CORROSION PERFORMANCE OF POLY ( BITHIOPHENE ) COATINGS ON STAINLESS STEEL ELECTRODES

IN NaCl MEDIUM

Nurgül Karadaş, Kübra Cınkıllı, Aziz Yağan, Nuran Özçiçek Pekmez

Hacettepe University,Department of Chemistry,Beytepe,Ankara,TURKEY nurgulk@hacettepe.edu.tr,npekmez@hacettepe.edu.tr

ABSTRACT: Conducting polymers as a protective film on oxidizable metallic surfaces such as iron (1),mild steel (2) and stainless steel (3) against corrosion have been a subject of a great deal research for the past twenty years because of easy synthesis, and cheapness.Conducting polymer coatings on stainless steel inhibit the formation of pitting corrosion in chloride medium (4). In this study , the synthesis of polybithiophene (PBTh) by electropolymerization as a new anticorrosive coating on stainless steel (SS) in aqueous oxalic acid solutions and sodium dodecyl sulfate (SDS) is described . Polybithiophene (PBTh) film was grown by potentiodynamic synthesis technique on SS. Smooth and adherent film was obtained on the steel surfaces. The corrosion performances of stainless steel with and without polymer top coats were investigated in 0.5 M NaCl solution, by using electrochemical impedance spectroscopy (EIS), open circuit potential (OCP) and Tafel test technique, respectively. The polymer coating behaved like a physical barrier and exhibited the most effective anti-corrosive protection for SS against corrosion. Keywords: Polybithiophene; Aqueous medium; SDS; Stainless Steel; Corrosion ÖZET: İletken polimerler demir, yumuşak çelik ve paslanmaz çelik gibi yükseltgenebilen metalik yüzeyler üzerine kolayca sentezlenebilmeleri ve ekonomik olmalarından dolayı korozyona karşı koruyucu film olarak kulanılması ile ilgili araştırmalar 20 yıldan beri yapılmaktadır. Paslanmaz çelik üzerindeki iletken polimer kaplamalar klorür içeren korozif ortamlarda çukur korozyonunun oluşumunu önler. Bu çalışmada ,okzalik asit, SDS ve bitiyofen monomeri içeren sulu çözeltide paslanmaz çelik üzerine elektropolimerizasyon tekniği ile antikorozif kaplama olarak PBTh sentezlendi. Çelik yüzeyinde düz ve yapışkan film olarak biriktirilen PBTh potansiyodinamik sentez tekniği ile elde edildi. Polimer kaplı paslanmaz çeliğin korozyon performansları, EIS, OCP ve Tafel test teknikleri kullanılarak 0.5 M NaCl çözeltisinde araştırıldı ve kaplanmamış elektrodun verileriyle karşılaştırıldı. PBTh kaplama fiziksel bariyer gibi davrandı ve paslanmaz çeliğe çok etkili koruma sağladığı gösterildi. Anahtar Kelimeler: Polibitiyofen; Sulu ortam; SDS; Paslanmaz çelik; Korozyon

1. INTRODUCTION Electrochemical synthesis of polythiophene has attracted much attention for many years (5–7). But the use of polythiophene as a coating material against corrosion has not been studied at all. Due to the high oxidation potential value of thiophene, there has been few studies related to synthesis of polythiophene films on oxidizable metals (8-11). Substituted polythiophene coating was first shown to offer corrosion resistance property by Deng and coworkers (8). Ren et al. achieved the synthesis of poly(3-methylthiophene) films on 430 stainless steel in nonaqueous medium galvanostatically and showed that the film protected the substrate efficiently in 0.5 M sulphuric acid (10). Most of these studies were carried out in nonaqueous solution, because such monomers present a serious drawback regarding their solubility, particularly in water. Many efforts have been made with a view to improving this property(12–14). Lagrost et al. showed that electropolymerization of bithiophene and thiophene derivatives on Pt and a glassy carbon electrode could be performed in water in the

presence of either sodium dodecyl sulfate (SDS) or hydroxypropyl-β-cyclodextrin under potentiostatic conditions(15). In contrast to what happens in organic media in water in the presence of a surfactant, the polymerization process appears to be critically dependent on the potential.

