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Keywords:

  • corrosion;
  • polyaniline;
  • polypyrrole;
  • stainless steel, sulphuric acid

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental
  5. 3 Result and discussion
  6. 4 Conclusions
  7. References

The electrodeposition of polypyrrole and polyaniline has been performed in aqueous solutions of sodium saccharinate and sulphuric acid, respectively. The coatings were obtained by voltamperometric and galvanostatic methods. The corrosion behaviour of polypyrrole and polyaniline coated stainless steel (SS) in sulphuric acid medium was investigated by linear polarisation, open circuit potential and electrochemical impedance spectroscopy. The polyaniline coatings doped with sulphate anions (SO42−) showed more resistance to protect SS to corrosion than the polypyrrole coatings doped with saccharinate anions (C7H4NO3S).

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental
  5. 3 Result and discussion
  6. 4 Conclusions
  7. References

Stainless steel (SS) revolutionised most of the modern industries like medicine, transportation, commercial building, hygienic and pharmacy, because its good strength, durability, high glass transition temperature and nonmagnetic properties, and easy manufacturing a desired shape at low cost [1]. Several studies were performed hoping to replace the graphite bipolar plates for SS in proton exchange membrane fuel cell (PEMFC) systems at a price about half [2, 3]. However, the dissolution of SS in fuel cell environment composed by sulphuric acid is the major drawback of this material. Effectively, the fuel cell membrane is influenced by the metallic ions during the SS oxidation in H2SO4 solution, which decreased its ionic conductivity and consequently the PEMFC efficiency.

Several researchers have concentrated their efforts in the investigation of the corrosion mechanism of SS and its protection by organic compounds [4-9]. Some reports show that the heterocyclic compounds like rhodanine azosulpha and substituted pyrazolones inhibit the corrosion of SS in hydrochloric acid [4] and sulphuric acid [5], respectively. This author stated that the inhibition action of those compounds occurs via adsorption on the steel surface through the active centres contained in their structure. The mechanism of inhibition was interpreted on the basis of the inductive and mesomeric effects of the substituent.

Hür et al. [10] have investigated polyaniline, poly(2-toluidine) (PT), and poly(aniline-co-2-toluidine) (co-PT) coatings in the corrosion protection of SS in HCl solution. These coatings were successfully achieved on SS by electrochemical synthesis from acetonitrile solution in the presence of tetrabuthylammonium perchlorate as supporting electrolyte and a few amount of perchloric acid. They found that PANi coatings are able to provide much better protection than PT and co-PT conducting polymers.

This work investigated the electrochemical behaviour of coated polypyrrole (PPy) or polyaniline (PANI) and uncoated SS in H2SO4 aqueous solutions by cyclic voltammetry, Tafel polarisation and electrochemical impedance spectroscopy (EIS) methods.

2 Experimental

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental
  5. 3 Result and discussion
  6. 4 Conclusions
  7. References

Sodium saccharinate salt (C7H4NNaO3S.2H2O) and pyrrole (98%) were purchased from Sigma, aniline (99%) from Aldrich and sulphuric acid (H2SO4, 96%) from Riedel–deHaën. The products were used as received and water was distilled twice before use.

The electrochemical experiments were performed in a one-compartment cell with three electrodes connected to Autolab model PGSTAT20 potentiostat/galvanostat with pilot integration controlled by GPES 4.4 software or to Voltalab 80 Model PGZ 402 run by Voltamaster 4 software. The SS (Cr 20%, Ni 9%) rod (1 cm diameter) and rectangular sheet constitute the working electrode. The electrodes were mechanically polished with abrasive paper (1200-grade) and rinsed in water and acetone before each electrochemical experiment. A stainless-steel plate was used as auxiliary electrode. All the potentials were measured versus an Ag/AgCl (0.1 M KCl) reference electrode.

EIS experiments were carried out by means of Voltalab 80 Model PGZ 402. The Nyquist plots were recorded at instantaneous open circuit potentials.

Scanning electron microscopy (SEM) micrographs and electron dispersion spectroscopy (EDS) analysis were obtained on a JEOL JSM-6301F instrument. The microscope chamber was maintained at a pressure between 4 and 10 Pa. The distance between the sample and the objective lens is remained at 15 mm (working distance). The SEM filament was operated at variable current and a voltage of 15 kV using magnifications 300×, 3000× and 10,000×.

