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Sacrificial effect of titanium powder on the corrosion by hydrogen fluoride in LiF-NaF-KF
Fusion Engineering and Design xxx (xxxx) xxx–xxx
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Sacrificial effect of titanium powder on the corrosion by hydrogen fluoride in LiF-NaF-KF ⁎
Gaku Yamazakia, , Juro Yagia,b, Teruya Tanakaa,b, Takashi Watanabeb, Akio Sagaraa,b a b
SOKENDAI (The Graduate University for Advanced Studies), 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Molten salt blanket Corrosion Hydrogen fluoride Titanium Sacrificial protection
Sacrificial corrosion protection, a new use of Ti powder in molten salt blankets, is proposed. Redox reactions of mixed Ti powder into molten fluoride salts are predicted to suppress the TF corrosion of structural materials. Stainless Steel 430 (SS430) is immersed into FLiNaK containing HF at 823 K for 6 h. Before the immersion, the Ti powder either exposed to air or not exposed to air at all is mixed into FLiNaK. It has been found that either types of mixed Ti powder can suppress the HF corrosion of SS430.
1. Introduction
less expensive and easier to process. Although Ti is not as strong as Be as a reducing agent, the reaction of Ti with TF by the following chemical reaction, for example, is predicted in molten fluoride salts,
FFHR series, D-T fusion helical reactors proposed by National Institute for Fusion Science, have been designed to be equipped with self-cooled liquid blankets [1]. One of the coolants through these blankets has been molten fluoride salts such as FLiNaBe (LiF-NaF-BeF2). In blankets, the reaction of molten fluoride salts consisting of LiF with neutrons produces tritium fluoride (TF) by the following nuclear reactions, 6 7
LiF + n→ TF + 4 He,
LiF + n→ TF + 4 H e+ n.
(1)
TF is a strong oxidizing agent, and it influences the dissolution type corrosion of structural materials (M) caused by the following chemical reaction,
M+ zTF → MFz +
z T2 2
(2)
where z is a valence of dissolved M ions. In order to remove this TF, redox reactions caused by reducing agents have been studied. Table 1 shows the standard electrode potential of the metal to be used in blankets [2]. The lower this potential is, the stronger the metal is as a reducing agent. Beryllium (Be) rods react with hydrogen fluoride (HF) quickly enough to suppress the HF corrosion of iron immersed into FLiBe (LiF-BeF2) [3–6]. Redox reactions caused by Be rods are also predicted to remove TF in molten fluoride salts. In this experiment, attention is focused on titanium (Ti) which is
⁎
Ti + 3TF → TiF3 +
3 T2. 2
(3)
There are several advantages of powder over rods when Ti is mixed into molten fluoride salts. It is possible to accurately control the amount of added Ti in coolants and to spread Ti throughout blankets. And because no additional structure is necessary, it is easy to insert the powder into current blanket designs. The idea of mixing powder into molten fluoride salts is based on the system (called “S-system”) proposed by Sagara, in which the hydrogen absorption metal powder mixed into molten fluoride salts effectively increases their low hydrogen solubility [1]. By the use of Ti as a hydrogen absorption metal and FLiNaK (LiF-NaF-KF), Yagi et al. practically demonstrated the increase in hydrogen effective solubility [7], and Nishiumi et al. practically demonstrated the decrease in the hydrogen permeation through structural materials [8]. By the existence of Ti powder throughout coolants, the sacrificial reaction of Ti powder with TF (Eq. (3)) occurs instead of the corrosion of structural materials by TF (Eq. (2)). In this experiment, HF is used as a simulant of TF, and FLiNaK is used because FLiNaK is easy to handle compared to FLiNaBe. Further, the hydrogen behavior of FLiNaK resembles that of FLiNaBe. The sacrificial effect of Ti powder on the HF corrosion of structural materials in FLiNaK is investigated.
Corresponding author. E-mail address: [email protected] (G. Yamazaki).
https://doi.org/10.1016/j.fusengdes.2018.02.054 Received 15 November 2017; Received in revised form 8 February 2018; Accepted 16 February 2018 0920-3796/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Yamazaki, G., Fusion Engineering and Design (2018), https://doi.org/10.1016/j.fusengdes.2018.02.054
