Electrosynthesis of a polyaniline/zeolite nanocomposite coating on copper in a three-step process and the effect of current density on its corrosion protection performance

Abstract

A polyaniline/zeolite nanocomposite (PZN) was successfully electrosynthesized at a copper (Cu) electrode in a sodium oxalate solution to generate a homogeneous and adherent coating. During the formation of the PZN coatings, three stages (electro-adsorption of monomer and electrolyte and initiation of formation of the passive film, growth and impingement of the passive film and decomposition of the latter and formation of composite coatings) are observed and three times related to these stages are reported. The coatings were characterized by SEM, XRD, FTIR and UV-vis spectroscopy. The corrosion behavior of copper with the PZN coatings in 3.5 wt% NaCl solution was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The effect of the applied current density on the protective properties of the PZN coatings has been investigated, and it was shown that the protection efficiency depends on the applied current density. The corrosion current density decreased from 43.56 μA cm⁻² for un-coated copper to 0.34 μA cm⁻² for PZN-coated copper under optimal conditions. It was observed that the PZN coating provided efficient protection (99.2%) to copper in 3.5 wt% NaCl solution. The corrosion rate for the PZN-coated copper was significantly lower than the bare copper (about 130 times).

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1. Introduction

Copper is widely used in many applications in the field of electronic and communications, such as in electrical power lines, pipelines for domestic and industrial water utilities, heat conductors, and heat exchangers.^1,2 Apart from the electrical and thermal properties, copper also exhibits good corrosion resistance. However, in certain media copper may suffer attack, such as in acidic media in the presence of oxidizing and/or complexing and then it is important to improve the corrosion resistance of the metal and this issue has attracted the attention of researchers.^1–8

One of protection processes against corrosion of metals often involves the elimination of contact with the corrosive medium resorting to the use of paints or organic coatings. Flaws in the coating have been found to produce accelerated corrosion of the metal. Within coating technology, there is increasing interest in the development of efficient anticorrosive coating that is able to replace the conventional inorganic anticorrosive pigments usually added to paints, which may have detrimental effects on both environment and health. Researchers have invented a revolutionary corrosion control system using conducting polymers in the last two decades.⁹

Since DeBerry¹⁰ reported that an electrochemically deposited polyaniline film could provide anodic protection for stainless steel, conductive polymers have been candidates for metal protection against corrosion.^10–15

Polyaniline is one of the most extensively investigated conducting polymers because of its good stability, low cost, low toxicity and valuable electronic properties. Polyaniline exhibit different chemical structures that are both pH and potential dependent.¹⁶ The emeraldine form has excellent air and thermal stability¹⁷ and thus has been extensively studied for corrosion control either as a thin primer, pigment or coating on inorganic pigment. Various corrosion protection mechanisms of polyaniline coatings have been proposed such as barrier protection, adsorption, anodic protection and shift of electrochemical interface.¹⁸ The exact mechanism that operates depends on many experimental factors: coating type, emeraldine form (conductive or non-conductive), corrosive environment, etc. This is one of the reasons why there is still much debate regarding the exact mechanism of protection of polyaniline.

Conducting polymer coatings can be prepared either chemically or electrochemically. Electrochemical techniques including potentiostatic method,¹⁹ galvanostatic method²⁰ and cyclic voltammetric method²¹ are widely employed for electropolymerization of monomer. In electrochemical synthesis methods, the main problem connected with using active metals as substrates arises from the fact that due to anodic polarization of the electrode, two simultaneous processes take place on the electrode: (i) electrodeposition of the polymer and (ii) dissolution of the substrate metal. If the latter process is faster, the polymer film cannot be deposited onto the electrode surface. Such a situation often takes place when the polymer is electrodeposited from an aqueous solution. Therefore, it is necessary to find suitable electrochemical conditions to inhibit the dissolution of the working electrode without preventing the electropolymerization of polymer. This problem can be solved by selecting appropriate dopant, pretreating the metal surface, and/or varying polymerization parameters, such as the concentration of monomer, the type and concentration of dopant, the pH of the solution and current density.²²