In this work, PBTh coatings were obtained by potentiodynamic synthesis technique as a conducting polymer coating on 304 stainless steel. PBTh was synthesized at optimum electrosynthesis conditions determined before. The corrosion performance of these coatings was investigated and compared with that for uncoated stainless steel by Tafel test, OCP and EIS in 0.5 M NaCl solutions. 2. EXPERIMENTAL Bithiophene monomer supplied from Fluka was used directly. Oxalic acid and sodium dodecyl sulfate (SDS) were received from Merck and Sigma-Aldrich, respectively. In all electrosynthesis experiments, an aqueous solution containing the monomer, SDS and oxalic acid was prepared by using doubly distilled water. Electrochemical measurements were carried out in a single compartment three-electrode cell with SS disc as working electrode, platinum foil as counter electrode and saturated calomel electrode (SCE) as reference electrode. The 304- stainless steel disc electrode (0.07 cm2) used for electrosynthesis. Before each experiment, the working electrode was polished with a series of wet sanding of different grit sizes (100, 320, 600, 800, 1000, 1200, 2000). After polishing, SS electrode was washed with doubly distilled water and dried at room temperature. The coatings prepared electrochemically were immersed in distilled water to remove adsorbed electrolyte, monomer and the soluble oligomers formed during electropreparation of the coating and then dried for 1 h at room temperature before investigating the corrosion protection effect. All electrochemical studies were carried out with CHI 660B electrochemical analyzer under computer control. Anti-corrosion control of the coated electrodes was carried out by Tafel, open circuit potential (Eocp)-time and EIS measurements in 0.5 M NaCl solution. Tafel tests were carried out by polarizing from cathodic to anodic potentials with respect to open circuit potential at a scan rate of 1 mV/s. Electrochemical impedance measurements were carried out in the frequency range of 105 to 10−3 Hz with amplitude of 7 mV at the corrosion potential of polybithiophene coating and uncoated electrodes.

3.RESULTS AND DISCUSSION 3.1. Electrosynthesis of polybithiophene coatings Electrosynthesis of PBTh was carried out in aqueous 0.1 M SDS solution containing 0.05 M bithiophene, 0.1 M oxalic acid with a scan rate 20 mV s-1. The effect of the scanning potential limits during the electrosynthesis of PBTh coatings on the SS electrode in aqueous oxalic acid solutions containing SDS was investigated. The potentiodynamic polarization curves of coatings here were recorded in 0.5 M NaCl corrosive medium and obtained the current density values at 1.6 V. According to the results, the current density value of coated surfaces are much lower than that of uncoated surface, which indicates better protection against dissolution of SS at positive potentials. The smallest dissolution current value was obtained with the film under potential sweeps between -0.2 and 1.2 V vs. SCE in NaCl.

3.2. Anticorrosive properties of PBTh coated stainless steel The homogeneous and adhesive electroactive PBTh coatings were obtained by potentiodynamic synthesis technique and used for corrosion tests. Tafel test: The anticorrosive properties of PBTh coatings and uncoated SS electrodes were tested by polarizing from cathodic to anodic potentials with respect to open circuit potential at a scan rate of 1 mV/s in 0.5 M NaCl solution. As can be seen in the Figure 1, the cathodic Tafel line of PBTh coated stainless steel and uncoated steel have similar current density values. Although there isn’t high differences between current densities, the corrosion potential shifts to more positive values from -0.366 V to 0.059 V. This result was giving evidence for very effective barrier property of the PTh coating.

Figure 1. Tafel curves of (a) uncoated (b) PBTh coated stainless steel obtained which was obtained under potentiodynamic conditions with a potential sweep between -0.20 V and 1.20 V vs. SCE in 0.5 M NaCl solution (v = 1 mV s-1) Table 1 shows the corrosion current densities (Icorr) and corrosion potentials (Ecorr) determined by extrapolation of the linear portions of the anodic and cathodic Tafel curves in Figure 1. Analysis of these data shows that Icorr value of coated electrode decreases and corrosion potential shifts to the positive potentials significantly when compared with that of uncoated electrode. As a result, PBTh coating restricts the anodic and cathodic reactions of stainless steel in aggressive media. Table 1: Corrosion potential (Ecorr) and corrosion currents (Icorr) obtained from polarization curves (Figure 1) in 0.5 M NaCl medium for uncoated and PBTh coated surfaces under its optimum conditions.

Ecorr / V Icorr / mA cm-1

PTh 0.059 2.82 x 10-3

Uncoated Stainless steel -0.366 3.08 x 10-3

The anodic and cathodic events occurring in the metal/polymer/solution interface are proposed to be:

Fe → Fe2+ + 2e−

(Ry+Ay)n + 2nye− → (R)n + nyA;

where A represents C2O42− and also SDS; R is the polymer.

