Zinc molybdenum phosphate belongs to the so-called sec- ond generation of phosphate pigments essentially. It is cam posed of zinc phosphate with zinc ...
Feature: Corrosion
High performanceanticorrosiveepoxy paints pigmentedwith zinc molybdenumphosphate R R o n m g n o l i , B d e l A m o , V F Vetere, L V~leva CIDEPINT
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Centro de Investigacidn y Desarrollo en Tecnologia de Pinturas, (CIC-CONICET), Calle 52 e / 1 2 1 y 122, ( 1 9 0 0 ) L a P l a t a , A r g e n t i n a
Inlroduclion Zinc molybdenum phosphate belongs to the so-called second generation of phosphate pigments essentially. It is c a m p o s e d of zinc phosphate with zinc m d y b d a t e (expressed as MoO 5) added up to 1% and it is claimed to have equal or better anticorrosive behaviour than chromates and undoubtedly better than zinc phosphate aloneA -6 The active iifllibi tire group in this pigment is the molybdate anion which is thought to repassivate corrosion pits in steel7 Little irdormation is available in the literature about the anticorrosive performance of zinc molybdenum phosphate. Adrian and Bittner ~,5,6 reported the behaviour of zinc molybd e n u m phosphate in alkyd paints compared with zinc phosphate and zinc ctlromate. The performance of zinc molyb d e n u m phosphate and other pigments belonging to the second generation of phosphate pigments series was also tested 6 in primers that comply with the relevant legislation. The purpose of the present research was to study the anticorrosive properties of zinc molybdenum phosphate in solvent-borne epoxy paints at two anticorrosive pigment loadings. The effect of incorporating zinc oxide was also studied. Zinc oxide was selected because of its ability to
Summaries High pertormae(e antkorrosive epoxy paints pigmented with zinc molybdenum phosphate The din of this researchwas to sludy the anlicorrosive performance of zinc molybdenun~ phosphGte in solvent-borneepoxy Nints at lwo Gnlicorrosivepigment ladings The effect of incorporating zinc oxide was also sludied. The anlicorrosive efficiency of the different pen systems wGs assessedby accelerGtedend dedtochemicd tests.
Hochleislnmjsepoxkl korrosionsschnlzheschkhlungssloffepigmenlierl rail Zinkmolybdiinphosphat. Der lweck dieser Forschungwar die Korrosionsschulzleislungvan linhnolybdiJnphosphat in 16semittelhdtigen EpoxidfGrbenbei Nei Gehdte GnKorrosionsschutzpigment.Die Wirkung der Einfuhrung van Zinkoxid wurde auch studied. Die Korrosionsschutzftihigkeit verschiedenerAnsltichstoffssytemewurde dutch schnelle und elekltochemische PrOfungenbewertet.
Peinlures bFoxydklueSantkorrosion de haul rendemenl pigment~es ave( zinc molylxl~ne phosphate. Le but de ces recherches6tail 1'6~de do rendement anficorr~ion de zinc molybd6ne phosphGte en peintures 6poxydiquesGuxsolvents ~ deux teneurs en pigment Gnlicorrosion. L'efficGdt4 Gnlicorrosionde differents syst6mes de peinlme 6tGit4vdu4eperdes essds acc616r~set &ctro6imiques.
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polarise cathodic areas by precipitating sparingly soluble salts. 8 The anticorrosive behaviour of the different pigment mixtures employed in this research was evaluated by means of corrosion potential measurements and polarisation curves. Painted panels with either the anticorrosive paint alone or with a complete paint system were subjected to the salt spray, humidity chamber and accelerated weathering tests. Also electrochemical tests were performed on steel panels coated only with the anticorrosive paint to avoid higher barrier effects due to a complete paint system.
Experimenlal Binder The film forming material polyamide resin.
was
an
epoxy
bisphenol
Pigment Micronised zinc molybdenum phosphate (average particle diameter 1 pm) was employed as anticorrosive pigment at two different loadings, 10 and 30% by volume with respect to the total pigment content. The complementary pigments were ferric oxide and barium sulphate and the mixture in which 20% by volume of these pigments had b e e n replaced by zinc oxide. Ferric oxide was also introduced in the p i ~ ment formula because its colloidal particles are said to interact with metallic substrates increasing its coverage. 9-11Solids contained in the anticorrosive paints tested is shown in Table 1.