X-ray photoelectron spectroscopy (XPS) analysis was carried out with a Vacuum Generators VG Scientific Escalab 200 A spectrometer with Pisces software for data acquisition and analysis. For analysis, an achromatic Al (Kα 1486.6 eV) X-ray source operating at 15 KV (300 W) was used. Pressures in the analysis chamber ranged from 10−6 to 10−7 Pa and the analysed area is about 10 mm2 size. The spectrometer, calibrated with Ag 3d5/2 (368.27 eV) reference, was operated in CAE mode with 20 eV pass energy. The spectra was corrected for the surface charging effect by taking the C 1s peak (Eb = 285 eV) as an internal reference. Spectra analysis was performed using peak fitting with Gaussian–Lorentzian peak shape and linear type background subtraction. SEM/EDS and XPS analysis have been performed in CEMUP (Centro de Materiais da Universidade do Porto).

3 Result and discussion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental
  5. 3 Result and discussion
  6. 4 Conclusions
  7. References

3.1 Electrochemical behaviour of SS in H2SO4

The voltammograms of SS polarised in 0.1 and 0.5 M H2SO4 are presented in Fig. 1. The curves show three complexes anodic peaks I, II and III and a passive domain. The first peak at −0.2 V can be the result of the oxidation of the alloy elements, Fe, Ni and Cr. It is reported that amounts of iron ions were detected in the solution during the anodic peak I by atomic absorption analysis conversely to Ni or Cr ions. This does not mean that chromium and nickel were not corroded, but more probably they remain on the electrode surface as oxides and sulfides. The broad anodic peak II that begins around 0.08 V and observed for 0.5 M H2SO4 is attributed to the chromium oxidation leading to the formation of chromate ions:

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Figure 1. Electrochemical behaviour of stainless steel polarised in 0.5 M H2SO4 () and 0.1 M H2SO4 (- - -) at potential scan rate of 100 mV/s

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The peak III localised at 1.2 V is attributed to oxidation of Fe2+ to Fe3+ and Cr3+ to Cr6+ to form the iron oxyhydroxide and chromate ions:

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To confirm the interpretation of the results, the surface electrode was analysed by XPS (Fig. 2). The Fe 2p3/2 spectrum is fitted with components at 707.15, 709.1, 710.6, 711.45, 713.1 and 715 eV. The main peak located at 707.15 eV corresponds to sulphur bonded Fe (II) [11] and the peak at 709 eV can be attributed to oxygen bonded Fe (II). The peaks at 710.6, 711.25 and 713.1 eV can be assigned to Fe2O3, FeOOH and Fe2(SO4)3, respectively [12, 13].

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Figure 2. XPS spectra of Fe 2p3/2, Cr 2p3/2 and Ni 2p3/2 after potentiodynamic polarisation (1st cycle) of stainless steel in 0.5 M H2SO4

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The Cr 2p3/2 signal is composed by the following peaks: at 574.1 eV corresponds to metallic Cr, at 576.9 and 575.7 eV attributed to Cr2O3 and at 578.3 eV assigned to CrO3 [11, 14].

The Ni 2p3/2 spectrum is composed of Ni metal at 852.43 eV, NiO at 853.5 eV and NiO3 at 855.7 eV [15].

The XPS analysis shows that the passivation domain of SS can be the result of the formation of iron, chromium and nickel sulphide, oxide and hydroxide. Thus, the breakdown of the passive film can be related with the sulphide oxidation, since it is knowledge [16] that dissolved oxygen and ferric iron are the principal oxidants of pyrite, and conversion of Cr2O3 in chromate ions according to the following reactions:

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The surface of SS electrode after scanning the potential during the first cycle in 0.5 M H2SO4 is also characterised by SEM. The micrograph (Fig. 3) reveals the etching of the alloy in sulphuric acid. These holes are formed at positive potential before oxygen evolution where it is energetically favourable for bisulfide to collect electrons from the interface [17].

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Figure 3. SEM micrograph of stainless steel polarised in 0.5 M H2SO4 during 1st cycle between −700 and 1400 mV at 100 mV/s scan rate

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The activity of SS is increased with sulphuric acid concentration as is deduced from Fig. 1. Indeed, the current density of the peak I and III is reduced to the half by decreasing the acid concentration from 0.5 to 0.1 M, and the peak II disappears.

