GREEN CHEMISTRY – PURIFIED TANNIN MOLECULES FOR THE PROTECTION OF
MILD STEEL CLOSED-LOOP SYSTEMS
M. Dargahi a,b, R. Gaudreaultb*, A. L. J. Olssona, N. Tufenkjia
a Department of Chemical Engineering, McGill University, Montreal, QC, Canada
b TGWT Clean Technologies Inc., Longueuil, QC, Canada
Tannins extracted from renewable resources are green molecules that protect steam boilers much above
ASME guidelines, for more than four decades. Using purified tannin-based corrosion inhibitors reduce
water and energy consumption, greenhouse gases emissions, and contaminants to the effluents; while
reducing the environmental footprint of Industry.
This paper investigates surface interactions of purified tannins (TG 3300) corrosion inhibitor with mild
steel surfaces, using Electrochemical Impedance Spectroscopy (EIS) and Quartz Crystal Microbalance
with Dissipation (QCM-D), to characterize the performance of the tannin-based layer.
The results show the formation of an effective and stable tannin-based protective layer on mild steel within
the first 5-15 minutes. The adsorption isotherm showed the optimum bulk concentration of TG 3300 for
protecting mild steel surfaces. High corrosion protection of mild steel by TG 3300 was also evidenced by
the field results (i.e.”excellent” referred to AWT Guideline).
This work is a significant advancement in the field of tannin-based corrosion inhibition of mild steel
because it links the laboratory results with four decades of industrial empirical optimization.
Green chemistry, purified tannin molecules, surface adsorption kinetics and thermodynamics, corrosion
inhibition, mild steel, heating and cooling closed-loop systems.
Approximately 549 billion dollars of USA national income is spent on corrosion prevention and the
maintenance or replacement of products lost or contaminated as a result of corrosion which is far more
than the annual budget of some countries (1).
Corrosion and corrosion-induced safety problems are among the main issues in water industry (2).
Moreover, due to the world situation in water scarcity, it is necessary to recycle water as much as possible.
However, by increasing the number of cycles in some equipment, corrosive salts concentration
(conductivity) increases which led to higher corrosion and corrosion induced problems (3). Consequently,
the industrial equipment cannot perform their required tasks for a targeted length of time. Corrosion is a
natural process; hence, it is not possible to prevent it thoroughly. To minimize corrosion and corrosion
induced-risks and maximize equipment’s life expectancy, it is crucial to understand the nature and
mechanisms by which corrosion occurs and then inhibit it as much as possible.
One of the most common techniques to minimize corrosion in water industry is the application of corrosion
inhibitors, which form a protective (blocking) layer on the metal surface and minimizes the access of
corrosive electrolytes to the surface (4). Unfortunately, conventional corrosion inhibitors (e.g., phosphates,
and sulfites) are neither renewable resources nor having a reliable performance in highly conductive
environment. Developing new, highly protective, and environmentally friendly corrosion inhibitors for steam
boilers, hot-water closed-loop systems, pipelines, and tanks is critical. To address this problem, TGWT Clean
Technologies Inc. has a portfolio of renewable/green corrosion inhibitors that work under much higher
conductive/corrosive environment (8000-10000 µS cm-1) than conventional chemistries (< 3000 µS cm-1).
Using purified tannin-based corrosion inhibitors results in reduced water and energy consumption, greenhouse
gases emissions, and contaminants to the effluents while reducing the environmental footprint of Industry.
Even though, the physicochemical properties of molecular-colloidal tannins have been widely studied (5-9),
the corrosion inhibition properties are still poorly understood.
This work investigates the adsorptive and protective behavior of tannin-based corrosion inhibitors for mild
steel (MS) hot-water closed-loop systems / boilers.
2- MATERIALS AND METHOD
2-1- Chemicals and Solutions
The corrosion inhibitor solution, purified tannin (TG 3300), was prepared by diluting concentrated TG
3300 1:1 using deionized water (DI) and then further diluting to desired concentration in tap water.
