J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Corrosion characteristic of AlN, Y2O3, Er2O3 and Al2O3 in Flinak
for molten salt blanket system(Received: 20 November 2009 / Accepted: 12 April 2010)

For liquid breeder blanket system using molten salts, tritium barrier, anti-corrosion barrier, tritium recovery
system and heat exchanger make important roles. Especially in the heat exchanger, the tritium permeationfrom primary coolant loop to secondary coolant loop must be inhibited by the tritium barrier for safety. To
develop it, it is essential to optimize chemical stability of the barrier material against molten salts. In thepresent study, the corrosion on bulk specimens of AlN, Y2O3 Er2O3 and Al2O3 in Flinak was investigated. The
static corrosion tests were performed for 230 and 1010 hours using these specimens with high purity Flinak at600C in Ar atmosphere. Weight measurement and surface analysis of the specimens were carried out before
and after the corrosion test. Chemical analysis of the Flinak also did before and after the corrosion test. AlN
showed corrosion resistance, even though the surface was slightly oxidized and corroded. Er2O3, Y2O3 andAl2O3 dissolved into Flinak. In Er2O3 corrosion, Zr in the matrix as an impurity might affect on the corrosion.
The Y2O3 corrosion was small. The weight loss due to the corrosion was affected by the breaking of thespecimens with porous structure. And the Al2O3 corrosion was large. The surface structure changed to roughsurface. These corrosion behaviors agreed with prediction based on thermodynamic evaluation.

layer was formed after certain time for the corrosion viaCr dissolution [6]. And the corrosion resistance of Ni
based alloy was demonstrated [7]. The corrosion could becontrolled by the impurity control.
Ceramic coating to prevent tritium permeability has
also been studied because of the low hydrogen diffusivity
and the low hydrogen surface recombination constants [8].
Ceramics such AlN, Y2O3, Er2O3 and Al2O3 are promising
as candidate materials for tritium barrier. But the stability
in molten salt is not made clear so far. To develop liquidblanket system using molten salts, we hence have tooptimize the heat and mass transfer property between
Flinak and the metal specimen coated with the barriermaterial. In the present study, the corrosion resistance of
those ceramics in Flinak was studied by static corrosion
test.

Force Free Helical type Reactor (FFHR) with
self-cooled liquid blanket system has been designed [1].
Molten salt is known as a promising self cooling tritium
breeder for the system for fusion reactor because of theattractive advantages on safety aspects: low tritium
solubility, low reactivity with air and water, low pressure
operation, and low MHD resistance. LiF-BeF2 (Flibe) is
planned to be employed as tritium breeder and coolant of
the blanket due to inherent safety and high thermal
efficiency operated above 500C [2].
46.5LiF-11.5NaF-42KF (mol%) called as Flinak
possesses similar characteristics to Flibe. It would meanpotentiality to be used as a simulant for the heat and masstransfer investigation. Flinak would be also one ofalternative candidates of the coolant and tritium breeder ofthe blankets [3]. For the blanket system, the corrosion of
the structural materials and the development of tritiumbarrier, anti-corrosion barrier, tritium recovery system andheat exchanger are key issues. Especially in the heatexchanger, the permeation of tritium to secondary coolant
loop must be inhibited by the tritium barrier [4]. And
deterioration of heat transfer efficiency of the heatexchanger by the barrier must be prevented concurrently.
Corrosion of several candidate structural materials in
Flinak has been studied for the blanket. It was found thatthe corrosion of reduced activation ferritic steel,
JLF-1(Fe-8.92Cr-2W) is caused by the impurity as H2O,
O2 and HF dissolved in the molten salts [5]. In thecorrosion of austenitic steel, Ni rich corrosion resistant

2. Experimental

2.1 Specimen preparation
The bulk specimens were prepared as follows: afterAlN powder including polyvinyl butyral as a binder wasshaped to a tablet by compression at 500 tons, it wascalcined at 1850 qC for 4 hours. After Er(NO3)3㨯5H2O
was calcined at 1000 qC for 5 hours, it was ground. The
obtained powder was shaped to a tablet by compression at60 kg/cm2 and calcined at 1700 qC for 2 hours. Y2O3powder was shaped to a tablet at 60 kg/cm2 and calcined at
1650 qC for 5 hours. Al2O3 power including polyvinyl
alcohol as a binder was shaped to a tablet by hand pressmachine at 80 qC. It was calcined at 400 qC for 3hours,
800 qC for 3 hours and finally 1600 qC for 5 hours step by342©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

