Inhibition of nickel coated mild steel corrosion by electrosynthesized polyaniline coatings

Article abstract of DOI:10.1016/j.electacta.2010.12.096

Polyaniline (PANI) coatings were electrochemically synthesized on nickel (Ni) coated mild steel (MS) and their corrosion protection properties were investigated. In this work, the Ni layer (～1 μm thick) was electrodeposited on MS under galvanostatic condi

Full text of DOI:10.1016/j.electacta.2010.12.096

Electrochimica Acta 56 (2011) 3049–3059
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Inhibition of nickel coated mild steel corrosion by electrosynthesized
polyaniline coatings
Sudeshna Chaudhari∗, P.P. Patil
Department of Physics, North Maharashtra University, Jalgaon 425001, Maharashtra, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2010
Received in revised form
28 December 2010
Accepted 28 December 2010
Available online 11 January 2011
Polyaniline (PANI) coatings were electrochemically synthesized on nickel (Ni) coated mild steel (MS)
and their corrosion protection properties were investigated. In this work, the Ni layer (∼1 ␮m thick) was
electrodeposited on MS under galvanostatic condition. Thereafter, the PANI coating was deposited over
the Ni layer from aqueous salicylate medium by using cyclic voltammetry. These bi-layered composite
coatings were characterized by cyclic voltammetry, UV–vis absorption spectroscopy and Fourier trans-
form infrared (FTIR) spectroscopy. The corrosion protection properties of Ni coated MS (Ni/MS) and PANI
coated Ni/MS (PANI/Ni/MS) were investigated in aqueous 3% NaCl by using open circuit potential (OCP)
measurements, potentiodynamic polarization technique and electrochemical impedance spectroscopy
(EIS). It was shown that the top layer of PANI exhibits a lower porosity behavior with respect to Ni coating
and reduces the corrosion rate of Ni/MS almost by a factor of 3500 and increases the lifetime of Ni coating.
Keywords:
Mild steel
Nickel coating
Cyclic voltammetry
Polyaniline
EIS
© 2011 Elsevier Ltd. All rights reserved.
Corrosion protection
1. Introduction
approaches such as synthesis of substituted conducting polymers,
copolymers and bilayers have been attempted.
During the last decade, several research groups studied the
electrochemical synthesis of conducting polymer coatings on oxi-
dizable metals for corrosion protection purposes [1–19]. The
common feature of these studies is that the electrochemical
synthesis of conducting polymer coatings on oxidizable metals
is preceded by the dissolution of the base metal at a poten-
tial lower than the oxidation potential of monomer. Hence,
a successful electrochemical synthesis of conducting polymer
coatings on oxidizable metal demands a careful choice of the
solvent and/or supporting electrolyte and the establishment
of electrochemical parameters, which will strongly passivate
the metal without impeding the electropolymerization pro-
cess.
Only few electrolytes such as oxalate [9,19], salicylate [17],
tartrate [20], malate [21] and sulphate [10] suitable for the elec-
trochemical synthesis of conducting polymers have been reported.
However, the extent of using the conducting polymers is lim-
ited due to the exclusivity of the monomers that are essential for
their synthesis. To overcome this limitation, different synthesis
Our group has developed appropriate electrochemical
polymerization procedures to synthesize strongly adherent
poly(o-anisidine) (POA) [15,17,22], poly(o-toluidine) (POT)
[16,23] and poly(2,5-dimethylaniline) (PDMA) [24,25] coat-
ings on MS and copper (Cu). It was shown that these coatings
have remarkable capability to protect MS and Cu against cor-
rosion. Pawar et al. [26] reported recently, the synthesis of
poly(aniline-co-o-toluidine) coatings on MS from aqueous
salicylate solution using cyclic voltammetry and investi-
gated the corrosion properties of these copolymer coatings
in aqueous 3% NaCl. It was found that the poly(aniline-co-
o-toluidine) coatings provide better protection for MS against
corrosion than the corresponding homopolymer coatings. The cor-
rosion rates of PANI, POT and poly(aniline-co-o-toluidine) (with
feed ratio of o-toluidine as 0.5) coated MS are found to be ∼3, 20 and
50 times, respectively, lower than that observed for uncoated MS.
Bilayer coating consists of either a top coat of conducting poly-
mer on the layer of the other conducting polymer or a top layer
of conducting polymer on the metallic coating such as nickel.
Recently, we electrochemically synthesized the bi-layered com-
posites of poly(o-anisidine) (POA) and PANI on MS from aqueous
salicylate medium and investigated the corrosion protection per-
formance of these bi-layered composites in aqueous 3% NaCl
solution [27]. It was shown that the combinations of the bi-layered
composite coatings provide better protection to MS than single
∗
Corresponding author. Present address: School of Materials Science and Engi-
neering, Nanyang Technological University, Singapore 639798, Singapore.
Tel.: +91 2572234329.
E-mail address: sudeshna6480@yahoo.co.in (S. Chaudhari).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.12.096

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S. Chaudhari, P.P. Patil / Electrochimica Acta 56 (2011) 3049–3059
layered PANI or POA coatings. However, the corrosion protection
offered to MS depends on the deposition order of polymer layers in
the composite. Tuken and his research group [28] studied the elec-
trochemical synthesis of PANI and PPY coatings on nickel plated MS
and Cu substrates from aqueous media.
The objectives of the present study are: (i) to develop an appro-
priate electrochemical polymerization recipe to synthesize PANI
layer on Ni coated MS from aqueous media; (ii) to find potentially
good, low cost and easily available supporting electrolyte for the
electropolymerization of aniline on Ni coated MS; (iii) to synthesize
uniform, compact and strongly adherent PANI layer on Ni coated
MS and (iv) to examine the corrosion protection performance of
PANI coated Ni/MS in aqueous 3% NaCl solution.
Pawar et al. [29] have investigated the electrochemical poly-
merization of aniline on MS from the aqueous salicylate solution by
cyclic voltammetry. It was shown that the aqueous salicylate solu-
tion is a suitable medium for the electropolymerization of aniline
and it results in the deposition of uniform and strongly adherent
PANI coatings on MS. Therefore, in the present work an aqueous
solution of sodium salicylate was chosen as the supporting elec-
trolyte for the electrochemical synthesis of PANI on Ni coated MS.
To the best of our knowledge, there are no reports in the literature
dealing with the electrochemical synthesis of corrosion protective
PANI coatings on Ni coated MS from aqueous salicylate medium by
using cyclic voltammetry.