So far, polyaniline coatings have provided corrosion protection for several materials such as silver,²³ aluminum,²⁴ carbon steel,²⁵ iron,^26–28 mild steel,^29,30 and especially stainless steel.^31–33 Although there is an extensive literature on the corrosion protection properties of conducting polymers, such as polyaniline and polypyrrole on iron or iron-based alloys there are a few reports devoted to the corrosion protection properties of these polymers when applied to copper or copper-based alloys.^34–36 This case partly results from the difficulty in the electropolymerization of the monomer to generate the conducting polymer on copper. Some researchers propose the use of oxalic acid,³⁷ sodium oxalate,³⁸ or sodium salicylate³⁹ as the electrolyte, and in that case, it was demonstrated that the growth of polymer films is possible after the initial oxidation of the copper electrode that generated a copper oxalate/salicylate pseudopassive layer. This pseudolayer has the capability of reducing the metal dissolution rate without hindering the electron transfer process, which is necessary for polymer formation.⁴⁰

The use of polyaniline as corrosion protection coating of metals has been investigated in many articles. However, the porosity and the ion exchange ability of conducting polymer coatings might be disadvantageous for corrosion protection performance. The charges stored in the polymer layer can be irreversibly consumed during the system's redox reactions, and the ability of the protection with polymer coatings may be lost with time. Therefore, nanostructure materials such as nanocomposites have been synthesized due to their specific properties. To date, many papers have been published dealing with the chemical⁴¹ and electrochemical^42,43 synthesis of nanocomposites and their corrosion protection properties. Electrochemically synthesis methods are favored because of the direct synthesis of a polymer on the metal surface without any organic additives and capability of preparing smaller and size-controllable particles by adjusting the applied potential or the current density.

Inorganic/organic composites have been investigated extensively in recent years due to a wide range of potential use of these materials.^44,45 One important class of such hybrid materials is that in which the organic fraction is composed of conducting polymers, such as polyaniline or polypyrrole. It is hopeful to obtain composite materials with synergetic or complementary behaviors between polymer and inorganic matrices. For example, polyaniline/inorganic composites with electrical and magnetic properties⁴⁶ have potential applications in batteries, electrochemical display devices, electromagnetic interference shielding, electro-magnetorheological fluids,⁴⁷ microwave-absorbing materials, and anticorrosion materials⁴⁸ because of their synergetic behaviors between conducting polymer and the inorganic magnetic nanoparticles.

As far as authors are aware, no previous study on the synthesis of polyaniline/zeolite nanocomposite onto copper by galvanostatic technique has been stated so far, and in this work, we demonstrate for the first time that polyaniline/zeolite nanocomposite coatings can be electrogenerated on copper from the basic aqueous electrolyte. The effect of the current density on the corrosion properties of the electrosynthesized nanocomposite coatings was also studied by polarization and EIS techniques in 3.5 wt% NaCl solution. We also present the characterization of the coatings by UV-vis absorption spectrometry, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM).

2. Experimental

Zeolite Y was purchased from Sigma-Aldrich. Other chemicals were purchased from Merck. Aniline was freshly distilled and stored in the dark. All solutions were freshly prepared from analytical grade chemical reagents using doubly distilled water. For each run, a freshly prepared solution and a cleaned set of electrodes were used, and all experiments were carried out at room temperature. Electrochemical experiments and corrosion tests were carried out using an AUTOLAB PGSTAT 35 potentiostat/galvanostat connected to a Pentium IV personal computer through a USB electrochemical interface. A conventional three-electrode cell was used. The working electrodes employed were prepared from a Cu sheet (99% purity). The metal sheet was cut into rectangular samples of 1 cm² area, soldered with Cu-wire for an electrical connection and mounted onto the epoxy resin to offer only one active flat surface exposed to the corrosive environment.

2.1. Pretreatment of the working electrode

Before each experiment, the working electrode was polished with a sequence of emery papers of different grades (400, 800, 1000 and 1200), washed with doubly distilled water and degreased with acetone. Before electropolymerization, the surface of the clean electrode was pretreated in 0.2 M sodium oxalate solution, between −0.5 and +1.4 V at a 20 mV s⁻¹ scan rate by applying 3 cycles.

