Black Wattle Tannin As Steel Corrosion Inhibitor

Abstract

In order to reduce the environmental impacts caused by chemical substances harmful to the environment and human health, the black wattle tannin can be used as an environmentally friendly corrosion inhibitor in acid and near neutral media. This paper provides information on the application of black wattle tannin as an inhibitor against the corrosion of carbon steel. The inhibition was evaluated using potentiodynamic polarization (PP) and electrochemical impedance spectroscopy (EIS) at room temperature in aerated 0.1 mol L
−1
Na
2
SO
4
(pH 6.0 and 2.5). The black wattle tannin when used as a corrosion inhibitor is more effective at acidic pH, its efficiency being dependent on its concentration. At the higher pH value (pH 6.0), a blue-black film (ferric tannate) with a short-term protection against corrosion was formed in aerated aqueous solution. At pH 2.5, this blue-black film was not observed.

Full text

International Scholarly Research Network
ISRN Corrosion
Volume 2012, Article ID 937920, 9 pages
doi:10.5402/2012/937920
Research Article
Black Wattle Tannin As Steel Corrosion Inhibitor
Rafael Silveira Peres,
1
Eduardo Cassel,
2
and Denise Schermann Azambuja
1
1
Laborat
´
orio de Eletroqu
´
ımica, Instituto de Qu
´
ımica, UFRGS, Avenida Bento Gonc¸alves 9500,
90619-900 Porto Alegre, RS, Brazil
2
Departamento de Engenharia Qu
´
ımica, Faculdade de Engenharia, PUCRS, Avenida Ipiranga 6681,
90619-900 Porto Alegre, RS, Brazil
Correspondence should be addressed to Denise Schermann Azambuja, denise@iq.ufrgs.br
Received 31 January 2012; Accepted 22 February 2012
Academic Editors: L. Bazzi, G. Bereket, A. Kalendov
´
a, and E. E. Oguzie
Copyright © 2012 Rafael Silveira Peres et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the or iginal work is properly
cited.
In order to reduce the environmental impacts caused by chemical substances harmful to the environment and human health, the
black wattle tannin can be used as an environmentally friendly corrosion inhibitor in acid and near neutral media. This paper pro-
vides information on the application of black wattle tannin as an inhibitor against the corrosion of carbon steel. The inhibition
was evaluated using potentiodynamic polarization (PP) and electrochemical impedance spectroscopy (EIS) at room temperature in
aerated0.1molL
−1
Na
2
SO
4
(pH 6.0 and 2.5). The black wattle tannin when used as a corrosion inhibitor is more effective at acidic
pH, its efficiency being dependent on its concentration. At the higher pH value (pH 6.0), a blue-black film (ferric tannate) with a
short-term protection against corrosion was formed in aerated aqueous solution. At pH 2.5, this blue-black film was not observed.
1. Introduction
Tannins comprise two different classes of polyphenolic com-
pounds; hydrolysable and condensed tannins. The condens-
ed tannins are found in substantial concentration in the
wood and bark of several trees, for instance, black wattle [1].
The tannin extracted from the bark of the black wattle tree
contains flavonoid units such as (
−)-robinetinidol, (+)-cate-
chin, and (+)-gallocatechin [2]. The flavonoid units are tree-
ring flavonols with fifteen carbons. The molecular weight of
condensed tannins ranges from around 500 to over 20,000
[1–3].
The hydrolysable tannins are obtained from the fruit,
pod, and wood of se veral trees (fruit of Terminalia chebula,
pod of Caesalpinia coriaria, and wood of Castanea valonea)
[1–4]. This class of tannin is made up mainly of gallic and
digallic a cids, which are often esterified to polyols [3], and
have molecular weights of up to 3000 [1].
Due to the OH
−
groups in the ortho position on the aro-
matic rings, tannins are able to form chelates with iron and
other metallic cations (e.g., copper) [5]. Yahya et al. reported
the formation of ferric tannate with both condensed and
hydrolysable tannins [6]. When Fe
3+
ions react with OH
−
groups in the orthoposition in aerated aqueous solution, a
highly insoluble and blue-black complex (ferric tannate) is
formed [7].
The application of tannins in corrosion studies has been
investigated by several authors and their efficiency is contro-
versial. According to Favre et al. the presence of hydrolysable
tannin (gallic acid) inhibits the formation of magnetite when
lepidocrocite is reduced [8, 9]. Electrochemical impedance
spectroscopy (EIS) experiments performed by Galvan et al.
showed no substantial increase in the polar ization resistance
(R
p
) of steel due to treatment of a rusted surface with tannins
[10]. On the other hand, some authors have developed a tan-
nin primer that exhibited excellent anticorrosive properties
[11–14]. Rahim et al. found that the mangrove tannin and
its flavonoid monomers are potential corrosion inhibitors for
steel in acidic medium [15]. According to Ross and Francis,
some of the rust is converted to more stable, inert, and adhe-
rent products [16]. These contradictory results may be due to
the diversity of material used in the different studies [17, 18].
Martinez investigated the mechanism of the adsorption
of mimosa tannin onto low-carbon steel in sulphuric acidic
solutions. At pH 1 and 2, the value of the free energy of adsor-
ption suggests a chemisorption mechanism. This mechanism

