Green Technology Tannin-Based Corrosion Inhibitor for Protection of Mild Steel

Abstract

For more than four decades, tannins extracted from renewable resources have been used to protect steam boilers at levels significantly above ASME guidelines. Using tannin-based (green) corrosion inhibitors reduces water and energy consumption, greenhouse gases emissions, and contaminants in effluent wastewaters, while reducing the environmental footprint of industrial processes. The surface adsorptive and corrosion protective properties of a commercial tannin-based corrosion inhibitor (TG 3300) for mild steel were investigated in an alkaline environment using quartz crystal microbalance with dissipation monitoring, open circuit potential, and electro-chemical impedance spectroscopy (EIS). The results showed the formation of an effective and stable tannin-based protective layer on mild steel within the first 5 min to 15 min of adsorption. It was found that adsorption of TG 3300 on mild steel can be well described by the Langmuir isotherm. Highly negative values of apparent Gibbs free energy of adsorption (DG ads = –47.36 kJ/mol), indicate a spontaneous and strong adsorption of TG 3300 onto the mild steel surface. The results suggest the formation of a TG 3300 protective layer on mild steel, where the highest value of inhibition efficiency was 85% under the applied experimental conditions.

Full text

CORROSION SCIENCE SECTION
CORROSION—Vol. 71, No. 11 1321
Submitted for publication: May 15, 2015. Revised and accepted:
August 11, 2015. Preprint available online: August 11, 2015,
http://dx.doi.org/10.5006/1777.
‡ Corresponding author. E-mail: rgaudreault@tgwt.com.
* Department of Chemical Engineering, McGill University, Montreal,
QC, Canada.
** TGWT Clean Technologies Inc., Longueuil, QC, Canada.
Green Technology: Tannin-Based Corrosion
Inhibitor for Protection of Mild Steel
M. Dargahi,*,** A.L.J. Olsson,* N. Tufenkji,* and R. Gaudreault‡,**
ABSTRACT
For more than four decades, tannins extracted from renew-
able resources have been used to protect steam boilers at
levels signicantly above ASME guidelines. Using tannin-
based (green) corrosion inhibitors reduces water and energy
consumption, greenhouse gases emissions, and contaminants
in efuent wastewaters, while reducing the environmental
footprint of industrial processes. The surface adsorptive and
corrosion protective properties of a commercial tannin-based
corrosion inhibitor (TG 3300) for mild steel were investigated
in an alkaline environment using quartz crystal microbalance
with dissipation monitoring, open circuit potential, and electro-
chemical impedance spectroscopy (EIS). The results showed
the formation of an effective and stable tannin-based protec-
tive layer on mild steel within the rst 5 min to 15 min of
adsorption. It was found that adsorption of TG 3300 on mild
steel can be well described by the Langmuir isotherm. Highly
negative values of apparent Gibbs free energy of adsorption
(
D
Gads = –47.36 kJ/mol), indicate a spontaneous and strong
adsorption of TG 3300 onto the mild steel surface. The results
suggest the formation of a TG 3300 protective layer on mild
steel, where the highest value of inhibition efciency was 85%
under the applied experimental conditions.
KEY WORDS: corrosion inhibition, green technology, mild
steel, surface adsorption kinetics and thermodynamics, tannin
molecules
INTRODUCTION
Corrosion and corrosion-induced safety problems
are among the main issues in the water industry1
and because of a global scarcity of water, it is often
necessary to recycle process water as much as pos-
sible. This, in turn increases the number of cycles in
steam boilers, thereby increasing the concentration
of corrosive salts inducing higher corrosion rates and
other process problems.2 To minimize corrosion and
corrosion-induced risks and maximize equipment life
expectancy, it is crucial to understand the nature and
mechanisms by which corrosion occurs and then in-
hibit it as much as possible.
One of the most common techniques to mini-
mize corrosion in the water industry is the applica-
tion of corrosion inhibitors, which form a protective
(blocking) layer on the metal surface and minimize
the access of corrosive electrolytes to the surface.3
Unfortunately, conventional corrosion inhibitors (e.g.,
phosphates and sultes) are neither renewable nor have
a reliable performance in highly conductive environ-
ments. Therefore, developing new, highly protective, and
environmentally friendly corrosion inhibitors for steam
boilers, hot-water closed-loop systems, pipelines, and
tanks is critical.
