Electrochemical behavior, characterization and corrosion protection properties of poly(bithiophene+2-methylfuran) copolymer coatings on A304 stainless steel

3.1 Electropolymerization of bithiophene on A304 stainless steel

The successive cyclic voltamperograms relative to bithiophene 10⁻²m in CH₃CN/LiClO₄ (10⁻¹m), on an A304 stainless steel electrode, recorded between the potential range 0 and 2 V/SCE, with a scan rate of 50 mV·s⁻¹ are shown in Figure 1.

Figure 1:

Cyclic voltammograms (cycling) corresponding to the bithiophene 10⁻²m in CH₃CN/LiClO₄ (10⁻¹m) solution, recorded between 0 V and 2 V vs. SCE, at a scan rate of 50 mV·s⁻¹, on an A304 electrode (Ø=2 mm).

It is observed that during the positive potential sweep, a shoulder which appears only in the first cycle at 1.32 V vs. SCE and a second at 1.65 V vs. SCE, are attributed, respectively, to the first and second oxidation of BTh. During the negative potential scan, a cathodic peak was observed around 0.75 V vs. SCE preceded by a shoulder at 1.41 vs. SCE which disappears during the successive scanning of the potential, corresponding to the reduction of the formed polymer film PBTh/A304. After the second cycle, the vague and peak sets are observed as a single intensive peak around 1.55 V vs. SCE.

The intensity of the latter increases during the cycling. This indicates that the polymer is in the process of settling on the A304 electrode, which is covered completely by a film of PBTh. This kinetic behavior has previously been observed by the work Nessark et al. (23) when they filed the terthiophen on 304-stainless steel. The polymer film is accompanied by a reversible change in color that changes from red brick during oxidation to blue night during the reduction.

3.2 Electropolymerization of 2-methylfuran on A304 stainless steel

Figure 2 shows the cyclic voltammograms of MeFu in a CH₃CN/LiClO₄ solution (10⁻¹m) recorded between 0 V and 2 V vs. SCE, at a scan rate of 50 mV·s⁻¹.

Figure 2:

Cyclic voltammograms (cycling) corresponding to the 2-methylfuran 10⁻²m in CH₃CN/LiClO₄ (10⁻¹m) solution, recorded between 0 V and 2 V vs. SCE, at a scan rate of 50 mV·s⁻¹, on an A304 electrode (Ø=2 mm).

The voltammogram shows an anodic wave at 1.54 V vs. SCE in positive scan potential, this is characteristic of MeFu monomer oxidation. This wave disappeared after the second cycle when a film of PMeFu was obtained on the electrode, and a low current was observed from the second cycle. This shows that the PMeFu plays an important role and the kinetics help protect the metal against corrosion, as was shown with other studies (24). The negative cathodic scan peak at 0.64 V vs. SCE was attributed to the reduction of PMeFu/A304. A slight decrease of the current intensities of the anodic and cathodic peaks was observed. This is probably due to the inhibition of the surface by an insulating polymeric film which was carried out on the electrode. Nessark et al. (25) also suggested that the current intensity of the anodic peak decreases during cycling and it stabilizes after many cycles when the electrode surface is coated by an insulating film.

3.3 Electropolymerization of (bithiophene +2-methylfuran) copolymer

Figure 3 represents the cyclic voltammograms relative to an A304 steel electrode in CH₃CN/LiClO₄ (10⁻¹m) solution, containing BTh (10⁻²m) and MeFu for different concentrations (10⁻⁴, 10⁻³ and 10⁻²m), which were obtained at a scan rate recording at v=50 mV·s⁻¹, between 0 V and 2 V vs. SCE.

Figure 3:

Cyclic voltammograms relating to a solution of bithiophene 10⁻²m dissolved in CH₃CN/LiClO₄ (10⁻¹m), obtained for different concentrations of MeFu [(a) 10⁻⁴, (b) 10⁻³ and (c) 10⁻²m], recorded at v=50 mV·s⁻¹, between −1.2 V and 2 V vs. SCE, on an A304 electrode (superposition of the first cycles).

The obtained cyclic voltammograms during the positive scan potential, show an anodic peak at 1.35 V followed by a shoulder at 1.46 V vs. SCE which are characteristic for the oxidation of the two monomers (bithiophene and 2-methylfuran) present in the solution. During cathodic scan potential, there was a peak at 0.72 V vs. SCE corresponding to the reduction of the copolymer.

