Self-healing capability of porous polymer film with corrosion inhibitor inserted for corrosion protection

Corrosion Science 53 (2011) 4118–4123

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Self-healing capability of porous polymer film with corrosion inhibitor inserted for corrosion protection Akihiro Yabuki ⇑, Toshinori Nishisaka Faculty of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan

a r t i c l e

i n f o

Article history: Received 26 January 2011 Accepted 8 August 2011 Available online 19 August 2011 Keywords: A. Steel B. EIS B. Scratching electrode B. SEM C. Polymer coatings

a b s t r a c t Porous polymer films with varying pore sizes were prepared by changing the evaporation time of an organic solvent. A specimen was prepared consisting of porous polymer film containing corrosion inhibitor coated onto carbon steel. The specimens were scratched with a knife-edge, and the polarization resistance was monitored in a sodium chloride solution. An increase in polarization resistance was confirmed, and the films with larger-sized pores demonstrated a higher self-healing capability. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Chromate conversion coatings have historically been applied as a surface treatment for metallic materials such as steel and for aluminum and magnesium alloys, because they bestow excellent corrosion protective properties. An important requirement of these types of coatings is the ability to self-heal, so that if the coating suffers mechanical damage, and degradation of the bare metal surface by corrosive species in the environment begins, the damaged surface is automatically repaired by a chemical component of the coating. It is well understood that the repairing effect of the film in chromate conversion coatings is due to a hexavalent chromium ion, which has high reactivity. Environmental concerns, however, have necessitated the reduction and discontinuation of this process in recent years. The addition of cerium, molybdic acid, phosphoric acid and colloidal silica to coating solutions has reportedly been effective as an alternative technology for chromate conversion coatings [1–8]. Several new approaches based on the encapsulation of inhibiting compounds prior to their addition to corrosion-protection systems have been suggested [9]. A porous oxide interlayer doped with an organic corrosion inhibitor increased the active corrosion-protection ability of thin hybrid sol–gel films on an aluminum alloy substrate [10,11]. Inhibitor-containing oxide particles have also been used as nanocarriers of a corrosion inhibitor [12]. Nanocontainers that regulated the storage and release of a corrosion inhibitor were constructed with nanometer-scale precision by use of the layer⇑ Corresponding author. Tel./fax: +81 82 424 7852. E-mail address: [email protected] (A. Yabuki). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.08.022

by-layer (LbL) method [13]. Such self-healing corrosion protective coatings based on LbL-assembled nanocontainers have been developed and demonstrated [14–16]. A novel, layered, double-hydroxide, (LDH)-based nanocontainer of a corrosion inhibitor allowed the controlled release of vanadate ions from nanocrystalline LDHs [17]. Self-healing polymer coating systems based on an electrospun coaxial healing agent have been demonstrated using polysiloxanebased healing agents and an acrylate matrix [18]. The release of organic inhibitors from a hybrid sol–gel matrix can be described as a pH-dependent triggered release mechanism [15,19]. Self-healing corrosion protective coatings using polymer and metal powders [20,21], fluoro-organic compound [22], and casein as a pH-sensitive organic agent [23] have been also reported. The key to the development of self-healing coatings is the ability to control both the storage and release of the added inhibitors. Applications in gas storage and separation have been reported for porous polymer networks [24]. Microporous polymer membranes have been developed for general filtration, diagnostic kits, and other applications where a custom membrane is needed [25]. The pore sizes of the polymer membranes can be controlled for the above applications [26]. Storage materials using porous polymer film have a high capacity for corrosion inhibition because they can release much more of the inhibitor that resides in the many spaces of the porous polymer film. In the present study, the self-healing properties of porous polymer films with corrosion inhibitor inserted were investigated. The pore size of polymer films was controlled by changing the evaporation time of the organic solvent. Film was coated onto a steel plate, and then it was covered to prevent the release of the inhibitor. Changes in the polarization resistance of the specimen, scratched

