The effect of electrolyte concentration on corrosion behavior of
polyaniline coated iron and mild steel electrodes
Aziz Yağana,¥, Nuran Özçiçek Pekmezb
yagan@dicle.edu.tr; npekmez@hacettepe.edu.tr
Dicle University, Chemistry Education, 21280, Diyarbakır, Turkey
Hacettepe University, Department of Chemistry, 06532 Beytepe, Ankara, Turkey
a
b
ABSTRACT: Polyaniline (PAni) coatings on pure iron and mild steel surfaces have been electrodeposited from
0.1 to 0.5 M aqueous oxalic acid solutions containing 0.1 M aniline using potentiodynamic synthesis technique.
The effect of oxalic acid concentration on the corrosion behavior of PAni coated iron and mild steel surfaces
were investigated by linear anodic potentiodynamic polarization and electrochemical impedance spectroscopy
(EIS) techniques in 0.5 M NaCl solution. Corrosion test results showed that corrosion resistance of PAni
coatings increases with increasing concentrations of oxalic acid in polymerization solution.
Keywords: Polyaniline; Electropolymerization; Iron; Mild steel; Impedance Spectroscopy
¥
: This paper is dedicated to Atilla Yıldız by Aziz Yağan
1. Introduction
Conducting polymer coatings for corrosion prevention of oxidizable metals have
potential for replacing conventional coating systems. It has been shown that conducting
polymer coatings such as polyaniline (PAni), polypyrrole can be easily synthesized
electrochemically in aqueous oxalic acid solutions (1-3). Synthesis, characterization and
corrosion protective properties of these polymer coatings have been studied (1-8). Uniform
films with desirable thicknesses and good adhesion properties can be easily electrosynthesized
on oxidizable metals in aqueous oxalic acid solutions containing monomer (4). The increased
conductivity and thickness of the coating and type of the doping anion are important for the
efficiency of protective conducting polymer coatings against corrosion (4,5). It was shown
that conducting polymers do not only behave as barrier coatings on common metals in
corrosive media, they also prevent easy reach of corrosive ions on metal surface due to its
own redox electroactivity in corrosive media (6,7). Fe2+ ions which are produced during
anodic polarization of iron form iron(II)oxalate electrolyte. Conducting polymer layers are
grown on iron oxalate layers in acidic solution containing monomer (6). This oxalate
interlayer is especially responsible for effective protection of conducting polymer coated steel
electrodes in corrosive medium (6-8). It was claimed that decreasing repassivation peak
intensity is an evidence for the formation of more protective iron(II)oxalate interphase.
Furthermore, conducting polymer layer is responsible for stabilization of this interphase,
because iron(II)oxalate layer obtained by polarization in monomer-free oxalic acid solution on
oxidizable metals exhibits worse protective properties than those obtained in oxalic acid
solution containing monomer (4).
In this study, the corrosion behavior of PAni coatings on Fe and mild steel were
compared in NaCl medium and the effect of the oxalic acid concentration used during
electrosynthesis on the stability of interphase layer is investigated.
2. Experimental
Aniline (Ani) supplied from BDH was doubly distilled under vacuum before use. Oxalic acid
dihydrate (C2H2O4.2H2O) was received from Merck. In all electrosynthesis experiments, an
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aqueous solution of the Ani (0.1 M) and oxalic acid (0.1 and 0.5 M) were prepared by using
doubly distilled water. Electrochemical measurements were carried out in a single
compartment three-electrode cell with Fe and mild steel discs as working electrode (0.07
cm2), platinum foil as counter electrode and saturated calomel electrode (SCE) as reference
electrode. The disc electrodes was Fe and mild steel rods with 3 mm diameter embedded in
Teflon holder. Before each experiment, the working electrode was polished with a series of
wet sandings of different grit sizes (320, 400, 600, 800, 1000 and 1200). After polishing, the
mild steel electrode was washed with doubly distilled water and dried at room temperature.
Electrodeposition was performed by using potentiodynamic techniques. The films were
coated with polarization in the potential region between -0.5 and 1.5 V vs. SCE at a 10 mV s-1
scan rate. 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 at room temperature before corrosion tests.
