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Ultra-fast electropolymerization of pyrrole in aqueous media on oxidizable metals in a one-step process
www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 478 (1999) 92 – 100
Ultra-fast electropolymerization of pyrrole in aqueous media on oxidizable metals in a one-step process J. Petitjean a,b, S. Aeiyach a, J.C. Lacroix a, P.C. Lacaze a,* a
Institut de Topologie et de Dynamique des Syste`mes de l’Uni6ersite´ Paris 7 -Denis Diderot, associe´ au CNRS (UPRES-A 7086), 1 rue Guy de la Brosse, 75005 Paris, France b SOLLAC-LEDEPP, Groupe Usinor, 17 A6 des Tilleuls, BP 70011, 57191 Florange Cedex, France Received 20 April 1999; received in revised form 1 October 1999; accepted 8 October 1999
Abstract A general one-step electrosynthesis process for polypyrrole (Ppy) films applicable to a large range of oxidizable metals is described. The process is based on the use of an aqueous medium containing a salicylate salt and pyrrole. The salicylate salt is quite specific: it passivates the substrate without preventing electropolymerization of pyrrole and, therefore, makes it possible to obtain strongly adherent PPy films, with controlled thickness, either by cyclic voltammetry or by electrolysis at constant current. In the latter case the process does not involve any induction period corresponding to a preliminary dissolution of the metal, the current efficiencies are close to 100%, and the deposition rate can be very high (1 mm s − 1, corresponding to current densities as high as 500 mA cm − 2). © 1999 Elsevier Science S.A. All rights reserved. Keywords: Electropolymerization; Zinc; Mild steel; Polypyrrole; Corrosion; Inhibition
1. Introduction In recent years several conducting polymers such as polyaniline (PANI), polypyrrole (PPy) and polythiophene (PT) have been shown to be promising candidates as material coatings for the protection of common metals against corrosion [1 – 44]. Among these polymers, PPy and its derivatives have attracted much attention owing to their high stability in the oxidized state, low potential for polymer formation and ease of synthesis in aqueous solution in a wide range of pH between 4 and 10 [27,29]. Generally, PPy films are easily synthesized at inert anodes such as platinum, gold, glassy carbon or stainless steel [45,46] by electrochemical oxidation of pyrrole or its derivatives in aqueous or organic media. Transferring the electropolymerization of these monomers to oxidizable metals is much more difficult. Indeed, the oxidation potentials of these metals are much more negative than that of the pyrrole and its derivatives and, consequently, dissolution of the metal will occur before electropolymeriza* Corresponding author. Fax: +33-1-4427-6814. E-mail address: [email protected] (P.C. Lacaze)
tion begins. Therefore, to form PPy films on oxidizable metals, it is necessary to find electrochemical conditions which will strongly passivate the metal without impeding electropolymerization. Several research groups have studied this problem [18–35]. Beck et al. [20,21,24,25] reported that the use of NO− 3 or oxalate ion as the electrolyte in aqueous solution allowed electropolymerization of pyrrole on Fe or mild steel and yielded adherent, smooth PPy films. However, in both cases, the adherence of the PPy films did not satisfy the usual requirements for good anticorrosion protection and, in particular, for the electrodeposition of cataphoretic paint on top of the films; these films peeled off the substrate when their reduction potential reached a value of − 0.5 V versus SCE. The adhesive strength of the PPy coatings was markedly improved when the iron or mild steel surface was pretreated with MnO− 4 ions [24,29] and electropolymerization was performed in aqueous oxalic acid solution. However, in spite of this improvement, it must be noted that electropolymerization began only after a lengthy induction period corresponding to metal dissolution. Approximately at the same time, our group found that strongly adherent PPy films could be deposited on
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J. Petitjean et al. / Journal of Electroanalytical Chemistry 478 (1999) 92–100
iron or mild steel without an induction period when the substrate was pretreated with dilute nitric acid and pyrrole electropolymerized in an aqueous solution con− taining NO− 3 , C2O4 or p-toluenesulfonate ions [26,27]. The quality of the PPy coatings was improved by adding small amounts of substituted pyrroles (COOH and NH2 groups at the 2-position) to the pyrrole solution, followed by heat treatment of the modified PPy films. The adherence of the PPy coatings was strong enough to allow paint deposition by the cataphoretic technique. The resistance of these novel coatings to salt spray tests (PPy+cataphoretic paint) was as good as that of the usual cataphoretic coatings obtained on phosphated mild steels [28,34,35] More recently we attempted to deposit PPy coatings on zinc or zincated steel using the same technique. Unfortunately, the procedure failed completely with zinc substrates. Zinc and zincated steel are much more electropositive than iron or steel, and under these conditions zinc dissolution remains extremely high and prevents pyrrole oxidation. Therefore, in order to deposit PPy films on these metals two different strategies were adopted. The first consisted of a two-step process where, prior to pyrrole electropolymerization, the zinc surface was chemically or electrochemically pretreated with a sulfide solution and then pyrrole was electropolymerized in an aqueous solution containing oxalate ions [30,32]. The second, more direct method, required only one step in which the treatment of the Zn surface and the electropolymerization of pyrrole occur simultaneously when small amounts of sulfide ion are mixed with the oxalate solution at pH 5 [33]. However, in spite of this significant improvement, which led to very adherent PPy films on zinc, the process remains quite specific and cannot be applied to other oxidizable metals. The aim of the present work is to find a general process for PPy coating, which can be applied to any metal and, in particular, to zinc which is one of the most easily oxidized metals. We show in this work that sodium salicylate in the presence of pyrrole constitutes a solution to this problem and allows the formation of strongly adherent PPy coatings on various oxidizable metals with very high current densities and without an induction period corresponding to metal dissolution.
2. Experimental All chemicals such as sodium orthophthalate, sodium 2-mercapto benzoate, sodium oxalate and sodium salicylate were purchased from Aldrich or Janssen and used as received without further purification. Electrodeposition was performed in a single-compartment cell using stainless steel as the counter-electrode and saturated calomel as the reference electrode. The working
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electrode was a 3 cm2 plate of iron or zinc (Weber metals) with an iron or zinc content greater than 99.5%. Mild steel, electrozincated mild steel, zinc/nickel, zinc/ cobalt and zinc/aluminum alloys were provided by Sollac Usinor. The substrates were degreased with acetone under ultrasonic conditions and rinsed with ethanol. FTIR characterization of the films was performed on a Nicolet 60 SX spectrometer. Raman spectra were recorded on a Dilor XY spectrometer with 514.5 nm laser light excitation at low power (5 mW) in order to prevent photodegradation of the films. XPS measurements were made on a VG ESCALAB MK1 spectrometer with a non-monochromatic source under the following conditions: Mg–Ka beam, source power 200 W, input energy 20 eV, area analyzed 0.4 cm2. The binding energy was referenced using the C1s signal at 285 eV and relative atomic concentrations were obtained from the areas of the corresponding signals by using, for C1s, O1s, N1s and Zn2p, sensitivity coefficients determined by analysis of model compounds of known stoichiometry and equal to 1, 3.2, 1.6 and 20, respectively. The PPy film thickness was estimated either by mass measurement or by scanning electron microscopy (SEM). In the first case, the weight of the PPy deposit was determined and converted into film thickness using a mean density of 1.5 [24]. Direct thickness measurements by SEM were made on micrograph cuttings.