Fusion Engineering and Design xxx (xxxx) xxx–xxx
G. Yamazaki et al.
the apparatus are chosen as follows. Outward-facing parts are made of Stainless Steel 304 or 316, whereas parts directly in contact with FLiNaK or HF are made of nickel (Ni) or Ni-based alloys in order not to react with HF. A Ni crucible is used to contain FLiNaK. To prevent the galvanic corrosion by the contact between the sample and the Ni crucible, a sample is hung, and PTFE insulation is provided on an outer wall of the apparatus. A Monel 400 tube guides an argon (Ar) gas containing H2 and HF to FLiNaK. A thermocouple wrapped in an Inconel 600 sheath monitors the FLiNaK temperature. Table 2 shows types of samples used in this corrosion experiment. The amount and particle diameters of mixed Ti powder and the amount of injected HF are different. Note that wt% is based on the mass of FLiNaK. Before the corrosion experiment, 30 mL of FLiNaK (LiF-NaF-KF: 46.5–11.5–42.0 mol%; melting point 727 K) is prepared in the Ni crucible by the following steps. At first, LiF, NaF, and KF powder (Kanto Chemical Co.), the purity of which is 98.0%, 99.0%, and 99.0%, respectively, are mixed to produce powdered FLiNaK. Referring to Table 2, one of the following two types of Ti powder is used. One type is the Ti powder prepared by an electrochemical method. Because this Ti powder is not exposed to air at all, it can increase in hydrogen effective solubility in molten fluoride salts [7]. In other words, this Ti powder is favorable for mixture into molten fluoride salts. FLiNaK containing 2 wt% this Ti powder was prepared in melted FLiNaK by I’MSEP Co. by the use of “Plasma-Induced Cathodic Discharge Electrolysis” which allows Ti not to be exposed to the air at all [9–11]. This Ti powder has particle diameters of 100 nm or less, and is nano-sized Ti powder (hereinafter called “nTi”). The amount of nTi in FLiNaK is adjusted by the dilution with the powdered FLiNaK. The other type is the Ti powder mixed directly into the powdered FLiNaK, which is exposed to air. This Ti powder has particle diameters of 45 μm or less and 99.98% purity (Nilaco Co.), and is micro-sized Ti powder (hereinafter called “μTi”). A certain amount of μTi is mixed into the powdered FLiNaK. The average particle diameter of μTi is about 12 μm according to an observation by a scanning electron microscope (SEM). In either case, the amount of mixed Ti powder is determined based on the entire amount of produced HF. In this experiment, 0.25 wt% is the amount of the mixed Ti powder reacting with the entire amount of produced HF, and 0.50 wt% is a large enough amount compared to it. A block of FLiNaK containing Ti powder is prepared in the Ni crucible by raising temperature to 827 K and cooling it to the room temperature. To reduce the corrosion by impurities such as H2O, the block of FLiNaK containing Ti powder is fabricated in an Ar atmosphere with 20 ppm or less H2O concentration. The samples are Stainless Steel 430 (SS430) as a simulant of structural materials. They are rectangles of 40 × 10 mm2 cut out from one single SS430 plate containing 16–18 wt% Cr (Nilaco Co.), the thickness of which is 0.30 mm. In order to equalize initial states, the polish of the samples is not given. After ultrasonic cleaning with ion exchanged water for 30 min, the weight, M0 [g], and the average lengths of the two sides, a0 and b0 [mm], of the sample are measured, and ultrasonic cleaning is performed again for 30 min. HF is produced by the following chemical reaction in which NiF2 is reduced to Ni at 823 K with an Ar gas containing 1.02 vol% H2,
Table 1 Standard electrode potential of main metals [2] used in molten salt blankets. Although not as strong as Be, Ti can also become a reducing agent in molten salt blankets. Half-Cell Reaction
Standard Electrode Potential [V]
Be2+ + 2e− ⇌ Be Ti2+ + 2e− ⇌ Ti V2+ + 2e− ⇌ V Cr2+ + 2e− ⇌ Cr Fe2+ + 2e− ⇌ Fe
−1.97 −1.63 −1.13 −0.90 −0.44
Fig. 1. Schematic diagram of an HF corrosion apparatus. So that a sample is corroded only by HF or FLiNaK, Ni or Ni based alloys are used, insulation is provided, and the sample is hung.
Table 2 Samples used in this experiment and the amount of mixed micro- or nano-sized Ti powder (μTi or nTi, respectively) and injected HF. All the samples are SS430. (A)–(E) and (O) are immersed into FLiNaK at 823 K for 6 h. Sample
μTi [wt%]
nTi [wt%]
Injected HF [mol/s]
(A) (B) (C) (D) (E) (O) (Z)
0.00 0.25 0.50 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.25 0.50 0.00 0.00
4 × 10−7 4 × 10−7 4 × 10−7 4 × 10−7 4 × 10−7 0 Before experiment
H2 + NiF2 → Ni + 2HF.
(4)
The equilibrium constant of this reaction is thermodynamically calculated as 7.3 × 104Pa. The HF-containing gas through FLiNaK reaches a two-stage water bubbler (see Fig. 1), and HF is completely collected in the bubbler because the solubility of HF into water is adequately high. The amount of produced HF in 6 h was 8 × 10−3 mol measured by titration. This value roughly agrees with 9 × 10−3 mol obtained from thermodynamic calculation. A production rate of HF is 4 × 10−7mol/s calculated by dividing 8 × 10−3 mol by 6.0 h. Assuming that HF is uniformly produced in 30 mL of FLiNaK, the production rate of HF per unit volume of molten salts is roughly 1000 times
Fig. 2. Corrosion weight loss and calculated corrosion rates of SS430 immersed at 823 K for 6 h into FLiNaK either with μTi or without μTi. Mixing μTi into FLiNaK can suppress the HF corrosion of SS430.