Montreal city tap water (2013 results: pH of 7.6 (min./6.5; max./8.0); conductivity of 302 µS cm-1
(min/257; max. /356); Ryznar Index of 8.7 (min./7.9; max./9.4); Langelier’s saturation index of -0.04;
Agressivity index of 12.0; Calcium: 29.97 mg L-1; TH: 115 mg L-1 CaCO3; Chlorides: 25.32 mg L-1;
Sulfates 24.41 mg L-1; Silica: 0.94 mg L-1; Total Phosphates: 0.030 mg L-1), was used as a corrosive liquid
in the corrosion cell, in the absence and presence of the TG 3300 inhibitor. For pH adjustment, aqueous
0.1 M sodium hydroxide, and 0.1 M sulfuric acid, were used. pH 10.5 was used for QCM-D and EIS
measurements, while pH 8 to 11 was used for the pH dependent measurements.
2-2- Quartz Crystal Microbalance with Dissipation (QCM-D)
An E4 QCM-D unit from Q-Sense (Q-Sense AB, Göteborg, Sweden) was used for the QCM-D
experiments. MS (C1050) coated AT-cut quartz crystals (QSX301) were used as the model substrate. The
crystals were cleaned by soaking and sonicating for at least 10 min in a 2% Hellmanex solution before
being rinsed with DI water and dried with nitrogen gas. The crystals were exposed to ultraviolet light for
20 min before the experiment.
A flow rate of 50 µL min-1 was maintained through the QCM-D using a peristaltic pump (RegloDigital,
Ismatec). The temperature was controlled by the QCM-D at 72°F (22°C). Before beginning the
experiments, a frequency and dissipation baseline in tap water was allowed to stabilize. Then the desired
solution was flowed across the crystal surface to measure any adsorption on the crystal. After each
experiment, the QCM-D setup was cleaned by flowing a 2% Hellmanex solution 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.
The Sauerbrey equation was used to calculate the mass of adsorbed molecules on the QCM-D sensor
2-3- Electrochemical/Corrosion Cell and Equipment
A standard three-electrode electrochemical/corrosion cell was used in the electrochemical experiments.
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 manuscript are expressed with
respect to the SCE. The working electrode (WE) was prepared from a C1010 (mild steel) rod (Metal
Samples), and sealed with epoxy resin to give a two-dimensional surface exposed to the electrolyte. Table
1 shows the chemical composition of alloy used in this study.
Table 1: The chemical composition (%, w/w) of mild steel.
Lab (for electrochemistry experiments)
Lab (for QCM-D experiments)
Electrochemical measurements were performed using a Solartron 1287 Electrochemical Interface
potentiostat/galvanostat and Solartron 1260 Impedance/Gain-Phase Analyzer. To ensure complete
characterization of the interface and the surface processes, electrochemical impedance spectroscopy (EIS)
measurements were made over a frequency range, from 100 kHz to 100 mHz with the alternating current
(AC) voltage 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 electrode was then immersed in the test electrolyte and equilibrated for
3 hrs at 158°F (70°C) at open-circuit potential (OCP), followed by the electrochemical measurements. All
the solutions were mixed by magnet stirrer. All data reported in this work represent mean values of four
to six replicates.
3- RESULTS AND DISCUSSION
3-1- Kinetics of Purified Tannins (TG 3300) Adsorption on MS Surface
It is interesting to investigate the kinetics of corrosion inhibitor adsorption to find the time scale within
which the equipment surface will be covered by varying degrees of inhibitors. Figure 1 shows that at a
constant inhibitor bulk concentration, the surface concentration of adsorbed inhibitor increases rapidly,
and then gradually levels off to a plateau. The adsorption equilibrium is reached after ca. 5-15 min,
depending on the inhibitor bulk concentration. Note that the difference in surface concentration
corresponding to 275 and 1100 ppm is within the experimental error.
Figure 1. Purified tannins (TG 3300) surface concentration as a function of time and inhibitor bulk concentration, obtained
from QCM-D at pH 10.5 and room temperature.
The kinetic data in Figure 1 were then modeled. For this purpose, the inhibitor surface concentrations were
converted into the corresponding apparent surface coverage values. Then, a two-step model (Figure 2) was
used to model the adsorption of inhibitor onto mild steel surface. In this model, the first adsorption step is
reversible, and the inhibitor’s surface conformation is assumed to resemble one in the bulk solution, i.e.
the native conformation. However, the inhibitor with this surface conformation can either desorb (Step 1)
or adopts a more thermodynamically favorable surface conformation (Step 2) (11-13) and assumed
Figure 2. Schematic representation of a two-step kinetic model for the adsorption of purified tannins (TG 3300) on MS.
represents surface coverage by the inhibitor adsorbed conformation. This molecule can either desorb back into the solution,
or reconform into a more thermodynamically favorable conformation, (11-13).
