Electrochemical corrosion behavior of carbon steel coated by polyaniline copolymers micro/nanostructures

Abstract

A series of polyaniline (PANI) copolymers micro/nanostructures were prepared in order to investigate the effect of different groups with different hydrophilicities on the corrosion protection behavior of polyanilines. Hydrophilic groups (–SO₃H and –COOH) and hydrophobic groups (–CH₃ and –C₂H₅) were introduced into the polyaniline molecular structure by the copolymerization of aniline (ANI) and 3-aminobenzenesulfonic acid, 3-aminobenzoic acid, 3-toluidine and 2-ethyl aniline using ammonium persulfate as an oxidant. The carbon steel coated by the resultant PANI copolymers micro/nanostructures were investigated and compared regarding their electrochemical corrosion in 0.1 M H₂SO₄. Potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) were used. It was found that neither PANI nor PANI copolymers micro/nanostructures provide anodic corrosion protection for carbon steel, but they can effectively restrict the cathodic reaction. Moreover, the ability to restrict the cathodic reaction depends on the surface wettability of the PANI and PANI copolymers micro/nanostructures. The corrosion protection of PANI and PANI copolymers increased along with their water repellency. A well-fitting correlation between the contact angle (θ) of the PANI or PANI copolymers micro/nanostructures coatings and their corrosion protection efficiency (η) was found to be η = 102.22 − 102.78 exp(−θ/84). The PANI copolymer with the hydrophobic –C₂H₅ group (PANI-C₂H₅) showed the largest static water contact angle (CA = 125°) and the most effective protection with an inhibition efficiency of 78.98%.

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1. Introduction

Corrosion poses a serious economic and industrial threat, as well as potential danger to humans. One of the current industrial practices for corrosion protection is to treat the surface of metals with chromium-containing compounds. However, chromium-containing coatings will be eventually banned^1,2 because of its adverse health and environmental effects. Conducting polymers have been considered to replace environmentally unfavorable materials because of their potential environment-friendly metal anticorrosion coatings. Among the conducting polymers used for the corrosion protection of metals, polyaniline (PANI) and its derivatives are the most studied ones. In recent years, several papers have reported that PANI and its derivatives display good corrosion inhibiting properties.^3–10 Besides PANI,^11,12 substituted polyanilines, e.g., poly(2-ethoxyaniline),¹³ poly(N-methylaniline),¹⁴ poly(2-methoxyaniline),¹⁵ poly(N-ethylaniline)¹⁶ and poly(2,5-dimethylaniline),¹⁷ as well as PANI copolymers, e.g., poly(aniline-co-2-anisidine), poly(aniline-co-2-iodoaniline) and poly(aniline-co-2-chloroaniline) were investigated and showed good corrosion protection properties. Various explanations have been suggested for this behavior. Some investigators believe that PANI acts as a simple aniline-type inhibitor, blocking the active points on the metals surface.¹⁸ Wessling et al. reported that PANI protected the metal by forming a protective passivation oxide layer on the metal surface induced by its redox catalytic capability, rather than by acting as a barrier.^5–10 Despite the existing different points of view about the protection mechanism of PANI or its derivatives, there is still not enough data in the literature to systematically explain how the different groups in PANI derivatives affect the electrochemical corrosion. In this paper, hydrophobic (–CH₃ and –C₂H₅) and hydrophilic (–SO₃H, –COOH) groups were introduced into PANI molecular chains by the copolymerization of aniline and the appropriate substituted aniline, and thus PANI copolymers micro/nanostructures were obtained. The electrochemical corrosion of a carbon steel electrode coated with PANI and PANI copolymer micro/nanostructures in 0.1 M H₂SO₄ was investigated and compared using potentiodynamic and electrochemical impedance measurements. The effect of the PANI molecular chains-substituted groups (–SO₃H, –COOH, –CH₃ and –C₂H₅) on the wettability and its possible influence on corrosion protection are also discussed.

2. Experimental

2.1 Materials

Aniline (ANI, Beijing Chem. Co.) was distilled under reduced pressure prior to use. The other reagents, metanilic acid (MA), 3-aminobenzoic acid (ABA), 3-methyl aniline (MA), 2-ethyl aniline (EA) and ammonium persulphate (APS) were purchased from Beijing Chem. Co. and used without further purification. The aqueous solutions used in the experiments were prepared using deionized water. The composition (wt%) of the carbon steel was C (0.14–0.22%), Mn (0.3–0.65%), Si ≤ 0.3%, S ≤ 0.05%, and P ≤ 0.045%.