2.2. Electrosynthesis of polyaniline/zeolite coatings on copper

As a typical procedure for the preparation of polyaniline/zeolite nanocomposite coatings with 1 wt% of zeolite, an appropriate amount of zeolite was introduced into 100 mL of distilled water and was magnetically stirred for 3 hours at room temperature. Then, a mixture of 0.2 mol of sodium oxalate and 0.15 mol of aniline monomer was added into the zeolite solution under magnetic stirring for 1 h. Subsequently, the obtained solution was ultrasonicated for 20 min in order to increase its uniformity. Electropolymerization was carried out by galvanostatic polarization from 10 mL of the prepared solution at pH 12.5.³⁶ Electrosynthesis of polyaniline/zeolite coatings over the copper surface were carried out by imposing a fixed current for certain duration of time. In this regard, current densities viz. 2, 4, 6, 8 and 10 mA cm⁻² for the time duration of 900 s were applied and the corresponding potential transients were recorded. Pre-treated Cu sample was used as the working electrode in the conventional three-electrode assembly, with a graphite rod as the counter electrode and a silver/silver chloride (Ag/AgCl (3 M Cl⁻)) as the reference electrode. After the electropolymerization reaction, the green colored, homogeneous and adherent polyaniline/zeolite nanocomposite coatings were successfully obtained on the Cu surface.

2.3. Characterization of electrosynthesized polyaniline/zeolite nanocomposites

The FTIR spectra of the electrosynthesized polyaniline/zeolite nanocomposite over the Cu surface was obtained and compared to polyaniline and zeolite using a Shimadzu Varian 4300 spectrophotometer in KBr pellets. The electrosynthesized polyaniline/zeolite nanocomposite was dissolved in pure N-methylpyrrolidone (NMP) and UV-vis spectra of the polymer solution were recorded on a Perkin Elmer Lambda2S UV-vis spectrometer. The morphologies of the electropolymerized coatings on Cu surfaces were analyzed using a PHILIPS model XL30 scanning electron microscope instrument operating at 10 kV. XRD patterns were recorded by a Rigaku D-max C III X-ray diffractometer using Ni-filtered Cu-Ka radiation.

2.4. Corrosion tests

The copper samples with electrosynthesized polyaniline/zeolite nanocomposite coatings were evaluated for their corrosion resistance properties in 3.5 wt% NaCl solution by Tafel polarization and electrochemical impedance spectroscopy (EIS). The coated-Cu electrode was used as the working electrode in the conventional three-electrode assembly, with a platinum sheet as the counter electrode and a silver/silver chloride (Ag/AgCl (3 M Cl⁻)) as the reference electrode. The working electrode was first immersed in the corrosion test solution (3.5 wt% NaCl solution) for 24 hours. In the case of Tafel polarization, the potential was scanned at ±200 mV versus open circuit potential (OCP) at a scan rate of 0.5 mV s⁻¹. From the anodic and cathodic polarization curves, the Tafel regions were identified and extrapolated to the corrosion potential (E_corr) to obtain the corrosion current density (i_corr) using the NOVA software. In the case of electrochemical impedance spectroscopy, a.c. signals of 20 mV amplitude and various frequencies from 100 kHz to 100 Hz at open circuit potentials were impressed to the coated-Cu electrode. A Pentium IV-powered computer and NOVA software were applied for analyzing impedance data. All experiments were performed in a Faraday cage to minimize noise interferences.

3. Results and discussion

3.1. Electrochemical synthesis of PZN coatings on copper

The experiments were performed under galvanostatic conditions. This mode of electrosynthesis agrees to industrial practice for economic reasons because it allows control of the film thickness, i.e., the cost of the process, and allows also the deposition of the polymeric coating in a few seconds.

All attempts at electropolymerization of the aniline to generate a polymeric coating in an acidic medium, at the copper electrode, failed as these high acidic conditions promote dissolution of copper before the growth of the polymer can be achieved. However, the copper electrodes were passivated in oxalate solution, enabling electropolymerization of polyaniline and the growth of an adherent and homogenous PZN coating.