Tannin-based corrosion inhibitors are becoming
popular in the water industry because of their renew-
able/green nature and their ability to perform under
much higher conductive/corrosive environments
(8,000 µS/cm to 10,000 µS/cm) than the ASME
guidelines (< 3,000 µS/cm).4
ISSN 0010-9312 (print), 1938-159X (online)
15/000217/$5.00+$0.50/0 © 2015, NACE International

CORROSION SCIENCE SECTION
1322 CORROSION—NOVEMBER 2015
Although the physicochemical properties of tan-
nin molecules have been studied previously,5-9 the
corrosion inhibition properties of these molecules are
still poorly understood. In addition, studies on corro-
sion protective properties of tannin-based corrosion
inhibitors have been mostly performed in highly acidic
conditions (pH 0.5 to 4.0),10-12 but rarely in alkaline
conditions, which prevail in chilling and heating
closed-loop systems.
This work investigated the adsorptive and protec-
tive behavior of the tannin-based TG 3300 inhibitor for
the protection of mild steel (MS) in alkaline conditions.
MATERIALS AND METHODS
Chemicals and Solutions
The tannin-based TG 3300 inhibitor is a commer-
cial blend of 100% natural molecules such as hydrolys-
able and condensable tannins (Figure 1, top and bottom
respectively6,13), having a charge density of –1.6 equiv-
alent/kg (dry basis) at pH 9.0. The common charac-
teristic of these natural molecules is that they have
aromatic rings with hydroxyl and carboxylic groups,
and are negatively charged in alkaline pH. Hydrolys-
able tannins refer to the hydrolysis propensity in highly
acidic or alkaline conditions. For example, the hydro-
lysis of tannic acid (TA), could yield by-products such
as 1-3-6-tri-O-galloyl-β-D-glucose (TGG), as well as
the ultimate sub-unit, i.e., gallic acid (GA) (Figures 1[a]
through [c]). On the other hand, condensed tannins are
the most abundant polyphenols, found in virtually all
families of plants and comprising up to 50% of the dry
weight of leaves. Tannins of tropical woods tend to be
of a cathetic nature, e.g., catechin (Figures 1[d] and
[e]), rather than of the gallic-type present in temper-
ate woods.14 Theoretical molecular modeling methods
have been described in previous work.6,13
The tannin-based TG 3300 inhibitor stock solution
was prepared by diluting 1 part of concentrated in-
hibitor with 1 part of deionized water (DI, 18.2 MΩ·cm
resistivity), and then further diluting to the desired
concentration in DI water to be used as the corrosive
solution. For pH adjustment, aqueous 0.1M sodium
hydroxide and 0.1 M sulfuric acid were used. The pH
of 10.5 was used for quartz crystal microbalance with
dissipation (QCM-D) monitoring, open circuit poten-
tial (OCP), and electrochemical impedance spectros-
copy (EIS) measurements, while pH 8 to 11 was used
for the pH-dependent study.
Quartz Crystal Microbalance with Dissipation
Monitoring
An E4 QCM-D† unit from Q-Sense was used
for the adsorption experiments. MS (C1020, UNS
G10200)(1) coated AT-cut quartz custom-made crys-
tals (QSX999) were used as the model substrate. The
crystals were cleaned by soaking and sonicating for
a minimum of 10 min in a 2% Hellmanex† solution
(cleaning solution) before being thoroughly rinsed with
DI water and dried with nitrogen gas. Then, to remove
the organic compounds/contaminations, crystals
were exposed to a UV/ozone treatment for 20 min be-
fore each experiment. The temperature was controlled
by the QCM-D at 22°C and the ow rate of 50 µL/min
was maintained using a peristaltic pump (RegloDigi-
tal†, Ismatec). Before beginning the experiments, a
frequency and dissipation baseline in DI water was al-
(1) UNS numbers are listed in Metals and Alloys in the Unied Num-
bering System, published by the Society of Automotive Engineers
(SAE International) and cosponsored by ASTM International.
FIGURE 1. Molecular structures of hydrolysable tannins: (a) top view of one possible structure of TA, and (b) TGG; (c)
GA;6,13 and (d) and (e) condensed tannins, e.g., catechin (courtesy of Professor M.A. [Tony] Whitehead and K. Conley,
McGill University).