2.3.2. Electrodeposition of PANI on Ni/MS
The PANI was deposited over the Ni layer from 0.1 M
aqueous salicylate solution containing 0.3 M aniline to pre-
pare
a PANI/Ni/MS bi-layered structure. The electrochemical
synthesis was carried out under cyclic voltammetric condi-
tions in a single compartment three electrode cell with MS
as
a
working electrode (150 mm2), platinum as
a
counter
ref-
electrode and saturated calomel electrode (SCE) as
a
erence electrode. The cyclic voltammetric conditions were
maintained using the electrochemical measurement System
(SI 1280B, Solartron, U.K.) controlled by corrosion software
(CorrWare, Electrochemistry/Corrosion Software, Scribner Asso-
ciates Inc. supplied by Solartron, U.K.) [30]. The synthesis
of PANI was carried out by cycling the electrode poten-
tial between −1.0 V and 1.8 V with the scan rate of 0.02 V/s.
After deposition, the working electrode was removed from the
electrolyte and rinsed with double distilled water and dried
in air.
2.4. Characterization of the coatings
The FTIR transmission spectrum was recorded with a Perkin
Elmer spectrometer (1600 Series II, U.S.A.) in horizontally atten-
uated total reflectance (HATR) mode in the spectral range of
4000–400 cm−1. The UV–vis absorption study was carried out ex
situ at room temperature in the wavelength range of 300–1100 nm
using a microprocessor controlled double beam UV–vis spec-
trophotometer (Model U 2000, Hitachi, Japan). The thickness of the
PANI was measured by a conventional magnetic induction based
microprocessor controlled coating thickness gauge (Minitest 600,
ElectroPhysik, Germany). The error in the thickness measurements
was less than 5%.
2. Experimental
2.1. Materials
All chemicals were of analytical grade. The monomer aniline
was procured from Fluka and was doubly distilled prior to being
used for the synthesis. Sodium salicylate (NaC7H5O3), nickel sul-
phate (NiSO4), nickel chloride (NiCl2) and boric acid (H3BO3) were
procured from Merck and used as received without further purifi-
cation. Bi-distilled water was used to prepare all the solutions.
2.5. Evaluation of corrosion protection performance
The corrosion protection properties of the Ni/MS and bi-layered
structure PANI/Ni/MS were evaluated in 3% NaCl solution by using
OCP measurements, potentiodynamic polarization technique and
EIS using an electrochemical measurement system (SI 1280B,
Solartron, U.K.). For these measurements, a Teflon holder was
used to encase the substrates so as to leave an area of ∼0.4 cm2
exposed to the solution. All the measurements were repeated
at least four times and good reproducibility of the results was
observed.
The potentiodynamic polarization measurements were per-
formed by sweeping the potential between −0.25 V and 0.25 V
from OCP with the scan rate of 0.002 V/s. Before polarization the
substrates were immersed into the solution and the OCP was mon-
itored until a constant value was reached. The potentiodynamic
polarization curves were analyzed by using Corr-view software
from Scribner Associates which performs the Tafel fitting and
calculates the values of the corrosion potential (Ecorr), corrosion
current density (jcorr) and corrosion rate (CR) in mm per year
[30].
2.2. Substrate preparation
The MS substrates (size ∼10 mm × 15 mm and 0.5 mm thick)
were polished with a series of emery papers of different grit sizes
(180, 400, 600, 800 and 1200). After polishing, the substrates were
cleaned with acetone and double distilled water and dried in air.
Prior to any experiment, the substrates were treated as described
and freshly used with no further storage. The chemical composition
(by wt%) of the MS used in this study was: 0.03% C, 0.026% S, 0.01%
P, 0.002% Si, 0.04% Ni, 0.002% Mo, 0.16% Mn, 0.093% Cu and 99.64%
Fe.
2.3. Synthesis of bi-layered structure of PANI and Ni on MS
The bi-layered PANI/Ni coating was synthesized on MS using the
following procedures.
The EIS measurements were carried out at the OCP in the
frequency range from 0.1 Hz to 20 kHz with amplitude of super-
imposed AC signal of 0.010 V. The spectra were also recorded after
different immersion times in aqueous 3% NaCl solution. The analy-
sis of the impedance spectra was done by fitting the experimental
results to equivalent circuits using Z-view software from Scrib-
ner Associates [31]. The quality of fitting to equivalent circuit was
judged firstly by the Chi-square value (ꢀ2, i.e. the sum of the square
of the differences between theoretical and experimental points)
and secondly by limiting the relative error in the value of each
element in the equivalent circuit to 5%.
2.3.1. Electrodeposition of Ni layer on MS
The Ni layer was first deposited on MS substrate from an aque-
ous bath containing NiSO4 (30 wt%), NiCl2 (1 wt%) and H3BO3
(1.25 wt%) [28]. The pH of the electrolyte bath was found to be
∼5.6–6.2. The thickness of Ni layer was controlled by monitoring
the amount of charge passing for the deposition under constant
applied cathodic current of 10 mA. The deposition process was car-
ried out in open atmosphere and under stirring conditions using Ni
electrode (99.9% purity, 3 cm2 surface area) as anode.

S. Chaudhari, P.P. Patil / Electrochimica Acta 56 (2011) 3049–3059
3051
100
80
60
40
20
0
160
120
80
B
40
0
C
-40
-80
C
1
2
-1.0 -0.5 0.0 0.5 1.0 1.5 2.0
POTENTIAL vs SCE/V
(c)
(b)
A
B
(a)
C
-20
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
POTENTIAL vs SCE/V
Fig. 1. Cyclic voltammograms of (a) first, (b) second and (c) third scans recorded
during the polarization of MS electrode in 0.1 M aqueous salicylate solution. Inset
depicts the cyclic voltammogram of first scan recorded during the polarization of Pt
electrode under identical experimental conditions.
Fig. 2. XRD pattern of (a) as received MS and (b) polarized MS substrates. Inset
shows the XRD pattern of polarized Pt electrode. (X indicates the diffraction peaks
corresponding to iron salicylate.)
be attributed to the partial stabilization of the MS electrode sur-
face due to the formation of the complexation of Fe2+ cations by
salicylate counter-ions.
3. Results and discussion
3.1. Synthesis of PANI coatings on Ni/MS
In order to understand the observed cyclic voltammetry results,
we have performed the XRD measurements of the MS electrode
polarized in 0.1 M aqueous salicylate solution. The XRD patterns of
the uncoated MS and the polarized MS electrode are shown in Fig. 2.
Apart from the characteristic peak of MS, the XRD pattern of the MS
electrode polarized in 0.1 M sodium salicylate solution [Fig. 2(b)]
indicates the presence of diffraction peaks at 2ꢁ values of 11.90◦,
18.10◦, 28.80◦ and 38.10◦ due to iron salicylate (JCPDS # 01-055).
