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α-Al2O3 coating on 21-6-9 austenitic stainless steel
Fusion Engineering and Design 148 (2019) 111280
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Optimum preparation of Fe-Al/α-Al2O3 coating on 21-6-9 austenitic stainless steel
T
Li Hua,b, Guikai Zhanga,b, Huan Wanga,b, Feilong Yanga, Xin Xianga,b, Meijuan Hua,b, Jiangli Caoc, ⁎ Tao Tanga,b, a
Institute of Materials, China Academy of Engineering Physics, P.O.Box 9071, Jiangyou 621907, China Science and Technology on Surface Physics and Chemistry Laboratory, P.O.Box 9-35, Mianyang 621908, China c Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Fusion Fe-Al/Al2O3 coating Tritium permeation barrier Oxidation kinetics Volmer-Weber model
To study the formation and growth mechanism of α-Al2O3 scales, the oxidation behavior of Fe-Al diffusion coatings on 21-6-9 austenitic stainless steel has been conducted within different oxygen partial pressure (PO2 ) at 980 °C. The pure α-Al2O3 is evident to form with PO2 ranging from 100 Pa to 1 kPa. The growing dynamics of αAl2O3 reveals two distinct stages including an initial linear oxidation regime and a later parabolic one. The initial oxidation rate constant is larger than that of the parabolic stage, and decreases with the increasing PO2 from 10 Pa to 1 kPa. The Volmer-Weber model is proposed to reveal the growing mechanism of α-Al2O3 scales on Fe-Al coating under different oxygen partial pressure. The deuterium permeation reduction factor (PRF) of Fe-Al/αAl2O3 coating increased by 2–3 orders of magnitude in the temperature ranging from 450 °C to 650 °C.
1. Introduction Tritium (T or 3H, also known as hydrogen-3) is a radioactive isotope of hydrogen. Tritium is used as a radioactive tracer, in radioluminescent light sources for watches and instruments, and, along with deuterium, as a fuel for nuclear fusion reactions with applications in energy generation. Tritium figures prominently in studies of controlled nuclear fusion because of its favorable reaction cross section and the large amount of energy (17.6 MeV) produced through its reaction with deuterium in both magnetic confinement and inertial confinement fusion reactor designs, such as the experimental fusion reactor ITER and the National Ignition Facility (NIF). However, naturally occurring tritium is extremely rare on Earth, where trace amounts are formed by the interaction of the atmosphere with cosmic rays. It can nearly only be produced by irradiating lithium metal or lithium-bearing ceramic pebbles or heavy-water in a nuclear reactor for practical applications. Tritium is an isotope of hydrogen, which allows it to readily bind to hydroxyl radicals, forming tritiated water (HTO), and to carbon atoms. Since tritium is a low energy beta emitter, it is not dangerous externally (its beta particles are unable to penetrate the skin), but it can be a radiation hazard when inhaled, ingested via food or water, or absorbed through the skin. Consequently, the confinement of tritium within the fuel cycling system is required for the safe application of tritium
⁎
especially in tritium-pumping pipes of fusion engineering and design [1,2]. It should be taken into account that the prevention of tritium permeation through structural materials [2]. Besides the high reliability of the structural design of tritium confinement and tritium handling systems, the tritium permeation barrier (TPB) coated on structural steel surface is one of the most effective method to minimize tritium permeation [3]. The aluminide coating on stainless steel with Fe-Al transition layer is generally considered to be one of the most effective coatings as tritium permeation barrier in fusion reactors. The hydrogen permeation reduction factor (PRF) is evaluated up to 103 or even tens of thousands [4]. The Fe-Al layer acting as a functional gradient transition layer can relieve thermal mismatch between Al2O3 ceramic and steel, moreover Fe-Al layer is evident to supply aluminum for self-healing of Al2O3 film when it is broken [5]. The preparation process of Fe-Al/Al2O3 coating generally involves two steps including aluminizing and oxidation. In the aluminizing step, the preparation technology can be classified as physical vapor deposition (PVD) [4], chemical vapor deposition (CVD) [6], hot-dipping aluminizing (HDA) [7], electrochemical deposition (ECD) [8], pack cementation (PC) [9] and sol–gel [10], etc. In the oxidation process, the formed Fe-Al transition layer needs to be selectively oxidized to form Al2O3 film.
Corresponding author at: Institute of Materials, China Academy of Engineering Physics, P.O.Box 9071, Jiangyou 621907, China. E-mail address: [email protected] (T. Tang).
https://doi.org/10.1016/j.fusengdes.2019.111280 Received 23 March 2019; Received in revised form 23 July 2019; Accepted 26 July 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Scheme of apparatus for gas driven permeation experiment.
Fig. 2. Typical SEM images of aluminum layer deposited on 21-6-9 stainless steel substrate at a current density of 10 mA/cm2 for 100 min: (a) surface and (b) cross section.
Fig. 3. EDS (a) and XRD (b) patterns of Fe-Al coating on 21-6-9 steel substrate after heat treatment at 750 °C for 4 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Comparing with alumina, the diffusion coefficient of tritium in FeAl alloy is higher, so the Al2O3 thin film of Fe-Al/Al2O3 composite coating is the key factor to determining the tritium resistant performance [11]. Al2O3 includes one stable phase (α) and a variety of metastable phases (β, γ, θ, etc.). The stable α phase is considered as the most effective tritium permeation barrier [12]. Previous investigation shows that PRF of α-Al2O3 is over 103 while γ-Al2O3 is only 40–70 [13].