3. Results and discussion When a zinc electrode is polarized at a potential slightly higher than −1 V in a usual electrolyte such as K2SO4 or KCl, immediate, strong dissolution of the metal occurs, and there is no possibility of carrying out the electrochemical oxidation of any compound whose oxidation potential is above this value. As stated previously, using an electrolyte with an anion capable of giving an insoluble salt with Zn2 + is the best way to passivate the zinc surface and to manage a potential window in which the oxidation of pyrrole can be performed. Accordingly, various salts known to form insoluble salts with Zn2 + were tested. Anionic ligands containing oxygen, sulfur or nitrogen atoms were preferred since they give stable complexes with iron and zinc [47]. Depending on the choice of the electrolytic salt, three types of voltammetric behavior were observed corresponding to various passivation phenomena. In the case of strongly Zn2 + -chelating salts, capable of forming a pseudo-five atom ring with Zn2 + , sharp passivation of the zinc surface was observed. A typical voltammetric curve obtained with sodium oxalate (Fig. 1(a)) exhibits a passivation peak between − 1 and 0 V and a wide potential range from 0 up to more than 2 V
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where the current density is very low. During the back-sweep the current density remains very small, indicating that an insulating insoluble zinc oxalate layer has precipitated on the surface and prevents zinc and pyrrole oxidation (Fig. 1(a)). Similar behavior is observed with sodium 2-mercapto benzoate, sodium 2amino-benzoate, sodium 2-mercapto-nicotinate and sodium 2-hydroxy-nicotinate. The opposite situation is found with salts with weak chelating properties, as is shown by sodium orthophthalate (Fig. 1(b)). In this case, the current density rises steadily from −1 to 2 V and decreases reversibly with the potential during the back-sweep. Clearly, this behavior, characteristic of progressive zinc dissolution without any marked passivation, can be attributed to the fact that zinc-orthophthalate is much too soluble in water to precipitate on the surface and to inhibit zinc oxidation. Consequently, it is obvious that this behavior is just as incompatible with pyrrole oxidation as the previous one is with oxalate. Therefore, a compromise must be found between these two extremes. An electrolyte which would form a non-blocking passivating layer with Zn2 + and would be able to inhibit the dissolution of the metal without preventing the oxidation of electroactive species such as pyrrole should be suitable. Such a compromise was found with sodium salicylate whose voltammetric curve (Fig. 1(c)) exhibits behavior intermediate between those of sodium oxalate and orthophthalate.
3.1. Voltammetric beha6ior of a zinc substrate in a 1 M sodium salicylate solution Fig. 2 shows the voltammetric curve recorded when the potential of a platinum (Fig. 2(a)) or a zinc substrate (Fig. 2(b)) is swept at 10 mV s − 1 in a 1 M
Fig. 1. Voltammetric curve at 10 mV s − 1 on a 3 cm2 zinc electrode in: (a) 0.1 M sodium oxalate; (b) 0.1 M sodium orthophthalate; (c) 0.1 M sodium salicylate.
sodium salicylate solution. On platinum, an irreversible peak is observed at 1 V due to the oxidation of salicylate ions. In this case, a thin yellowish film appears on the electrode after polarization in the region of 1 V and colored soluble products diffuse into the electrolytic medium. A further potential sweep indicates that the deposited film is non-conductive, since the salicylate oxidation signal decreases slightly with the number of potential sweeps. Most of the reaction products appear to be soluble in the electrolytic media and this is probably due to the presence of carboxylic acid groups on the oligomers generated. When a zinc substrate is used, sweeping the potential between − 1.1 and 2.8 V shows three oxidation peaks at − 0.9, − 0.65 and + 1 V and the beginning of an oxidation wave at + 2.3 V. The first two peaks can be attributed to substrate dissolution leading to Zn2 + , followed by chemical precipitation of the metallic cation with salicylate, which leads to a thin insoluble layer, corresponding to complexation of the Zn2 + cation salicylate anions. This insoluble layer has passivating properties towards the zinc substrate; the peak at 1 V and the wave at 2.3 V do not involve zinc oxidation but that of salicylate and water, respectively, similarly to what is observed on platinum under the same conditions. Upon reversal of the potential sweep, two new oxidation peaks appear at + 0.8 and −0.95 V attributed to salicylate and zinc oxidation, respectively. These peaks disappear when the sweep rate is faster than 200 mV s − 1. In this latter case (Fig. 2(c)) the observed current stays at a value close to zero during the whole backsweep, indicating that reactivation of the zinc electrode is slow. The second potential sweep is also highly dependent on the sweep rate. At 200 mV s − 1, the passivation state is maintained during the second potential sweep and only one signal at +1 V is detected (Fig. 2(b)). On the contrary, when the sweep rate is 10 mV s − 1, the peaks at − 0.9 and − 0.7 V reappear, indicating that the initial passivating layer has been progressively wiped off the substrate, allowing the zinc to dissolve again (Fig. 2(c)). All these results show that the surface modifications induced by the first potential sweep on the zinc substrate are controlled kinetically and, therefore, it differs dramatically from iron or mild steel electrodes, for which the surface modifications induced by the first potential sweep remain quite stable when the sweep rate is kept at 10 mV s − 1 (Fig. 2(d)). Similar non-blocking passivating properties were obtained with various metals such as zinc/cobalt, zinc/ nickel, zinc/aluminum, aluminum, copper and tin. It therefore appears clearly that the use of sodium salicylate as an electrolytic salt is very general, allowing full passivation of the metallic substrates by the formation of non-blocking layers. Oxidation of the adsorbed sali-
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Fig. 2. Voltammetric curve in a 1 M sodium salicylate solution on: (a) platinum (10 mV s − 1); (b) zinc (10 mV s − 1); (c) zinc (200 mV s − 1); (d) iron (10 mV s − 1).
cylate remains possible on any substrate and, surprisingly, as will be shown below, pyrrole can be electropolymerized on these modified electrodes at very low potentials.
