BORIC-ACID CORROSION OF CARBON STEEL BELOW …

17th International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors August 9-13, 2015, Ottawa, Ontario, Canada

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BORIC-ACID CORROSION OF CARBON STEEL BELOW PERFORATED STAINLESS STEEL CLADDING

Derek Lister1 and Pimsiree Phromwong2 1 Department of Chemical Engineering, University of New Brunswick, Fredericton, NB, Canada

2 EM&I Stantec Ltd., Halifax, NS, Canada

ABSTRACT

A polished carbon steel plate was fastened to a polished stainless steel plate perforated with four drilled holes of different diameters. The assembly formed the base of a vessel containing a solution of boric acid and lithium hydroxide and was mounted on a hotplate. Short, carbon-steel rods as corrosion specimens were immersed in the warm solution and in a separate room-temperature solution. The solutions evaporated in air for 14 days but were topped up regularly with fresh solution. After exposure, the cleaned and descaled rods and plates were assessed for corrosion gravimetrically and by surface profiling respectively. The plate experiment was repeated twice. The rod in the warm solution corroded 16 times faster than that at room temperature, while attack on the carbon steel plate extended slightly beyond the confines of the holes and increased rapidly with hole area. Corrosion products were mostly lepidocrocite and goethite with some magnetite. The results indicate that in an opened PWR component such as a steam generator channel head, pooled boric acid solution evaporating on a warm surface could damage the steel beneath perforations in the cladding, although the measured rate of attack alone was insufficient to account for observations in an operating PWR.

Keywords: PWR steam-generator channel head; defected stainless-steel cladding; boric-acid corrosion; low-alloy steel.

1. INTRODUCTION

1.1 Background

During the 2011 refuelling outage at a PWR that had operated since 1987, visual inspection of the channel head of one of the three steam generators revealed defects in the type-308L stainless-steel (SS) cladding and apparent attack of the underlying low-alloy steel (LAS) [1]. The visible degradation comprised six perforations in the cladding on the cold-leg side of the channel head, in the vicinity of the drain line and the “mouse-hole” at the bottom of the divider plate. The configuration of the channel head is shown in Figure 1. The largest defect measured 7.5 mm by 14.4 mm, and the total defect area was estimated to be 116 mm2. Ultrasonic examination of the channel head from the outside indicated extensive wastage of the LAS below the defects. There was a single, irregularly-shaped cavity of maximum depth 28 mm and area 7,350 mm2. The cause of the cladding defects was not known, but corrosion mechanisms were the suspected cause of the cavity. In 2012 January, Westinghouse sent their customers a Nuclear Safety Advisory Letter (NSAL-12-1) informing them of the observations and recommending appropriate inspection procedures for their plants and for Combustion Engineering plants, and in 2013 October the US Nuclear Regulatory Commission (NRC) issued an information notice bringing the matter to the attention of its licensees [2].

It is well known that the general corrosion of primary heat transport system (PHTS) materials during PWR operation is low. Since the introduction of chemical shim for reactivity control in the 1960s, it has been amply demonstrated that at ~300°C solutions of boric acid at concentrations of about 1,000 ppm (parts per million by weight) with lithium hydroxide to high-temperature pH values of 7 or so induce corrosion rates of 300-series SS cladding of a few µm/a (for example, see [3]). Similarly, the corrosion of LAS exposed to primary coolant after repairs to component penetrations has been deemed acceptably low by the NRC [4].


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On the other hand, corrosion of LAS induced by boric acid solutions leaking from the PHTS can be severe [5]. Under such conditions, the boric acid concentrates by evaporation and becomes aerated on contact with the environment, producing a very corrosive solution – especially on hot surfaces [6]. A prime example of potentially catastrophic boric-acid corrosion occurred at the Davis Besse PWR in 2002, where the leak of primary coolant through a cracked penetration in the reactor pressure-vessel head created a large cavity in the LAS [7]. Most of the damage was thought to have occurred in the later stages, possibly after continuing undetected for many months, when the leak had developed into a substantial jet disturbing pooled boric acid and impacting the sides of the growing cavity. Laboratory studies indicated that a type of flow-accelerated corrosion, possibly exacerbated by galvanic effects at the region close to the interface with the SS cladding, was responsible [8].