A. Yabuki, T. Nishisaka / Corrosion Science 53 (2011) 4118–4123

with a knife-edge, were monitored, and the surface appearance of the specimen was observed after the corrosion test to elucidate the self-healing property of the films. The surface appearance of the scratched specimen was observed using SEM. 2. Experimental 2.1. Preparation of the specimens A cold-rolled steel plate (C < 0.15 wt.%, Mn < 0.60 wt.%, P < 0.10 wt.%, S < 0.05 wt.%, JIS G 3141, SPCC-SD, Nippon Testpanel Co., Ltd.) was used as a substrate. Samples measuring 12  12 mm were cut from a steel plate with a thickness of 0.8 mm, as received. Cellulose acetate was used to form the porous polymer film. A corrosion inhibitor was inserted into the porous film, and then it was adhered to the steel substrate. A top coating was used on the specimen to prevent the release of the corrosion inhibitor into the test solution. In order to prepare a porous polymer film of cellulose acetate, a mixed solution of cellulose acetate, CAF (Kanto Chemical Co., Inc.) of 17.9 wt.%, formamide (Sigma–Aldrich Co., Inc.) of 21.4 wt.% and acetone (Sigma–Aldrich Co., Inc.) of 60.7 wt.% was coated onto a glass plate using a doctor-blade with a gap of 150 lm. The film was let set for a certain period to allow the acetone to evaporate from the polymer, followed by immersion in deionized water to stop the formation of pores in the film. The prepared porous polymer film was then peeled off the glass plate. The size of the pores in the polymer film was controlled by changing the evaporation times of the acetone: 0.1, 0.5, 1.0, and 3.0 min. The prepared porous polymer films were labeled CAF-T0.1, CAF-T0.5, CAF-T1.0, and CAFT3.0, according to their respective evaporation times. The thickness of the film was approximately 15–30 lm. Sodium benzoate (SB) (Kanto Chemical Co., Inc.) was used as a corrosion inhibitor to be inserted into the pores of the porous polymer film. The prepared porous polymer film was immersed in an SB solution of 26 wt.% at 20 °C for 8 h, and then it was dried for 8 h in air. The prepared film was labeled as follows: CAFT0.1 + SB, CAF-T0.5 + SB, CAF-T1.0 + SB, and CAF-T3.0 + SB. The amount of corrosion inhibitor inserted in the film was calculated from the difference in the mass of the film before and after the immersion in the SB solution. A plain porous polymer film, CAFT0.1, and bare substrate, were used for a reference. The bare substrate was used after abrading with emery paper #2000. The porous polymer film with corrosion inhibitor inserted was adhered to a substrate using an adhesive agent (Vylonal MD1480, Toyobo Co., Ltd.). As a top coating, vinylester polymer (Ripoxy RT-833DA, Showa Highpolymer Co., Ltd.) was coated onto the film using a spin coater at 2000 rpm for 15 s. The curing process was initiated by the addition of hardening agents: methyl ethyl ketone peroxide at 2 wt.% as an initiator (radical source) and cobalt naphthenate at 0.5 wt.% as a catalyst. Curing was carried out at room temperature for 1 day. The thickness of the top coating was approximately 30 lm. The total thickness of the prepared coatings was approximately 45–60 lm. 2.2. Evaluation of self-healing properties Specimens with a top coating of adhered porous polymer film were scratched with the knife-edge of a scratch tester (IMC1552, Imoto Machinery Co., Ltd.). The scratch load, which was intended to expose the substrate, was 500 g. The length of the scratch was approximately 7 mm. The specimens were immersed in 0.5 wt.% sodium chloride solution that was air-saturated using an air pump and maintained at 35 °C. The pH of the solution was 6.0. The self-healing properties of the specimens were evaluated