3.2. Voltammetric beha6ior of a zinc substrate in a 1 M sodium salicylate and 0.5 M pyrrole solution The voltammetric behavior of a zinc substrate in a 1 M sodium salicylate and 0.5 M pyrrole solution is characterized by three oxidation peaks at −0.9, − 0.65 and + 0.5, and by an oxidation wave at +0.6 V (Fig. 3(a)). The first two peaks (− 0.9 and − 0.65 V) are similar to those observed without pyrrole and are attributed to zinc dissolution followed by the formation of a passive layer on the electrode. Moreover, these peak currents decrease strongly when pyrrole is in the solution, a result that indicates that pyrrole is involved in the passivation process. The new peak at 0.5 V suggests the oxidation of an adsorbed species, which obviously results from the presence of pyrrole in the solution. The sharp oxidation wave at 0.6 V can be
attributed to pyrrole oxidation since a black, uniform film is generated on the zinc substrate. Upon reversal of the potential sweep an irreversible cathodic peak at − 0.9 V attributed to partial reduction of the polypyrrole film is observed. The second potential sweep shows a stronger passivation of the electrode as compared to the first cycle or to the behavior of the substrate when no polypyrrole film was deposited (Fig. 3(a)). No signals characteristic of zinc dissolution are detected, and polypyrrole coverage of the surface appears to inhibit zinc oxidation. Surprisingly, no or very little oxidation of the polypyrrole film occurs; this confirms that polypyrrole reduction at − 0.9 V is irreversible. The pyrrole oxidation signal at + 0.6 V is still present but is markedly decreased probably due to the presence of a mixed insulating layer consisting of PPy and zinc salicylate. This phenomenon can be avoided if the first reversed potential sweep is stopped at − 0.5 V before PPy reduction (Fig. 3(b)). In this case the second positive cycle again exhibits a strong pyrrole oxidation signal at 0.6 V and polypyrrole growth can be achieved by multiple potential sweeps
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between −0.5 and 0.6 V, allowing a high control of the PPy film thickness.
3.3. Gal6anostatic beha6ior of a zinc substrate in a 1 M sodium salicylate and 0.5 M pyrrole solution Depending on the current density, different galvanostatic curves were observed (Fig. 4). For a current density of 1 mA cm − 2 the potential of the zinc electrode remains at around −0.8 V and continuous oxidation of the substrate occurs. For current densities between 2 and 4 mA cm − 2 the electrode potential increases up to a maximum of about 0.4 V, less than the pyrrole oxidation potential, then decreases and stabilizes at a negative value around −0.2 V. The zinc substrate is oxidized and no film growth is observed. At 4 mA cm − 2 the chronopotentiometric curve exhibits two plateaus: the first at around 0.6 V for 50 s and
Fig. 3. Voltammetric behavior (2 cycles) of a zinc substrate in a 1 M sodium salicylate, 0.5 M pyrrole solution: (a) final potential − 1 V; (b) final potential −0.5 V.
without film formation, the second at 0.75 V with polypyrrole deposition. For current densities greater than 5 mA cm − 2 the chronopotentiometric curve of the zinc electrode is similar to that observed on platinum. The potential stabilizes at 0.75 V and PPy deposition occurs immediately. It is worth noting that, in the latter case, deposition starts without an induction period in contrast to other procedures which have been reported in the literature for iron or mild steel [20,21,24,25,29].
3.3.1. Current efficiency and growth rate When high current densities (\ 20 mA cm − 2) are used or when the electrolysis time is longer than 4 min, crystals of salicylic acid precipitate on the film surface. This effect is attributed to massive production of hydronium ion induced in the vicinity of the electrode by the polymerization and can be eliminated by stirring the solution vigorously. The current efficiencies of the electropolymerization were measured for samples prepared at different current densities. The theoretical mass of the polypyrrole coating was calculated using the following electrochemical equation (Scheme 1) where y is the doping level of the polypyrrole film and A − the counter ion (salicylate) that maintains electroneutrality in the material. At high current densities, current efficiencies are found to be greater than 100% because of salicylic acid precipitation, which increases the mass of the deposit. When the solution was stirred, PPy yields were 103 to 105% and were found to be independent of the current density. The process appears to be very selective and no pyrrolic by-products are produced during the electrolysis. Film deposition occurs without pollution of the electrolytic solution, which confirms that soluble oligomers are not formed. The same electrolytic solution can be used for several days without any degradation as long as the pyrrole concentration is maintained at 0.5 M through continuous addition of monomer, a result which is particularly important for industrial application, which uses continuous lines. The PPy growth rate was measured for films prepared at 5 mA cm − 2 (without stirring). The results given by gravimetric and SEM measurements are in good agreement, as shown in Fig. 5. The observed thickness variations are fairly linear with the quantity of electricity and for a current density of 5 mA cm − 2 the film growth rate is 1 mm min − 1. The PPy film growth rate is also linear with increasing current density up to the point where salicylic acid precipitates on the electrode. With vigorous agitation of the solution using an electrode rotating at 4000 rpm it was possible to deposit strongly adherent PPy films up to 800 mA cm − 2, a result which is quite compatible with an industrial process. For example, a PPy film 2.5 mm thick can be obtained in less than 3 s at 300 mA cm − 2. This result is in marked contrast with those reported in wet
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Fig. 4. Galvanostatic behavior for various current densities (mA cm − 2) of a zinc substrate in a 1 M sodium salicylate, 0.5 M pyrrole solution.
Scheme 1.
Fig. 5. PPy growth rate for films prepared at 5 mA cm − 2. () gravimetric measurements ( ) SEM measurements.
acetonitrile where a reaction limited current density of a few mA cm − 2 was observed [48] and is probably due to solvent effects even though a specific effect of salicylate cannot be excluded. Indeed, it has been shown recently, using molecular modeling, that solvation has a dramatic impact on the relative and absolute rates of oligopyrrole radical-cation coupling reactions [49,50].
Another important aspect of the process is its independence of the surface roughness and the composition of the substrate. Several zinc electrodes (provided by Sollac) with various surface states were tested. In all cases, film deposition was achieved under the same conditions as with polished samples. Furthermore, the process was successful with various metallic substrates
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such as iron, mild steel, zinc/cobalt, zinc/nickel, zinc/ aluminum, aluminum, copper and tin, and the same electrochemical behavior as with zinc substrate was observed.
3.3.2. Film characterization The black films are uniform and cover the substrate completely. They have a shiny and a plastic appearance. At very high magnification the morphology of the films is cauliflower like. There are no marked morphological differences between films of different thickness or prepared on various metallic substrates. Furthermore, in all cases, the SEM confirms that the deposit is homogeneous. Films generated using current densities of 5 mA cm − 2 were analyzed using IR spectroscopy (oxidized state). The films were scraped off the electrode, ground to a fine powder and analyzed as KBr pellets. The IR spectrum of a film deposited on a zinc electrode shows the same features as one deposited on platinum. It shows the principal features of a polypyrrole film in an oxidized state [51 – 54]. Strong IR absorption in the 2000 to 4000 cm − 1 region characteristic of a conductive material is seen and disappears upon reduction of the film. Furthermore, a 1700 cm − 1 band is also observed; this can be attributed to carbonyl groups often seen for polypyrrole synthesized in aqueous media [55,56]. No spectroscopic differences between films prepared at low current densities and those prepared at high current densities were formed which suggests that the PPy films are not over-oxidized. Raman spectroscopy was also used in order to confirm the polypyrrole structure of the deposited films. The Raman spectra of films generated on mild steel, zinc or a platinum electrode are identical and show the principal bands of polypyrrole reported in the literature [57,58]. Its very simple appearance with very few and strong bands, despite the large number of vibrational degrees of freedom, indicates that an extended p-conjugated chain has been generated. These two spectroscopic techniques clearly indicate that the films deposited on zinc are polypyrrole.