2 ISRN Corrosion
4400
4390
4380
4370
1500
1000
500
0
0 0.5 1 1.5 2
R
P
(Ohm·cm
2
)
Tannin concentration (g L
−1
)
Figure 1: Influence of tannin concentration i n the polarization
resistance (R
p
) for carbon steel samples immersed 1 day in aerated
0.1 mol L
−1
Na
2
SO
4
solutions: () pH 2.5 medium and (•)pH6
medium.
occurs due to the formation of an adsorption bond between
the oxygen lone-pair electrons of the tannin
−
OH group and
the metal surface. At pH
≥ 3, ferric tannate is formed and
the value of the free energy of adsorption suggests a physisor-
ption m echanism of ferric tannate adsorption onto the steel
surface [19].
The aim of this study was to evaluate the anticorrosive
properties of tannin extracted from the bark of the black
wattle tree (Acacia mearnsii De Wild.) in 0.1 mol L
−1
Na
2
SO
4
at pH 6.0 and 2.5 for application on carbon steel. The corro-
sion tests were carried out in aerated sulphate solution at
room temperature, using electrochemical impedance spe c-
troscopy (EIS) and potentiodynamic polarization (PP).
2. Experimental Procedure
2.1. Material. The tannin sample used in this work was sup-
plied by TANAC (Montenegro, Brazil). The chemical com-
position of the carbon steel samples is given in Ta ble 1.All
solutions and samples were prepared with analy tical grade
reagent. Sulphuric acid (Synth, Brazil) was used to adjust the
pH value. Sodium sulphate (VETEC, Brazil) was employed
in the preparation of the electrolyte solutions.
2.2. Sample Preparation and Exper imental Setup. Disc-work-
ing electrodes with an area of approximately 1.0 cm
2
were
prepared from the steel samples by cold resin (epoxy) embed-
ding. The surface was prepared by grinding with silicon car-
bide paper up to grade number 1200 foll owed by degreasing
with an acetone/chloroform mixture (1 : 1) and drying under
ahotairstream.
A satur ated calomel electrode (E
= 0.241 V/NHE) was
used as the reference electrode, and all potentials are referred
to it. The auxiliary electrode was a Pt wire. The experiments
were carried out under naturally aerated conditions at 25
◦
C.
Table 1: Chemical composition of carbon steel sample.
Chemical composition
Element C Mn P S Cu Cr
wt.% 0.103 0.46 0.013 0.096 0.01 0.18
The electrochemical measurements were performed using a
potentiostat (AUTOLAB PGSTAT 30, (Echo Chemie, The
Netherlands)) coupled to a frequency response analyser
(FRA 2). The software used for analysis of impedance spectra
was FRA 4.9 (Echo Chemie, The Netherlands). The potentio-
dynamic polarization curves were recording a t a scan rate of
1mVs
−1
. The corrosion potentials (E
corr
) and the polariza-
tion resistances (R
p
) were calculated at a scan rate of
0.1 mVs
−1
with potential range of E
corr
±20 mV. This method
was based on ASTM G59 [20] and ASTM G102 [21] stand-
ards.
The EIS measurements were performed in potentiostatic
mode at the open circuit potential, OCP. The OCP after
potential stabilization is referred to in this study as the corro-
sion potential, E
corr
. The amplitude of the EIS perturbation
signal was 10 mV, and the frequency range studied was from
10
5
to 10
−2
Hz.
Fourier transform infrared (FTIR) analysis was p erform-
ed with a Perkin Elmer Spectrum 1000 spectrometer.
3. Results and Discussion
3.1. Influence of Black Wattle Tannin Concentration. Figure 1
shows the polarization resistance (R
p
) for carbon steel sam-
ples immersed for 1 day in a erated 0.1 mol L
−1
Na
2
SO
4
solu-
tions (pH 2.5 and 6) with different concentrations of black
wattle tannin. The R
p
values were obtained from stepwise
potentiostatic polarization using a single small potential step
(ΔE)of
±20 mV.
At concentrations of up to 2 g L
−1
of tannin, the R
p
values
of the steel samples increase for both mediums (pH 2.5 and
pH 6). Concentrations above 2 g L
−1
were not tested due to
the presence of nondissolved black wattle tannin particles in
Na
2
SO
4
solutions at room temperature. Thus, in this study,
the concentration of tannin used as an inhibitor was 2 g L
−1
in aerated 0.1 mol L
−1
Na
2
SO
4
solution. This concentration
is not very different from that reported by Rahim et al. who
tested the efficiency of mangrove tannin [15, 22].
3.2. Study at pH 6.0. After one hour of immersion in aerated
0.1 mol L
−1
Na
2
SO
4
solution (pH 6.0), a nonadherent blue-
black film on the steel surface and a blue-black precipitate on
the bulk of the electrolyte were observed. Some authors have
attributed the formation of these blue-black products to
ferric-tannate formation [23]. Fourier transform infrared
(FTIR) analysis was performed in order to verify whether
these blue-black products were ferric tannate.
3.2.1. Results of Fourier Transform Infra red (FTIR) Analysis.
The FTIR spectrum of the black wattle tannin (Figure 2(a))
shows a broad absorption band with a maximum absorbance