2. Experimental Fig. 1 shows a schematic diagram of an apparatus for the HF corrosion experiment. The size of the apparatus is about 30 × φ15 cm. So that a sample is corroded only by HF or FLiNaK, materials and shapes of 2
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Fig. 3. SEM surface images of SS430 immersed at 823 K for 6 h into FLiNaK either with μTi or without μTi. It can be predicted that the HF corrosion of SS430 is more suppressed by mixing 0.50 wt% μTi than by mixing 0.25 wt%.
with ion exchanged water for 30 min. The sample is split, and the weight, M1 [g], and lengths of two sides, a1 and b1 [mm], of a part of the sample which is in FLiNaK during the six-hour immersion are measured. The corrosion weight loss of the sample, W [mg/cm2], is derived by the following equation: a b
W=
M0 a 1 b1 − M1 0 0
2a1 b1
× 105.
(5)
Note that the area of the sample is regarded as only its front and back rectangles, ignoring its side faces. Because b0 = b1 in all the samples, the error in Eq. (5), δW[mg/cm2], is calculated by the following equation using the partial differential and measurement accuracy of each parameter, Fig. 4. Corrosion weight loss and calculated corrosion rates of SS430 immersed at 823 K for 6 h into FLiNaK either with nTi or without nTi. Mixing nTi into FLiNaK can suppress the HF corrosion of SS430.
higher than that of TF estimated in FFHR blankets. The Ni crucible containing the block of FLiNaK and the sample are attached to the apparatus, and the crucible is heated up to 823 K by a heater wound around the apparatus. After FLiNaK is melted, the sample, the gas injection tube, and the thermocouple are immersed into melted FLiNaK via Ultra-TORR (Swagelok®) feedthrough, and then the production of HF is started. The total gas flow rate is 5.0 × 10−7 m3/s. The immersion is maintained for 6.0 h. The sample, the gas injection tube, and the thermocouple are thereafter raised, and the apparatus is cooled down to the room temperature in several hours. The sample is extracted from the apparatus and subjected to the ultrasonic cleaning
δW =
∂W ∂W ∂W ∂W ∂W δM0 + δM1 + δa0 + δa1 + δb0 ∂M0 ∂M1 ∂a0 ∂a1 ∂b0
=
δM0 δM1 M δa M δa M0 M1 ⎞ + + 0 2 0 + 12 1 + ⎜⎛ − ⎟ δb0 2 2a0 b0 2a1 b0 2a0 b0 2a1 b0 2a1 b02 ⎠ ⎝ 2a 0 b0
(6)
where δM0, δM1, δa0, δa1, δb0 represent the measurement accuracy of M0, M1, a0, a1, b0, respectively. Assuming entire surface corrosion, the corrosion rate of the sample, v [mm/year], is calculated from the following equation:
v=
24 × 365 W × 10−2 = 1.9W 6.0 7.7
(7)
where the immersion time and density of SS430 is set to 6.0 h and 7.7 g/cm3, respectively. Surfaces of the samples are observed by SEM (1000 x or 3000 x magnification) and analyzed by an Energy Dispersive X-ray
Fig. 5. SEM surface images of SS430 immersed at 823 K for 6 h into FLiNaK either with nTi or without nTi. Protuberances on surfaces of SS430 immersed with nTi may be the results of the less corrosion by HF.
3
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spectrometry (EDX).
SS430 is not suppressed to the same extent to which HF does not exist.
3. Results and discussion
3.3. Comparing two types of Ti powder
3.1. Mixing micro-sized Ti powder
The HF corrosion of SS430 in FLiNaK can be suppressed by both mixing μTi and mixing nTi. It has been found that the corrosion is suppressed by Ti powder regardless of its exposure to air. Moreover, the following outcome may be predicted. The Ti powder with from 12 μm to 100 nm of particle diameters can suppress the corrosion. Comparing SEM images in Figs. 3 and 5, even when the initial mass of mixed Ti powder is same, the extent of the suppression of the HF corrosion of SS430 in FLiNaK differs between μTi and nTi. It can be considered that the average particle diameter and number density of initial Ti powder influence the extent of the corrosion suppression. In order to evaluate the extent of the corrosion suppression by Ti powder from the corrosion weight loss of SS430, it is necessary to improve the measurement accuracy of this experiment.
At first, μTi, which is exposed to air, is investigated. Fig. 2 shows the weight loss and rates of the corrosion of SS430 immersed with or without using μTi. The left Y-axis represents the corrosion weight loss of SS430 obtained from the Eq. (5) whereas the right Y-axis represents corrosion rates of SS430 obtained from the Eq. (7). Error bars indicate the measurement accuracy obtained from the Eq. (6). Sample names, production rates of injected HF, and the amount of mixed μTi are described below each bar. The corrosion rate of (O), without HF injection, agrees within an error compared to that of JLF-1 by Kondo, 0.050 mm/year (tested in non-purified FLiNaK) [12], or that of Stainless Steel 316L by Sellers, 0.020 mm/year [13], which implies that the results obtained by this experiment are valid although the measurement accuracy of this experiment is inferior to that of the previous researches. The corrosion weight loss of (B) and (C) is less than that of (A). It has been found that by mixing μTi into FLiNaK, the HF corrosion of SS430 is suppressed. Fig. 3 shows SEM surface images of SS430 immersed either with μTi or without μTi. An observation area of upper images is 50 μm square while that of lower images is 15 μm square. Sample names, production rates of injected HF, and the amount of mixed μTi are described above each image. Compared to (Z), all the outermost surfaces of (C) exist without pit and slight grain boundary corrosion, which are close to surfaces of (O), whereas parts of the outermost surfaces of (B) and all the outermost surfaces of (A) are corroded and peeled off. It has been confirmed by these images as well that by mixing μTi into FLiNaK, the HF corrosion of SS430 is suppressed. Judging from these images, corrosion rates of samples seem slower in the order of (C), (B), and (A). The following outcome may be predicted. The corrosion of SS430 by HF is more suppressed by mixing 0.50 wt% μTi than by mixing 0.25 wt%. Considering Figs. 2 and 3, the corrosion weight loss and surface image of (C) are close to those of (O). The following outcome may be predicted. By mixing about 0.50 wt% μTi into FLiNaK, the HF corrosion of SS430 is suppressed to the same extent to which HF does not exist.