The effect of current density on the process of coating formation was studied by carrying out the polymerization at current densities varying from 2 to 10 mA cm⁻² for 900 s. A typical chronopotentiometric (galvanostatic) curve of electrosynthesis of PZN coating on the copper electrode from aqueous solution of 0.2 M sodium oxalate and 0.15 M aniline containing 1 wt% zeolite at pH 12.5 is given in Fig. 1. As seen from the figure, the formation of PZN coating on copper involves three distinct stages: (I) electro-adsorption of monomer and electrolyte and initiation of formation of passive film; (II) growth and impingement of the passive film and decomposition of the latter; (III) formation of composite coatings.⁴⁹ During stage I, it can be seen a jump in the potential after a short period of time (t₁) attributed to dissolution of copper, adsorption of the electrolyte anions and initiation of formation of a Cu oxalate film.⁵⁰ It is followed by a subsequent increase in potential (Region II) and constancy in potential (Region III). The increase in potential at stage II is attributed partly to the growth of Cu oxalate film and the contribution from the exposed copper substrate due to the dissolution of the Cu oxalate film. After a period of induction time (t₂), the decomposition of the passive film is followed by the initiation and formation of PZN coating and the associated fixity in the cell potential (Region III). Chronopotentiometric (galvanostatic) curves of electrosynthesis of PZN coatings on copper electrode from aqueous solution of 0.2 M sodium oxalate and 0.15 M aniline containing 1 wt% zeolite at pH 12.5, obtained by different current densities during 900 s are given in Fig. 2. The three distinct regions for the development of the nanocomposite coating are clearly shown in Fig. 2.

Fig. 1 E–t curves showing the development of PZN and polyaniline coatings.

Fig. 2 E–t curves of the formation of the polyaniline/zeolite nanocomposite coatings on the Cu electrode in 0.2 M sodium oxalate and 0.15 M aniline containing 1 wt% zeolite at pH 12.5 under galvanostatic polymerization conditions at various current densities.

For the mentioned three stages, their periods of time (t₁, t₂ − t₁ and t₂, respectively) are reported in the Table 1 and the changes in these times are shown in Fig. 3. As seen in the table and Fig. 3, the higher rates of PZN formation can be obtained with the higher deposition current densities, as expected. The potential increases rapidly for the first 10 s by applying 10 mA cm⁻² current density. Shorter induction times can be achieved by applying higher current densities. Passivation and polymerization potential values, on the other hand partially increase with increasing applied current density.

Table 1 The time values of three stages for the applied current densities

Iapplied (mA cm^−2)	t1 (s)	t2 − t1 (s)	t2 (s)
2	100	263	363
4	25	86	111
6	15	13	28
8	9	11	20
10	2	9	11

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Fig. 3 The changes in time of three stages for the applied current densities.

For comparative purposes, the polyaniline coating also electrosynthesized on the copper electrode at the same conditions and its E–t curve is also shown in Fig. 1. As seen in the figure, the induction time for polyaniline formation is shorter than PZN formation and the polymerization potential value of polyaniline is also lower than PZN coating formation (about 0.5 V). These results show that the conditions of electrosynthesis for PZN coatings are more difficult than the electrosynthesis of polyaniline that it could be related to the presence of zeolite particles in the electrodeposition bath.

Adherent coatings with homogeneous appearance were obtained at 2–10 mA cm⁻² current densities. However, it has been found that the corrosion resistant properties of these electrosynthesized PZN coatings in 3.5 wt% NaCl solution have been decreased when the applied current density increased at electrosynthesis stage.

The used galvanostatic procedure generated a green coating, characteristic of polyaniline in the emeraldine oxidation state, at the surface of copper. Photographs of un-coated and PZN-coated copper electrodes are shown in Fig. 4.

Fig. 4 The photographs of (a) un-coated and (b) PZN-coated copper electrodes.

3.2. Characterization of the PZN coatings

The FTIR spectra of zeolite, polyaniline, and polyaniline/zeolite nanocomposite containing 1 wt% zeolite are shown in Fig. 5. In the spectrum of the polyaniline/zeolite nanocomposite, characteristic absorbance bands of polyaniline and zeolite occurred at the following locations: the aromatic C–H stretching vibration at about 2924 cm⁻¹, C–N stretching vibration at 1304 cm⁻¹, and the in-plane C–H bending at about 1153 cm⁻¹ reveal the characteristic bands of polyaniline. The peak at about 752 cm⁻¹ is characteristic of the para-disubstituted aromatic rings that indicate polymer formation. The bands at approximately 1503 and 1594 cm⁻¹ are due to the benzenoid and quinoid ring units. It was found that the polyaniline film formed by galvanostatic conditions contained both benzenoid and quinoid moieties. The Al–O stretching vibration at about 1024 cm⁻¹ and O–T–O bending vibration at about 467 cm⁻¹ confirm the presence of zeolite in the polyaniline/zeolite nanocomposite.^51,52 Therefore, FTIR spectra of polyaniline/zeolite nanocomposite exhibits bands characteristic of polyaniline as well as of zeolite which confirms the presence of both components in the PZN nanocomposite.