Fig. 6. Variation of the applied force versus displacement (three point flexion test) for a 1 mm PPy film deposited on zinc.
XPS spectroscopy was used to evaluate the doping level of the as-grown film. The N1s, signals of the polypyrrole surface facing the solution can be decomposed into four components centered at 398.3, 399.9, 401.2 and 402.9 eV with relative intensities of 0.13, 1, 0.26 and 0.06, respectively. The 399.9 eV signal is attributed to normal pyrrole nitrogen, which does not carry a positive charge [59,60]. The lowest energy signal (398.3 eV) has been observed frequently in other polypyrrole XPS studies, and is attributed to deprotonated imine-type nitrogen in the polymer [51,55,61,62]. The two highest binding energy signals characterize nitrogen atoms with positive charges and are correlated with the doping level of the polymer. The intensity ratio of these components and the whole nitrogen signal is 22% and represents the doping level in the as-grown polymer [63,64]. The C1s signal is highly asymmetric and less easy to analyze. In agreement with literature results [65] it was decomposed into four components at 285, 286.5, 288.1 and 289.3 eV with the following attributions: 285 eV, b carbons of the pyrrolic chains and aromatic carbons of the salicylate anions; 286.5 eV, a carbons of the pyrrolic chains; 288.1 eV, carboxylic carbons due mainly to carbon atoms of the salicylate anions linked to the OH groups; 289.3 eV, carboxylic acid type carbons. They characterize the presence of salicylate anions as doping ions in the as-grown material, and confirm the doping yield of about 22%. Two principal components at 532 and 533.5 eV are seen in the O1s signal. They can be attributed to CO and OCO− [59] and represent the contribution of the salicylate anions. The intensity ratio between the O1s signal and three times the N1s signal is 28%, and this agrees fairly well with the doping level evaluation based on the analysis of the N1s signal and confirms the O1s signal attribution. The factor of three originates from the number of oxygen atoms in one salicylate counterion. Finally, no Zn2p signal can be detected in the XPS analysis of the film surface. This clearly indicates that the film covers the zinc substrate fully and that Zn2 + does not migrate across the deposited film.
3.3.3. Mechanical properties The adherence of polypyrrole was tested by two procedures: the AFNOR NFT 30038 Sellotape test before and after criss-crossing the covered metallic surface and a three point flexion test [66,67]. Four series of five samples each were run. They differed in the polypyrrole thicknesses, which were 1, 2, 3 and 4 mm. All samples were prepared galvanostatically using 5 mA cm − 2 current density. The thicknesses of the samples were controlled by adjusting the electrolysis time. When the film is in its oxidized state the adherence is 100% according to the AFNOR NFT 30038 Sellotape
J. Petitjean et al. / Journal of Electroanalytical Chemistry 478 (1999) 92–100 Table 1 Maximum forces applied just before rupture for samples of various thicknesses Thickness/mm Average Fmax/N
1 125
2 135
3 131
4 129
test. Fig. 6 shows the variation of the applied force versus displacement (three point flexion test) and clearly indicates that the fracture is abrupt and occurs in the elastic domain of the sample. SEM examination of the delaminating zone shows that the beginning and the propagation of the fracture occur at the interface between the metal and the film. The process can be described as an adhesive fracture and the measured force represents the adherence between the metal and the film. Table 1 shows the maximum forces applied just before rupture for the 20 samples. The results are not fully reproducible but an average force between 120 and 140 N (sample area =5 cm2) can be estimated. Interestingly, this force does not depend on the film thickness, which confirms that the rupture occurs at the metal film interface. When the film is fully reduced electrochemically or by means of Na2SO3 or NaOH treatment, the adherence falls to 0% and the film can even be peeled off the metal. This seems to indicate that the adherence results from ionic interactions between the oxidized PPy and the surface, and falls to zero when the PPy is reduced into its neutral state. Consequently, to improve the adherence of PPy films under cathodic polarization, it will be necessary to graft onto pyrrolic monomers polar functional groups capable of adhering strongly with the metallic surface whatever the applied potential.
4. Conclusions From several salts investigated to perform the electropolymerization of pyrrole on oxidizable metals in aqueous media, sodium salicylate was the only one to allow the formation of adherent and homogeneous PPy films. This unusual property results from the fact that sodium salicylate yields with the metal ion arising from electrode oxidation a thin protective layer, which slows dissolution of the metal considerably but does not prevent oxidation of the monomer. In contrast to previous systems, film formation occurs in a one-step process, and does not need any preliminary treatment of the metal. The current efficiencies of the polymerization reaction are more than 100%, and the deposition of PPy coatings on oxidizable metals is ultra fast (1mm s − 1 at 500 mA cm − 2), which corresponds to industrial requirements. The coating materials obtained from this new process are promising and might lead to industrial applications in the protection of the oxidizable substrates against corrosion [68].
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[39] S. Aeiyach, A. Kone´, M. Dieng, J.J. Aaron, P.C. Lacaze, J. Chem. Soc. Chem. Commun. (1991) 822. [40] U. Barsch, F. Beck, Synth. Met. 55 (1993) 1638. [41] P. Lang, Z. Mekhalif, F. Garnier, R. Re´gis, in: P.C. Lacaze (Ed.), Organic Coatings, vol. 354, AIP Press, Woodbury, New York, 1996, p. 176. [42] M. Sato, S. Tanaka, K. Kaeriyama, Synth. Met. 14 (1986) 279. [43] S. Aeiyach, E.A. Bazzaoui, P.C. Lacaze, J. Electroanal. Chem. 434 (1997) 153. [44] E.A. Bazzaoui, S. Aeiyach, P.C. Lacaze, Synth. Met. 83 (1996) 159. [45] T.A. Skotheim (Ed.), Handbook of Conducting Polymers, vols. 1 and 2, Marcel Dekker, New York, 1997, and references therein. [46] H.S. Nalwa (Ed.), Organic Conductive Molecules and Polymers, Vol. 2, Wiley, New York, 1996, and references therein. [47] R.L. Leroy, Corrosion 34 (1978) 98. [48] F. Beck, M. Oberst, R. Jansen, Electrochim. Acta 35 (1990) 1841. [49] J.C. Lacroix, R.-J. Valente, F. Maurel, P.C. Lacaze, Chem. Eur. J. 4 (1998) 1667. [50] J.C. Lacroix, F. Maurel, P.C. Lacaze, Synth. Met. 101 (1999) 675. [51] E.T. Kang, K.G. Noeh, Y.K. Ong, K.L. Tan, B.T.G. Tan, Synth. Met. 39 (1990) 69. [52] E.T. Kang, K.G. Noeh, Y.K. Ong, K.L. Tan, B.T.G. Tan, Polymer 32 (1991) 1354.
