Effect of solution chemistry on the electropolymerization and the electrochemical properties of polypyrrole coatings on aluminum is studied by means of electrochemical techniques, scanning electron microscopy (SEM), and x-ray photoelectron spectroscopy. It is shown that the protection effect of the coating in long-term exposures and when exposed to more concentrated NaCl solutions depends on the chemistry of electropolymerization electrolyte. The results show that nitrate anions passivate the aluminum substrate during the electropolymerization process. The resulting coating is less prone to blistering in a NaCl solution and probably due to its higher electrochemical activity presents a higher anodic protection effect. The galvanic interaction of polypyrrole coating with aluminum in a NaCl solution is directly observed using focused ion beam-assisted SEM.

INTRODUCTION

Conductive polymers including polythiophene, polyaniline, and polypyrrole are able to conduct electricity thanks to the existence of conjugated bonds.1 

These polymers have numerous electrochemical applications such as batteries, supercapacitors, electrochromic devices, fuel cells, light and emitting diodes, to name a few. Among other conductive polymers, polypyrrole has been the subject of many studies due to its ease of preparation, high electrical conductivity, and good redox properties.2  Within a certain potential range (the so-called “stability window”), its redox property is reversible. Therefore, it can be cycled repeatedly without any degradation.3 

One less common application of conductive polymers is in the area of corrosion protection. David W. DeBerry suggested the application of conductive polymers for corrosion protection for the first time in 1985.4  He deposited polyaniline coating on stainless steel and showed its ability to keep the substrate passivated in acidic solutions.

Application of polypyrrole coatings as a corrosion protective layer has been investigated for various metals including steel,5-13  magnesium,14-17  and aluminum alloys.18-34 

The corrosion protection mechanism of polypyrrole coatings can be attributed to several phenomena including (i) passivation of the substrate by the conductive polymer as an oxidizer, and (ii) shifting the oxygen reduction site from the substrate/coating to the coating/electrolyte interface.21,31,35  However, irreversible consumption of the charge stored in the conductive polymer, which is responsible for the passivation effect, poor barrier effect, permeability and anion-exchange properties, and poor adhesion to the metal substrate, especially to active metals, limit the anticorrosion application of conductive polymers.35 

These are not the only problems concerning the application of conductive polymers in corrosion protection. While literature has demonstrated the potential of polypyrrole to protect aluminum alloys from corrosion,27-28,36  the effective corrosion protection mechanism and the possible protection in chloride containing environment are still controversial. Some papers have cast doubt on the passivation mechanism in the presence of large defects.37-38  Moreover, the galvanic interaction at the conductive polymer/metal interface has been considered as a serious obstacle in achieving effective corrosion protection.39 

There are ways to improve the protection effectiveness of these coatings. Doping polypyrrole film with different anions, especially voluminous ones, results in coatings with more efficient anticorrosive performances in aggressive environments. These bigger anions will be trapped within the polymer coating, hindering the exchange with external (electrolyte-containing) aggressive chloride ions.9,25,40-41  Conductive polymers can also act as a reservoir of inhibitor anions.42-45 

Direct electrodeposition of polypyrrole on aluminum is challenging due to the simultaneous dissolution of the electrode (substrate) during the electropolymerization process, which must be conducted at high anodic potentials (well above the aluminum corrosion potential). By using electron transfer mediators such as 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt (DHBDS, commercially known as Tiron), a significant reduction in the polypyrrole deposition potential (by ca. 500 mV) can be achieved, alleviating the dissolution of the aluminum substrate during deposition.46 

Following one of the first attempts to electrochemically deposit polypyrrole on aluminum by Hülser, et al.,47  supporting electrolytes for electrodeposition of polypyrrole on aluminum (alloys) have mostly contained oxalic acid20,27,31,33-34  and other organic acids including tartaric and citric acids.26  A few examples of using inorganic acids such as nitric19  and sulfuric acids28  are also available in the literature. The supporting electrolyte must passivate the electrode surface at low enough potentials to alleviate its anodic dissolution (preferably) before the electropolymerization.48 

A limited number of researchers have used alkaline solutions containing sodium nitrate or sodium sulfate to investigate the effect of doping the conductive polymer coating by nitrate or sulfate ions and/or their passivating effect during the electropolymerization process.18,20 