Thus, the XRD result clearly reveals the formation of iron salicylate
on the MS electrode surface. The XRD pattern of the polarized Pt
electrode is shown in the inset of Fig. 2. It exhibits the presence of
diffraction peaks at 2ꢁ values of 40.10◦ and 46.60◦ corresponding
to the Pt (JCPDS # 04-0802). We do not observe the presence of
any other peak due to salicylate complex, which confirmed that the
polarization of the MS electrode in 0.1 M sodium salicylate solution
results in the passivation of the electrode surface via the formation
of iron salicylate phase.
The MS electrodes were first polarized in 0.1 M aqueous sal-
icylate solution (without aniline) by cycling continuously the
electrode potential between −1.0 and 1.8 V versus SCE at a potential
scan rate of 0.02 V/s in order to understand the different processes
occurring at the electrode surface. The first scan of the voltammetric
response of the MS electrode in 0.1 M aqueous salicylate solution is
shown in Fig. 1(a). The first positive cycle is characterized by (i) an
anodic peak (A) at −0.498 V versus SCE; (ii) a small oxidation peak
(B) at ∼1.05 V versus SCE and beyond this potential high anodic cur-
rent flows. The anodic current decays very sharply and a negligibly
small current is observed till 0.043 V versus SCE. The negative cycle
terminates with the reduction peak C at ∼−0.838 V versus SCE.
To discriminate between the origin of the different peaks, the
platinum (Pt) electrode was polarized under identical experimental
conditions and the corresponding first scan of the cyclic voltam-
mogram is presented in the inset of Fig. 1. When the Pt electrode
was used, the first positive cycle is characterized by the reversible
oxidation peak B at ∼1.04 V versus SCE which is attributed to
the oxidation of the salicylate electrolyte. The absence of peak A
in the voltammogram corresponding to the Pt electrode polarized
under the same electrolytic conditions proves that in the case of
MS this peak comes from the oxidation of the MS electrode.
Hence, the anodic peak A is attributed to the dissolution of the
MS electrode surface which produces Fe2+ ions in its vicinity. These
ions interact with salicylate, corresponding to complexation of the
Fe2+ cation by salicylate counter-ions which inhibit the dissolu-
tion of the electrode surface [32]. As a result, just after the peak A,
the current density decreases and attains a negligibly small value.
The soluble colored products of the reaction diffuse into the elec-
trolyte. Indeed, the color of the electrolyte gradually changes to
brownish-pink during the polarization process. It seems that the
major portion of the reaction products is soluble which may be due
to the presence of carboxylic acid groups on the oligomers produced
during the polarization process. The oxidation peak B is assigned to
the oxidation of the salicylate electrolyte.
The cyclic voltammogram of the first scan recorded during
the synthesis of PANI coating on the MS is shown in Fig. 3(a).
70
A1
60
(b)
50
40
30
20
10
0
(a)
-10
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
On repetitive cycling, the voltammograms identical to that of
first scan are obtained. However, the position of the peak A is shifted
in the positive direction and the current density corresponding to
it increases with an increase in the number of scans. This may
POTENTIAL vs SCE/V
Fig. 3. Cyclic voltammograms of the first scan recorded during the synthesis of PANI
coating on (a) uncoated MS and (b) Ni/MS.

3052
S. Chaudhari, P.P. Patil / Electrochimica Acta 56 (2011) 3049–3059
70
60
50
40
30
20
10
0
A1
(a)
(b)
(c)
-10
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
POTENTIAL vs SCE/V
Fig. 4. Cyclic voltammogram of the (a) first, (b) second and (c) tenth scans recorded
Fig. 5. FTIR spectrum of the PANI coating synthesized on Ni/MS.
during the synthesis of PANI coating on Ni/MS.
This voltammogram is significantly different from that recorded
in aqueous salicylate solution without aniline (cf. Fig. 1(a)). Indeed,
the anodic peak A corresponding to the dissolution of the MS sub-
strate was not observed and the first potential sweep indicates the
anodic peak A1 at 1.36 V versus SCE. During reverse cycle, the anodic
current density decreases rapidly and a negligibly small current
density is observed until the end of the cycle. The non-observance
of the anodic peak A corresponding to the dissolution of the MS
when aniline is in the solution indicates that aniline is involved
in the passivation process and the electrochemical polymerization
takes place without dissolution of the electrode surface. The anodic
peak A1 corresponds to the oxidation of aniline and formation of
radical cations, which is considered to be the first step in the poly-
merization of the conducting polymers [33]. The non-observance
of the reduction peaks implies the occurrence of the irreversible
oxidation of aniline on the uncoated MS.
The cyclic voltammogram of the first scan recorded during the
synthesis of PANI coating on the Ni/MS is shown in Fig. 3(b). This
cyclic voltammogram exhibits good resemblance with the first
potential sweep [cf. Fig. 3(a)] recorded during the synthesis of PANI
layer on uncoated MS. In the first potential sweep, a well-defined
anodic peak at ∼1.42 (peak A1) versus SCE is observed, which obvi-
ously attributed to the oxidation of aniline. The non-observance
of the reduction peak implies the occurrence of the irreversible
oxidation of aniline on the Ni/MS.
tion was found to be ∼12.7 ␮m, which is fairly in agreement with
that measured by using a conventional magnetic induction based
microprocessor controlled coating thickness gauge.
The FTIR spectrum of PANI coating synthesized on Ni/MS
recorded in HATR mode is shown in Fig. 5. Main characteristic
bands of PANI are assigned as follows [35–38]: a broad band at
∼3360 cm−1 is due to the N–H stretching mode, the C N and C
C
stretching modes for the quinoid (Q) and benzoid (B) rings occur
at 1594 and 1491 cm−1, respectively, a band at ∼1380 cm−1 is
assigned to the C–N stretching, a band at ∼1299 cm−1 is assigned to
the presence of carboxylic groups of sodium salicylate, the bands at
1177 and 1023 cm−1 are attributed to the 1–4 substitution on the
benzene ring and the bands between 800 and 700 cm−1 reveal the
occurrence of the 1–3 substitutions.
The UV–vis absorption spectrum of PANI coating synthesized
on uncoated MS is shown in Fig. 6(a). It exhibits a shoulder peak
at ∼540 nm, which is attributed to the formation of pernigraniline
base (PB) form of PANI. The PB is the fully oxidized form of PANI and
is insulating in nature [39]. The UV–vis absorption spectrum of the
PANI coating on Ni/MS [Fig. 6(b)] does not exhibit major changes as
compared to the spectrum shown in Fig. 6(a), except the decrease
in the overall absorbance level.
4
The cyclic voltammograms of the first, second and tenth scans
recorded during the synthesis of PANI coating on Ni/MS are shown
in Fig. 4. The cyclic voltammogram of the first scan is also presented
in Fig. 4(a) for comparison. The cyclic voltammogram of the second
scan [Fig. 4(b)] is identical to that of first scan. However, the current
density corresponding to the oxidation peak decreases gradually
with the number of scans. The cyclic voltammogram of the tenth
scan [Fig. 4(c)] does not show well-defined redox peaks.
