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Applied Surface Science 305 (2014) 492–497

Contents lists available at ScienceDirect

Applied Surface Science

journa l h om epa ge: www.elsev ier .com/ locate /apsusc

he use of trivalent chromium bath to obtain a solar selective blackhromium coating

. Surviliene ∗, A. Cesuniene, R. Juskenas, A. Selskiene, D. Bucinskiene, P. Kalinauskas,. Juskevicius, I. Jureviciute

enter for Physical Science and Technology, A.Gostauto 9, 01108 Vilnius, Lithuania

r t i c l e i n f o

rticle history:eceived 9 January 2014eceived in revised form 18 March 2014ccepted 19 March 2014vailable online 28 March 2014

eywords:lack chromiumlectrodeposition

a b s t r a c t

Black chromium coatings were electrodeposited from a trivalent chromium bath using a ZnO additiveas a second main component. Black chromium was electrodeposited on steel and copper plates andsubstrates plated with bright nickel prior to black chromium electrodeposition. The black chromiumcoatings were characterized by XRD and SEM. The XRD data suggest that the phase structure of blackchromium may be defined as a zinc solid solution in chromium or a chromium solid solution in zincdepending on the chromium/zinc ratio in the deposit. The role of substrate finish was evaluated throughthe corrosion resistance and reflectance of black chromium. According to corrosion tests the samplesplated with bright nickel prior to black chromium deposition have shown the highest corrosion resistance.

hase structureorphology

orrosion

The electrodeposited black chromium possesses good optical properties for the absorption of solar energy.The absorption coefficient of black chromium was found to be over 0.99 for the samples obtained withoutthe Ni undercoat and below 0.99 for those obtained with the use of Ni undercoat. However, the use ofnickel undercoat before black chromium plating is recommended because it remarkably improves thecorrosion resistance of samples.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Solar selective coatings absorb solar radiation and convert itnto thermal energy. Black deposits containing chromium are useds commercial finishes under the name “black chromium”. Blackhromium is widely used in solar collectors due to its high absorp-ance, good stability and high thermal resistance. Traditionally,he solar selective coatings are deposited on metallic substratesNi, Cu, steel) from a hexavalent chromium bath using pulse cur-ent electrolysis to improve the optical properties of the coatings1]. The data on electrodeposition of selective black chromiumoatings, their microstructures, optical properties and degradations absorbers have been published in several works [2–9]. As foromposition of black chromium, there is no general agreementmong authors. One view holds that as-deposited black chromiumonsists of chromium hydroxide and metallic Cr, both of which con-

ert into oxides during thermal annealing at temperatures about00 ◦C [7,8], whereas, no evidence for metallic chromium was found

n other works [10,11]. The earlier sources [12,13], which were

∗ Corresponding author. Tel.: +370 67617825.E-mail address: sveta@ktl.mii.lt (S. Surviliene).

ttp://dx.doi.org/10.1016/j.apsusc.2014.03.122169-4332/© 2014 Elsevier B.V. All rights reserved.

devoted to the research in the field of black coatings electrode-position, suppose that metal was incorporated into the oxide filmas fine separate grains that formed light absorption centres. Inthe case when the metal content in the deposit is low, the opti-cal constants are such as to give low reflectance at a sufficientlylarge thickness. The appearance of black deposits arises from adecrease in reflectance. The authors suggest that appreciably thicklayers of a dielectric containing metal in the form of dispersed par-ticles smaller than the wave-length of light may look black eventhough the metal content is not too high [13]. It was found [6]that the solar absorptance of black chromium is dependent on thevolume ratio of Cr to Cr + Cr2O3 in the deposit and the coatingthickness. The substrate influences the solar absorption as well.The best suited thickness of black chromium was found to beabout 1.0 �m for the copper substrate and about 0.7 �m for thenickel one [14]. The substrate morphology influences the struc-ture and thermal stability of black chromium [10,15]. The blackchromium coatings are stable at temperatures under 350 ◦C; how-ever, a stainless steel/black chromium system is stable up to 450 ◦C

[16]. Various substrate treatments, which are used prior to plat-ing, affect the structure, morphology, chemical composition andselective characteristics of black chromium [17]. It was found thatelectropolishing of the substrate allows obtaining coatings with

rface Science 305 (2014) 492–497 493

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Table 1The base composition of the black Cr(III) bath.

