Polypyrrole films on stainless steel

Surface & Coatings Technology 200 (2006) 4713 – 4719 www.elsevier.com/locate/surfcoat

Polypyrrole films on stainless steel Tunc¸ Tu¨ken* The University of C¸ukurova, Arts and Science Faculty, Chemistry Department, 01330 Adana, Turkey Received 11 March 2005; accepted in revised form 7 April 2005 Available online 17 May 2005

Abstract Polypyrrole (PPy) films have been deposited on stainless steel substrates from two different solutions; 0.15 M LiClO4 in acetonitrile (ACN-LiClO4) and 0.3 M oxalic acid in water. For this aim cyclic voltammetry technique was used and approximately 1.7 Am thick PPy coating was obtained, applying the same potential range for the film growth in both solutions. The corrosion behaviour of coated and uncoated samples was investigated in 3.5% NaCl solution, using ac impedance spectroscopy and anodic polarisation curves. It was shown that the film obtained from ACN-LiClO4 had less porous structure and higher stability than that obtained from aqueous oxalic acid solution. The ac impedance results showed that both coatings exhibited important anodic protection behaviour and enhanced the self passivation of steel with protective oxide film. D 2005 Elsevier B.V. All rights reserved. Keywords: Polypyrrole; Stainless steel; Corrosion; ac impedance

1. Introduction Electrochemically synthesized conducting polymer coatings have long been studied for their possible use in anticorrosive applications [1 –8]. They simply constitute a physical barrier against the attack of corrosive environment and reduce the corrosion rate of the substrate. It was also reported that conducting polymer films can act as polymeric anodic inhibitors, accelerating and stabilizing the formation of protective metal oxides [9 – 11]. Hence, the degree of corrosion protection afforded by a conducting polymer coating depends on both its structural and electronic properties. These properties are also strongly related to electrochemical synthesis conditions [12]. Polypyrrole (PPy) is one of the most promising conducting polymers in view of its high conductivity, stability and ease of synthesis. The good electronic

conductivity (¨ 1 S cm 2) allows the electrochemically synthesis of a top coating on PPy in order to improve protection efficiency [12,13]. There are also some studies subjecting the use of PPy as a primary coating, Blackwood et al. reported that the PPy/PANI coating provided excellent protection to both localized and general corrosion of 304 stainless steel in artificial sea water [14]. Another study reported by Lacaze et al. showed that electrochemically synthesized PPy on mild steel can successfully be used in automotive industry as a primer coating under the cataphoretic painting [15]. In this study, the electrochemical synthesis of PPy coating has been achieved on stainless steel samples from equimolar monomer containing solutions of oxalic acid in water and LiClO4 in acetonitrile. Then the corrosion behaviours of coated samples have been investigated using electrochemical methods.

2. Experimental * Tel.: +90 322 338 60 81; fax: +90 322 388 60 81. E-mail address: [email protected]. 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.04.011

All the chemicals were purchased from Merck; pyrrole was distilled before use and stored in the dark, all the

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T. Tu¨ken / Surface & Coatings Technology 200 (2006) 4713 – 4719

Fig. 1. The CVs of stainless steel in monomer free (a) and 0.1 M pyrrole containing oxalic acid solution (b), scan rate: 50 mV/s.

solutions were prepared with distilled water. The electrochemical cell is consisted of a three-electrode cell where the auxiliary electrode was a platinum sheet and Ag/AgCl electrode (saturated with KCl) was used as the reference. All the potential values are referred to this electrode. Stainless steel (SS) samples were cylindrical rods measuring 0.65 cm in the radius and had the following composition (wt.%). C