Fig. 5 FT-IR spectra of zeolite, pure polyaniline and polyaniline/zeolite nanocomposite.

Fig. 6 shows the UV-vis absorption spectra for polyaniline and polyaniline/zeolite nanocomposite in pure NMP solvent. This spectrum shows the characteristic peak at 310 nm and a broad peak at 580 nm. The first one corresponds to the π–π* transition and π–n* transition. The second one at 580 nm can be assigned to the exciton transition which is rather weak in these polymers.⁵³ The UV-vis absorption spectra of the polyaniline/zeolite nanocomposite revealed a blue shift from the peak location for pure polyaniline, reflecting a decreased conjugated chain length of polyaniline in the polyaniline/zeolite nanocomposite.⁵⁴

Fig. 6 UV-vis spectra recorded for the electrosynthesized pure polyaniline and polyaniline/zeolite nanocomposite on the Cu electrode.

The X-ray diffraction patterns were recorded for polyaniline, polyaniline/zeolite nanocomposite as shown in Fig. 7. The intensity of the XRD pattern peaks can be influenced by crystallinity or by polyaniline chain order in the nanocomposite structure. According to the XRD patterns of polyaniline, it can be seen that polyaniline has a relatively amorphous structure, but by encapsulation of polyaniline in the zeolite channels the alignment and arrangements of polyaniline chains were improved and as a result, the intensity of the peaks related to the nanocomposite was increased.⁵⁵

Fig. 7 X-ray diffraction patterns of polyaniline and the polyaniline/zeolite nanocomposite.

3.3. Corrosion protection performance of the PZN coatings

3.3.1. Potentiodynamic polarization test. Potentiodynamic polarization curves of copper and copper coated by electrochemically synthesized PZN coatings, after 24 h of immersion in 3.5 wt% NaCl solution are given in Fig. 8. Current densities at E_corr (i_corr), were determined by extrapolating the Tafel lines at E_corr. The corrosion potential of bare copper was −0.314 V, while the corrosion potential of PZN-coated copper is nobler. The E_corr value for PZN coatings is shifted to the positive side as compared with the un-coated Cu electrode. The anodic shift observed (about 0.173 V) supports the anodic protection of copper through a galvanic couple between polyaniline and copper.

Fig. 8 Polarization behavior of the electrosynthesized polyaniline/zeolite nanocomposite coated on copper at various current densities (mA cm⁻²) in 3.5 wt% NaCl solution.

The values of the corrosion potential (E_corr), corrosion current density (i_corr), polarization resistance (R_p), corrosion rate (CR) and protection efficiency (PE) obtained from these curves are given in Table 2. The corrosion current density of the copper electrode was considerably decreased by the PZN coating, from 43.56 μA cm⁻² to 0.34 μA cm⁻² under optimal conditions. Also, the corrosion rate of copper is significantly reduced as a result of the reduction in i_corr. The CR of the PZN-coated Cu is found to be 0.004 mm yr⁻¹, which is ∼130 times lower than that observed for bare Cu. As seen from the data, the corrosion current density of the coated-copper electrodes slightly increases with increasing current density applied in the electro-polymerization stage.

Table 2 Electrochemical parameters of electropolymerized coatings on copper in aqueous 3.5 wt% NaCl solution under the galvanostatic conditions at different current densities for 900 s

Iapplied (mA cm^−2)	Ecorr (mV)	icorr (μA cm^−2)	Rp (kΩ)	CR (mm yr^−1)	PE (%)	Porosity^a (%)
a (ba: 0.156 V dec^−1).
Bare	−314	43.56	2.83	0.510	—	—
PZN
2	−141	0.34	31.28	0.004	99.2	0.70
4	−148	1.72	13.77	0.019	96.0	1.88
6	−150	2.34	12.99	0.027	94.6	1.94
8	−174	1.83	13.37	0.024	95.8	2.77
10	−187	1.81	13.59	0.021	95.9	3.19
Polyaniline
2	−130	0.84	25.67	0.009	98.1	0.73

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From the measured corrosion current density values, the protection efficiency was obtained from the following equation,⁵⁶

where i_corr and i_corr(C) are the corrosion current density values in the absence and presence of the coating respectively.

