Characterization of polyaniline films electrodeposited on mild steel in aqueous p-toluenesulfonic acid solution

Journal of Electroanalytical Chemistry 445 (1998) 117 – 124

Characterization of polyaniline films electrodeposited on mild steel in aqueous p-toluenesulfonic acid solution J.L. Camalet, J.C. Lacroix, S. Aeiyach, P.C. Lacaze * Institut de Topologie et de Dynamique des Syste`mes de l’Uni6ersite´ Paris 7 -Denis Diderot, Associe´ au CNRS (URA 34), 1 rue Guy de la Brosse, 75005 Paris, France Received 14 July 1997

Abstract Aqueous p-toluenesulfonic acid solution is used to electrosynthesize a polyaniline (PANi) film on mild steel. Polarization of the substrate in this medium leads to passivation of the surface mainly via the formation of an iron oxide layer. When aniline is added to the solution, electropolymerization is not hindered and a dark green deposit is obtained in high yield (80%). Spectrochemical techniques (IR, XPS and UV) and mass determination (SEC and MALDI) indicate that it has the same properties as reported for PANi. Evaluation of the anticorrosion performances of the film shows that the corrosion current is divided nearly by ten, suggesting that this polymer coating could be used for corrosion protection. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Electropolymerization; Iron; Mild steel; Polyaniline; Corrosion; Inhibition

1. Introduction Recently, the use of conducting polymers on oxidizable metals to protect them against corrosion was reported. Like other polymeric coatings, these materials can constitute a physical barrier toward corrosive reagents. Moreover, as they carry polar groups, they may also act as macromolecular inhibitors and shift the potential of the substrate to a value where its rate of corrosion is reduced. In industry, treatments of mild steel prior to painting use conversion steps (phosphatation and/or chromatation) that have a strong environmental impact. Electropolymerization may be a clean and cheap alternative treatment which could eliminate the pollution associated with these processes and take advantage of the electrodeposition baths already used. However, to be of industrial interest, these new conductive coatings must ensure good adhesion of the subsequent paint layers and improve the corrosion resistance of the painted * Corresponding author. 0022-0728/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 2 2 - 0 7 2 8 ( 9 7 ) 0 0 5 2 6 - 3

metal. Furthermore, to minimize the environmental impact, the coating must be formed in an aqueous electrochemical bath. On oxidizable substrates, the anodic electrodeposition of conducting polymers is not easy since thermodynamic data predict that the metal will oxidize before the oxidation potential of the monomer is reached. To form a film therefore, it is necessary to find electrochemical conditions which lead to passivation of the metallic surface. This approach has allowed the deposition of conducting polymers, mainly polypyrrole and polyaniline, on metals like stainless steel, titanium, nickel or aluminium [1–7]. Although iron or mild steel have technological importance, their use as substrates has been less investigated because a passivation state allowing the formation of a conducting polymer is more difficult to obtain [7–15]. Nevertheless, in spite of this, electrosynthesis of PANi films on mild steel from aqueous nitric acid has been already reported [8]. However, the results were unsatisfactory, since the films had poor mechanical and anticorrosion properties. More recently, we have shown [14,15] that PANi films elec-

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trodeposited in aqueous oxalic acid solution could be used to protect a substrate but the rather slow deposition rate of the process restricts its application. Using another approach, Wessling et al. [16,17] reported that mild steel sheets dip-coated with chemically prepared PANi showed a real improvement in their behaviour in corrosive media and suggested the use of PANi for corrosion protection. In this work, the electrochemical path for the coating of the substrates is investigated; we report another electrolytic bath suitable for the deposition of PANi on iron or mild steel: an aqueous p-toluenesulfonic (tosylic) acid. This medium allows passivation of the surface without preventing the electropolymerization of aniline. The passivation processes of oxalic and tosylic acids are compared and the deposited films are characterized by various spectroscopic techniques.

2. Experimental

2.1. Electrochemical methods A single compartment cell using stainless steel as the counter-electrode and a saturated calomel as the reference electrode was used for the electrochemical studies. The working electrode was either a 3 cm2 iron plate (Weber metals) with %Fe\ 99.5 or a mild steel plate provided by SOLLAC. The experiments were conducted with a PAR 173 potentiostat connected to a PAR 175 programmer.