2.2 Preparation of PANI copolymers micro/nanostructures

PANI copolymer micro/nanostructures were prepared by copolymerization of aniline and a ring-substituted aniline. Taking poly(aniline-co-methyl aniline) (PANI-CH₃) as an example, a typical synthesis procedure of a PANI copolymer micro/nanostructures was performed as follows: aniline monomer (0.1 mL, 1 mmol) and 2-methyl aniline (0.12 mL, 1 mmol) were mixed in 10 mL of deionized water in an ice bath under ultrasonic stirring for 1 min to form white emulsion. 5 mL of H₃PO₄ (1 mmol) solution was added to the above emulsion. Then, an aqueous solution of APS (0.46 g in 5 mL of deionized water) was added to the above mixture. The polymerization was carried out for 12 h in the ice bath. A green solid of poly(aniline-co-methyl aniline) was obtained after rinsing with H₂O, CH₃OH, and CH₃CH₂OCH₂CH₃ three times. Other PANI copolymers, poly(aniline-co-ethyl aniline) (PANI-C₂H₅), poly(aniline-co-3-metanilic acid) (PANI-SO₃H), poly(aniline-co-3-aminobenzoic acid) (PANI-COOH) and PANI micro/nanostructures were synthesized using a similar method.

2.3 Characterization of PANI copolymers micro/nanostructures

The morphology of the products was characterized by field-emission scanning electron microscopy (FESEM, JSM-6700F). The molecular structures were characterized by FTIR, UV-vis spectroscopy and X-ray diffraction. The FTIR spectra were recorded using an IFS-113V instrument. UV-vis spectroscopy was measured by Hitachi UV3100. X-ray diffraction was determined by a Micscience Model M18XHF diffractometer (MAC SCIENCE, JAPANI).

2.4 Electrochemical measurements

The potentiodynamic and electrochemical impedance measurements were performed on commercially available A₃ carbon steel sheets (10 mm × 10 mm). A copper wire was spot welded to the carbon steel and encapsulated with an epoxy resin. The carbon steel electrodes were sequentially polished by emery paper (1000 grit and 5000 grit), and they were then treated with acetone and ethanol to degrease prior to coating.

The PANI or PANI copolymers coatings were prepared as follows: PANI or PANI copolymers micro/nanostructures (20 mg) were dispersed in 2 mL of n-butyl alcohol and ultrasonically treated for 5 min. The dispersion was drop-cast onto the carbon steel electrode and allowed to dry at room temperature to form PANI or PANI copolymer coatings. All coatings had a thickness in the range of 20–22 μm.

The potentiodynamic and electrochemical impedance measurements on the samples were run in a typical three-electrode system consisting of the carbon steel electrode as the working electrode (WE), platinum sheet as the counter electrode (CE) and a saturated calomel electrode as the reference electrode. The reference electrode was connected to a Luggin capillary. The corrosion environment was provided by 0.1 M H₂SO₄ aqueous solutions. Potentiodynamic polarization curves were obtained by automatically changing the electrode potential from −250 mV_SCE to +250 mV_SCE, versus open circuit potential with a constant sweep rate of 1 mV s⁻¹. Electrochemical impedance measurements were performed in the frequency range of 10⁵ to 10⁻² Hz with an amplitude of 10 mV. The real (Z′) and imaginary (Z′′) components of the impedance spectra in the complex plane were analyzed using a non-linear least squares (NLS) fitting program to estimate the parameters of the equivalent electrical circuit. The electrochemical measurements were run on an AUTOLAB PGSTAT302N.

From the Tafel polarization studies, the protection efficiency was obtained using the following equation:

Protection efficiency (%) = [i_corr − i_corr(C)] × 100/i_corr

where i_corr and i_corr(C) are the corrosion current density values in the absence and presence of PANI or PANI copolymers coatings.

All electrochemical measurements were repeated 3–5 times at room temperature.