The most effective protection against corrosion in an aqueous neutral corrosive medium was accomplished when the applied current density was 2 mA cm⁻². In other words, the minimum corrosion rate and corrosion current values were obtained when the applied current density was a minimum value in the electro-polymerization stage.

The coating porosity is an important parameter, which strongly governs the anticorrosive behavior of the coatings. In this work, the porosity of PZN coatings on copper substrates was determined from potentiodynamic polarization measurements. The coating porosity (P) was calculated from Elsener's empirical equation:⁵⁷

where P is the total porosity, R_puc is the polarization resistance of the uncoated Cu, R_pc is the measured polarization resistance of coated Cu, ΔE_corr is the difference between the corrosion potentials and b_a is the anodic Tafel slope for the uncoated Cu substrate. The calculated porosity values of the coatings are also given in Table 2. As seen in the Table, the porosity value of the coating increases with an increase in the applied current densities, indicating that the PZN coating will deposit with worse uniformity, regardless of the increase in the applied current density. Then, the coatings obtained at low currents are of the best quality than those obtained at high currents. High protective efficiency (higher than 99%) and low porosity (0.7) of the PZN coating confirmed the excellent performance as a protective layer to the copper.

For comparison aims, the potentiodynamic polarization curves for uncoated, polyaniline and PZN-coated Cu electrodes in an aqueous 3.5 wt% NaCl solution are shown in Fig. 9. The polarization parameters for polyaniline coating are also reported in Table 2. It can be seen that the corrosion current of PZN-coated sample is lower than for pure polyaniline-coated sample. Therefore, it was found that the presence of zeolite in polyaniline matrix, promotes the anticorrosive efficiency of PZN coating on Cu samples. Comparison of the corrosion rate and porosity of PZN-coated Cu showed that the anticorrosive properties of the PZN coating in 3.5 wt% NaCl solution are better than pure polyaniline-coated Cu. However enhanced corrosion protection of PZN compared to pure polyaniline coated samples might result from zeolite particles dispersed in polyaniline matrix which increase the tortuosity of the diffusion pathway of corrosive agents such as oxygen gas, hydrogen, hydroxide and chloride ions.

Fig. 9 Polarization curves for uncoated, polyaniline and polyaniline/zeolite nanocomposite coated copper electrodes in 3.5 wt% NaCl solution.

3.3.2. Electrochemical impedance spectroscopy (EIS). In order to investigate the anticorrosive properties of the obtained coatings, electrochemical impedance spectroscopy was employed, due to its non-disturbing and informative properties.

The typical Nyquist impedance plots of the polyaniline/zeolite nanocomposite and polyaniline-coated Cu synthesized under galvanostatic conditions at the current density of 2 mA cm⁻² are shown in Fig. 10. These impedance plots were modeled by the equivalent circuit depicted in Fig. 11. The equivalent circuit consists of the electrolyte resistance (R_s), the coating pore resistance (R_pore), coating capacitance (CPE_c), charge transfer resistance (R_ct) and double layer capacitance (CPE_dl). Instead of capacitance, a constant phase element (CPE) was used. The CPE represents the deviation from the true capacitance behavior.

Fig. 10 Typical Nyquist impedance plots for uncoated, polyaniline and polyaniline/zeolite nanocomposite coated copper in 3.5 wt% NaCl solution.

Fig. 11 Equivalent circuit model.

The impedance of a phase element is defined as Z_CPE = 1/[Q(jω)ⁿ], where the exponent n of the CPE is related to the non-equilibrium current distribution due to the surface roughness and surface defects. Q is a constant which represents the true capacitance of the oxide barrier layer. The CPE (which represents a deviation from the true capacitor behavior) is used here instead of an ideal double layer capacitance. The impedance of CPE with the value of n is often associated with a non-uniform current distribution due to the porous oxide layer. CPE describes an ideal capacitor for n = 1, an ideal resistor for n = 0, and a pure inductor for n = −1.⁵⁸

The Nyquist plots of the PZN-coated Cu synthesized under galvanostatic conditions at various current densities are shown in Fig. 12. Table 3 gives the values of the representative parameters of the best fit to the experimental data using the circuit shown in Fig. 11 as a function of applied current density for the galvanostatic deposition of the polyaniline/zeolite nanocomposite and pure polyaniline on Cu.