The microstructural and compositional features of the substrate can affect electrodeposition and the electrochemical properties of polypyrrole coating as well.27-28,31,33  As an example, the catalytic effect of copper in AA2024 (UNS A92024(1)) on the electrodeposition of polypyrrole is elaborated in the papers by Volpi, et al.,33  and Rizzi, et al.,31  and also briefly mentioned in the research performed by Arenas, et al.27  In Rizzi, et al.,31  the effect of copper has been attributed to its migration from the alloy and then its interaction with the pyrrole ring through oxalate dopant ions (due to the application of oxalic acid in the solution). It is further shown that the presence of copper oxo-complexes in the polypyrrole film has a positive influence on its protection efficiency for the AA2024 (UNS A92024) substrate. This clearly connects the effect of compositional features to those of the solution chemistry.

Arenas, et al.,27  have considered the influence of alloying elements, including copper, on the modification of the aluminum oxide layer, which creates conductive pathways for the electrodeposition of polypyrrole.

Similarly, it has been shown that the presence of intermetallic precipitates such as Mg3Si6Al8 or Mg2Si in AA6061 (UNS A96061) has a positive effect on promoting the electropolymerization of polypyrrole film. As they facilitate the electrical connection between the metal and the electrolyte.28 

Considering these results and the controversy about the corrosion protection effect of polypyrrole coatings, to simplify and eliminate the effect of substrate composition, this research was primarily performed on a simple (almost pure) aluminum alloy.

Despite the available research on the electrodeposition of polypyrrole coating on aluminum, the electrochemical properties and the possible corrosion protection effect of this coating in highly concentrated sodium chloride solution have not been fully investigated especially by electrochemical impedance spectroscopy (EIS).

In the current research, we have investigated the electropolymerization of polypyrrole coatings on aluminum alloy (AA) 1050 (UNS A91050), using different solutions containing DHBDS, sodium dodecyl sulfate (SDS), sodium nitrate (NaNO3), and citric acid (C6H8O7). The effect of the presence of different anions on the electrodeposition and the corrosion resistance of polypyrrole coatings was studied. Corrosion resistance of the coatings was examined in sodium chloride solutions of 0.1 M and 0.6 M using EIS measurements and open circuit potential (OCP) monitoring. The possible corrosion protection effect was examined using focused ion beam-assisted scanning electron microscopy (FIB-SEM) and x-ray photoelectron spectroscopy (XPS).

EXPERIMENTAL

Substrate Preparation

Samples of AA1050 (UNS A91050), with minimum 99.5% aluminum, were used in the current research. They were cut in squares of 2 cm × 2 cm from the main sheet of 1 mm thickness. Prior to the electropolymerization process, samples were wet-abraded using 1200 grit SiC abrasive paper. Subsequently, they were ultrasonically cleaned in acetone for 10 min and then rinsed with deionized (DI) water. As the final pretreatment step, each sample was etched for 15 s in a solution of 40 g/L NaOH, at room temperature.

Electropolymerization of Polypyrrole Coatings

Based on some initial experiments and in accordance with a similar research,18  cyclic voltammetry (CV) was selected rather than galvanostatic or potentiostatic methods to deposit polypyrrole coatings on the aluminum substrates. The latter two methods result in localized deposition of the coating, while CV produces more homogenously deposited coatings.

A three-electrode 100 mL electrochemical cell was used. The applied potential ranged from 0 V to 0.9 VAg/AgCl (3 M KCl) as the reference electrode, while a graphite rod was used as the counter electrode. The scan rate was 10 mV/s, and 20 cycles were applied to form each coating.

Electrolytes used for the electropolymerization process are listed in Table. 1. The main reagents in each electrolyte were the pyrrole monomer (Py), SDS, and DHBDS.

Table 1.