3. Results and discussion

3.1 DC polarization and EIS

Fig. 1 shows the polarization curves of carbon steel coated by PANI and PANI copolymer micro/nanostructures obtained in 0.1 M H₂SO₄. The corresponding corrosion current density (i_corr) and inhibition efficiency (η) obtained from the extrapolation of anodic and cathodic Tafel lines are given in Table 1. It can be seen from Fig. 1 that the anodic reactions of carbon steel coated with PANI or PANI copolymers micro/nanostructures have no obvious change compared with that of uncoated carbon steel. This result suggests that PANI and PANI copolymers micro/nanostructures cannot provide anodic protection for carbon steel. However, the cathodic currents decrease when the carbon steel is coated with PANI or PANI copolymers micro/nanostructures. The current density decreases in the following order: PANI-SO₃H > PANI-COOH > PANI > PANI-CH₃ > PANI-C₂H₅. Compared with the bare carbon steel (0.2350 mA cm⁻²), the corrosion current density of PANI-SO₃H (0.1956 mA cm⁻²), PANI-COOH (0.1889 mA cm⁻²), PANI (0.1802 mA cm⁻²), PANI-CH₃ (0.1499 mA cm⁻²) and PANI-C₂H₅ (0.04939 mA cm⁻²) is decreased by 16.77%, 19.62%, 23.32%, 36.19% and 78.98%, respectively, (Table 1). This result shows that PANI and PANI copolymer coatings can efficiently restrict the cathodic reaction in 0.1 M H₂SO₄. Therefore, we can conclude that the corrosion protection of PANI and PANI copolymer micro/nanostructures derives from the cathodic protection in this potential range. It is clear that PANI-CH₃ and PANI-C₂H₅ coatings provided a more effective restriction of the cathodic current density compared to PANI and PANI copolymers with hydrophilic groups (PANI-SO₃H and PANI-COOH). PANI-C₂H₅ had the highest inhibition efficiency (78.98%).

Fig. 1 Tafel polarization curves of uncoated carbon steel (a) and carbon steel coated with PANI and different PANI copolymers micro/nanostructures in 0.1 M H₂SO₄, (b) PANI-SO₃H, (c) PANI-COOH, (d) PANI, (e) PANI-CH₃ and (f) PANI-C₂H₅.

Table 1 Tafel polarization curves parameters and impedance data for bare carbon steel and carbon steel coated by different PANI copolymers in 0.1 M H₂SO₄

Sample	Contact angle (°)	Polarization method			Impedance method
		Ecorr (mv)	icorr (mA cm^−2)	η (%)	Rs (Ω cm^2)	Rct (Ω cm^2)	CPE		RL (Ω cm^2)	L (kH cm^−2)
							YO	n
Bare carbon steel	—	−496.58	0.2350	—	2.97	50.0	304.39	0.833
PANI-SO3H	15	−508.04	0.1956	16.77	4.53	101.24	114.84	0.877
PANI-COOH	18	−515.18	0.1889	19.62	3.06	109.72	117.43	0.865
PANI	24	−513.40	0.1802	23.32	3.71	115.9	115.55	0.881
PANI-CH3	36	−510.87	0.1499	36.19	6.35	151.36	101.57	0.867
PANI-C2H5	125	−517.51	0.04939	78.98	11.5	270.0	79.6	0.82	650	11.0

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The electrochemical impedance technique was used to investigate the corrosion processes, which occur at a covered electrode/electrolyte interface. For PANI-SO₃H, PANI-COOH, PANI and PANI-CH₃, the Nyquist plots with almost one capacitive loop (Fig. 2A(b–e)). The impedance diagram of PANI-C₂H₅ consists of a large capacitive loop at high frequencies and an inductive loop at low frequencies (Fig. 2A(f)). This inductive loop may be originated from the relaxation process that occurs when species as H⁺ and SO₄²⁻ are absorbed on the surface of the PANI or PANI copolymers.¹⁹ The Bode plots are shown in Fig. 2B. The impedance data were analyzed by the equivalent circuit shown in Fig. 3. In the equivalent circuit model: R_s is solution resistance, R_ct is charge transfer resistance from the electrolyte to the coating.²⁰ PANI and PANI copolymers have similar features to those of the bare carbon steel, with a depressed, capacitive-like semicircle and its diameter along the real axis (Fig. 2). It is clear that the size of the diameter of the capacitive-like semicircle associated with the polarization resistance varies when the carbon steel electrode is coated by PANI and PANI copolymer micro/nanostructures. These diameters increased in the following order: PANI-SO₃H < PANI-COOH < PANI < PANI-CH₃ < PANI-C₂H₅. EIS measurements suggest that the corrosion resistance (R_ct) increases in the following order: PANI-SO₃H < PANI-COOH < PANI < PANI-CH₃ < PANI-C₂H₅. PANI-C₂H₅ showed the best corrosion protection. This result is in good agreement with that obtained by potentiodynamic polarization experiments.