4. Conclusions Sacrificial corrosion protection, a new use of Ti powder, is proposed for the protection of structural materials from TF corrosion in molten fluoride salt blankets. The apparatus in which a sample is corroded only either by HF, which is a simulant of TF, or by FLiNaK is prepared. SS430, a simulant of structural materials, is immersed into FLiNaK at 823 K for 6 h. Before the immersion, either the micro-sized Ti powder exposed to air or the nano-sized Ti powder not exposed to air at all are mixed into FLiNaK. It has been found that both mixed micro-sized or mixed nano-sized Ti powder can suppress the HF corrosion of SS430 in FLiNaK, leading to the suppression occurring regardless of the exposure of Ti powder to air. In order to derive the exact relationship between the average particle diameter and corrosion suppression of Ti powder, further investigation, including the improvement of measurement accuracy, is required. References [1] A. Sagara, et al., Helical reactor design FFHR-d1 and c1 for steady-state DEMO, Fusion Eng. Des. 89 (2014) 2114–2120. [2] The Electrochemical Society of Japan, Electrochemical Handbook Ver.6 (in Japanese), Maruzen, Tokyo, 2013. [3] D. Olander, Redox condition in molten fluoride salts Definition and control, J. Nucl. Mater. 300 (2002) 270–272. [4] J.R. Keiser, et al., The Corrosion Resistance of Type 316 Stainless Steel to Li2BeF4, ORNL/TM-5782, Oak Ridge National Laboratory, 1977. [5] P. Calderoni, et al., Control of molten salt corrosion of fusion structural materials by metallic beryllium, J. Nucl. Mater. 386 (2009) 1102–1106. [6] S. Delpech, et al., Molten fluorides for nuclear applications, Mater. Today 13 (2010) 34–41. [7] J. Yagi, et al., Hydrogen solubility in FLiNaK mixed with titanium powder, Fusion Eng. Des. 98 (2015) 1907–1910. [8] R. Nishiumi, et al., Hydrogen permeation through fluoride molten salt mixed with Ti powder, Fusion Sci. Technol. 72 (2017) 747–752. [9] M. Tokushige, et al., Plasma-induced cathodic discharge electrolysis to form various Metal/Alloy nanoparticles, Russ. J. Electrochem. 46 (2010) 657–665. [10] Y. Ito, et al., Plasma-induced molten salt electrolysis to form functional fine particles, in: M. Gaune-Escard, K.R. Seddon (Eds.), Molten Salts and Ionic Liquids: Never the Twain? John Wiley & Sons, Inc., Hoboken, 2010, pp. 169–180. [11] T. Oishi, et al., Formation and size control of titanium particles by cathode discharge electrolysis of molten chloride, J. Appl. Electrochem. 32 (2002) 819–824. [12] M. Kondo, et al., Corrosion of reduced activation ferritic martensitic steel JLF-1 in purified Flinak at static and flowing conditions, Fusion Eng. Des. 85 (2010) 1430–1436. [13] R.S. Sellers, et al., Corrosion of 316L stainless steel alloy and hastelloy-N superalloy in molten eutectic liF-NaF-KF salt and interaction with graphite, Nucl. Technol. 188 (2014) 192–199.