CORROSION SCIENCE SECTION
CORROSION—Vol. 71, No. 11 1323
lowed to stabilize. Then, the solution with the desired
inhibitor bulk concentration was owed across the
crystal surface. As molecular deposition occurs on the
crystal surface, a negative shift in the resonance fre-
quency (Df) can be measured in real time. After each
experiment, the QCM-D setup was cleaned by ow-
ing the cleaning solution and then DI water through
the QCM-D for at least 10 min each. The water was
purged from the setup using air and nitrogen gas.
To determine whether the Sauerbrey equation
provides an adequate estimation of the adsorbed
mass, the ratio of the dissipation factor and the nor-
malized frequency shift (normalized with respect to
overtone number) was obtained. The threshold value
for the Sauerbrey to be valid is 10–7 according to the
supplier and 4×10–7 according to Reviakine, et al.15 In
the present study, the ratio DD/Df was always lower
than 10–7 (–7.6×10–8 to –5.1×10–9 Hz–1) for all of the
studied inhibitor bulk concentrations, indicating that
the protective layer was rigid (Figure 2[a]), and justify-
ing the use of the Sauerbrey equation to calculate the
adsorbed mass. Moreover, the frequency shifts were
practically identical at all overtones and therefore only
the data from the third overtone is presented.
Consequently, the mass of adsorbed molecules
on the QCM-D sensor surface was calculated from
the Df using the Sauerbrey equation, referred to as
the Sauerbrey mass.15
Electrochemical/Corrosion Cell and Equipment
A standard three-electrode electrochemical/cor-
rosion cell was used in the electrochemical experi-
ments. The counter electrode (CE) was a graphite rod.
The reference electrode (RE) was a saturated calomel
electrode (SCE) separated from the cell by a glass frit.
All potentials in this article are expressed with respect
to the SCE. The working electrode (WE) was prepared
from a C1010 (mild steel, UNS G10100) rod (Metal
Samples Company†), and sealed with epoxy resin to
yield a two-dimensional surface exposed to the elec-
trolyte. Table 1 shows the chemical composition of the
alloy used in this study.
Electrochemical measurements were performed
using a Solartron 1287† electrochemical interface
(b)(a)
(c) (d)
Time (min)
010203040
–25
–20
–15
–10
–5
0
0
1
2
3
4
(1)
(2)
Time (min)
010203
040
0
1
2
3
4
5
17 ppm
34 ppm
275 ppm
1,100 ppm
0 ppm
Time (min)
010203040
0
1
2
3
4
5
275 ppm
17 ppm
Time (min)
010203
04
0
0
20
40
60
80
100
θ
θ2
θ1
Δf (Hz)
ΔD (10–6)
Surface Concentration, CS (mg/m2)
Surface Coverage, θ (%)
Surface Concentration, CS (mg/m2)
FIGURE 2. (a) Time dependence of adsorbed layer normalized frequency shift and dissipation factor upon adsorption
of tannin-based TG 3300 inhibitor obtained from QCM-D in a solution containing 68 ppm of the inhibitor at pH 10.5 and
room temperature. (b) Inhibitor surface concentration as a function of time and inhibitor bulk concentration, obtained from
QCM-D at pH 10.5 and room temperature. (c) The kinetics of inhibitor adsorption onto the MS surface modeled by the
two-step model. Solid lines represent the experimental data, while the circles show simulated data using the kinetic model
described by Equations (1) through (3). (d) Time dependence of total relative surface coverage of inhibitor (
q
, solid line),
relative surface coverage with native (
q
1, dashed line), and reconformed (
q
2, dotted line) adsorbed on MS obtained in a
solution containing 68 ppm of the inhibitor.

CORROSION SCIENCE SECTION
1324 CORROSION—NOVEMBER 2015
potentiostat/galvanostat and 1260 impedance/gain-
phase analyzer. To ensure complete characteriza-
tion of the interface and the surface processes, EIS
measurements were made over a frequency range of
100kHz to 10mHz, with an alternating current volt-
age amplitude of ±10mV.
Prior to each experiment, the WE surface was
polished with 600-gradation abrasive sandpaper, and
then thoroughly rinsed with ethanol. After this, the
electrode was kept in an ultrasonic bath for 5min
in ethanol, and then rinsed with DI water. The elec-
trode was then immersed in the test electrolyte and
equilibrated for 3h at 70°C at OCP, followed by the
electrochemical measurements. All of the solutions
were mixed using a magnetic stirrer. All data reported
in this work represent mean values of four to six rep-
licates.