Component g l−1

CrCl3·6H2O 250NH2CH2COOH 18.75H3BO3 30.0NaCl 60.0NaNO3 3.0ZnO 5.0

S. Surviliene et al. / Applied Su

ore metallic chromium, whereas, mechanical polishing resultsn a coating with more oxides of chromium. This suggests that these of electropolishing after mechanical polishing of the substrateakes it possible to improve selectivity of black chromium.It is known [18], that monodispersed spherical particles

f �-Cr2O3, which are synthesized from a diluted solution ofCr(SO4)2·12H2O using the aqueous chemical growth (ACG) tech-ique, may be used for solar absorber applications. The structural,orphological and chemical characteristics of the black Cr-

r2O3 coatings deposited on the tantalum substrate using thehysical–chemical ACG method and modified by annealing to Cr/�-r2O3 were discussed in [19]. The black Cr-Cr2O3 coating, whichonsists of metallic Cr, Cr2O3, hydrated oxides and hydroxides ofr, are transformed during annealing into the polycrystalline Cr/�-r2O3 of high absorptivity. It should be noted that the hydratedr2O3 nH2O is amorphous, whereas, �-Cr2O3 is a stable corun-um phase. The results of electron spin resonance (ESR) studyhow that dehydration/rehydration of Cr2O3 nH2O is possible dur-ng the heating-cooling cycles [20]. According to the publishedata [21], the Cr/�-Cr2O3 coatings deposited on Cu or stainlessteel substrates are good candidates for solar absorbers. The filmicrostructure, surface composition and morphology affect the

ptical properties of the black coating.The solar selective black coatings may be electrodeposited from

rivalent chromium baths [22]. It is known [23], that ions of metalsf high density such as Ni(II), Co(II) or Fe(II) were used as a secondain component to improve the efficiency of Cr(III) bath. However,

he information on the use of zinc as a second component in a triva-ent black chromium bath is lacking. Taking into account that zincs an anodic metal as compared to nickel, iron or other cathodic

etals, it was expected that co-deposition of zinc with chromiumill improve the protection of the substrate. As impurities of hydro-

en, oxygen and other elements influence the properties of blackhromium [24–26]; our interest was to clarify the influence of zincn the electrodeposition and properties of the black chromium.

The aim of this research was to obtain solar selective blackhromium using a trivalent chromium bath and to study the prop-rties of the deposits in order to optimize the bath composition andlating parameters.

. Experimental

Black chromium coatings were electrodeposited on steel con-aining 99.4% of iron (Steel-3), copper and nickel undercoat10 �m). Before electrodeposition the substrates were mechani-ally polished with further electropolishing. The Cu substrate wasechanically degreased by grinding with 600-grit SiC paper and

hen electropolished in a commercial degreasing solution at 20 ◦Cnder a current density of 0.05 A cm−2 for 3 min, using the substrates an anode. After that, samples were washed with distilled waternd immediately placed in the plating bath. The steel substrate waslso treated mechanically and electropolished in the commercialolution containing 27.5 g l−1 sodium carbonate, 17.5 g l−1 sodiumripolyphosphate, 2.5 g l−1 sodium silicate, 2.5 g l−1 synthanol DS-0 (CAS No 12679-83-3) and 50 g l−1 sodium hydrate at a currentensity of 0.05 A cm−2 for 5 min at 50 ◦C, using the substrate as annode and a Ti plate as a cathode. After that samples were washedith distilled water, soaked in a hot 20 vol.% HCl solution, washed

gain with distilled water and then immediately placed in the plat-ng bath. A bath of a volume of 1 l with two vertical Pt anodes and

cathode between them was maintained at a constant tempera-

ure. The cathode (substrate) and anode were disposed within theath at a distance of 25 mm. The current density during platingas 0.2–0.4 A cm−2 and the temperature was 20–35 ◦C. A currentensity of 0.4 A cm−2 and a temperature of 20 ◦C were selected as a

pH = 1.2Temperature 18–20 ◦Cic = 0.2–0.4 A cm−2

compromise between efficiency and deposit quality. The electrode-position rate of black chromium was determined gravimetrically.

