INHIBITION OF CORROSION BY POLY(BITHIOPHENE) BILAYERS WITH
POLY(ANILINE) COATINGS OF STAINLESS STEEL ELECTRODES
IN AGRESSIVE MEDIA
Kübra CINKILLI , Nurgül KARADAŞ, Aziz YAĞAN, Nuran ÖZÇİÇEK PEKMEZ
Hacettepe University,Department of Chemistry,Beytepe,Ankara,TURKEY
kubracinkilli@gmail.com,npekmez@hacettepe.edu.tr
ABSTRACT: There has been recently interest on the possible use of conducting polymers as either film forming
corrosion inhibitors or in protective coatings. In general, these materials exhibit not only high conductivity but also
excellent stability in the oxidized state, which are expected to be useful properties within the field of corrosion
protection.This is the first time the electrochemical synthesis of an anticorrosive PBTh and its bilayers with PAni
coating by potentiodynamic synthesis technique in aqueous oxalic acid solutions containing SDS on stainless steel
has been demonstrated. The tests for corrosion protection of these polymers coated and uncoated stainless steel
substrates were performed in HCl solution by electrochemical impedance measurements (EIS), open circuit potential
(OCP) and Tafel test technique. It was shown that bilayer coatings served as a stable host matrix on stainless steel
against corrosion.
Keywords: Polybithiophene-Polyaniline bilayers; Stainless Steel; Corrosion;
ÖZET: Son zamanlarda iletken polimerlerin kullanımına olan ilgi hem koruyucu kaplamalar hem de korozyon
inhibitor film özelliğinden dolayı artmıştır. Genelde bu malzemelerin yüksek iletkenliği ve yükseltgenmiş formda
mükemmel kararlılıkları nedeniyle korozyon koruma alanında kullanılabilir kılmaktadır. Anti-korozif polibitiyofenpolianilin bilayer kaplamaların paslanmaz çelik üzerine ilk kez SDS içeren sulu oksalik asit çözeltisinde
elektrokimyasal sentezi potansiyodinamik teknik ile yapılmıştır. Polimer kaplı ve kaplanmamış paslanmaz çeliğin
korozyon testleri HCl çözeltisinde EIS, OCP, Tafel test teknikleri kullanılarak yapılmıştır. Bilayer kaplamaların
korozyona karşı paslanmaz çelik üzerinde kararlı host-makriks sergilediği gözlenmiştir.
Anahtar Kelimeler: Polibitiyofen-Polianilin bilayer kaplamalar; Paslanmaz Çelik; Korozyon
1. INTRODUCTION
Corrosion of metals is an enormous problem throughout the world (1,2). Several techniques
have been used to protect metals from corrosion. Among them, polymer coatings may be the
most widely used technique. Conducting polymer coatings such as polyaniline (PAni) (3–9) and
polypyrrole (10,11) on SS electrodes can be obtained electrochemically and these coatings
provide important protective properties against corrosion. In general, these materials exhibit not
only high conductivity but also excellent stability in the oxidized state, which are expected to be
useful properties within the field of corrosion protection. Conducting polymers, their copolymers
and bilayers can be deposited on most of the active metals like iron, aluminium, zinc, copper.
nickel and their alloys, chemically or electrochemicaly, in organic solvents or water solutions in
spite of the simultaneous processes of anodic metal dissolution (12). It was proven that
conducting polymer coatings on stainless steel lead to improved the corrosion performance by
formation of a stable passive oxide layer between polymer and metal, barrier effect against
corrosive ions and electroactive interactions of adhesive polymer layer with corrosive ions
(13,14). Hermas et al. (15) reported that polyaniline (PANi) enhanced the passivity of stainless
steel by increasing the Cr2O3 content in the passive film and inhibiting the interaction of the oxide
with solution.
In 1985 DeBerry reported that PANI coating on passivated steel in a strong acidic solution
enhanced the corrosion protection of SS (16). Substituted polythiophene (PTh) coating was first
shown to offer corrosion resistance property by Deng and coworkers (17) In 1992, Ren and
Barkey showed that electrochemically synthesized PTh film could provide important protection
to stainless steel, under acidic conditions (18). 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 (19–21). 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 (22).