ISRN Corrosion 3
4000 3600 3200 2800 2400 2000 1600 1200 800 400
Wavenumber (cm
−1
)
(a)
(b)
T (%)
Figure 2: (a) FTIR spectrum of the black wattle tannin and (b)
FTIR spectrum of the blue-black precipitate formed in the bulk
of the electrolyte after one hour of immersion of steel in aerated
0.1 mol L
−1
Na
2
SO
4
solution (pH 6.0) in the presence of acacia
tannin.
at 3413 cm
−1
which is due to the presence of hydroxyl groups
[15]. Peaks occurring between 1600 and 1450 cm
−1
are char-
acteristic of aromatic compounds [15]. Various peaks in the
600–1300 cm
−1
correspond to substituted benzene rings
[15].
Figure 2(b) shows the FTIR spectrum of the blue-black
precipitate. The reduced intensity (11.90% to 20.02% of
transmittance) of the broad peak at around 3413 cm
−1
shows
the reduction of free OH groups [24]. These hydroxyl groups
in aromatic rings enable the tannins to form ferric-tannate
complexes. In this way, the formation of ferric tannate was
detected. The peaks occurring at 1110 cm
−1
and 619 cm
−1
are
characteristic of sulphate groups (electrolyte solution) [25,
26].
3.2.2. Pote ntiodynamic Polarization. The potentiodynamic
polarization curves of steel immersed in aerated 0.1 mol L
−1
Na
2
SO
4
solution (pH 6.0), in the presence and absence of
tannin, were obtained from 600 mV up to E
corr
and 300 mV
down to E
corr
after different periods of immersion (Figure 3).
According to Figure 3, the anodic current densities dec-
rease in the presence of tannin in all immersion times, while
the cathodic current densities show no significant modifica-
tion. The presence the black w attle tannin shifts the E
corr
to
more positive values in whole immersion times.
The corrosion current densities ( j
corr
)inTa ble 2 were cal-
culated from the extrapolation of the anodic and cathodic
Tafel li nes (Figure 3). The inhibition efficiencies IE (%) were
calculated by the polarization resistance according to (1)
[15]:
IE
(
%
)
=
R
p
tannin
− R
p
R
p
tannin
× 100, (1)
where R
p
tannin
and R
p
are the polarization resistance value with
the presence and absence of black wattle tannin, respectively.
0
−0.2
−0.4
−0.6
−0.8
−1
−8 −7 −6 −5 −4 −3 −2 −1
Potential (V)
log j (A cm
−2
)
(1)
(2)
(a)
−8 −7 −6 −5 −4 −3 −2
−1
log j (A cm
−2
)
−0.2
−0.4
−0.6
−0.8
−1
Potential (V)
(1)
(2)
(b)
−0.2
−0.4
−0.6
−0.8
−1
Potential (V)
−8 −7 −6 −5 −4 −3 −2 −1
log j (A cm
−2
)
−9
(1)
(2)
(c)
Figure 3: Potentiodynamic polarization curves of steel immersed
in aerated 0.1 mol L
−1
Na
2
SO
4
solution (pH 6.0) in the presence of
tannin (1) and absence of tannin (2) for (a) 1 day, (b) 3 days, and 7
days.

