Journal of
Electroanalytical
Chemistry
Journal of Electroanalytical Chemistry 583 (2005) 133–139
www.elsevier.com/locate/jelechem
Corrosion protection by polyaniline-coated latex microspheres
Yousuf Mohammad Abu, Koichi Aoki
*
Department of Applied Physics, University of Fukui, 3-9-1 Bunkyo, Fukui-shi 910-8507, Japan
Received 18 April 2005; received in revised form 24 May 2005; accepted 24 May 2005
Available online 7 July 2005
Abstract
The iron plate covered with films of polyaniline-coated polystyrene latex (PANI–PS) microspheres were protected against corrosion in 0.01 mol dm3 HCl and 3% (w/v) NaCl aqueous solution. PANI–PS particles 1.85 ± 0.06 lm in diameter were synthesized
by polymerizing chemically aniline on polystyrene (PS) latex in the suspension. The conducting state (emeraldine salt) of PANI film
shifted the corrosion potential of the underlying iron toward a positive from the insulating state of PANI (leucoemeraldine form).
The film from which core-polystyrene was removed by dissolution in tetrahydrofuran had the similar anti-corrosive properties. Tafel
plots, open circuit potential–time diagrams were used to examine the corrosion properties of PANI–PS coated and uncoated
electrodes.
2005 Published by Elsevier B.V.
Keywords: Corrosion; Iron; Polyaniline; Latex; Microspheres; Tafel plot
1. Introduction
Conducting polymers and their derivatives have
shown to have protection of corrosion in the past two
decades, expecting a replacement of chromium-containing materials [1–5]. Since conducting polymers make
good adhesion on the metal surfaces, they work not only
as physical barrier against oxygen but also as an electrochemical barrier against aggressive ions [6–11]. Polyaniline (PANI) has been the most widely studied of
conductive polymers because of its ease of synthesis
[12–14], redox and thermal stability and high corrosion
resistance [15–20]. PANI based blends on the surface
of stainless steel [19,20] and on iron [21,22] have shown
corrosion inhibition and the mechanistic behavior has
been reported. However, it is not easy to make large
polyaniline films for coating metal surfaces because
polyaniline is brittle, insoluble in water, sparingly solu-
*
Corresponding author. Tel.: +81 776 27 8665; fax: +81 776 27 8494.
E-mail address: d930099@icpc00.icpc.fukui-u.ac.jp (K. Aoki).
0022-0728/$ - see front matter 2005 Published by Elsevier B.V.
doi:10.1016/j.jelechem.2005.05.014
ble in organic solvents such as m-cresol and N-methylpyrrolidone (NMP) and non-fusible even by heating
up to their decomposition temperature. The electropolymerization offers some advantages over other coating techniques [23–32] in getting uniformly thin and
adhesive films on the metal surfaces with simple instruments. However, corrosion-susceptible metals are often
oxidized or dissolved in the potential domain of the electropolymerization of polyaniline.
A well-known technique of coating the large metal
surface is to spread suspensions of microparticles and
to dry them [33,34], like painting. If PANI–PS microspheres are spread on a substrate to form a film, the
problems of the poor processability and roughness of
film thickness on a large area will be solved. Synthetic
techniques of nearly mono-dispersed PANI–PS are to
polymerize chemically aniline on PS latex surface in
the presence of surfactant, such as poly(vinyl alcohol),
poly(N-vinylpyrrolidone) (PVP), poly(vinyl methyl
ether), poly(ethylene oxide) or cellulose ethers [35–42].
Since the PANI–PS is dispersed in aqueous phase, no
organic solvent is needed for the film formation, in
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Y.M. Abu, K. Aoki / Journal of Electroanalytical Chemistry 583 (2005) 133–139
contrast with using m-cresol or NMP for the organic
solvents-aid coating.
The PANI–PS has a core-shell structure, that is, a PS
core and a PANI shell [42]. This structure is ascribed to
the immiscibility of the hydrophobic PS and the hydrophilic PANI. Since the PANI–PS is stable in a suspension form, it has been applied to voltammetry of
colloidal suspensions [41–44]. The suspensions have
shown diffusion-controlled voltammetric responses
[41,42], and sometimes associated with periodic current
[43,44]. Self-standing PANI film of mono-particle-layer
has been generated from the PANI–PS. The electrochemical properties are similar to those of the electrochemically polymerized films [45,46] if the PANI–PS is
purified at low temperature. This film may be helpful
for corrosion inhibition as for conventional polyaniline
films. In this article, we report the protection properties
of the PANI–PS-coated iron electrode in chloride-containing solutions.