Details of Electropolymerization Solutions

Details of Electropolymerization Solutions
Details of Electropolymerization Solutions

SDS, as an amphiphilic anionic surfactant, is considered to have two major roles in the electrodeposition process: (i) providing DS, a voluminous monovalent anion, incorporated into the polymer layer;9  and (ii) forming micelles at the aluminum surface which helps the electrodeposition of polypyrrole.49  These micelles are formed in the way that the hydrophilic group of the surfactant molecule is attached to the aluminum oxide layer while the hydrophobic end is a preferential site for the pyrrole monomer to be concentrated at, due to its hydrophobic nature. This encourages the lateral growth of the polypyrrole.49 

Citric acid (C6H8O7) and sodium nitrate (NaNO3) were added to the main solution to investigate the effect of electrolyte and the presence of different anions on the electrodeposition, electrochemical, and anticorrosive properties of the polypyrrole coating. All the chemicals used in this work were of analytical grade.

In Table 1, X refers to the coating deposited from the plain solution, while X-Cit and X-Nit refer to the coatings deposited from the solutions containing citric acid or sodium nitrate, respectively.

Coating Characterization

Morphological features of the polypyrrole coated samples were examined using SEM and FIB-SEM. The FIB milling conditions were as follows: 30 kV, 5 nA for rough milling, and 1 nA for final polishing. Beam energy of 10 kV was employed for energy dispersive x-ray spectroscopy analysis and mapping.

XPS analysis was performed on coated samples to examine the composition of polypyrrole coatings. These measurements consisted of the acquisition of a wide spectrum at a pass energy of 160 eV. Higher-energy resolution was obtained analyzing the core lines of interest at a pass energy of 20 eV, which led to an energy resolution of ∼0.3 eV. Thick coatings needed charge compensation which was performed by adjusting the flood gun working conditions to minimize the peak FWHM (full width at half maximum) while maximizing its intensity.

XPS analysis was performed on the surface of each coating as well as at the polypyrrole/aluminum interface. To analyze the interface, coatings were sputtered using an Ar+ ion gun operated at 3,800 V and 20 mA current that applied a raster of 3 mm × 3 mm. To reduce the sputtering time, thinner coatings were deposited by applying either six (X) or four (X-Cit and X-Nit) CV cycles for the deposition process. The coatings thicknesses were as follows: X: 1.2±0.2 μm, X-Cit: 2.6±0.4 μm, and X-Nit: 5.1±0.6 μm.

As sputtering is an energetic process and could induce a change in the chemical state of the elements, the obtained results were confirmed by repeating the same analysis on the surface of unsputtered thin coatings. In this case, coatings with thicknesses lower than the XPS sampling depth (∼7 nm to 8 nm) were deposited by applying two scans in the CV process.

The coatings thicknesses were measured using cross-sectional SEM images. Five images were taken of each sample. A Java-based image processing program was employed for the measurement.

Electrochemical Studies

The corrosion resistance and electrochemical stability of AA1050 (UNS A91050) protected by polypyrrole coatings were investigated in sodium chloride solutions of 0.1 M and 0.6 M by means of EIS and OCP monitoring for immersion times of 24 h and 168 h (7 d). In the EIS measurements (performed at the OCP), the frequency range was from 100 kHz to 10 mHz, with an amplitude of 10 mV (RMS).

All the electrochemical measurements were performed in a traditional three-electrode cell, containing the sample (with the exposure area of 1 cm2) as the working electrode, standard Ag/AgCl (3 M KCl) as the reference electrode and a platinum counter electrode.

The measurements (both electropolymerization and electrochemical characterizations) were performed using a commercial computer-controlled potentiostat.

The repeatability of electrochemical data in this work is guaranteed by repeating the measurements for three times.

RESULTS AND DISCUSSION

Electropolymerization, Morphology, and Composition

CV curves related to the electropolymerization of three different polypyrrole coatings are depicted in Figures 1(a), (b), and (c). According to these curves, in the first cycle, oxidation of pyrrole and the formation of polypyrrole film on the aluminum electrode start at around 0.6 (VAg/AgCl). Similar oxidation voltages for pyrrole have been reported by other researchers.18-19  At this point, the current density increases sharply and it reaches its highest value at the upper limit of the scanning range. The oxidation potential (during the first scan) is approximately shown as the point where the slope of the graph starts changing. While performing the experiments, at this point formation of polypyrrole film could be observed.

FIGURE 1.