Time / min
010 20 30 40
Surface Concentration, Cs / mg m-2
Expressing the inhibitor surface concentration in terms of its apparent surface coverage,, the two-step
adsorption kinetic model can be formulated as:
where is the fraction of apparent surface covered by inhibitor in both the
thermodynamically unstable () and stable () conformations, (mol L-1) is the TG 3300 inhibitor bulk
concentration, (mol-1 L min-1) is the adsorption constant, (min-1) is the desorption constant, and
(min-1) is the reconformation (surface rearrangement) constant.
The experimental data (Figure 1) were fit using the kinetic model in Eqs. (1)-(3) and a good agreement
between the model (Figure 3, circles) and the experimental data (Figure 3, solid lines) was obtained in all
cases. This shows the applicability of the proposed model to describe the kinetics of TG 3300 corrosion
inhibitor adsorption onto MS under these experimental conditions. Table 2 and Figure 4 shows the
corresponding kinetics constants.
There is a possibility of desorption of reconformed more stable tannin molecules. However, the two-step
model gives an accurate approximation of the system’s behavior.
Figure 3. The kinetics of purified tannins (TG 3300) adsorption onto mild steel surface presented in term of the dependence
of apparent inhibitor surface concentration as a function of time at different bulk concentrations, pH 10.5 and room
temperature. Solid lines represent the experimental data, while the circles show simulated data using the two-step adsorption
kinetic model described by Eqs. (1)-(3) and Figure 2.
Table 2. Kinetic rate constants for the adsorption of purified tannins (TG 3300) onto mild steel as a function of inhibitor bulk
concentration. The parameters were determined by fitting the experimental data from QCM-D experiments.
[TG 3300] / ppm
ka × 10-5 / mol -1 L min-1
kd × 10+1 / min-1
kf × 10+2 / min-1
Figure 4 shows that for all inhibitor bulk concentrations, the adsorption rate constants are considerably
bigger than desorption and reconformation rate constants, showing a strong affinity of TG 3300 for MS.
Figure 4 also shows that the kinetic rate constants depend on the inhibitor bulk concentration in a regular
manner, i.e. they decrease with an increase in the inhibitor bulk concentration, indicating that they are
apparent rate constants. Namely, the proposed kinetic model does not take into account the intermolecular
interactions of adsorbing inhibitor molecules with those that are already on the surface, but only the
Time / min
0 5 10 15 20 25 30 35
Surface Concentration, Cs / mg m-2
occupancy of the surface with the latter molecules. At higher inhibitor bulk concentrations, the substrate
surface becomes covered by inhibitor faster, which increases the probability for the occurrence of the
intermolecular interactions. Consequently, the apparent kinetic rate constants change. The decrease in their
value shows that the intermolecular interactions with the already adsorbed tannin molecules “inhibit
(molecular steric hindrance)” further adsorption from the solution, or desorption from the substrate, and
reconformation of the inhibitor molecules on the surface.
Figure 4. Dependence of the (a) adsorption, (b) desorption, and (c) reconformation rate constants on the purified tannins (TG
3300) bulk concentration. The data were obtained by modeling the experimental kinetic data using the kinetic model
presented in Figure 2.
The proposed inhibitor adsorption kinetic model described in Figure 2 and Eqs.(1)-(3) shows that the
substrate surface coverage by native conformation () and reconformed () inhibitor changes with time.
It became interesting to examine the kinetics of this molecular reconformation. For that purpose, the
ka x 10-5 / mol-1 L min-1
kd x 10+1 / min-1
[TG 3300] / ppm
0500 1000 1500 2000 2500
kf x 10+2 / min-1
modeled kinetic data were deconvoluted into the two contributions ( and ), as shown in Figure 5 and
then separately in Figure 6a and b.