2. Experimental
2.1. Chemicals
Styrene (Wako) was purified by distillation under
vacuum at 60 C. Poly(n-vinyl pyrrolidone) (PVP)
(Wako) of 360 kg mol1 was used as a stabilizer or a
surfactant during the dispersion polymerization of styrene. Ammonium peroxydisulfate (Wako), aniline
hydrochloride (Kanto), a-azoisobutyronitrile (AIBN)
(Kanto), 2-propanol (Wako), tetrahydrofuran (THF)
(Wako), hydrazine monohydrate (Kanto) and sulfuric
acid and PVP were used as received. All aqueous solutions were prepared with deionized water made by an
Advantec ultra pure water system (CPW – 100).
2.2. Synthesis of PVP-stabilized polystyrene latex
A mixture of 120 cm3 2-propanol and 2.1 g PVP was
heated to 70 C with bubbling nitrogen during mechanical stirring for 24 h in a three-necked round-bottomed
flask to remove trace of oxygen. To this solution was
added styrene (15 g) containing AIBN (0.15 g) drop-wise
for 40 min. The solution was maintained at 70 C for 24
h, during which the polymerization occurred. The resulting milky-white mixture was centrifuged with a centrifuge, SRX – 201 (TOMY, Tokyo) equipped with a
cooling system. The supernatant was decanted and replaced by deionized water to yield white sediment of
PS latex particles. This centrifugation–redispersion cycle
was repeated several times in order to remove residual
reactants. Finally the particles were kept in aqueous
medium with 3% (w/v) PVP. Then this latex suspension
was used for the next coating process. Features of the
suspension of the PS latex particles were observed by
using a video microscope, VH-Z450 (Keyence, Osaka).
2.3. Coating the latex particle with PANI
About 1.11 g aniline hydrochloride was dissolved in
100 cm3 of the PVP-stabilized 3% (w/v) PS latex suspension. The mixture was stirred and cooled to 0 C in an
ice bath for 2 h in order to adsorb aniline on the surface
of latex prior to polymerization. The polymerization
was made at 0 C for 12 h after adding 2.45 g
(NH4)2S2O8 as the oxidant. The solution was kept at
room temperature for further 24 h during stirring the
mixture to complete the polymerization. Centrifugation
of the suspension at 4 C yielded three layers. The second layer contained fragments of PANI. The bottom
layer, which contained a high portion of the PANIcoated latex, was redispersed in 0.l M H2SO4 solution.
This suspension was centrifuged several times until the
H2SO4 solution layer became transparent. Then we obtained the purified PANI–PS latexes and stored in 0.1 M
H2SO4 and 1% PVP solution.
The particle under the reduced state was made by
adding hydrazine monohydrate to the suspension from
which H2SO4 and PVP had been removed by centrifugation and redispersion. After being kept for few hours,
the particle was washed by distilled water and stored
in 1% PVP solution. Both oxidized and reduced
PANI–PS particles were used for making films on
electrodes.
2.4. Coating of iron electrode surface
A certain amount of 0.5% (w/v) PANI–PS suspension
including 0.05% (w/v) PVP was spread on the iron electrode surface. Upon drying it at slow evaporation rate in
an open atmosphere with 70% humidity at 25 C, a uniform multi-particle film was obtained on the electrode
surface. The PANI–PS coated-iron (PANI–PS–Fe) electrode was inserted into THF for 45 min to remove the
soluble PS-core. It was washed carefully by distilled
water for several times and was dried. Then a thin PANI
film without PS was obtained on the electrode surface.
The electropolymerization of aniline on the Fe electrodes was carried out in 0.1 M aniline including 0.3
M oxalic acid, as described by Tüken et al. [47]. Firstly,
the electrode surface was passivated by applying a single
forward scan from 0.5 to 0.3 V at scan rate of 4
mV s1. Then one potential cycle was taken in the range
from 0.0 to 1.6 V at a scan rate of 10 mV s1 and then
150 cycles were applied between 0.2 and 0.95 V at 50
mV s1. The coated electrodes were used as the working
electrode for the measurement of corrosion.
Y.M. Abu, K. Aoki / Journal of Electroanalytical Chemistry 583 (2005) 133–139
2.5. Electrodes
The disk electrodes were made of an iron rod 1.2 mm
in diameter. The cylindrical wall of the rod was covered
with glass tube. A space between the inner wall of the
glass tube and the iron rod was insulated by epoxy resin.