Fig. 2 Nyquist plots (A) and Bode plots (B) of carbon steel-coated with PANI copolymers and PANI in 0.1 M H₂SO₄. (a) Blank, (b) PANI-SO₃H, (c) PANI-COOH, (d) PANI, (e) PANI-CH₃ and (f) PANI-C₂H₅.

Fig. 3 Equivalent electric circuits used to simulate the EIS results for (a) bare carbon steel, PANI-SO₃H, PANI-COOH, PANI and PANI-CH₃ and (b) PANI-C₂H₅.

Both the potentiodynamic polarization and the electrochemical impedance experiments show that corrosion protection of PANI and PANI copolymer coatings to the carbon steel changed in the following order: PANI-SO₃H < PANI-COOH < PANI < PANI-CH₃ < PANI-C₂H₅. It is obvious that the substituent groups on the PANI molecular chains have an important effect on the corrosion protection of PANI copolymers, and hydrophobic groups facilitated anticorrosion. The effect of PANI molecular chain substituent groups on the corrosion protection may be related to their wettability. The contact angles of PANI or PANI copolymers coatings increase from 15° to 125° when the substituent groups changed from hydrophilic –SO₃H to hydrophobic –C₂H₅. Correspondingly, the corrosion protection efficiency increased from 16.77% to 78.98% and R_ct obtained from Nyquist plots increased from 101.24 to 270.00 Ω cm². The PANI-C₂H₅ coatings have the largest contact angle and offered the best corrosion protection to carbon steel. It is noted that there is a well-fitting correlation between the contact angle (θ) of the PANI or the PANI copolymer micro/nanostructures coatings and their corrosion protection efficiency (η), as shown in Fig. 4 (solid squares). η can be expressed as

η = 102.22 − 102.78 exp(−θ/84) (1)

where η is the corrosion protection efficiency and θ is the contact angle (0° < θ < 180°). Eqn (1) indicates that an increased corrosion protection efficiency of PANI or PANI copolymers micro/nanostructures coatings can be accomplished by improving their contact angle.

Fig. 4 The effect of the contact angle (θ) of the PANI or PANI copolymers micro/nanostructures coatings on their corrosion protection efficiency (η).

In order to confirm the applicability of the formula, 2-methoxyaniline is chosen to prepare PANI copolymers [i.e. poly(aniline-co-2-methoxyaniline)] coatings to protect carbon steel. It was found that the contact angle of poly(aniline-co-2-methoxyaniline) micro/nanostructures coatings is 41°, and its corrosion protection efficiency is 42.43%. This was in good agreement with the theoretical 39.13% calculated using eqn (1). Thus, the formula is also applicable when poly(aniline-co-2-methoxyaniline) micro/nanostructures is used to protect carbon steel (Fig. 4, open square). In order to further prove the universality of the formula, 3-aminobenzeneboronic acid was chosen to prepare poly(aniline-co-3-aminobenzeneboronic acid) micro/nanostructures. As shown in Fig. 4 (triangle), the contact angle of the poly(aniline-co-2-methoxyaniline) micro/nanostructures coatings is 26°, and its corrosion protection efficiency is 23.16%, which also fits the formula well. Eqn (1) indicates that the corrosion protection efficiency increases along with the contact angles of the PANI or PANI copolymers coatings. The change in the corrosion protection efficiency with the changes in the contact angles of the PANI or the PANI copolymers coatings can be confirmed by electrochemical impedance measurements.