2.2. Other techniques UV-Vis absorption spectra were obtained using a Perkin Elmer Lambda 2 spectrophotometer. All other techniques used for the characterization of the metallic surface or the polyaniline films, XPS, IR, SEC (Size Exclusion Chromatography) and MALDI-TOF (Mass Assisted Laser Desorption Ionization), have been described elsewhere [15].

3. Results and discussion

3.1. Polarization of mild steel in 1 M tosylic acid 3.1.1. Electrochemical response of the electrode Cyclic voltammograms (sweep rate, 6 =50 mV s − 1) of a mild steel electrode polarized in a 1 M tosylic acid solution are given in Fig. 1. The first positive scan (between − 0.54 V and 1.8 V) shows an oxidation peak (A) at − 0.1 V, the beginning of the oxidation wave of the electrolytic medium (B) at 1.2 V; between 0.1 V and 1.2 V, no electrochemical signal is observed. During the reverse scan, the oxida-

Fig. 1. Potentiodynamic polarization (6 = 50 mV s − 1) of mild steel electrode in 1 M aqueous tosylic acid solution.

tion current takes small values except in the potential range 0.3 to 0 V where a sharp oxidation peak (C) is observed. For the next cycles, flat voltammograms are obtained with the exception of the oxidation wave B and the oxidation peak C which shifts to a fixed potential of 0.16 V and whose height decreases. This behaviour has been already reported by Yang and Teng [18] and is roughly similar to that observed for mild steel polarized in oxalic acid solution [15,19]. In particular, the current density and the position of peak A also vary linearly with the square root of 6, which is consistent with the formation of a passivating film under ohmic resistance control [20]. Therefore, peak A can be attributed to an electrode passivation process and peak C to another oxidation of the substrate due to partial destruction of the passive layer [19]. The report for three different media of the charge Qp and the current density jp associated with peak A (Table 1) shows that the extent of oxidation of the metal varies with the nature of the electrolyte. Differences in the establishment of the passivation state can explain these results. For the oxalic acid solution, the formation of a precipitate and the influence of its solubility product on Qp have been shown Table 1 jp and Qp values of peak A recorded during potentiodynamic polarization of a mild steel electrode in various electrolytic media 1 M Tosylic acid jp / mA cm−2 98 Qp / mC cm−2 545

0.3 M Oxalic acid

1 M Sulphuric acid

35.2 238

590 7500

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Moreover, Eq. (1) implies that the higher the current density the less anodic dissolution of the metal occurs. Therefore, the extent of oxidation of the metal can be monitored by controlling j, which is not the case for oxalic acid. Polarization in this latter medium leads to dissolution of a thickness of iron of ed = 0.1 mm, whatever j, while in tosylic acid ed depends on j and will be less than 0.1 mm when j \40 mA cm − 2, which is the order of magnitude needed for industrial applications.

Fig. 2. Chronopotentiometric curves of mild steel electrode polarized at various current densities in 1 M aqueous tosylic acid solution. (a) j= 20 mA cm − 2, (b) j =7 mA cm − 2.

[15]. If such a mechanism were accepted for the other media of the study, the various stabilities of the precipitates would give different values of Qp. The structure of the anion must also play a part in the passivation of the surface: the presence of an aromatic nucleus can limit the oxidation of the metal by blocking its oxidation sites [21] and can account for the values of Qp in the tosylic and sulfuric solutions. From a practical point of view, it is obvious that a limited oxidation of the metal and, therefore, small values of Qp are better. In what follows only tosylic acid will be studied, a previous paper having been devoted to oxalic acid [15]. When mild steel is polarized in the galvanostatic mode, the chronopotentiometric curves (Fig. 2) obtained are characteristic of a passivation process: after an induction time tp during which oxidation of the substrate occurs, an abrupt rise of the potential is observed. This behaviour is similar to that obtained during polarization of mild steel in oxalic acid [15] but there are some differences in the relation between tp and j. Indeed, in the tosylic acid solution: − 2 0.5 jt 0.5 s p = Constant=116 mA cm

3.1.2. Nature of the mild steel surface after polarization The chemical composition of the mild steel surface after polarization in 1 M aqueous tosylic acid has been studied by XPS. The carbon, oxygen, sulfur and iron signals have been analysed to determine the modifications of the surface after its passivation. No sulfur signal was observed on the samples, so tosylate ions do not seem to take part in the formation of a specific compound directly responsible for the passivation of the substrate. This result is in contrast with those of Yang and Teng [18], who postulate formation of an iron sulfide compound that inhibits oxidation of the metal and then leads to the decrease in the anodic current. The iron spectrum shows (Fig. 3) a component with a binding energy at 707 eV, characteristic of metallic iron and a broad one at 711 eV which corresponds to various metallic oxide/hydroxide and oxyhydroxide forms [24]. The shift of the iron satellite in the spectrum is, however, indicative of a strong contribution of iron(III) oxide. The analysis of the oxygen signal confirms the presence of these oxides: besides a component attributed to contamination (at  532 eV), a low binding energy component ( 530 eV) characteristic of iron oxide [24] is detected.