Fig. 12 Nyquist impedance plots for the polyaniline/zeolite nanocomposite coated on copper at various current densities (mA cm⁻²) in 3.5 wt% NaCl solution.

Table 3 Impedance parameter values of the electrosynthesized polyaniline/zeolite nanocomposite extracted from the fit to the equivalent circuit for the impedance spectra, recorded in aqueous 3.5 wt% NaCl solution as a function of applied current density

Iapplied (mA cm^−2)	Rs (Ω cm^2)	Q1		Rpore (Ω cm^2)	Rct (kΩ cm^2)	Q2		PE (%)
		n	Y (Ω^−1 cm^−2 s^n)			n	Y (Ω^−1 cm^−2 s^n)
Bare	6	0.5	987	—	7.5	—	—	—
PZN
2	29	0.7	10	61.3	61.2	0.6	561	87.7
4	14	0.4	51	34.0	31.4	0.6	803	76.1
6	26	0.7	173	15.4	14.3	0.6	985	47.5
8	11	0.6	169	21.3	18.8	0.7	911	60.1
10	15	0.7	143	26.2	23.6	0.6	940	68.2
Polyaniline
2	17	0.6	31	48.4	45.1	0.6	480	83.4

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As seen in the Table, the R_ct values increased from 7.5 kΩ cm² for uncoated Cu to 61.2 and 45.1 kΩ cm² for the PZN and polyaniline-coated Cu, respectively, at a lower applied current density. This indicates that the presence of zeolite into the polyaniline matrix exhibited better charge transfer resistances than for pure polyaniline-coated Cu electrodes. This was related to the increase of ohmic resistance and barrier properties, which cause a delay in transport of water, oxygen and ion species through the coating layer. Nyquist diagrams of PZN coatings electrosynthesized at varied current densities show that, much protective results were obtained when the copper electrode was coated at lower current densities. It was deduced from the Nyquist diagrams that PZN coating synthesized at 2 mA cm⁻² current density has the best corrosion resistance that this result was also obtained from polarization tests.

The coating pore resistance (R_pore) generally reflects the barrier ability of a particular coating in electrolyte solution. Thus, R_pore is an important parameter in the evaluation of the corrosion resistance of a coating.^59,60 The R_pore values of polyaniline and PZN coatings are given in the Table 3. R_pore decreased with the increase in the applied current density. The higher R_pore of PZN coating than that of polyaniline coating indicated the higher barrier ability of PZN because of the presence of the zeolite in the coating matrix. In the other words, zeolite improved the barrier ability of PZN coating. Electrosynthesized PZN coating at the 2 mA cm⁻² current density exhibited the highest R_pore values, indicating that it had the highest barrier ability against the electrolyte and the best corrosion protective properties on the copper surface.

The protection efficiency (PE) of the coating was obtained from the relationship and the measured charge transfer resistance values,⁵⁶

where R_ct(C) and R_ct are the charge transfer resistance values in the presence and absence of the coating, respectively. The protection efficiency values decreased with increasing applied current density, which confirms the obtained results of potentiodynamic polarization. This can be ascribed to the increase of the corrosion reaction rate, possibly through the presence of further pores in the coating or an increase in the area exposed at the base of the existing pores or flaws.