All electrochemical studies were carried out with CHI 660B electrochemical analyzer under
computer control. Anti-corrosion control of the coated electrodes was carried out in 0.5 M
NaCl aqueous solutions by linear potentiodynamic polarization and electrochemical
impedance spectroscopy (EIS) methods. Linear potentiodynamic polarization curves were
obtained by sweeping the potential region between −0.5 and 1.7 V vs. SCE at a 10 mV s −1
scan rate (6). 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 PAni coated
electrodes.
3. Result and Discussion
3.1. Electrosynthesis of PAni coatings on Fe and mild steel electrodes
Figure 1 shows the first scan of cyclic voltammogram recorded during the synthesis of
PAni coatings on Fe electrodes between -0.5 to 1.5 V vs. SCE with a scan rate of 10 mV /s in
a) 0.1 M and b) 0.5 M oxalic acid solutions containing 0.1 M Ani.
a
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b
Fig. 1. Potentiodynamic polarization curves of Fe electrode in 0.1 M Ani solution containing a) 0.1 and b) 0.5 M
oxalic acid in the potential region between –0,5 and 1.5 V vs. SCE (v=10 mV s-1).
a
During the first forward potential sweep an oxidation peak between –0.50 and -0.20 V
vs. SCE is observed, which is attributed to electrodissolution of Fe (6-8). The dissolved Fe+2
ions interact with the oxalate ions to form adherent and insoluble iron (II) oxalate film on the
electrode surface. Beyond this oxidation peak the mild steel surface is passivated. Increase in
the oxidation current density between 0.35 and 1.5 V vs. SCE is attributed to the formation of
radical cations of Ani monomers and its oligomers during the first scan (9). More soluble iron
(III) oxalate is formed during this initial positive scan above 0.6 V vs. SCE which creates
micropores on the surface. Oxidation of the underlying iron to iron (III) oxide occurs within
these pores. During the first reverse scan, sharp oxidation peak (repassivation peak) about 0.0
(Fig.1a) and 0.1 V (Fig.1b) vs. SCE is observed. This repassivation peak corresponds to the
transition of the film to iron (II) oxalate. This peak decreases gradually during subsequent
scans. Decrease of repassivation peak intensity during next scans is attributed to formation of
more homogeneous and stable iron (II) oxalate interphase between metal and polymer (6-8).
Fig.2 shows voltammograms recorded during electrosynthesis of PAni on mild steel electrode
in a) 0.1 and b) 0.5 M oxalic acid solutions containing 0.1 M Ani. As can be seen in Fig. 1a
and 2a, repassivation peak did not appear in curves recorded at 0.1 M monomer and 0.1 M
electrolyte concentrations. Repassivation peak potential (Erep) values are shifted cathodically
with decreasing electrolyte concentration. The value of charge density (Qdiss) consumed
during repassivation decreases with the decrease in electrolyte concentration of
polymerization solution due to the formation of more effective anti-corrosive interphase
between polymer coating and metal surface (6-8).
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b
Fig. 2. Potentiodynamic polarization curves of mild steel electrode in 0.1 M Ani solution containing a) 0.1 and
b) 0.5 M oxalic acid in the potential region between –0,5 and 1.5 V vs. SCE (v=10 mV s-1).
3.3. Corrosion performance
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3.3.1. Linear anodic potentiodynamic polarization in 0.5 M H2SO4
PAni coated Fe and mild steel substrates were tested against corrosion in 0.5 M H2SO4
medium using potentiodynamic polarization experiments. These tests are only preliminary for
evaluation of anticorrosive properties of PAni coatings. As can be seen in Fig. 3A(a) and
B(a), respectively, uncoated Fe and mild steel underwent anodic dissolution with much higher
current values between −0.50 and 0.33 V vs. SCE, and then electrode surfaces is passivated
after about 0.33 V vs. SCE (6,9-11). Fig. 3 and 4 compare the linear potentiodynamic
polarization curves of PAni coated Fe and mild steel surfaces in 0.5 M H2SO4 medium,
respectively. The polymer coatings on Fe and mild steel were prepared under
potentiodynamic conditions in 0.1 (curve b in Fig. 3 and 4) and 0.5 M (curve c in Fig. 3 and
4) oxalic acid solution containing 0.1 M Ani monomer. Fig. 3 compares the potentiodynamic
polarization curves of uncoated (a) and PAni coated (b, c) Fe surfaces. Fig. 4 also compares
the polarization curves of uncoated (a) and PAni coated (b, c) mild steel surfaces. The
smallest dissolution current value was obtained with the films obtained in 0.1 M oxalic acid
solution containing 0.5 M Ani (curves b in Fig. 3 and 4). The presence the polymer also
causes about 70 mV shift of the passivation potential (Epass) towards negative values. PAni
coated Fe electrodes show lower Imax values in 0.5 M H2SO4 as seen in Fig. 7B(b, c). It was
also observed that Epass value was shifted to more positive potentials similar to PAnicoatings
(Fig. 7A(b, c)). As can be also clearly seen in Fig 3 and 4, the acidity of the polymerization
solution was found to be influenced by the anticorrosive behavior of the PAni coatings. The
PAni coatings obtained from different polymerization solution cause a considerable change in
Imax values in corrosive medium. While the electrolyte concentration of the polymerization
solution is increased, Imax values for PAni coated substrates are decreased which indicates a
more protective film against corrosion. However, Breslin (8) and Sazou (6) reported that PAni
coating prepared in 0.1 M oxalic acid solution containing 0.1 M Ani was found the most
stable against corrosion in 0.5 M H2SO4 solution.