(a)
(b)
3
2
1
0
The thickness of PANI layer was estimated by using the equation
[34]:
QM
d =
(1)
2Fꢂ
assuming a two electron mechanism based on the monomer
involved in the polymerization process and a current efficiency
of 100%. In this equation, Q is the specific overall charge for the
electrochemical polymerization, ꢂ is the density of the PANI, M
is the molar mass and F is the Faraday constant (96,500 C). The
thickness of PANI layer (20 sweeps) calculated by using this equa-
300
400
500
600
700
800
900
1000
1100
Wavelength/nm
Fig. 6. UV–vis absorption spectra of the PANI coated (a) MS and (b) Ni/MS.

S. Chaudhari, P.P. Patil / Electrochimica Acta 56 (2011) 3049–3059
3053
-0.40
-0.45
-0.50
-0.55
-0.60
-0.65
-0.70
0.2
a
(d)
(b)
(c)
-0.2
-0.6
(a)
0
4
8
12
16
20
-1.0
1x10-8
1x10-5
1x10-2
1x101
1x104
IMMERSION TIME /h
CURRENT DENSITY /A/m-2
-0.50
-0.55
-0.60
-0.65
-0.70
b
Fig. 8. Potentiodynamic polarization curves recorded in aqueous 3% NaCl solution
for (a) uncoated MS, (b) Ni/MS, (c) PANI/MS and (d) PANI/Ni/MS.
that of the uncoated electrode. Thus, the PANI protects the MS in
3% NaCl for almost 8 h.
In case of PANI coated Ni/MS [Fig. 7(b)], initially the OCP of PANI
coated Ni/MS was measured to be ∼−0.555 V versus SCE, which is
more positive than that of the uncoated MS by up to ∼0.155 V versus
SCE. In the early stages of the immersion, the potential decreases
sharply and it attains a plateau wherein the potential remains fairly
constant at ∼−0.586 V versus SCE during the first 5 h of immersion.
This plateau is observed for 77 h during which the PANI coating
exhibits barrier behavior by limiting the diffusion of the corrosive
species towards the underlying steel substrate. After 77 h of immer-
sion the OCP decreases sharply and a second plateau is seen around
−0.592 V versus SCE. The duration of the second plateau is ∼122 h
and finally the potential decreases sharply to −0.615 V versus SCE,
which is close to the corrosion potential of the MS. Thus, the PANI/Ni
protects the MS in 3% NaCl for almost 199 h.
0
50
100
150
200
250
IMMERSION TIME /h
Fig. 7. Open circuit potential–time curves recorded for (a) PANI/MS and
(b) PANI/Ni/MS.
3.2. Evaluation of corrosion protection performance
3.2.2. Potentiodynamic polarization measurements
The corrosion protection performance of PANI coating on Ni/MS
was evaluated in 3% NaCl solution by using OCP measurements,
potentiodynamic polarization technique and EIS.
The behavior of the uncoated MS, Ni/MS and PANI coated MS and
PANI coated Ni/MS (12.7 ␮m thick) in aqueous 3% NaCl under the
potentiodynamic polarization conditions is presented in Fig. 8. The
values of the Ecorr, jcorr, Tafel constants (ˇa and ˇc), polarization
resistance (Rpol) and CR obtained from these curves are given in
Table 1.
The potentiodynamic polarization curve for Ni/MS is shown
in Fig. 8(b). It is observed that the jcorr decreases from
3.07 × 10−5 A/cm2 for uncoated MS to 1.74 × 10−5 A/cm2 for Ni/MS.
The Ecorr increases from −0.710 V versus SCE for uncoated MS to
−0.389 V versus SCE for Ni/MS. The positive shift of 0.321 V in Ecorr
indicates the protection of the MS surface by the Ni plating. The CR
of Ni/MS is found to be ∼0.20 mm/year which is ∼1.75 times lower
than that observed for uncoated MS.
3.2.1. OCP measurements
The evolution of OCP for PANI/MS and PANI/Ni/MS as a func-
tion of immersion time in 3% NaCl was studied. The corresponding
OCP–time curves are shown in Fig. 7. The OCP of PANI/MS [Fig. 7(a)]
was measured to be ∼−0.483 V versus SCE, which is more positive
than that of the uncoated MS by up to ∼0.227 V versus SCE. The
initial decrease in the potential is associated with the initiation of
the water uptake process in the coating. After 8 h of immersion
the potential decreases sharply and it remains fairly constant at
∼−0.608 V versus SCE, which is close to the corrosion potential of
MS. The protection time of the coating is determined by measuring
the elapsed time until the OCP of the polymer coated MS drops to
The potentiodynamic polarization curve for PANI coated MS
recorded in aqueous 3% NaCl solution is shown in Fig. 8(c). The
Table 1
Result of potentiodynamic polarization measurements.
Sample
Ecorr (Va)
jcorr (A/cm2)
ˇa (V dec−1
)
ˇc (V dec−1
)
Rp (ꢃ)
CR (mm year−1
)
% P
Uncoated MS
−0.710
−0.389
−0.502
−0.143
3.07 × 10−5
1.74 × 10−5
9.60 × 10−7
1.49 × 10−8
0.084
0.197
0.225
0.130
0.185
0.659
0.399
0.156
8.17 × 102
3.78 × 103
6.51 × 104
2.07 × 106
0.35
0.20
0.01
0.0001
–
–
Ni/MS (∼1 ␮m)
PANI/MS (∼2.90 ␮m)
4.19 × 10−3
PANI/Ni/MS (∼12.7 ␮m)
7.10 × 10−9
a
Potentials versus SCE.

3054
S. Chaudhari, P.P. Patil / Electrochimica Acta 56 (2011) 3049–3059
-0.75
Ecorr value increases from −0.710 V versus SCE for uncoated MS to
−0.502 V versus SCE for PANI coated MS. The jcorr decreases from
3.07 × 10−5 A/cm2 for uncoated MS to 9.6 × 10−7 A/cm2 for PANI
coated MS. The CR of PANI coated MS is found to be ∼0.01 mm/year
which is ∼35 times lower than that observed for uncoated MS.