Table 1: Solidsin anticorrosivepaint compositions(% by volume) Paint
1
2
3
4
Zinc oxide 5.6 7.2 Zinc molybdenum phosphate 12.1 4.1 12.1 4.1 Ferricoxide 14.1 18.1 11.3 14.5 Epoxy resin 59.7 59.7 59.7 59.7 Barium sulpMte 14.1 18.1 11.3 14.5 N0le:lt~es0~enlmixlureempbvedforep0xv~inlswaslduem/melt~/Iis0b~l kel0m/b~l dmhol (36/52/12, by weight) The inhibitive properties of the resulting pigment mixtures were evaluated by measuring the corrosion potential of an AISI 1010 steel electrode in the pigment suspension, after 24 hours exposure, against a saturated calomel electrode (SCE). The electrolyte was a 0.025 M sodium perchlorate solution. Anodic and cathodic polarisation curves of an iron electrode (working electrode) in the pigment suspensions were also obtained. The sweep began in the vicinity of corrosion potential, at a scan rate of 5 mV.s L A saturated calomel elec-
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Feature: Corrosion trade was u s e d as reference and a platinum grid was the counter electrode. Tim pigments were s u s p e n d e d in a 0, 5 M sodium perctflorate solution for 24 hours before introducing the working electrode which was maintained in contact with the suspension for 8 hours prior to plotting the polarisation curve, Cathodic curves were obtained immediately after the anodic sweep.
Paintmanufactureand application It was carried out b y ball milling in a 3.3 lilies jar. The resin solution was a d d e d first, and the pigments later; the charge was milled for 24 hours to achieve an acceptable degree of dispersion, grade 4 on the Hegrnan scale, n A sealer a n d a finishing coat, the composition of w h i c h can b e seen in Table 2, were p r e p a r e d b y a similar method, In the case of the sealer, the n o n leafing aluminium was incorporated after pigment dispersion to avoid reaction with chlorinated rubber resin,
by determination of m e a n value of the results obtained in the test. Accelerated tests were carried out o n panels coated with the anticorrosive paint alone a n d with different paint systems (Table 3).
Humidity cabinet test (ASTM D 2247) Panels were p l a c e d in the humidity c h a m b e r at 38 + I~ for 2650 hours. The degree of blistering was determined accord ing to the ASTM D 714 Standard.
Acceleratedageing test (ASTM G 26) The accelerated weathering of painted samples was carried out in an Atlas Weather Ometer (Xenon arc type). The test prograrrtme consisted of a 102 minutes light cycle followed by 18 minutes light water spray cycle, The overall time of each cycle was 2 hours a n d that of the complete test 1100 hours, Tim degree of rusting was evaluated according to the a b o v e - m e n t i o n e d Standard.
Table 2: Sealer and finishing coal composilions expressed as % by volume Companenls
Percenlage
Ferric oxide Barium sulphate Non leafing aluminium Zinc oxide Titanium oxide Alkyd resin (solids) Chlorinatedrubber (RIO) Chlorinatedparaffin (42%) Pine oil Xylene Aromasol H
Corrosionpotential measurements
Sealer
Finishing coat
2.2 2.2 4.7 0.8 5.6 16.8 7.2 0.5 40.0 20.0
7.3 20.8 4.8 2.1 0.5 43.0 21.5
Test panels, cut from AISI 1010 steel, were previously sandblasted to Sa 2 1/2 (SIS 05 59 00-67) attaining 20 + 4 p m maximttm roughness, d e g r e a s e d with toluene a n d coated with a w a s h primer (SSPC-PT 3-64). Paints a p p l i e d b y means of a spray gun o n AISI 1010 steel panels (15.0 x 7,5 x 0.2 cm) a n d three different paint systems were studied (Table 3) to assess their anticorrosive performance.