After curing the resin the surface of the electrode was
polished with 500, 1000, 1500 and 2000 mesh sand papers successively and washed with distilled water several
times in an ultrasonic bath before each use. A platinum
coil and saturated calomel electrode (SCE) were employed as a counter and a reference electrode, respectively. The potentiodynamic experiments were carried
out with the coated and uncoated electrodes using the
AutoLab potentiostat in 3% (w/v) NaCl solutions applying a sweep rate of 1 mV s1 at room temperature.
3. Results and discussion
The PANI–PS was dark green, very similar to the color of electropolymerized PANI films. The aqueous suspension through a video microscope showed that most
particles were so well isolated each other that no lump
of particles was generated and that each was subject to
the Brownian motion. The acidic suspension was stable
for a few days without gravitational sedimentation. The
sedimented particles could be easily redispersed by simply mixing. The PANI–PS was nearly mono-dispersed
and spherical with 1.85 ± 0.06 lm in diameter. Since
the PS latex particle before loading PANI had
1.80 ± 0.05 lm, the thickness of PANI layer is ca.
0.025 lm [=(1.85 1.80)/2].
Fig. 1(a) and (b) are optical microscope photographs
of the bare iron and PANI–PS–Fe electrodes after 7 days
immersion in 3% (w/v) NaCl solutions, respectively. The
coated electrode (b) was more anti-corrosive than the
bare iron (a). The color of the PANI–PS film on the iron
changed from dark green of the emeraldine salt to pale
green of leucoemeraldine, whereas no color change was
135
observed for the film on the epoxy resin and glass coated
area. PANI–PS particles coated at the iron electrode
were found to be reduced by the corrosion as evidenced
from the color change. The corrosion started near at the
interface of Fe and the shielding material, as shown as a
white product in Fig. 1(b), and the whole film turned redbrown finally. This behavior was observed also for the
electropolymerized PANI coated iron (PANI–Fe) electrode, as shown in Fig. 1(c).
Open circuit potential, Eoc of coated and uncoated
samples were measured, and plotted against the immersion time in 0.01 M HCl solution in Fig. 2. The PANIcoated electrodes (b)–(d) showed more positive potentials than the potential at the bare electrode (a), where
the coating materials are (b) the reduced PANI–PS, (c)
the electrochemically polymerized PANI, and (d) the
oxidized PANI–PS. Consequently, the PANI film works
as protection of the corrosion [48–53]. The oxidized
PANI is more effective for the corrosion protection than
the reduced form.
Fig. 3 shows the Tafel lines (a), (b), (c) and (d) for uncoated-Fe, reduced PANI–PS–Fe, PANI–Fe and oxidized PANI–PS–Fe electrodes, respectively, in 3% (w/
v) NaCl medium at the scan rate of 1 mV s1. The films
were not removed from the Fe surface even at 1.2 V by
hydrogen gas evolution. Indeed no gas evolution was
observed. The corrosion potentials, Ecorr for all the three
coated electrodes are more positive than that observed
in the uncoated-Fe. The most anodic value of Ecorr indicates that the oxidized PANI–PS–Fe electrode should
have the highest corrosion protection.
The polarization resistances, Rp were evaluated from
the Tafel plots, according to the Stearn–Geary equation
[54],
Rp ¼ ba bc =2.303ðba þ bc ÞI corr .
ð1Þ
Here, Icorr is the corrosion current determined by an
intersection of the linear portions of the anodic and
cathodic curves, and ba and bc are anodic and cathodic
Tafel slopes (DE/Dlog I), respectively. The protection
Fig. 1. Photographs of: (a) Fe electrode, (b) PANI–PS–Fe electrode, (c) PANI–Fe electrode, after 7 days immersed in 3% (w/v) NaCl aqueous
solutions. The curve in (b) was drawn in order to display the location of the interface between the iron electrode and the shielding material (epoxy
resin). White lumps in (b) and (c) are corroded products after 7 days of immersion.
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Y.M. Abu, K. Aoki / Journal of Electroanalytical Chemistry 583 (2005) 133–139
-0.5
-0.6
(d)
Ecorr / V vs. SCE
EOC / V vs.SCE
-0.5
(c)
-0.6
(b)
(e)
-0.7
-0.7
-0.8
(a)
0
1000
2000
t/s
-0.9
-11
-10
-9
-8
-7
-6
ln(W / g)
Fig. 2. Time-variations of open circuit potential, Eoc at: (a) Fe, (b)
reduced PANI–PS–Fe, (c) PANI–Fe, (d) oxidized PANI–PS–Fe, and
(e) Fe connected with PANI-coated Pt electrodes (Fig. 8) in 0.01 M
HCl.