4 ISRN Corrosion
3
2.5
2
1.5
1
0.5
0
32.521.510.5
10
8
6
4
2
0
1086420
Z

(kOhm·cm
2
)
Z

(kOhm·cm
2
)
Z

(kOhm·cm
2
)
Z

(kOhm·cm
2
)
Figure 4: Nyquist plots for steel immersed in 0.1 mol L
−1
Na
2
SO
4
solution (pH 6.0) for 1 day (closed triangle), 3 days (closed circle),
and 7 days (closed star) in the presence of tannin; and for 1 day
(open square), 3 days (open lozenge), and 7 days (open circle) in
the absence of tannin.
Table 2: Polarization parameters for the corrosion of carbon steel
immersed in 0.1 mol L
−1
Na
2
SO
4
solution (pH 6.0), in the presence
and absence of tannin, for different periods.
j
corr
(μAcm
−2
)
E
corr
(V)
R
p
(Ω cm
2
)
IE (%)
1 day 39.66 −0.73 486.46
91.06
1 day with tannin 0.883
−0.50 5446.0
3 days 20.52
−0.73 461.37
93.03
3 days with tannin 1.512
−0.72 6622.2
7 days 13.58
−0.75 481.21
75.74
7 days with tannin 3.241
−0.72 1984.3
The j
corr
value decreases significantly in the presence
of tannin during the first three days of immersion. In the
seventh day, the difference between the blank j
corr
and the
inhibitor j
corr
decreases. The inhibition efficiency remains
almost constant for the first and third days, though decreased
on the seventh day of immersion.
3.2.3. Elect rochemical Impedance Spectroscopy (EIS) . The EIS
spectra for the steel in aerated 0.1 mol L
−1
Na
2
SO
4
solution
(pH 6.0), in the presence and absence of tannin, for different
periods are shown in Figures 4 and 5.
After1,3,and7daysofimmersioninthepresenceoftan-
nin the plots show two time constants, one at high frequency
and one at low frequency with maximum phase angles
of around
−20
◦
and −45
◦
, respectively. The evolution of
the EIS spec tra may be related to the adsorption of the
inhibitor on the metal surface resulting in an increase in
the resistance of the steel to polarization. The experimental
data shown in Figures 4 and 5 were fitted using two different
equivalent circuits (EC) depending on the immersion time as
shown in Figure 6. The fitting quality was evaluated based on
− Phase angle (deg)
log f (Hz)
60
40
20
0
−2 −1012345
(a)
4
3.5
3
2.5
2
1.5
log f (Hz)
−2 −1012345
log |Z| (Ohm·cm
2
)
(b)
Figure 5: Bode plots for steel immersed in 0.1 mol L
−1
Na
2
SO
4
solution (pH 6.0) for 1 day (closed triangle), 3 days (closed circle),
and 7 days (closed star) in the presence of tannin; for 1 day (open
square), 3 days (open lozenge), and 7 days (open circle) in the
absence of tannin.
the error percentage associated with each component, show-
ing errors smaller than 5%. The software used to simulate the
EIS data was NOVA 1.7 (Echo Chemie, The Netherlands).
The simulated data obtained from the fittings are given in
Table 3.
The equivalent circuit (EC) proposed for fitting the EIS
diagram after 1, 3, and 7 days of immersion in the presence
of tannin is R
s
(Q
1
[R
1
(Q
2
R
2
)]) where R
s
represents the ohmic
resistance between the reference and working electrodes, R
1
and R
2
represent the resistance or charge transfer in physical
meaning [27]. The Q
1
and Q
2
parameters are the impedance
related to a constant phase element (CPE) and can
be attributed electrode surface or adsorbed species [27]. The
CPE impedance takes into account the phenomena due to
the surface heterogeneities. The CPE impedance is given by
(2)[27]:
1
Z
CPE
= Q

