Int. J. Electrochem. Sci., 8 (2013 ) 3589 – 3601 [602461]

Int. J. Electrochem. Sci., 8 (2013 ) 3589 – 3601

International Journal of
ELECTROCHEMICAL
SCIENCE
www.electrochemsci.org

Electrochemical Study of Haematoxylin Inhibitory Activity to
Control Carbon Steel Corros ion in Natrium Nitrate Solution

Adriana Samide*, Bogdan Tutunaru, C ătălina Ionescu, Cristian Tigae, Anca Moanță
University of Craiova, Faculty of Sciences, Department of Chemistry, Calea Bucuresti 107 i, Craiova,
Romania
*E-mail: [anonimizat]

Received: 10 January 2013 / Accepted: 9 February 2013 / Published: 1 March 2013

The corrosion inhibition of carbon steel in 0.1 M NaNO 3 solution using haematoxylin (HX), IUPAC
name 7,11b -Dihydroindeno[2,1 -c]chromene -3,4,6a,9,10(6 H)-pentol was studied using elec trochemical
measurement s such as: potentiodynamic polarization and electrochemical impedance spectroscopy
(EIS) in association with UV -Vis spectrophotometry. Microscopic images were used to examine the
surface morphology. Electrochemical results showed tha t, for 1.0 mM HX in natrium nitrate solution,
the corrosion current density ( icorr) had the lowest value (11.5 μA cm-2), and the highest polarization
resistance ( Rp) was obtained (498.6 Ω cm2), and consequently, an inhibition efficiency ( IE) of
81.6±2% for HX was reached. A synergic action mechanism of HX was proposed related to the
organic film formation consisting in haematoxylin and its oxidized form haematein (HT) adsorption,
supplemented by evidence of HT complexes with Fe3+, and an additional process of HX
electropolymerization. By optical microscopy, the feature of the uniform layer on the surface was
nuanced for carbon steel corroded in 0.1 M NaNO 3 solution containing 1.0 mM HX.

Keywords: corrosion inhibition; haematoxylin; electrochemical measurem ents; actio n mechanism;
surface morphology

1. INTRODUCTION

The destruction of iron and iron alloys by chemical and electrochemical reactions with their
environment is a major industrial problem that has attracted researchers’ interest . Hydrochloric and
sulphuric acids are widely used for pickling or cleaning in industrial applications and because of their
highly corrosive nature they may cause serious problems.
A useful method, among others, to protect steels and iron in aggressive acidic environments, is
the addition of inhibitors in the solutions with which the metals come into contac t, in order to inhibit
the corrosion reactions and to reduce the corrosion rate. Organic inhibitor molecules can physically or