Fig. 4. Plot of corrosion potential, Ecorr against the logarithm of the
amount of PANI loaded on the electrodes for (circles) PANI–Fe
(squares) oxidized PANI–PS–Fe and (triangles) PANI–PS–Fe after
THF treatment. In all cases corrosion potentials were measured in 3%
(w/v) NaCl aqueous solutions.
efficiency (PEF%) values were estimated using the following equation [55]:
These corrosion parameters were calculated from the
Tafel plots for several ratios of the amount the reduced
PANI–PS and the oxidized PANI–PS, and were listed in
Table 1. The polarization resistance as well as the protection efficiency increased with an increase in the fraction of the emeraldine form of PANI. The corrosion
currents for the coated-Fe electrodes are smaller than
the current at the uncoated-Fe.
Fig. 4 shows the dependence of the corrosion potential, Ecorr on the logarithm of the amount of PANI, W,
loaded on the Fe surfaces. The positive shift of Ecorr
with an increase in W indicates the strong protection
of the corrosion at a thicker film. Since thicker films
can be fabricated by use of PANI–PS more easily than
by electrochemical polymerization, the protection by
PANI–PS is more efficient than electrochemical
fabrication.
The oxidized PANI–PS can react with ferrous or ferric ion to yield corrosion-protective Fe–PANI complex
[56]. According to the reaction proposed [56],
1
1
P EF % ¼ 100½R1
p ðuncoatedÞ Rp ðcoatedÞ=Rp ðcoatedÞ.
ðn=mÞFe þ ½PANInþ ! ðn=mÞFemþ þ ½PANI0
(a)
(b)
(c)
log(|I| / A)
-6
(d)
-8
-10
-1.2
-1
-0.8
-0.6
-0.4
E /V vs.SCE
Fig. 3. Tafel plots at: (a) Fe, (b) reduced PANI–PS–Fe, (c) PANI–Fe,
(d) oxidized PANI–PS–Fe electrodes in 3% (w/v) NaCl aqueous
solutions at a potential scan rate of 1 mV s1.
ð2Þ
! ðFeÞn=m ½PANI;
ð3Þ
Table 1
Some corrosion parameters obtained in 3% (w/v) NaCl solutions, varying with the amount ratio of the oxidized PANI–PS to the reduced PANI–PS
at a common value of the total loaded amount of PANI–PS
Samples
W(ox):W(rd)
Ecorr/V vs. SCE
Icorr/lA cm2
Rp/kX
PEF %
Uncoated-Fe
PANI–PS/Fe
PANI–PS/Fe
PANI–PS/Fe
PANI–PS/Fe
PANI–PS/Fe
PANI–PS/Fe
PANI–PS/Fe
–
0:1
1:4
2:3
1:1
3:2
4:1
1:0
0.880
0.865
0.812
0.755
0.723
0.698
0.645
0.614
15.3
17.0
8.2
7.4
6.3
5.3
3.4
1.9
22
76
180
372
552
694
833
1120
–
70
87
94
96
96
97
98
Y.M. Abu, K. Aoki / Journal of Electroanalytical Chemistry 583 (2005) 133–139
where m = 2 or 3. Letting jFe and jPANI are the current
densities for the oxidation and reduction of Fe and
PANI, respectively, we can write:
ð4Þ
þ
jPANI ¼ k PANI ½PANI exp½bnFE=RT ;
ð5Þ
where a and b are transfer coefficients. Since the anodic
current is compensated with cathodic one in corrosion,
summation of cathodic and anodic current densities
yield,
jFe þ jPANI ¼ 0.
-0.7
-0.8
ð6Þ
-0.9
-13
From Eqs. (4)–(6) we can write,
Ecorr
-0.6
Ecorr / V vs.SCE
jFe ¼ k Fe expðamFE=RT Þ;
137
RT
fln½PANIþ þ const.g
¼
F ðam þ bnÞ
ð7Þ
Values of am and bn were obtained from the Tafel slopes
in Fig. 3(a). The value of RT/F(am+bn) is 0.052 V,
which is close to the value of the slope in Fig. 4. Therefore the linearity in Fig. 4 is based on the simple combination of Eqs. (4)–(6).
Fig. 5 shows the variation of Ecorr against the logarithm of the ratio of the amount of the oxidized
PANI–PS to the reduced one, where the amounts of
loaded PANI–PS were common. A Nernst-like variation
is found, showing the slope of 0.060 V. If the linear relation were to be regarded as the Nernst equation, the value of n is 0.4. This is within the conventional domain of
n from 0.2 to 0.5 [57,58]. Therefore, Ecorr at the PANI–
PS–Fe electrode is strongly controlled by the redox reaction of PANI.