Table 3: Tesled paint systems Identification
Paint systems (lhkkness is put between brackets) anlkorrosive paint alone (75 _+ 5 pm ) anlkorrosive paint (40 + 5 pm) +
sealer (30 § 5 pm) anlkorrosive paint (40 + 5 pm) +
sealer ( 30 § 5pm) +
finishingcoat (40 + 5pro)
Salt spray test (ASTM B 117) After 1500 hours exposure, panels were evaluated to deter mine the degree of oxidation according to ASTM D 610. In all cases, experiments were carried out in triplicate, followed
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The cells for the performance of these measurements were constructed by marking out 3 cm 2 circular zones o n the painted surface. A cylindrical o p e n acrylic tube, with one fiat end, 7 cm long, was then p l a c e d o n the specimen a n d the electrolyte (0, 5 M sodium perctflorate s d u t i o n ) p l a c e d in the tube. The measurements of the corrosion potential of the painted steel substrata with respect to the SCE were m a d e employing a high impedance voltmeter. Electrochemical tests were carried out o n steel panels covered only with ant~ corrosive paints.
Ionic resistancemeasurements The resistance b e t w e e n the steel substrata and a platinum electrode was m e a s u r e d e m p l o y i n g the cells described previously and an ATI Orion, m o d e l 170, conductivity meter which operates at a 1000 Hz frequency.
Polarisation resistancemeasurements The polarisation resistance of p a i n t e d specimens was deter m i n e d as a function of the immersion time employing an electrochemical cell with three electrodes, Tile reference electrode was the SCE a n d the counter electrode a platinum grid. The voltage s w e e p was + 10 mV, starting from the corrosion potential, Measurements were carried out employing an EG&G PAR Potentiostat/Galvanostat, Model 273A, and the software SOFTCORR 352. Tim polarisation resistance of u n c o a t e d steel was also m o n i t o r e d as a function of the inmlersion time.
Resultsand Discussion Pigments mixtures The measured corrosion potential for the tested pigment mixrares was as follows: 532 (nmnber 1); q540 (number 2); -540 (number 3); -571 (number 4). A higher zinc m o l y b d e n u m phosphate content caused the corrosion potential to move towards higher values, indicating a higher degree of protection, For the same zinc m o l y b d e n u m phosphate content, the presence of zinc oxide impaired the anticorrosive properties
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Feature: Corrosion of the coating by lowering the corrosion potential as was observed for the paint containing the pigment mixture 4. Anodic polarisation curves (Figure 1) reveal that pigment mixtures 1 and 3 (with the highest zinc m o l y b d e n u m phosphate content) have the best inhibitive properties because they have the lowest passivation current for the second peak. The presence of zinc oxide was detrimental, because it made it difficult to obtain an effective steel passivation (pigment mixture 2). However, zinc oxide reduced the effect of lowering the zinc m d y b d e n u m phosphate content (pigment mix ture 4). It is thought that the passivated film formed b y corrosion is similar to that formed at potentials previous to the second peak (which corresponds to the oxidation of ferrous to ferric ions); in the case of mixtures 1 and 3 a more protective film seemed to b e formed because, as was said before, the current corresponding to the second p e a k decreased notably.
Figure I: .Anadicpalarisalian curvesaf the iron electrode in the different pigment mixtures (Table I}. Sweep rate 3 mrs ]
Figure 2: Cathodicpdar[satbn curvesaf the [ran eledrade in the different pigment mixtures (Table 1). Sweep rate 3 mrs -1 0.0 -0.1 -0.2 .7
_,i
-0.3 -0.4
....... .....