In the case of the corrosion protection of metals provided by conducting polymers, it was reported that conducting polymers act as a barrier coating alone or promote to form a protective passivation layer on the surface of the metal because of the redox chemistry of conducting polymers and their good electronic conductivity^5,9 or because they act as an active coating, participating in the reaction occurring on the polymer-coated metal electrolyte interface.^21,22 However, the corrosion potential of carbon steel coated by PANI and PANI copolymers shifts negatively in our work. Similar results have been reported before.^23,24 This negative shift of corrosion potential is not hard to understand. The passivation layer is usually formed on the surface of those metals that are passivated easily (e.g. Al and stainless steel). This leads to a positive shift of corrosion potential.^25–27 In the case of carbon steel, a higher current is required to passivate it. PANI or PANI copolymers cannot provide such a high current for the carbon steel to stay in the passivation region and the carbon steel stays in the active corrosion region. The fact that no passivation layer is formed on the surface of the carbon steel can be confirmed by the polarization curve. As suggested by the polarization curve, the PANI and the PANI copolymer coatings only provided cathodic protection to the carbon steel in our work. Regarding the conductive properties of PANI or PANI copolymers, it is reasonable to believe that the cathodic hydrogen evolution reaction mainly occurs at the surface of PANI (or PANI copolymers) micro/nanostructures. A possible corrosion mechanism was proposed as follows:

The anodic corrosion process:

Fe → Fe²⁺ + 2e⁻ (2)

The cathodic process:

H₃O⁺ → H⁺ + H₂O (3)

H⁺ + PANI(e) → PANIH (4)

PANIH + PANIH → H₂ + 2PANI (5)

PANIH + H⁺ + PANI(e) → H₂ + 2PANI (6)

The anodic process is driven by the dissolution of the metal and formation of soluble Fe²⁺ ions. The cathodic reaction involves the evolution of hydrogen gas. In the cathodic processes, hydronium ions (H₃O⁺) diffuse towards the PANI or PANI copolymers coatings at first, and they are absorbed by their surface. Then H⁺ receives an electron to become a hydrogen atom. H₂ can be formed by the reaction of hydrogen atoms and released from the surface of PANI or PANI copolymers. The evolution of hydrogen gas depends on reaction (4), which is the decisive one. It is obvious that this plays a vital role in reaction (4). Therefore, it is expected that the effective corrosion protection capability of the PANI and PANI copolymers results from the resistance of H⁺ absorbed onto the surface of their micro/nanostructures. In the case of the PANI-C₂H₅ coatings, the hydrophobic surface (CA = 125°) can stop the water molecules absorbing onto their surface, which is the decisive step in the cathodic process. Therefore, PANI-C₂H₅ coatings can effectively inhibit cathodic process because of their hydrophobic surface. Thus, PANI-C₂H₅ coatings offered the best corrosion protection to carbon steel. This result is in accordance with those reported in previous literature,^28,29 i.e., good repellent properties of protection coatings could provide an effective barrier to stop water and H⁺ from absorbing on the surface of the protection coatings, thus resulting in better corrosion protection. This is in good agreement with the result obtained from eqn (1).

It is well known that the substitute groups in PANI copolymers not only affect the wettability of their coatings, but also their conductivity and porosity. The conductivity and porosity of the conducting polymer coating will also affect the corrosion protection. It can be seen from the Tafel polarization curves (Fig. 1) that the anodic reactions of carbon steel coated with different PANI copolymers micro/nanostructures have no obvious change. On the other hand, the cathodic currents decrease when the carbon steel is coated with PANI copolymers micro/nanostructures. This result suggests that PANI copolymers micro/nanostructures have little effect on the anodic protection of carbon steel. Thus, it is reasonable to assume that the conductivity and porosity of the PANI copolymers coatings will affect the cathodic reaction. Even though the conductivity of the PANI copolymers may affect the evolution of hydrogen gas from the PANI copolymers surface, the influence can be neglected because the evolution of hydrogen gas is not the decisive process. Regarding the conductive properties of PANI or PANI copolymers, it is reasonable to believe that cathodic hydrogen evolution reaction mainly occurs at the surface of their micro/nanostructures. The holes in the PANI or PANI copolymers can be occupied by the released H₂, and therefore, the porosity of the coatings can be ignored. We believe that effective corrosion protection capability of the hydrophobic PANI copolymers results from the resistance of H⁺ absorbed onto the surface of PANI copolymer micro/nanostructures. The corrosion protection mechanism details will be further investigated in our future work.