In the case of the under-clad damage of the LAS in the steam-generator channel head, there was no leak from the PHTS. Moreover, it is unlikely that primary coolant during operation would have been corrosive enough to cause the attack [3, 4], especially when any effects of flow or fluid turbulence penetrating through the 3-6 mm–thick SS cladding within the limited confines of the defects would have been small. Attack by remnant boric acid exposed to air and evaporating during shutdown was therefore suspected. Such attack might have been exacerbated by partial delamination of the weld-overlaid cladding.

1.2 Purpose of Research

The cause of the LAS degradation in the steam generator is of interest to PWR plant operators and regulators. To determine whether the static situation at plant shutdown, when warm, aerated boric acid solution is evaporating above perforations in the stainless steel cladding, could induce enough corrosion to account for the observed attack, laboratory tests were undertaken as described below.

2. EXPERIMENT

2.1 Method

The test assembly comprised a plate of type 316 SS, 3 mm thick and 100 mm square, perforated with four drilled holes of diameter 9.5, 6.4, 3.2 and 1.6 mm, in contact with a plate of ASTM type A36 carbon steel (CS), 9 mm thick and also 100 mm square. The composition of the CS is given in Table 1. The contact surfaces of the plates had been cleaned and lightly polished with 600-grit emery paper then rinsed with acetone before the plate and sheet were fastened together tightly with eight countersunk SS screws evenly spaced around the periphery. The roughness of the CS surface below each of the holes in the SS was determined before fastening by scanning with a Surtronic 25 profilometer from Taylor Hobson Ltd., UK. The plates in contact before fastening are shown in Figure 2.

Aluminum-foil tape (labelled “HVAC”) was used to build a dam, 30 mm high, around the edge of the SS and to cover the heads of the screws. The resulting shallow vessel or tray (shown in Figure 3) was filled with 200 mL of a stock solution of boric acid (2,500 ppm as B) and lithium hydroxide (2 ppb as Li) and put onto a hotplate to keep the solution temperature at 50ºC. A corrosion specimen comprised a rod machined from ASTM type A106-B CS, 7 mm diameter and 10 mm long. The steel composition is presented in Table 1. The rod was weighed and put into a separate 200-mL aliquot of the stock solution in a beaker at room temperature. The beaker and heated assembly were put in a fume hood and after two days a similar CS rod to the first was immersed in the warm solution in the tray but insulated from direct contact with the SS with a plastic disc. Over a 14-day exposure, the tray was topped up with fresh solution every day to compensate for slight leakage (under the dam) and evaporation, although for the last day there was no addition and the contents of the tray dried out. The beaker was topped up every other day. Throughout the exposure the addition of new solution tended to shift the position of the rod in the tray; however, the holes were never obscured and the corrosion product on the rod was not visibly disturbed. The effect on its corrosion was assumed to be negligible.


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Rusting of the CS below the holes could be seen through the solution in the tray from the first day; the solution became increasingly stained to yellow then orange-brown as the exposure progressed and then as boric acid precipitated. Staining of the solution in the beaker also occurred, but there was no boric acid precipitation. At the end of the exposure the CS rod in the tray ended up buried in the precipitated boric acid. Figure 4 shows the appearance of the tray as the experiment progressed from Day 1 to Day 14 and Figure 5 shows the appearance of the beaker over the same period. There was no significant damage to the Al-foil tape.