jω

n
,(2)

ISRN Corrosion 5
Table 3: Fitting parameters used to simulate the EIS plots for steel immersed for 1, 3, and 7 days in 0.1 mol L
−1
Na
2
SO
4
solution (pH 6.0) in
the presence and absence of tannin.
Immersion time Chi-square Equivalent circuit
Fitting parameters
R
s
(Ω cm
2
)
R
1
(kΩ cm
2
) Q
1
(F cm
2
) n
1day 1.3 × 10
−3
69.2
1.85 1.48
× 10
−4
0.80
3days 0.9
× 10
−3
R
s
(R
1
Q
1
) 61.4
2.13 3.01
× 10
−4
0.73
7days 1.1
× 10
−3
58.6
2.30 2.24
× 10
−4
0.78
R
s
(Ω cm
2
) Q
1
(F cm
2
) n
1
R
1
(kΩ cm
2
) Q
2
(F cm
2
) n
2
R
2
(kΩ cm
2
)
1 day with tannin 3.1 × 10
−3
64.1 2.34 × 10
−6
0.64 0.369 1.84 × 10
−4
0.57 53.7
3 days with tannin 2.6
× 10
−3
R
s
(Q
1
[R
1
(Q
2
R
2
)]) 62.3 3.10 × 10
−6
0.60 0.378 1.57 × 10
−4
0.63 66.5
7 days with tannin 3.4
× 10
−3
65.6 2.68 × 10
−6
0.60 0.112 7.83 × 10
−5
0.73 6.56
R
s
represents the ohmic resistance between the reference and working electrodes; R
1
and R
2
represent the resistance or charge transfer; Q
1
and Q
2
are the im-
pedance related to a constant phase element (CPE) and can be attributed to electrode surface or adsorbed species; n, n
1
,andn
2
represent a CPE exponent.
Capacitor for n
= 1, a resistor for n = 0, and a diffusion process for n = 0.5.
R
s
R
s
R
1
R
1
R
2
CPE
(a) (b)
CPE
1
CPE
2
Figure 6: Equivalent electric circuits proposed to simulate the ex-
perimental data of steel immersed in 0.1 mol L
−1
Na
2
SO
4
(pH 6.0)
for (a) 1, 3, and 7 days in the absence of tannin; for (b) 1, 3, and 7
days in the presence of tannin.
Table 4: Black wattle tannin inhibition efficiencies obtained from
electrochemical impedance spectroscopy (EIS) and potentiodyna-
mic polarization (PP) in 0.1 mol L
−1
Na
2
SO
4
solution (pH 6.0).
Potentiodynamic
polarization (PP)
Electrochemical impedance
spectroscopy (EIS)
IE (%) IE (%)
1 day 91.06 96.57
3 days 93.03 96.81
7 days 75.74 65.52
where Z
CPE
is the impedance and ω is the angular frequency.
The CPE represents a capacitor for n
= 1, a resistor for n = 0,
and a diffusion process for n
= 0.5.
The EC proposed for 1, 3, and 7 days in the presence of
tannin has two time constants. The first constant R
1
Q
1
repre-
sents the time constant of hig h frequency which is relative to
the charge transfer reactions and double layer. The second
constant R
2
Q
2
represents the time constant of low frequency
which is relative to the adsorption processes. For a ll times of
immersion in the absence of tannin, the EC proposed was
R
s
(R
1
Q
1
), which contains one time constant which is related
with charge transfer reactions and double layer.
The Nyquist diagram (Figure 4) shows a smal l variation
in the polarization resistance during the first three days of
immersion in the absence of tannin. On the seventh day,
Table 5: Polarization parameters for the corrosion of carbon steel
immersed in 0.1 mol L
−1
Na
2
SO
4
solution (pH 2.5), in the presence
and absence of tannin, for different periods.
j
corr
(μAcm
−2
)
E
corr
(V) R
p
(Ω cm
2
)IE(%)
1 day 103.4 −0.65 202.76
55.7
1 day with tannin 25.90
−0.60 457.58
3 days 123.73
−0.68 76.863
62.0
3 days with tannin 25.93
−0.63 202.23
7 days 122.90
−0.68 68.152
68.7
7 days with tannin 29.20
−0.63 217.72
there is a small increase in the resistance value which can be
attributed to deposition of corrosion products on the metal
surface. The R
p
values were obtained from the simulated
parameters considering that, in all cases, R
p
represents the
overall resistance. Thus, for the R
s
(Q
1
[R
1
(Q
2
R
2
)]) circuit,
this value was obtained from the sum of R
1
and R
2
. The high-
est values for polarization resistance were found on the first
and third day of immersion in the presence of tannin.
Table 4 shows the inhibition efficiencies (IE) obtained from
electrochemical impedance spectroscopy (EIS) and Potentio-
dynamic Polarization (PP) by (1). The IE values obtained
from both methods are in agreement and show a decrease in
corrosion resistance with longer immersion times. This fact
can be attributed due to the inhibition mechanism at this pH
value which provides a formation of nonadherent and por-
ous ferric tannate complex layer.
3.3. Study at pH 2.5 . In the case of the immersion in aerated
0.1 mol L
−1
Na
2
SO
4
solution (pH 2.5), the formation of the
blue-black ferric tannate on the steel surface and in the elec-
trolyte was not observed. According to Martinez and St
˘
ern
[23], the formation of ferric tannate is not detected at this
pH.
3.3.1. Pote ntiodynamic Polarization. The potentiodynamic
polarization curves of steel immersed in aerated 0.1 mol L
−1
Na
2
SO
4
solution (pH 2.5), in the presence and absence of