[53] C.A. Ferreira, S Aeiyach, M. Delamar, PC Lacaze, Surf. Interface Anal. 20 (1993) 749. [54] K. Hyodo, A.D. Macdiarmid, Synth. Met. 11 (1985) 167. [55] J. Lei, Z. Cai, C.R. Martin, Synth. Met. 46 (1992) 53. [56] S.J. Hahn, W.J. Gajda, P.O. Vogelhut, M.V. Zeller, Synth. Met. 14 (1986) 89. [57] J.P. Marsault, K. Fraoua, S. Aeiyach, J. Aubard, G. Levi, P.C. Lacaze, J. Chim. Phys. 89 (1992) 1167. [58] R. Erlandsson, I. Lundstrom, J. Phys. Colloque C3 (Suppl. 44) 6 (1983) 713. [59] J.E. Muilenberg (Ed), Handbook of X-ray Photoelectron Spectroscopy, Perkin – Elmer, Eden Prairie, 1978 [60] L.F. Warren, D.P. Anderson, J. Electrochem. Soc. 134 (1987) 103. [61] R. Erlansson, O. Inganas, I. Lundstrom, W.R. Salaneck, Synth. Met. 10 (1985) 2199. [62] T.A. Skotheim, Synth. Met. 14 (1986) 31. [63] S.A. Chen, Y.C. Tsai, S.H. Chen, Synth. Met. 28 (1989) C151. [64] W. Liang, J. Lei, C.R. Martin, Synth. Met. 52 (1992) 227. [65] J.J. Pireaux, J. Riga, P. Boulanger, P. Snauwaert, Y. Novis, M. Chtaib, C. Gregoire, F. Fally, E. Beelen, R. Caudano, J. Verbist, J. Electron. Spectrosc. 52 (1990) 423. [66] D. Benazet, A. Roche, F. Gaillard, Adhecom, 89, 360. [67] SOLLAC-LEDEPP quality procedure ABIR 001 [68] S. Aeiyach, P.C. Lacaze, M. Hedayatullah, J. Petitjean, ITODYS-SOLLAC Patent no. 9315395, 1993; extension USA– Japan, 1995.
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Ultra-fast electropolymerization of pyrrole in aqueous media on oxidizable metals in a one-step process J. Petitjean a,b, S. Aeiyach a, J.C. Lacroix a, P.C. Lacaze a,* a
Institut de Topologie et de Dynamique des Syste`mes de l’Uni6ersite´ Paris 7 -Denis Diderot, associe´ au CNRS (UPRES-A 7086), 1 rue Guy de la Brosse, 75005 Paris, France b SOLLAC-LEDEPP, Groupe Usinor, 17 A6 des Tilleuls, BP 70011, 57191 Florange Cedex, France Received 20 April 1999; received in revised form 1 October 1999; accepted 8 October 1999
Abstract A general one-step electrosynthesis process for polypyrrole (Ppy) films applicable to a large range of oxidizable metals is described. The process is based on the use of an aqueous medium containing a salicylate salt and pyrrole. The salicylate salt is quite specific: it passivates the substrate without preventing electropolymerization of pyrrole and, therefore, makes it possible to obtain strongly adherent PPy films, with controlled thickness, either by cyclic voltammetry or by electrolysis at constant current. In the latter case the process does not involve any induction period corresponding to a preliminary dissolution of the metal, the current efficiencies are close to 100%, and the deposition rate can be very high (1 mm s − 1, corresponding to current densities as high as 500 mA cm − 2). © 1999 Elsevier Science S.A. All rights reserved. Keywords: Electropolymerization; Zinc; Mild steel; Polypyrrole; Corrosion; Inhibition
1. Introduction In recent years several conducting polymers such as polyaniline (PANI), polypyrrole (PPy) and polythiophene (PT) have been shown to be promising candidates as material coatings for the protection of common metals against corrosion [1 – 44]. Among these polymers, PPy and its derivatives have attracted much attention owing to their high stability in the oxidized state, low potential for polymer formation and ease of synthesis in aqueous solution in a wide range of pH between 4 and 10 [27,29]. Generally, PPy films are easily synthesized at inert anodes such as platinum, gold, glassy carbon or stainless steel [45,46] by electrochemical oxidation of pyrrole or its derivatives in aqueous or organic media. Transferring the electropolymerization of these monomers to oxidizable metals is much more difficult. Indeed, the oxidation potentials of these metals are much more negative than that of the pyrrole and its derivatives and, consequently, dissolution of the metal will occur before electropolymeriza* Corresponding author. Fax: +33-1-4427-6814. E-mail address: [email protected] (P.C. Lacaze)
tion begins. Therefore, to form PPy films on oxidizable metals, it is necessary to find electrochemical conditions which will strongly passivate the metal without impeding electropolymerization. Several research groups have studied this problem [18–35]. Beck et al. [20,21,24,25] reported that the use of NO− 3 or oxalate ion as the electrolyte in aqueous solution allowed electropolymerization of pyrrole on Fe or mild steel and yielded adherent, smooth PPy films. However, in both cases, the adherence of the PPy films did not satisfy the usual requirements for good anticorrosion protection and, in particular, for the electrodeposition of cataphoretic paint on top of the films; these films peeled off the substrate when their reduction potential reached a value of − 0.5 V versus SCE. The adhesive strength of the PPy coatings was markedly improved when the iron or mild steel surface was pretreated with MnO− 4 ions [24,29] and electropolymerization was performed in aqueous oxalic acid solution. However, in spite of this improvement, it must be noted that electropolymerization began only after a lengthy induction period corresponding to metal dissolution. Approximately at the same time, our group found that strongly adherent PPy films could be deposited on
0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 9 9 ) 0 0 4 1 8 - 0
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iron or mild steel without an induction period when the substrate was pretreated with dilute nitric acid and pyrrole electropolymerized in an aqueous solution con− taining NO− 3 , C2O4 or p-toluenesulfonate ions [26,27]. The quality of the PPy coatings was improved by adding small amounts of substituted pyrroles (COOH and NH2 groups at the 2-position) to the pyrrole solution, followed by heat treatment of the modified PPy films. The adherence of the PPy coatings was strong enough to allow paint deposition by the cataphoretic technique. The resistance of these novel coatings to salt spray tests (PPy+cataphoretic paint) was as good as that of the usual cataphoretic coatings obtained on phosphated mild steels [28,34,35] More recently we attempted to deposit PPy coatings on zinc or zincated steel using the same technique. Unfortunately, the procedure failed completely with zinc substrates. Zinc and zincated steel are much more electropositive than iron or steel, and under these conditions zinc dissolution remains extremely high and prevents pyrrole oxidation. Therefore, in order to deposit PPy films on these metals two different strategies were adopted. The first consisted of a two-step process where, prior to pyrrole electropolymerization, the zinc surface was chemically or electrochemically pretreated with a sulfide solution and then pyrrole was electropolymerized in an aqueous solution containing oxalate ions [30,32]. The second, more direct method, required only one step in which the treatment of the Zn surface and the electropolymerization of pyrrole occur simultaneously when small amounts of sulfide ion are mixed with the oxalate solution at pH 5 [33]. However, in spite of this significant improvement, which led to very adherent PPy films on zinc, the process remains quite specific and cannot be applied to other oxidizable metals. The aim of the present work is to find a general process for PPy coating, which can be applied to any metal and, in particular, to zinc which is one of the most easily oxidized metals. We show in this work that sodium salicylate in the presence of pyrrole constitutes a solution to this problem and allows the formation of strongly adherent PPy coatings on various oxidizable metals with very high current densities and without an induction period corresponding to metal dissolution.