RESULTS AND DISCUSSION
Kinetics of Tannin-Based TG 3300 Inhibitor
Adsorption on Mild Steel Surface
The kinetics of corrosion inhibitor adsorption
was studied to nd the time scale within which the
MS surface could be covered by varying levels of the
inhibitor. Figure 2(b) shows that at a constant inhibi-
tor bulk concentration, the surface concentration of
adsorbed inhibitor increased rapidly, and then gradu-
ally leveled off to a quasi-steady state. The adsorption
equilibrium was reached after ~5 min to 15 min, de-
pending on the inhibitor bulk concentration.
The kinetic data in Figure 2(b) were modeled us-
ing a two-step mode.16-17 For this purpose, the inhibi-
tor surface concentrations were converted into the
corresponding relative surface coverage values (q =
(Cs,t/Cs,max)), where Cs,t is the inhibitor surface con-
centration at time t, and Cs,max (~4.25 mg/m2) is the
maximum inhibitor surface concentration obtained
from the adsorption isotherm (see next section). In the
two-step model, the rst adsorption step is reversible,
and the inhibitor’s surface conformation is assumed
to resemble that in the bulk solution, i.e., the native
conformation. However, the inhibitor with this surface
conformation can either desorb (Step 1) or adopt a
more thermodynamically favorable surface conforma-
tion (Step 2),16-18 which was assumed to be irrevers-
ible.
Expressing the inhibitor surface concentration in
terms of its relative surface coverage, q, the two-step
adsorption kinetic model can be formulated as:
θ=θ
θ+
θ
d
dt
k[C](1 ––)–(k k)
1
ai 12
df
1 (1)
θ
=θ
d
dt
k
2
f1
(2)
θ=θ+θ
=θ
θ
d
dt
d
dt
d
dt
k[C](1 –)–k
12
ai
d1
(3)
where q = q1 + q2 and 0 ≤ q ≤ 1 are the fraction of
relative surface covered by inhibitor in both the ther-
modynamically unstable (q1) and stable (q2) conforma-
tions, Ci (mol/L) is the inhibitor bulk concentration, ka
(M–1 min–1) is the adsorption constant, kd (min–1) is the
desorption constant, and kf (min–1) is the reconforma-
tion (surface rearrangement) constant.
The experimental data (Figure 2[b]) were tted
using the kinetic model in Equations (1) through (3),
and a good agreement between the model (Figure 2[c],
circles) and the experimental data (Figure 2[c], solid
lines) was obtained in all cases. This shows the appli-
cability of the proposed model to describe the kinetics
of inhibitor adsorption onto MS under these experi-
mental conditions (Table 2).
Table 2 shows that for all inhibitor bulk concen-
trations, the adsorption rate constants were consider-
ably larger than the desorption and reconformation
rate constants, showing a strong afnity of inhibi-
tor for MS. In addition, the adsorption kinetic rate
constant decreased with an increase in the inhibitor
bulk concentration. At higher inhibitor bulk concen-
trations, the substrate surface became covered by
inhibitor faster, which increased the probability for
the occurrence of intermolecular interactions. The
TABLE 1
The Chemical Composition (%, w/w) of Mild Steel
Name Grade C Fe Mn P S
MS C1010 Lab (for electrochemistry experiments) 0.02-0.08 Bulk 0.30-0.60 0-0.04 0.05
MS C1020 Lab (for QCM-D experiments) 0.18-0.23 Bulk 0.30-0.60 0.04 0.05
TABLE 2
Kinetic Rate Constants for the Adsorption of Tannin-Based
TG 3300 Inhibitor onto Mild Steel as a Function
of Inhibitor Bulk Concentration(A)
[TG 3300]/ppm ka×10–5/M–1 min–1 kd×10/min–1 kf×102/min–1
17 2.30 2.31±0.08 1.71±0.08
34 2.76 3.36±0.22 1.71±0.31
64 2.94 2.41±0.14 4.07±0.24
275 0.59 0.82±0.23 5.52±0.37
412 0.53 1.64±0.22 3.75±0.18
825 0.21 1.79±0.28 3.83±0.15
1,100 0.12 0.91±0.19 2.32±0.19
1,375 0.11 1.28±0.11 2.41±0.12
(A) The parameters were determined by fitting the experimental data
from QCM-D experiments.