6 ISRN Corrosion
Table 6: Fitting parameters used to simulate the EIS plots for steel immersed for 1, 3, and 7 days in 0.1 mol L
−1
Na
2
SO
4
solution (pH 2.5) in
the presence and absence of tannin.
Immersion time Chi-square
Equivalent
circuit
Fitting parameters
R
s
(Ω cm
2
)
R
1
(kΩ cm
2
)
Q
1
(F cm
2
) n
1 day with tannin 1.9 × 10
−3
48.1
1.76
4.73 × 10
−6
0.91
3 days with tannin 2.1
× 10
−3
49.6
2.34
5.68 × 10
−6
0.91
7 days with tannin 1.4
× 10
−3
R
s
(R
1
Q
1
)
57.4
3.08
4.43
× 10
−6
0.89
1 day without tannin 2.3
× 10
−3
63.6
0.71
2.80 × 10
−5
0.76
3 days without tannin 2.1
× 10
−3
49.8
0.70
9.21 × 10
−5
0.66
R
s
(Ω cm
2
) Q
1
(F cm
2
) n
1
R
1
(Ω cm
2
) Q
2
(F cm
2
) n
2
R
2
(kΩ cm
2
)
7 days without
tannin
4.5
× 10
−3
R
s
(R
1
Q
1
)(R
2
Q
2
) 47.2 1.53 × 10
−5
0.74 112 5.49 × 10
−4
0.8 0.68
R
s
represents the ohmic resistance between the reference and working electrodes; R
1
and R
2
represent the resistance or charge transfer; Q
1
and Q
2
are the
impedance related to a constant phase element (CPE) and can be attributed to electrode surface or adsorb ed species; n, n
1
, and n
2
represent a CPE exponent.
Capacitor for n
= 1, a resistor for n = 0, and a diffusion process for n = 0.5.
tannin, were obtained from 600 mV up to E
corr
and 300 mV
down to E
corr
after different periods of immersion (Figure 7).
The anodic current density values decreased in the pres-
ence of tannin for all exposure periods tested. Accordingly,
the corrosion potential (E
corr
) of the metal was shifted to
a more positive value and the polarization resistance (R
p
)
increased in the presence of tannin as observed in Tab le 5.
The corrosion current densities (j
corr
)inTable 5 were cal-
culated from the extrapolation of the anodic and cathodic
Tafel li nes (Figure 7). The inhibition efficiencies IE (%) were
calculated by the polarization resistance according to (1).
The j
corr
value decreases significantly in the presence of
tannin during al l days of immersion. The inhibition efficien-
cy increases with the increases of exposure days.
3.3.2. Elect rochemical Impedance Spectroscopy (EIS) . The EIS
spectra for the steel immersed in aerated 0.1 mol L
−1
Na
2
SO
4
solution (pH 2.5), in the presence and absence of tannin, at
OCP, for different periods are shown in Figures 8 and 9.
TheNyquistplots(Figure 8) show a decrease in the R
p
value for steel with and an increase in the time of immersion
in the aerated 0.1 mol L
−1
Na
2
SO
4
solution (pH 2.5). In the
presence of tannin, the R
p
shows the opposite behaviour (in-
crease with increased time of immersion). This can be