3.2. Mixing nano-sized Ti powder Next, nTi, which is not exposed to air at all, is investigated. Fig. 4 shows the weight loss and rates of the corrosion of SS430 immersed with or without using nTi. Definitions of axes, error bars and results of (A) and (O) in Fig. 4 are the same as in Fig. 2. The corrosion weight loss of both (D) and (E) is less than that of (A). It has been found that by mixing nTi into FLiNaK, the HF corrosion of SS430 is suppressed. Fig. 5 shows SEM surface images of SS430 immersed either with nTi or without nTi. Sample names, production rates of injected HF, and the amount of mixed nTi are described above each image. The images of (A), (O), and (Z) are the same as in Fig. 3. Compared to (A), protuberances are observed on the surfaces of both (D) and (E). According to EDX analysis, these protuberances have the composition of SS430. It can be considered that these protuberances are the results of the less corrosion by HF. Considering Figs. 4 and 5, the following outcome may be predicted. Even if up to 0.50 wt% nTi is mixed into FLiNaK, the HF corrosion of
4
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Sacrificial effect of titanium powder on the corrosion by hydrogen fluoride in LiF-NaF-KF ⁎
Gaku Yamazakia, , Juro Yagia,b, Teruya Tanakaa,b, Takashi Watanabeb, Akio Sagaraa,b a b
SOKENDAI (The Graduate University for Advanced Studies), 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Molten salt blanket Corrosion Hydrogen fluoride Titanium Sacrificial protection
Sacrificial corrosion protection, a new use of Ti powder in molten salt blankets, is proposed. Redox reactions of mixed Ti powder into molten fluoride salts are predicted to suppress the TF corrosion of structural materials. Stainless Steel 430 (SS430) is immersed into FLiNaK containing HF at 823 K for 6 h. Before the immersion, the Ti powder either exposed to air or not exposed to air at all is mixed into FLiNaK. It has been found that either types of mixed Ti powder can suppress the HF corrosion of SS430.
1. Introduction
less expensive and easier to process. Although Ti is not as strong as Be as a reducing agent, the reaction of Ti with TF by the following chemical reaction, for example, is predicted in molten fluoride salts,
FFHR series, D-T fusion helical reactors proposed by National Institute for Fusion Science, have been designed to be equipped with self-cooled liquid blankets [1]. One of the coolants through these blankets has been molten fluoride salts such as FLiNaBe (LiF-NaF-BeF2). In blankets, the reaction of molten fluoride salts consisting of LiF with neutrons produces tritium fluoride (TF) by the following nuclear reactions, 6 7
LiF + n→ TF + 4 He,
LiF + n→ TF + 4 H e+ n.
(1)
TF is a strong oxidizing agent, and it influences the dissolution type corrosion of structural materials (M) caused by the following chemical reaction,
M+ zTF → MFz +
z T2 2
(2)
where z is a valence of dissolved M ions. In order to remove this TF, redox reactions caused by reducing agents have been studied. Table 1 shows the standard electrode potential of the metal to be used in blankets [2]. The lower this potential is, the stronger the metal is as a reducing agent. Beryllium (Be) rods react with hydrogen fluoride (HF) quickly enough to suppress the HF corrosion of iron immersed into FLiBe (LiF-BeF2) [3–6]. Redox reactions caused by Be rods are also predicted to remove TF in molten fluoride salts. In this experiment, attention is focused on titanium (Ti) which is
⁎
Ti + 3TF → TiF3 +
3 T2. 2
(3)
There are several advantages of powder over rods when Ti is mixed into molten fluoride salts. It is possible to accurately control the amount of added Ti in coolants and to spread Ti throughout blankets. And because no additional structure is necessary, it is easy to insert the powder into current blanket designs. The idea of mixing powder into molten fluoride salts is based on the system (called “S-system”) proposed by Sagara, in which the hydrogen absorption metal powder mixed into molten fluoride salts effectively increases their low hydrogen solubility [1]. By the use of Ti as a hydrogen absorption metal and FLiNaK (LiF-NaF-KF), Yagi et al. practically demonstrated the increase in hydrogen effective solubility [7], and Nishiumi et al. practically demonstrated the decrease in the hydrogen permeation through structural materials [8]. By the existence of Ti powder throughout coolants, the sacrificial reaction of Ti powder with TF (Eq. (3)) occurs instead of the corrosion of structural materials by TF (Eq. (2)). In this experiment, HF is used as a simulant of TF, and FLiNaK is used because FLiNaK is easy to handle compared to FLiNaBe. Further, the hydrogen behavior of FLiNaK resembles that of FLiNaBe. The sacrificial effect of Ti powder on the HF corrosion of structural materials in FLiNaK is investigated.
Corresponding author. E-mail address: [email protected] (G. Yamazaki).
https://doi.org/10.1016/j.fusengdes.2018.02.054 Received 15 November 2017; Received in revised form 8 February 2018; Accepted 16 February 2018 0920-3796/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Yamazaki, G., Fusion Engineering and Design (2018), https://doi.org/10.1016/j.fusengdes.2018.02.054