Values of Ecorr for several values of W(ox)/W(rd)
showed a linear variation of the logarithm of the corrosion current, as shown in Fig. 6. The slope was 0.13 V,
which is equivalent to bn = 0.21. Since n = 0.4, we got
b = 0.5. This belongs to the conventional charge transfer
process.
-12
-11
ln( Io / A)
Fig. 6. Variation of corrosion potential with logarithm of the
corrosion current at PANI–PS–Fe electrodes.
When the PS-core was dissolved in THF from the
PANI–PS–Fe electrode [41,42] the resulting film exhibited the linear variation of Ecorr with ln W, as shown in
Fig. 4 (triangles). We succeeded in fabricating a large
PANI film (5 · 4 cm2) by removing PS-core from the
PANI–PS film in THF. This film was uniform on the
scale of 0.5 mm, as is shown in Fig. 7. However, it
Ecorr / V vs.SCE
-0.6
-0.7
-0.8
-1
0
1
ln[ W(ox) / W(rd) ]
Fig. 5. Dependence of corrosion potential on the logarithm of the
amount ratio of the oxidized PANI–PS to reduced PANI–PS at a
common value of the total loaded amount of PANI–PS.
Fig. 7. Photograph of large (5 · 4 cm2) PANI–PS film on Fe-plate
after removing the PS-core by THF. White part is the uncovered Feplate. The inset is a magnified photograph.
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Y.M. Abu, K. Aoki / Journal of Electroanalytical Chemistry 583 (2005) 133–139
a
b
bare
bare
Electrode
Electrode
Fig. 8. Illustration of: (a) the presence of the bare part of Fe surface
and (b) local covering of the bare part with PANI.
had a pattern of the PANI-shell on the scale of 10 lm, as
is shown in the inset of Fig. 7. Since the domain among
the grains in the inset was still green, it should be filled
with PANI. Although the film is heterogeneous in the
grain form on the micrometer scale, it covers the electrode surface completely.
A question may arise about whether the PANI–PS
particles can cover uniformly the iron plate coated with
spherical PANI–PS and whether the corrosion may occur at the uncoated part, as is illustrated as ‘‘bare’’ in
Fig. 8(a). Since a sphere comes in contact with a plane
at a point geometrically, most area of the iron should
not be coated with PANI. However, we observed that
the PANI–PS prevented hydrogen evolution at 0.0 V
vs. SCE from the platinum electrode in the PANI–PS
suspension [38], indicating that the bare part has been
coated with invisible PANI film automatically. This
property is due to strong adsorption of PANI on surfaces. A PANI film has bridged two separated electrodes
electrically over the insulating wall [59,60]. PANI may
transfer from the surface of adsorbed PANI–PS particle
onto the iron surface to make an anti-corrosive PANI–
iron interface, as illustrated in Fig. 8(b).
In order to elucidate the corrosion mechanism we
consider the two systems in the solution: (A) an iron
plate of which surface is covered with a PANI film,
and (B) an iron block connected with a PANI block
through a platinum wire, as illustrated in Fig. 9. The
two systems are different in the presence (A) and the absence (B) of iron–PANI interface. We constructed a
model (B) by connecting electrically a PANI-coated Pt
wire and an iron plate. The model immersed in chloride
solution exhibited the white precipitate from the iron
Pt
Fe
PANI
Chloride solution
Fig. 9. Illustration of a model electrode or the corrosion. The potential
is measured against a reference electrode, e.g., SCE.
plate. The open circuit potential was 0.52 V (Fig.
2(e)), being between Eoc at only the iron 0.65 V and
Eoc at only the PANI film 0.51 V. The redox state of
the iron for (B) may be the same as the state at which
0.52 V is applied to the iron by a potentiostat. Thus,
the protection of the corrosion can be ascribed the interface between Fe and PANI. PANI forms a complex with
Fe at the interface [56], as is shown by reaction (3). The
complex may make an anti-corrosive film.
4. Conclusion
The PANI–PS particles on the iron plate were useful
to protect iron against corrosion. The films were stable
at the iron surface at potential region where gas evolution occurred. The oxidized form of PANI–PS film
showed better corrosion protection than its reduced
form. The corrosion protection was demonstrated with
the positive shift of the open circuit potential and the
corrosion potential and by the decrease in the corrosion
current. The corrosion potential had the linear relation
with the logarithm of the amount ratio of the oxidized
PANI to the reduced PANI. Pure polyaniline film on
iron was obtained by removing PS latex by THF. This
film showed anti-corrosion similar to the PANI–PS film
and electrochemically synthesized PANI coated on iron.
We can fabricate a large corrosion-protection film by
painting the PANI–PS aqueous suspension on iron
objects.
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