Several mechanisms have been suggested during recent years that basically consider a conducting polymer either as a barrier coating alone or as an active coating participating to the reactions taking place across the polymer-coated metal and electrolyte interface. The specific structures of polyaniline have a redox catalytic effect which lead to the formation of metal oxide layers. Upon immersion in 3.5 wt% NaCl solution, the electrochemical reactions at the metal/coating interface must be considered in the following sequences;⁶¹

Cu → Cu⁺ + e⁻ (1)

Cu⁺ → Cu²⁺ + e⁻ (2)

EM²⁺ + 2e⁻ ↔ LE (3)

O₂ + 2H₂O + 4e⁻ → 4OH⁻ (4)

As indicated in equations, copper is directly oxidized, leading to the formation of Cu₂O and later to a mixture of Cu₂O and CuO and acts as passivating layers. The formation of a thin oxide layer at the interface between the metal and the conducting polymer, which is believed to be the result of electrochemical reaction between the polymer and the copper surface, prevents significantly the movement of the electrolyte solution to the copper surface. Besides, it is reported^62,63 that in chloride media, the formation of cuprous chloride species is dominant and the dissolution of Cu is represented by a two step reaction in the following equations:

Cu + Cl⁻ → CuCl + e⁻ (5)

CuCl + Cl⁻ → CuCl₂⁻ (6)

Consequently, the corrosion rate of copper diminishes upon the formation of Cu(I)-complexes, Cu(I) and Cu(II) oxides with polyaniline.

It is also reported that in chloride media, the conducting polymer could act as a reservoir for releasing corrosion inhibiting ions and the increase of the corrosion protection properties might be a consequence of cathodic de-doping of anions from polymer film or substituting the anions by the chloride ions in the conducting polymer backbone, according to following equations:³⁴

(PANI^y+(A⁻)_y)_n + nye → (PANI)_n + nyA⁻ (7)

(PANI^y+(A⁻)_y)_n + nCl⁻ → (PANI^y+(Cl⁻)_y)_n + nA⁻ (8)

Also, these occurrences enhance and stabilize the formation of protective insoluble copper compounds.

3.4. Investigation of surface morphology

In order to observe the effects of corrosion media on surface morphology of the coating on copper, SEM micrographs were taken before and after immersion in NaCl solution, and given in Fig. 13. A comparison of images a and b show that numerous large pits and inequalities were formed after exposure to corrosion media, which reveals severe damage on the surface due to metal dissolution. Image c shows that the polyaniline/zeolite nanocomposite coating was electrodeposited on the surface and protected it from the corrosion after immersion in NaCl solution.

Fig. 13 Scanning electron micrographs of abraded Cu (a), pre-treated Cu after corrosion in 3.5 wt% NaCl solution (b) and the polyaniline/zeolite nanocomposite coating after corrosion in 3.5 wt% NaCl solution (c).

4. Conclusions

In this article, we found that electrosynthesized polyaniline/zeolite nanocomposite coating at low zeolite loading showed an advanced corrosion protection effect compared to pristine polyaniline through a series of electrochemical measurements on copper in 3.5 wt% aqueous NaCl solutions.

It was found that polyaniline/zeolite nanocomposite coatings can provide considerable protection, as well as a physical barrier against corrosive environments in which the metal are exposed. The corrosion rate for the PZN-coated copper was significantly lower than the bare copper (∼130 times).

The presence of the zeolite in the polyaniline matrix was confirmed by FTIR, UV-vis and XRD studies. It is indicated that these films can improve the corrosion resistance of the substrate in a 3.5 wt% NaCl solution at room temperature. The influence of current density applied at the electrosynthesis stage was examined and it is indicated that the coatings obtained by applying a current density of 2 mA cm⁻², have better corrosion resistance properties than other coatings.

It was also found that the formation of PZN coatings on copper involved three distinct stages: electro-adsorption of monomer and electrolyte and initiation of formation of passive film; growth and impingement of the passive film and decomposition of the latter and formation of composite coatings. These coatings, with a uniform and green colored surface, reduced the corrosion current density from 43.56 μA cm⁻² to 0.34 μA cm⁻² and shifted positively corrosion potential from −0.314 V to −0.141 V.

It can be seen from the results presented that adherent and homogenous PZN coatings can be electrodeposited onto Cu in the presence of oxalate anions. These coatings exhibit significant corrosion protection properties in highly aggressive chloride-containing solutions. The formation of polyaniline at the copper surface appears to be associated with the passivation of the copper surface by an oxalate layer. All attempts to electropolymerize polyaniline, from an acid solution, failed because the copper electrode remained active in these media. However, this oxalate layer appears to be sufficiently protective to decrease the rate of copper dissolution, but also not sufficiently insulating to inhibit the electropolymerization process.

Acknowledgements

The authors are grateful to University of Kashan for supporting this work by Grant No. (434066-5).

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