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based on the corrosion behavior at the scratched portion of the specimens. The impedances of the specimens in the corrosive solution were measured for 8 h using a platinum counter electrode and a Ag/AgCl reference electrode connected to a potensiostat (HABF-5001, Hokuto Denko Co.), a frequency response analyzer (5010A, NF Co.), and a personal computer. Sine wave voltages (10 mV rms) at frequencies ranging from 20 kHz to 50 mHz were superimposed on a given electrode potential. A computer software program was used to control the measurements through a General Purpose Interface Bus (GPIB). The plot of the measured impedances did not show fully semicircular, so that the polarization resistance was calculated by fitting a semicircular plot to the measured data using EIS measurement software, ZPlot (Solartron Co., Ltd.). The measured polarization resistance was normalized to the surface area of the scratch alone, which was calculated from the depth and length of the scratched portion of each specimen by using optical micrographs (VH-8000, Keyence Co.). Thus, the data presented below have also been normalized. After the corrosion test, the surface appearance of the scratched specimens was observed using a Scanning Electron Microscope (SEM, S-3000N). In addition, the scratched portion of each of the specimens was analyzed by energy dispersive X-ray fluorescence spectrometry (EDX). 2.3. Analyzing porous polymer film The prepared porous polymer films were cut with a microtome, and then the cross-sections of the films were observed. The size and structure of the pores in the prepared porous polymer films of cellulose acetate was evaluated using Field Emission–Scanning Electron Microscopy (FE–SEM, JSM-6340F). The average diameter of the pores in the film was calculated by measuring the diameter of more than 50 pores by SEM observation. In order to measure the penetration distance of the solution from the scratched portion of the film, a dye solution of approximately 0.03 mL was dropped onto the center of the scratched portion of the porous polymer film. After 30 min, the distance from the scratch to the portion where the color had changed was determined to be the penetration distance of the solution in the porous film. 3. Results and discussion 3.1. Film preparation The surface appearance of the porous polymer film prepared at various evaporation durations is shown in Fig. 1. The CAF-T0.1 film was white and the CAF-T3.0 film was transparent. The transparency of the film increased with evaporation time. The SEM images of a cross-section of porous polymer films prepared at various evaporation times, 0.1 min (CAF-T0.1), 0.5 min (CAF-T0.5), and 3.0 min (CAF-T3.0), are shown in Fig. 2. Relatively homogeneous, 3-D, reticulation structures were observed on each film. These pores seemed to be connected to one another. The pores in the CAF-T0.1 film consisted of two types of large pores of approximately 1 lm, and small pores of approximately 0.1 lm. Although the small pores were opened on the walls of the large pores, large pores without small pores were also observed throughout. The size of the large pores was reported as the pore size, since the size must be related to the release of the corrosion inhibitor. The mean diameters of the pores in the CAF-T0.1, CAF-T0.5, CAF-T1.0, and CAFT3.0 films were 0.83, 0.54, 0.20, and 0.03 lm, respectively. The pore sizes decreased with increasing evaporation time [26,27]. The differences in color, as shown in Fig. 1, were dependant on

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Fig. 1. Surface appearance of porous polymer film prepared by varying the evaporation time of acetone: (a) 0.1 min (CAF-T0.1), (b) 0.5 min (CAF-T0.5), (c) 1.0 min (CAF-T1.0), (d) 2.0 min (CAF-T2.0) and (e) 3.0 min (CAF-T3.0).

(a)

(b)

2 µm

(c)

2 µm

2 µm

Fig. 2. Cross-sectional SEM images of porous polymer films prepared by varying the evaporation times of acetone: (a) 0.1 min (CAF-T0.1), (b) 0.5 min (CAF-T0.5) and (c) 3.0 min (CAF-T3.0).

the scattering of light due to the pore size of the film [28]. The thicknesses of the CAF-T0.1, CAF-T0.5, CAF-T1.0, and CAF-T3.0 films were 30, 25, 20, and 15 lm, respectively. Thus, the thickness of the porous film decreased with evaporation time. This was due to the size of the pores generated in the film (Fig. 2); that is, larger pores had a lower apparent density, which resulted in thicker film. A cross-section of the porous polymer film with corrosion inhibitor inserted, CAF-T0.1 + SB, is shown in Fig. 3. Compared to the porous polymer film without corrosion inhibitor, CAF-T0.1 (Fig. 2a), the insertion of the corrosion inhibitor into pores is recognizable, although it is slight. Measuring the mass gain after the insertion of the corrosion inhibitor, it was confirmed that the CAF-T0.1 + SB film contained 3.21 mg/cm2 of corrosion inhibitor. CAF-T0.5 + SB, CAF-T1.0 + SB, and CAF-T3.0 + SB films contained 1.05, 0.75, and 0.67 mg/cm2 of corrosion inhibitor, respectively.