Initially, the chemical composition of the plating bath andworking conditions were designed taking into account the colour,homogeneity and adhesion of the coating. Inexpensive materi-als such as chromium chloride, zinc oxide, boric acid, sodiumchloride and sodium nitrate were used to prepare the platingbath. All solutions were prepared from analytical grade chemicals(manufacturer AppliChem gmbh) without further purification usingdistilled water. To improve the optical properties of a black coat-ing the additive materials were selected experimentally. Glycine(NH2CH2COOH) was found to be the proper complexing agentfor Cr(III) and ZnO was used to improve the quality of the blackchromium coatings. The final chemical composition of plating bathis presented in Table 1.

The quality of the deposits was assessed visually and by using anoptical microscope. A deposit of good appearance and good adhe-sion was defined as a high quality deposit for further research.

The black chromium coatings were examined for morphologyand chemical composition using scanning electron microscopy(SEM) and energy disperse spectroscopy (EDS), respectively. Inorder to choose a typical picture for each coating, three–four loca-tions (in the central zone) were taken. The phase composition ofblack chromium was examined by using X-ray diffraction (XRD).The XRD 2� scans of the coatings were measured using an X-raydiffractometer SmartLab (Rigaku) equipped with a 9 kW rotatingCu anode X-ray tube. The grazing incidence (GIXRD) method wasused in the 2� range 30–75◦. The angle between the parallel beamof X-rays and the specimen surface (� angle) was adjusted to0.5◦.

Reflectance measurements were performed by using a UVVIS spectrophotometer SPECORD® 2500 with variable anglereflectance attachment from analytic Jena (Germany). The Wave-length range in which this Spectrophotometer may be employedis 190–1100 nm. The angle of reflection is variable in the range of11–60◦. Based on our practice, reflection measurements under anincidence of 12◦ are the most stable compared to those under 8–11◦.Therefore, in our experiments the angle of incident light is 12◦.

The corrosion resistance of the coatings was tested according toLST EN ISO 9227:2012 standard [27], using a salt-spray chamber“Q-Fog CCT 600-CE”. The samples were submitted to the salt spraytest and evaluated according to LST EN ISO 10289:1999 by the timeuntil the emergence of the first corrosion points. The temperaturein the testing chamber was maintained at 35 ± 2 ◦C.

3. Results and discussion

It is known that trivalent chromium has a tendency to formvarious complexes in aqueous solutions [28]. In order to prevent

the formation of undesirable complexes, it is essential to use aweak complexing agent. The optimal electrolyte composition andoperating conditions of black chromium electrodeposition weredetermined experimentally. According to experiments, formic acid

494 S. Surviliene et al. / Applied Surface Science 305 (2014) 492–497

Fig. 1. (a) XRD pattern for black chromium deposited at 0.4 A cm−2 from the Cr(III) bath containing 5 g l−1 ZnO. Positions of pure chromium XRD peaks are indicated accordingt at 0.X ern forZ 2 data

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o PDF-2 database card No 06-0694. (b) XRD pattern for black chromium depositedRD peaks are indicated according to PDF-2 database card No 06-0694. (c) XRD pattnO at 0.2 A cm−2. Positions of pure zinc XRD peaks are indicated according to PDF-

nd oxalic acid were found not suitable as complexing agents forlack chromium electrodeposition. The formate-urea bath was alsoejected because of greyish colour of the coatings obtained. Aminocetic acid (NH2CH2COOH) was found to be a proper complexinggent during formation of black chromium. In this case 18 g l−1 is ainimum concentration of NH2CH2COOH which should be added

o the Cr(III) bath to obtain the deposit of black colour. The presencef NaNO3 as an oxidizing agent is also important during electrode-osition of black chromium. The results of experiments testify thathe coatings are of deep black colour at a concentration of about

g l−1 NaNO3. To find the proper concentration of Cr3+ ion andts joint anion, chromium sulphate and chromium chloride werenvestigated using the concentration in the range of 100–300 g l−1.s a result, CrCl3·6H2O was selected with an optimum concen-

ration of 250 g l−1. Substitution of chloride for sulphate made itmpossible to obtain the black deposit. The working conditions oflating were investigated and an additive which could improve theptical properties of black chromium was selected experimentally.t was found that using ZnO as a second main component the coat-ng became much darker. According to experiments a noticeablemprovement in the black colour of samples was observed afterddition of 2–5 g l−1 ZnO. Any further increase in ZnO concentrationid not show improvement in colour of the coating. A black coat-

ng of good appearance without grey or rainbow tones and withouturning on the sample edges may be deposited from the electrolyteresented in Table 1.