Figures 5 and 6a demonstrate that the initial increase (within the first 5 to 10 minutes of adsorption) in
total inhibitor surface coverage, (solid line in Figure 5), is mostly due to the formation of an inhibitor
layer with assumed native conformation, (dashed line in Figure 5). The surface coverage with the native
inhibitor initially increases sharply and then reaches a plateau value, which occurs at earlier times at higher
bulk solution concentrations. On the other hand, the surface coverage with a reconformed inhibitor
(Figures 5 and 6b), (dotted line in Figure 5), gradually increases in the entire time interval. Figures 5
and 6 also show that the transfer from the native conformation () into the reconformed inhibitor ()
does not get completed within the time interval studied.
Figure 5. Time dependence of total apparent surface coverage by purified tannins (TG 3300) inhibitor (, solid line), is the
sum of apparent surface coverage with native (, dashed line) and reconformed (, dotted line) adsorbed inhibitor on MS
obtained in a solution containing 64 ppm inhibitor at pH 10.5 and room temperature. The data were obtained by modeling the
experimental data in Figure 2 using Eqs.(1)-(3).
In addition, Figure 6c shows the values of the ratio of surface coverage of native to reconformed adsorbed
. The results demonstrate that with an increase in adsorption time, the relative surface
ratio also increases. The trend can be approximated by a second-order polynomial function behavior. Also,
the values at a particular adsorption time are different at different inhibitor bulk solution concentrations.
This indicates that the inhibitor bulk solution concentration does influence the kinetics of the inhibitor
surface reconformation (from to ).
Time / min
0 5 10 15 20 25 30 35
Surface Coverage, / %
Figure 6. Time dependence of mild steel (a) apparent surface coverage with native (), (b) reconformed (), and the ratio of
surface coverage of native to reconformed adsorbed inhibitor (
) obtained at different purified tannins (TG 3300) bulk
solution concentrations: (1) 17, (2) 34, (3) 64, and (4) 275 ppm.
1 / %
2 / %
Time / min
010 20 30 40
3-2- Equilibrium of Purified Tannins (TG 3300) Adsorption on MS Surface
The inhibitor affinity with MS can be determined by the Gibbs free energy of adsorption, through
adsorption isotherms (11). The mean value of the QCM-D crystal frequency shift, after reaching
equilibrium at each inhibitor bulk solution concentration, was used for further equilibrium calculations.
Figure 7 shows that the inhibitor surface concentration increases with an increase in inhibitor bulk
concentration, and then levels off to a plateau (maximum surface concentration, ≈4.25 mg m-2) at
an inhibitor bulk concentration of ca. 275 ppm. Interestingly, this bulk concentration converges toward the
optimum value observed after four decades of empirical optimization in industrial MS boilers, treated with
tannin-based chemistries. Moreover, the steep initial slope of the adsorption isotherm (raising part of the
isotherm) shows the high affinity of TG 3300 molecules toward MS surface (14).
Figure 7. Adsorption isotherm for purified tannins (TG 3300) onto MS at pH 10.5 and room temperature. Circles represent
the experimental data, while the solid line represents the corresponding value from Langmuir isotherm. Inset: Experimental
data (circles) show an excellent correlation with the linearized Langmuir isotherm.
The plot in Figure 7 resembles a shape of a unimodal adsorption isotherm (11). The corresponding
Langmuir isotherm equation, for further investigation of TG 3300 adsorption onto MS surface, is:
[TG 3300] / ppm
0500 1000 1500 2000 2500 3000
Surface Concentration, Cs / mg m-2
[TG 3300] / M
020 40 60 80 100
[TG 3300] / M
Cs,max=4.25 mg m-2
where (mg m-2) is the surface concentration at a particular TG 3300 bulk concentration, (ca.
4.25 mg m-2) is a maximum surface concentration, (L mol-1) is the adsorption affinity constant at
constant temperature and (mol L-1) the TG 3300 bulk concentration.
To verify whether the experimental data in Figure 7 can be described by the Langmuir adsorption isotherm,
widely used to study gas phase and colloidal systems, it would be more convenient to use a linearized form
of Eq. 4. Thus, dividing the equation by the corresponding maximum value,
the ratio by apparent surface coverage, , a linearized form of the isotherm is obtained:
For the Langmuir isotherm to be considered valid, a plot of versus should yield a straight line
with a slope of one and an intercept
. Indeed, the inset of Figure 7 shows that a linear behavior from
adsorption isotherm with the corresponding slope value close to unity (0.96), in agreement with Eq. (5).