In recent years, much efforts were expended trying to prepare the Fe (Cr)-Al/α-Al2O3 layer on various structural steels, such as China low activation martensitic (CLAM) steel [14], P91 steel–reduced activation ferritic martensitic steels (RAFMS) [15], Fe-Cr-Al ferritic steel [16] and SUS430 stainless steel [17]. According to Ellingham-Richardson diagrams, the Al2O3 thin film is more easily formed on Fe-Al surface under lower oxygen partial pressure [18]. It indicates that oxygen partial 2
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2. Experiments 2.1. Preparation of Fe-Al diffusion layer Fe-Al coating on substrate was prepared by a 2-step process of Alelectroplating from ionic liquid followed by heat treatment [8]. The substrate material was 21-6-9 austenitic steel with a nominal composition of Fe–0.03C–1Si–21Cr–6Ni–8Mn (mass%). The specimens were cut into size of 3 mm × 4 mm × 5 mm for TGA experiment and Φ20 mm × 1 mm for gas driving permeation (GDP) experiment. Polished samples with a mirror-like surface were used for other analysis, which were 20 mm × 20 mm × 4 mm. All specimens were ultrasonically cleaned in acetone followed by alcohol for 10 min. Afterwards, they were quickly transferred into the argon-gas glovebox with an atmosphere of water and oxygen below 1 ppm at room temperature. Then a current density of 10 mA/cm2 was employed for 100 min on the electro-deposition of Al on the specimen with an ionic liquid (molar ratio 1:2) of 1-ethyl-3-methylimidazolium chloride (EMIC) and aluminum chloride (AlCl3). Finally, the Al coated specimen was heated at 750 °C for 4 h in Ar and then cooled in furnace.
Fig. 4. XRD pattern of the specimen oxidized under different oxygen partial pressure at 980 °C for 1 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.2. Oxidation of Fe-Al diffusion layer The Fe-Al coating on substrate was oxidized in a furnace at 980 °C for 1 h under different oxygen partial pressures (1 Pa, 10 Pa, 100 Pa, 1 kPa, 10 kPa, 20 kPa) and then cooled in furnaces. To study the oxidation kinetics of the Fe-Al diffusion layer, the thermo-gravimetric analyzer (STA-449C, mass resolution is 0.1 μg) was used to measure the isothermal oxidation kinetics of Fe-Al diffusion layer at 980 °C for 14 h. The optimized carrier gas flow rate was 20 ml/min mixing with the oxygen partial pressure of 1 Pa, 10 Pa, 100 Pa, 1 kPa, 10 kPa, 20 kPa, respectively. 2.3. Gas driving permeation experiment Fig. 5. XPS spectrum of Fe2p3 oxidized under different oxygen partial pressure at 980 °C for 1 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Deuterium permeability of uncoated and coated sample was measured by helium leak detector (Lab 1000+). As shown in Fig. 1 the driven pressure of deuterium with a nominal purity of 99.7% was 105 Pa. The sample was heated in furnace, and the temperature was set at 450 °C, 500 °C, 550 °C, 600 °C and 650 °C respectively in GDP experiment.
pressure always plays an important role in the selective oxidation of Alcontaining alloys [19]. In present work, specimens of Fe-Al diffusion coating on 21-6-9 austenitic stainless steel were oxidized at high temperature and under different oxygen partial pressure (1 Pa–20 kPa) hoping to form the stable α phase. Thermo-gravimetric analysis (TGA) isothermal oxidation kinetic experiments with different characterization technology such as reflection spectrum, grazing incidence angle X-ray diffraction (GXRD) and scanning electron microscope (SEM) were used to investigate the growth mechanism of oxide film on Fe-Al diffusion layer.
2.4. Characterization techniques Phase structure of the oxide scale was analyzed by GXRD (X Pert pro, incident angle of 0.25° and 2θ of 20°–90°). The chemical states and the thickness of oxidized scale were identified by X-ray photoelectron spectroscopy (XPS) and reflection spectrum (Ocean optics, HR4000).
Fig. 6. Reflection spectrum (a) from the top of specimen and the thickness (b) of alumina thin film oxidized under different oxygen partial pressure at 980 °C for 1 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 3
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Fig. 7. SEM images of surface of specimen oxidized under different oxygen partial pressure at 980 °C for 1 h: (a)1 Pa; (b)10 Pa; (c)100 Pa; (d)1 kPa; (e)10 kPa; (f) 20 kPa. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Oxidation kinetic curves of specimen oxidized under different oxygen partial pressure at 980 °C for 1 h: (a)1 Pa; (b)10 Pa; (c)100 Pa; (d)1 kPa; (e) 10 kPa; (f)20 kPa. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. XPS spectrum of Fe2p3 oxidized in 20 kPa O2 at 980 °C for 700 min. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Results and discussion
The Cross-section and surface morphology of the coatings were observed using scanning electronic microscope (SEM, Sirion-200), chemical composition was analyzed with energy dispersive spectroscopy (EDS). The samples of the alumina film for cross-section TEM were prepared by FIB (FEI, Helios NanoLab 600i). The selected area diffraction was identified by transmission electron microscope(JEM2100F).