1st, 5th, 10th, 15th, and 20th CV curves in the solution (a) X, (b) X-Cit, and (c) X-Nit. (d) two successive cycles in the same solutions without the monomer (scanning rate was 10 mV/s).

FIGURE 1.

1st, 5th, 10th, 15th, and 20th CV curves in the solution (a) X, (b) X-Cit, and (c) X-Nit. (d) two successive cycles in the same solutions without the monomer (scanning rate was 10 mV/s).

The polymer oxidation voltage is almost independent of pH of the solution (these pH values are written in Table 1).

For the next CV cycles, the inflection potential shifts to lower potentials (as shown in Figure 1[a]) demonstrating the easier formation of the polymeric film after the first cycle. For the X-Nit coating, during polymerization, the current density in the upper anodic region is higher than the other two coatings.

During 20 scans, the changes in the current density, especially for the solutions X and X-Cit, are negligible.

To see the effect of the supporting electrolyte on the electrochemical behavior of aluminum substrate, two successive CV scans, in a similar manner to the ones shown in Figures 1(a) through (c), were performed in the same solutions without the pyrrole monomer.

According to this graph, the values of current densities are substantially lower in comparison to the condition when the monomer is present in the solution. As it is obvious, the oxidation of pyrrole has a higher contribution to the increase in the current density, observed in Figures 1(a) through (c) in the high anodic region, compared to the oxidation/dissolution of the aluminum substrate.

According to Figure 1(d), following the first forward scan a decrease in the anodic current density can be observed, which is probably due to the passivation of the electrode. Afterward, a wide passive region is recorded. The initial decrease in the current could be due to the fact that the voltage range is behind the active/passive potential for the substrate. Figure 1(d) shows that the solutions containing C6H8O7 and NaNO3 yield lower current densities in both forward and reverse scans. During the second scan, a significant decrease in the current density is observed for these two solutions compared to the X solution.

In fact, citric acid is a corrosion inhibitor for aluminum as citrate anions are able to be adsorbed on the surface and make different complexes with Al3+ cations.50  The formation of these surface compounds can inhibit the dissolution of aluminum without preventing the electropolymerization process. Nitrate anions, on the other hand, form resistive transitory compounds such as Al(NO3)3, which contribute to the inhibition of the anodic dissolution of aluminum electrode.51 

As an example, the surface morphology of the coating X-Nit is shown in Figure 2(a). The three coatings show the so-called cauliflower morphology. All the coatings are homogenous and no specific difference in the size or the shape of polypyrrole globules is observed. The morphology of a polypyrrole film depends on the reactivity of pyrrole radical cations which is comparable in the three electropolymerization solutions we used in this work.52  All coatings display a compact homogeneous microstructure as visible in the cross-sectional image of the X-Nit coating in Figure 2(b). Few voids are observed at the polypyrrole/aluminum interface almost for all coatings. We suspect that some of them may have been caused by the ion beam polishing.

FIGURE 2.

(a) SEM-SE image of surface morphology of the coating X-Nit and (b) SEM-FIB (BS) image of cross-sectional view of the same coating.

FIGURE 2.

(a) SEM-SE image of surface morphology of the coating X-Nit and (b) SEM-FIB (BS) image of cross-sectional view of the same coating.

The thickness of the three coatings was measured as follows: X: 7.1±0.7 μm; X-Cit: 10.4±1.0 μm; and X-Nit: 12.0±0.9 μm. The trend in coatings thicknesses correlates well with the current densities observed during the electrodeposition process in Figures 1(a) through (c).

XPS analysis was used to study the chemical composition of the three polypyrrole coatings and also the composition state of the polypyrrole/aluminum interface.

Figure 3 depicts the high resolution XPS spectra (C 1s, N 1s, and S 2p) for the three coatings. The three spectra are similar.

FIGURE 3.

High-resolution XPS spectra of the polypyrrole coatings (a) C 1s, (b) N 1s, and (c) S 2p.

FIGURE 3.

High-resolution XPS spectra of the polypyrrole coatings (a) C 1s, (b) N 1s, and (c) S 2p.