Consequently, the Langmuir isotherm was deemed applicable in describing the adsorption of TG 3300 on
MS. However, the adsorption of TG 3000 on MS surface is considered irreversible, which is not in
accordance with the original assumption of the Langmuir isotherm on the reversibility of an adsorption
process. Nevertheless, for quantitative / comparative / engineering scaling-up purposes, it is still
appropriate to use the Langmuir isotherm to describe adsorptive type of interactions.
From this, apparent Gibbs free energy of adsorption values, (J mol-1), were calculated through:
where (mol L-1) is the molar concentration of the solvent, which in this case corresponds to water
(= 55.5 (mol L-1)), is the gas constant (8.314 (J mol-1 K-1)), and (K) is the temperature.
The calculated adsorption affinity constant and corresponding apparent Gibbs free energy of adsorption
values obtained from QCM-D measurements are 4.39 × 106 L mol-1 and -47.36 kJ mol-1, respectively. The
relatively big negative apparent Gibbs free energy of adsorption value, obtained from equilibrium
experiments, indicated a spontaneous and relatively strong adsorption of TG 3300 onto the mild steel
surface which confirms the conclusion from the kinetic measurements (11, 15).
3-3- Open-Circuit Potential Measurements
Figure 8 shows that when TG 3300 bulk concentration increases the OCP also increases to more noble
values, indicating the adsorption of TG 3300 on the mild steel surface and its protective influence on MS
Figure 8. Open-circuit potential (OCP) of MS as a function of purified tannins (TG 3300) bulk concentration, recorded after 3
hrs incubation at pH 10.5 and 158°F.
3-4- Electrochemical Impedance Spectroscopy Measurements
Electrochemical impedance spectroscopy (EIS) was applied to investigate the electrode/electrolyte
interface and processes that occur on the mild steel surface at OCP in the presence and absence of
TG 3300 in the solution, most notably the general corrosion resistance of MS. Figure 9 shows that the
diameter of the EIS spectra semicircle increases with the increase in inhibitor bulk concentration,
indicating an increase in corrosion resistance of the material (16, 17). In order to extract qualitative
information, a nonlinear least squares (NLLS) fit analysis was used to model the spectra, employing
electrical equivalent circuits (EECs) presented in Figure 10 (18, 19). In these EECs, represents the
ohmic resistance between the WE and RE; is the charge transfer resistance related to the corrosion
reaction at OCP, while CPE is the capacitance of the electric double-layer at the electrode/electrolyte
interface. The EEC in Figure 10 was used to fit the spectra recorded in the absence and presence of TG
3300 in the solution.
[TG 3300] / ppm
0200 400 600 800 1000 1200 1400 1600
OCP / V
Figure 9. Nyquist plot of a MS recorded at different bulk concentrations of purified tannins (TG 3300) at 158°F and pH 10.50
after 3 hrs of incubation at OCP. Circles are experimental data and solid lines represent the simulated spectra.
Figure 10. EEC model used to fit EIS data recorded MS in absence and presence of the inhibitor.
Taking into account the physical meaning of the EEC parameters of the circuits in Figure 10, the corrosion
resistance of the bare (control) and covered MS surface is equivalent to the charge-transfer resistance, ,
consequently, the corrosion inhibition efficiency of TG 3300 (), was calculated by comparing the total
resistance value, , recorded at various concentrations of TG 3300, and the value recorded in the
absence of TG 3300 (control sample):
Figure 11a shows that with an increase in the inhibitor concentration in the bulk solution, the corrosion
inhibition efficiency also increases and reaches a maximum value of ca. 82% indicating high surface
corrosion protection by TG 3300 molecules. Figures 11b and 11c also show that in the absence of the
corrosion inhibitor, the electrode if fully corroded. However, in presence of approximately 275 ppm of TG
3300 in the solution, the sample is highly protected and there is no visible corrosion mark on the electrode.