The potentiodynamic polarization curve for PANI coated Ni/MS
recorded in aqueous 3% NaCl solution is shown in Fig. 8(d). The
Ecorr value increases from −0.710 V versus SCE for uncoated MS to
−0.143 V versus SCE for PANI coated Ni/MS. Further, the jcorr value
for PANI coated Ni/MS is observed to be far lower than the corre-
sponding values for uncoated MS as well as for Ni/MS indicating the
corrosion resistant feature of the PANI coating. The jcorr decreases
from 3.07 × 10−5 A/cm2 for uncoated MS to 1.49 × 10−8 A/cm2 for
PANI coated Ni/MS. The CR of PANI coated Ni/MS is found to be
∼0.0001 mm/year which is ∼3500 times lower than that observed
for uncoated MS. The protection efficiency (PE) of the coating was
calculated by using the expression:
a
-0.50
-0.25
0.00
0.00
0.25
0.50
0.75
Z (re) /kΩ
ꢀ
ꢁ
-4
-3
-2
-1
0
Rpol (coated) − Rpol (uncoated)
Rpol (coated)
PE% =
× 100
(2)
b
where Rpol (uncoated) is the polarization resistance of uncoated
MS and Rpol (coated) is the measured polarization resistance for
coated MS. The PEs of Ni/MS, PANI/MS and PANI/Ni/MS calculated
from potentiodynamic polarization data are found to be ∼78%, 98%
and 99%, respectively.
The porosity in the coating is a very important parameter to
determine whether a coating is suitable or not to protect the sub-
strate against corrosion. In order to calculate the porosity of these
deposits we have used the relationship [40]:
Rpol (uncoated)
|/ˇ
a
)
corr
P =
× 10−(|ꢄE
(3)
Rpol (coated)
0
1
2
3
4
where P is the total porosity, ꢄEcorr is the difference between cor-
rosion potentials and ˇa is the anodic Tafel slope for uncoated MS
substrate. Table 1 also gives the porosity values in the polymer
deposits. The porosity in PANI coating synthesized on MS and Ni/MS
calculated from potentiodynamic polarization data is found to be
∼4.19 × 10−3% and 7.01 × 10−9%, respectively. Thus, PANI coating
synthesized on Ni/MS is more compact and the porosity in this
coating is almost 5.9 × 105 times lower than the porosity in PANI
synthesized on uncoated MS.
Z (re) /kΩ
Fig. 9. Nyquist impedance plots for (a) uncoated MS and (b) Ni/MS recorded in
aqueous 3% NaCl solution.
semicircle in the low frequency region is attributed to the Ni/MS
interface and it is characterized by the Rct for the charge transfer
reactions occurring at the bottom of the pores in the coating and
the CPEdl
.
3.2.3. EIS studies
The Nyquist impedance plot and phase angle–frequency Bode
curve of PANI/Ni/MS recorded in aqueous 3% NaCl solution are
shown in Fig. 11(a) and (b), respectively. This impedance plot
was modeled by the electrical equivalent circuit, model B, shown
in Fig. 10(b). The first semicircle is attributed to the character-
istics of the PANI/electrolyte interface and it is characterized by
the Rp and CPEc. The second semicircle in the low frequency
region is attributed to the PANI/Ni interface and it is character-
ized by the Rct for the charge transfer reactions occurring at the
bottom of the pores in the coating and the CPEdl. The values of
the impedance parameters of the best fit to the experimental
impedance plots for uncoated MS, Ni/MS and PANI/Ni/MS are given
in Table 2.
The values of Rct for Ni/MS, and PANI/Ni/MS are found to be
∼3.379 and 97.457 kꢃ, respectively, which are about ∼5 and 149
times higher than that observed for uncoated MS. The protective
effect of Ni and PANI/Ni is obvious as the Rct values are signifi-
cantly higher as compared to the uncoated MS. However, PANI/Ni
bi-layered composite provides effective protection to MS against
corrosion than Ni coating, which is in agreement with the poten-
tiodynamic polarization results. The lower values of CPEc and CPEdl
for the PANI/Ni/MS provide further support for the protection of
MS by the PANI/Ni coating. Thus, the higher values of Rct and Rp
The Nyquist impedance plot of uncoated MS recorded in 3% NaCl
is shown in Fig. 9(a) and is modeled by an electrical equivalent
circuit [41–43] model A depicted in Fig. 10(a), where Rs represents
the electrolyte resistance, CPEp the phase element that represents
all the frequency dependent electrochemical phenomena, namely
double layer capacitance (Cdl) and diffusion processes and Rct the
charge transfer resistance connected with corrosion processes. The
constant phase element, CPE, is introduced in the circuit instead of
a pure capacitor to give a more accurate fit. Thus, the impedance
plot of the uncoated MS can be fitted with a semicircle, which is
attributed to the processes occurring at the MS surface.
The Nyquist impedance plot of Ni/MS recorded in aqueous 3%
NaCl solution is shown in Fig. 9(b). The electrical equivalent circuit
model B depicted in Fig. 10(b) is used to model this impedance plot.
It consists of the electrolyte resistance (Rs), pore resistance (Rp),
coating capacitance (Cc), Rct and Cdl. The constant phase element
(CPE) was used instead of capacitance. The CPE represents the devi-
ation from the true capacitance behavior. Thus, the impedance plot
of Ni/MS was fitted with two semicircles, a smaller one at high fre-
quency range followed by a larger one at lower frequencies. The first
semicircle is attributed to the characteristics of the Ni/electrolyte
interface and it is characterized by the Rp and CPEc. The second

S. Chaudhari, P.P. Patil / Electrochimica Acta 56 (2011) 3049–3059
3055
-160
CPEp
a
a
Rs
-120
-80
Rct
CPEc
b
2
Rs
-40
3
CPEdl
1
4
5
6
0
Rp
0
40
80
120
160
Z (re) /k
-70
-60
-50
-40
-30
-20
-10
0
as it is
4 h
48 h
72 h
96 h
240 h
b
CPEc
c
2
Rs
3
CPEdl
CPEdiff
6
Rp
4
1
5
Rct
Rdiff
Fig. 10. Electrical equivalent circuit models: (a) model A, (b) model B and (c) model
C used for the fitting of impedance plots.
-1
10
100
101
102
103
104
105
and lower values of Cc and Cdl indicate the excellent corrosion per-
formance of the PANI/Ni coating. The PE was also calculated by
FREQUENCY /Hz
using the expression (2) from the EIS data with the Rpol = Rp + Rct
.
Fig. 11. Nyquist impedance (a) and Bode plots (b) for PANI/Ni/MS recorded after 0 h
(curve 1), 4 h (curve 2), 48 h (curve 3), 72 h (curve 4), 96 h (curve 5) and 240 h (curve
6) of immersion in aqueous 3% NaCl solution.
The PEs calculated from EIS data for Ni/MS and PANI/Ni/MS were
found to be ∼85 and 99%, which is fairly in agreement with the
potentiodynamic polarization results.
the coating and consequently, the onset of corrosive processes at
the surface of the Ni coated MS.
3.2.4. Immersion tests
In order to gain further insight into corrosion protection proper-
ties, the impedance plots of Ni/MS and PANI/Ni/MS were recorded
as a function of immersion time in 3% NaCl solution.