The C 1s peak is fitted using six different Gaussian components to account for the aromatic ring and the CN bond of pyrrole, the CHx bonds and the different carbon-oxygen bonds formed during the electropolymerization process. Similarly, the N 1s peak is described by N in an aromatic ring similar to pyridine. Nitrogen in the pyrrole ring can be in neutral, ionized form, and the NO3 bond. This last component is present in all three coatings, although very weak, which indicates that its presence in the coating is not necessarily related to the presence of NaNO3 in one solution. Finally, the S 2p core line in the three coatings is mainly located at low binding energies. The two deconvoluted peaks at ∼169 eV are assigned to NaSO4. A small variation in the intensity of the S 2p core line appears at ∼164 eV, which can be assigned to the 1/2 to 3/2 spin orbit components of sulfur in Na2SO4. These XPS results suggest incorporation of DHBDS and SDS into the polymer. However, no strong proof is found for the incorporation of citrate and nitrate anions into the X-Cit and X-Nit coatings, respectively.

A difference between the X-Nit coating and the other two coatings becomes evident when analyzing the polypyrrole/aluminum interface. Figure 4 compares the Al 2p core lines after sputtering the polypyrrole coatings. The spectra are noisy since as soon as the Al 2p peak appeared sputtering was switched off to preserve the interface and its chemistry. Therefore, the aluminum substrate is partially masked by the residual polypyrrole coating. The aluminum spectrum is described utilizing two Gaussian components. The prominent shoulder at the high binding energy corresponds to the aluminum oxide (Alox), while the sharper peak at the lower energy corresponds to the crystalline bulk aluminum (Al0).

FIGURE 4.

Aluminum core line at the polypyrrole/aluminum interface.

FIGURE 4.

Aluminum core line at the polypyrrole/aluminum interface.

As it appears from Figure 4, the intensity of the nonoxidized metallic peak on the X and X-Cit substrates is higher with respect to the oxidized one, while the two components are similar in the case of the X-Nit substrate. Table 2 summarizes the contribution of the Al0 and Alox components (in percent) for the different coatings.

Table 2.

Contribution of the Metallic and Oxidized Components to the Total Spectral Power of the Al 2p Spectrum

Contribution of the Metallic and Oxidized Components to the Total Spectral Power of the Al 2p Spectrum
Contribution of the Metallic and Oxidized Components to the Total Spectral Power of the Al 2p Spectrum

Based on these results, the ratio of the aluminum oxide to the metallic aluminum is higher at polypyrrole/substrate interface of the X-Nit sample compared to the other two samples.

This suggests the passivating effect of nitrate anions on the aluminum surface during the electropolymerization process. While, according to Figure 1(d), both nitrate and citrate anions should provide this effect, based on the XPS results nitrate anions are passivating the substrate more efficiently.

One could argue that sputtering may modify the oxidation state of aluminum. However, these results are confirmed by the XPS spectra acquired on the unsputtered thin films deposited by applying two CV cycles. These results are presented in Table 2 as well and the modulation of the oxidized and bulk aluminum intensities shows a similar behavior in the three samples.

After applying two cycles of CV, to the naked eye, the treated electrode seems to be only partially covered by the polypyrrole coating and there are areas on the sample that the coating thickness can be possibly lower than the sampling depth of XPS (∼7 nm to 8 nm). The measurement was repeated several times on different areas to guarantee the reliability of the data and the same results were always obtained.

Electrochemical Properties

EIS responses of the three coatings were measured to investigate the degradation of the coatings and to monitor their electrochemical interaction with the aluminum substrate.

Two concentrations of 0.1 M and 0.6 M NaCl were chosen to investigate the effect of chloride ion concentration on the coatings’ behavior and stability.

OCP was also monitored during immersion in both NaCl solutions (Figure 5). The coated samples were kept immersed in the more concentrated solution for longer times (up to 7 d) to assess the failure time (Figure 5[c]).

FIGURE 5.

OCP values of polypyrrole coated and bare AA1050 (UNS A91050) samples vs. time immersed in (a) 0.1 M, (b), and (c) 0.6 M NaCl solution (due to overlapping of standard deviation bars, to make it clearer for the reader, the bars are shown only in the negative direction).

FIGURE 5.