3.1. Preparation of Fe-Al diffusion layer The aluminum coatings on 21-6-9 steel displayed smooth, brightness and good combination with the surface of substrate by visual inspection. Fig. 2(a) showed the surface morphology of aluminum coating. The substrate was covered with continuous and uniform 4
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Table 1 TGA curve fitting results denoted by relationship between specimen mass gain (y) and oxidation time (t) as simulation formula y = f(t) under different oxygen partial pressure (PO2 ). PO2 /Pa
Kp1
Kp2
1.0
y = f(t) R2
y = 6.44 × 10−3t − 3.18 × 10−2 0.98777
y = 0.25 lg(5.41t − 427.74) 0.99011
10
y = f(t) n R2
y = 4.73 × 10−3t + 2.64 × 10−3 – 0.98127
yn = 1.58 × 10−4t + 2.94 × 10−3 4.08 0.97857
1k
y = f(t) n R2
y = 1.58 × 10−3t + 9.59 × 10−2 – 0.98980
yn = 1.06 × 10−4t + 4.11 × 10−3 3.21 0.99609
10k
y = f(t) n R2
y = 3.24 × 10−3t − 9.97 × 10−3 – 0.99051
yn = 3.66 × 10−5t − 1.93 × 10−2 2.93 0.99757
20k
y = f(t) n R2
y = 9.90 × 10−3t − 0.19 – 0.97175
yn = 1.27 × 10−3t + 5.43 × 10−4 2.54 0.99848
Fig. 10. The EDS mapping of O element on the surface of specimen oxidized under different oxygen partial pressure at 980 °C for 1 h. (a)1 Pa; (b)10 Pa; (c)100 Pa; (d) 1 kPa; (e)10 kPa; (f)20 kPa. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
27 μm was formed on the substrate surface. The typical cross section microstructure of the aluminized coating was shown in Fig. 3(a). The aluminized coating showed a double-layer structure with a higher Al content in the outer layer according to EDS result (Fig. 3(a)). The firm metallurgical bonding between 21-6-9 stainless steel and aluminized
aluminum coating growing in a spherical or pyramidal shape. The thickness of aluminum layer measured by cross section examination was about 20 μm as shown in Fig. 2(b). It was obvious that the aluminum layers well adhered to the substrate without cracks or voids. After heat treatment, an aluminized coating with a thickness of 5
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Fig. 11. The Volmer-Weber model of alumina film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
shown in Fig. 5. As a result, selective oxidation of aluminum was more easily occurred under lower oxygen partial pressure, which was in good accordance with literature [19], and the α-Al2O3 formed under the oxygen partial pressure ranging from 100 Pa to 1 kPa. The reflection spectrum on the surface of specimen was illustrated in Fig. 6(a) which showed a typical interference pattern inferring that the thickness of Al2O3 scale was about 500–700 nm (Fig. 6(b)). With the increase in oxygen partial pressure, the thickness of Al2O3 film decreased first and then increased, reaching the minimum while PO2 ≈ 1 kPa . Surfaces of specimens oxidized under low oxygen partial pressure (1 Pa–1 kPa) showed dense and homogeneous alumina scale morphology, with no spallation (Fig. 7(a)–(d)). However, under high oxygen partial pressure, some nodules (Fig. 7(e)) and pits (Fig. 7(f)) were observed. Boggs [22] also found nodules on Fe10Al at 600 °C and their analysis showed the nodules were consisted of Fe2O3, FeO and FeAl2O4. Saegusa [23] noted the distribution of nodules was located at highly stressed corners and edges of specimens. High aluminum content contributed to restraining the iron oxide nodules discovered by Tomaszewicz [21]. Combining with the XRD results given in Fig. 4 and XPS spectrum in Fig. 5, the nodules observed in Fig. 7(e) might be iron oxide nodules.
Fig. 12. Deuterium permeability of Fe-Al/Al2O3 coated and uncoated 21-6-9 steel piece. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
coating was obtained without visual cracks or any visible metallurgy flaw. A flat interface between the aluminized coating and substrate indicated that the interdiffusion between Al atoms and Fe atoms was homogeneous. The XRD pattern (Fig. 3(b)) identified that the aluminized coating was consisted of FeAl2, FeAl and Fe3Al. This kind of structure with high Al content could provide the Al source for selfhealing [20].
3.3. Oxidation kinetics of Fe-Al coating An important method to study the growth mechanism of film is the isothermal oxidation kinetics. Fig. 8 showed the corresponding TGA curves of Fe-Al transition layer oxidation reaction under different oxygen partial pressure (1 Pa–20 kPa) at 980 °C for 14 h. First, the net mass gain of the samples increased linearly with oxidation time. Afterwards, the mass increasement rate decreased, indicating that the formed oxide film on surface had an excellent resistance to high temperature oxidation and also exhibited a good compactness of film. However, the curve of 20 kPa raised again after 700 min, indicating that long exposure time at such condition caused the alumina film to break and the iron oxide was formed as shown in Fig. 9. Lee [24] suggested that the mass gain of Fe-Al alloys during oxidation process followed a relationship of the form:
3.2. Characterization of oxidized coating Tomaszewicz [21] reported that a complete protective Al2O3 scale was formed only when aluminum content in Fe-Al alloys was more than 6.9 wt.%, i.e. a critical concentration of Al selective oxidation on Fe-Al alloys surface should be more than 6.9 wt.%. In present work, the Al content of as-prepared Fe-Al transition layer varying from 64.0–70 atm. % (resulted from the EDS analysis in Fig. 3(a)), which met such critical concentration, so the selective oxidation should occur theoretically. After oxidation at 980 °C for one hour, the surface of specimen was integrity and without peeling or spallation by visual observation. The specimens oxidized at the low oxygen partial pressure (1 Pa–1 kPa) displayed a reddish green color with metal luster, whereas the specimens oxidized at the oxygen partial pressure of 10 kPa and 20 kPa O2 became dull. XRD pattern (Fig. 4) identified that the coatings were mainly consisted of Fe3Al, FeAl and α-Al2O3. In addition, the γ-Al2O3 was only detected under low oxygen partial pressure (1 Pa and 10 Pa), indicating that the γ phase was not completely transformed into α-Al2O3 phase in such low O2 pressure. By increasing the PO2 to 100 Pa, alumina film presented almost pure α phase. Additionally, as the PO2 reached to 10 kPa, the iron oxide was evident to form by employing the XPS as
(ΔM )n = Kp t + C
(1)
where ΔM is the mass gain, n is the rate exponent, Kp is the isothermal rate constant, t is the exposure time at particular temperature and C is constant. Table 1 showed the simulated parameters in Eq. (1). The obtained results showed that the growth of Al2O3 film exhibits two distinct stages: (a) a linear oxidation stage; (b) a parabolic oxidation stage. Interesting, the Kp1 decreased first and then increased, reaching the minimum of 1.58 × 10−3 mg/min in 1 kPa O2, with the increase in oxygen partial pressure from 1 Pa to 20 kPa, in good accordance with reflection spectrum results (Fig. 6). As shown in Table 1, the Kp1 6
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Fig. 13. TEM selection area diffraction pattern of the surface oxide film at 1 kPa oxygen partial pressure at 980 °C for 1 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
ranging from 1.58 × 10−3 mg/min to 9.90 × 10−3 mg/min was larger than the Kp2, which showed that the completely covered, dense and homogeneous alumina scale formed in the initial of oxidation, which prevented further oxidation in the later stage, resulting into the smaller Kp2. However, The significantly increasing Kp2 of the oxidation at the oxygen partial pressure of 20 kPa indicated that the oxidation-resistance of the film became poor, suggesting that the film should not consist of dense and homogeneous alumina. This result is in good accordance with surface SEM results (Fig. 7(f)). Thus, the completely covered α-Al2O3 scale should form more faster in the initial of oxidation with the oxygen partial pressure increasing from 10 Pa to 1 kPa.