3.2. Corrosion inhibition properties of a scratched specimen Fig. 4 shows the Nyquist plots of the typical electrochemical impedance spectroscopy of scratched specimens with porous polymer film containing corrosion inhibitor adhered to the surface

2 µm Fig. 3. Cross-sectional SEM images of porous polymer film containing sodium benzoate as a corrosion inhibitor. Evaporation time of the acetone in the film was 0.1 min (CAF-T0.1 + SB).

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cm2)

104

200

Polarization resistance (

2)

300

- ZIm (

CAF-T0.5 + SB

100

1000 103

CAF-T3.0 + SB

0 0

100

200

300

400

500

600

2)

ZRe (

CAF-T0.1 + SB

Fig. 4. Nyquist plots of scratched specimens coated with porous polymer film containing corrosion inhibitor after an 8 h immersion.

(CAF-T0.5 + SB, CAF-T3.0 + SB). Since semicircular plots could not be obtained from measurements up to 0.05 Hz, polarization resistance was obtained by adjustment to the dotted curve in the figure using EIS measurement software, ZPlot. The same behavior was observed on the bare substrate. Deviation of this type, often referred to as the frequency dispersion, was attributed to the roughness and inhomogeneities of the solid surface [29–31]. Therefore, a constant phase element (CPE), instead of a capacitive element, was used to get a more accurate fit of the experimental data sets using generally more complicated equivalent circuits. Usually, the CPE behavior could be treated as a ‘‘x space fractality,’’ i.e., as a manifestation of the self-similarity in the frequency domain [32]. The CPE impedance is given by the following formula [32,33].

Z CPE ¼ A1 ðixÞn

ð1Þ

where A is the CPE constant, x is the angular frequency (in rad s1), i2 = 1 is the imaginary number and n is a CPE exponent that can be used as a gauge of the heterogeneity, or roughness, of the surface [34]. Depending on the value of n, CPE can represent resistance (n = 0, A = R), capacitance (n = 1, A = C), inductance (n = 1, A = L) or Warburg impedance (n = 0.5, A = W) [35]. The transfer function is thus represented by an equivalent circuit, having only one time constant (Fig. 5). Parallel to the double layer capacitance (simulated by a CPE) is the charge transfer resistance (Rct), and Rs is the electrolyte resistance. Excellent fit was obtained with this model for all experimental data. The Rct was used as the polarization resistance of each specimen. Fig. 6 shows the polarization resistance of the scratched specimens coated with various porous polymer films with or without corrosion inhibitor and bare substrate. Because the polarization resistance of the scratched specimens CAF-T0.1 + SB, CAFT0.5 + SB, CAF-T1.0 + SB, and CAF-T3.0 + SB after a 5 min immersion could not be measured owing to potential instability, the change in polarization resistance of scratched specimens at the initial stage is shown as a dotted curve, and the polarization resistance of the bare substrate exposed to corrosive solution should be similar. The resistance of the specimen coated with plain porous polymer film, CAF-T0.1, largely decreased, becoming almost constant after 3 h, although it should be similar to the bare substrate. This was due to the enlargement of the exposed surface

ZCPE Rs

Rct Fig. 5. Equivalent circuit used to represent the impedance results.

CAF-T0.5 + SB

CAF-T1.0 + SB CAF-T3.0 + SB

100 102

Substrate

CAF-T0.1

10

0

1

2

3

4

5

6

7

8

9

Immersion time (h) Fig. 6. Polarization resistance of scratched specimens coated with porous polymer films containing corrosion inhibitor (CAF-T0.1 + SB, CAF-T0.5 + SB, CAF-T1.0 + SB, CAF-T3.0 + SB), plain porous polymer film (CAF-T0.1) and bare substrate.