OCP values of polypyrrole coated and bare AA1050 (UNS A91050) samples vs. time immersed in (a) 0.1 M, (b), and (c) 0.6 M NaCl solution (due to overlapping of standard deviation bars, to make it clearer for the reader, the bars are shown only in the negative direction).

The OCP value of a polypyrrole-coated aluminum sample can be an average of the potentials of the electrochemical reactions taking place in the system, including the reduction of the polypyrrole film and the corrosion of aluminum.

According to Figure 5, OCP values of all coated samples in both solutions are considerably nobler in comparison to the bare sample. The average difference is around 0.8 V, which is quite significant.

The OCP values recorded in 0.1 M NaCl solution (Figure 5[a]) are comparable for the three samples of polypyrrole coatings up to 10 h of immersion. However, at longer times OCP decreases for the X and X-Cit coatings, while it remains quite stable for the X-Nit coating. The transition time of 10 h to 11 h was observed for two samples out of three for the coating X. For the coating X-Cit, the transition time was extended to 16 h for one of the samples. The third sample of both coatings did not show the transition time for 24 h. It can also be seen that the OCP values for the X-Nit coating show smaller standard deviation indicating better reproducibility of this coating.

The difference between OCP values of the three coatings is more distinct in 0.6 M NaCl solution. As it can be observed from Figure 5(b), OCP initially decreases linearly for the X coating, suddenly dropping after 8 h and remaining constant up to 18 h, when again drops and continues to decrease. The X-Cit coating appears to be more stable, starting to deteriorate after ca. 18 h of immersion. As shown in Figure 5(c), the final failure for these two samples of coatings occurs after 18 h (coating X) and 52 h (coating X-Cit), as at these points OCP values decrease sharply until they reach values similar to that of the bare sample. The coating X-Nit, however, retains its OCP value constant for 124 h (5 d).

The failure times of the other two samples of coating X were 20 h and 40 h.

The long immersion test was repeated two more times and the failure times of 18 h and 103 h for the coating X-Cit, and 144 h and 125 h for the coating X-Nit were recorded.

Considering the results in Figure 5, the protective performance of the coatings can be ranked as follows: X-Nit ≫ X-Cit > X.

Bode presentations of representative EIS spectra (performed at the OCP) for the bare and coated aluminum samples during 24 h of immersion in 0.1 M and 0.6 M NaCl solutions are depicted in Figures 6 and 7, respectively.

FIGURE 6.

EIS spectra of polypyrrole coated and bare AA1050 (UNS A91050) samples after (a) 1 h, (b) 6 h, (c) 12 h, and (d) 24 h of immersion in 0.1 M NaCl solution.

FIGURE 6.

EIS spectra of polypyrrole coated and bare AA1050 (UNS A91050) samples after (a) 1 h, (b) 6 h, (c) 12 h, and (d) 24 h of immersion in 0.1 M NaCl solution.

FIGURE 7.

EIS spectra of polypyrrole coated and bare AA1050 (UNS A91050) samples after (a) 1 h, (b) 6 h, (c) 12 h, and (d) 24 h of immersion in 0.6 M NaCl solution (The solid lines are fitted data).

FIGURE 7.

EIS spectra of polypyrrole coated and bare AA1050 (UNS A91050) samples after (a) 1 h, (b) 6 h, (c) 12 h, and (d) 24 h of immersion in 0.6 M NaCl solution (The solid lines are fitted data).

The total impedance values of the polypyrrole coatings are lower compared to the bare aluminum. These results do not mean that there is worse performance of the polymer coatings but represent the (semi) conductive nature of the polypyrrole film. Other researchers have reported similar results.28,53 

The difference of total impedance values of the three polypyrrole coatings is less significant in the more concentrated NaCl solution while it increases over time.

Potentially, the EIS spectra of the polypyrrole coatings in 0.1 M and 0.6 M NaCl solutions, as shown in Figures 6 and 7, represent two time constants at the high and medium frequency ranges together with a Warburg element and/or a time constant at the low frequency range.

The difference between the EIS spectra in the two different concentrations appears as the (occasional) absence of the time constant at the high frequency range in 0.1 M NaCl solution for all coatings. This time constant was always absent for samples of the coating X-Cit and two samples (out of three) of the coatings X and X-Nit did not show it.