3.2 Electrochemical behaviour of SS in C7H4NNaO3S

The polarisation of SS in 0.1 M and 0.5 M sodium saccharinate by scanning the potential between −700 and 1400 mV at 100 mV/s scan rate is represented in Fig. 4. The voltammogram obtained at 0.5 M sodium saccharinate shows two regions: a passive domain between −700 and 400 mV which can be the result of saccharinate inhibitor acting on the electrode surface, and an active domain above 400 mV that is related with the oxidation of the alloy elements and the formation of oxyhydroxides before oxygen evolution region (above 1.3 V).

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Figure 4. Electrochemical behaviour of stainless steel polarised in 0.5 M C7H4NNaO3S () and 0.1 M C7H4NNaO3S (- - -) at 100 mV/s scan rate

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The XPS analysis of the metal surface (Fig. 5) reveals the presence of metallic iron at 707.1 eV, iron oxide at 711.2 eV, ferric sulphate at 713.1 eV [12, 13, 18], being the peak arising at 715 eV a broad Fe (II) satellite [19]. As has just been described, the Cr 2p3/2 XPS shows the presence of metallic Cr and Cr oxides [15, 20]. On the other hand, the SEM image (Fig. 6) shows a SS surface more uniform compared to that observed in sulphuric acid medium, which is in line with the inhibition imposed by ion saccharinate.

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Figure 5. XPS spectra of Fe 2p3/2 and Cr 2p3/2 after potentiodynamic polarisation (1st cycle) of stainless steel in 0.5 M C7H4NNaO3S

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Figure 6. SEM micrograph of stainless steel polarised in 0.5 M C7H4NNaO3S during 1st cycle between −700 and 1400 V at 100 mV/s scan rate

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3.3 Potentiodynamic polymerisation of aniline (0.5 M H2SO4 + 0.1 M aniline)

The electropolymerisation of aniline on SS was performed using cyclic voltammetry with a scan rate of 100 mV/s within different potential regions by varying the final potential (Ef = 1.4, 1.2 and 1 V; Fig. 7). The electropolymerisation was achieved only for conditions where the potential reached values above 1.2 V. Indeed, the anodic peak located at 1.1 V and attributed to aniline oxidation is not detected in Fig. 7c. During the first scan a strong anodic peak at −0.25 V is observed comparable to that obtained in sulphuric acid without monomer that is attributed to the surface oxidation of the SS electrode. During the following sweeping the anodic peak disappears and the electrode remains passive, allowing the growth of PANi coating on its surface. During the first 10 scans the voltammograms reveal three anodic peaks at 0.25, 0.60 and 0.95 V related to transformation of leucoemeraldine (LE) to oxidised emeraldine (EM), oxidation of dimer being present as intermediates, and the transition of LE to pernigraniline (PE) state, respectively [21]. The potential of anodic peaks (Ep,a) is shifted towards more positive values with increasing the scan number. It was reported that a slower diffusion of anions in the electrolyte solution and in the porous film may avoid polymer oxidation resulting in displacement of the oxidation potentials [21].

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Figure 7. Potentiodynamic polymerisation of aniline on stainless steel in 0.5 M H2SO4 + 0.1 M aniline at 100 mV/s scan rate

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3.4 Potentiodynamic polymerisation of pyrrole (0.1 M C7H4NNaO3S + 0.5 M pyrrole)

The electropolymerisation of pyrrole on SS was performed using cyclic voltammetry with a scan rate of 100 mV/s within different potential regions by varying the final potential (Ef = 1.4 and 1 V; Fig. 8). The iE curve is characterised by a passivation region and an oxidation wave that takes place at 0.6 V in the first potential scan attributed to the pyrrole oxidation. During the following sweeping the oxidation potential of the monomer is shifted to lower values due to the catalytic effect of the PPy layer formed on the surface during the first sweeping. The obtained coating is homogeneous and adherent.

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Figure 8. Potentiodynamic polymerisation of polypyrrole on stainless steel in 0.1 M C7H4NNaO3S + 0.5 M pyrrole at 100 mV/s scan rate.

