

W5254


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