It is also shown that there was a slight decrease of the current intensities of the anodic and cathodic peaks. This is accompanied by a slight shift of the anodic peak potential towards less positive values and the cathodic peak towards more negative values, with an increase in MeFu concentration. This suggests that the presence of the latter inhibits the electropolymerization reaction of bithiophen and contributes to the formation of a less conducting film. A similar behavior was observed on a platinum electrode (26).

3.4 Potentiodynamic polarization curves

Although the Tafel extrapolation method is usually insufficient to understand the protective properties of organic polymer coatings drivers, this method can be used to compare corrosion performances of various types of coating polymers (27).

In this part we examine the evolution of electrochemical parameters of A304, PBTh film in the absence and presence of various concentrations of 2-methylfuran. Before each immersion, A304 electrode was cleaned by polishing with sand paper. Then it was rinsed with distilled water. The Tafel curves were recorded at a scan rate of 50 mV·s⁻¹, the film was analyzed in a corrosive medium (H₂O/H₂SO₄ 1 N) after immersion for 30 min.

Figure 4 shows the Tafel curves corresponding to A304 with and without a coating of the copolymer being studied. The electrochemical parameters such as corrosion potential (E_corr), corrosion current density (i_corr), anodic and cathodic Tafel slopes (b_a and b_c), polarization resistance (R_p) values and protection efficiencies (η_p) are summarized in Table 1. The percentage protection efficiencies (η_p) of coating were calculated using the relationship:

Figure 4:

Tafel curves corresponding to the electrode modified poly(BTh+MeFu)/A304 in a solution of H₂O/H₂SO₄ 1 N.

ηp %=icorr0−icorricorr0×100

where icorr0 and i_corr are the corrosion current densities of uncoated and coated electrodes.

We see that E_corr values increased from −380.4 mV (A304) to −344.4 mV (PBTh/A304) it increases with an increase in the concentration of MeFu and these values were −314.5, −286.7 and −242.8 mV, respectively. It moves toward more positive values (20). On the other hand, the values of the i_corr decrease with the addition of MeFu, initially. However, the value of this current begins to increase after from the second concentration 10⁻⁴m (see third curve), while remaining less than that observed for the metal alone. In light of these results we can say that films of poly(BTh+MeFu) improve the corrosion resistance of steel in H₂SO₄ 1 N medium. Rajendran et al. (28) have reported similar behavior of poly(N-(methacryloyloxymethyl)benzotriazole-coglycidylmethacrylate) coatings on mild steel in 0.1 m HCl. We note here that the increase of the corrosion potential towards more positive values, accompanied by an increase in the corrosion current from the third curve, result in the modification of the physicochemical properties of the film (nature of the film, real surface), which vary as 2-methylfuran is added in the solution. Thus, the current in no way could be read as a current of corrosion of the metal alone, but it is a current of the whole (stainless steel and copolymer).

3.5 Electrochemical impedance measurements (EIS)

The impedance measurement is a widespread method in the anti-corrosion coating processes (29) and an in situmethod for characterizing polymer coated metal and changes in their performance during exposure in corrosive environments (30).

Figure 5 shows the Nyquist plots of A304 and poly(BTh+MeFu)/A304 electrodes obtained for different concentrations of MeFu immersed in a sulfuric medium (H₂O/H₂SO₄ 1 N). As shown the impedance diagrams obtained are complex and they consist of a large capacitive loop in the high frequencies region followed by a small inductive loop at low frequencies.

Figure 5:

Nyquist diagrams corresponding to poly(BTh+MeFu)/A304 in (H₂SO₄ 1 N) obtained on a frequency range between 100 kHz and 100 mHz.

The capacitive loop observed at high frequencies, is characteristic of the charge transfer resistance (R_ct) of the corrosion process and the double layer capacitance (C_dl) of the liquid/metal interface (31, 32). In the literature, the low frequencies inductive loop is assigned to the relaxation process of the film that is coated on the surface of the electrode (33, 34). The diameter of the capacitive loop of (BTh+MeFu) copolymer increases with the increase in MeFu concentration suggesting an augmentation in the charge transfer resistance (R_ct) and thus a decrease of the double layer capacitance (C_dl) of (BTh+MeFu) copolymer film, which indicates a decrease in the porosity of the film and a barrier effect of the performance improvement for the protection against corrosion of the steel substrate (35).