As it was noted before the spectra presented in Figures 6 and 7 are the representative results. In the 0.1 M NaCl solution, the spectra from one sample to another for all coatings were more reproducible. While in the more concentrated solution, time of failure was so different for various samples of the coatings X and X-Cit.

To interpret the EIS results (performed in 0.6 M NaCl solution), these impedance responses were fitted using an equivalent electrical circuit (Figure 8) and the fitting parameters are analyzed as a function of time. As the coating X-Nit presents the best and the most reproducible results fitting was performed for this coating. To check the reproducibility of the data, the fitting was repeated for three samples.

FIGURE 8.

Equivalent circuit for fitting the EIS responses of polypyrrole coatings.

FIGURE 8.

Equivalent circuit for fitting the EIS responses of polypyrrole coatings.

The equivalent circuits employed by other researchers to fit the EIS responses of polypyrrole/metal systems (aluminum and steel) are commonly based on the interpretation of a time constant (RC constant) at the medium/high and a Warburg tail at low frequency ranges.8,47,54-55  These are typically attributed to the coating (pore) resistance, charge transfer resistance of the redox reaction of the polymer, and diffusional process through the film, respectively.10,54,56  Some have also considered the contribution of a passive oxide layer.10,13,55 

Considering the controversial results available in the literature and the electroactive nature of polypyrrole proposing a certain equivalent circuit is difficult. Based on the presence of three time constants in the EIS spectra of the coating X-Nit (Figure 7) an equivalent circuit is suggested and shown in Figure 8.

We suggest that the time constant at the high frequency range is probably attributed to the (localized) galvanic interaction of polypyrrole film and aluminum which leads to passivation (oxidation) of the substrate. A resistance (Rga) and a constant phase element (CPEga) represent this contribution. As it was mentioned before, the presence of this time constant and therefore the galvanic interaction can be affected by the chloride concentration.

The time constant at the medium frequency range can be related to the redox reaction of the polypyrrole film that is modeled by a resistance (Rf) and another constant phase element (CPEf).

Possibly, at the low frequency range there is transition between a mass-transport-related constant (Warburg element) and a separate time constant (presented by CPEdl (none ideal double layer capacitance of the coating/solution interface) and Rct (charge transfer resistance). This time constant at the low frequency range can probably be related to the reduction of oxygen at the polypyrrole/electrolyte interface.21,57 

The results of data fitting for a 24-h immersion test of three samples of the X-Nit coating in 0.6 M NaCl solution are presented in Table 3. In addition, the fitted spectra are presented as solid lines in Figure 7.

Table 3.

Summary of Results Derived from Fitting the EIS Responses of X-Nit Polypyrrole Coatings in 0.6 M NaCl Solution

Summary of Results Derived from Fitting the EIS Responses of X-Nit Polypyrrole Coatings in 0.6 M NaCl Solution
Summary of Results Derived from Fitting the EIS Responses of X-Nit Polypyrrole Coatings in 0.6 M NaCl Solution

For all of the coatings, values of Rga slightly increase by the immersion time, which may imply that the galvanic interaction is slowed down/decreased due to the decrease in the coating conductivity. Rf which can be related to the reduction reaction of the polypyrrole film decreases by the immersion time. The reduction of polypyrrole film is accompanied by the release of the dopant anions (e.g., and DS) while its reoxidation is accompanied by the uptake of anions such as chloride to maintain the polymer electroneutrality.58 

If we consider that Rf is related to the coating resistance, its decrease displays easier ion mobility through the coating with the immersion time. The entry of an electrolyte during the immersion can also decrease the coating resistance.58  The increase in the capacity of the coating can confirm this as well.57,59 

According to Table 3, the charge transfer resistance usually decreases from the first to the last hour of immersion. If this resistance is to be considered for oxygen reduction, its decrease shows a faster reaction and vice versa.

For three different samples of X-Nit coating, the same trend in various variables is observed. Although sample No. 2 seems to behave differently its final failure occurs after 144 h.

The longer protection provided by the coating X-Nit in the more concentrated solution may be attributed to the better quality of the polypyrrole/aluminum interface due to the passivating effect of nitrate ions, which leads to a better galvanic interaction (anodic protection).