attributed to the kinetics mechanism of the adsorption of
tannin onto the metal surface and/or the formation of corro-
sion products from the reaction of iron with tannin. The
phase angles in the presence of tannin are higher than those
in the absence of tannin, reaching a maximum value of
−65
◦
which indicates an increase in the capacitive character of the
film formed.
The experimental data presented in Figures 8 and 9 were
fitted using the two proposed equivalent circuits (EC) shown
in Figure 10. The software used to simulate the EIS data was
NOVA 1.7 (Echo Chemie, The Netherlands). The simulated
data obtained from the fittings are given in Tabl e 6.
The R
s
represents the ohmic resistance between the refer-
ence and working electrodes, R
1
and R
2
represent the resist-
Table 7: Black wattle tannin inhibition efficiencies obtained from
electrochemical impedance spectroscopy (EIS) and potentiodyna-
mic polarization (PP) in 0.1 mol L
−1
Na
2
SO
4
solution (pH 6.0).
Potentiodynamic
Polarization (PP)
Electrochemical impedance
spectroscopy (EIS)
IE (%) IE (%)
1 day 55.7 59.6
3 days 62.0 70.1
7 days 68.7 74.3
ance or charge transfer in physical meaning [27]. The Q
1
and
Q
2
parameters are the impedance related to a constant phase
element (CPE) and can be attributed to electrode surface or
adsorbed species. The EC proposed for all immersion times
in the presence of tannin, and for 1 and 3 days, and in the
absence of tannin, was R
s
(Q
1
R
1
). The R
1
Q
1
represents the
time constant which is relative to the charge transfer reac-
tions and double layer. For fitting, the EIS diagram for 1 day
of immersion in the absence of tannin the EC used was
R
s
(Q
1
R
1
)(Q
2
R
2
), which has two time constants. The fi rst
constant R
1
Q
1
represents the time constant of high frequency
which is relative to the charge transfer reactions and double
layer. The second constant R
2
Q
2
represents the time constant
of low frequency which is relative to the adsorption pro-
cesses.
A comparison between the capacitance values obtained
after 1 and 3 days of immersion in solutions with and without
tannin shows that this parameter decreased in the presence of
tannin, which is probably due to the adsorption of inhibitor
on the metal surface. Accordingly, higher values of R
p
were
found in the presence of tannin (EIS parameters in Tab le 6).
Table 7 shows the inhibition efficiencies (IE) obtained
from electrochemical impedance spectroscopy (EIS) and
potentiodynamic p olarization (PP). The IE values obtained
from both methods are in agreement and show an increase