CORROSION SCIENCE SECTION
CORROSION—Vol. 71, No. 11 1325
decrease in the adsorption rate constants suggests
that the intermolecular interactions with the already
adsorbed inhibitor molecules inhibit (molecular steric
hindrance) further adsorption from the solution.
Molecular reconformation kinetics of the inhibitor
were also examined. For that purpose, the modeled
kinetic data were deconvoluted into the two contribu-
tions (q1 and q2), as shown in Figure 2(d) and then
separately in Figures 3(a) and (b). Figures 2(d) and
3(a) demonstrate that the initial increase (within the
rst 5 min to 10 min of adsorption) in total inhibitor
surface coverage, q (solid line in Figure 2[d]), is mostly
a result of the formation of an inhibitor layer with as-
sumed native conformation, q1 (dashed line in Figure
2[d]). The surface coverage with the native inhibitor
initially increased sharply and then reached a quasi-
steady state, which occurred at earlier times at higher
bulk solution concentrations. On the other hand, the
surface coverage with a reconformed inhibitor (Figures
2[d] and 3[b]), q2 (dotted line in Figure 2[d]), gradually
increased in the entire time interval. Figures 2(d) and
3 also show that the transfer from the native confor-
mation (q1) into the reconformed inhibitor (q2) was not
completed within the time interval studied.
In addition, Figure 3(c) shows the values of the
ratio of surface coverage of native to reconformed
adsorbed inhibitor (q2/q1). The results demonstrated
that with an increase in adsorption time, the relative
surface ratio also increases. The trend can be approxi-
mated by a second-order polynomial function. Also,
the values at a particular adsorption time were differ-
ent at different inhibitor bulk solution concentrations.
This indicates that the inhibitor bulk solution concen-
tration inuenced the kinetics of the inhibitor surface
reconformation (from q1 to q2).
Equilibrium of Tannin-Based TG 3300 Inhibitor
Adsorption on Mild Steel Surface
The inhibitor afnity with MS can be determined
by the Gibbs free energy of adsorption (DGads), through
adsorption isotherms.18 The mean value of the QCM-D
crystal frequency shift, after reaching a quasi-steady
state (average of the last 15 min within 1 h of adsorp-
tion) at each inhibitor bulk solution concentration,
was used for further equilibrium calculations.
Figure 4 shows that the inhibitor surface concen-
tration increased with an increase in inhibitor bulk
concentration, and then leveled off to a quasi-steady
state (maximum surface concentration, Cs,max ≈ 4.25
mg/m2) at an inhibitor bulk concentration of approxi-
mately 275 ppm. Moreover, the steep initial slope of
the adsorption isotherm (rising part of the isotherm)
showed the high afnity of TG 3300 molecules for the
MS surface.19
The plot in Figure 4 resembles the shape of a
unimodal adsorption isotherm.18 The corresponding
Langmuir isotherm equation, for further investigation
of inhibitor adsorption onto MS surface, is:
=+
C
CB[C ]
1B [C ]
s,i
s,maxads i
adsi (4)
where Cs,i (mg/m2) is the inhibitor surface concentra-
tion at a particular inhibitor bulk concentration, Cs,max
(~4.25 mg/m2) is the maximum inhibitor surface con-
centration, Bads (L/mol) is the adsorption afnity con-
stant at constant temperature, and [Ci] (mol/L) is the
TG 3300 bulk concentration.
To verify whether the experimental data in Figure
4 can be described by the Langmuir adsorption iso-
therm, it is more convenient to use a linearized form
of Equation (4). Thus, dividing the equation by the
0
20
40
60
80
100
(a)
0
20
40
60
80
100(b)
010203
040
0
1
2
3
4
5(c)
(2)
(3)
(4)
(1)
(1)
(2)
(3)
(4)
(1)
(3)
(4)
(2)
Time (min)
θ
1
(%)θ
2
(%)θ
2
(θ
1
)
FIGURE 3. Time dependence of MS (a) relative surface coverage
with native (
q
1), (b) reconformed (
q
2), and (c) the ratio of surface
coverage of native to reconformed adsorbed inhibitor (
q
2/
q
1)
obtained at different tannin-based TG 3300 inhibitor bulk solution
concentrations: (1) 17 ppm, (2) 34 ppm, (3) 68 ppm, and
(4) 275 ppm.