growth of Al2O3 film should follow island model namely Volmer-Weber model [25], in which there were three distinctive growth steps (Fig. 11). Firstly, the film started with an growth by nucleation of discrete islands. Secondly the coalescence process occurred. Finally, the third phase of the Volmer-Weber growth should be dominated by the post-coalescence process, where the formed film continued to increase until a predefined thickness reached. In the first step the distribution of islands was discrete. This should lead to more oxygen contacting the substrate, so the net mass gain reached maximum initially. The increasing the oxygen partial pressure could accelerate the transition from nucleation of discrete islands to coalescence process [14]. Thus, the first step transferred more quickly to the second step with the oxygen partial pressure increasing, and the formed complete alumina film could prevent further oxidation, resulting in the reduction of mass gain. However the formed iron oxides in high oxygen partial pressure (10 kPa, 20 kPa) reduced the high temperature resistance to oxygen, leading to the increase of mass gain again.
3.4. Island (Volmer-Weber) model of alumina film EDS maps of O element under different oxygen partial pressure were shown in Fig. 10. The O element dispersed on the surface of film at oxygen partial pressure of 1–10 Pa, and became gradually dense with the increase in the oxygen partial pressure and was full of film surface as the oxygen partial pressure higher than 1 kPa. This indicated 21-6-9 substrate surface exposed to oxygen should decrease with the oxygen partial pressure increasing, leading to the better oxidation resistance of substrate at higher oxygen partial pressure, in agreement with that kp1 of the initial oxidation stage decreases from 6.44 × 10−3 (mg/min) to 1.58 × 10−3 (mg/min) (Table 1). Thus, the thickness of scale would decrease with the oxygen partial pressure increasing while exposing the same time and reached the minimum at 1 kPa O2. This was consistent with the reflection result showed in Fig. 6(a). Whereas kp1 of the initial oxidation stage at 10 kPa and 20 kPa sharply increased due to poor oxidation-resistance of mix-oxides of Al and Fe (Fig. 5), resulting in the scale thickness increasing at the oxygen partial pressure of 10Pa–20 kPa. Thus, combining with the EDS mapping, oxidation net mass gain (Fig. 8) and reflection results(Fig. 6), we proposed a hypothesis that the
3.5. Deuterium permeation properties The piece for GDP experiment was oxidized under oxygen partial pressure of 1 kPa. Fig. 12 showed the deuterium permeation rate of FeAl/Al2O3 coated 21-6-9 steel and uncoated piece as a comparison. While the oxidation time was 1 h, the deuterium permeability was only reduced by one order of magnitude. With the increase of oxidation time, the PRF increased. As the oxidation time increased to 3 h, the coated piece showed good resistance to deuterium, giving a reduction by 2–3 orders of magnitude, i.e. the PRF is about 1261 at 450 °C and 491 at 650 °C. The diffraction pattern of piece oxidized for 1 h showed α-Al2O3 and γ-Al2O3 shown in Fig. 13. This mixed coating structure led to the decreasing of PRF: many lattice distortions existed between αAl2O3 and amorphous phases of the coating and deuterium transport 7
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was possible along boundaries of the α-Al2O3 crystals. Such results were corresponding to Levchuk [13]. However, γ-Al2O3 was not detected by XRD (Fig. 4), indicating that the amount of γ-Al2O3 was small.
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4. Conclusion The α-Al2O3 thin film with the thickness of 500–700 nm was formed on Fe-Al diffusion coating. Selective oxidation of aluminum occurred under lower oxygen partial pressure of 1 Pa–10 kPa. The stable-state αAl2O3 was more easily to form under oxygen partial pressure of 100 Pa–1 kPa. The oxidation kinetics of Fe-Al layer illustrated the oxidation process had two stages, a linear oxidation stage in the initial followed by a parabolic oxidation stage. The Fe-Al/α-Al2O3 coating performed good high temperature oxidation resistance under oxygen partial pressure of 1 Pa–10 kPa. The Volmer-Weber model can be well used to explain the growth mechanism of alumina thin film. The deuterium permeation rate through the piece with oxidized for 3 h was reduced by 2–3 orders of magnitude at 450°C ∼650°C, and the ability of deuterium permeation resistance was increased with the oxidation time increased. Funding This work is supported by National Natural Science Foundation (No. 21471137) and National Magnetic Confinement Fusion Science Program (No. 2017YFE0300304) of China. References [1] I. Cristescu, I. Cristescu, L. Doerr, M. Glugla, D. Murdoch, Tritium inventories and tritium safety design principles for the fuel cycle of ITER, Nucl. Fusion 47 (2007) S458–S463. [2] S.-E. Wulf, N. Holstein, W. Krauss, J. Konys, Influence of deposition conditions on the microstructure of Al-based coatings for applications as corrosion and anti-permeation barrier, Fusion Eng. Des. 88 (9) (2013) 2530–2534. [3] R. Causey, R. Karnesky, C. San Marchi, 4.16 – tritium barriers and tritium diffusion in fusion reactors, in: R.J. Konings (Ed.), Comprehensive Nuclear Materials, vol. 4, Elsevier, Oxford, 2012, pp. 511–549. [4] E. Serra, P.J. Kelly, D.K. Ross, R.D. Arnell, Alumina sputtered on MANET as an effective deuterium permeation barrier, J. Nucl. Mater. 257 (2) (1998) 194–198. [5] J. Könys, W. Krauss, N. Holstein, Development of advanced Al coating processes for future application as anti-corrosion and T-permeation barriers, Fusion Eng. Des. 85 (10) (2010) 2141–2145.