In this study, PBTh bilayers with PAni on SS in aqueous oxalic acid solution containing
bithiophene and aniline monomers in the presence of SDS was synthesized under potentiostatic
conditions. Corrosion protection effect of PBTh/ PAni and PAni/ PBTh bilayers for stainless steel
electrodes was investigated by comparing with that for uncoated stainless steel by using Tafel test,
open circuit potential (OCP) and EIS in 0.5 M HCl solutions.
2. EXPERIMENTAL
Aniline (Riedel-de Haen) monomer was distilled under vacuum before use. 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 threeelectrode 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)
and rod electrode (0.539 cm2) used for electrosynthesis. Before each experiment, the working
electrode was polished with a series of wet sanding of different grit sizes (320 -1200). 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 HCl 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 10−2 Hz with amplitude of 7 mV at the corrosion potential of bilayer coatings and
uncoated electrodes.
3.RESULTS AND DISCUSSION
3.1. Electrosynthesis of bilayer coatings
In order to improve the conductivity properties of protective PBTh coating, the
electrosynthesis of its bilayer with PAni, which has higher conductivity value, was considered.
To synthesize PBTh/ PAni and PAni/ PBTh bilayers, the first layers have been
electropolymerized under their own electrosynthesis conditions obtained above. Second layers
were obtained with a scan rate 20 mV s-1 by examining the scanning potential limits during the
electrosynthesis in polymerization solution containing the other monomer. Electrosynthesis of
PBTh/PAni bilayer was carried out in aqueous 0.1 M SDS solution containing 0.05 M
bithiophene, 0.1 M oxalic acid and then containing 0.075 M aniline, 0.2 M oxalic acid and 0.1 M
SDS. On the other hand, PAni/PBTh bilayer was obtained by inverting the process. When the
effect of the scanning potential limits during the electropolymerization of second layer for
PBTh/PAni was investigated, the smallest current density value was obtained with the film under
potential sweeps between -0.2 and 1.1 V vs. SCE according to the potentiodynamic polarization
curves taken in 0.5 M NaCl corrosive medium
3.2.Corrosion Behaviour of Bilayer and Its Homopolymer Coatings
Tafel tests:
The corrosion performance of bilayer coatings and uncoated SS electrodes were studied by
polarizing from cathodic to anodic potentials with respect to open circuit potential at a scan rate
of 1 mV/s by using Luggin capillary in 0.5 M HCl solution. Figure 1 shows the polarization
curves for PBTh, PAni, PBTh/PAni, PAni/PBTh coated and uncoated electrodes. After about
−0.34 V, while the potential increases, the current of uncoated SS decreases, which is related to
the formation of a passive film on the SS surface. The uncoated electrode starts to dissolve with
Figure 1. Tafel curves of uncoated, PAni, PBTh, PBTh/PAni and PAni/PBTh bilayers coated SS electrodes
in 0.5 M HCl solution (v = 1 mV s-1)
breakdown of passive film after about −0.15V and the current density increases. Current density
of cathodic and anodic Tafel lines of homopolymer and bilayer coated SS electrodes decrease
significantly compared to that of uncoated steel, in spite of the contribution originating from the
electroactivity of protective conducting polymer coating (23). As can be seen in Figure 1, the
corrosion potentials of PBTh, PAni/PBTh, PAni and PBTh/PAni coatings SS electrodes shift in
the positive direction compared to that of the uncoated electrode. The dissolution potential of
coated electrodes at anodic polarization also shifted toward positive direction. Table 1
summarizes that the corrosion current densities (Icorr) and corrosion potentials (Ecorr) are
determined by extrapolation of the linear portions of the anodic and cathodic Tafel curves in
Figure 1. Analysis of these data shows that Icorr values of coated electrodes decrease significantly
when compared with that of uncoated electrode. The homopolymer and bilayer coatings restrict
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 HCl medium for PBTh and PAni homopolymers and their bilayers coated surfaces under their own
optimum conditions.