The Nyquist impedance plots of PANI/Ni/MS recorded after 4, 48,
72, 96 and 240 h of immersion times in aqueous 3% NaCl are shown
in Fig. 11(a) (curves 2–6). These impedance plots exhibit system-
atic variations in terms of the values of the impedance parameters,
which are the result of the changes in the dielectric characteristics
of the coating due to electrolyte penetration through the pores in
3.2.4.1. Immersion time: from 4 to 48 h. The Nyquist impedance plot
recorded at 4 h of exposure time during the immersion in aqueous
3% NaCl solution is shown in Fig. 11(a) (curve 2). This plot is ana-
lyzed in terms of the same equivalent circuit model B, as shown
in Fig. 10(b). It is fitted with two semicircles with diameters Rp (at
higher frequency range) and Rct (at lower frequencies). The Rp is
related to the penetration of the electrolyte into the micropores
Table 2
Impedance parameter values extracted from the fit to the equivalent circuit for the impedance spectra recorded in aqueous 3% NaCl solution.
Immersion time
Rp (ꢃ)
Cc (F)
Rct (ꢃ)
Cdl (F)
Rdiff (ꢃ)
Cdiff (F)
Uncoated MS
As it is
Ni/MS
As it is
PANI/Ni/MS
As it is
4 h
16 h
48 h
72 h
96 h
–
–
6.54 × 102
7.99 × 10−4
9.1 × 10−5
–
–
–
–
1087
1.95 × 10−5
3.379 × 103
5408
4576
4021
3891
3497
2589
1790
845
1.03 × 10−6
1.30 × 10−6
1.46 × 10−6
2.88 × 10−6
4.47 × 10−6
5.02 × 10−6
1.38 × 10−5
1.55 × 10−5
97.46 × 103
20.06 × 104
20.30 × 104
24.07 × 104
15.90 × 103
60.55 × 102
45.44 × 102
33.34 × 102
4.53 × 10−7
1.21 × 10−6
1.65 × 10−6
2.44 × 10−6
1.57 × 10−5
2.03 × 10−5
2.53 × 10−5
2.91 × 10−5
–
–
–
–
–
–
–
–
21.38 × 103
23.23 × 103
27.03 × 103
32.48 × 103
2.44 × 10−5
3.22 × 10−5
2.94 × 10−6
2.69 × 10−5
144 h
240 h

3056
S. Chaudhari, P.P. Patil / Electrochimica Acta 56 (2011) 3049–3059
of the coating. The Rp value decreases slightly during this immer-
sion time. The slight decrease in the value of the Rp is attributed
to the entry of the electrolyte into the micropores in the coating
[41]. The Rct is related to the charge transfer process occurring at
the metal substrate beneath the coating [41]. In the present study,
the charge transfer process is attributed to anodic oxidation of Ni to
its oxide rather than the underlying MS, under accelerating effect
of top PANI layer. The Rct value is observed to increase during this
immersion period with respect to Ni/MS. This observation reveals
an efficient barrier property of the PANI layer against corrosion and
it is attributed to the oxidation of the Ni by the PANI, which results
in the formation of Ni oxide compounds at the PANI/Ni interface
and also to partial reduction of the polymer film.
It is known that the phase angle is more sensitive to the
state change of both coating and metal surface. The phase
angle–frequency Bode curve recorded for PANI/Ni/MS at 4 h expo-
sure time during immersion in aqueous 3% NaCl solution is shown
in Fig. 11(b) (curve 2). It is seen that the value of the phase angle is
maximum (∼60◦) at the middle frequencies and then it decreases
sharply to ∼13◦ at the lowest frequency. This observation provides
the evidence for a limited diffusability through the PANI/Ni com-
posite coating.
The Nyquist impedance plot recorded for PANI/Ni/MS after 48 h
of exposure time during the immersion in aqueous 3% NaCl solution
is shown in Fig. 11(a) (curve 3). This plot is also fitted satisfacto-
rily to the equivalent circuit model B, as shown in Fig. 10(b). The Rct
value is observed to increase further, whereas the Rp decreases fur-
ther during this immersion time. Further decrease in the Rp value
during this immersion time indicates the increase in the amount
of the electrolyte solution held within the pores of polymer coat-
ing. When Ni layer comes into contact with sufficient amount of
electrolyte solution, dissolution of Ni starts. In the mean time the
polymer film is reduced at the PANI/Ni interface. As a result of
these events, the thickness of the Ni oxide layer increases and the
polymer film reduces, consequently the increase in the Rct value is
observed. The Rct value after this immersion time was found to be
240.74 kꢃ. The Ecorr value at this immersion time is measured to be
−0.557 V versus SCE, which is more positive than that of the cor-
rosion potential for the uncoated MS by up to 0.153 V. Therefore,
it can be said that even after 48 h of exposure time, the detectable
corrosion processes were not started at the MS surface under the
coating.
at the bottom of the pores which interacts with the electrolyte
solution and results in the formation of corrosion products which
diffuse towards the coating surface through the pores in the coating.
More recently, we investigated the corrosion protection per-
formance of POA coatings on MS in 3% NaCl through immersion
tests by EIS. It was observed that a single equivalent circuit was
inadequate toexplain the various physical and electrochemical pro-
cesses occurring at different immersion times. It was shown that
a diffusion combination, which consists of the diffusion resistance
(Rdiff) and the diffusion capacitance (Cdiff) related to the diffusion
of corrosion products from steel surface towards coating must be
considered in the equivalent circuit after a certain immersion time
[41]. The same equivalent circuit, model C, depicted in Fig. 10(c)
was used to fit the impedance plot of PANI/Ni/MS recorded after
72 h of immersion. The model C consists of three time constants in
which the first time constant is a representative of the characteris-
tics of the PANI coating, while the second and third capacitive loops
are attributed to the complex reactions occurring at the surface of
the Ni coated MS. The second loop is characterized by charge trans-
fer resistance (Rct) and double layer capacitance (Cdl) and the third
loop is correlated to diffusion processes (Rdiff and Cdiff) caused by
the presence of corrosion products. It is found that the fitting curve
gives a good fit to the experimental impedance plot after 72 h of
immersion when the equivalent circuit model C is used.
After 72 h of immersion, the Rct value is observed to decrease
and its value is found to be ∼15.896 kꢃ. However, it is higher than
that of the uncoated MS. The decrease in the value of Rct may be
due to the dissolution of the passive oxide layer on the MS surface.
The diffusion resistance is the resistance against the diffusion of the
corrosion products from MS surface towards coating and its value
is observed to be ∼21.376 kꢃ.
The phase angle–frequency Bode curve recorded for PANI/Ni/MS
at 72 h exposure time during immersion in aqueous 3% NaCl solu-
tion is shown in Fig. 11(b) (curve 4). It is clearly observed that the
value of the phase angle exhibits two maxima and then it decreases
sharply. Therefore, it is expected to observe two separate semi-
circles in the Nyquist impedance plot; however these semicircles
cannot be resolved well.