2. Experimental All chemicals such as sodium orthophthalate, sodium 2-mercapto benzoate, sodium oxalate and sodium salicylate were purchased from Aldrich or Janssen and used as received without further purification. Electrodeposition was performed in a single-compartment cell using stainless steel as the counter-electrode and saturated calomel as the reference electrode. The working
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electrode was a 3 cm2 plate of iron or zinc (Weber metals) with an iron or zinc content greater than 99.5%. Mild steel, electrozincated mild steel, zinc/nickel, zinc/ cobalt and zinc/aluminum alloys were provided by Sollac Usinor. The substrates were degreased with acetone under ultrasonic conditions and rinsed with ethanol. FTIR characterization of the films was performed on a Nicolet 60 SX spectrometer. Raman spectra were recorded on a Dilor XY spectrometer with 514.5 nm laser light excitation at low power (5 mW) in order to prevent photodegradation of the films. XPS measurements were made on a VG ESCALAB MK1 spectrometer with a non-monochromatic source under the following conditions: Mg–Ka beam, source power 200 W, input energy 20 eV, area analyzed 0.4 cm2. The binding energy was referenced using the C1s signal at 285 eV and relative atomic concentrations were obtained from the areas of the corresponding signals by using, for C1s, O1s, N1s and Zn2p, sensitivity coefficients determined by analysis of model compounds of known stoichiometry and equal to 1, 3.2, 1.6 and 20, respectively. The PPy film thickness was estimated either by mass measurement or by scanning electron microscopy (SEM). In the first case, the weight of the PPy deposit was determined and converted into film thickness using a mean density of 1.5 [24]. Direct thickness measurements by SEM were made on micrograph cuttings.
3. Results and discussion When a zinc electrode is polarized at a potential slightly higher than −1 V in a usual electrolyte such as K2SO4 or KCl, immediate, strong dissolution of the metal occurs, and there is no possibility of carrying out the electrochemical oxidation of any compound whose oxidation potential is above this value. As stated previously, using an electrolyte with an anion capable of giving an insoluble salt with Zn2 + is the best way to passivate the zinc surface and to manage a potential window in which the oxidation of pyrrole can be performed. Accordingly, various salts known to form insoluble salts with Zn2 + were tested. Anionic ligands containing oxygen, sulfur or nitrogen atoms were preferred since they give stable complexes with iron and zinc [47]. Depending on the choice of the electrolytic salt, three types of voltammetric behavior were observed corresponding to various passivation phenomena. In the case of strongly Zn2 + -chelating salts, capable of forming a pseudo-five atom ring with Zn2 + , sharp passivation of the zinc surface was observed. A typical voltammetric curve obtained with sodium oxalate (Fig. 1(a)) exhibits a passivation peak between − 1 and 0 V and a wide potential range from 0 up to more than 2 V
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where the current density is very low. During the back-sweep the current density remains very small, indicating that an insulating insoluble zinc oxalate layer has precipitated on the surface and prevents zinc and pyrrole oxidation (Fig. 1(a)). Similar behavior is observed with sodium 2-mercapto benzoate, sodium 2amino-benzoate, sodium 2-mercapto-nicotinate and sodium 2-hydroxy-nicotinate. The opposite situation is found with salts with weak chelating properties, as is shown by sodium orthophthalate (Fig. 1(b)). In this case, the current density rises steadily from −1 to 2 V and decreases reversibly with the potential during the back-sweep. Clearly, this behavior, characteristic of progressive zinc dissolution without any marked passivation, can be attributed to the fact that zinc-orthophthalate is much too soluble in water to precipitate on the surface and to inhibit zinc oxidation. Consequently, it is obvious that this behavior is just as incompatible with pyrrole oxidation as the previous one is with oxalate. Therefore, a compromise must be found between these two extremes. An electrolyte which would form a non-blocking passivating layer with Zn2 + and would be able to inhibit the dissolution of the metal without preventing the oxidation of electroactive species such as pyrrole should be suitable. Such a compromise was found with sodium salicylate whose voltammetric curve (Fig. 1(c)) exhibits behavior intermediate between those of sodium oxalate and orthophthalate.
3.1. Voltammetric beha6ior of a zinc substrate in a 1 M sodium salicylate solution Fig. 2 shows the voltammetric curve recorded when the potential of a platinum (Fig. 2(a)) or a zinc substrate (Fig. 2(b)) is swept at 10 mV s − 1 in a 1 M
Fig. 1. Voltammetric curve at 10 mV s − 1 on a 3 cm2 zinc electrode in: (a) 0.1 M sodium oxalate; (b) 0.1 M sodium orthophthalate; (c) 0.1 M sodium salicylate.
sodium salicylate solution. On platinum, an irreversible peak is observed at 1 V due to the oxidation of salicylate ions. In this case, a thin yellowish film appears on the electrode after polarization in the region of 1 V and colored soluble products diffuse into the electrolytic medium. A further potential sweep indicates that the deposited film is non-conductive, since the salicylate oxidation signal decreases slightly with the number of potential sweeps. Most of the reaction products appear to be soluble in the electrolytic media and this is probably due to the presence of carboxylic acid groups on the oligomers generated. When a zinc substrate is used, sweeping the potential between − 1.1 and 2.8 V shows three oxidation peaks at − 0.9, − 0.65 and + 1 V and the beginning of an oxidation wave at + 2.3 V. The first two peaks can be attributed to substrate dissolution leading to Zn2 + , followed by chemical precipitation of the metallic cation with salicylate, which leads to a thin insoluble layer, corresponding to complexation of the Zn2 + cation salicylate anions. This insoluble layer has passivating properties towards the zinc substrate; the peak at 1 V and the wave at 2.3 V do not involve zinc oxidation but that of salicylate and water, respectively, similarly to what is observed on platinum under the same conditions. Upon reversal of the potential sweep, two new oxidation peaks appear at + 0.8 and −0.95 V attributed to salicylate and zinc oxidation, respectively. These peaks disappear when the sweep rate is faster than 200 mV s − 1. In this latter case (Fig. 2(c)) the observed current stays at a value close to zero during the whole backsweep, indicating that reactivation of the zinc electrode is slow. The second potential sweep is also highly dependent on the sweep rate. At 200 mV s − 1, the passivation state is maintained during the second potential sweep and only one signal at +1 V is detected (Fig. 2(b)). On the contrary, when the sweep rate is 10 mV s − 1, the peaks at − 0.9 and − 0.7 V reappear, indicating that the initial passivating layer has been progressively wiped off the substrate, allowing the zinc to dissolve again (Fig. 2(c)). All these results show that the surface modifications induced by the first potential sweep on the zinc substrate are controlled kinetically and, therefore, it differs dramatically from iron or mild steel electrodes, for which the surface modifications induced by the first potential sweep remain quite stable when the sweep rate is kept at 10 mV s − 1 (Fig. 2(d)). Similar non-blocking passivating properties were obtained with various metals such as zinc/cobalt, zinc/ nickel, zinc/aluminum, aluminum, copper and tin. It therefore appears clearly that the use of sodium salicylate as an electrolytic salt is very general, allowing full passivation of the metallic substrates by the formation of non-blocking layers. Oxidation of the adsorbed sali-
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Fig. 2. Voltammetric curve in a 1 M sodium salicylate solution on: (a) platinum (10 mV s − 1); (b) zinc (10 mV s − 1); (c) zinc (200 mV s − 1); (d) iron (10 mV s − 1).