T. Watanabe et al., Corrosion Characteristic of AlN, Y2O3, Er2O3 and Al2O3 in Flinak for Molten Salt Blanket Systemstep. These specimens were cut to adjust into the size of10 mm u 15 mm u 5.0 mm. Initial composition of thespecimens was analyzed by X-Ray Fluorescence
Spectroscopy (XRF). Table 1 shows initial composition ofthe specimens.

compared to that before the corrosion test. X-ray
diffraction (XRD) patterns were measured before and after
the corrosion test. The surface was observed by scanningelectron microscopy (SEM). Chemical analysis at thespecimen surface was carried out by X-ray photoelectron
spectroscopy (XPS). Chemical composition of Flinak wasalso analyzed by inductively coupled plasma mass
spectrometry (ICPMS) and Karl Fischer titration.

2.2 Test condition
The Flinak used in the present corrosion tests waspurified by electrolytic refining method, and had initial
impurity as shown. The Flinak contained lower metal
impurity and moisture than that in the previous study [5].
Figure 1 shows a capsule for corrosion test made fromSS316L. The specimens were putted in SS316L crucibles
filled with Flinak [7] and packed in dry Ar atmosphere.
The capsule was heated at 600C [4]. After 230 hour and1010 hour immersion, the specimens were extracted fromthe capsules.
The specimens were rinsed in pure water for 4 days atroom temperature to remove Flinak from the specimens.
After drying the specimens, the weight was measured
using micro-balance with 0.1mg in accuracy, andAlN
Er2O3
Y 2O 3
Al2O3

Al49.63
Er37.91
Y39.61
Al39.68

Y
0.47
Zr
1.24
Fe0.02
Ce0.09CapsuleTube
Cap of swagelok

Specimen
Flinak

Fig.1 Capsule for corrosion test

Table 1 Composition of specimens (at%)

Si0.16
Pb0.15Si0.06
Si0.07

Mg
0.08
Y0.09
Al0.03
S0.03

CrucibleS0.01
Al0.17
Mg
0.04
Mg
0.04

Ca0.01
Si0.11O
60.22
Cl0.03

O
0.00
P0.07

N49.63O
60.25

Sr0.02

K0.01

Ca0.02O
60.01

3. Results
3.1 Surface observation

Figure 2 shows the photos of specimens before andafter the corrosion test. The initial AlN specimen was gray.
Although it got slightly light gray after the corrosion test,
no shape change was significantly observed. In case ofEr2O3, the specimen was pink before corrosion test and thecolor has been kept through the corrosion test. It, however,got fragile after corrosion test and chipped off especiallyat the corners. Y2O3 specimens were white before
corrosion test. It broke and got blackish after the test.
Al2O3 specimens were white before corrosion test. Afterthe test, the specimens colored to gray.

e spattering time is corresponding to
the depth from surface. The spattering rate was about 3
nm/sec for SiO2. The actual spattering rate for the tested
ceramics could be close to this value.

343

T. Watanabe et al., Corrosion Characteristic of AlN, Y2O3, Er2O3 and Al2O3 in Flinak for Molten Salt Blanket System

AlN

Er2O3

Y2O3

Al2O3

Beforecorrosion
testBroken

After
230h

corrosion
test
BrokenBroken

After
1010h

corrosion
test

10mm
Fig.2

Photos of specimens before and after corrosion test

Table 2 Weight Change for corrosion test (unit: g/m2)
Time230 hours
1010 hours
AlN-0.233-0.0928
Al2O3+981+74.0
Y2O3+195-58.9 (broken)
Er2O3-446 (broken)-8.21 (slightly broken)
3.2 Weight change of specimens
Table 2 shows weight change per unit area before
and after the corrosion test. The weight loss in AlN
specimen was small. The losses in the Er2O3 specimens
and the Y2O3 specimen after l010 hour corrosion test
were large and their specimens were broken. The weightloss includes loss of fragments due to breaking during thecorrosion test and water rinse. On the other hand, theY2O3 specimen after 230 hour corrosion test and theAl2O3 specimens gained in weight. These specimens
possess porous structure. The gain of the weight ispossibly due to the Flinak and water permeated into thepores of those specimens.

testing time after 230 hours although these results containthe breaking of specimens placed in the Flinak.
4. Discussion
4.1 Thermodynamic stability
Table 4 shows standard Gibbs energy for reaction
about these ceramics with LiF and KF, respectively. LiF
occupies the largest part in Flinak used in this study. It is
more stable than NaF and KF. KF was most unstable due
to largest free energy for formation in the Flinak. Theenergy values provide us the equilibrium tendency about
the reactions in the condition in this study and allow us
prediction of thermodynamical stability in Flinak.
According to the reaction energy estimation, AlN could
be stable in Flinak if it mainly reacted with LiF, which
had the largest part in the Flinak. On the other hand,
Er2O3, Y2O3 and Al2O3 would be unstable.