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chemically adsorb on metal surface and form a surface layer that protects the metal from corrosion.
Inhibition efficiency is closely related to certain factors: the presence of functional groups, the
presence of heteroatoms, lone electron pairs in the heteroatoms, molecular size, electron density, s teric
factor (i.e. planarity), π -electron s, conjugated bonds [1 -9].
In the past years, many classes of organic compounds have been tested as inhibitors. Increased
attention has been dedicated to eco -friendly corrosion inhibitors, which have been exploited as a green
alternative to synthetic and toxic chemicals [10, 11].
These compounds include amino acids and their derivatives. Weight loss, potentiodynamic
polarization, electrochemical impedance spectroscopy, AFM and EDX analyses have been used to
study the corrosion inhibition of adenine and L -tryptophan [12, 13]. The application of anionic,
cationic or gemini surfactants, have remarkable inhibition efficiency, near their critical micellar
concentration [14 -17].
Imidazolium, alkilimidazolium and some pyrid inium – or vinyl – based ionic liquids have been
tested as corrosion inhibitors for steel , using potentiodynamic polarization, linear polarization and
weight loss methods. This type of compounds present s relatively high inhibition efficiencies (80 -96 %)
[18-20]. Monoethanolamine was used to convert polyethylene terephthalate waste into a water soluble
amide as corrosion inhibitor for carbon steel in 1M HCl [21]. Few articles are reported on the
corrosion inhibition of carbon steel with polymeric inhibitors : polyvinyl pyrrolidone, poly(ethylene
glycol) methyl ether, polyacrylamide [22 -24]. Organic dyes have been reported as effective corrosion
inhibitors of carbon steel in acidic media. The inhibition effect of Methylene Blue, Methyl Red, New
Fuchsin, Nile Blue , Indigo Carmine, A lizarin Yellow, Bromophenol Blue, Thymol Blue, Basic yellow
13, Basic yellow 28, has been studied [25 -32].
The use of antibiotics and other drugs has been investigated and inhibition efficiencies are
correlated with their heterocyclic st ructure. A theoretical study on the electronic and molecular
structures of sulfaacetamide, sulfapyridine, sulfamerazine, and sulfathiazole was used to determine the
relationship between some quantum chemical parameters and inhibition efficiencies [33]. The
inhibitory properties of Penicillin G, Chloroquine, Cloxacillin and Trimethoprim have also been
reported [34 -37].
Several studies have been carried out on the inhibition of carbon steel corrosion by plant
extracts of Asteriscus Graveolens [38], Jatropha c urcas [39], Sesamum indicum [40], Phyllanthus
Amarus [41] species.
Haematoxylin, IUPAC name: 7,11b -Dihydroindeno[2,1 -c]chromene -3,4,6a,9,10(6 H)-pentol, is
a natural compound extracted from the heartwood of the Central American logwood Haematoxylon
campech ianum Linnaeus.
In this work, the inhibition activity of haematoxylin (HX) has been evaluated for carbon steel
corrosion in 0.1 M NaNO 3 solution using electrochemical measurements associated with UV -Vis
spectrophotometry.

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2. MATERIALS AND METHOD S
2.1. Materials
The used material was a carbon steel plate (20mm x 20mm x 2mm) with the following
composition (weight %): C=0.1%; Si=0.035%; Mn=0.4%; Cr=0.3%; Ni=0.3%; Fe =the remainder. The
samples were mechanically polished with emery paper, degreased with acet one and dried. Corrosion
tests were performed in 0.1 M NaNO 3 blank solution and 0.1 M NaNO 3 containing different
concentrations of haematoxylin: 4.10-4 M, 6.10-4 M, 8.10-4 M and 10-3 M. All reagents were obtained
from Merck.

2.2. Electrochemical measureme nts
Potentiodynamic polarization was used to determine the corrosion rate of carbon steel in the
presence and in the absence of haematoxylin. All the electrochemical measurements were obtained
using a VoltaLab 40 potentiostat/galvanostat, VoltaMaster 4 sof tware, connected to a personal
computer. A glass corrosion cell with a platinum counter electrode, a saturated Ag/AgCl sat reference
electrode and carbon steel sample as working electrode were used. The immersion time of the plates in
the respective media was of 4 minutes in open circuit, at room temperature. The polarization curves
were registered with a scan rate of 1 mV/sec.
The electrochemical impedance spectra, corresponding to steel corrosion in the absence and in
the presence of haematoxylin were rec orded in the frequency range of 100 kHz – 10 mHz, with an
amplitude of 10 mV.

2.3. UV -Vis spectroscopy
Samples of the 0.1M NaNO 3 solutions containing various concentrations of haematoxylin (HX)
were used for spectrophotometric analysis, before and after t heir usage in the corrosion tests of carbon
steel. UV -VIS analysis reports were obtained using a UV -VIS spectrophotometer, VARIAN -CARY 50
type.
Procedure: The solutions were placed in the UV -VIS beam and a graph of absorbance versus
the wavelength was obta ined; for each sample analysis, reports were obtained using the soft CARY
WIN UV. Alternatively, the solutions were prepared in known concentrations of HX and they were
read by the UV -VIS spectrophotometer. Results are graphed to make a calibration curve f rom which
the unknown concentration of can be determined by its absorbance.