Fusion Engineering and Design xxx (xxxx) xxx–xxx
G. Yamazaki et al.
the apparatus are chosen as follows. Outward-facing parts are made of Stainless Steel 304 or 316, whereas parts directly in contact with FLiNaK or HF are made of nickel (Ni) or Ni-based alloys in order not to react with HF. A Ni crucible is used to contain FLiNaK. To prevent the galvanic corrosion by the contact between the sample and the Ni crucible, a sample is hung, and PTFE insulation is provided on an outer wall of the apparatus. A Monel 400 tube guides an argon (Ar) gas containing H2 and HF to FLiNaK. A thermocouple wrapped in an Inconel 600 sheath monitors the FLiNaK temperature. Table 2 shows types of samples used in this corrosion experiment. The amount and particle diameters of mixed Ti powder and the amount of injected HF are different. Note that wt% is based on the mass of FLiNaK. Before the corrosion experiment, 30 mL of FLiNaK (LiF-NaF-KF: 46.5–11.5–42.0 mol%; melting point 727 K) is prepared in the Ni crucible by the following steps. At first, LiF, NaF, and KF powder (Kanto Chemical Co.), the purity of which is 98.0%, 99.0%, and 99.0%, respectively, are mixed to produce powdered FLiNaK. Referring to Table 2, one of the following two types of Ti powder is used. One type is the Ti powder prepared by an electrochemical method. Because this Ti powder is not exposed to air at all, it can increase in hydrogen effective solubility in molten fluoride salts [7]. In other words, this Ti powder is favorable for mixture into molten fluoride salts. FLiNaK containing 2 wt% this Ti powder was prepared in melted FLiNaK by I’MSEP Co. by the use of “Plasma-Induced Cathodic Discharge Electrolysis” which allows Ti not to be exposed to the air at all [9–11]. This Ti powder has particle diameters of 100 nm or less, and is nano-sized Ti powder (hereinafter called “nTi”). The amount of nTi in FLiNaK is adjusted by the dilution with the powdered FLiNaK. The other type is the Ti powder mixed directly into the powdered FLiNaK, which is exposed to air. This Ti powder has particle diameters of 45 μm or less and 99.98% purity (Nilaco Co.), and is micro-sized Ti powder (hereinafter called “μTi”). A certain amount of μTi is mixed into the powdered FLiNaK. The average particle diameter of μTi is about 12 μm according to an observation by a scanning electron microscope (SEM). In either case, the amount of mixed Ti powder is determined based on the entire amount of produced HF. In this experiment, 0.25 wt% is the amount of the mixed Ti powder reacting with the entire amount of produced HF, and 0.50 wt% is a large enough amount compared to it. A block of FLiNaK containing Ti powder is prepared in the Ni crucible by raising temperature to 827 K and cooling it to the room temperature. To reduce the corrosion by impurities such as H2O, the block of FLiNaK containing Ti powder is fabricated in an Ar atmosphere with 20 ppm or less H2O concentration. The samples are Stainless Steel 430 (SS430) as a simulant of structural materials. They are rectangles of 40 × 10 mm2 cut out from one single SS430 plate containing 16–18 wt% Cr (Nilaco Co.), the thickness of which is 0.30 mm. In order to equalize initial states, the polish of the samples is not given. After ultrasonic cleaning with ion exchanged water for 30 min, the weight, M0 [g], and the average lengths of the two sides, a0 and b0 [mm], of the sample are measured, and ultrasonic cleaning is performed again for 30 min. HF is produced by the following chemical reaction in which NiF2 is reduced to Ni at 823 K with an Ar gas containing 1.02 vol% H2,
Table 1 Standard electrode potential of main metals [2] used in molten salt blankets. Although not as strong as Be, Ti can also become a reducing agent in molten salt blankets. Half-Cell Reaction
Standard Electrode Potential [V]
Be2+ + 2e− ⇌ Be Ti2+ + 2e− ⇌ Ti V2+ + 2e− ⇌ V Cr2+ + 2e− ⇌ Cr Fe2+ + 2e− ⇌ Fe
−1.97 −1.63 −1.13 −0.90 −0.44
Fig. 1. Schematic diagram of an HF corrosion apparatus. So that a sample is corroded only by HF or FLiNaK, Ni or Ni based alloys are used, insulation is provided, and the sample is hung.
Table 2 Samples used in this experiment and the amount of mixed micro- or nano-sized Ti powder (μTi or nTi, respectively) and injected HF. All the samples are SS430. (A)–(E) and (O) are immersed into FLiNaK at 823 K for 6 h. Sample
μTi [wt%]
nTi [wt%]
Injected HF [mol/s]
(A) (B) (C) (D) (E) (O) (Z)
0.00 0.25 0.50 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.25 0.50 0.00 0.00
4 × 10−7 4 × 10−7 4 × 10−7 4 × 10−7 4 × 10−7 0 Before experiment
H2 + NiF2 → Ni + 2HF.
(4)
The equilibrium constant of this reaction is thermodynamically calculated as 7.3 × 104Pa. The HF-containing gas through FLiNaK reaches a two-stage water bubbler (see Fig. 1), and HF is completely collected in the bubbler because the solubility of HF into water is adequately high. The amount of produced HF in 6 h was 8 × 10−3 mol measured by titration. This value roughly agrees with 9 × 10−3 mol obtained from thermodynamic calculation. A production rate of HF is 4 × 10−7mol/s calculated by dividing 8 × 10−3 mol by 6.0 h. Assuming that HF is uniformly produced in 30 mL of FLiNaK, the production rate of HF per unit volume of molten salts is roughly 1000 times
Fig. 2. Corrosion weight loss and calculated corrosion rates of SS430 immersed at 823 K for 6 h into FLiNaK either with μTi or without μTi. Mixing μTi into FLiNaK can suppress the HF corrosion of SS430.