PBTh
PAni
PBTh/PAni
PAni/PBTh
Stainless steel
Ecorr / V
-0.34
-0.053
-0.058
0.23
-0.37
Icorr / mA cm-1
3.40 x 10-3
2.27 x 10-3
6.01 x 10-3
8.92 x 10-3
0.121
Open Circuit Potential:
The Eocp values measured in 0.5 M HCl solution for uncoated and PBTh, PAni, PBTh/PAni
and PAni/PBTh coated SS electrodes after various immersion times were ploted against time and
are given in Figure 2 and Table 2. Initial Eocp value of uncoated electrode was −0.432 V and
remains almost constant after 100 minutes of immersion time. However initial Eocp values of
PBTh, PAni, PBTh/PAni and PAni/PBTh coated electrodes were +0.066 V, +0.201 V, +0.198 V
Figure 2. Eocp – time curves of uncoated, PAni, PBTh, PBTh/PAni and PAni/PBTh bilayers coated SS electrodes in
0.5 M HCl solution
and +0.258 V respectively, which were positive with respect to uncoated electrode. The Eocp
values of coated SS electrodes were found to decrease continuously, within exposure time. After
nearly 7 hours of immersion time Eocp values of PBTh, PBTh/PAni and nearly 8 hours of
immersion time Eocp values of PAni, PAni/PBTh coatings SS electrodes shifted towards that of
uncoated electrodes. This could be explained by partially break down of passivity of electrode.
PAni
PBTh
PBTh/PAni
PAni/PBTh
Stainless Steel
Eint /V
0.201
0.066
0.198
0.258
-0.432
E (50min ) /V
-0.392
-0.036
-0.168
-0.127
-0.392
E (250 min ) /V
-0.330
-0.346
-0.372
-0.162
-0.367
E (660 min ) /V
-0.318
-0.376
-0.349
-0.355
-0.359
Table 2. Eocp values obtained from Eocp - time curves (Figure2) in 0.5 M HCl medium for PBTh and PAni
homopolymers and their bilayers coated surfaces under their own optimum conditions.
Electrochemical impedance spectroscopy:
Impedance measurements provide information on both the resistive and capacitive behavior
of the interface and makes possible to evaluate the performance of polymer coatings as a
protective layer against metal corrosion (24). Figure 3,4,5,6,7 shows the Nyquist plots and
corresponding Bode plots of uncoated SS and PBTh, PAni, PBTh/PAni, PAni/PBTh coating SS
electrodes recorded after various exposure time in 0.5 M HCl, respectively. As can be seen
Figure 3a the impedance diagram uncoated SS is in the shape of a depressed semicircle and Rct
value is equal to the diameter of the semicircle, which is 285.9 ohm. Uncoated steel exhibited a
low polarization resistance due to the easy attack of the corrosive chloride ions on steel surfaces
(25). Rct values which is refer to charge transfer resistance of the steel/electrolyte interface and
Figure 3.a. Impedance spectra b. Bode plots of stainless steel electrodes in 0.5 M HCl after 15 minutes
immersion time
the conducting polymer coatings resistance for PBTh, PAni, PBTh/PAni, PAni/PBTh coating SS
in HCl solution are significantly higher than those of the uncoated steel due to inhibition of
electron transfer to polymer from metal (Figure 4,5,6,7 and Table 3,4). This indicates that the
protection mechanism involves the formation of a high-resistance oxide film induced by
electroactive polymer coating. Consequently, the insertion and the transport rate of anions to
interface of Polymer/SS system in aqueous acidic chloride solution are difficult. The electroactive
and conducting PBTh, PAni, PBTh/PAni, PAni/PBTh coatings exhibit effectively protective
properties and performance HCl solution.