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3.5 Chronopotentiometric synthesis of PANi (0.5 M H2SO4 + 0.1 M aniline) and PPy (0.1 M C7H4NNaO3S + 0.5 M pyrrole)

Figure 9 shows the galvanostatic results of electrochemical synthesis of PANi and PPy at 3 mA/cm2 during 10 min.

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Figure 9. Chronopotentiometric curves of PANi (a) and PPy (b) electrodeposited on stainless steel at 3 mA/cm2

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The Et curves exhibit three regions: (1) the induction period characterised by the dissolution of the electrode and a potential increase with time until it reaches a maximum value; (2) the nucleation period corresponding to a decreased potential related with the formation of polymer nucleus on the active sites of the SS surface; (3) a final stage where the potential is stabilised and during which electropolymerisation develops. In both cases, the obtained films are homogeneous and adherent. However, the structure of PANi is completely different from PPy. The SEM images show a fibrilar texture to PANi and globular texture to PPy (Fig. 10). Increasing the current density increases the thickness and the particle size of the coating.

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Figure 10. SEM micrographs of PANi (a1, a2) and PPy (b1, b2) electrosynthesised on SS at a1, b1) 5 mA/cm2 and a2, b2) 10 mA/cm2 during 10 min

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The induction period in sodium saccharinate is instantaneous while in aqueous medium of sulphuric acid is larger about 14 s. The previous cyclic voltammetry results have already shown that the SS is more active in sulphuric acid solution than in sodium saccharinate.

3.6 Corrosion performance

Figure 11 shows the linear polarisation curves for coated SS with PPy or PANi in 0.1 M H2SO4 at two temperatures. The corrosion potential is strongly shifted toward positive values in comparison with the naked SS electrode. The corrosion potential of coated SS decreases with increasing temperature, even so its value remains at positive potentials. The effect of temperature from 25 to 60 °C in sulphuric acid medium is much more pronounced in naked SS: the corrosion potential shifts from −225 to −425 mV and the corrosion current density from 2.1 to 9 µA/cm2. In the case of the coated electrode the corrosion potential decreases from 155 to 10 mV for PPy/SS, and from 210 to 160 mV for PANi/SS. As can be seen, it is difficult to draw conclusions about the results of Tafel extrapolation regarding the effect of coatings on the corrosion current density. In fact, the current estimated by Tafel polarisation can be the result of PPy and PANi redox phenomena in the polymers structure as well as dissolution of the electrode.

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Figure 11. Linear polarisation of (a) stainless steel, (b) PPy/SS and (c) PANi/SS in 0.1 M H2SO4 at 25 and 60 °C.

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The corrosion test by immersion in 0.1 M H2SO4 during 12 days was achieved by analysing the metal content in solution by atomic absorption, Table 1. It is clear that the PANi and PPy coating inhibit the dissolution of the SS electrode; meanwhile PANi appears to be better than PPy considering the level of iron ions detected in the acidic solution. Furthermore, the electropolymerisation time has a great effect on the protection of the SS against corrosion. The thicker is the conducting polymer the better is the corrosion protection. Impedance curves of SS electrodes without and with immersion in 0.1 M H2SO4 during 12 days are gathered in Fig. 12. The Nyquist diagram of SS presents a depressed semicircle characteristic of a predominantly capacitive behaviour. The charge transfer resistance Rct is determined by the diameter of the semicircle after its extrapolation.

Table 1. Analysis of metals in 0.1 M H2SO4 solution after 12 days of immersion of electrodes
SamplesConducting polymers thickness/μmFe/(μg/ml)Cr/(μg/ml)Ni/(μg/ml)
SS0.550.17n.d.
PPy/SS1.30.440.04n.d.
PPy/SS2.4n.d.n.d.n.d.
PANi/SS0.60.16n.d.n.d.
PANi/SS1.1n.d.n.d.n.d.
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Figure 12. Impedance spectra (Nyquist plots) of stainless steel in 0.1 M H2SO4. (a) without electrode immersion and (b) with immersion of the electrode in 0.1 M H2SO4 during 12 days

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After immersing the electrode in 0.1 M H2SO4 during 12 days the EIS spectrum shows Warburg behaviour at media frequencies leading to the line of 45° formed. This confirms the diffusion of metallic ions from the electrode to the solution through the passive layer developed during the electrode immersion.

