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Effects of oxidation on the deuterium permeation behavior of the SIMP steel
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Effects of oxidation on the deuterium permeation behavior of the SIMP steel Yi-Ming Lyu a,b, Yu-Ping Xu a,*, Hao-Dong Liu a,b, Xin-Dong Pan a,b, Hai-Shan Zhou a, Xiao-Chun Li a, Yanfen Li c, Yiyin Shan c, Qian Xu a, Zhong-Shi Yang a, Guang-Nan Luo a,b a
Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, China Science Island Branch of Graduate, University of Science & Technology of China, Hefei, 230031, China c Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China b
article info
abstract
Article history:
Reduced activation ferritic/martensitic (RAFM) steels are the primary candidate structural
Received 21 February 2019
materials for the blanket in future fusion power plants. In the recent years, a novel 9e12%
Received in revised form
Cr modified F/M steel named SIMP steel has been developed. The deuterium permeation
5 May 2019
behavior of the SIMP steel with and without oxide layer has been evaluated using a gas
Accepted 7 May 2019
driven permeation (GDP) device. The permeability of original and oxidized SIMP steel has
Available online 13 June 2019
been derived and compared. In order to characterize the oxide layer, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS)
Keywords:
experiments have been carried out. GDP results show that the SIMP steel has similar
RAFM steel
deuterium permeability with the CLF-1 steel. The deuterium permeability of the oxidized
Oxidation
SIMP steel is lower than that of the original SIMP steel. Heat treatment during the oxi-
Permeation
dization process has limited influence on the deuterium permeability of the bare SIMP
Hydrogen isotope
steel. The cross-section SEM pictures present that with the increase of the oxidation time, the thickness of the oxide layer is increased. The oxide layers are observed to be porous. The results of XRD and EDS show that the oxide layer mainly consists of chromium oxide and accumulation of chromium can be detected in the surface region. The formation of the chromium oxide on the surface of the SIMP steel could be the reason of the decrease of the deuterium permeability of the oxidized SIMP steel. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen isotopes permeation in the structural material of fusion blanket will cause uncontrolled tritium (T) recycling, resulting in T safety and T economy problems [1]. Therefore, the proper confinement of the T is one of the most important
requirements for the safe and economic operation of the fusion facility [2]. For materials working in T environment, the permeation behavior of T through these materials should be noticed for the safely control of T [3e12]. Reduced activation ferritic/martensitic (RAFM) steels are the primary candidate structural materials for the blanket in future fusion power plants and lots of work have been done to study the
* Corresponding author. E-mail address: [email protected] (Y.-P. Xu). https://doi.org/10.1016/j.ijhydene.2019.05.046 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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permeation behavior of hydrogen isotopes through the RAFM steels [13e16]. In real fusion reactor environment, RAFM steels can react with oxygen gas and water during manufacture and installation. Besides, in the solid breeding blankets, tritiated water can be released by the breeding ceramic pebbles [17], and RAFM steel can be oxidized by the high temperature tritiated water [18]. Oxide layer on the steels might influence the hydrogen isotopes permeation behavior significantly. Many researches have been performed to investigate the oxidation behavior of steels or alloys [19e24], but the influence of the oxidation on the hydrogen isotopes permeation behavior of the RAFM steel is still unsettled. A recently developed 9e12% Cr modified F/M steel named SIMP steel has been developed by Institute of Metal Research, Chinese Academy of Sciences [25e28]. Using lower activation Ta and W elements to substitute the higher activation Nb and Mo elements, the concentration of Ni and Mo in the SIMP steel is reduced to lessen the activation. In addition, the SIMP steel has excellent resistance to corrosion [29]. In this study, the deuterium permeation behavior of the original SIMP steel, oxidized SIMP steel and thermal aging SIMP steel has been first investigated mainly employing a gas driven permeation (GDP) device. The oxide layer formed on the SIMP steel has been characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and EnergyDispersive X-ray Spectroscopy (EDS). The mechanism of the effect of oxidation on the deuterium permeation behavior of the SIMP steel has been discussed.