Mn

Si

S

P

Ni

Cr

Mo

N2

Co Cu

0.025 1.880 0.530 0.010 0.039 10.400 18.590 2.100 0.040 –

–

The working area was 1.327 cm2 while the rest of electrode was embedded in a thick polyester block. The exposed surfaces were polished to a 1200 grit finish using SiC paper, degreased with 1:1 ethanol/water mixture and washed with distilled water. The PPy film was synthesized electrochemically by using cyclic voltammetry technique from 0.1 M pyrrole containing solutions of 0.3 M oxalic acid in water and 0.15 M LiClO4 in acetonitrile (ACN-LiClO4). Electrochemical Impedance Spectroscopy (EIS) and anodic polarization curves were used to investigate the corrosion performances

of these coatings. The Nyquist plots were recorded at instantaneous open circuit potential values in a frequency range of 1 mHz – 100 kHz, the amplitude was 8 mV. The polarization curves were recorded with a scan rate of 2 mV/ s, where the initial potential was the corrosion potential value reached after 4 h of exposure time in 3.5% NaCl solution.

3. Results and discussion Fig. 1 shows the voltammograms obtained for SS electrode in 0.3 M oxalic acid solution in the absence and presence of monomer. In monomer free solution, two well defined peaks were observed in the forward direction and peak I (at around  0.20 V) was attributed to oxidation – passivation behaviour of the surface [16 – 18]. The surface of SS electrode should be covered with stable oxide layer due to oxidation of chromium and nickel components of the electrode, prior to this measurement, as soon as it was exposed to atmosphere and humidity. Therefore, the peak I was attributed to oxidation and passivation process occurring through the pores of this oxide layer. The peak II was attributed to degradation of

Fig. 2. The CVs of stainless steel in monomer free (a) and 0.1 M pyrrole containing ACN-LiClO4 (b), scan rate: 50 mV/s.

T. Tu¨ken / Surface & Coatings Technology 200 (2006) 4713 – 4719 0,40

þ  Cr2 O3 þ 5H2 O Y 2HCrO 4 þ 8H þ 6e E ¼ 0:928 þ 0:0197log½HCrO 4

0,20

E/V

mainly chromium based oxide layer according to following reactions [16].

4715

0,00

Cr2 O3 þ

2 4H2 O Y Cr2 O 7

þ

þ 8H þ 6e



2 E ¼ 0:906 þ 0:0098log½Cr2 O 7 

Cr2 O3 þ

2 5H2 O Y 2CrO 4

þ 10H þ þ 6e

-0,20 0

2 E ¼ 0:926 þ 0:0197log½CrO 4 

Then, the current value started to increase continuously at the potential value of 1.60 V, this event was related to continuous oxygen gas evolution [16]. In the case of monomer containing solution, the current value started to increase rapidly at + 0.60 V potential values due to monomer oxidation [17,18]. This oxidation process overlapped the degradation peak of oxide layer described above. It must be noted that the degradation peak decreased at the second and the third cycles, as the polymer film grows and covers the surface. Also, the reduction of polymer film was observed at the reverse scan at around +0.10 V, as a cathodic peak [17]. In monomer free ACN-LiClO4 solution (Fig. 2), there was no significant current increase up to + 1.30 V. Then the current value started to increase rapidly, due to oxidation of the electrode surface leading to degradation of formerly formed oxide layer and anodic dissolution of substrate. In presence of 0.1 M pyrrole, the monomer oxidation process was found to start at around + 0.60 V and the current value increased continuously. Fig. 3 shows the successive voltammograms recorded during the polymer film growth on SS electrode. The potential range applied for the polymer film growth was between + 0.30 and +0.90 V for both solutions. The samples coated in ACN-LiClO4 solution were named as SS/PPy(I) and the samples coated in aqueous oxalic acid solution were named as SS/PPy(II). The thickness of polymer coatings was estimated to be 1.7 Am approximately. For this aim, the

20

40

60

80

100

120

t/h



Fig. 4. The variation of open circuit potential values (E ocpt) for bare SS: 0, SS/PPy (I): r and SS/PPy(II): ? samples.