Figure 6 shows the assumed equivalent circuit. The equivalent circuit presented in Figure 6 refers to the work of Yuan et al. (36). The impedance behavior of the uncoated electrode can be represented by the equivalent circuit shown in Figure 6A, where R_Ω is the electrolyte resistance, the conventional double-layer capacitance is replaced by a constant phase element (CPE) in order to give a more accurate fit to the experimental results (37) at high frequency and the Warburg diffusion impedance (W) at low frequency.

Figure 6:

Equivalent circuit of: uncoated electrode A304 (A), and coated electrode poly(BTh+MeFu)/A304 (B).

As can be seen in Figure 6B, the correspondent circuit model of poly(BTh+MeFu) coated electrode, this is the most frequent circuit model used to describe the corrosion mechanism of metal/polymer electrodes (38). In this circuit (Figure 6B), C_dl is the double layer capacitance, R_ct the interfacial charge-transfer resistance, L the inductance and R_L the inductive resistance (39).

3.6 Scanning electron microscopic (SEM) analyses

SEM images are shown in Figure 7. The single A304 steel surface is polished before being exposed to the aggressive environment and where the polishing lines of the surface of the steel are visible (40). The microstructures of the surfaces in the presence of the PBTh film clearly show that the morphology of PBTh on steel has a porous, spongy structure (Figure 7A,A′), a similar behavior was observed for the polyterthiophene coated A304 (23). Concerning the PMeFu, the coating surface presents higher porosity and the film formed is very thin (transparent), suggesting that the latter partially covers the surface of the substrate (Figure 7B,B′). In some cases, acceleration of corrosion was also noticed due to the presence of micropores in the electrodeposed PMeFu coating (24).

Figure 7:

SEM images of (A, A′) A304, (B, B′) PBTh/A304, (C, C′) PMeFu/A304 and (D, D′) copolymer (scale bar: 10 μm and 1 μm).

The morphology of poly(BTh+MeFu) copolymer (Figure 7C,C′) reveals that the film is formed by granules of the both polymers, this resulted in a reduced rate of corrosion and a higher corrosion protection performance of the coating on the metal substrate.

3.7 Fourier transforms infrared spectroscopy (FTIR)

FTIR spectra of PBTh and PMeFu and poly(BTh+MeFu) copolymer recorded in the wave number range 400–4000 cm⁻¹ are presented in Figure 8.

Figure 8:

FT-IR absorption spectra of the: (a) PMeFu, (b) PBTh and (c) poly(BTh+MeFu) copolymer.

For the spectrum of the PBTh, the absorption band located at 790 cm⁻¹ is due to the stretching vibration of the C-S-C bond (41), the band at 1600 cm⁻¹ belongs to C=C stretching modes of vibrations in the thiophene ring (42). The band at 1085 cm⁻¹ represents the C-H ring breathing (43). The band near 1350 cm⁻¹ is probably due to a stretching vibration of C-C bonds (44). The bands in the region 2810–2970 cm⁻¹ are possibly due to the symmetric and asymmetric stretch of methyl groups (44). The absorption bands in the region 2300–2400 cm⁻¹ are assigned to the C=C stretching mode and prove the presence of polyconjugation in polybithiophene molecules (44).

In addition, the weak absorption around 1640 cm⁻¹ corresponding to the C=O stretching vibration shows that some defects do exist in the polymer film. The presence of this band probably indicates the ring opening of 2-methyl furan with the consequent generation of unconjugated structures (25) such as a 2-methylfuran ring break which was also noted by Carrillo et al. (45). The band located around 905 cm⁻¹ is assigned to C-O stretching vibration. However, the P(BTh+MeFu) copolymer spectrum is almost similar to those of PBTh, the absorption band of the C=C bond in the P(BTh+MeFu) copolymer appears in 1600 cm⁻¹. The slight low-intensity peak at 3060 cm⁻¹ can be ascribed to the aromatic C-H stretching vibrations (46). Furthermore, the bands at 795 cm⁻¹ indicates that α-α′ coupling of the radical cations has taken place in the copolymerization. This is a characteristic of α-substituted five-membered heterocyclic compounds (47).