3.2 The morphology and wettability of PANI and PANI copolymers coatings

The PANI copolymer micro/nanostructures were prepared by the copolymerization of aniline and ring-substituted anilines at [ANI]/[ring-substituted ANI] = 1. Fig. 5 shows the typical SEM images of the as-synthesized PANI copolymers. For PANI, the products were composed of uniform fibers with average diameters of ∼110 nm and length of up to several micrometers. Nanoparticles were also observed on the surface of the PANI fibers. As seen in Fig. 5, ring-substituted anilines have obvious effects on the morphologies of the PANI copolymers. In the case of PANI and PANI-COOH, products were of similar fibrous morphologies (Fig. 5b) with a smaller average diameter of ∼60 nm. Furthermore, the PANI-COOH fibers interconnected to form net-like nanostructures. When the PANI copolymer was prepared by aniline and 3-metanilic acid, the products were made up of granular particles with diameters of 100–300 nm and significant agglomeration occurred, as shown in Fig. 5a. PANI-CH₃ (Fig. 5d) is dominated by nanofibers (100–300 nm in diameter), and only a small portion of granular particles is formed. Fig. 5e presents the SEM images of PANI-C₂H₅, and the products consist of accumulated flakes, blocks and granular particles.

Fig. 5 SEM images and static water contact angles of PANI and PANI copolymers synthesized at [ANI]/[ring-substituted ANI] = 1. (a) PANI-SO₃H, (b) PANI-COOH, (c) PANI, (d) PANI-CH₃ and (e) PANI-C₂H₅.

The wettability of PANI and PANI copolymers micro/nanostructures was evaluated in terms of water contact angle (CA) measurements using 2–3 μL of water, as shown in Fig. 5. CA for the PANI micro/nanostructures was estimated to be 24° and exhibited a strong hydrophilic character (Fig. 5c inset). PANI-SO₃H and PANI-COOH have even lower contact angles than PANI, 15°and 18°, respectively. On the contrary, PANI-CH₃ (CA = 36°) and PANI-C₂H₅ (CA = 125°) are more hydrophobic than PANI. It is well known that CA depends not only on the chemical structures but on the surface roughness of the coatings.³⁰ All the PANI copolymers and PANI coatings have rough surfaces. For a rough surface, Cassie and Baxter proposed the relationship between the CA of a water droplet on (θ_r) in air can be shown in the following:³¹

cos θ_r = f₁ cos θ − f₂ (7)

where f₁ and f₂ are the fractions of solid surface and air in contact with water, respectively (f₁ + f₂ = 1). It is easy to deduce from eqn (7) that an increase in f₂ can cause an increase in θ_r. So a relatively rough surface could have a large CA because it can let air fill in the aperture, thus reducing the contact area between water and the coatings. Therefore, the fact that PANI-SO₃H and PANI-COOH copolymers with hydrophilic groups (–SO₃H and –COOH) have lower contact angles than PANI may be due to the existence of the hydrophilic groups on the PANI backbone and the smoother surface (Fig. 5a and b). The fact that PANI-CH₃ and PANI-C₂H₅ show better hydrophobic properties than PANI may result from the hydrophobicity of the –CH₃ or –C₂H₅ groups in the PANI copolymers. Especially, PANI-C₂H₅ could greatly increase the hydrophobicity, and the contact angle reaches up to 125° (which is characteristic of a hydrophobic surface). Coatings with a high contact angle can be expected to prevent the absorption of water and thus to effectively inhibit the corrosion reaction.