2. Experimental Fig. 1 shows a schematic diagram of an apparatus for the HF corrosion experiment. The size of the apparatus is about 30 × φ15 cm. So that a sample is corroded only by HF or FLiNaK, materials and shapes of 2
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Fig. 3. SEM surface images of SS430 immersed at 823 K for 6 h into FLiNaK either with μTi or without μTi. It can be predicted that the HF corrosion of SS430 is more suppressed by mixing 0.50 wt% μTi than by mixing 0.25 wt%.
with ion exchanged water for 30 min. The sample is split, and the weight, M1 [g], and lengths of two sides, a1 and b1 [mm], of a part of the sample which is in FLiNaK during the six-hour immersion are measured. The corrosion weight loss of the sample, W [mg/cm2], is derived by the following equation: a b
W=
M0 a 1 b1 − M1 0 0
2a1 b1
× 105.
(5)
Note that the area of the sample is regarded as only its front and back rectangles, ignoring its side faces. Because b0 = b1 in all the samples, the error in Eq. (5), δW[mg/cm2], is calculated by the following equation using the partial differential and measurement accuracy of each parameter, Fig. 4. Corrosion weight loss and calculated corrosion rates of SS430 immersed at 823 K for 6 h into FLiNaK either with nTi or without nTi. Mixing nTi into FLiNaK can suppress the HF corrosion of SS430.
higher than that of TF estimated in FFHR blankets. The Ni crucible containing the block of FLiNaK and the sample are attached to the apparatus, and the crucible is heated up to 823 K by a heater wound around the apparatus. After FLiNaK is melted, the sample, the gas injection tube, and the thermocouple are immersed into melted FLiNaK via Ultra-TORR (Swagelok®) feedthrough, and then the production of HF is started. The total gas flow rate is 5.0 × 10−7 m3/s. The immersion is maintained for 6.0 h. The sample, the gas injection tube, and the thermocouple are thereafter raised, and the apparatus is cooled down to the room temperature in several hours. The sample is extracted from the apparatus and subjected to the ultrasonic cleaning
δW =
∂W ∂W ∂W ∂W ∂W δM0 + δM1 + δa0 + δa1 + δb0 ∂M0 ∂M1 ∂a0 ∂a1 ∂b0
=
δM0 δM1 M δa M δa M0 M1 ⎞ + + 0 2 0 + 12 1 + ⎜⎛ − ⎟ δb0 2 2a0 b0 2a1 b0 2a0 b0 2a1 b0 2a1 b02 ⎠ ⎝ 2a 0 b0
(6)
where δM0, δM1, δa0, δa1, δb0 represent the measurement accuracy of M0, M1, a0, a1, b0, respectively. Assuming entire surface corrosion, the corrosion rate of the sample, v [mm/year], is calculated from the following equation:
v=
24 × 365 W × 10−2 = 1.9W 6.0 7.7
(7)
where the immersion time and density of SS430 is set to 6.0 h and 7.7 g/cm3, respectively. Surfaces of the samples are observed by SEM (1000 x or 3000 x magnification) and analyzed by an Energy Dispersive X-ray
Fig. 5. SEM surface images of SS430 immersed at 823 K for 6 h into FLiNaK either with nTi or without nTi. Protuberances on surfaces of SS430 immersed with nTi may be the results of the less corrosion by HF.
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spectrometry (EDX).
SS430 is not suppressed to the same extent to which HF does not exist.
3. Results and discussion
3.3. Comparing two types of Ti powder
3.1. Mixing micro-sized Ti powder
The HF corrosion of SS430 in FLiNaK can be suppressed by both mixing μTi and mixing nTi. It has been found that the corrosion is suppressed by Ti powder regardless of its exposure to air. Moreover, the following outcome may be predicted. The Ti powder with from 12 μm to 100 nm of particle diameters can suppress the corrosion. Comparing SEM images in Figs. 3 and 5, even when the initial mass of mixed Ti powder is same, the extent of the suppression of the HF corrosion of SS430 in FLiNaK differs between μTi and nTi. It can be considered that the average particle diameter and number density of initial Ti powder influence the extent of the corrosion suppression. In order to evaluate the extent of the corrosion suppression by Ti powder from the corrosion weight loss of SS430, it is necessary to improve the measurement accuracy of this experiment.
At first, μTi, which is exposed to air, is investigated. Fig. 2 shows the weight loss and rates of the corrosion of SS430 immersed with or without using μTi. The left Y-axis represents the corrosion weight loss of SS430 obtained from the Eq. (5) whereas the right Y-axis represents corrosion rates of SS430 obtained from the Eq. (7). Error bars indicate the measurement accuracy obtained from the Eq. (6). Sample names, production rates of injected HF, and the amount of mixed μTi are described below each bar. The corrosion rate of (O), without HF injection, agrees within an error compared to that of JLF-1 by Kondo, 0.050 mm/year (tested in non-purified FLiNaK) [12], or that of Stainless Steel 316L by Sellers, 0.020 mm/year [13], which implies that the results obtained by this experiment are valid although the measurement accuracy of this experiment is inferior to that of the previous researches. The corrosion weight loss of (B) and (C) is less than that of (A). It has been found that by mixing μTi into FLiNaK, the HF corrosion of SS430 is suppressed. Fig. 3 shows SEM surface images of SS430 immersed either with μTi or without μTi. An observation area of upper images is 50 μm square while that of lower images is 15 μm square. Sample names, production rates of injected HF, and the amount of mixed μTi are described above each image. Compared to (Z), all the outermost surfaces of (C) exist without pit and slight grain boundary corrosion, which are close to surfaces of (O), whereas parts of the outermost surfaces of (B) and all the outermost surfaces of (A) are corroded and peeled off. It has been confirmed by these images as well that by mixing μTi into FLiNaK, the HF corrosion of SS430 is suppressed. Judging from these images, corrosion rates of samples seem slower in the order of (C), (B), and (A). The following outcome may be predicted. The corrosion of SS430 by HF is more suppressed by mixing 0.50 wt% μTi than by mixing 0.25 wt%. Considering Figs. 2 and 3, the corrosion weight loss and surface image of (C) are close to those of (O). The following outcome may be predicted. By mixing about 0.50 wt% μTi into FLiNaK, the HF corrosion of SS430 is suppressed to the same extent to which HF does not exist.