7 -0.5
Rgmenl mixture 1 Rgment mixture 2 Rgmentmixture 3 Rgment mixture 4
-0.6
1.5
"%
Pigment mixture I Pigment mixture 2 Pigment mixture 3 Pigment mixture 4
1.0
I
-1400
,t,
ir i ,t
i
.t, u
0.5
7""
"
0.0
I
I
I
I
0
I
200
I
400
I
I
I
600
800
1000
E/mY(SCE) From the cathodic polarisation curves (Figure 2), it m a y be seen that the higher the zinc m o l y b d e n u m p h o s p h a t e con tent in the p i g m e n t mixture the lower the oxygen diffusion current is. The presence of zinc oxide in mixture 3 was detrimental because it increased the o x y g e n diffusion current instead of polarising cathodic areas as it was suggested pre viously, n It must be r e m e m b e r e d that the higher the o x y g e n diffusion current the higher the corrosion rate w o u l d be. As the cathodic curve was obtained after the anodic one, it was c o n c l u d e d that the film f o r m e d o n the electrode surface during the anodic scan b l o c k e d the active sites for oxygen reduction, According to the foregoing discussion, it m a y be expected that paints formulated with the p i g m e n t mixture 1 must s h o w the best anticorrosive performance and paints containing mixtures 2 a n d 4 the worst,
Salt spray test (ASIM B 117) Results obtained in the salt spray cabinet after 1500 hours are s h o w n in TaMe 4. The anticorrosive protection was m o r e efficient for paints with the highest zinc m d y b d e n u m p h o s p h a t e content (Paint 1). The incorporation of zinc oxide
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I
-1000
I
-800
I
-600
I
-400
I
-200
E/mY (SCE) i
-800 -600 -400 -200
-1200
did not improve coating p e r f o m l a n c e as w o u l d have b e e n e x p e c t e d from data in the literature 12 and previous results obtained with alkyd paints, is This is due to the fact that zinc oxide has a relatively high water absorption la a n d that it reduces zinc m o l y b d e n u m p h o s p h a t e solubility by the comm o n ion effect. Evidently, these characteristics prevailed over the protection p r o v i d e d b y this pigment. Paint system 2 (anticorrosive paint + sealer) p r o v e d to b e as efficient as system i (two coats of the anticorrosive paint); this m a y be attributed to initial high barrier effect d e v e l o p e d for anticorrosive paints as will b e said later. The presence of a finishing coat (Paint system 3) e n h a n c e d the barrier effect and all systems s h o w e d high efficiency including those containfllg zinc oxide, Results were comqrmed after removal of the paint system. Blistering, as a general rule, was higher for panels coated solely with the anticorrosive paint, The application of a fin ishing coat p r e v e n t e d blistering e x c e p t for those paints con mining zinc oxide.
Humidity cabinet test (ASIM D 2247) The blistering results are presented in Table 5. Paint with the higher anticorrosive pigment content ( Paint 1, Table 1) did not present blistering after 2640 hours exposure, Blistering increased with the zinc oxide content. Signs of corrosion under the blisters were n o t o b s e r v e d except for Paint 4,3 after 1500 hours (Tables 4 and 5), In all cases the blistering was higher in system 3, b u t the blisters originated at the anticorrosive sealer paint interface.
Acceleratedageing lesl (ASTM G 26) All paints exhibited g o o d behaviour after 1100 hours e x p o sure, No signs of corrosion were o b s e r v e d either o n the painted surface or on the steel substrate. Blistering was, as a general role, low and increased only slightly for paints containfllg zinc oxide, It m a y be e x p e c t e d that these paint sys terns w o u l d p e r f o r m acceptably o n outdoor e x p o s u r e for at least three years without showing signs of corrosion,
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Feature: Corrosion Table 4: Rusling {ASTM D 610) and blislering degree (ASTM D 714} after exposure in Ihe salt sprm/lest Time (hours)
Paint systems 360 Rusting
720
Blistering
Rusting
960
Blistering
Rusting
1300
Blistering
Rusling
1500
Bl[sleffng
1.1 10 10 10 6F 10 6F 9 4F 1.2 10 10 10 8F 10 6F 10 6F 1.3 10 10 10 10 10 10 10 10 2.1 10 4M 10 2M 10 2M 8 2M 2.2 10 6F 10 6F 9 6F 8 6F 2.3 10 10 10 6F 10 6F 10 6F 3.1 10 6F 10 4F 10 4M 8 4M 3.2 10 10 10 10 10 10 9 10 3.3 10 10 10 10 10 10 10 10 4.1 10 6F 10 6F 10 6F 7 3F 4.2 10 10 10 10 10 6F 7 4F 4.3 10 10 10 10 10 8D 10 8D Note.:In the paintsystemcolumn,lhefirstnumbermrrespandsto the~ntimrrosivepainttested(Table1) oW thesecondoneidentifiesthepaintsystem.