2.4. Surfaces morphology
The surface morphologies of the uncorroded and corroded carbon steel, in the absence and in
the presence of the inhibitor, were examined by Euromex micros cope with Canon camera and included
ZoomBrowser – EOS Digital software

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3. RESULTS AND DISCUSSION
3.1. Potentiodynamic measurements
The presence of HX in corrosion environment produces changes in both the shape of
polarization curves, as well as of their po sitions, which were shifted in other fields of potential
compared with the polarization curve obtained in the absence of HX. From Figure 1, it can be seen that
the corrosion potentials are shifted in positive direction, starting with 0.4 mM concentration o f HX in
aggressive medium. However, polarization curve corresponding to concentration of 0.4 mM, is moved
in a higher current region than that obtained for the blank solution. This is associated with an increase
in corrosion current indicating that, at the concentration of 0.4 mM, HX accelerates the corrosion
process of carbon steel in NaNO 3 medium. In the concentration range from 0.6 mM to 1.0 mM,
polarization curves are shifted in lower current areas. Both processes, anodic oxidation and cathodic
reaction of oxygen reduction, are strongly modified, especially for the last two concentrations of HX,
0.8 mM and 1.0 mM, respectively. Taking these data into account, we may conclude that the corrosion
current density decreases starting with the 0.6 mM concentrat ion of HX, reaching the lowest value for
the solution containing 1.0 mM HX.

-10123
-950 -850 -750 -650 -550
E (mV vs. Ag / AgCl)log i (mA cm-2)
w ithout HX
0.4 mM HX
0.6 mM HX
0.8 mM HX
1.0 mM HX

Figure 1. The potentiodynamic curves of carbon steel corroded in 0.1 M NaNO 3 solution in the
absence and in the presence of different concentratio ns of HX.

These observati ons indicate the fact that HX inhibits the corrosion of carbon steel in NaNO 3 by
blocking the active sites of metal surface by forming a thin organic film [42], which is interposed at the
metal/electrolyte interface . It can also be noted that, in the poten tial range between -710 mV and -660
mV, the current density is constant forming a pla teau on the polarization curve of carbon steel
corroded in nitrate solution containing 0.8 mM HX, indicating that in this region the metal ionization
can be greatly suppre ssed. At potential values higher than -660 mM, carbon steel becomes active,
perhaps with the help of the dissolved oxygen reduction reaction. At lower current density, in the

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potential range between -680mV and -630 mV, a similar plateau can be observed on the polarization
curve of carbon stee l corroded in nitrate solution containing 1.0 mM HX.
The electrochemical parameters such us: corrosion potential ( Ecorr), corrosion current density
(icorr), anodic and cathodic Tafel lines ( ba & bc), as well as inhibition efficiency ( IE) as a function of
HX concentration (C-HX) are presented in Table 1.
The corrosion current density ( icorr) was estimated to intersection of anodic and cathodic Tafel
lines at corrosion potential, ba & bc being estimated for the segments value of 90 mV dec-1 using
VoltaMaster 4 software. The inhibition efficiency percentage ( IE) of HX was determined from
polarization measurements according to the following equation, Eq. 1 [43 -45]:

100
0
corrcorrcorr
ii i
IE0
(1)

where i0
corr and icorr are the corrosion current densities of carbon steel in 0.1 M NaNO 3 solution
without and with HX, respectively.

Table 1. Electrochemical parameters and inhibition efficiency obtained from Tafel polarization for
carbon steel corroded in 0.1 M NaNO 3 solution in the absence and in the presence of different
concentration of HX.

C-HX/
mmol L-1 Ecorr/mV
vs.Ag/AgCl icorr/
μA cm-2 ba/
mV dec-1 bc/
mV dec-1 IE/
%
0 -843 70.0 262.0 98.0 0
0.4 -774 113.0 218.0 245.0 0
0.6 -712 45.2 114.0 225.0 35.4
0.8 -732 20.2 122.0 118.0 71.1
1.0 -717 11.5 132.0 100.0 83.6

By analysing t he data from Table 1 we may conclude that the corrosion current density
decreases with the increase in HX concentration starting with 0.6 mM HX in corrosive medium, and
consequently, the inh ibition efficiency increases, reaching a maximum value of 83.6% at 1.0 mmol L-1
HX in 0.1 M NaNO 3 solution.