(1)

while in oxalic acid: jtp = Qp = 325 mC cm − 2

(2)

These relations are often found in the galvanic polarization of oxidizable metals but their significance is still unclear [22,23].Eq. (1) can be rewritten: jQp =(Constant)2 which shows that in the tosylic acid solution passivation does not occur after consumption of a constant charge. The passivation processes in oxalic and tosylic acids are then different though the passive layers are both formed under ohmic resistance control.

Fig. 3. Fe2p XPS spectrum of mild steel electrode polarized at E= 0.5 V in 1 M aqueous tosylic acid solution.

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All these observations seem to indicate that the passivation of the surface results from iron oxide layers formed by the reaction of the metal with the electrolytic solution. This is in sharp contrast with mild steel polarized in 0.3 M oxalic acid [15], where an insoluble iron(II) oxalate is precipitated: XPS results indicate that the precipitate covers the metallic surface. Although a similar compound is not observed in the surface polarized in tosylic acid, the formation of an iron tosylate precipitate during the passivation process cannot be excluded. This precipitate would favour, as in oxalic acid, the formation of metallic oxides leading to the final passivation of the electrode. Such a process has already been suggested for various metals, and their passive films would be multilayers with a nature and a structure that are still a matter of controversy. However the authors agreed that the upper porous layer consists of a precipitate from the anions of the electrolyte and the metallic cations and that the inner layer is a compact phase of metallic oxides responsible for the passivity of the substrate [25]. It is worthwhile mentioning that the metallic precipitate is rarely detected, probably because it is generally soluble and it is not very adherent to the surface: Wang et al. [26] indicate that the amount of sulfur detected by XPS in passivated iron samples is variable and depends on the rinsing procedure. Therefore, in the case of the oxalic and tosylic acid solutions, the difference between the composition of the passivated surfaces probably reflects the fact that iron tosylate is much more soluble than iron(II) oxalate and is eliminated by the rinsing procedure that precedes XPS analysis.

3.2. Electropolymerization of aniline 3.2.1. The potentiodynamic mode Adding aniline to the electrolytic solution does not modify appreciably the shape of the first voltammogram. One can again observe the oxidation peak A of the metal whose position and intensity still vary linearly with the square root of the sweep rate. This indicates that the passivation process is hardly modified by the presence of the aniline. Above 0.1 V, the oxidation current remains low until wave B is reached. During the back sweep, no electrochemical signal is observed with the exception of peak C that is still present at 0.1 to 0.2 V. The second cycle of polarization reveals three oxidation peaks attributed to a new oxidation of the metal (at −0.3 V) and to the oxidation of aniline and/or its electropolymerization products (at 0.2 and 0.75 V). During the potentiodynamic scans, the current density between 0.2 and 1.2 V increases continually and an electroactive and electrochromic film is deposited: it is pale green when E B0 V and becomes green then blue-black when the potential is greater than 0.4 V.

Fig. 4. Potentiodynamic polarization (6 = 50 mV s − 1) during the growth of PANi film on mild steel in aqueous solution of 1 M tosylic acid+ 0.3 M aniline.

As for steel polarized in oxalic acid [15], film growth has been studied by reducing the potential window to between 0.25 and 0.65 V (Fig. 4). The increase in the oxidation and reduction currents observed during the polarization indicates the regular growth of a PANi film. A redox couple can be observed at 0.55 and 0.35 V. These peaks could be attributed to electroactive by-products trapped in the film [27]. The increase of the polarization window to higher potentials does not provide more information (the increase of the current at 0.8 to 0.85 V corresponds to aniline oxidation) and the presence of peak C at 0.15 to 0.2 V prevents any observation at potentials below 0.25 V.