At the end of the exposures (designated Run 1), the tray assembly was removed from the hotplate, cleaned of boric acid with deionised water and disassembled. The CS rods were removed from their containers and similarly cleaned. The holes in the SS were found to be blocked with corrosion product coloured orange-brown with flecks of black. This was sampled and, along with that covering the rods and that in the damaged areas or depressions on the CS below the holes in the SS, analysed by laser-Raman spectroscopy (inVia Confocal Raman microscope, Renishaw plc, UK). An ASTM procedure [9] involving Clarke’s solution (2 g Sb2O3 and 5 g SnCl2 in 100 mL concentrated HCl) was then used to descale the plates and rods. A clean, unexposed rod specimen was included for a blank correction in the descaling procedure

The corrosion of the CS plate was assessed by surface profilometry. This involved a series of parallel scans at 1 mm intervals followed by a similar series at right angles across the damaged area beneath each of the hole confines. The volume of metal corroded below each hole was estimated by integrating each scan profile, summing the integrated profiles and averaging the two orthogonal results. The corrosion of the rods was assessed by weighing.

The plate exposure in Run 1 was repeated twice. Both repetitions, Runs 2 and 3, involved reassembling the descaled plates after the previous run, fabricating a new tray with the Al tape, filling the tray with the boric acid solution and replacing it on the hot plate to maintain the temperature at 50ºC. Again, exposures continued for 14 days, with topping-up of solution every day except at the end. After Run 3, the underside of the cleaned SS plate was also scanned with the profilometer by taking two single scans at right angles across each hole. The rod specimens were not included in the repeat runs.

2.2 Results

The contact surfaces of the disassembled SS and CS plates after Run 1 are shown in Figure 6. On the left (a) can be seen the four holes in the SS blocked with corrosion product along with evidence that the solution had spread beyond the confines of the holes. That spread also shows on the surface of the CS, seen on the right (b), where there is damage and corrosion product within the regions below the holes. The tapped holes for the SS screws around the periphery of the CS are corroded, indicating contact with boric acid. Solution could have found its way under the Al tape and through the screw holes in the SS and also to the underside of the CS by the slight leakage under the dam and down the edge of the assembly. Penetration between the SS and CS from the edge does not seem to have reached the screw holes, as indicated by Figure 6. In any case, the only access of the solution to the CS below the four test holes was clearly through those four holes. The appearance of the CS plate and rods after descaling is shown in Figure 7.

Figure 8 is a typical Raman spectrum of corrosion product, in this case sampled from the 9.5 mm hole in the SS after Run 1. It indicates peaks (with the expected [10] most intense peak for each species listed first followed by the next intense) as follows: lepidocrocite (γ-FeOOH) at 252 cm-1 and 379 cm-1; goethite (α-FeOOH) at 386 cm-1 and 300 cm-1; haematite (α-Fe2 O3) at 1,350 cm-1 (broad peak) and 292 cm-1; and, magnetite (Fe3O4) at 667 cm-1 and 532 cm-1. Traces of boric acid (B(OH)3) are indicated by the peak at 881 cm-1. Samples from the other holes showed increasing proportions of boric acid as the hole size decreased – presumably due to the increasing difficulty of washing it out of more confined spaces during the post-exposure cleaning. The composition of corrosion product from the rods was similar.


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The profiles across the holes on the contact side of the SS plate after exposure showed no significant corrosion, in spite of the penetration of the boric acid between the plates around the holes and the resulting discolouration of the surface visible in Figure 6. The edges remained quite sharp. As an example, the profile across the 6.2 mm hole after Run 3 is presented in Figure 9.

Surface profiles through the centres of the four hole positions on the CS plate after descaling in Run 1 are shown in Figure 10. Similar profiles, but indicating widening and deepening depressions, were obtained after Runs 2 and 3. The damage in terms of measured volume of metal corroded below each hole is presented in Table 2. Also tabulated are the calculated rates of loss of volume and weight (via the steel density of 7,900 kg·m-3) and rates of penetration in terms of: assumed uniform penetration beneath the hole confines (“nominal penetration rate”); assumed uniform penetration over the measured damaged areas below the holes (“uniform penetration rate”); and, actual measured deepest penetrations from the minima in the profilometer scans (“maximum penetration rate”). The averages of the corrosion rates for each hole size are also presented. The dependence of loss of volume on hole size and exposure time is pictured in Figure 11.