It was found experimentally that the percentage of Zn in blackhromium increases with ZnO concentration in Cr(III) bath andecreases with an increase in the current density of electrolysis. Tolucidate the relationship between the composition and structure

f black chromium, XRD tests were performed for samples with dif-erent amounts of main components (chromium and zinc). Fig. 1ahows a typical XRD pattern for black chromium in which the per-entage of chromium is much higher than that of zinc (Cr/Zn = 1.8).

4 A cm−2 from the Cr(III) bath containing 10 g l−1 ZnO. Positions of pure chromium black chromium deposited on the steel substrate from the Cr(III) bath with 10 g l−1

base card No 04-0831.

Two broad peaks are seen on this pattern, whose positions do notmatch the positions of pure chromium. As the lattice parameter ofa body-centred cubic structure a is 0.28839 nm for pure chromium,it is reasonable to believe that the value a = 0.2997 ± 0.0003 nmobtained for black chromium corresponds to the CrZn phase whichis defined as a solid solution. It is evident that these peaks could beattributed to the zinc solid solution in chromium.

The XRD pattern for black chromium with equal amounts of bothchromium and zinc (Cr/Zn = 1) is presented in Fig. 1b. There are twobroad peaks which are also associated with the zinc solid solutionin chromium with the lattice parameter a = 0.2995 ± 0.0003 nm. Inthis case we deal with the zinc solid solution in chromium whichis similar to the above-mentioned one.

Fig. 1c shows a XRD pattern for black chromium witha higher atomic percentage of zinc as compared to that ofchromium (Cr/Zn = 0.7). This pattern shows peaks which may beattributed to the hexagonal structure peculiar to zinc. Taking intoaccount the values of the lattice parameters of a hexagonal closepacked structure of zinc (a = 0.2665 nm, c = 0.4947 nm) and thoseobtained for the given black chromium (a = 0.2690 ± 0.0003 nm,c = 0.4825 ± 0.0003 nm), it is believed that this is a chromium solidsolution in zinc. For comparison, the Cr/�-Cr2O3 coating, whichis recommended for solar absorbers applications, is defined as �-Cr2O3 particles containing amorphous chromium. The crystalline�-Cr2O3 particles, which are obtained by the AGC method fol-lowed by heating of hydrated Cr2O3 at 500 ◦C, show a rhombohedralstructure with the parameters a = 0.4961 and c = 1.3599 nm [18].According to the published data, chromium oxide coating, annealedat 500 ◦C, was completely transformed into the polycrystalline�-Cr2O3 phase, whereas, our attempts to detect the crystalline �-

Cr2O3 phase in the electrodeposited black chromium coatings withvarious amounts of zinc have not been successful, both before andafter heating at 500 ◦C. Despite this, the black chromium coatingsstudied exhibit good optical properties.

S. Surviliene et al. / Applied Surface Science 305 (2014) 492–497 495

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ig. 2. Surface morphology of the black chromium coatings deposited from the Cr(It (b,d,f). Black chromium was deposited at 0.4 A cm−2 (a–d) and 0.2 A cm−2 (e, f).