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Optimum preparation of Fe-Al/α-Al2O3 coating on 21-6-9 austenitic stainless steel
T
Li Hua,b, Guikai Zhanga,b, Huan Wanga,b, Feilong Yanga, Xin Xianga,b, Meijuan Hua,b, Jiangli Caoc, ⁎ Tao Tanga,b, a
Institute of Materials, China Academy of Engineering Physics, P.O.Box 9071, Jiangyou 621907, China Science and Technology on Surface Physics and Chemistry Laboratory, P.O.Box 9-35, Mianyang 621908, China c Institute of Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Fusion Fe-Al/Al2O3 coating Tritium permeation barrier Oxidation kinetics Volmer-Weber model
To study the formation and growth mechanism of α-Al2O3 scales, the oxidation behavior of Fe-Al diffusion coatings on 21-6-9 austenitic stainless steel has been conducted within different oxygen partial pressure (PO2 ) at 980 °C. The pure α-Al2O3 is evident to form with PO2 ranging from 100 Pa to 1 kPa. The growing dynamics of αAl2O3 reveals two distinct stages including an initial linear oxidation regime and a later parabolic one. The initial oxidation rate constant is larger than that of the parabolic stage, and decreases with the increasing PO2 from 10 Pa to 1 kPa. The Volmer-Weber model is proposed to reveal the growing mechanism of α-Al2O3 scales on Fe-Al coating under different oxygen partial pressure. The deuterium permeation reduction factor (PRF) of Fe-Al/αAl2O3 coating increased by 2–3 orders of magnitude in the temperature ranging from 450 °C to 650 °C.
1. Introduction Tritium (T or 3H, also known as hydrogen-3) is a radioactive isotope of hydrogen. Tritium is used as a radioactive tracer, in radioluminescent light sources for watches and instruments, and, along with deuterium, as a fuel for nuclear fusion reactions with applications in energy generation. Tritium figures prominently in studies of controlled nuclear fusion because of its favorable reaction cross section and the large amount of energy (17.6 MeV) produced through its reaction with deuterium in both magnetic confinement and inertial confinement fusion reactor designs, such as the experimental fusion reactor ITER and the National Ignition Facility (NIF). However, naturally occurring tritium is extremely rare on Earth, where trace amounts are formed by the interaction of the atmosphere with cosmic rays. It can nearly only be produced by irradiating lithium metal or lithium-bearing ceramic pebbles or heavy-water in a nuclear reactor for practical applications. Tritium is an isotope of hydrogen, which allows it to readily bind to hydroxyl radicals, forming tritiated water (HTO), and to carbon atoms. Since tritium is a low energy beta emitter, it is not dangerous externally (its beta particles are unable to penetrate the skin), but it can be a radiation hazard when inhaled, ingested via food or water, or absorbed through the skin. Consequently, the confinement of tritium within the fuel cycling system is required for the safe application of tritium
⁎
especially in tritium-pumping pipes of fusion engineering and design [1,2]. It should be taken into account that the prevention of tritium permeation through structural materials [2]. Besides the high reliability of the structural design of tritium confinement and tritium handling systems, the tritium permeation barrier (TPB) coated on structural steel surface is one of the most effective method to minimize tritium permeation [3]. The aluminide coating on stainless steel with Fe-Al transition layer is generally considered to be one of the most effective coatings as tritium permeation barrier in fusion reactors. The hydrogen permeation reduction factor (PRF) is evaluated up to 103 or even tens of thousands [4]. The Fe-Al layer acting as a functional gradient transition layer can relieve thermal mismatch between Al2O3 ceramic and steel, moreover Fe-Al layer is evident to supply aluminum for self-healing of Al2O3 film when it is broken [5]. The preparation process of Fe-Al/Al2O3 coating generally involves two steps including aluminizing and oxidation. In the aluminizing step, the preparation technology can be classified as physical vapor deposition (PVD) [4], chemical vapor deposition (CVD) [6], hot-dipping aluminizing (HDA) [7], electrochemical deposition (ECD) [8], pack cementation (PC) [9] and sol–gel [10], etc. In the oxidation process, the formed Fe-Al transition layer needs to be selectively oxidized to form Al2O3 film.
Corresponding author at: Institute of Materials, China Academy of Engineering Physics, P.O.Box 9071, Jiangyou 621907, China. E-mail address: [email protected] (T. Tang).
https://doi.org/10.1016/j.fusengdes.2019.111280 Received 23 March 2019; Received in revised form 23 July 2019; Accepted 26 July 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Scheme of apparatus for gas driven permeation experiment.
Fig. 2. Typical SEM images of aluminum layer deposited on 21-6-9 stainless steel substrate at a current density of 10 mA/cm2 for 100 min: (a) surface and (b) cross section.
Fig. 3. EDS (a) and XRD (b) patterns of Fe-Al coating on 21-6-9 steel substrate after heat treatment at 750 °C for 4 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Comparing with alumina, the diffusion coefficient of tritium in FeAl alloy is higher, so the Al2O3 thin film of Fe-Al/Al2O3 composite coating is the key factor to determining the tritium resistant performance [11]. Al2O3 includes one stable phase (α) and a variety of metastable phases (β, γ, θ, etc.). The stable α phase is considered as the most effective tritium permeation barrier [12]. Previous investigation shows that PRF of α-Al2O3 is over 103 while γ-Al2O3 is only 40–70 [13].