3.3 Chemical analysis
The chemical composition of Flinak after thecorrosion test was compared with the initial that of Flinak
in Table 3. While the Flinak used for AlN 230 hourcorrosion test contains Al at 61 wppm, those used for
Er2O3, Y2O3 and Al2O3 corrosion tests contain Er at 1.47
104 wppm, Y at 3.30103 wppm and Al at 930 wppm,respectively. In 1010-hour corrosion test, similartendency was also observed. These results mean that
Er2O3, Y2O3 and Al2O3 dissolved into Flinak and that thecorrosion is not negligible on Er2O3, Y2O3 and Al2O3.
Figure 4 shows relationship between impurity
concentration in Flinak and testing time on corrosion test.
It shows no increase of the impurity concentration with

4.2 Solubility of oxide and nitride in Flinak
The corrosion tests for the ceramics were carried out
at 600qC for 230 hours and 1010 hours. The results ofweight loss measurement in these tests did not clearlyshow time dependence on the corrosion, because theweight loss was significantly affected by breaking of thespecimens, particularly in the specimens of Er2O3 andY2O3. The specimen surface structure after the 230 hour
test was similar to that for 1010 hour test. Although therepeatability of the corrosion characteristics were344

T. Watanabe et al., Corrosion Characteristic of AlN, Y2O3, Er2O3 and Al2O3 in Flinak for Molten Salt Blanket System

confirmed by the tests, significant time dependence on
the corrosion was not obtained.
One possible reason is saturation of the dissolved
elements within 230 hours. The corrosion might besuppressed after reaching it. The Flinak contained 950
wppm of Al after the immersion of Al2O3 for 1010 hours,
which is comparable with Al amount in Flinak after 230
hour immersion. If the corrosion products were dissolved
to Flinak via the fluoridation shown in Table 4, Al
Beforecorrosion test

After 230-hourcorrosion test

60

Al40N%

200 O
0204060
Time / min8040Al20080

N 1sF0O60
40Er20%0204060
Time / min

80

204060Time/minAtomic concentration (%)

Atomic concentration (%)

Y%
204060Time / min

80

60

Atomic concentration (%)

80
60%

40O

200Al0204060
Time / min

80

O40
Al

200020

O%

2000F
Er

204060Time / min

K80

5Pm

10080%60

O40

Y

200F
0204060
Time / min

80

5Pm

10080
60
O40

Al

200F

Li0%
20Na-4060
Time / min80

Fig.3 Surface images of specimen by SEM and surface element analysis by XPSbefore and after corrosion test in Flinak at 600qC for 230 and 1010 hours.

345N%

405Pm

100

60805Pm
Atomic concentration (%)

Y2O3
Al2O3O80

5Pm

5Pm

80

080

100

1005Pm10020LiNaKC5Pm
Atomic concentration (%)

80

05Pm

O

60

100

40

After soaking in water for
5 days at room temperature

Atomic concentration (%)

Atomic concentration (%)

Atomic concentration (%)

5Pm

80

60

After 1010-hourcorrosion test

1005PmAtomic concentration (%)

Er2O3AlN5Pm
1000amount in Flinak after the 1010 hour corrosion test wouldcorrespond to solubility of Al in Flinak. Considering Al
dissolving into Flinak via fluoridation, it would mean thesolubility of fluoride, AlF3. It is 2.95u103 wppm obtained
from molecular weight ratio of Al and AlF3.
In case of Er and Y, the concentration would similarly
give us the solubilities of ErF3 and YF3, which are 321
wppm and 180 wppm, respectively.

4060
Time / min80

T. Watanabe et al., Corrosion Characteristic of AlN, Y2O3, Er2O3 and Al2O3 in Flinak for Molten Salt Blanket System


Impurity concentration / wppm
105


Er in Flinak of Er O corrosion test

104

2

3

Y in Flinak of Y O corrosion test

3210

310

2

Al in Flinak of Al O corrosion test

10

Al in Flinak of AlN corrosion test

2

3

10.1
0.01
0.001

200 400 600 800 1000 1200
Time / hours
Fig.4 Impurity concentrations in Flinak before and after corrosion test.

