electrochemically synthesized polypyrrole films as primer for protective coatings on carbon steel

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Surface & Coatings Technolog

Electrochemically synthesized polypyrrole films as primer for protective

coatings on carbon steel

S.U. Rahmana,*, M.A. Abul-Hamayela, B.J. Abdul Aleemb

aChemical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi ArabiabMechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

Received 20 July 2004; accepted in revised form 6 April 2005

Available online 23 May 2005

Abstract

The use of electrochemically synthesized polypyrrole (a conducting polymer) film is investigated as a primer for protective coating on

carbon steel. It provides excellent adherence and corrosion resistance, and is more environment-friendly. Polypyrrole was galvanostatically

synthesized on carbon steel, and epoxy paint top coat was applied on it. The corrosion performance was evaluated using salt spray test, Tafel

plots, and electrochemical impedance spectroscopy. The performance was compared to that of a commercial zinc primer. These tests

coherently demonstrate that the use of polypyrrole film inhibits corrosion better than a zinc primer in salt and acid environments.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Polypyrrole films; Primer; Carbon steel; Corrosion protection

1. Introduction

Conventional protective coating on carbon steel involves

environmentally unsafe chrome rinse and inorganic primers.

Therefore, there is a need for a primer that provides

excellent adherence and corrosion resistance, and is

environmentally safer. The use of conducting polymers,

which have been shown to pose corrosion potential of

oxidizable metals in passive range, is an attractive option.

The anodic protection provided by conducting polymers

inhibits or delays corrosion of coated metals initiated by the

coating defects, which are usually responsible for eventual

coating failures.

Several researchers have studied the deposition of

various conducting polymers on metal surfaces for corrosion

protection purposes. Deberry [1] electrochemically synthe-

sized polyaniline in perchloric acid on type 410 and 430

stainless steels. The coated samples remained passive for a

longer period of time in acid solutions in which they are

0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2005.04.012

* Corresponding author. Fax: +966 3860 4234.

E-mail address: [email protected] (S.U. Rahman).

normally active and subject to corrosion. Deng et al. [2]

demonstrated the use of poly(3-methyl thiophene) (P3MT)

coating on a Ti–TiO2 surface to hold the corrosion potential

of Ti metal in the passive region. Ren and Barkey [3]

deposited a P3MT film on SS430 in organic electrolyte

galvanostatically. Adherence of the film to the surface was

better on phosphated SS430 samples. The coating was

stable in 1.0 N H2SO4 for long periods. Haase and Beck [4]

used electropolymerization to deposit polypyrrole, P3MT,

poly(bisthiophene), and polyaniline on iron in various

aqueous and non-aqueous electrolytes. Ahmad and Mac-

Diarmid [5] utilized electrochemically synthesized polyani-

line for corrosion protection of iron and steel. Kinlen et al.

[6] studied polyaniline coating on carbon steels with an

epoxy top coat. Jiang et al. [7] studied corrosion protection

of AZ91 magnesium alloys by coating them with poly-

pyrrole in alkaline solutions and observed that the coating

had changed the corrosion potential of the substrate alloys.

Iroh and Su [8] electrosynthesized polypyrrole on low

carbon steel and concluded that poly(N-methylpyrrole) is a

better candidate for corrosion protection. Ferreira et al. [9]

studied a bilayer of polypyrrole and cataphoretic paint on

carbon steel and electrozincated steel coupons by exposing

y 200 (2006) 2948 – 2954

S.U. Rahman et al. / Surface & Coatings Technology 200 (2006) 2948–2954 2949

them to salt spray test. Lenz et al. [10] prepared

polypyrrole– titanium oxide films on carbon steel and

recommended using them in place of phosphatized layers

on carbon steel. Tan and Blackwood [11] proposed multi-

layered coating involving polypyrrole and polyaniline for

corrosion protection.

In this work, the possibility of using electrosynthesized

polypyrrole as primer has been explored. Conventional

primer contains toxic heavy metal and therefore is not

environmentally safe. Use of polypyrrole as primer will

obviate the need of traditional toxic heavy metal primers.

Since the application of a conducting polymer keeps the

potential of the entire surface in passive range, the passive

film will be maintained even in the presence of painting

defects like pinholes. Therefore, the initiation of corrosion

on these defects will be inhibited or delayed. A commercial

zinc primer is compared with polypyrrole films, with and

without an epoxy topcoat.