Faraday’s law based on electricity consumption to obtain the deposit was used [19 –22]. It was assumed that the two electron mechanism based on the monomer molecule implied in the process the current efficiency 100%, the density of polypyrrole (q) 1.5 g/cm3 and a pyrrole molar mass of 67 g/mol. The thickness was calculated according to following equation, where Q was the specific overall charge for electropolymerization (C/cm2) and the F value was 96500 C. d¼

QM 2Fq

The value of Q was obtained from the sum of charges passing in monomer oxidation potential region (between + 0.60 and 0.95 V) of successive cycles applied for the film growth [22]. The variations of open circuit potential values (E ocp) are given in Fig. 4 as a function of exposure time to corrosive test solution. Within the first hours of immersion time, the E ocp values of SS/PPy(I) and (II) samples were both nobler than the bare SS electrode. This was simply explained with effective barrier behaviour of these coatings. Then these values started to decrease due to initiation of water up taking process through the polymer film. Thus, ionically conducting paths are formed along the pores of polymer film. The corrosive species (dissolved oxygen and chloride ions) are transported to metal surface with help of water diffusion

Fig. 3. The CVs of stainless steel during the film growth in ACN-LiClO4+ 0.1M pyrrole (a) and 0.3 M oxalic acid + 0.1 M pyrrole (b), scan rate: 50 mV/s.

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T. Tu¨ken / Surface & Coatings Technology 200 (2006) 4713 – 4719

Fig. 5. The anodic polarization curves obtained for bare SS: 0, SS/PPy(I): ? and SS/PPy(II): > electrodes in 3.5% NaCl solution.

along these paths. When sufficient amount of solution reaches the metal surface, the corrosion process starts to take place under the polymer film. As a result the E ocp value shifts to less noble values. The E ocp value decreases further and approaches to the uncoated sample, as the amount of electrolyte solution held by polymer film increases. It was apparent from Fig. 4 that the E ocp value of SS/PPy(I) remained quite noble for a considerable period and tended to plot a plateau between 20 and 60 h. While the E ocp value of SS/PPy(II) electrode decreased soon and became close to uncoated sample. This case could only be explained with less permeable (less porous) structure of PPy(I) film when compared to PPy(II) film. Fig. 5 shows the anodic polarization curves recorded for SS, SS/PPy(I) and SS/PPy (II) electrodes in 3.5% NaCl solution. In the case of uncoated sample, the corrosion potential value (E corr) was + 0.08. The current value increased steadily due to anodic dissolution of metal, up to +0.15 V potential value. The highest current value was reached at this point and then it started to decrease, as a result of surface passivation provided with stable nickel and chromium compounds [16]. The current value exhibited almost a plateau between +0.35 and + 1.10 V potential values. This region exhibited some fluctuations due to presence of chloride ions [16,23,24], however it could be considered as a passivity region. It was apparent

that the passivity started to break down after 1.10 V potential value. In presence of PPy coating, the E corr value moved to nobler region significantly, it was determined to be + 0.32 V for SS/PPy (I) and + 0.30 V for SS/PPy (II) electrode. This case was simply related to physical barrier behaviour of polymer coating between the corrosive environment and substrate metal. Also, the coated samples exhibited much lower current values than the uncoated sample, at the initial region of potential scan. It was also apparent that the current values of SS/PP(I) electrode were lower than SS/PPy(II) electrode. Then, the current value started to increase at around + 0.45 V, for both samples. This increase was attributed to dissolution of underlying substrate, at the bottom of the pores of polymer film. At higher potentials, the polymer film also undergoes degradation (irreversible oxidation leading to cleavage of some polymeric bonds present in film) and makes contribution to measured current value [25]. This also results with less protective film. At this point of view, PPy coating obtained from ACN-LiClO4 was said to have higher durability when compared to that obtained from aqueous oxalic acid solution. Since, it gave quite lower current values even at highly anodic potentials. Both the metal dissolution and polymer film degradation are strictly related to permeability (porosity) of coating. The diffusion of electrolyte (corrosive) solution must happen along the pores of polymer coating for the initiation and continuation of these electrochemical processes [26,27]. In this context, it could be stated that the ACN-LiClO4 solution gave less permeable and more stable polymer film. Fig. 6 shows the EIS results of uncoated SS sample, as a function of immersion time. In the Nyquist plots, two regions of distinct electrochemical response can be seen clearly. At high frequency region, a depressed semicircle was obtained from which the charge transfer resistance (R ct) and the film resistance R f can be obtained [28 – 31]. The R f arises from the chromium and nickel oxides formed due to oxidation of metal. R ct arises from a kinetically controlled electrochemical reaction such an electron transfer between the metal and corrosive environment at the bottom of the pores of oxide film. At lower frequency region the response is indicative of diffusion process presented by a straight line