In recent years, much efforts were expended trying to prepare the Fe (Cr)-Al/α-Al2O3 layer on various structural steels, such as China low activation martensitic (CLAM) steel [14], P91 steel–reduced activation ferritic martensitic steels (RAFMS) [15], Fe-Cr-Al ferritic steel [16] and SUS430 stainless steel [17]. According to Ellingham-Richardson diagrams, the Al2O3 thin film is more easily formed on Fe-Al surface under lower oxygen partial pressure [18]. It indicates that oxygen partial 2
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2. Experiments 2.1. Preparation of Fe-Al diffusion layer Fe-Al coating on substrate was prepared by a 2-step process of Alelectroplating from ionic liquid followed by heat treatment [8]. The substrate material was 21-6-9 austenitic steel with a nominal composition of Fe–0.03C–1Si–21Cr–6Ni–8Mn (mass%). The specimens were cut into size of 3 mm × 4 mm × 5 mm for TGA experiment and Φ20 mm × 1 mm for gas driving permeation (GDP) experiment. Polished samples with a mirror-like surface were used for other analysis, which were 20 mm × 20 mm × 4 mm. All specimens were ultrasonically cleaned in acetone followed by alcohol for 10 min. Afterwards, they were quickly transferred into the argon-gas glovebox with an atmosphere of water and oxygen below 1 ppm at room temperature. Then a current density of 10 mA/cm2 was employed for 100 min on the electro-deposition of Al on the specimen with an ionic liquid (molar ratio 1:2) of 1-ethyl-3-methylimidazolium chloride (EMIC) and aluminum chloride (AlCl3). Finally, the Al coated specimen was heated at 750 °C for 4 h in Ar and then cooled in furnace.
Fig. 4. XRD pattern of the specimen oxidized under different oxygen partial pressure at 980 °C for 1 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.2. Oxidation of Fe-Al diffusion layer The Fe-Al coating on substrate was oxidized in a furnace at 980 °C for 1 h under different oxygen partial pressures (1 Pa, 10 Pa, 100 Pa, 1 kPa, 10 kPa, 20 kPa) and then cooled in furnaces. To study the oxidation kinetics of the Fe-Al diffusion layer, the thermo-gravimetric analyzer (STA-449C, mass resolution is 0.1 μg) was used to measure the isothermal oxidation kinetics of Fe-Al diffusion layer at 980 °C for 14 h. The optimized carrier gas flow rate was 20 ml/min mixing with the oxygen partial pressure of 1 Pa, 10 Pa, 100 Pa, 1 kPa, 10 kPa, 20 kPa, respectively. 2.3. Gas driving permeation experiment Fig. 5. XPS spectrum of Fe2p3 oxidized under different oxygen partial pressure at 980 °C for 1 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Deuterium permeability of uncoated and coated sample was measured by helium leak detector (Lab 1000+). As shown in Fig. 1 the driven pressure of deuterium with a nominal purity of 99.7% was 105 Pa. The sample was heated in furnace, and the temperature was set at 450 °C, 500 °C, 550 °C, 600 °C and 650 °C respectively in GDP experiment.
pressure always plays an important role in the selective oxidation of Alcontaining alloys [19]. In present work, specimens of Fe-Al diffusion coating on 21-6-9 austenitic stainless steel were oxidized at high temperature and under different oxygen partial pressure (1 Pa–20 kPa) hoping to form the stable α phase. Thermo-gravimetric analysis (TGA) isothermal oxidation kinetic experiments with different characterization technology such as reflection spectrum, grazing incidence angle X-ray diffraction (GXRD) and scanning electron microscope (SEM) were used to investigate the growth mechanism of oxide film on Fe-Al diffusion layer.
2.4. Characterization techniques Phase structure of the oxide scale was analyzed by GXRD (X Pert pro, incident angle of 0.25° and 2θ of 20°–90°). The chemical states and the thickness of oxidized scale were identified by X-ray photoelectron spectroscopy (XPS) and reflection spectrum (Ocean optics, HR4000).
Fig. 6. Reflection spectrum (a) from the top of specimen and the thickness (b) of alumina thin film oxidized under different oxygen partial pressure at 980 °C for 1 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 3
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Fig. 7. SEM images of surface of specimen oxidized under different oxygen partial pressure at 980 °C for 1 h: (a)1 Pa; (b)10 Pa; (c)100 Pa; (d)1 kPa; (e)10 kPa; (f) 20 kPa. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. Oxidation kinetic curves of specimen oxidized under different oxygen partial pressure at 980 °C for 1 h: (a)1 Pa; (b)10 Pa; (c)100 Pa; (d)1 kPa; (e) 10 kPa; (f)20 kPa. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. XPS spectrum of Fe2p3 oxidized in 20 kPa O2 at 980 °C for 700 min. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3. Results and discussion
The Cross-section and surface morphology of the coatings were observed using scanning electronic microscope (SEM, Sirion-200), chemical composition was analyzed with energy dispersive spectroscopy (EDS). The samples of the alumina film for cross-section TEM were prepared by FIB (FEI, Helios NanoLab 600i). The selected area diffraction was identified by transmission electron microscope(JEM2100F).