Table 1

Description of corrosion coupons

Designation Description

PRIM Primer only

PRIMTC Primer and top coat

15PPY Polypyrrole deposited for 15 min




15PPYTC Polypyrrole deposited for 15 min and a top coat




2. Experimental

2.1. Materials

Carbon steel [0.184% (C), 0.070% (Si), 0.29% (Mn),

0.097 (Cr), 0.071 (Ni), 0.021 (Mo), 0.065 (Cu), 0.014 (V),

0.012 (P), 0.029% (S), and 99.15% (Fe)], oxalic acid

(AnalR, no. 10174, BDH), sodium bicarbonate (AR no.

27778293, BDH), sulfuric acid (Acculute Standard Solution

no. 007152586, BDH), pyrrole (no. 807492, Merck), zinc

phosphate primer (no. 90044, United Industrial Company

for Paints, Jeddah, KSA), and epoxy enamel (Hempatex

Ename 5636, Hempel Paints SA, Dammam, KSA) were

used. All chemicals were used as-received, except pyrrole,

which was distilled. All solutions were prepared in

deionized water.

2.2. Electrosynthesis of polypyrrole

The electrosynthesis of polypyrrole was carried out

galvanostatically on flag-shaped carbon steel coupons in a

single compartment cell. Stainless steel bar and saturated

calomel electrodes (SCE) were used as counter and

reference electrodes, respectively. These three electrodes

were connected to a potentiostat (EG&G PARC Model

273A), which was controlled using electrochemical soft-

ware (M270, EG&G PARC). Carbon steel coupons were

first cleaned mechanically with increasing grades of emery

paper having grit sizes 100, 400, 600, and 1500. Sub-

sequently, acetone was used to remove any oil or grease.

The stem of the coupon was insulated with an insulation

paint so as to expose the desired deposition area. Coupons

of two different sizes were fabricated (1.0 cm1.0 cm and

4.0 cm4.0 cm). The larger samples were used for salt spay

and solution exposure tests, while the smaller ones were

used for electrochemical tests.

2.3. Sample preparation and nomenclature

A batch from both sizes of samples was coated with zinc

phosphate primer on both sides. The primer was applied by

a spray gun of standard nozzle size no. 20. The spraying was

articulately controlled by a skilled painter so as to obtain a

uniform thickness. After several trials, uniform thickness of

the primer within a tolerance limit was achieved. The

thickness of the primer was 30 Am. Polypyrrole films were

deposited by galvanostatic electrosynthesis on another batch

of coupons. A number of coupons from both batches were

coated with epoxy top coat. The thickness of the topcoat

was controlled by paint application using a spray gun. The

thickness of the top coat was 60 Am. The details and

nomenclature of these coupons are given in Table 1.

2.4. Salt spray and salt exposure tests

The test was conducted as per ASTM B117-85. A scratch

was made through the coating with a sharp knife so as to

expose the underlying metal before testing. The salt solution

was prepared by dissolving 3.5% by weight of sodium

chloride in distilled water. The sodium chloride used was

substantially free of nickel and copper. It contained not more

than 0.1% of sodium iodide and not more than 0.3% of total

impurities on a dry basis. The exposure zone of the salt

spray chamber was maintained at about 35 -C. The

specimens were gently washed in clean running water to

remove salt deposits after the exposure period. Visual

inspection was done periodically to determine parameters

such as scribe activity, blister size, and blister density. The

specimens were also exposed to 3.5% sodium chloride

solution at room temperature to study the corrosion

performance in stagnant solution.

2.5. Electrochemical tests

Corrosion rates of all samples were determined using

Tafel plots. For this purpose, an electrochemical cell was

prepared in a 200-ml beaker with carbon steel sample,

graphite rod, and saturated calomel electrode (SCE),

functioning as working electrode, counter electrode, and

reference electrode, respectively. The electrodes were

Fig. 1. SEM micrograph of polypyrrole deposited on carbon steel.

log(I), A

-6 -4 -2 0

E, m

v S

CE

-800

-700

-600

-500

-400

-300

-200

-10015PPY30PPY45PPY60PPYPRIM

Fig. 2. Tafel plots for primer and various polypyrrole films.

S.U. Rahman et al. / Surface & Coatings Technology 200 (2006) 2948–29542950

connected to a potentiostat, which was controlled by a

software (Softcorr-III, EG&G Princeton Applied Research).

All Tafel tests were performed at room temperature and in

0.5 N H2SO4 solution. The scan rate was maintained at 1

mV/s in all experiments.