3.2. Action mechanism of HX as corrosion inhibitor
To estimate the most likely mechanism of the carbon steel inhibition by H X, the UV -Vis scans
of corrosive media, before and after corrosion processes, were accomplished. In Figure 2, the
molecular structure of HX and UV -Vis scans of 0.1 M NaNO 3 solution containing 0.4 mM HX and 1.0
mM HX, before and after corrosion, are presented. Note that HX has two adsorption peaks, at 295 nm

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and 436 nm, corresponding to the haematoxylin (HX) and its oxidized form, haematein (HT),
respectively.
Before corrosion, for both above mentioned concentrations, HT is presented as a flattened
shoulder, slightly more nuanc ed for 1.0 mM HX, indicating that the HT amount in corrosive media is
very small. After corrosion, it can be observed that, HX absorbance decreases without altering the
wavelength of adsorption maximum, but at the same time, the corresponding peak of HT in creases in a
considerable manner and a new peak at 558 nm is observed. In our previous studies [46, 47] we
showed that, under these experimental conditions, the main product of corrosion is formed by Fe3+
species which are similar to those shown by Fe3+ oxide/oxyhydroxides. By considering those
previously stated , we can say that during potentiodynamic polarization, two simultaneous phenomena
appear, as follows: (i) the oxidation of HX to HT (Scheme I); (ii) the formation of a strongly coloured
complex of HT with Fe3+ ions. In other studies the absorption bands of haematein and haematein
complexes were recognized at 445 nm and 556 nm, respectively [48, 49]. Also, this was confirmed by
the change of the reaction environment colour during the electrochemical me asurements, as follows:
light yellow HX solution → red solution (indicating the presence of HT) → violet solution (indicating
the presence of the HT complex with Fe3+).

00.40.81.21.6
250 450 650 850
Wavelength (nm)Absorbance0.4 mM HX before corrosion 1.0 mM HX before corrosion
1.0 mM HX after corrosion 0.4 mM HX after corrosion

Figure 2. The HX molecular structure and the UV -Vis scans of 0.1 M NaNO 3 solution c ontaining 0.4
mM HX and 1.0 mM HX, before and after corrosion tests

Based on these data, we consider that HX acts as inhibitor for carbon steel corrosion in nitrate
solution through several mechanisms related to the formation of a thin layer, such as t he pure
adsorption of HX and HT, as well as complexes formation between HT and iron (III) ions that may
provide a good stability of this layer.
Moreover, from Figure 1 it can be observed that the shape of polarization curves obtained for
the carbon steel corroded in nitrate solution containing 0.8 mM HX and 1.0 mM HX are very modified
compared with the polarization curves obtained for carbon steel corroded in 0.1 M NaNO 3 without and

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with lower concentrations of HX. This can be explained by occurrence of ot her phenomena which
could change the composition of the thin layer formed on carbon steel surface. Several studies reported
that the electropolymerization of HX is also possible under different experimental conditions such as:
slightly and neutral pH, 10- 4 haematoxylin concentration in the potential range of -0.5 ÷ 2 V [50, 52],
resulting a poly -haematoxylin film (p -HX) with bioelectrocatalytic activity [50 -52]. So, the
polymerization of HX should be considered as an additional process contributing to the formation of a
protective film on the metal surface.
The proposed mechanisms of HX oxydation and electropolymerization are presented below
(Scheme 1).

Scheme 1. Haematein (HT) formation and propagation steps of the electrochemical haematoxylin
polymer ization mechanism.

From Scheme 1, it can be observed that haematoxylin molecule loses an electron and a proton
with formation of an active radical (I), for which resonance structures can be written. In a second

