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

Surface & Coatings Technology 200 (2006) 2948 – 2954 www.elsevier.com/locate/surfcoat

Electrochemically synthesized polypyrrole films as primer for protective coatings on carbon steel S.U. Rahmana,*, M.A. Abul-Hamayela, B.J. Abdul Aleemb a Chemical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Mechanical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

b

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 synthesized 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

* Corresponding author. Fax: +966 3860 4234. E-mail address: [email protected] (S.U. Rahman). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.04.012

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 MacDiarmid [5] utilized electrochemically synthesized polyaniline 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 polypyrrole 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

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

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 multilayered 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.

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 software (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. Subsequently, 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 cm  1.0 cm and 4.0 cm  4.0 cm). The larger samples were used for salt spay and solution exposure tests, while the smaller ones were used for electrochemical tests.

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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 Table 1 Description of corrosion coupons Designation

Description

PRIM PRIMTC 15PPY 30PPY 45PPY 60PPY 15PPYTC 30PPYTC 45PPYTC 60PPYTC

Primer only Primer and top coat Polypyrrole deposited Polypyrrole deposited Polypyrrole deposited Polypyrrole deposited Polypyrrole deposited Polypyrrole deposited Polypyrrole deposited Polypyrrole deposited

for for for for for for for for

15 30 45 60 15 30 45 60

min min min min min min min min

and and and and

a a a a

top top top top

coat coat coat coat

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-100 -200

E, mv SCE

-300

15PPY 30PPY 45PPY 60PPY PRIM

-400

-500 -600

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

-700

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 impedance 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

-800 -6

-4

-2

0

log(I), A Fig. 2. Tafel plots for primer and various polypyrrole films.

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 concluded that using oxalic acid as supporting electrolyte gives strongly adherent, compact, and homogeneous films with relatively lesser defects. Rahman and Ba-

Table 2 Tafel parameters for different polypyrrole films and zinc primer with and without top coat Sample PRIM PRIMTC 15PPY 30PPY 45PPY 60PPY 15PPYTC 30PPYTC 45PPYTC 60PPYTC

Corrosion potential (mV SCE) 542 515 494 482 463 425 503 502 513 517

Corrosion current (A/cm2) 5.100  10 1.432  10 7.460  10 6.870  10 6.250  10 4.765  10 3.428  10 1.987  10 1.454  10 1.173  10

4 5 4 4 4 4 5 5 5 5

|b a| (V/decade)

|b c| (V/decade)

0.083 0.069 0.087 0.089 0.083 0.080 0.073 0.078 0.084 0.078

0.133 0.127 0.155 0.208 0.145 0.161 0.221 0.270 0.254 0.156

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

-200

E, mv SCE

3.2. Salt spray and salt solution exposure

15PPYTC 30PPYTC 45PPYTC 60PPYTC PRIMTC

-300

-400

-500

-600

-700 -7

-6

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-5

-4

-3

-2

log(I), A

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 observations, 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

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 temperature, 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. Galvanostatic experiments were carried out using an electrochemical 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 polypyrrole sample is shown by SEM in Fig. 1. The micrograph shows grains of less than 5 Am. The surface was visually defect-free.

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 (I corr), corrosion potential (u corr), and anodic and cathodic Tafel slopes (b a and b c). 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

Cc

RS Cdl RPO

Rp

Warburg

W Fig. 4. Equivalent circuit model of corroding metal covered with a polymeric film.

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150

500

15PPY 30PPY 45PPY 60PPY

15PPY 30PPY

300

45PPY 60PPY PRIM

PRIM

|Z|, Ω

Zimag, Ω

100

400

200

50 100

0 0 0

50

100

150

200 10 -2

Zreal, Ω

10 -1

10 0

101

10 2

10 3

10 4

10 5

Frequency, Hz

Fig. 5. 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 delocalization, 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

Fig. 6. Total impedance Bode plots for Nyquist plots for different polypyrrole films and zinc primer.

observations are in agreement with those from salt spray tests. 3.4. Electrochemical impedance spectroscopy Corroding carbon steel coupons with polymeric coatings can be modeled using an electrical circuit shown in Fig. 4. The circuit includes the polarization resistance R p, pore resistance R po, coatings capacitance C C, double layer capacitance C dl, and Warburg impedance W. Here, R p and C dl can be correlated to coatings delamination and the onset of corrosion at the interface. Corrosion resistance of a coating system is associated with high R p, high R po, and low C c [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.

Table 3 Parameter of equivalent circuit model for different polypyrrole films and zinc primer with and without top coat R s (V) 15PPY 30PPY 45PPY 60PPY PRIM 15PPYTC 30PPYTC 45PPYTC 60PPYTC PRIMTC

3.743 3.087 2.321 2.696 1.743 9.495 15.14 9.166 21.77 9.41

C C (F)

R po (V) 5

8.17  10 9.89  10 5 1.01 10 4 9.88  10 5 9.34  10 5 1.76  10 6 4.2  10 7 5.90e 07 2.6  10 7 1.1 10 6

21.82 17.9 20.4 18.1 12.9 56.26 80.75 94.15 142.3 152.6

C dl (F) 1.60  10 1.73  10 1.70  10 1.67  10 5.31 10 7.47  10 3.40  10 2.80  10 1.90  10 1.65  10

3 3 3 3 5 6 6 6 6 6

R p (V)

Warburg (V/s0.5)

34.8 37.5 46.3 69.2 31.6 309.2 403.4 504 652.5 556.5

2.99  10 3.17  10 2.97  10 2.55  10 1.00  10 1.89  10 9.36  10 2.01 10 1.37  10 9.66  10

2 2 2 2 2 2 3 2 2 3

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

500

15PPYTC 30PPYTC 45PPYTC 60PPYTC

400

Zimag, Ω

PRIMTC 300

200

100

0 0

200

400

600

800

1000

1200

Zreal, Ω Fig. 7. Nyquist plots for different polypyrrole films and zinc primer with epoxy paint.

1200

|Z|, Ω

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 (R p) was in a narrow range for all samples. The double layer capacitance (C dl) was also almost constant as expected. The polarization resistance was small but increased with increasing deposition 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 decreasing 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 R p 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

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1000

15PPYTC 30PPYTC

800

45PPYTC 60PPYTC PRIMTC

600

400

200

0

10 -2

10 -1

10 0

10 1

10 2

10 3

10 4

10 5

Frequency, Hz Fig. 8. Total impedance Bode 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 synthesized 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 spectroscopy in 0.5 N H2SO4 were carried out. All of these tests coherently indicate that galvanostatically synthesized polypyrrole 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 polypyrrole 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 comparatively 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

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

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