The Nyquist results for the coated SS (Fig. 13) show a capacitive loop at higher frequencies attributed to the relaxation time constant of the charge-transfer resistance followed by capacitive straight lines of nearly 90° at lower frequencies illustrating a pure capacitive behaviour. This means that the coating acts as an intact capacitor prohibiting permeation of corrosive species such as water, oxygen, and other ions towards the surface of the metal. It should be noted that the charge transfer resistance recorded in the case of conducting polymers coated SS is related with the resistance to electrode dissolution as well as the polymers resistance that depends on their oxidation state.

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Figure 13. Nyquist plots of PPy/SS (a) and PANi/SS (b) in 0.1 M H2SO4. (A) without electrode immersion and (B) with immersion of the electrode in 0.1 M H2SO4 during 12 days

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After 12 days immersion in 0.1 M H2SO4 the PANi coated electrode stays intact leading to an EIS spectrum similar to that without immersion. Meanwhile, with PPy the EIS spectrum is transformed to capacitive loop showing that the PPy coating is affected by the immersion time, and its resistance to corrosion started to decrease. These results agree well with the variation of the corrosion current densities obtained by Tafel extrapolation.

The open circuit (OCP) test (Fig. 14) shows that in acid solution during about 6 h the conducting polymers maintain the potential of SS in the passive region. The potential achieved by PANi/SS is higher than that reached by PPy/SS in agreement with the linear polarisation. Frang et al. [22] showed that the OCP response of PANi/SS in 0.2 M H2SO4 depends of the thickness of the coating.

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Figure 14. Open circuit potential of SS, PPy/SS and PANi/SS in 0.1 M H2SO4

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In literature, the most discussed aspect of metals protection by ICP is based on active protection including anodic protection, the mediator of electron transfer and the role as oxygen reduction catalysts [23-28]. The oxidising action of PPy or PANi in metal may be described by the following reactions:

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Recently, a broader concept based on anion-exchange capacity was proposed [29-32]. The released anions, from polymer reduction induced by corrosion at metal defect or by concentration gradient, may act as inhibitors. The large dopant anion, as saccharine ion, has difficulty in release the polymer network and then a cation insertion into the polymer backbone occurs to compensate the negative charge [33]. In this case, PPy reduction is described by the following reaction:

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According to Michalik and Rohwerder [34], the insertion of the cation is dominant for PPy coatings doped with molybdate, tosylate or perchlorate. This behaviour is due to the better mobility of cation in reduced PPy. Thus, although anion release takes place, its mobility is so low in comparison with the cation, and/or the amount transferred too small for the area to protect that it cannot possibly passivate the defect. In presence of small cations all conducting polymers have to become cation permselective and hence are prone to fast reduction/delamination if the defect cannot be passivated [35-37]. The recently findings on this subject suggest that an effective corrosion protection by conducting polymers will only work inside a composite where IPC is distributed as isolated microscopic clusters inside a non-conductive matrix [36-38]. However, the coatings investigated here have no larger defects. Hence according to Michalik and Rohwerder [34] then good protection can be possible, as observed in this work. The observed differences between PANI and PPy may lie in differences in ion conductivity with the investigated coatings.

According to the Tafel polarisation, atomic absorption analysis, EIS studies and open circuit potential, PANi shows a better performance to protect the SS against corrosion. The distribution of conducting polymer nucleus sites on the SS surface, the porosity of the backbone structure, the anion dopant, the ratio of active surface to active ICP as well as the amount of EM constitute an important parameter in corrosion protection as it was reported in the literature [35, 36, 38-41].

4 Conclusions

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental
  5. 3 Result and discussion
  6. 4 Conclusions
  7. References

The electrodeposition of PANi and PPy has been achieved from aqueous solutions, respectively, of sulphuric acid and sodium saccharinate. The obtained coatings under cyclic voltammetry and chronopotentiometry are homogeneous and adherent. The corrosion tests attained by linear polarisation, open circuit potential and EIS showed that the coatings impose a good resistance to SS against corrosion in sulphuric acid medium. It is concluded that PANi coating is better than PPy as anticorrosion treatment on SS. This result could be related with the differences in doping anions in PPy and PANi coatings. Therefore, conducting polymers coated SS can be an interesting alternative material as electrodes in fuel cell system.

References

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Experimental
  5. 3 Result and discussion
  6. 4 Conclusions
  7. References