CORROSION SCIENCE SECTION
1326 CORROSION—NOVEMBER 2015
corresponding maximum value (Cs,i/Cs,max) and replac-
ing this ratio with the relative surface coverage, qi, a
linearized form of the isotherm is obtained:
θ=+
[C ]
[C ]
1
B
i
i
i
ads (5)
For the Langmuir isotherm to be considered valid, a
plot of [Ci]q–1 versus [Ci] should yield a straight line
with a slope of 1 and an intercept B–1
ads. Indeed, the
inset of Figure 4 shows a linear behavior with the
corresponding slope close to unity (0.96), in agree-
ment with Equation (5). Consequently, the Langmuir
isotherm was deemed applicable in describing the
adsorption of inhibitor on MS. From this, the apparent
Gibbs free energy of adsorption, DGads (J / mol), was
calculated:
Bads =1
csolvent
exp –ΔGads
RT
( )
(6)
where csolvent (mol/L) is the molar concentration of
the solvent (cwater = 55.5 mol/L), R is the gas constant
(8.314 J/mol·K), and T (K) is the temperature.
The calculated adsorption afnity constant and
corresponding apparent Gibbs free energy of adsorp-
tion, are (4.44±0.21)×106 L/mol and –47.36±0.88 kJ/
mol, respectively. The relatively large negative appar-
ent Gibbs free energy of adsorption indicated a spon-
taneous and strong adsorption of inhibitor onto the
MS surface.18,20
Other studies10-11 that investigated the adsorption
of tannin-based chemistries on MS and aluminum,
using gravimetric weight loss at pH 1.0 and 30°C,
reported apparent Gibbs free energy of adsorption of
–31.10 kJ/mol and –18.64 kJ/mol, respectively. Al-
though a difference is to be expected as a result of the
different surfaces, sensitivity of the techniques, and
the overall difculty of reproducing similar experi-
mental conditions among different research groups,
the DGads obtained in the present study were more
negative than the literature values, hence indicating
a stronger bond between the TG 3300 inhibitor mol-
ecules and the MS substrate.
Open Circuit Potential
Figure 5(a) shows the variation of the OCP of MS
with time in absence and presence of 137 ppm of tan-
nin-based TG 3300 inhibitor at pH 10.5 and 70°C. In
the absence of the inhibitor, the OCP sharply dropped
and then reached the quasi-steady state, which indi-
cated MS corrosion in the control solution. However,
in the presence of the inhibitor, the OCP value rst
increased and then reached a plateau. In the rst ap-
proximation, this trend indicated that the corrosion
reaction slowed down with time, and then reached a
quasi-steady state rate within the time interval pre-
sented here. This indicated blocking of the MS surface
by the inhibitor molecules which resulted in less elec-
tron transfer between the MS substrate and the bulk
solution. Similar results were obtained at other inhibi-
tor bulk concentrations (results not shown here). The
mean value of the OCP after reaching a plateau at
each inhibitor bulk solution concentration was plotted
versus inhibitor bulk concentration (Figure 5[b]).
As expected, by increasing the inhibitor bulk concen-
tration, the OCP values approached zero, indicating
the adsorption of tannin-based TG 3300 inhibitor
on the MS surface and its inuence on the corrosion
reaction.21-22
Electrochemical Impedance Spectroscopy
EIS was applied to investigate the electrode/
electrolyte interface and processes that occur on the
MS surface at OCP in the presence and absence of
the tannin-based TG 3300 inhibitor in solution, most
notably the general corrosion resistance of MS. Fig-
ure 5(c) shows the EIS spectra of the MS electrode
recorded in the absence and presence of the inhibitor
at different bulk concentrations. The spectra were re-
corded at 70°C after the stabilization of the electrode
at OCP for 3 h. Figure 5(c) shows that the diameter of
the semicircle increased with an increase in inhibitor
bulk concentration, indicating an increase in corro-
sion resistance of the material.23-24
In order to extract quantitative information,
a nonlinear least squares t analysis was used to
model the spectra, using electrical equivalent circuits
(EECs), presented in Figure 5(d).25-26 In these EECs,
Rel represents the ohmic resistance between the WE
and RE, Ri is the charge transfer resistance related to
the corrosion reaction at OCP, and CPE is the capaci-
tance of the electrical double-layer at the electrode/
FIGURE 4. Adsorption isotherm for tannin-based TG 3300
inhibitor onto mild steel at pH 10.5 and room temperature. Circles
represent the experimental data, while the solid line represents
the corresponding value from the Langmuir isotherm. Inset:
experimental data (circles) show an excellent correlation with the
linearized Langmuir isotherm.