To clarify the electrochemical behavior, we examined the coating surfaces and their cross-sectional view after the immersion tests in NaCl solutions. These observations provide a better insight regarding the electrochemical reactions at the polypyrrole/aluminum interface and facilitate understanding of the protection effect. The related images are presented in Figure 9.

FIGURE 9.

SEM(-FIB) (SE/BS) image of the coating (a) X, (b) X-Cit, and (c) X-Nit after 24 h. immersion in 0.1 M NaCl solution and of the X-Nit coating after (d) 24 h and (e) 168 h of immersion in 0.6 M NaCl solution.

FIGURE 9.

SEM(-FIB) (SE/BS) image of the coating (a) X, (b) X-Cit, and (c) X-Nit after 24 h. immersion in 0.1 M NaCl solution and of the X-Nit coating after (d) 24 h and (e) 168 h of immersion in 0.6 M NaCl solution.

Figures 9(a), (b), and (c) depict the surfaces of the three polypyrrole coatings after 24 h of immersion in 0.1 M NaCl solution. On all of the surfaces, the presence of some blisters is evident. The size of these blisters varies in the same order as the performances of the coatings (with respect to the OCP results in Figure 5[a]). Therefore, the coating X-Nit exhibits the smallest and the coating X the biggest blisters.

Cross sectioning these blisters using FIB elucidates corrosion/oxidation reactions occurring at the polypyrrole/aluminum interface. SEM (BS) images of the polypyrrole/aluminum interfaces inside the blisters after 1-d immersion in 0.1 M NaCl solution are shown in Figures 9(a), (b), and (c) as well. With some variations, the presence of three distinct layers including the aluminum substrate, an oxide layer, and the polypyrrole film is noticeable at all cross sections. This observation shows the inevitable galvanic interaction of polypyrrole coating with the aluminum substrate.39  This can keep the surface passivated but locally causes serve oxidation and forms blisters.

The polypyrrole/aluminum interface outside the blister was unchanged.

The surface of the X-Nit coating was examined after 24 h and 168 h of immersion (after failure) in 0.6 M NaCl solution (Figures 9[d] and [e]). The blisters have significantly spread on the coating surface after 24 h and finally caused failure after 5 d.

The formation of blisters at both chloride concentrations for all the coatings is inevitable, however, their formation is significantly limited on the X-Nit coating in 0.1 M NaCl solution compared to the other two coatings. In this solution, the chloride ions are present at the interface, but the X-Nit coating keeps the surface passivated and protected.

In other words, the addition of NaNO3 to the electropolymerization solution results in the formation of the coating with a reduced number of blisters (after immersion) and longer protection provided in more concentrated NaCl solutions.

CONCLUSIONS

Polypyrrole coatings were successfully electropolymerized on substrates of AA1050 (UNS A91050) using CV. Different electropolymerization solutions containing Py, SDS, DHBDS, C6H8O7, and NaNO3 were used. The main conclusions drawn are as follows.

  • Solutions containing passivating anions (citrate or nitrate) lead to the deposition of thicker coatings.

  • The presence of nitrate anions effectively leads to the passivation of the aluminum electrode during the electropolymerization step.

  • Polypyrrole coatings are able to relatively protect the surface in 0.1 M and 0.6 M NaCl solutions.

  • The coating deposited from the nitrate containing solution presents the best and most reproducible performance demonstrating that the protection efficiency can be improved by altering the solution chemistry which affects the polymer/metal interface and probably the electrochemical activity of the coating.

  • Based on the EIS results, the passivation corrosion protection can be suggested for the polypyrrole coatings.

  • In the presence of chloride ions, the formation of blisters as a result of severe (localized) galvanic interaction of polypyrrole coating with aluminum is a serious problem and cause of the final failure.

  • The application of polypyrrole in corrosion protection needs specific considerations in tailoring the galvanic interaction of the conductive polymer and metal.

Trade name.

(1)

UNS numbers are listed in Metals and Alloys in the Unified Numbering System, published by the Society of Automotive Engineers (SAE International) and cosponsored by ASTM International.

ACKNOWLEDGMENTS

This project was partially funded by European Cooperation in Science and Technology (E-COST) by the e-minds action [MP1407].

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