The electrochemical cell for the electrochemical impe-

dance measurement was similar to the one used for Tafel

tests but without a reference electrode. The carbon steel

coupon and a graphite rod were held vertically in a small

beaker containing 0.5 N H2SO4. A lock-in amplifier (Model

2810, EG&G PARC) created potential sine waves of desired

frequency and amplitude. The waveforms were sent to the

potentiostat (Model 283), which applied it to the test cell..

The equipment were controlled by a software (Power Sine,

EG&G PARC), which allowed to choose a range of

frequencies and amplitude of the potential waveforms. In

the present experiments, the range of frequencies was 1

mHz to 120 kHz, while the amplitude was 10 mV with

respect to open circuit potential. This big frequency range

was chosen to capture most of the Nyquist plot.

3. Results and discussion

3.1. Electrosynthesis of polypyrrole

The electrodeposition of polypyrrole on oxidizable

metals such as iron is not easy, as the metal tends to

Table 2

Tafel parameters for different polypyrrole films and zinc primer with and withou

Sample Corrosion potential (mV SCE) Corrosion c

PRIM 542 5.10010

PRIMTC 515 1.43210

15PPY 494 7.46010

30PPY 482 6.87010

45PPY 463 6.25010

60PPY 425 4.76510

15PPYTC 503 3.42810

30PPYTC 502 1.98710

45PPYTC 513 1.45410

60PPYTC 517 1.17310

dissolve before the electropolymerization potential of the

monomer (pyrrole) is reached. The oxidation potential of

the metal is much more negative than that of pyrrole.

Therefore, metal dissolution occurs and stabilizes the

potential of the electrode at a negative value, preventing

monomer oxidation. Thus, to achieve the deposition of

the polypyrrole on iron or other oxidizable metals, it is

necessary to find electrochemical conditions that lead to

a partial passivation of the metal and decrease its

dissolution rate without preventing electropolymerization.

The supporting electrolyte must be carefully chosen to

avoid anodic dissolution of the substrate and allow the

formation of the polymer film. Polypyrrole has been

synthesized electrochemically on iron/steel using sodium

sulfate [12,13], oxalic acid [14–18], potassium oxalate

[13], benzene sulphonate [19], sodium toluene sulphonate

[20,21], dodecylbenzene sulphonic acid [22], and malate

[23].Ba-Shammakh [24] has done a comparative study to

find out the best and convenient technique and con-

cluded that using oxalic acid as supporting electrolyte

gives strongly adherent, compact, and homogeneous

films with relatively lesser defects. Rahman and Ba-

t top coat

urrent (A/cm2) |ba| (V/decade) |bc| (V/decade)

4 0.083 0.1335 0.069 0.1274 0.087 0.1554 0.089 0.2084 0.083 0.1454 0.080 0.1615 0.073 0.2215 0.078 0.2705 0.084 0.2545 0.078 0.156

log(I), A-7 -6 -5 -4 -3 -2

E, m

v S

CE

-700

-600

-500

-400

-300

-20015PPYTC30PPYTC45PPYTC60PPYTCPRIMTC

Fig. 3. Tafel Plots for primer and polypyrrole films with top coats.


Shammakh [25] and Ba-Shammakh [26] have studied the

effects of various parameters including pH and temper-

ature, and concluded that the deposition of polypyrrole

in alkaline pH is easier and comparatively better. Based

on these studies, a solution of 0.1 M pyrrole and 0.1 M

oxalic acid was used in the present work. The pH was

raised to 8.2 by adding sodium bicarbonate. Galvano-

static experiments were carried out using an electro-

chemical cell at room temperature, as described in the

previous section. Electropolymerization was done for

four different durations, namely, 15, 30, 45, and 60

min at 4 mA/cm2. The resultant polypyrrole films were

black in color, compact, and very adherent to carbon

steel coupons. Visually the surface was uniform, smooth,

and had some gloss. Morphology of a typical poly-

pyrrole sample is shown by SEM in Fig. 1. The

micrograph shows grains of less than 5 Am. The surface

was visually defect-free.

Cc

RPO

RS

Fig. 4. Equivalent circuit model of corroding

3.2. Salt spray and salt solution exposure

Two coupons from each of PRIM, PRIMTC, 15PPYTC,

30PPYTC, 45PPYTC, and 60PPYTC were exposed in a

standard salt spray chamber as described earlier. A set of

these coupons was also submerged in a standard 3.5% NaCl

solution at room temperature. Changes in surface color and

texture were observed and recorded. From these observa-

tions, it was inferred that the use of commercial zinc

phosphate primers and top coat was able to withstand the

salt spray environment for more than 120 h. The polypyrrole

films did not work well when the thickness of the

polypyrrole was small. However, when the deposition time

was 60 min, the polypyrrole film worked almost like the

commercial primer. Nevertheless, these tests were not

deemed adequate to discriminate between the commercial

primer and the polypyrrole films.