Table 5: Blislering degree (ASTM D 714) afler exposure in Bumidily lesl Paint Systems 720
960
1500
1872
2640
1.1 1.2 1.3 2.1 2.2
10 10 10 10 10 10 10 10 10 8F 8F 8F 1OF 8F 8F 8F 8F 8F 8F 8M 8M 8M 8M 8M 6F 6F 6M 6M 4MD 4MD 2.3 4F 4MD 4D 4D 4D 4D 3.1 8F 8F 8F 8F 8F 8F 3.2 8F 8MD 8MD 8D 6F 6F 3.3 6F 6F 6F 6F 6F 6F 4.1 8F 8MD 8MD 8MD 8MD 8MD 4.2 8F 8D 8D 6F 6F 6F 4.3 6F 6MD 6MD 6MD 6MD 6MD Nole:Thenumeralrepresentsblislersize:10 no blislering8 lhesmdleslsizeblisler,elc Lellers indicdethe.frequel~y:D,deme;MD,n~ium deme;M, n~ium; F,few.
4F 6F 10 2M 6F 6F 4M 10 10 3F 4F 8D
200
0
-200
400 9 o 9 []
~00
-800 0 because, as was stated in the literature, 1100 hours of accel erated weathering m a y b e considered, on average, equiva lent to 3 years outdoor exposure.S5 The paint systems tested in this research were being e x p o s e d in a tropical climate without showhlg signs of corrosion after one year as this p a p e r ends.
Corrosion potential measurements (Figure 3) The b e s t anticorrosive protection was afforded b y Paints 1 a n d 3 and the high barrier effect s h o w e d b y these paints was e v i d e n c e d b y the shffthlg of the corrosion potential to abnormally high levels. It must b e p o i n t e d out that the error introduced in these measurements b y the presence of the impervious coating is about 25%. As was p r e d i c t e d by p d a r isation curves obtained with the iron electrode immersed in the different p i g m e n t suspensions, reduction of the zinc m o l y b d e n u m p h o s p h a t e content impaired the paint performance (Paint 2) as well as the incorporation of zinc oxide. In b o t h cases the corrosion potential of painted panels acquired more negative values during the test period.
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Bl[sler[ng
8 9 10 7 6 10 6 7 10 6 6 8
Figure 3: Corrosbn potential of painled AISI 1010 sled panels, as a fund[on of lhe expasure time, [n 0.5 M sadWm perchlorale salution
~me(haurs) 360
Rusl[ng
Paint 1 Paint 2 Paint 3 Paint 4 i 20
i 40 t/days
i 60
i 80
Ionic resistance measurements (Figure 4) The m e a s u r e d resistance is m a d e u p of two components: the solution resistance a n d the paint film resistance. As the solution resistance is l o w (84 ~1) the paint film resistance is responsible for most of the values measured. P d a r i s a t i o n effects may be neglected at the frequency e m p l o y e d in this test. The ionic resistance of Paints 1 and 3 was higher than l0 s and 107 ~ cm 2 respectively, giving full protection m the steel substrate b y a barrier effect. Paints 2 and 4 s h o w e d high ionic resistance at the beginning of the test, b u t this barrier effect was lost as time e l a p s e d (Figure 4), The m o r e impervious films c o r r e s p o n d e d to the highest anticorrosive pigment content; this l e d to the authors thinking that pigment-binder interaction was responsible for the higher barrier effect of Paints 1 a n d 3. In the case of paint 3 this was slightly impaired b y the presence of zinc oxide in the film, causing it to b e m o r e susceptible to electrolyte penetration.