3.1. Preparation of Fe-Al diffusion layer The aluminum coatings on 21-6-9 steel displayed smooth, brightness and good combination with the surface of substrate by visual inspection. Fig. 2(a) showed the surface morphology of aluminum coating. The substrate was covered with continuous and uniform 4
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Table 1 TGA curve fitting results denoted by relationship between specimen mass gain (y) and oxidation time (t) as simulation formula y = f(t) under different oxygen partial pressure (PO2 ). PO2 /Pa
Kp1
Kp2
1.0
y = f(t) R2
y = 6.44 × 10−3t − 3.18 × 10−2 0.98777
y = 0.25 lg(5.41t − 427.74) 0.99011
10
y = f(t) n R2
y = 4.73 × 10−3t + 2.64 × 10−3 – 0.98127
yn = 1.58 × 10−4t + 2.94 × 10−3 4.08 0.97857
1k
y = f(t) n R2
y = 1.58 × 10−3t + 9.59 × 10−2 – 0.98980
yn = 1.06 × 10−4t + 4.11 × 10−3 3.21 0.99609
10k
y = f(t) n R2
y = 3.24 × 10−3t − 9.97 × 10−3 – 0.99051
yn = 3.66 × 10−5t − 1.93 × 10−2 2.93 0.99757
20k
y = f(t) n R2
y = 9.90 × 10−3t − 0.19 – 0.97175
yn = 1.27 × 10−3t + 5.43 × 10−4 2.54 0.99848
Fig. 10. The EDS mapping of O element on the surface of specimen oxidized under different oxygen partial pressure at 980 °C for 1 h. (a)1 Pa; (b)10 Pa; (c)100 Pa; (d) 1 kPa; (e)10 kPa; (f)20 kPa. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
27 μm was formed on the substrate surface. The typical cross section microstructure of the aluminized coating was shown in Fig. 3(a). The aluminized coating showed a double-layer structure with a higher Al content in the outer layer according to EDS result (Fig. 3(a)). The firm metallurgical bonding between 21-6-9 stainless steel and aluminized
aluminum coating growing in a spherical or pyramidal shape. The thickness of aluminum layer measured by cross section examination was about 20 μm as shown in Fig. 2(b). It was obvious that the aluminum layers well adhered to the substrate without cracks or voids. After heat treatment, an aluminized coating with a thickness of 5
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Fig. 11. The Volmer-Weber model of alumina film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
shown in Fig. 5. As a result, selective oxidation of aluminum was more easily occurred under lower oxygen partial pressure, which was in good accordance with literature [19], and the α-Al2O3 formed under the oxygen partial pressure ranging from 100 Pa to 1 kPa. The reflection spectrum on the surface of specimen was illustrated in Fig. 6(a) which showed a typical interference pattern inferring that the thickness of Al2O3 scale was about 500–700 nm (Fig. 6(b)). With the increase in oxygen partial pressure, the thickness of Al2O3 film decreased first and then increased, reaching the minimum while PO2 ≈ 1 kPa . Surfaces of specimens oxidized under low oxygen partial pressure (1 Pa–1 kPa) showed dense and homogeneous alumina scale morphology, with no spallation (Fig. 7(a)–(d)). However, under high oxygen partial pressure, some nodules (Fig. 7(e)) and pits (Fig. 7(f)) were observed. Boggs [22] also found nodules on Fe10Al at 600 °C and their analysis showed the nodules were consisted of Fe2O3, FeO and FeAl2O4. Saegusa [23] noted the distribution of nodules was located at highly stressed corners and edges of specimens. High aluminum content contributed to restraining the iron oxide nodules discovered by Tomaszewicz [21]. Combining with the XRD results given in Fig. 4 and XPS spectrum in Fig. 5, the nodules observed in Fig. 7(e) might be iron oxide nodules.
Fig. 12. Deuterium permeability of Fe-Al/Al2O3 coated and uncoated 21-6-9 steel piece. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
coating was obtained without visual cracks or any visible metallurgy flaw. A flat interface between the aluminized coating and substrate indicated that the interdiffusion between Al atoms and Fe atoms was homogeneous. The XRD pattern (Fig. 3(b)) identified that the aluminized coating was consisted of FeAl2, FeAl and Fe3Al. This kind of structure with high Al content could provide the Al source for selfhealing [20].
3.3. Oxidation kinetics of Fe-Al coating An important method to study the growth mechanism of film is the isothermal oxidation kinetics. Fig. 8 showed the corresponding TGA curves of Fe-Al transition layer oxidation reaction under different oxygen partial pressure (1 Pa–20 kPa) at 980 °C for 14 h. First, the net mass gain of the samples increased linearly with oxidation time. Afterwards, the mass increasement rate decreased, indicating that the formed oxide film on surface had an excellent resistance to high temperature oxidation and also exhibited a good compactness of film. However, the curve of 20 kPa raised again after 700 min, indicating that long exposure time at such condition caused the alumina film to break and the iron oxide was formed as shown in Fig. 9. Lee [24] suggested that the mass gain of Fe-Al alloys during oxidation process followed a relationship of the form:
3.2. Characterization of oxidized coating Tomaszewicz [21] reported that a complete protective Al2O3 scale was formed only when aluminum content in Fe-Al alloys was more than 6.9 wt.%, i.e. a critical concentration of Al selective oxidation on Fe-Al alloys surface should be more than 6.9 wt.%. In present work, the Al content of as-prepared Fe-Al transition layer varying from 64.0–70 atm. % (resulted from the EDS analysis in Fig. 3(a)), which met such critical concentration, so the selective oxidation should occur theoretically. After oxidation at 980 °C for one hour, the surface of specimen was integrity and without peeling or spallation by visual observation. The specimens oxidized at the low oxygen partial pressure (1 Pa–1 kPa) displayed a reddish green color with metal luster, whereas the specimens oxidized at the oxygen partial pressure of 10 kPa and 20 kPa O2 became dull. XRD pattern (Fig. 4) identified that the coatings were mainly consisted of Fe3Al, FeAl and α-Al2O3. In addition, the γ-Al2O3 was only detected under low oxygen partial pressure (1 Pa and 10 Pa), indicating that the γ phase was not completely transformed into α-Al2O3 phase in such low O2 pressure. By increasing the PO2 to 100 Pa, alumina film presented almost pure α phase. Additionally, as the PO2 reached to 10 kPa, the iron oxide was evident to form by employing the XPS as
(ΔM )n = Kp t + C
(1)
where ΔM is the mass gain, n is the rate exponent, Kp is the isothermal rate constant, t is the exposure time at particular temperature and C is constant. Table 1 showed the simulated parameters in Eq. (1). The obtained results showed that the growth of Al2O3 film exhibits two distinct stages: (a) a linear oxidation stage; (b) a parabolic oxidation stage. Interesting, the Kp1 decreased first and then increased, reaching the minimum of 1.58 × 10−3 mg/min in 1 kPa O2, with the increase in oxygen partial pressure from 1 Pa to 20 kPa, in good accordance with reflection spectrum results (Fig. 6). As shown in Table 1, the Kp1 6
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Fig. 13. TEM selection area diffraction pattern of the surface oxide film at 1 kPa oxygen partial pressure at 980 °C for 1 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
ranging from 1.58 × 10−3 mg/min to 9.90 × 10−3 mg/min was larger than the Kp2, which showed that the completely covered, dense and homogeneous alumina scale formed in the initial of oxidation, which prevented further oxidation in the later stage, resulting into the smaller Kp2. However, The significantly increasing Kp2 of the oxidation at the oxygen partial pressure of 20 kPa indicated that the oxidation-resistance of the film became poor, suggesting that the film should not consist of dense and homogeneous alumina. This result is in good accordance with surface SEM results (Fig. 7(f)). Thus, the completely covered α-Al2O3 scale should form more faster in the initial of oxidation with the oxygen partial pressure increasing from 10 Pa to 1 kPa.