by separation between the film and the substrate, as shown later in Fig. 7c. However, the polarization resistance of the scratched specimens coated with porous polymer films containing corrosion inhibitor, CAF-T0.1 + SB, CAF-T0.5 + SB, CAF-T1.0 + SB, and CAFT3.0 + SB, increased with time. A shorter evaporation time equated to a higher polarization resistance. The resistance of the scratched specimens CAF-T0.5 + SB, CAF-T1.0 + SB, and CAF-T3.0 + SB was increased at the initial stage and became almost constant after a 2 h immersion. The resistance of the scratched specimen CAF-T0.1 + SB was drastically increased at 1 h, then decreased at 2 h, but gradually increased again, and kept increasing after 8 h. The increase was caused by the release of corrosion inhibitor from the pores of the film. In particular, the high polarization resistance of the scratched specimen CAF-T0.1 + SB at the initial stage was due to the dissolution of a large amount of corrosion inhibitor. Thus, the CAF-T0.1 + SB specimen demonstrated two types of release properties: a large release of corrosion inhibitor at the initial stage and a continuous release during the second stage. Fig. 7a,b shows the SEM images of the scratched portions of a specimen, CAF-T0.1 + SB, coated with porous polymer film containing corrosion inhibitor before and after testing. In order to observe the surface of the scratched portion, the top coating was removed from the substrate by means of peeling off from the notch applied in the interface between the top coating and the porous film using a knife-edge. Fig. 7c shows the SEM image of the substrate of a scratched specimen coated with plain film, CAF-T0.1, after testing. The image shows only the substrate of the scratched specimen, since the porous polymer film and top coatings of the specimen were easily removed because of poor adhesion, so the surface of the substrate could be exposed. The scratched portion of the CAF-T0.1 specimen shows the resultant accumulation of corrosion products near the scratched portion. However, relatively thick film and rod-like corrosion products were observed at the scratched portion of the CAFT0.1 + SB specimen. EDX analyses of the scratched portions of the CAF-T0.1 + SB and CAF-T0.1 specimens are shown in Figs. 8ab. Fe was detected on the scratched portion of the CAF-T0.1 and the CAF-T0.1 + SB specimens. However, C and O was also detected on the scratched portion of the CAF-T0.1 + SB specimen. This was attributed to the insertion of corrosion inhibitor into the pores of the porous polymer film. The corrosion product formed on the scratched portion of the CAF-T0.1 + SB specimen was expected to be a protective deposition of Fe(C6H5COO)x(OH)3x [36–38]. Thus, the increase in the polarization resistance of the scratched specimen was due to the release of the corrosion inhibitor, and the increase in the polarization resistance of the specimen was related to the evaporation time, which equated to the amount of corrosion inhibitor in

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(c)

(b)

(a)

30 µm

30 µm

30 µm

Fig. 7. SEM images of the scratched areas of specimens coated with porous polymer films containing corrosion inhibitor (CAF-T0.1 + SB) before the corrosion test (a), after an 8 h immersion (b), and plain porous polymer film (CAF-T0.1) after an 8 h immersion (c).

(b)

(a) Fe

Intensity (a.u.)

C Intensity (a.u.)

Fe

O

Fe

Fe 0

2

4

6

8

0

2

Energy (keV)

4

6

8

Energy (keV)

Fig. 8. EDX analysis of the scratched portions of specimens coated with porous polymer films containing corrosion inhibitor (CAF-T0.1 + SB) (a), and plain porous polymer film (CAF-T0.1) (b) after an 8 h immersion.

the film, as described in Section 3.1. The ratio of the amount of corrosion inhibitor in the CAF-T0.1 + SB and CAF-T0.5 + SB films was approximately 3, and the ratio of the polarization resistance of these films was more than 5. Taking into account the ratio and the characteristic behavior of the polarization resistance of the scratched CAFT0.1 + SB specimen (Fig. 6), the penetration of the solution through the pores into the film should be investigated.