3.3. Tafel tests

Tafel plots from all coupons, after 20 min of exposure to

0.5 N H2SO4 solution at room temperature, were obtained as

described earlier. The plots were analyzed to estimate

corrosion current (Icorr), corrosion potential (ucorr), and

anodic and cathodic Tafel slopes (ba and bc). These values

are given in Table 2. Fig. 2 shows Tafel plots for samples

15PPY, 30PPY, 45PPY, 60PPY, and PRIM. The corrosion

potential of coupon with zinc phosphate primer was the

most active (542 mV SCE). The potential was more

positive when samples had polypyrrole coatings. Its value

was 425 mV SCE for the thickest polypyrrole film (i.e.,

with the highest synthesis time). The shift towards the

positive side was due to stored charge in polypyrrole. A

thicker film of polypyrrole stores more charge and thus

makes the potential of the carbon steel poise in a more

positive range. The shift also resulted in reduced corrosion

rates. The corrosion rates of the coupons with films obtained

by 45 and 60 min of electropolymerization were comparable

to that of zinc phosphate primer. This performance of

polypyrrole is partly due to the presence of conjugated

Cdl

WarburgRp

W metal covered with a polymeric film.

Zreal, Ω

Zim

ag, Ω

0 50 100 150 2000

50

100

15015PPY

30PPY45PPY60PPY

PRIM

Fig. 5. Nyquist plots for different polypyrrole films and zinc primer.

Frequency, Hz10-2 10-1 100 101 102 103 104 105

|Z|,

Ω

0

100

200

300

400

500

15PPY30PPY

45PPY

60PPYPRIM

Fig. 6. Total impedance Bode plots for Nyquist plots for different

polypyrrole films and zinc primer.


double bonds and polar –NH group in the pyrrole ring.

They act as corrosion inhibitors. It is believed that

compounds with conjugated bonds adsorb better on metal

surface due to a higher number of electrons. In addition,

charge conduction in the pyrrole facilitates charge delocal-

ization, which hinders formation of localized anodic or

cathodic regions. Therefore, the surface is more stable and

inhibits corrosion reaction, which requires localization of

charge [8].

Tafel plots for samples with polypyrrole and zinc

phosphate primer, and with epoxy topcoat are shown in

Fig. 3. It is evident that the corrosion potential of all these

samples is in a narrow range, indicating that it is not

significantly affected by the primer. A surface covered

with a top coat is likely to have fewer defects in

comparison to a thin coat of only primer. Therefore, the

effect of stored charge in the polypyrrole is not perceptible

in a sample with topcoat. However, the effect of primer on

corrosion rates after 20 min of exposure to an acidic

environment is noticeable. The polypyrrole film with 60

min of deposition time, working as primer, exhibited lesser

corrosion rate than the zinc phosphate primer. The

Table 3

Parameter of equivalent circuit model for different polypyrrole films and zinc pri

Rs (V) CC (F) Rpo (V)

15PPY 3.743 8.1710 5 21.82

30PPY 3.087 9.8910 5 17.9

45PPY 2.321 1.0110 4 20.4

60PPY 2.696 9.8810 5 18.1

PRIM 1.743 9.3410 5 12.9

15PPYTC 9.495 1.7610 6 56.26

30PPYTC 15.14 4.210 7 80.75

45PPYTC 9.166 5.90e07 94.15

60PPYTC 21.77 2.610 7 142.3

PRIMTC 9.41 1.110 6 152.6

observations are in agreement with those from salt spray

tests.

3.4. Electrochemical impedance spectroscopy

Corroding carbon steel coupons with polymeric coat-

ings can be modeled using an electrical circuit shown in

Fig. 4. The circuit includes the polarization resistance Rp,

pore resistance Rpo, coatings capacitance CC, double layer

capacitance Cdl, and Warburg impedance W. Here, Rp and

Cdl can be correlated to coatings delamination and the

onset of corrosion at the interface. Corrosion resistance of

a coating system is associated with high Rp, high Rpo, and

low Cc [27].