The Nyquist impedance plot recorded at 96 h of exposure time
during the immersion in aqueous 3% NaCl solution is shown in
Fig. 11(a) (curve 5). The equivalent circuit model C [as shown in
Fig. 10(c)] is used to fit the experimental data. After 96 h of immer-
sion time, the Rct value is observed to decrease further to 6.055 kꢃ.
However, it is higher than that of the uncoated MS. It is impor-
tant to note that the Rdiff value is observed to increase during this
immersion time and it is found to be ∼23.230 kꢃ. The corrosion
products are partially deposited into the pores in the coating and
as a result, further diffusion of the corrosion products towards the
coating surface is hindered. Consequently, the increase in the Rdiff
is observed during this immersion time.
The Nyquist impedance plot recorded at 240 h of exposure time
during the immersion in aqueous 3% NaCl solution is shown in
Fig. 11(b) (curve 6). After 240 h of immersion time, the significant
decrease in the Rct value is observed and its value is found to be
∼3.334 kꢃ, which is still higher than that of the uncoated MS. As
expected the Rdiff value increases significantly and its value is found
to be ∼32.483 kꢃ. During this immersion time, the diffusion of the
corrosion products is hindered and they are accumulated at the
bottom of the coating, resulting in the formation of a loose corro-
sion product layer. However, the detachment of the coating was
not observed even after 240 h of immersion.
The phase angle–frequency Bode curve recorded for PANI/Ni/MS
at 48 h exposure time [Fig. 11(b) (curve 3)] is similar to that
observed at 4 h of exposure time. This suggests that the PANI/Ni/MS
composite has a limited diffusability for corrosive species even after
48 h of immersion.
3.2.4.2. Immersion time: from 72 to 240 h. Further increasing the
exposure time does not result in obvious variation in the shape of
the Nyquist impedance plots. The Nyquist impedance plot for the
PANI/Ni/MS at 72 h of exposure time during the immersion in the
aqueous 3% NaCl solution is shown in Fig. 11(a) (curve 4). As seen
earlier, the Nyquist impedance plots before 72 h of immersion are
fitted satisfactorily to the equivalent circuit model B. However, the
experimental Nyquist impedance plot after 72 h of immersion can-
not be well fitted to model B. During immersion, the electrolyte
penetrates via the pores in the coating and develops the electrolyte
pathways with time through coating. The corrosive species along
with the water diffuse through these paths towards the Ni surface.
When the sufficient amount of electrolyte reaches the Ni surface,
the corrosion processes are initiated at the PANI/Ni interface and
as a consequence the Ecorr value shifts to less noble value. The mea-
sured open circuit potential value at this moment is found to be
∼−0.515 V versus SCE. Therefore, it can be said that after 72 h of
immersion the anodic dissolution of the passive oxide layer begins
3.2.5. Evolution of impedance parameters with immersion time
The variation of Rp for Ni/MS and PANI/Ni/MS as a function of
immersion time is shown in Fig. 12(a). It is seen that the value of
Rp decreases rapidly and continuously for Ni/MS thereby indicating

S. Chaudhari, P.P. Patil / Electrochimica Acta 56 (2011) 3049–3059
3057
4.0
3.5
3.0
2.5
2.0
1.5
-3.0
Ni/MS
Ni/MS
a
b
PANI/Ni/MS
PANI/Ni/MS
-3.5
-4.0
-4.5
-5.0
-5.5
-6.0
0
50
100
150
200
250
0
4
8
12
16
(IMMERSION TIME)1/2/h1/2
IMMERSION TIME /h
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Ni/MS
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
c
250
200
150
100
50
PANI/Ni/MS
Ni/MS
d
PANI/Ni/MS
0
0
50
100
150
200
250
0
50
100
150
200
250
IMMERSION TIME /h
IMMERSION TIME /h
0.9
0.6
0.3
0.0
300
250
200
150
100
50
Ni/MS
Ni/MS
PANI/Ni/MS
f
e
PANI/Ni/MS
0
0
50
100
150
200
250
0
50
100
150
200
250
IMMERSION TIME /h
IMMERSION TIME /h
Fig. 12. (a) Variation of pore resistance Rp for Ni/MS and PANI/Ni/MS as a function of immersion time in 3% NaCl solution; (b) variation of coating capacitance Cc for Ni/MS
and PANI/Ni/MS as a function of immersion time in 3% NaCl solution; (c) volume fraction of the electrolyte in Ni/MS and PANI/Ni/MS coating as a function of immersion time
in 3% NaCl solution; (d) variation of charge transfer resistance Rct for Ni/MS and PANI/Ni/MS as a function of immersion time in 3% NaCl solution; (e) variation of double layer
capacitance Cdl for Ni/MS and PANI/Ni/MS as a function of immersion time in 3% NaCl solution; (f) delamination area for Ni/MS and PANI/Ni/MS as a function of immersion
time in 3% NaCl solution.
the least corrosion protection offered by Ni plating to MS. In the case
of PANI/Ni/MS composite, almost constant value of Rp was observed
up to 96 h of immersion and thereafter a slow decrease in the value
of Rp was seen to occur. This observation reveals that development
ofelectrolytepathwaysthroughthecompositereached steadystate
since the first measurements [42,43]. Moreover, even after 240 h of
immersion, the values of Rp for PANI/Ni/MS composite were found
to be higher than those observed for Ni/MS.
The coating capacitance Cc is the capacitance of the intact coat-
ing layer or the capacitance of the areas where rapid solution uptake
does not occur [41]. The evolution of the Cc with the immersion
time is shown in Fig. 12(b). The square root of time, t1/2, is used in
the figure to improve the resolution of the time axis. The increase
in the values of Cc for Ni/MS and PANI/Ni/MS can be interpreted
as consequence of continuous water uptake. The values of Cc for
the bi-layered PANI/Ni/MS composite are significantly lower than

3058
S. Chaudhari, P.P. Patil / Electrochimica Acta 56 (2011) 3049–3059
those observed for Ni/MS even after 240 h of immersion. Thus, in
the case of PANI/Ni/MS composite the water permeation is very
small up to 240 h of immersion.
The volume fraction of the water in the polymer coating when
in contact with 3% NaCl solution as a function of time is determined
using Brasher–Kingsbury equation [44]:
Ecorr of the PPY coated Ni/MS after 24 h of immersion in 3.5% NaCl
and the Icorr value is not reported.
In the present study, it is observed that the OCP of the
PANI/Ni/MS remains fairly constant at ∼0.592 V for a longer period
of time (more than 120 h) under the given experimental conditions.