The corrosion of the rods was estimated from the weight losses during the 14-day exposure (for the room-temperature solution) and the 12-day exposure (for the solution in the tray at 50°C). The rod in the boric acid in the tray at 50°C corroded at an average rate of 753 mg·dm-2·day-1 (corresponding to a uniform penetration rate of 3.48 mm·a-1) and that in the boric acid in the beaker at room temperature at an average rate of 54.8 mg·dm-2·day-1 (0.253 mm·a-1).

3. DISCUSSION

Tests on carbon steel coupons immersed in aerated solutions of 2,500 ppm boric acid carried out by Westinghouse [6] indicated that long-term corrosion rate increased with temperature from 0.05 mm·a-1 at 21°C, through 0.18 mm·a-1 at 38°C to 0.38 mm·a-1 at 60°C. The data showed that initial rates were much higher, with averages for the first 12-14 days of exposure increasing from approximately 0.12 mm·a-1 at 21°C through 0.41 mm·a-1 at 38°C to 0.76 mm·a-1 at 60°C. The more rapid corrosion within the same temperature range of the rods in both our beaker test at room temperature (0.253 mm·a-1) and our tray test at 50°C (3.48 mm·a-1) no doubt reflects the effect of the increasing concentrations of boric acid as our solutions evaporated. It is of interest that the Westinghouse tests included coupons of CS coupled to type 304 SS. The results showed no difference in corrosion rate between coupled and isolated coupons, indicating that there were no discernible galvanic effects.

Our data for the plate corrosion in Table 2 indicate that corrosion rates below the holes in terms of volume of metal lost were somewhat greater in Run 1 than in the other two runs, although for the 9.5 mm hole the difference is hardly significant (in fact, the corrosion below the 9.5 mm hole was remarkably consistent for all three runs). We expected Run 1 to show lower corrosion, since in Runs 2 and 3 the surfaces would have been roughened by the previous exposure and would have been more susceptible to attack. Any suppression of corrosion is tentatively ascribed to the descaling process, which could have left traces of inhibitor on the surface after Runs 1 and 2. For the sake of comparison with other studies, however, the rates have been averaged.

Not surprisingly, the volume of metal lost by corrosion (and therefore the average rate of loss in volume or mass units) increased with the size of the hole (Table 2 and Figure 11). Besides the columns in the table showing those actual rates, the last three columns show penetration rates. The nominal penetration rate is the measured rate of volume loss divided by the area of the hole. Comparing this with the next column, uniform penetration rate, gives an idea of the effect of the area of damage beneath the hole as measured during the profilometer scans. The actual damage area beneath each hole increased from run to run and the proportional increase decreased with increasing hole size (the smallest hole, 1.6 mm diameter, had the greatest damage area relative to its own area, amounting to a factor of 5.4 after Run 3). The uniform penetration rates below the largest hole (9.5 mm), averaging 2.28 mm·a-1, are expected to provide


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the best comparison with the rod corrosion of 3.48 mm·a-1. The difference would have arisen from restricted mass transport to and from the plate as the hole became blocked with corrosion product. In fact, the variation in all the measured penetration rates no doubt arose from restricted access through holes blocked to different degrees.

We note that the damaged steam generator head had perforations in the SS similar to the size of our experimental perforations, with the largest (7.5 mm x 14.4 mm) having an area of approximately 70 mm2 (estimated roughly from an assumed lenticular shape, there being no detailed information on the geometry available). Our 9.5 mm hole had a very similar area of 70.9 mm2. At 116 mm2, the total perforation area in the steam generator cladding was similar to our total perforation area of 113.1 mm2 in the SS plate; however, the damage area below the steam generator cladding was 7,350 mm2 whereas after Run 3 the total damage area below all four holes was 237.0 mm2.