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oxidation step of radiacal I, haematein (HT) is formed, this being the stable oxidized form of
haematoxylin.
Five different tautomeric forms can be written for hematein, but the values of their free Gibbs
energies indicate that the most stable form among them is the HT structure presented in Scheme 1 [53].
Therefore, an oxidation mechanism that would first lead to one of the other four tautomeric forms, that
would subsequently tautomerize to the known stable structure of HT, is not excluded either.
The radical (I) may attack a new molecule, forming a haematoxylin dimmer [50, 52]. This
dimmer is easily oxidized to a radical -dimer [52]. The radical -dimmer attacks a new molecule of
haematoxylin forming a trimer, and this chained process continues until obtaining the stable poly –
haematoxylin macromolecu le [52].
Taking these data into account, we assume that the action mechanism of HX as inhibitor for
carbon steel corrosion in nitrate solution is difficult to assess, but concrete phases of organic film
formation could be given, as follows:
– the pure absor tion of HX;
– the pure adsorbtion of HT;
– the complexes formation between HT and iron (III) and their adsorption on carbon steel
surface;
– electrosynthesis of p -HX film which could insert the HX and HT molecules, different ions,
such as: OH-, NO 3-, Fe3+, as we ll as iron compounds and its complexes.

3.3. Electrochemical impedance spectroscopy
Figure 3 shows the Nyquist (Fig.3a) and Bode (Figs.3b and 3c) plots for carbon steel in 0.1 M
NaNO 3 solution without and with different concentrations of HX; it can be se en that the impedance
response of carbon steel in 0.1 M NaNO 3 solution shows a significantly change after haematoxylin
addition, indicating that, in low frequency range, the carbon steel impedance increases with increasing
the inhibitor concentration (Fig. 3b) and consequently, the inhibition efficiency increases.
It is also apparent from these plots that Nyquist curves (Fig.3a) are consisted in one capacitive
loop and one phase angle maximum in Bode format (Fig.3c). For both plots more pronounced
frequency arcs were obtained for the samples which were immersed in 0.1 M NaNO 3 solution
containing various HX concentrations higher than 0.4 mM.
This behaviour is usually assigned to changes in density and composition of substrate layer
[54-57]. It is clear that HX presence produced a higher polarization resistance ( Rp) value, which is
interpreted in terms of formation of an effective protective layer which is controlled by the process of
adsorption and desorption of inhibitor molecule at low frequencies [58, 59], with adsorption
predominance. An equivalent circuit is suggested in Figure 3a relating the best fitting of EIS
measurements, where Rs is the solution resistance of the bulk electrolyte and Ccoat is the capacitance of
the coating. Typical coating capacitan ces are on the order of 1μF cm-2 [54]. The value can vary based
on the thickness of the coating, as well as its dielectric constant [54]. The dielectric constant and
thickness can both change over time when exposed to solution, because coating often adsorb s water

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[54]. Rcoat is the resistance of the coating that often has very small pores which contain electrolyte,
providing a conduction path of ions through the coating [54]. Cdl represents the double layer
capacitance of the electrolyte at the metal surfac e. Rp is the polarization resistance of carbon steel. The
impedance parameters derived from EIS measurements and respective fitting results are given in Table
2 and Figure 3, respectively.

0100200300
0 100 200 300 400 500
Zr (Ω cm 2)-Zi (Ω cm 2)
2134 5measured
1. w ithout HX
2. 0.4 mM HX
3. 0.6 mM HX
4. 0.8 mM HX
5. 1.0 mM HXa
simulated

01234
-2 0 2 4 6
log Frequency (Hz)log Z ( Ω cm 2) .1. w ithout HX
2. 0.4 mM HX
3. 0.6 mM HX
4. 0.8 mM HX
5. 1.0 mM HX21345bsimulated

-80
-60
-40
-20
0-1 1 3 5
log Frequency (Hz)Phase (degree) 2
13451. w ithout HX
2. 0.4 mM HX
3. 0.6 mM HX
4. 0.8 mM HX
5. 1.0 mM HXcsimulated

Figure 3. Nyquist plots, equivalent circuit model (a) and Bode plots (b, c) for carbon steel immersed in
0.1 M NaNO 3 blank solution and in 0.1 M NaNO 3 solution containing various concentrations of
HX

The fitting results show that Rcoat and Rp increase with increasing HX concentration, suggesting
that the amount of adsorbed species increases, forming a thin organic film. The decrease in Ccoat and
Cdl could be attributed to the decrease in local dielectric constant and/or to the increase in thickness of
the electrical double layer, signifying that an organic film at the interface of metal/solution was formed
[54, 60].
The change in Rcoat, Rp, Ccoat and Cdl values was caused by the gradual replacement of water
molecules by adsorption of the organic film on the metal surface [60]. The Rp was used to calculate the
inhibition efficiency from Eq. 2:

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1000




pp p
RRRIE
(2)

where Rp and Rp0 represent the corrosion resistance in the presence and in absence of inhibitor,
respectively.