Experimental Material preparation The studied material chosen in this experiment was the SIMP steel. The detailed chemical composition of the SIMP steel is shown in Table 1. Disks with a diameter of 20 mm and a thickness of 0.85 mm were cut and mechanically polished by SiC papers up to 2500 grade, then polished to mirror face using the diamond polishing powder and polishing cloth for the GDP experiments. Samples with the dimensions of 10 10 0.85 mm3 have been prepared for the XRD and SEM/ EDS experiments. After these processing steps, the disks were ultrasonically cleaned in acetone and alcohol. The sample after electro-polished in 10 wt % HCLO4 alcoholic solution at ~253 K was obtained for the surface characterization using SEM method. The precipitates are found accumulating on the surface in Fig. 1. For the cross-section SEM observation, the samples were fixed using resin and mechanical polished to obtain a primary cross-section. Then the samples were put into the Leica EM TIC 3X equipment to obtain a clean and smooth cross section. In order to study the oxidation behavior of the SIMP steel under extreme service environment, 1073 K
was chosen as the oxidation temperature. The isothermal oxidation was conducted for 1 h and 210 h under air condition. Curves of weight gain versus oxidation time at 1073 K in air for up to 210 h are shown in Fig. 2. The weight gain of unite area of oxidized SIMP steel is increased with the increase of the oxidation time. The increase rate of the weight gain of unite area of the SIMP steel is decreased slightly with the increase of oxidation time. But until 210 h, the increase of the weight is not saturated. In order to evaluate the influence of heat treatment process on the deuterium permeation behavior of the SIMP steel excluding the influence of oxide layer, the same heat treatment process in the vacuum (thermal aging) has been performed for the SIMP steel.
Deuterium GDP experiments Deuterium GDP experiments have been performed for the original SIMP steel, oxidized SIMP steel and thermal aging SIMP steel at 670e831 K. The D2 gas driving pressure was set between 104-105 Pa. Before the permeation experiments, all the samples were pre-heated to 773 K for 10 h in the high vacuum to degas the sample. Detailed description of the GDP setup and experimental procedure has been described in our previous papers [30,31]. In order to obtain a quantitative data of the permeation signal, the QMS was calibrated by a D2 standard leak [19].
Sample characterization The chemical states of the oxide layers were confirmed by XRD using an installation coupled with CueKa radiation source, and scans were performed at a range of 0.06 /s in the 2q interval ranging from 30 to 80 . The microstructure and morphology of the oxide layer on the SIMP steel were investigated using SEM, the chemical composition of the coatings was identified using EDS.
Results and discussion Deuterium gas-driven permeation Shown in Fig. 3 is the correlation between the steady-state permeation flux and the square root of upstream deuterium gas pressure. According to the studies [32,33], the steady-state permeation flux (J∞ ) is related to the square root of driving gas pressure (p1/2) through P J∞ ¼ p1=2 d
(1)
where P means the permeability of deuterium in material; d represents the thickness of the sample. As the thickness of all the samples is same in this study, the phenomenon that the slope of fitting line is steeper with a higher permeation
Table 1 e Detailed chemical composition of the SIMP steel [29] samples. Element
Cr
Nb
W
Ta
V
C
Mn
Si
Fe
SIMP wt%
10.8
0.01
1.2
0.11
0.19
0.25
0.54
1.43
Balance
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Fig. 1 e Surface morphology of the SIMP steel after electropolished in 10 wt % HCLO4 alcoholic solution at ~253 K.
Fig. 2 e Weight gain curves vs. time of the SIMP steel in air at 1073 K for up to 210 h.