The inhibition efficiency was evaluated from the measured charge transfer resistance Rct
values as Rct1 is the so-called high-frequency ionic charge transfer resistance at the polymer–
electrolyte interface or electron transfer resistance at the SS/polymer interface. Rct2 is the lowfrequency electron transfer resistance of the redox reactions. The slightly suppressed nature of the
semicircle may indicate that the interfacial impedance results from more than one
electrochemical process possible related to the polymeric film and to the changes of the double
layer (24). CPE is employed to instead of the double layer capacitance (Cdl) reflected an ideal
capacitor to describe the non-homogeneities in the system. Rct and CPE jointly belong to the
electrochemistry of corrosion at the polymer-metal interphase after coating penetration by
corrosive anions. The CPE comprises the capacitance of both double layer and the conductive
film. CPE = [Q(wj) a] –1. Q is the frequency independent constant, and w is the angular frequency,
a values are the correlation coefficients for the CPE (0 < a < 1). The corrosion kinetic parameters
obtained uncoated and polymer coated stainless steel in 0.5 M HCl solution are given in Table 3
& 4.
(a)
(b)
Figure 4 a. Impedance spectra b. Bode plots of PAni coated stainless steel electrodes in 0.5 M HCl at different
immersion time
Although PBTh coating was effective barrier coating for SS in short period time (15 min),
PBTh/PAni bilayer exhibited better protection than PBTh in long period time in HCl medium.
(a)
(b)
Figure 5.a. Impedance spectra b. Bode plots of PBTh coated stainless steel electrodes in 0.5 M HCl at different
immersion time.
(a)
(b)
Figure 6.a. Impedance spectra b. Bode plots of PAni/ PBTh coated stainless steel electrodes in 0.5 M HCl at
different immersion time
(a)
(b)
Figure 7.a. Impedance spectra b. Bode plots of PBTh/ PAni coated stainless steel electrodes in 0.5M HCl at
different immersion time
Table 3. The Rs , Rct , CPEand σ values for Pani, PBTh coated electrodes after various immersion times in 0.5 M HCl
t(h)
Rs / ohm Rct / ohm
uncoated
15 min 2.692
2.859x102
CPE /µF cm-2 σ / Ω.cm-2.s-1/2
34.74
PAni
15 min
2h
4h
48 h
120 h
2.861
9.668
4.063
4.216
4.170
2.410 x102
1.214 x103
7.261 x102
4.384 x102
3.785 x102
77.23
66.36
26.59
306.6
418.8
0.013
----------
PBTh
15 min
2h
6h
24 h
72 h
120 h
50.44
52.99
30.04
8.916
6.773
5.944
7.812 x104
8.534 x103
2.796 x103
1.410 x103
6.991 x102
6.492 x102
20.20
93.48
159.9
187.7
596.6
727.5
-------------------
Table 3. The Rs , Rct , CPE values for uncoated and PAni/PBTh , PBTh/Pani coated electrodes after various
immersion times in 0.5 M HCl
t(h)
Rs 1 / ohm Rct 1 / ohm
PAni/PBTh
15 min 23.20
1.708 x103
2h
24.57
2.322 x102
Rs 2 / ohm Rct 2 / ohm CPE1 /µF cm-2 CPE 2/µF cm-2
---129.2
---6.216 x102
337.6
1.126 x103
718.5
24 h
48 h
13.50
5.898
PBTh/PAni
15 min 35.68
2h
44.39
6h
54.16
24 h
35.69
72 h
6.393
96 h
6.172
6.026 x102
4.410x102
1.745 x103
2.491 x103
2.202 x103
7.090 x102
8.090 x102
6.627 x102
-------
-------
350.1
137.9
-------
5.461 x102
1.694 x103
1.341 x103
4.304 x102
-------
1.840 x104
1.108 x104
3.794 x103
2.373 x103
-------
2.597
1.042
8.163
62.54
96.03
170.2
65.53
82.87
209.3
238.6
-------
4.CONCLUSIONS
The electroactive PBTh, PAni, PBTh/PAni, PAni/PBTh have been electrodeposited on 304
stainless steel electrode from aqueous oxalic acid solution with SDS by using potentiodynamic
synthesis technique The corrosion behavior of these coating polymers under immersion in highly
aggressive HCl solution was studied by Tafel Test, Open Circuit Potential and EIS techniques.
The results show that PBTh, PAni, PBTh/PAni, PAni/PBTh coating on SS as conducting polymer
coatings exhibited effective protective behavior in highly corrosive media containing acidic
chloride anions and decreased the corrosion rate of stainless steel but PBTh/PAni bilayer exhibit
better protection than the other coatings in long period time in HCl medium.
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