Table 2. Impedance parameters for carbon steel in 0.1 M NaNO 3 solution in the absence and in
presence of different concentrations of HX, at room temperature

C-HX/
mML-1 Rs/
mΩ cm2 Ccoat/
μF cm-2 Rcoat/
Ω cm2 Cdl /
μF cm-2 Rp/
Ω cm2 IE/
%
0 38.50 2.5 2.2 1100.0 101.8 0
0.4 11.60 2.2 3.1 819.5 78.9 0
0.6 9.90 1.8 5.3 812.0 172.7 41.1
0.8 6.68 1.6 7.8 714.0 291.8 65.1
1.0 6.20 1.3 9.1 459.0 498.6 79.6

The presence of HX leads to an approx. 80.0% inhibition efficiency, a closely value to those
obtained from potentiodynamic curves (Table 2).

3.4. Surface characteriz ation
The microscopic images of carbon steel surfaces before and after potentiodynamic curves
obtained in 0.1 M NaNO 3 solution without and with various concentrations of HX are presented in
Figure 4. The reference sample reveals a characteristic morphology of the carbon steel surface before
corrosion process (Fig.4a). The microscopic image from Figure 4b shows that after corrosion, carbon
steel surface was coated with large spots which change its texture, but the feature of a film adsorbed on
the surface is not nuanced. Moreover, large portions of the surface keep the morphology of the
reference sample, indicating that nitrate ions have some protective effect due to their oxidant action.
From Figure 4c to Figure 4f the microscopic images are completely diffe rent compared with
those presented above, showing a characteristic morphology of protected surfaces. In Figure 4c
corrosion spots are equally emphasized, as in Figure 4b, indicating that the formation of anodic areas
on which corrosion processes are enhanc ed, is possible. These confirm the results obtained by
electrochemical measurements which showed that 0.4 mM HX in nitrate solution is not the optimal
concentration for corrosion inhibition of carbon steel. Figures 4d and 4e show the image of some
surfaces covered by a discontinuous layer that could be attributed to an adsorbed organic film. All
organic components, such as: HX, HT, p -HX could act as “an incipient rust transformer”, favouring
the formation of a “superficial closed layer”. It is known that th e organic components of the rust
transformers penetrate the rust and decelerate the rusting process in this way [61]. The surface
morphology shown in Figure 4f is different compared with those commented above. The feature of an

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organic film, which could be attributed to a polymer layer on carbon steel surface, is more obvious and
its uniformity is also evidenced.

Figure 4. Microscopic images of carbon steel surfaces: a – before corrosion; b – after corrosion in 0.1 M
NaNO 3 blank solution; c – after corro sion in 0.1 M NaNO 3 solution containing 0.4 mM HX; d –
after corrosion in 0.1 M NaNO 3 solution containing 0.6 mM HX; e – after corrosion in 0.1 M
NaNO 3 solution containing 0.8 mM HX; f – after corrosion in 0.1 M NaNO 3 solution containing
1.0 mM HX

4. CO NCLUSIONS
The acti on mechanism of haematoxylin as inhibitor for carbon steel corrosion in 0.1 M NaNO 3
solution was studied using potentiodynamic polarization and electrochemical impedance spectroscopy
associated with UV -Vis spectrophotometry.
Both, potent iodynamic polarization and electrochemical impedance spectroscopy
measurements showed that HX acts as corrosion inhibitor of carbon steel under the above mentioned
laboratory conditions, starting with the concentration value of 0.6 mM.
UV-Vis spectrophoto metry confirms the oxidation process of haematoxylin to haematein
followed by its Fe3+ complexes formation.

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A synergic mechanism of corrosion inhibition was proposed, consisting of an organic film
formation based on HX and HT pure adsorbtion supplemented b y HT complexes with Fe3+ ions
occurrence, and HX electropolymerization.

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