ISRN Corrosion 7
−7 −6 −5 −4 −3 −2 −1
log j (A cm
−2
)
0
−0.2
−0.4
−0.6
−0.8
−1
Potential (V)
(1)
(2)
(a)
0
−0.2
−0.4
−0.6
−0.8
−1
Potential (V)
−7 −6 −5 −4 −3 −2 −1
log j (A cm
−2
)
(1)
(2)
(b)
−7 −6 −5 −4 −3 −2 −1
log j (A cm
−2
)
0
−0.2
−0.4
−0.6
−0.8
−1
Potential (V)
(1)
(2)
(c)
Figure 7: Potentiodynamic polarization curves of steel immersed
in aerated 0.1 mol L
−1
Na
2
SO
4
solution (pH 2.5) in the presence of
tannin (1) and absence of tannin (2) for (a) 1 day, (b) 3 days, and 7
days.
3
2.5
3.5
2
1.5
1
0.5
0
032.521.510.5
0.80.60.40.20
0.8
0.6
0.4
0.2
0
Z

(kOhm·cm
2
)
Z

(kOhm·cm
2
)
Z

(kOhm·cm
2
)
Z

(kOhm·cm
2
)
Figure 8: Nyquist plots for steel immersed in 0.1 mol L
−1
Na
2
SO
4
solution (pH 2.5) for 1 day (closed triangle), 3 days (closed circle),
and 7 days (closed star) in the presence of tannin; for 1 day (open
square), 3 days (open lozenge), and 7 days (open circle) in the
absence of tannin.
log f (Hz)
−2 −1012345
− Phase angle (deg)
60
40
20
0
(a)
log f (Hz)
−2 −1012345
3.5
3
2.5
2
1.5
log |Z|
(b)
Figure 9: Bode plots for steel immersed in 0.1 mol L
−1
Na
2
SO
4
solution (pH 2.5) for 1 day (closed triangle), 3 days (closed circle),
and 7 days (closed star) in the presence of tannin; for 1 day (open
square), 3 days (open lozenge), and 7 days (open circle) in the ab-
sence of tannin.

8 ISRN Corrosion
R
s
R
s
R
1
R
1
CPE
(a)
R
2
CPE
1
CPE
2
(b)
Figure 10: Equivalent electric circuits proposed to simulate the
experimental data of steel immersed in 0.1 mol L
−1
Na
2
SO
4
solution
(pH 2.5) for (a) 1, 3, and 7 days in the presence of tannin, 1 and 3
days in the absence of tannin; for (b) 7 days in the absence of tannin.
in corrosion resistance with longer immersion times.
The EIS data demonstrated that at pH 2.5 a distinct be-
haviour was detected in the presence of tannin, with the for-
mation of a more protective film. The inhibition mechanism
of tannin at this pH value should be initially adsorbed on
metal surface by electron rich centers of molecule. With in-
crease of immersion time, a layer with adherent nature was
formed. That might be a reason of increasing of inhibition
efficiency with the increase of immersion time at pH 2.
4. Conclusions
(i) The inhibition action of black wattle tannin towards
the corrosion of steel is dependent on the concentra-
tion of tannin added and the pH value of the electro-
lyte.
(ii) A nonadherent ferric tannate complex was formed
on the steel surface at pH 6 in the presence of black
wattle tannin.
(iii) Ferric tannate was not formed on the steel sur f ace at
pH 2.5 in the presence of black wattle tannin.
(iv) The black acacia tannin showed the formation of a
layer with a short-term protection against corrosion
at pH 6.
(v) The black acacia tannin showed the best performance
as a corrosion inhibitor at pH 2.5. The inhibition
efficiency increased in the presence of tannin.
(vi) Through EIS, the characteristics and behaviour of the
films formed on the steel surface can be identified
more clearly. The porosity of the ferric-tannate film
and the stronger capacitive character of the film
formed in acid medium were identified.
Acknowledgments
The financial suppor t for this work provided by the Brazilian
Government Agency CNPq and the Laboratory of Polymer
Materials of the Federal University of Rio Grande do Sul
(UFRGS), which car ried the FTIR analysis, is gratefully ack-
nowledged.
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