AlN

Al2O3
Y2O3
Er2O3
ZrO40Table 4 Gibbs energy for reaction of ceramics material in Flinak at 600qCThermodynamic
Possible Reaction' Gr [J/K.mol]
Stability
AlN + 3LiF = AlF3+Li3N+2.015
StableFluoridation
AlN + 3KF = AlF3 + 3K + 1/2N2-148.607
Unstable
AlN + 3H2O =-318.676
Unstable
NH3 + 1/2Al2O3.3(H2O)(-325.219 at 300K)
Oxidation
AlN + 3/4O2 = 1/2Al2O3 + 1/2N2-51.351
Unstable
Al2O3+6LiF = 2AlF3+3Li2O-23.293
Unstable
Al2O3+6KF = 2AlF3+3K2O-18.280
Unstable
Y2O3 + 6LiF = 2YF3 + 3Li2O-2.227
Unstable+2.786 at 870K
Stable over 800KY2O3 + 6KF = 2YF3 + 3K2O
(negative under 800K)
Unstable under 800K
Fluoridation
Er2O3 + 6LiF = 2ErF3 + 3Li2O-24.365
Unstable
Er2O3 + 6KF = 2ErF3 + 3K2O
-19.355
Unstable
ZrO4+4LiF = ZrF4 + 2Li2O-16.484
Unstable
ZrO4+4KF = ZrF4 + 2K2O
-13.139
Unstable
However, the AlN might be corroded, since the weightloss of the specimen was slightly detected. According toref. [8], reaction of AlN with moisture forms Al(OH)3 andammonia. Moreover, the hydroxide might change to Al2O3

4. 3 Corrosion resistance of AlN in Flinak
The AlN specimens seem to possess corrosion
resistance against Flinak compared with Er2O3 and Y2O3.

346

T. Watanabe et al., Corrosion Characteristic of AlN, Y2O3, Er2O3 and Al2O3 in Flinak for Molten Salt Blanket Systemand/or its hydrate. After the corrosion test, the oxygen wasdetected on the surface by XPS as shown in Fig. 3. The
surface of AlN specimen could be thinly covered withAl(OH)3 and/or Al2O3. The oxygen rich layer could beformed in Flinak if there was reaction with moisture
and/or oxygen in the Flinak. As another possibility, theoxygen rich layer might be formed when the specimens
were rinsed in water after corrosion test. Then, to confirm
substitution from N to O at the AlN surface in water,
another AlN specimen (7.0 mmu 6.0 mm u 3.0 mm) cut
from the identical lot was soaked in water for 5 days.
Before and after the water soaking, XPS spectra weremeasured. The insets in Fig.3 show the depth profile ofatomic composition ratio measured with XPS and show
the replacement from N to O.
The AlN might be corroded if the surface waspreliminary oxidized or oxidized in the Flinak due to thereaction with dissolved oxygen in Flinak as shown in Fig.
5.

At the same time, fluorine was also detected on the surface
as shown in Fig.3. The surface was corroded via thefluoridation process. Er2O3 specimens contained Zr as animpurity as shown in Table 1. Since zirconia seems to be
thermodynamically unstable as shown in Table 4, zirconia
would promote the corrosion and brittleness.
Although the corrosion of Y2O3 specimen was small,the fluorine was slightly detected on the surface. Thissuggests that the surface could be corroded via
fluoridation process.
5. Conclusion
Major results are as follows;(1) AlN showed corrosion resistance in Flinak. The
surface could be oxidized by the oxygen or moisture.
(2) The corrosion of Al2O3 was large. The corrosion ofthe 316L type austenitic steel was promoted by theredox reaction caused by the dissolved Al from Al 2O3in Flinak.
(3) The corrosion of Er2O3 was caused via thefluoridation process. Zr oxide dissolved in the sampleof Er2O3 might be promoted the corrosion.
(4) The corrosion of Y2O3 was small, although weightloss due to the corrosion was affected by breaking of
the specimens with porous structure.
(5) The corrosion intensity of the ceramic materials in
Flinak agreed with the tendency of chemical reaction
indicated by the calculation of the Gibbs reactionenergy.
In conclusion, AlN might be suitable for surface coatingof the structural materials for blanket system using molten
salt like a Flinak.

4. 4 Corrosion of Al2O3, Er2O3 and Y2O3 in Flinak

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In Er2O3 corrosion test, Er dissolution was detected. It
was found that the molar ratio of Er was low at the surface.

347