This observation indicates barrier behavior of the PANI top coat on
Ni/MS, which limits the diffusion of the corrosive species towards
the underlying substrate. The most important result of the corro-
sion protection of Ni/MS by PANI top coat is the positive potential
shift (0.567 V versus SCE) in the Ecorr to noble value in comparison
to the uncoated Ni/MS. This potential shift to more noble values
is regarded as the anodic protection. Moreover, the jcorr values are
∼2060 times lower than that of the uncoated MS. The EIS study
indicates that the Rct value of PANI/Ni/MS is higher (∼149 times)
than that of the uncoated MS. Moreover, it is seen that water per-
meation is negligibly small up to 144 h of immersion, which shows
that the coating resists water uptake and it is corrosion resistant.
Thus, it is possible to deposit strongly adherent PANI top coat on
Ni/MS using sodium salicylate as a supporting electrolyte and it
can be considered as a potential coating material to protect Ni/MS
against corrosion in aqueous 3% NaCl.
It is now apparent that the conducting polymer accepts elec-
trons from the metal and gives them to oxygen. This reaction
generates the formation of a passive oxide layer at the poly-
mer/metal interface which lowers the corrosion rate and shifts the
Ecorr to more positive values. The result of the potentiodynamic
polarization measurements for the PANI coated MS shows a shift
in the Ecorr to more positive value (0.208 V) and reduces the cor-
rosion rate of MS almost by a factor of 35. The positive shift of
0.208 V versus SCE in Ecorr indicates the protection of the MS surface
by the PANI layer. Thus, the protection effect of PANI layer seems
to be related to its passivation effect as well as its barrier behav-
ior. The potentiodynamic polarization measurement of PANI/Ni/MS
exhibits a shift in the Ecorr to more positive value (0.567 V) and
reduces the corrosion rate of MS almost by a factor of 3500. In this
case, the Ni (and its oxide) acts as the additional barrier as they are
more stable and corrosion protective in neutral or near alkaline
conditions [46]. Besides, the PANI top coat on Ni/MS itself con-
stituted an effective barrier with low permeability on the surface.
Thus, corrosion protection performances of Ni/MS and PANI/Ni/MS
indicate that the bi-layered PANI/Ni coating provides much better
protection to MS for longer period.
log Ct/C0
log 80
ꢅ =
(4)
where Ct is the coating capacitance at an instant t and C0 is the
capacitance at time t = 0; the value 80 is the dielectric constant of
water at 25 ◦C. The volume fraction of water in the coating as a
function of immersion time is shown in Fig. 12(c). It is seen that
water permeation in the bi-layered PANI/Ni/MS composite is very
small up to 144 h of immersion, which shows that this composite
resists water uptake and it is corrosion resistant.
The variation of Rct as a function of the immersion time
is shown in Fig. 12(d). In the case of Ni/MS, the value of Rct
decreased rapidly with the immersion time up to 96 h and there-
after it remained fairly constant with the immersion time. It is
observed that for PANI/Ni/MS, the Rct value increased up to 72 h
of immersion and after 72 h of immersion, it decreased abruptly to
15.896 kꢃ. Thereafter, the value of Rct for PANI/Ni/MS remained
almost constant with the immersion time and after 240 h, it
was found to be ∼3.334 kꢃ which is about ∼5.1 times higher
than that observed for uncoated MS. Thus, the PANI/Ni/MS com-
posite provides better protection to MS against corrosion than
Ni plating.
The double layer capacitance Cdl represents the wet area under
the coating, which is the area in contact with the electrolyte. The
value of Cdl for Ni/MS (Fig. 12(e)) increases sharply after 96 h of
immersion which indicates an increase in the area at which delam-
ination and/or corrosion occurred under the coating. It is observed
that for PANI/Ni/MS composite the Cdl values are the lowest and
they remain almost constant up to immersion time of 240 h indi-
cating that this bi-layered composite provides better protection to
MS than PANI/Ni/MS composite.
The delamination area is evaluated by comparing the values
of the measured Cdl with a specific capacitance Cdl (measured on
uncoated substrate) [45]. The determined delamination area val-
ues as a function of immersion time [Fig. 12(f)] clearly indicate
negligible delamination up to 240 h as evident from the lowest
values of the order of 0.0003–0.008 cm2. This confirms the stabil-
ity of PANI/Ni composite coating on MS when exposed to 3% NaCl
solution.
4. Conclusions
Corrosion protective PANI coatings were successfully synthe-
sized on Ni coated MS by electrochemical polymerization of aniline
from aqueous salicylate solution. The electrochemical polymeriza-
tion process occurs without noticeable dissolution of MS. It was
shown that the sodium salicylate is a suitable supporting elec-
trolyte for the electrochemical polymerization of aniline on Ni
coated MS. The UV–vis absorption spectroscopy study of PANI
reveals the formation of the pernigraniline base (PB) form of PANI.
The potentiodynamic polarization and electrochemical impedance
spectroscopy studies reveal that the PANI acts as a corrosion pro-
tective layer on Ni coated MS in 3% NaCl solution. The top layer of
PANI exhibits a lower porosity behavior with respect to Ni coating
and reduces the corrosion rate of Ni coated MS almost by factors
of 3500 (relative to uncoated MS) and increases the lifetime of Ni
coating.
Thus, the experimental results of the present study reveal that
the PANI acts as a corrosion protective layer on Ni coated MS in
3% NaCl solution. The top layer of PANI exhibits a lower porosity
behavior with respect to Ni coating and reduces the corrosion rate
of Ni coated MS almost by factors of 3500 (relative to uncoated MS).
As mentioned earlier, Tuken et al. [28] studied the electrochem-
ical synthesis of PANI and PPY coatings on Ni plated MS from
aqueous oxalic acid medium. Corrosion tests, including EIS, anodic
polarization curves and OCP–time curves, in 3.5% NaCl, all indicated
that the PANI top coated Ni/MS provide better protection for longer
periods than the single PPY film. Although the potentiodynamic
polarization measurements indicate the positive shift of 0.398 V in
Ecorr of the PANI coated Ni/MS after 24 h of immersion in 3.5% NaCl,
the Icorr value (0.017 mA) is higher than that of Ni/MS. These results
emphasized that the PANI layer synthesized from aqueous oxalic
acid solution on Ni/MS is not protective in NaCl medium. These
authors also performed the long term immersion experiments and
found that after 120 h of exposure, the PANI layer saturates with
the electrolyte and consequently its efficiency diminished signifi-
cantly. In the case of PPY top coat on Ni/MS, the potentiodynamic
polarization measurements indicate the positive shift of 0.471 V in
Acknowledgement
The financial support from the University Grants Commis-
sion (UGC), New Delhi, India, through SAP-DRS-II programme No.
F.530/2/DRS/2010(SAP-I) is gratefully acknowledged.

S. Chaudhari, P.P. Patil / Electrochimica Acta 56 (2011) 3049–3059
3059
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