The last column in Table 2, maximum penetration rate, shows the average rate at which the deepest point of the depression was formed. Since the deepest depression in the damaged steam generator head was 28 mm whilst our maximum penetration rate was 4.87 mm·a-1 (from the 3.2 mm hole in Run 2), we conclude that it was unlikely that the damage mechanism at work in the steam generator was exactly the same as the corrosion in our tests – exposures amounting to over five years to warm, evaporating boric acid would not have occurred. Moreover, the much larger area of the cavity in the steam generator also indicates other mechanisms. It suggests that the SS cladding had delaminated from the underlying CS, possibly before operation. Defects in the cladding would then allow hot, pressurised coolant to enter the cavity during operation while during shutdown aeration and evaporation of pooled boric acid on the surface would have initiated corrosion as described in our experiments. Subsequent operation would have again forced hot coolant through the defects and disturbed the corrosion products, opening the areas to further corrosion during the next shut-down. Several such cycles would have caused significant corrosion, but not enough on its own to generate the size of cavity detected. Unfortunately, we have no information about the number of shutdowns at the reactor nor about shutdown procedures.

CONCLUSIONS

Warm, pooled boric acid evaporating for two weeks in air on the surface of a stainless steel plate fastened to a carbon steel plate significantly corroded the carbon steel below perforations in the stainless steel. The volume and mass of metal lost by corrosion increased with the size of the perforation. Corrosion products, mainly lepidocrocite and goethite, filled the perforations and the attack of the steel extended beyond the confines of the hole areas. There was no significant attack of the stainless steel.

Such corrosion could attack the low-alloy steel below defected cladding in an opened PWR steam generator head if boric acid solution were allowed to pool and evaporate there during a reactor shut-down; however, the attack should be obvious after a day or so by staining of the solution with corrosion products.

The mechanism demonstrated in the experiment is not sufficient on its own to account for the extent of damage in an operating PWR reported in the literature.

ACKNOWLEDGEMENTS

Funding from the Natural Sciences and Engineering Research Council of Canada and the CANDU Owners Group is acknowledged. Dr. Lihui Liu is thanked for the analyses and Dr. Thananchai Piroonpan for help in the laboratory.

REFERENCES

[1] EPRI. “Steam generator management program: steam generator channel head degradation failure modes and effects analysis”, EPRI Product Abstract: - Technical Report, Product ID 3002000473, Electric Power Research Institute, 2013 April 26.


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[2] NRC. “Steam generator channel head and tubesheet degradation”, NRC Information Notice 2013-20, US Nuclear Regulatory Commission, 2013 October 3.

[3] Cohen, P. “Water coolant technology of power reactors”, ANS Monograph, Gordon and Breach Science Publishers, 1969.

[4] Jones, R.L. and Hickling, J. “Topical report on boric acid corrosion”, Appendix B.18, Report NP SD-1198-P, US Nuclear Regulatory Commission, 2002 February.

[5] Miraglia, F. “Boric acid corrosion of carbon steel reactor pressure boundary components in PWR plants”, Technical Report 8803220364, US Nuclear Regulatory Commission, 1988 March.

[6] EPRI. “Boric acid corrosion guidebook, Revision 1; managing boric acid corrosion issues at PWR power stations”, EPRI Report 1000975, Electric Power Research Institute, 2001 November.

[7] NRC. “Davis-Besse reactor pressure vessel head degradation. Overview, lessons learned, and NRC actions based on lessons learned”, NUREG/BR-0353, Rev. 1. US Nuclear Regulatory Commission, 2008 August.

[8] Pongkuak, J., Lister, D.H., Cook, W., Hickling, J. and Ahluwalia, K. “Boric acid corrosion of reactor pressure vessel steel caused by an impinging jet of simulated PWR coolant”. Proc. Int. Conf. Water Chem. Nucl. Reactor Systems, Jeju Island, Korea, 2006 October.

[9] ASTM. “G1: Chemical cleaning procedure for removal of corrosion product”, ASTM Standards, Vol. 3.02, Chapter Iron and Steel. ASTM International, Philadelphia, USA, 2010.

[10] Cornell, R.M. and Schwertmann, U. “The iron oxides: Structures, properties, reactions, occurrences and uses; Second edition”, Wiley-VCH Verlag GmbH & Co., KGaA. On-line, 2004 January.