cylate remains possible on any substrate and, surprisingly, as will be shown below, pyrrole can be electropolymerized on these modified electrodes at very low potentials.
3.2. Voltammetric beha6ior of a zinc substrate in a 1 M sodium salicylate and 0.5 M pyrrole solution The voltammetric behavior of a zinc substrate in a 1 M sodium salicylate and 0.5 M pyrrole solution is characterized by three oxidation peaks at −0.9, − 0.65 and + 0.5, and by an oxidation wave at +0.6 V (Fig. 3(a)). The first two peaks (− 0.9 and − 0.65 V) are similar to those observed without pyrrole and are attributed to zinc dissolution followed by the formation of a passive layer on the electrode. Moreover, these peak currents decrease strongly when pyrrole is in the solution, a result that indicates that pyrrole is involved in the passivation process. The new peak at 0.5 V suggests the oxidation of an adsorbed species, which obviously results from the presence of pyrrole in the solution. The sharp oxidation wave at 0.6 V can be
attributed to pyrrole oxidation since a black, uniform film is generated on the zinc substrate. Upon reversal of the potential sweep an irreversible cathodic peak at − 0.9 V attributed to partial reduction of the polypyrrole film is observed. The second potential sweep shows a stronger passivation of the electrode as compared to the first cycle or to the behavior of the substrate when no polypyrrole film was deposited (Fig. 3(a)). No signals characteristic of zinc dissolution are detected, and polypyrrole coverage of the surface appears to inhibit zinc oxidation. Surprisingly, no or very little oxidation of the polypyrrole film occurs; this confirms that polypyrrole reduction at − 0.9 V is irreversible. The pyrrole oxidation signal at + 0.6 V is still present but is markedly decreased probably due to the presence of a mixed insulating layer consisting of PPy and zinc salicylate. This phenomenon can be avoided if the first reversed potential sweep is stopped at − 0.5 V before PPy reduction (Fig. 3(b)). In this case the second positive cycle again exhibits a strong pyrrole oxidation signal at 0.6 V and polypyrrole growth can be achieved by multiple potential sweeps
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between −0.5 and 0.6 V, allowing a high control of the PPy film thickness.
3.3. Gal6anostatic beha6ior of a zinc substrate in a 1 M sodium salicylate and 0.5 M pyrrole solution Depending on the current density, different galvanostatic curves were observed (Fig. 4). For a current density of 1 mA cm − 2 the potential of the zinc electrode remains at around −0.8 V and continuous oxidation of the substrate occurs. For current densities between 2 and 4 mA cm − 2 the electrode potential increases up to a maximum of about 0.4 V, less than the pyrrole oxidation potential, then decreases and stabilizes at a negative value around −0.2 V. The zinc substrate is oxidized and no film growth is observed. At 4 mA cm − 2 the chronopotentiometric curve exhibits two plateaus: the first at around 0.6 V for 50 s and
Fig. 3. Voltammetric behavior (2 cycles) of a zinc substrate in a 1 M sodium salicylate, 0.5 M pyrrole solution: (a) final potential − 1 V; (b) final potential −0.5 V.
without film formation, the second at 0.75 V with polypyrrole deposition. For current densities greater than 5 mA cm − 2 the chronopotentiometric curve of the zinc electrode is similar to that observed on platinum. The potential stabilizes at 0.75 V and PPy deposition occurs immediately. It is worth noting that, in the latter case, deposition starts without an induction period in contrast to other procedures which have been reported in the literature for iron or mild steel [20,21,24,25,29].
3.3.1. Current efficiency and growth rate When high current densities (\ 20 mA cm − 2) are used or when the electrolysis time is longer than 4 min, crystals of salicylic acid precipitate on the film surface. This effect is attributed to massive production of hydronium ion induced in the vicinity of the electrode by the polymerization and can be eliminated by stirring the solution vigorously. The current efficiencies of the electropolymerization were measured for samples prepared at different current densities. The theoretical mass of the polypyrrole coating was calculated using the following electrochemical equation (Scheme 1) where y is the doping level of the polypyrrole film and A − the counter ion (salicylate) that maintains electroneutrality in the material. At high current densities, current efficiencies are found to be greater than 100% because of salicylic acid precipitation, which increases the mass of the deposit. When the solution was stirred, PPy yields were 103 to 105% and were found to be independent of the current density. The process appears to be very selective and no pyrrolic by-products are produced during the electrolysis. Film deposition occurs without pollution of the electrolytic solution, which confirms that soluble oligomers are not formed. The same electrolytic solution can be used for several days without any degradation as long as the pyrrole concentration is maintained at 0.5 M through continuous addition of monomer, a result which is particularly important for industrial application, which uses continuous lines. The PPy growth rate was measured for films prepared at 5 mA cm − 2 (without stirring). The results given by gravimetric and SEM measurements are in good agreement, as shown in Fig. 5. The observed thickness variations are fairly linear with the quantity of electricity and for a current density of 5 mA cm − 2 the film growth rate is 1 mm min − 1. The PPy film growth rate is also linear with increasing current density up to the point where salicylic acid precipitates on the electrode. With vigorous agitation of the solution using an electrode rotating at 4000 rpm it was possible to deposit strongly adherent PPy films up to 800 mA cm − 2, a result which is quite compatible with an industrial process. For example, a PPy film 2.5 mm thick can be obtained in less than 3 s at 300 mA cm − 2. This result is in marked contrast with those reported in wet
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97
Fig. 4. Galvanostatic behavior for various current densities (mA cm − 2) of a zinc substrate in a 1 M sodium salicylate, 0.5 M pyrrole solution.
Scheme 1.
Fig. 5. PPy growth rate for films prepared at 5 mA cm − 2. () gravimetric measurements ( ) SEM measurements.
acetonitrile where a reaction limited current density of a few mA cm − 2 was observed [48] and is probably due to solvent effects even though a specific effect of salicylate cannot be excluded. Indeed, it has been shown recently, using molecular modeling, that solvation has a dramatic impact on the relative and absolute rates of oligopyrrole radical-cation coupling reactions [49,50].