According to the phase diagram of the CrZn system [29],nly two intermetallic compounds CrZn13 and CrZn17 have beennown until the present time. It is stated [29] that the phaserZn17 (5–6 at.% Cr), which may be prepared at 419 ◦C, has aexagonal structure with the lattice parameters a = 1.2916 nm and

= 3.0562 nm, whereas the phase CrZn13 (7 at.% Cr), crystallizing at temperature of about 550–850 ◦C in the monoclinic form with ahombic cross section, has a structure similar to that of FeZn13 (PDFo 00-045-1185) with a = ∼1.34, b = ∼0.76 and the lattice parameterarallel to the needle axis c = 0.524 nm. The XRD patterns for blackhromium coatings under investigation contain the peaks match-ng to cubic (bcc) or hexagonal structures (with different latticearameters), which could not be attributed to the above mentioned

ntermetallic compounds. No evidence was found in literature forhe existence of the CrZn phase composition with other atomic per-entages of Cr and Zn. It is not feasible to prepare the CrZn alloyith a high percentage of Cr by the metallurgical method, because

f large differences in the melting temperatures of Cr (1903 ◦C) and

n (419 ◦C). However, as electrochemical co-deposition of theseetals proceeds even at room temperature, there is a good prob-

bility that the CrZn coatings with a high percentage of Cr may beormed through electrolysis. Thus, it may be concluded that, the

(a,b) and Cr(III) + ZnO bath (c–f) on steel with the Ni undercoat (a,c,e) and without

zinc solid solution in chromium or the chromium solid solution inzinc may be formed during electrolysis in the Cr(III) bath contain-ing ZnO. This interpretation is supported by preliminary resultsobtained by SEM-FIB. A cross section of Cr–Zn layer showed thatthe whole layer (about 4 �m in thickness) is composed of Zn, Crand oxygen, however on the surface of the layer a greater quantityof oxygen is present.

Fig. 2 shows the morphologies of black chromium coatingsobtained on steel plated with a bright nickel coating (a,c,e) andjust on steel (b,d,f) substrates. The influence of such factors as sub-strate finish and addition of ZnO to the Cr(III) bath is evident. Blackchromium coatings have a microcracked structure with numerouslamellar crystals. It is seen that the samples obtained on steel platedwith nickel prior to black chromium have finer grained and lessrough structures compared to those of black chromium depositedjust on the steel substrate. It is evident, that the initial roughnessof steel surface is transformed into a less rough one after brightnickel deposition on steel. The effect of the current density on the

morphology of coating is perfectly seen when samples c, d are com-pared with samples e, f, respectively. It is seen that a decreasein current density leads to a change in cracks which becomelarger.

496 S. Surviliene et al. / Applied Surface Science 305 (2014) 492–497

Table 2The results of salt-spray test obtained for black chromium on: steel (1, 2, 7), steel with Ni undercoat (3, 4, 8) and Cu (5, 6) substrates. Black chromium coatings were depositedat ic: 0.4 A cm−1 (1–5) and 0.2 A cm−2 (6–8) for 3 min, a temperature 18 ◦C.

Sample no. Bath composition Substrate Current density A dm−2 The time until corrosion h

1 Cr(III) steel 40 52 Cr(III) + ZnO steel 40 153 Cr(III) steel plated by Ni 40 124 Cr(III) + ZnO steel plated by Ni 40 3705 Cr(III) + ZnO Cu 40 1706 Cr(III) + ZnO Cu 20 2007 Cr(III) + ZnO Steel 20 10

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Results of corrosion tests are presented in Table 2. The databtained show that addition of ZnO to the Cr(III) bath made itossible to markedly improve the corrosion resistance of blackhromium. Thus, addition of 5 g l−1 ZnO to the Cr(III) bath prolongshe time until the emergence of first corrosion points about three-old if black chromium is deposited just on steel (compare samples

and 2), whereas, using steel plated by nickel as a substrate, theime may be prolonged about thirty-fold (compare samples 3 and). The role of substrate finish becomes obvious when the resultsbtained for various substrates under the same electrolysis con-itions (bath composition, plating conditions) are compared. Theesults show that the first corrosion points appear on the surfacesf steel/black chromium (sample 1) and steel/nickel coating/blackhromium (sample 3) after 5 and 12 h, respectively. The time untilhe emergence of first corrosion points may be prolonged up to70 h when the Cr(III) + ZnO bath and steel plated with Ni aresed for black chromium plating (sample 4). The nickel under-oat slows down the penetration of aggressive substances to theteel surface due to both the barrier effect and peculiarity of theicrostructure of black chromium which is formed on the surface

f nickel coating. The results of corrosion tests testify that blackhromium deposited at a current density of 0.4 A cm−2 (sample 4)rovides a better corrosion protection compared to that of blackhromium plated at 0.2 A cm−2 (sample 8). Samples 5 and 6 showhat copper may be successfully employed as a substrate to plate

lack chromium coatings, however, the highest corrosion resis-ance is achieved using steel plated with Ni prior to black chromiumsample 4).