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Feature: Corrosion figure 4: Resistanceof painted AISI 1010 steel, as a functien of the expesurelime, in 0.5 M sodium per61erate selutbn 107 106 105. BE --
m m
m m
~
C 104. 103
Conclusions
102
101 .
m o l y b d e n u m p h o s p h a t e ) r e d u c e d the corrosion rate of iron.17 It was demonstrated that m o l y b d e n u m c o m p o u n d s increase the polarisation resistance of steel. This was evident in Figure 5 where the paint with a 30% p i g m e n t content s h o w e d higher polarisation resistance values. M o l y b d e n u m c o m p o u n d s also improve the corrosion resistance of the substrate in the presence of chlorides, is The addition of m o l y b d e n u m hexavalent anions decreases the critical cur rent density for passivation a n d increases the stability of passivated films. The presence of silicon in the steel e m p l o y e d in this research m a y have contributed to the enhancing effect of molybdenum.19
0 Pain12 9 Pain13 [] Pa[nl 4
100
~
~
~ "~ I
I
I
I
20
40 t/days
60
80
PolarJsationresistancemeasurements(Figure 5) The high barrier effect s h o w e d b y Paints 1 and 3 controlled paint behaviour. In these cases a linear response, indicating an ohmic behaviour, was o b t a i n e d a n d thus there was n o p o i n t in taking the variation of polarisation resistance as a function of time. However, once Paints 2 and 4 h a d lost part of their barrier properties, polarisation resistance was m e a sured (Figure 5). Polarisation resistance, as m e a s u r e d in this research, is n o t a true figure, because it includes the ionic resistance. However, in all cases it is higher than the ionic resistance, which indicates that the pigment tested has inhibitive p r o p erties, 16 which is in agreement with previous results where zinc p h o s p h a t e (the main c o m p o n e n t of the so called zinc
Ffgure 5: Pdarisatien resistanceof painted AISI 1010 steel, as a [unclien ef the exposure trine, in 0.5 M sodium per61erate sdutbn 400
1. The obtained results s h o w that the greatest anticorrosive effect is obtained by e m p l o y i n g 30% by volume of zinc m o l y b d e n u m p h o s p h a t e in the pigment. 2. Polarisation measurements s h o w that the anodic film f o m l e d on steel b l o c k e d the active sites for o x y g e n reduction. 3. The ferric oxide a n d barium sulphate e m p l o y e d as extenders gave an additional barrier effect. 4. Zinc oxide is n o t r e c o n m l e n d e d in e p o x y paints pig m e n t e d with zinc m o l y b d e n u m phosphate. This may be due to its high water absorption a n d to the fact that it reduces zinc m o l y b d e n u m p h o s p h a t e solubility b y the c o m m o n ion effect. 5. Polarisation curves of p i g m e n t mixtures m a y b e u s e d as a guideline to predict the p r o b a b l e anticorrosive coating performance in accelerated and electrochemical tests. However, the final decision o n p i g m e n t selection must b e taken o n the basis of accelerated trials.
Acknowledgements The authors are grateful to CONICET (Consejo Nacional de Investigaciones Cientificas y T~cnicas ), CIC (Comisi6n de Investigaciones Cientificas d e la Provincia de Buenos Aires) and UNLP (Universidad Nacional d e La Plata) for their s p o n s o r s h i p of this research. The authors also wish to thai~k Colores Hispania for providing the anticorrosive pigment.
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350
2. Meyer G, Parbe+Lac& 71, (2) 113, 1965.
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Leidheiser Jr H, J. Coat, Tech., 53, (678), 29, 1981.
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Gerhard A and Bittner A, J, Coat. Tech., 58, (740), 59, 1986.
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7. AmbroseJ R, Corrosion (NACE)~ 34, (1), 27, 1978.
O ~
[] Paint 4
8.
,
I
I
I
40
50
60
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Szklarska Smialowska Z and MankowskT J, Br Corros. f , 4, (9), 271, 1969.
9. Andrade EM, Molina FV and Posadas D, J. Colloid and tnwrface Science, 165, 450, 1994. 10. Andrade EM, Gordillo GJ, Molina FV and Posadas D, f Colloid and Interface Science, 173, 231, 1995.
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