3.2.2. The gal6anostatic mode When the electrode is polarized at constant current densities, the curves obtained still express passivation of the surface and are similar to those obtained without aniline. However, we can observe the formation of a dark green film once the potential stabilizes to values between 1.6 and 1.9 V and also the diffusion of green products into the solution. Concurrently with the electrodeposition of PANi, oxidation of the electrolytic solution and of the metal probably occur and may interfere with the electropolymerization process. The deposited film is green, relatively dusty (for thick films) and Sellotape tests indicate weak adhesion to the substrate. For j values between 5 and 45 mA cm − 2, a relation: − 2 0.4 jt0.4 s p = Constant= 84 mA cm

(3)

is derived. This law is similar to Eq. (1) and the lower value of the constant confirms that passivation is easier when aniline is added to the medium. In order to obtain quantitative information about the mass variation, Dm of the substrate during polarization at j= 10 mA cm − 2, the electrode was carefully rinsed in ethanol, then in water and dried under dynamic vacuum before being weighed. Two stages appear in the Dm= f(Q) curve, where Q stands for the charge consumed (Fig. 5):

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(1) Q B2 C cm − 2: the decrease in the mass of the electrode can be related to the anodic dissolution of the substrate according to the reaction Fe“Fe2 + +2e − . The experimental electrochemical equivalent gexp1 is evaluated from the slope of the straight line and is equal to 0.3 mg C − 1, a value close to the theoretical one gth1 = MFe/2F = 0.29 mg C − 1. (2) Q \ 2 C cm − 2: the deposition of a PANi film is associated with an increase in the mass of the electrode which occurs after time tp determined from Eq. (3): tp = 20 s :Q/j From the slope of the line we deduce gexp2 =0.56 mg C − 1. For the electropolymerization of aniline, the theoretical equivalent is equal to: gth2 =(MM + yMA)/(2 + y)F =0.71 mg C

−1

where MM and MA are the molar masses of the monomer unit ( = 91 g mol − 1) and the anion ( =171 g mol − 1), respectively, F is the Faraday constant and y is the degree of insertion of the dopant in the film (= 0.45). The discrepancy between the two values can be attributed to secondary processes like oxidation of the substrate or diffusion of some products into the solution. Nevertheless, an electrochemical yield of 80% is obtained for the electrosynthesis of PANi film which is better than the 35% obtained in the oxalic acid solution [15] according to MEB measurements. This electrochemical study shows that tosylic acid solution is a convenient medium for the electrodeposition of PANi. Although oxidation of the substrate appears to be greater than in oxalic acid, the former has two obvious advantages: The extent of metal corrosion occurring before deposition of the film can be controlled and diminishes with increase in the current density used. The yield of the deposition process is much greater.

Fig. 5. Mass variation Dm versus the charge consumed Qs during the polarization at j= 10 mA cm − 2 of mild steel electrode in aqueous solution of 1 M tosylic acid+ 0.3 M aniline.

Fig. 6. IR spectra of undoped PANi films prepared at j = 10 mA cm − 2 in aqueous solution of 1 M tosylic acid+ 0.3 M aniline and treated with 0.05 M NaOH: (a) on platinum electrode, (b) on mild steel electrode.

3.3. Film characterization and anticorrosion properties 3.3.1. IR analysis The films are deposited in the potentiodynamic or galvanostatic mode in an aqueous solution of 1 M tosylic acid +0.3 M aniline. After synthesis, they are rinsed in ethanol to eliminate low molar mass products. They are deprotonated in a 0.05 M NaOH solution, rinsed in distilled water, then in ethanol and dried under dynamic vacuum before being scraped from the metallic surface. Films are analysed in the form of KBr pellets. The IR spectrum (Fig. 6) corresponds to that obtained in the literature [28], and comparison with a spectrum of PANi electrosynthesized on platinum shows that the main peaks are identical. These spectra allow us, therefore, to identify deposited films on mild steel as being PANi having a regular linear structure with a 1,4 coupling between the aromatic nuclei. Bands at 1165, 1010 and 830 cm − 1 are attributed to in− plane vibrations (1165 and 1010 cm − 1) and out-of-plane (830 cm − 1) bending modes of C–H bonds of 1,4-disubstituted aromatic nuclei [28]. The lack of vibrations around 900 cm − 1 and at 800 to 700 cm − 1 indicates that 1,2- and 1,3-disubstituted nuclei are negligible. 3.3.2. XPS analysis Deprotonated films are obtained following the same protocol as for the IR study. The protonated form is obtained by rinsing the film in an aqueous solution of 0.1 M tosylic acid. Iron is not detected in the samples analysed, which indicates that the electrochemical deposition process allows full coverage of the metal. For the deprotonated film, the nitrogen signal (Fig. 7a) can be decomposed into two peaks at 398.6 eV and 399.7 eV which are attributed [29] to neutral imine and