Fig. 3. Linear anodic polarization curves obtained in 0.5 M H2SO4 solutions for (a) uncoated, PAni coated Fe in
(b) 0.1 and (c) 0.5 M oxalic acid solution containing 0.1 M monomer (10 mV s−1).
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Fig. 4. Linear anodic polarization curves obtained in 0.5 M H2SO4 solutions for (a) uncoated, PAni coated mild
stel Fe (b) 0.1 and (c) 0.5 M oxalic acid solution containing 0.1 M monomer (10 mV s−1).
3.3.2 Electrochemical impedance spectroscopy
Fig. 5 and 6 show impedance spectra recorded in 0.5 M NaCl solutions of PAni
coatings synthesized on Fe and mild steel in a) 0.1 and b) 0.5 M oxalic acid solutions
containing 0.1 M ANİ, respectively. The impedance response can be represented by the
capacitance of the interfacial double layer (Cdl) in parallel with charge transfer resistance (Rct)
at bottom of the pores (4,5,12-15). Rct, Cdl jointly represent the electrochemistry of corrosion
at the polymer-metal interface after coating penetration by corrosive anions (15). Rct is
obtained from the measured diameter of depressed semicircle.
Fig. 5. Impedance spectra of PAni coated Fe electrodes obtained in 0.1 M Ani solution containing 0.1 (a) and 0.5
M (b) oxalic acid in 0.5 M NaCl solution.
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Fig. 6. Impedance spectra of PAni coated mild steel electrodes obtained in 0.1 M Ani solution containing 0.1 (a)
and 0.5 M (b) oxalic acid in 0.5 M NaCl solution.
Increase in Rct values as increasing of electrolyte concentration in polymerization
solution is related to the increased degree of protection of Fe and mild steel in corrosive
solution. The penetration on electrode surface of corrosive chloride ions is inhibited by
conducting polymer coating. As electrolyte concentration is increased from 0.1 to 0.5 M in
polymerization solution, impedance of the coated substrates also increase in corrosive
medium. Increase in the concentration of oxalic acid in polymerization solution causes an
increase in the value of Rct in NaCl corrosive medium.
4. Conclusion
PAni on Fe and mild steel as an anti-corrosive coating has been electrodeposited from
solutions containing different oxalic acid concentrations using potentiodynamic method.
PANİ coatings provide more effective protection to Fe and mild steel in H2SO4 compared to
that in HCl solution according to results obtained by using DC polarization and EIS
techniques. Corrosion test results showed that protection effect of PAni coatings for Fe and
mild steel depended on pH of polymerization solution. The increased acidity of the
polymerization solution led to low corrosion rate by the polymer coating. PAni coatings
obtained in 0.5 M oxalic acid solution exhibited considerably higher corrosion resistance
compared to PANİ coatings obtained in 0.1 M oxalic acid solution. It is accepted that the
decrease in the intensity or disappearance of the repassivation peak means a better
anticorrosive behavior of the polymer coated iron or mild steel surfaces (6-8). The above
findings indicate, however, that this is not always the case. Rct values of PAni coated steel
surfaces show that the surface for which a repassivation peak is observed is much better
protected than that for which no repassivation peak is recorded. This implies that the degree
of conductivity and electroactivity of the protecting polymer layer must also be taken into
account in addition to the presence or absence of the repassivation peak.
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