3.3 Characterization

The molecular structures of the PANI copolymers were characterized by means of FTIR spectra, UV-vis spectra and XRD pattern. The typical FTIR spectra of the PANI copolymers is in the range 4000–400 cm⁻¹, as shown in Fig. 6. The peak at about 1580 cm⁻¹ is due to the C=C double bond of the quinoid rings,^32,33 whereas the peak at about 1500 cm⁻¹ arises from the vibration of the C=C bond associated with the benzenoid ring.^34,35 The presence of quinoid and benzenoid bands clearly shows that the PANI copolymers are composed of amine and imine units. The absorption peaks at 1310 cm⁻¹ and 1118 cm⁻¹ are characteristic of aromatic C–N stretching.³⁴ The peak at 816 cm⁻¹ is characteristic of para-disubstituted aromatic rings, indicating polymer formation.^25–37 Actually, these peaks are similar to those of PANI-HCl prepared by a common method.³⁸ The peak at 1030 cm⁻¹ (due to S=O stretching) confirms the presence of sulfonate groups in the polymers.³⁶ The S–O stretching bands are seen at 700 cm⁻¹, and the peaks at 630 cm⁻¹ are caused by the SO₃⁻ stretching vibrational mode. These absorptions (with maxima at 1030, 700 and 630 cm⁻¹) are consistent with the presence of SO₃⁻ groups attached to the aromatic rings.³⁷ These results suggest that the backbone structures of the copolymers obtained in this study are similar to each other and also to those of PANI-HCl³⁹ reported elsewhere.

Fig. 6 FTIR of PANI and PANI copolymers synthesized at [ANI]/[ring-substituted ANI] = 1. (a) PANI-SO₃H, (b) PANI-COOH, (c) PANI, (d) PANI-CH₃ and (e) PANI-C₂H₅.

Moreover, the UV-vis spectra of the copolymer micro/nanostructures dissolved in m-cresol solvent show that they are in the emeraldine oxidation state with absorption maxima around 310 nm and a weak peak at 800 nm with a long tail (Fig. 7). The band around 310 nm can be attributed to the overlap of the π–π* transition of the benzoid rings of PANI and the azobenzene moiety.⁸ The band at about 800 nm with a long tail is assigned to the polaron band, which typically characterizes protonation and is identical to that of the emeraldine salt of PANI. The band at 430 nm corresponds to the partial oxidation of PANI and represents the intermediate state between the leucoemeraldine form containing benzenoid rings and the emeraldine form containing conjugated quinoid rings in the main chain of the PANI.

Fig. 7 UV-vis absorption spectra of the PANI and PANI copolymers synthesized at [ANI]/[ring-substituted ANI] = 1. (a) PANI-SO₃H, (b) PANI-COOH (c) PANI, (d) PANI-CH₃ and (e) PANI-C₂H₅.

The XRD patterns of PANI copolymer micro/nanostructures are shown in Fig. 8. PANI-SO₃H and PANI-COOH showed two peaks centered at 2θ = 20° and 25°, which are attributed to the periodicity perpendicular and parallel to the PANI polymer chain, respectively.⁴⁰ One characteristic peak centered at 2θ = 25° is observed for PANI-CH₃ and PANI-C₂H₅ (Fig. 8d and e). This result indicates that the PANI copolymers are amorphous. However, PANI shows higher crystallinity than the PANI copolymers and has three characteristic peaks at 2θ = 15°, 21° and 25°. Especially, a sharp peak is observed at 2θ = 6.5°, which represents the periodical distance between the dopant and the N atom on the adjacent main chain,⁴¹ indicating that a short-range order exists between the counter anion and the polymer chain in the PANI micro/nanostructures.

Fig. 8 X-ray diffraction patterns of the PANI and PANI copolymers. (a) PANI-SO₃H, (b) PANI-COOH, (c) PANI, (d) PANI-CH₃ and (e) PANI-C₂H₅.

4. Conclusion

PANI and PANI copolymers were prepared by the copolymerization of aniline, and substituted PANI was prepared through a simple chemical polymerization method. The electrochemical measurements for PANI and PANI copolymers revealed that in the range of open potential, PANI or PANI copolymer micro/nanostructures do not provide anodic protection to carbon steel but can effectively restrict the cathodic reaction. The anti-corrosion performance of PANI copolymers increased along with the water repellency of the PANI copolymers, and a well-fitted relationship between the wettability of PANI copolymers and corrosion protection efficiency was observed. PANI copolymers micro/nanostructures with hydrophobic –C₂H₅ groups (PANI-C₂H₅) showed the largest static water contact angle (CA = 125°) and the most effective protection with an inhibition efficiency of 78.98%. It is believed that the hydrophobic surface can stop the water molecules absorbing onto the surface of PANI-C₂H₅ coatings, which is the decisive step in the cathodic process.

Acknowledgements

This project was supported by the National Natural Science Foundation of China (no. 50973098) and National Science and Technology Support Program (2012BAB15B02).

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