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neutral amine units respectively. These two components have roughly the same intensity ( 50% of the total nitrogen area), in agreement with the ideal emeraldine structure of PANi. Nevertheless, a weak peak can be observed for a binding energy greater than 400 eV. It is attributed to positively charged nitrogen [29,30] and its presence shows that the film remains slightly protonated. After protonation, the nitrogen signal becomes totally asymmetrical (Fig. 7b). The component attributed to neutral imine nitrogen disappears while that attributed to amine nitrogen is not affected, keeping the same area ( 50% of the total nitrogen area). At high binding energies, there is a large, broad signal indicative of positively charged nitrogen atoms N + . The disappearance of the imine component from the protonated PANi spectrum indicates that film protonation involves preferentially imine sites that are indeed more basic than amine sites [31]. We can, therefore, like Kang et al. [32], attribute the high bonding energy signal to the exclusive protonation of imine sites. The N + signal can be decomposed into three peaks at 400.8, 401.6 and 402.7 eV, in agreement with previous results [29,30]. Kumar et al. [29] and Yue and Epstein [30] consider that the conjugated structure of PANi induces a delocalization of the positive charge on the polymer backbone. The component at 400.8 eV is therefore attributed to nitrogen atoms with strong delocalization of the positive charge, whereas the 402.7 eV peak is due to nitrogen atoms bearing a localized charge. In this scheme, the 401.6 eV component corre-

Fig. 8. UV-visible spectra of PANi film dissolved in DMSO. PANi film was synthesized on mild steel electrode in aqueous solution of 1 M tosylic acid+0.3 M aniline: (a) deprotonated film (treated with 0.05 M NaOH), (b) protonated film (treated with 0.2 M tosylic acid).

sponds to an intermediate state. Nevertheless, these attributions are still a matter of some controversy and will not be discussed in more detail here. After protonation of the polymer, this signal becomes important showing that tosylate ions are incorporated in the film. The intensity of the sulfur signal is equal to the N + one, which indicates that tosylate ions are indeed the counter-ions of the film and compensate the positive charges on the polymer backbone.

3.3.3. UV analysis in solution in DMSO All UV spectra of deprotonated PANi films in solution in DMSO present (Fig. 8a) two absorption bands at 331 nm (3.75 eV) and 626 nm (2 eV) whose position and shape are not influenced by the polymer concentration. Spectra obtained are characteristic of the emeraldine base form and are identical to those reported in the literature [33–35]. The band at 331 nm is attributed to the aromatic nucleus of the chain and corresponds to the p –p* transition of the polymer skeleton [33]. The absorption in the visible (626 nm) indicates an interaction (exciton band) between benzene nuclei and the diimine quinoid structure on the PANi [33]. This band is characteristic of the degree of oxidation of the polymer because it does not appear in the spectra of the completely reduced form (without quinone diimine structure). Between 1 and 10 mg l − 1 the optical absorption of these two bands obeys the Beer-Lambert law and this allows an evaluation of a molar extinction coefficient: e330 = 6340 l mol − 1 cm − 1

and

e626 = 3790 l mol − 1 cm − 1

Fig. 7. N1s XPS spectra of PANi film synthesized in aqueous solution of 1 M tosylic acid + 0.3 M aniline at j =10 mA cm − 2 on mild steel electrode: (a) deprotonated film (treated in 0.05 M NaOH solution), (b) protonated film (treated in 0.2 M tosylic acid solution).

values slightly greater than those reported by Barbero et al. [35] (5400 l mol − 1 cm − 1 and 3200 l mol − 1 cm − 1, respectively). When the films are protonated (Fig. 8b), the spectrum exhibits three absorption bands at 330 nm (3.75

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eV), 425 nm (2.9 eV) and 825 nm (1.5 eV) and it is characteristic of the emeraldine salt form [33,34]. The two new bands result from the transformation of the neutral quinonediimine structure into radical cations and/or dications [33,34].