0 500 1,000 1,500 2,000 2,500
3,000
0
1
2
3
4
5
[TG 3300] (µM)
020406080100
0
20
40
60
80
100
120
R
2
= 0.99
Cs,max = 4.25 mg/m2
[TG 3300] (ppm)
Surface Concentration, CS (mg/m2)
[TG 3300] θ–1 (µM)

CORROSION SCIENCE SECTION
CORROSION—Vol. 71, No. 11 1327
electrolyte interface. The EEC in Figure 5(d) was
used to t the spectra recorded in the absence and
presence of the inhibitor in the solution and the cor-
responding EEC values, obtained by tting the experi-
mental spectra, are presented in Table 3.
The average CPE exponent (0.71±0.02) presented
in Table 3 supports the authors’ opinion that CPE
can be prescribed to the capacitive behavior of the
electrochemical double-layer and the adsorbed inhibi-
tor layer. Moreover, the generally decreasing trend of
CPE with increasing inhibitor bulk concentration is as
expected, and indicates the formation of an adsorbed
layer on the MS surface. The increase in the charge
transfer resistance Ri also leads to an increase of cor-
rosion inhibition efciency (ηi).
Taking into account the physical meaning of the
EEC parameters of the circuits in Figure 5(d), the cor-
rosion resistance of the bare (control) and covered MS
(d)
Time (min)
020406080100 120140
–0.8
–0.7
–0.6
–0.5
–0.4
–0.3
(1)
(2)
[TG 3300] (ppm)
0 200 400 600 800 1,000
1,200
–0.8
–0.7
–0.6
–0.5
–0.4
–0.3
0 200 400 600 800 1,000 1,200 1,400
–600
–500
–400
–300
–200
–100
0
0 200 400 600 800 1,000 1,200
0
20
40
60
80
100
0 200 400 600 800 1,000 1,200
0
1
2
3
4
5
pH
78910 11 12
0
20
40
60
80
100
(a) (b)
(c)
(e) (f)
[TG 3300] (ppm)
CS (mg/m2)
[TG3300] (ppm)
Zʹ (Ω·cm2)
Rel CPE
Ri
EOCP (VSCE)
Zʺ (Ω·cm2)Corrosion Inhibition, η (%)
Corr
osion Inhibition Ef
ficiency, η (%) EOCP (VSCE)
FIGURE 5. (a) OCP of MS as a function of time in the (1) absence and (2) presence (137 ppm) of tannin-based TG 3300
inhibitor at pH 10.5 and 70°C. (b) OCP of MS as a function of inhibitor bulk concentration, recorded after 3 h incubation.
(c) Nyquist plot of MS recorded at different bulk concentrations of inhibitor: (×) 0 ppm, (
¯
) 17 ppm, (
r
) 68 ppm,
(
¡
) 275 ppm, and (
¨
) 550 ppm. (d) EEC model used to fit EIS data recorded for MS in absence and presence of
the inhibitor. (e) Corrosion inhibition efficiency of inhibitor as a function of inhibitor bulk concentration. Inset: adsorption
isotherm for inhibitor onto mild steel. (f) MS corrosion inhibition efficiency as a function of pH. The EIS data were recorded
in the absence and presence of 275 ppm of inhibitor.