Fig. 6. The Nyquist plots obtained for bare SS after 8: >, 24: 0 and 72 h:

?

immersion periods in 3.5% NaCl solution.

T. Tu¨ken / Surface & Coatings Technology 200 (2006) 4713 – 4719

Fig. 7. The proposed equivalent circuit for the SS impedance system.

with respect to real axis and this region could be considered as Warburg impedance, Z w [29]. The overall corrosion process was said to be diffusion controlled. The corrosive species have to diffuse through the pores of oxide layer, to reach and corrode the metal surface. The proposed equivalent circuit is given in Fig. 7 [29]. The solution resistance (R s) is always a small constant for a given electrolyte. This value could not be resolved from Nyquist plots, thus it was measured experimentally and found to be 3.2 V. In logf –logZ diagram (Fig. 6), the R ct and R f values were clearly identified as two separate plateau region. The one representing charge transfer process in the high frequency range indicated to a much lower resistance than that of oxide film resistance depicted by the other plateau at lower frequency range. Thus, two semicircles and a diffusion tail would be expected on the Nyquist plot. However, the curves were found to consist of a depressed semicircular and a long tail in shape. It was considered that the R ct value was quite smaller than the R f value and the semicircle representing the oxide film merges with the charge transfer loop [28 – 30]. The R ct and R f values were determined from logf – logZ diagrams and given in Table 1. The resistance value depicted by the first plateau region (at high frequencies) included the solution resistance (R s) also. Therefore, the R ct value was obtained by subtracting the R s from the total resistance value corresponding to this first plateau. It was clearly seen that both R ct and R f values increased with time due to further oxide film growth on the surface. The R ct values of SS/PPy(I) were higher than the uncoated SS sample, while the R f values of uncoated SS were quite higher than those of SS/PPy(I). Therefore the semicircle representing the R ct was observed as a separate

4717

semicircle (partially seen) at the highest frequency region of the Nyquist plots (Fig. 8). The long diffusion tail was also observed for all the measurements and showed that the diffusion process was slower than the charge transfer process. Thus, this slower step controlled the rate of overall corrosion process. With increasing exposure time, the amount of solution interacting with the metal surface increased and as a result the R ct value decreased. This case was in resonance with the established role of conducting (electroactive) polymer coatings which is known to stabilize the passive layer of protective oxide formed on the surface. During this time, the PPy film undergoes a reduction process and accelerates the oxidation of metal according to following mechanism [32]. In these equations the term A represents the counter anion present in polymer film. anodic branch: Fe Y Fe2þ þ 2e YF eþ3 þ e cathodic branch: ðRyþ Ay Þn þ 2nye Y ðRÞn þ nyA 1=2O2 þ H2 O þ 2e Y 2OH The growth of oxide film under polymer coating increased the R f value, on the other hand the polymer film resistance increased due to reduction [32 – 34], as a result the total R f value increased. In the case of coated samples the measured R f value included both the polymer film and oxide film. Seventy-two hours later, the barrier property of polymer coating was still very good and the R ct value was still higher than the uncoated SS, while the R f value SS/ PPy(I) was lower than the uncoated sample. It could be concluded that a thinner oxide film with PPy top coating gave better protection than single oxide film under such aggressive conditions. In the case of SS/PPy(II) electrode (Fig. 9), both R ct and R f values were found to be lower than the SS/PPy(I) electrode. This case was explained with less effective barrier behaviour of PPy(II) coating when compared to PPy(I) film. This must be related to synthesis conditions; ACN-LiClO4 medium gave less porous PPy coating on stainless steel. The water up taking process takes place