3.3.4. Molecular mass determination DMF solutions of deprotonated PANi films were used for comparative molecular mass studies by the SEC and MALDI techniques. The SEC chromatogram (Fig. 9) shows a polymodal distribution with a broad peak A and two small peaks B and C. These latter correspond to PANi aggregates [36,37] and are not useful for quantitative analysis. Peak A is characteristic of the polymer in solution and its mass determination (by using polystyrene standards for calibration) gives: Mw =24 400 g mol − 1

Mn =17 100 g mol − 1

Fig. 10. MALDI spectra of PANi film dissolved in DMF solution. PANi film was synthesized on mild steel electrode in aqueous solution of 1 M tosylic acid+0.3 M aniline.

Ip = Mw/Mn = 1.4 The molar mass Mw is slightly lower than previously reported values [37] and, therefore, the electrodeposition of PANi on mild steel gives shorter polymer chains (DPn =190) than those obtained on platinum. The polydispersity index shows that the electrosynthesis of this conducting polymer leads to a relatively narrow mass distribution of the products. The MALDI spectrum obtained by analysing the same solution (Fig. 10) shows a broad massif centred around 2200 g mol − 1 which extends up to masses of 10 000 g mol − 1. An evaluation of the mean molar mass gives Mn =2900 g mol − 1. Comparison with the value determined in GPC shows that Mn(GPC)/

Mn(MALDI)\ 5. The large discrepancy between the two values points out, as reported earlier [15], that the GPC molecular masses are overestimated. The pattern of several secondary peaks in the MALDI spectrum corresponds to chains of various lengths and reflects, therefore, the complexity of the electropolymerization reaction. This has been already observed for PANi electrosynthesized on mild steel [15], and the mass difference Dm = 91 g mol − 1 between two main peaks can be related to an aniline motif for the polymer.

3.3.5. Corrosion resistance of the coated samples For the corrosion tests, the PANi-coated samples (S= 1 cm2) were dipped in a known volume of 0.1 M HCl+ 0.4 M NaCl. After 5 h, the amount of Fe2 + in the solution was measured by the o-phenanthroline method [38]. When the samples are dipped in the test solution bubbles appear immediately on the uncoated mild steel sheet and only later (after 5 h) for the coated surfaces. This process corresponds to the reaction: Fe+ 2H + “ Fe2 + + H2 and can be related to differences in the extent of the corrosion of the two substrates. The current density of corrosion jcor (mA cm − 2) was obtained from the Faraday law: jcor = 2FnFe/t

Fig. 9. GPC chromatogram of PANi film dissolved in DMF. PANi film was synthesized on mild steel electrode in aqueous solution of 1 M tosylic acid +0.3 M aniline.

where nFe is the number of mol of Fe2 + present in the solution. For an uncoated mild steel surface (used as reference), a current density of jcor = 1.3 mA cm − 2 was determined, while for a PANi-coated surface jcor =0.15

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mA cm − 2. This low value confirms the visual observations that PANi increases the corrosion resistance of mild steel. The inhibition properties of PANi have already been demonstrated [14 – 17,39] and the same decrease of jcor has been reported for mild steel coated with PANi electrodeposited from aqueous oxalic acid [15]. It should be noted that Sathiyanarayanan et al. [39] obtained a similar result for iron dipped in an acidic medium containing dissolved poly(methoxyaniline). The PANi acts then rather as a macromolecular inhibitor which, as suggested by Wessling et al. [16,17], promotes the formation of a passivating iron oxide layer.

4. Conclusion PANi film has been electrosynthesized on a mild steel electrode from an aqueous solution of tosylic acid. The deposition process occurs after passivation of the substrate. Chemical analysis of the surface shows that the passive layer is mainly an iron oxide, as already reported for the polarization of this substrate in sulfuric acid and in contrast to oxalic acid where a precipitate is formed on the metallic surface. From an industrial point of view, tosylic acid presents some advantages over oxalic acid because the extent of metal oxidation can be diminished by applying high current densities and the yield of the electropolymerization reaction is greater, nearly 80%. The film deposited on the oxidizable metal has the same structure (IR, XPS, UV) and molecular weight (SEC) similar to that of PANi electrosynthesized on platinum. Corrosion tests performed on PANi-coated electrodes confirm previous results and demonstrate the potential utility of this polymer as an anticorrosion coating. However, the general performance of the deposit (adhesion, resistance to deformation) remains to be improved by the formulation of appropriate electrochemical baths.

Acknowledgements The authors wish to thank SOLLAC (USINORSACILOR) for financial support of this work.

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