CORROSION SCIENCE SECTION
1328 CORROSION—NOVEMBER 2015
TABLE 3
Dependence of EEC Parameters on the Concentration of Tannin-Based TG 3300 Inhibitor Bulk Concentration(A),(B)
[TG 3300]/ppm 0 17.1 34.3 68.6 137 275 412 550 825 1,100
CPE×106/Ω–1·cm–2sn 586 361 328 274 276 254 271 276 281 252
±SD 35 26 38 43 27 54 19 16 31 42
n1 0.71 0.73 0.71 0.71 0.69 0.67 0.69 0.69 0.73 0.73
±SD 0.03 0.05 0.03 0.02 0.04 0.05 0.03 0.04 0.03 0.03
Ri/Ω·cm2 419 679 802 1,072 1,315 2,751 2,920 3,048 2,845 2,991
±SD 11 22 26 66 127 258 220 492 436 696
ηi/% 0 38.2 47.7 60.8 67.9 84.7 85.6 85.9 85.0 85.4
±SD 0 2.0 1.6 2.5 3.3 1.4 1.1 2.7 2.3 3.2
(A) The data were obtained by modeling EIS spectra recorded in absence and presence of inhibitor at pH 10.5 and 70°C. The table also lists the
corresponding corrosion inhibition efficiency values, ηi.
(B) SD: standard deviation. An average electrolyte resistance value was 586 Ω.
surface is equivalent to the charge transfer resistance,
Ri. Consequently, the corrosion inhibition efciency
(ηi) was calculated by comparing the resistance value,
Ri, recorded at various concentrations of inhibitor, and
the R0 value recorded in the absence of inhibitor (con-
trol sample; Table 3 and Figure 5[e]):
ηi=100 ×1−R0
Ri




 (7)
As shown in Figure 5(e), with an increase in the in-
hibitor bulk concentration, the corrosion inhibition
efciency also increased and reached a maximum
value of approximately 85%, indicating high surface
corrosion protection. The authors’ eld data on the
protection of MS closed-loop systems, using tannin-
based TG 3300 (data not shown), converge with the
laboratory results.27 In addition, the calculated trend,
in combination with the trend in CPE values (Table
3), indicates that the corrosion inhibition efciency
depends on the inhibitor surface concentration (cover-
age) that forms the protective layer.
Moreover, the onset of both corrosion inhibition
efciency and the adsorption isotherm occurred at
~275 ppm of inhibitor (Figure 4[e] and the inset). In-
terestingly, this bulk concentration converges toward
the optimum value observed after four decades of
empirical optimization in industrial MS boilers and
closed-loop systems, treated with tannin-based chem-
istries.
Effect of pH on Tannin-Based TG 3300
Corrosion Protection of Mild Steel Surface
The inuence of pH on MS corrosion inhibition
efciency was studied over a pH range from 8 to 11,
in the absence and presence of 275 ppm of tannin-
based TG 3300 inhibitor. Figure 5(f) shows that corro-
sion inhibition increased with pH and gave the highest
efciency at pH 11 (i.e., 86.4±1.60%). This could be
a result of the higher corrosion resistance of MS at
alkaline pH. The EIS data (not shown here) also con-
rm higher electron transfer resistance in absence (R0)
and presence (Ri) of inhibitor at alkaline pH. Similar
behavior was observed by the authors in previous
work.28
CONCLUSIONS
v The QCM-D method was used to investigate the
adsorption kinetics and thermodynamics of a tannin-
based TG 3300 inhibitor on MS. The kinetic experi-
ments showed that inhibitor adsorption reaches a
quasi-steady state within 5 min to 15 min, depending
on the inhibitor bulk concentration. The adsorption
kinetics follow a two-step model: the rst step repre-
sents the reversible inhibitor adsorption/desorption,
while the second step represents the molecular recon-
formation of tannins into a thermodynamically more
stable conformation. The adsorption process showed
an excellent correlation (R2 = 0.99) with the Langmuir
isotherm. The large negative apparent Gibbs free en-
ergy of adsorption (DGads = –47.36 kJ/mol) conrmed
spontaneous and strong adsorption of tannin-based
TG 3300 inhibitor on the MS surface. EIS showed that
high inhibition efciency (i.e., 85%) is achieved within
3 h of immersion of a freshly polished MS in the in-
hibitor solution. Laboratory results showed that by in-
creasing the pH, the MS surface protection increases.
Finally, this work is a signicant advancement in the
eld of tannin-based corrosion inhibition of MS be-
cause it links the laboratory results with four decades
of industrial empirical optimization.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the nancial
support from the Natural Sciences and Engineering
Research Council of Canada (NSERC). The authors
also thank Prof. Theo van de Ven from McGill Uni-
versity for valuable scientic insights and Mr. Louis-
Philippe Cloutier for valuable discussions.
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CORROSION SCIENCE SECTION
CORROSION—Vol. 71, No. 11 1329
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