0500 1000 1500 2000
0 ppm 34 ppm
Figure 11. (a) Corrosion inhibition efficiency of purified tannins (TG 3300) as a function of inhibitor bulk concentration for
MS after 3 hrs of incubation at 158°F and pH 10.50. Images of MS electrodes after incubation in the absence (b) and presence
(c) of 275 ppm of TG 3300.
3-5- Effect of pH on Purified Tannins (TG 3300) Corrosion Protection of Mild Steel Surface
The influence of pH on MS corrosion inhibition efficiency was studied in a pH range from 8 to 11, in the
absence and presence of 275 ppm of TG 3300. Figure 12 shows that corrosion inhibition increases with
pH and gives the highest efficiency at pH 11 (i.e. 80%). In addition, low corrosion inhibition efficiencies
at lower pH is likely due to the MS dissolution in lower alkalinity environment (20, 21). Similar behavior
was observed by the authors in previous work (22).
Figure 12. MS corrosion inhibition efficiency as a function of pH. The EIS data were recorded in the absence and presence of
275 ppm of purified tannins (TG 3300) at 158°F and 3 hrs.
[TG 3300] / ppm
0200 400 600 800 1000 1200 1400 1600
Corrosion Inhibiton Efficiency, / %
7 8 9 10 11 12
Corrosion Inhibition Efficiency, / %
3-6- Practical Results from the Field
An obvious challenge is to link the laboratory results with the field observations. Table 3 shows corrosion
coupons results from the field for heating and cooling water closed-loop MS systems, treated with TG
3300. Measurements from different locations showed very low corrosion rates; overall averages of 0.157
±0.276 mpy for 173 days (n = 16 samples), i.e. “Excellent” from AWT qualitative classification (QC) (23,
24). Consequently, this work highlights that our laboratory method (ca. 80% corrosion inhibition
efficiency, Figure 12) underestimates the performance of the field observations (Excellent, Table 3). Thus,
more laboratory work is needed to develop a method which accurately predicts the performance of tannin-
based corrosion inhibitor for the field applications.
Table 3. Mild steel water closed loop systems treated with purified tannins (TG 3300)
Days / #
/ μS cm-2
Ave. Corr. Rate
(Peterbilt Kenworth Trucks)
0.154 ± 0.112
Sherbrooke University / Sherbrooke
0.183 ± 0.344
Sherbrooke University / Longueuil
0.033 ± 0.020
NB: Average ± SD expressed as absolute values.
The QCM-D method was used to investigate the adsorption kinetics and thermodynamics of purified
tannins (TG 3300) corrosion inhibitor on mild steel (MS). The kinetic experiments showed that TG 3300
adsorption reaches equilibrium after 5-15 min, depending on the inhibitor bulk concentration. The purified
tannins (TG 3300) adsorption kinetics follows a two-step model; the first step represents the reversible
purified tannins (TG 3300) adsorption/desorption, while the second step represents the molecular
reconformation of purified tannins to a thermodynamically more stable conformation. The adsorption
process showed an excellent correlation (= 0.99) with the Langmuir isotherm. The large negative
apparent Gibbs free energy of adsorption ( = -47.36 kJ mol-1) confirmed spontaneous and strong
adsorption of purified tannins (TG 3300) on the MS surface. The adsorption isotherm showed that ca. 275
ppm bulk concentration converges toward the optimum value observed after four decades of industrial
empirical optimization of MS boilers, treated with tannin-based chemistries. Electrochemical impedance
spectroscopy (EIS) showed that high inhibition efficiency (i.e. 80%) is achieved within 3 hrs of the
immersion of a freshly polished MS in a purified tannins (TG 3300) solution. Laboratory results showed
that by increasing the pH, MS surface protection increases. High corrosion protection of MS by purified
tannins (TG 3300) was also evidenced by the field results; overall average corrosion rates of 0.157 ± 0.276
mpy in 173 days, i.e. “Excellent” from the perspective of the AWT qualitative classification. However,
future work is needed to improve the predictability of the lab results toward the field performance.
The authors gratefully acknowledge the financial support from the Natural Science and Engineering
Research Council of Canada (NSERC). The authors also thank Prof. Theo van de Ven form McGill
University for valuable scientific insights, and Mr. Louis-Philippe Cloutier for valuable discussions.
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