Table 1 The R ct, R f, E cor values determined from EIS results SS

E corr (V) R ct (V) R f (V) b fa (V) ha a

SS/PPy(I)

SS/PPy(II)

8

24

72

8

24

72

8

24

72

+0.08 9.8 1992 2330 44.7

0.02 16.2 2818 3350 46.3

0.03 37.2 4851 6820 45.8

+0.32 198.4 251 6460 52.4

+0.20 176.5 416 3370 51.3

+0.12 156.4 602 2397 49.3

+0.30 26.2 171 2600 51.1

+0.16 24.5 254 1660 50.2

+0.02 23.9 483 930 48.2

b f and h values are given for the 0.008 Hz frequency value.

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T. Tu¨ken / Surface & Coatings Technology 200 (2006) 4713 – 4719

Fig. 8. The Nyquist plots obtained for SS/PPy(I) after 8: >, 24: 0 and 72 h:

more rapidly through the PPy(II) coating and lower R ct values obtained. The adhesion of polymer coating on metal substrate must have decreased with time, due to water up taking process. It was clear that the adhesion of polymer film will be lost completely and single oxide film behaviour will be observed, at higher exposure periods. On the other hand, the long diffusion tail on the Nyquist plots was observed in every case. This region has been indicated as Z w (Warburg impedance) in equivalent circuit model (Fig. 7) previously. At very low frequencies, the capacitive components become negligible and the measured total impedance value, Z , is given as R s + R ct + R f + rx 1/2(1  j) [28,29]. The last term was equal to Z w, where r is the Warburg coefficient (V cm2 s1/2) and x is 2pf (rad s1). In this term, the imaginary portion ( rx  1/2j) reflects the information pffiffiffi of diffusion process and the Z w value was equal to 2rx  1/2. The Warburg impedance can be obtained from the value of Imag-axis at low frequency. In this study, the b f value (b f = rx  1/2) was used as the Warburg impedance for the comparison of coated and uncoated SS samples for their permeability. For a given x value, the larger b f value indicates that the coating has a less porous (permeable) structure and the diffusion of corrosive species is more

?

immersion periods in 3.5% NaCl solution.

difficult [29]. It was apparent from Table 1 that the b f value of SS sample increased with time due to further oxide film growth, while the b f values of PPy coated samples decreased with time. This event was explained with the loss of barrier property of polymer film with increasing exposure time, due to water up taking process. On the other hand, the PPy(I) coating gave higher b f values than PPy(II) coating. This also corroborated that that PPy(I) had less porous structure than PPy(II).

4. Conclusions The synthesis of polypyrrole coating on stainless steel was achieved from aqueous oxalic acid and ACN-LiClO4 solutions, electrochemically. The corrosion performance of these coatings was investigated using ac impedance spectroscopy and anodic polarization curves. It was shown that ACN-LiClO4 solution gave more protective polymer film which had lower permeability and higher stability than the other. Also, the ac impedance results showed that both coatings exhibited important anodic protection behaviour on passivation of steel. However, better efficiency was obtained with the film synthesized from ACN-LiClO4 solution.

Fig. 9. The Nyquist plots obtained for SS/PPy(II) after 8: >, 24: 0 and 72 h:

?

immersion periods in 3.5% NaCl solution.

T. Tu¨ken / Surface & Coatings Technology 200 (2006) 4713 – 4719

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