Another important aspect of the process is its independence of the surface roughness and the composition of the substrate. Several zinc electrodes (provided by Sollac) with various surface states were tested. In all cases, film deposition was achieved under the same conditions as with polished samples. Furthermore, the process was successful with various metallic substrates
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such as iron, mild steel, zinc/cobalt, zinc/nickel, zinc/ aluminum, aluminum, copper and tin, and the same electrochemical behavior as with zinc substrate was observed.
3.3.2. Film characterization The black films are uniform and cover the substrate completely. They have a shiny and a plastic appearance. At very high magnification the morphology of the films is cauliflower like. There are no marked morphological differences between films of different thickness or prepared on various metallic substrates. Furthermore, in all cases, the SEM confirms that the deposit is homogeneous. Films generated using current densities of 5 mA cm − 2 were analyzed using IR spectroscopy (oxidized state). The films were scraped off the electrode, ground to a fine powder and analyzed as KBr pellets. The IR spectrum of a film deposited on a zinc electrode shows the same features as one deposited on platinum. It shows the principal features of a polypyrrole film in an oxidized state [51 – 54]. Strong IR absorption in the 2000 to 4000 cm − 1 region characteristic of a conductive material is seen and disappears upon reduction of the film. Furthermore, a 1700 cm − 1 band is also observed; this can be attributed to carbonyl groups often seen for polypyrrole synthesized in aqueous media [55,56]. No spectroscopic differences between films prepared at low current densities and those prepared at high current densities were formed which suggests that the PPy films are not over-oxidized. Raman spectroscopy was also used in order to confirm the polypyrrole structure of the deposited films. The Raman spectra of films generated on mild steel, zinc or a platinum electrode are identical and show the principal bands of polypyrrole reported in the literature [57,58]. Its very simple appearance with very few and strong bands, despite the large number of vibrational degrees of freedom, indicates that an extended p-conjugated chain has been generated. These two spectroscopic techniques clearly indicate that the films deposited on zinc are polypyrrole.
Fig. 6. Variation of the applied force versus displacement (three point flexion test) for a 1 mm PPy film deposited on zinc.
XPS spectroscopy was used to evaluate the doping level of the as-grown film. The N1s, signals of the polypyrrole surface facing the solution can be decomposed into four components centered at 398.3, 399.9, 401.2 and 402.9 eV with relative intensities of 0.13, 1, 0.26 and 0.06, respectively. The 399.9 eV signal is attributed to normal pyrrole nitrogen, which does not carry a positive charge [59,60]. The lowest energy signal (398.3 eV) has been observed frequently in other polypyrrole XPS studies, and is attributed to deprotonated imine-type nitrogen in the polymer [51,55,61,62]. The two highest binding energy signals characterize nitrogen atoms with positive charges and are correlated with the doping level of the polymer. The intensity ratio of these components and the whole nitrogen signal is 22% and represents the doping level in the as-grown polymer [63,64]. The C1s signal is highly asymmetric and less easy to analyze. In agreement with literature results [65] it was decomposed into four components at 285, 286.5, 288.1 and 289.3 eV with the following attributions: 285 eV, b carbons of the pyrrolic chains and aromatic carbons of the salicylate anions; 286.5 eV, a carbons of the pyrrolic chains; 288.1 eV, carboxylic carbons due mainly to carbon atoms of the salicylate anions linked to the OH groups; 289.3 eV, carboxylic acid type carbons. They characterize the presence of salicylate anions as doping ions in the as-grown material, and confirm the doping yield of about 22%. Two principal components at 532 and 533.5 eV are seen in the O1s signal. They can be attributed to CO and OCO− [59] and represent the contribution of the salicylate anions. The intensity ratio between the O1s signal and three times the N1s signal is 28%, and this agrees fairly well with the doping level evaluation based on the analysis of the N1s signal and confirms the O1s signal attribution. The factor of three originates from the number of oxygen atoms in one salicylate counterion. Finally, no Zn2p signal can be detected in the XPS analysis of the film surface. This clearly indicates that the film covers the zinc substrate fully and that Zn2 + does not migrate across the deposited film.
3.3.3. Mechanical properties The adherence of polypyrrole was tested by two procedures: the AFNOR NFT 30038 Sellotape test before and after criss-crossing the covered metallic surface and a three point flexion test [66,67]. Four series of five samples each were run. They differed in the polypyrrole thicknesses, which were 1, 2, 3 and 4 mm. All samples were prepared galvanostatically using 5 mA cm − 2 current density. The thicknesses of the samples were controlled by adjusting the electrolysis time. When the film is in its oxidized state the adherence is 100% according to the AFNOR NFT 30038 Sellotape
J. Petitjean et al. / Journal of Electroanalytical Chemistry 478 (1999) 92–100 Table 1 Maximum forces applied just before rupture for samples of various thicknesses Thickness/mm Average Fmax/N
1 125
2 135
3 131
4 129
test. Fig. 6 shows the variation of the applied force versus displacement (three point flexion test) and clearly indicates that the fracture is abrupt and occurs in the elastic domain of the sample. SEM examination of the delaminating zone shows that the beginning and the propagation of the fracture occur at the interface between the metal and the film. The process can be described as an adhesive fracture and the measured force represents the adherence between the metal and the film. Table 1 shows the maximum forces applied just before rupture for the 20 samples. The results are not fully reproducible but an average force between 120 and 140 N (sample area =5 cm2) can be estimated. Interestingly, this force does not depend on the film thickness, which confirms that the rupture occurs at the metal film interface. When the film is fully reduced electrochemically or by means of Na2SO3 or NaOH treatment, the adherence falls to 0% and the film can even be peeled off the metal. This seems to indicate that the adherence results from ionic interactions between the oxidized PPy and the surface, and falls to zero when the PPy is reduced into its neutral state. Consequently, to improve the adherence of PPy films under cathodic polarization, it will be necessary to graft onto pyrrolic monomers polar functional groups capable of adhering strongly with the metallic surface whatever the applied potential.
4. Conclusions From several salts investigated to perform the electropolymerization of pyrrole on oxidizable metals in aqueous media, sodium salicylate was the only one to allow the formation of adherent and homogeneous PPy films. This unusual property results from the fact that sodium salicylate yields with the metal ion arising from electrode oxidation a thin protective layer, which slows dissolution of the metal considerably but does not prevent oxidation of the monomer. In contrast to previous systems, film formation occurs in a one-step process, and does not need any preliminary treatment of the metal. The current efficiencies of the polymerization reaction are more than 100%, and the deposition of PPy coatings on oxidizable metals is ultra fast (1mm s − 1 at 500 mA cm − 2), which corresponds to industrial requirements. The coating materials obtained from this new process are promising and might lead to industrial applications in the protection of the oxidizable substrates against corrosion [68].
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