Fig. 5 shows the Nyquist plots obtained from samples

15PPY, 30PPY, 45PPY, 60PPY, and PRIM. The data were

taken after 20 min of exposure to 0.5 N sulfuric acid at room

temperature. All of these plots exhibit two semicircles

merged into each other, indicating two time constants. These

semicircles represent the capacitance–resistance couples for

the coating and polarization at the metal–polymer interface.

mer with and without top coat

Cdl (F) Rp (V) Warburg (V/s0.5

1.6010 3 34.8 2.9910 2

1.7310 3 37.5 3.1710 2

1.7010 3 46.3 2.9710 2

1.6710 3 69.2 2.5510 2

5.3110 5 31.6 1.0010 2

7.4710 6 309.2 1.8910 2

3.4010 6 403.4 9.3610 3

2.8010 6 504 2.0110 2

1.9010 6 652.5 1.3710 2

1.6510 6 556.5 9.6610 3

)

Frequency, Hz10-2 10-1 100 101 102 103 104 105

|Z|,

Ω

0

200

400

600

800

1000

1200

15PPYTC30PPYTC

45PPYTC

60PPYTCPRIMTC

Fig. 8. Total impedance Bode plots for different polypyrrole films and zinc

primer with epoxy paint.


Straight lines in the low frequency regime represent the

presence of diffusion-limited reaction. The diameters of the

semicircles grow with the deposition time of polypyrrole,

indicating that the higher deposition time helps in inhibiting

overall corrosion reaction. More insight is obtained by

fitting impedance data into an equivalent electrical model. A

software (Zsimpwin, EG&G PARC) was used for this

purpose. The model parameters were retrieved and are given

in Table 3. The electrolyte resistance (Rp) was in a narrow

range for all samples. The double layer capacitance (Cdl)

was also almost constant as expected. The polarization

resistance was small but increased with increasing deposi-

tion time. A thicker polypyrrole film was expected to show

higher polarization resistance as described earlier. The zinc

phosphate had smaller polarization resistance. The pore

resistances of all polypyrrole films were comparable. Fig. 6

shows Bode plots for modulus of impedance. The decreas-

ing modulus with increasing frequency indicates that the

basic inhibition mechanisms were diffusion barrier and

charge transfer.

Fig. 7 shows Nyquist plots of carbon steel coupons with

different polypyrrole films and zinc primer with epoxy paint

after 20 min of exposure to 0.5 N H2SO4 at room

temperature. Corresponding Bode diagrams are given in

Fig. 8. The data were fitted in the equivalent circuit model

and the calculated parameters are given in Table 3. The

polarization resistances were larger for each case when

compared with samples without top coat. The data show an

increase in Rp with increasing deposition time. Polypyrrole

films with a deposition time of 45 min showed corrosion

protection almost similar to that of zinc phosphate primer;

deposition time of 60 min produced a primer with a

performance better than that of the commercial primer in

Zreal, Ω

Zim

ag, Ω

0 200 400 600 800 1000 12000

100

200

300

400

50015PPYTC

30PPYTC45PPYTC60PPYTC

PRIMTC

Fig. 7. Nyquist plots for different polypyrrole films and zinc primer with

epoxy paint.

question. The pore resistance was also comparable to the

zinc phosphate.

4. Conclusions

In this work, conductive polypyrrole films were synthe-

sized on carbon steel using oxalic acid as supporting

electrolyte for different lengths of time at a constant current

density of 4 mA/cm2. The corrosion protection ability of

such polypyrrole film as primer was compared to that of a

commercial zinc phosphate-based primer with, and without,

a commercial epoxy top coat. The salt spray test, Tafel plots

in 0.5 N H2SO4, and electrochemical impedance spectro-

scopy in 0.5 N H2SO4 were carried out. All of these tests

coherently indicate that galvanostatically synthesized poly-

pyrrole films are capable of working as primer for corrosion

protection. Thinner films resulting from a short synthesis

time could not compare well with zinc primer. However,

films synthesized for 45 and 60 min showed better corrosion

resistance, with and without epoxy top coat. The poly-

pyrrole films were able to demonstrate this performance

because the polymer was able to delocalize charge at the

metal surface and it was able to shift the corrosion potential

towards positive side. In addition to its better corrosion

inhibition capability, polypyrrole is environmentally com-

paratively safer than other inorganic primers used today.

Acknowledgements

Facilities and financial assistance provided by the King

Fahd University of Petroleum and Minerals under the ARI

Grant (ARI-006) are gratefully acknowledged. We thank


Mr. M. Ba-Shammakh for help in the electrosynthesis of

polypyrrole.

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electrochemically synthesized polypyrrole films as primer for protective coatings on carbon steel

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