growth of Al2O3 film should follow island model namely Volmer-Weber model [25], in which there were three distinctive growth steps (Fig. 11). Firstly, the film started with an growth by nucleation of discrete islands. Secondly the coalescence process occurred. Finally, the third phase of the Volmer-Weber growth should be dominated by the post-coalescence process, where the formed film continued to increase until a predefined thickness reached. In the first step the distribution of islands was discrete. This should lead to more oxygen contacting the substrate, so the net mass gain reached maximum initially. The increasing the oxygen partial pressure could accelerate the transition from nucleation of discrete islands to coalescence process [14]. Thus, the first step transferred more quickly to the second step with the oxygen partial pressure increasing, and the formed complete alumina film could prevent further oxidation, resulting in the reduction of mass gain. However the formed iron oxides in high oxygen partial pressure (10 kPa, 20 kPa) reduced the high temperature resistance to oxygen, leading to the increase of mass gain again.
3.4. Island (Volmer-Weber) model of alumina film EDS maps of O element under different oxygen partial pressure were shown in Fig. 10. The O element dispersed on the surface of film at oxygen partial pressure of 1–10 Pa, and became gradually dense with the increase in the oxygen partial pressure and was full of film surface as the oxygen partial pressure higher than 1 kPa. This indicated 21-6-9 substrate surface exposed to oxygen should decrease with the oxygen partial pressure increasing, leading to the better oxidation resistance of substrate at higher oxygen partial pressure, in agreement with that kp1 of the initial oxidation stage decreases from 6.44 × 10−3 (mg/min) to 1.58 × 10−3 (mg/min) (Table 1). Thus, the thickness of scale would decrease with the oxygen partial pressure increasing while exposing the same time and reached the minimum at 1 kPa O2. This was consistent with the reflection result showed in Fig. 6(a). Whereas kp1 of the initial oxidation stage at 10 kPa and 20 kPa sharply increased due to poor oxidation-resistance of mix-oxides of Al and Fe (Fig. 5), resulting in the scale thickness increasing at the oxygen partial pressure of 10Pa–20 kPa. Thus, combining with the EDS mapping, oxidation net mass gain (Fig. 8) and reflection results(Fig. 6), we proposed a hypothesis that the
3.5. Deuterium permeation properties The piece for GDP experiment was oxidized under oxygen partial pressure of 1 kPa. Fig. 12 showed the deuterium permeation rate of FeAl/Al2O3 coated 21-6-9 steel and uncoated piece as a comparison. While the oxidation time was 1 h, the deuterium permeability was only reduced by one order of magnitude. With the increase of oxidation time, the PRF increased. As the oxidation time increased to 3 h, the coated piece showed good resistance to deuterium, giving a reduction by 2–3 orders of magnitude, i.e. the PRF is about 1261 at 450 °C and 491 at 650 °C. The diffraction pattern of piece oxidized for 1 h showed α-Al2O3 and γ-Al2O3 shown in Fig. 13. This mixed coating structure led to the decreasing of PRF: many lattice distortions existed between αAl2O3 and amorphous phases of the coating and deuterium transport 7
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was possible along boundaries of the α-Al2O3 crystals. Such results were corresponding to Levchuk [13]. However, γ-Al2O3 was not detected by XRD (Fig. 4), indicating that the amount of γ-Al2O3 was small.
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4. Conclusion The α-Al2O3 thin film with the thickness of 500–700 nm was formed on Fe-Al diffusion coating. Selective oxidation of aluminum occurred under lower oxygen partial pressure of 1 Pa–10 kPa. The stable-state αAl2O3 was more easily to form under oxygen partial pressure of 100 Pa–1 kPa. The oxidation kinetics of Fe-Al layer illustrated the oxidation process had two stages, a linear oxidation stage in the initial followed by a parabolic oxidation stage. The Fe-Al/α-Al2O3 coating performed good high temperature oxidation resistance under oxygen partial pressure of 1 Pa–10 kPa. The Volmer-Weber model can be well used to explain the growth mechanism of alumina thin film. The deuterium permeation rate through the piece with oxidized for 3 h was reduced by 2–3 orders of magnitude at 450°C ∼650°C, and the ability of deuterium permeation resistance was increased with the oxidation time increased. Funding This work is supported by National Natural Science Foundation (No. 21471137) and National Magnetic Confinement Fusion Science Program (No. 2017YFE0300304) of China. References [1] I. Cristescu, I. Cristescu, L. Doerr, M. Glugla, D. Murdoch, Tritium inventories and tritium safety design principles for the fuel cycle of ITER, Nucl. Fusion 47 (2007) S458–S463. [2] S.-E. Wulf, N. Holstein, W. Krauss, J. Konys, Influence of deposition conditions on the microstructure of Al-based coatings for applications as corrosion and anti-permeation barrier, Fusion Eng. Des. 88 (9) (2013) 2530–2534. [3] R. Causey, R. Karnesky, C. San Marchi, 4.16 – tritium barriers and tritium diffusion in fusion reactors, in: R.J. Konings (Ed.), Comprehensive Nuclear Materials, vol. 4, Elsevier, Oxford, 2012, pp. 511–549. [4] E. Serra, P.J. Kelly, D.K. Ross, R.D. Arnell, Alumina sputtered on MANET as an effective deuterium permeation barrier, J. Nucl. Mater. 257 (2) (1998) 194–198. [5] J. Könys, W. Krauss, N. Holstein, Development of advanced Al coating processes for future application as anti-corrosion and T-permeation barriers, Fusion Eng. Des. 85 (10) (2010) 2141–2145.
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