(a)

(b)

3.3. Solution penetration into porous polymer film The behavior of a solution as it penetrates porous polymer film was investigated by measuring the penetration distance of dye on the scratched portion. The surface appearance of scratched films, 30 min after the application of a dye solution, were prepared with evaporation times of 0.1 min (CAF-T0.1) and 3.0 min (CAF-T3.0), and are shown in Fig. 9. In all portions of the scratched line in the uppermost example, CAF-T0.1, the penetration of the dye solution was almost uniform, although the color was diluted. The penetration distance of the dye in the CAF-T0.1 film was 1.9 mm. By contrast, dye barely penetrated the scratch in the CAF-T3.0 film. Fig. 10 shows the penetration distance of the dye solution in the scratched portion of each sample of film prepared using various evaporation times. The penetration distance was drastically shortened as the acetone evaporation time was increased to 3.0 min. The relationship between the pore sizes of porous polymer films and the penetration distance of the dye solution into the scratched portions is shown in Fig. 11. The penetration distance increased as the pore size of the film increased. The parabolic curve in the figure was theoretically calculated using the following equation. Assuming the multi pore was a micro tube, it could be applied to the following Lucas–Washburn Eq. (2), which is a theoretical equation for capillary penetration [39–41].

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dccosh l¼ t 4g

ð2Þ

2 mm

2 mm

Fig. 9. Surface appearance of scratched porous polymer film prepared by varying the evaporation time of acetone by 0.1 min CAF-T0.1 (a) and 3.0 min CAF-T3.0 (b) after penetration of a dye drop for 30 min.

where l is the penetration distance, d is the capillary diameter, c is the surface tension, h is the contact angle, g is the viscosity of the solution, and t is time. This assumes that constants c, h, g, t, and l can be expressed as a function of d. Consequently, the penetration distance, l, can be shown by the following Eq. (3) by fitting the measured penetration distance.

pffiffiffi l ¼ 4:36 d

ð3Þ

The penetration distance obtained from the equation agreed well with the experimental results. Thus, the porous polymer film with large pores, CAF-T0.1, could easily release corrosion inhibitor from the pores of the film, resulting in a high self-healing

A. Yabuki, T. Nishisaka / Corrosion Science 53 (2011) 4118–4123

chloride solution. The effect that pore size had on the insertion of corrosion inhibitor was evaluated based on the corrosion behavior of the scratched specimens. Porous polymer film of cellulose acetate with varying pore sizes could be prepared by changing the evaporation duration of the acetone, and sodium benzoate could be inserted into the pores of the prepared film as a corrosion inhibitor. The porous polymer film containing corrosion inhibitor coated onto carbon steels showed self-healing capability. The film with larger pores had higher self-healing capability, which was related to the penetration of the environmental solution into the pores of the film and to the diffusion release of the corrosion inhibitor from the film.

Penetration distance (mm)

2.0

1.6

1.2

0.8

0.4

0

4123

Acknowledgements 0

1

2

3

4

Evaporation time(min) Fig. 10. Penetration distances of dye solution from the scratched portion of each film prepared by various evaporation times.

This research was supported in part by a Grant from the Ministry of Education, Culture, Sports, Science and Technology, Grant-inAid for Scientific Research (C) (No. 21560747). References

Penetration distance (mm)

2.5

2.0

1.5

1.0

0.5

0

0

0.2

0.4

0.6

0.8

1.0

Pore diameter (µm) Fig. 11. Relationship between the pore sizes of porous polymer films and the penetration distance of dye solution from a scratched portion.

capability. Consequently, the polarization resistance of porous polymer film was due to the amount of corrosion inhibitor stored in the film and to the releasing behavior of corrosion inhibitor attributed to the penetration of solution into the pores. The polarization resistance measurements of the scratched specimens CAFT0.5 + SB, CAF-T1.0 + SB, and CAF-T3.0 + SB, as shown in Fig. 6, were dominated by the amount of corrosion inhibitor stored in the films, which resulted in corrosion healing. With respect to scratched specimen CAF-T0.1 + SB, a large and rapid release of corrosion inhibitor from the film at the initial stage generated a highbarrier healing film. The release was due to the large amount of corrosion inhibitor stored in the film and to the larger pore sizes of the film. The gradual increase in polarization resistance may be a characteristic structure of the film – Many pore openings on the walls of large pores, as shown in Fig. 2a. This was brought about by the slow release of corrosion inhibitor from the film. The resistance was gradually increased, but it was not enough to produce high resistance, so the slow and controlled release of inhibitor was important to improve the self-healing, corrosion inhibition qualities of the coatings.

4. Conclusions Corrosion tests for carbon steel coated with porous polymer film containing corrosion inhibitor were carried out in a sodium

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