4. Conclusions Sacrificial corrosion protection, a new use of Ti powder, is proposed for the protection of structural materials from TF corrosion in molten fluoride salt blankets. The apparatus in which a sample is corroded only either by HF, which is a simulant of TF, or by FLiNaK is prepared. SS430, a simulant of structural materials, is immersed into FLiNaK at 823 K for 6 h. Before the immersion, either the micro-sized Ti powder exposed to air or the nano-sized Ti powder not exposed to air at all are mixed into FLiNaK. It has been found that both mixed micro-sized or mixed nano-sized Ti powder can suppress the HF corrosion of SS430 in FLiNaK, leading to the suppression occurring regardless of the exposure of Ti powder to air. In order to derive the exact relationship between the average particle diameter and corrosion suppression of Ti powder, further investigation, including the improvement of measurement accuracy, is required. References [1] A. Sagara, et al., Helical reactor design FFHR-d1 and c1 for steady-state DEMO, Fusion Eng. Des. 89 (2014) 2114–2120. [2] The Electrochemical Society of Japan, Electrochemical Handbook Ver.6 (in Japanese), Maruzen, Tokyo, 2013. [3] D. Olander, Redox condition in molten fluoride salts Definition and control, J. Nucl. Mater. 300 (2002) 270–272. [4] J.R. Keiser, et al., The Corrosion Resistance of Type 316 Stainless Steel to Li2BeF4, ORNL/TM-5782, Oak Ridge National Laboratory, 1977. [5] P. Calderoni, et al., Control of molten salt corrosion of fusion structural materials by metallic beryllium, J. Nucl. Mater. 386 (2009) 1102–1106. [6] S. Delpech, et al., Molten fluorides for nuclear applications, Mater. Today 13 (2010) 34–41. [7] J. Yagi, et al., Hydrogen solubility in FLiNaK mixed with titanium powder, Fusion Eng. Des. 98 (2015) 1907–1910. [8] R. Nishiumi, et al., Hydrogen permeation through fluoride molten salt mixed with Ti powder, Fusion Sci. Technol. 72 (2017) 747–752. [9] M. Tokushige, et al., Plasma-induced cathodic discharge electrolysis to form various Metal/Alloy nanoparticles, Russ. J. Electrochem. 46 (2010) 657–665. [10] Y. Ito, et al., Plasma-induced molten salt electrolysis to form functional fine particles, in: M. Gaune-Escard, K.R. Seddon (Eds.), Molten Salts and Ionic Liquids: Never the Twain? John Wiley & Sons, Inc., Hoboken, 2010, pp. 169–180. [11] T. Oishi, et al., Formation and size control of titanium particles by cathode discharge electrolysis of molten chloride, J. Appl. Electrochem. 32 (2002) 819–824. [12] M. Kondo, et al., Corrosion of reduced activation ferritic martensitic steel JLF-1 in purified Flinak at static and flowing conditions, Fusion Eng. Des. 85 (2010) 1430–1436. [13] R.S. Sellers, et al., Corrosion of 316L stainless steel alloy and hastelloy-N superalloy in molten eutectic liF-NaF-KF salt and interaction with graphite, Nucl. Technol. 188 (2014) 192–199.
3.2. Mixing nano-sized Ti powder Next, nTi, which is not exposed to air at all, is investigated. Fig. 4 shows the weight loss and rates of the corrosion of SS430 immersed with or without using nTi. Definitions of axes, error bars and results of (A) and (O) in Fig. 4 are the same as in Fig. 2. The corrosion weight loss of both (D) and (E) is less than that of (A). It has been found that by mixing nTi into FLiNaK, the HF corrosion of SS430 is suppressed. Fig. 5 shows SEM surface images of SS430 immersed either with nTi or without nTi. Sample names, production rates of injected HF, and the amount of mixed nTi are described above each image. The images of (A), (O), and (Z) are the same as in Fig. 3. Compared to (A), protuberances are observed on the surfaces of both (D) and (E). According to EDX analysis, these protuberances have the composition of SS430. It can be considered that these protuberances are the results of the less corrosion by HF. Considering Figs. 4 and 5, the following outcome may be predicted. Even if up to 0.50 wt% nTi is mixed into FLiNaK, the HF corrosion of
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