ig. 3. Spectral reflectance measured for black chromium deposited on the steelubstrate from the Cr(III) bath (1) and Cr(III) + ZnO bath (2–4). Thickness of the coat-ng: 3.2 �m (1, 2); 2.6 �m (3); 4.2 �m (4). The current density 0.4 A cm−2 (1, 2) and.2 A cm−2 (3, 4).

20 160

The spectral reflectance of black chromium deposited on thesteel substrate is presented in Fig. 3. The spectra encompass a wave-length range 350–1100 nm, which is compatible with the range ofsolar radiation spectrum. Since the light diffuse scattering com-ponent is neglected for the bright and semi-bright black coatings,it is evident that losses related to the diffuse scattering compo-nent have little effect on the values of spectral reflectance for thesamples under investigation. It is seen that the variations in thespectral reflectance of samples versus the wavelength are distinct.In the visible range the reflectivity of the samples was found tobe very low: about 0.7–0.8% for black chromium from the Cr(III)bath and 0.25–0.55% for the coatings from the Cr(III) + ZnO bath.Such results may be conditioned by the structural and chemicalparameters of samples. Some increase in reflectance is observed inthe 700–1100 nm wavelength range, which means that the absorp-tance of the black coating decreases in this range. The location ofthe curves in Fig. 3 suggests that the absorption increases withthe thickness of black chromium layer. According to literature data[24], the absorption coefficient of the coatings may be determinedusing Kirchhoff’s law:

A + T + R = 1

where A is absorptance, T is transmittance, R is reflectance of thesurface.

As T = 0 for the opaque surface, therefore, in our case absorptanceis determined by the equation:

A = 1 − R

It is seen from Fig. 3 that the samples have an absorption coef-ficient around 0.99.

Fig. 4. Spectral reflectance measured for black chromium electrodeposited from theCr(III) + ZnO bath on steel (1, 2) and copper (3, 4) substrates with the Ni undercoat(2, 4) and without it (1, 3). Black chromium was deposited at 0.2 A cm−2 for 2 min.

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The role of substrate-surface finish evaluated through theeflectance of black chromium is shown in Fig. 4. It is seen thathe spectral reflectance of black chromium on the copper substratecurve 3) is higher compared to that on the steel substrate (curve 1).he effect of a bright Ni undercoat prior to black chromium electro-lating was investigated for both steel and copper substrates. Theesults have shown that in both cases the spectral reflectance oflack chromium deposited on the bright Ni coating is higher com-ared to that of black chromium deposited on the same substrateithout the Ni undercoat. It is clear from Fig. 4 that the absorption

oefficient is over 0.99 for samples without the Ni undercoat andt is somewhat lower for the samples with the Ni undercoat. Thus,here is no significant difference between the absorption coefficientf the samples with and without Ni undercoat.

. Conclusions

Black chromium coatings were electrodeposited from the triva-ent chromium bath using a ZnO additive as a second mainomponent. Black chromium was electrodeposited on steel, cop-er plates and substrates plated by bright nickel prior to blackhromium. The black chromium coatings were characterized byRD and SEM. The XRD results suggest that the phase structuref black chromium may be characterized as the zinc solid solutionn chromium or the chromium solid solution in zinc depending onhe chromium/zinc ratio in the deposit. The role of substrate finishas evaluated through the corrosion resistance and reflectance of

lack chromium. According to corrosion tests the samples platedith bright nickel before black chromium showed the best results.lack chromium exhibits a high solar absorptance. The absorptionoefficient was found to be over 0.99 for the samples without the Ni

ndercoat and below 0.99 for the samples with the Ni undercoat.owever, the use of nickel undercoat before black chromium plat-

ng is recommended because it remarkably improves the corrosionesistance of samples.

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Science 305 (2014) 492–497 497

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