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TABLES

Table 1. Composition of CS plate and rod (element %; balance: Fe + C)

Si P S V Cr Mn Co Cu As Mo

plate 0.478 0.015 0.029 0.0008 0.0623 1.067 0.0052 0.193 0.0072 0.0071

rod 0.269 0.030 0.025 0.0052 0.0192 0.623 0.0032 0.0098 0.0021 0.0037


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Table 2. Rate of loss of metal volume and mass and rates of penetration on CS plate (N.B.: measured penetration rate on CS rod exposed in tray = 3.48 mm·a-1 ≡ 1,033 mm3·a-1)

Hole diam.

(mm)

Run no.

Measured volume of metal lost (cumul.) (mm3)

Volume loss rate

(mm3·a-1)

Weight loss rate

(g·a-1)

Nominal penetrat-ion rate*

(mm·a-1)

Uniform penetrat-

ion rate **

(mm·a-1)

Maximum penetrat-

ion rate*** (mm·a-1)

9.5 1 9.10 237.4 1.88 3.35 2.35 3.91

2 18.17 236.6 1.87 3.34 2.29 4.17

3 27.03 231.2 1.83 3.26 2.20 3.28

Ave. 235.1 1.86 3.32 2.28 3.79

6.4 1 6.49 169.3 1.34 5.26 3.73 3.83

2 11.02 118.2 0.93 3.67 1.82 3.44

3 15.76 123.7 0.98 3.85 1.40 3.07

Ave. 137.1 1.08 4.26 2.32 3.45

3.2 1 2.85 74.4 0.59 9.25 4.37 4.41

2 5.52 69.7 0.55 8.62 4.07 4.87

3 7.99 64.4 0.51 8.01 2.40 3.31

Ave. 69.5 0.55 8.63 3.61 4.20

1.6 1 0.54 14.1 0.11 7.01 2.44 2.96

2 0.85 8.09 0.065 4.02 1.03 1.89

3 1.20 9.13 0.072 4.54 0.84 0.92

Ave. 10.4 0.082 5.19 1.44 1.92

*based on hole area and assuming uniform corrosion **based on measured area of damage and assuming uniform corrosion ***based on profile minimum and total exposure time to measurement


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FIGURES

Figure 1. Steam generator channel head (Ref. 2)

Figure 2. View of CS plate and SS overlay before fastening

Primary manways

Primary inlet

Primary outlet

Divider plate

“Mousehole” above drain line


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Figure 3. HVAC aluminum-foil dam with perforated SS plate forming base of tray

Figure 4. Appearance of tray from Day 1 (D1) to Day 14 (D14), Run 1


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Figure 5. Appearance of beaker from Day 1 (D1) to Day 14 (D14)

Figure 6. Contact surfaces of SS (a) and CS (b) after exposure in Run 1and disassembly


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Figure 7. The CS rods ((a) from tray, (b) from beaker) and CS plate (c) after descaling after Run 1

Figure 8. Raman spectrum of corrosion product sampled from 9.5 mm hole after Run 1

(c)


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0 1 2 3 4 5 6 7 8-10

-5

0

5

10 D

evia

tion

fro

m o

rigi

nal s

urfa

ce (m

)

Length (mm)

Hole

0 1 2 3 4 5 6 7 8-10

-5

0

5

10

Dev

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rom

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gina

l sur

face

(m

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Hole

Length (mm)

Figure 9. Scans from the left (a) and to the right (b) of the 6.4 mm hole on the contact side of the descaled SS plate after Run 3

Figure 10. Surface profiles across the centres of the damaged areas (measured diameters shown) on

descaled CS plate after Run 1; hole size: (a) 9.5 mm; (b) 6.4 mm; (c) 3.2 mm; (d) 1.6 mm

(a) (b)

(c) (d)

(a) (b)


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Figure 11. Dependence of CS volume loss on hole size and exposure time

BORIC-ACID CORROSION OF CARBON STEEL BELOW …

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