O2 + 2H2O + 4e− → 4OH− (16-21).

Open Circuit Potential: The Eocp values measured in 0.5 M NaCl solution for uncoated and PBTh coated SS electrodes after various immersion times were ploted against time and are given in Figure 2 and Table 2.

Figure 2. Eocp – time curves of a) uncoated, b) PBTh coated SS electrodes in 0.5 M NaCl solution Table 2: Open circuit potential values with different immersion times in 0.5 M NaCl solutions Eint /V E (30 min) /V E (160 min) /V E (910 min) /V

PTh 0.378 0.278 0.132 0.014

Stainless steel -0.218 -0.121 -0.068 -0.023

The initial Eocp value was measured to be 0.378 V. This value was quite nobler than the corrosion potential value of uncoated SS (Ecorr = −0.218 V) under the same conditions. This case was giving evidence for effective barrier behavior of PTh coating against the attack of corrosive environment. But, the Eocp value had started to decrease with the initiation of water up taking process and was measured to be 0.278 V after 30 min of exposure time. As the water amount held within the pores of the coating increased, the mobility of corrosive species through the film increased. The porosity of the coating has much importance for initiation and progression of corrosion phenomenon under the coating. The value of Eocp was measured to be 0.014 V after 910 min of exposure time. This value was still nobler than Ecorr value of SS. During this period, the barrier efficiency of the coating had diminished due to the increased

amount of electrolyte held by the coating, and this was indicating that the corrosion process had progressed under the coating.

Electrochemical impedance spectroscopy: EIS measurements were carried out to evaluate of performance against corrosion of polymer coated steel and uncoated steel immersed in 0.5 M NaCl solutions as a function of immersion time. Figure 3 shows impedance spectra of uncoated steel obtained in NaCl. The Nyquist plots of PBTh coating recorded during long exposure times in NaCl are given in Figure 4. Uncoated steel exhibited a low polarization resistance due to the easy attack of the corrosive chloride ions on steel surfaces (22). Figure 4 shows that PBTh coated steel exhibited much higher impedance value, which indicates the protective effect of coating in corrosive medium.

Figure 3. Impedance spectra plot of uncoated stainless steel electrode in 0.5 M NaCl after 15 min. immersion time

(a)

(b) Figure 4 Impedance spectrum plots of PBTh coated SS electrodes in 0.5 M NaCl at different immersion time.

The impedance diagram has the shape of a depressed semicircle and Rct value is equal to the diameter of the semicircle. In the beginning of immersion of PBTh coated electrode in NaCl, Warburg behavior was observed, which indicates the resistance of coating against the diffusion of corrosive ions in PBTh coated stainless steel electrode (in Figure 4). The results obtained from evaluation of the impedance diagrams of bare and polymer coated SS electrodes are given in Table 3. Rs, Rct and CPE are solution resistance, ionic charge-transfer resistance at the polymer–electrolyte interface and constant phase element, respectively. A

constant phase element (CPE) is employed instead of the double-layer capacitance (Cdl) to describe the non-homogeneities in the system:

CPE = [Q(jω)α ]−1

Q is the frequency independent constant, and ω the angular frequency. α values are the correlation coefficients for the CPE (0 < α < 1) and α values are obtained from slope of the Bode plot. Effective corrosion resistance is associated with high Rct and low CPE values. It can be seen from the Table 3 that the PBTh coating is able to offer protection to stainless steel during long immersion period in 0.5 M NaCl solution. Effective corrosion resistance of PBTh coating on electrochemically passivated stainless steel surfaces is due to an homogeneous coating, stability of electroactivity and occurrence of a homogeneous polymer/metal interphase during electrosynthesis (23–25). Table 3. The Rs , Rct , CPE and σ values for PBTh coated electrode after various immersion times in

0.5 M NaCl

Immersion time Rs / ohm Rct / ohm CPE / F cm-2 α σ / Ω.cm-2.s-1/2 uncoated

30 min 26.2 4.39 x 105 0.906 0.83

PBTh coating

15 min 24.95 7.80 x 106 0.618 0.76 303987

2 h 29.24 9.95 x 106 0.827 0.83

4 h 54.74 1.16 x 107 0.845 0.80

24 h 120.7 1.65 x 107 0.922 0.61

7 days 36.18 1.44 x 107 0.792 0.77

12 days 31.50 1.46 x 107 0.546 0.55

17 days 12.44 1.28 x 107 0.675 0.61

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