Fig. 3 e (a) Relationship between steady state permeation flux and the square root of upstream deuterium gas pressure of original SIMP steel and (b) the comparison of the steady state permeation flux of original SIMP steel, 1 h oxidized SIMP steel and 210 h oxidized SIMP steel in different upstream deuterium gas pressure at 831 K.
temperature can be corresponded to the faster deuterium permeation rate at the higher temperature. Good linear fittings are obtained for all the samples during different permeation temperatures, indicating that the volume diffusion process is the controlling process of the deuterium permeation during the deuterium GDP through the original SIMP steel, 1 h and 210 h oxidized SIMP steel [34]. Fig. 3(b) shows that the steady-state deuterium permeation flux is decreased with the increase of oxidation time. The slight change of background pressure in the GDP device owing to temperature change might be the reason why the fitted lines of the oxidized samples do not go through the origin. According to the measured steady-state permeation flux and the Eq. (1), the temperature dependence of permeability of deuterium can be obtained. Fig. 4 shows the permeability of deuterium in original SIMP steel, 1 h and 210 h oxidized SIMP steel and thermal aging SIMP steel vs. the reciprocal temperature compared with data of the CLF-1 steel [30], JLF1 steel [35], JLF-1 steel oxidized at 973 K in reduced-pressure atmosphere for 2 h [35], SS-316 steel [36], and SS-316 steel oxidized at 673 K in Ar gas with 1000 ppm oxygen for 50 h [19]. Clearly, the values of permeability of the SIMP steel, the
CLF-1 steel and the JLF-1 steel are similar, and they are higher than the permeability of the austenitic steel SS-316. The deuterium permeability of 1 h oxidized SIMP steel is about 4 times less than that of original SIMP steel, and for 210 h oxidized SIMP steel, the permeability is about 8 times less compared with that of the original SIMP steel. The oxidation temperature is lower than the phase transition temperature of the ferrite to martensite, but the heat treatment process might change the microstructure of the SIMP steel. Thus GDP experiments have been performed for the SIMP steel with same heat treatment process in the vacuum. The deuterium permeability of the thermal aging SIMP steel shows few difference with the original SIMP steel, which indicated that heat treatment in this condition has limited influence on the deuterium permeability of the SIMP steel. There are usually an original oxide layer on the steel surface, and impurities such as HDO in the D2 which is introduced in the GDP experiments may also oxidize the SIMP steel. Taking these conditions into consideration, the permeation reduction factor could be larger than 4e8 times. Tanaka et al. have studied the oxidation behavior of reduced activation ferritic steel JLF-1, and their deuterium
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Cross-section morphology of oxidized SIMP steel
Fig. 4 e Temperature dependence of permeability of deuterium in original SIMP steel, 1 h and 210 h oxidized SIMP steels and thermal aging SIMP steel compared with the CLF-1 steel [30], JLF-1 steel [35], JLF-1 steel oxidized at 973 K in reduced-pressure atmosphere for 2 h [35], SS-316 steel [36], SS-316 steel oxidized at 673 K in Ar gas with 1000 ppm oxygen for 50 h [19].
permeability has been evaluated [35]. The deuterium permeability of the JLF-1 steel oxidized at 973 K in reducedpressure atmosphere for 2 h is about 2 orders of magnitude lower than that of the original JLF-1 steel. The deuterium permeation reduction factor of the oxidized JLF-1 steel is much higher than that of the oxidized SIMP steel, which might relate to the different oxidation condition, such as the reduced-pressure atmosphere and lower oxidation temperature [35]. Oya et al. have studied the deuterium permeation behavior of the SS-316 steel oxidized at 673 K in Ar gas with 1000 ppm oxygen for 50 h, the deuterium permeation reduction factor of the oxidized SS-316 steel is about 1 orders [19]. Chromium content in SS-316 steel is much higher than the SIMP steel, which leads to the easier formation of chromium oxide. The temperature dependence of obtained permeability (mol/m/s/Pa1/2) of deuterium in the SIMP steel, 1 h and 210 h oxidized SIMP steel can be presented by 0:48±0:03½eV PSIMP ¼ 1:58 107 exp kT
(2)
0:48±0:01½eV PSIMP1 hoxidized ¼ 3:22 108 exp kT
(3)
PSIMP210 hoxidized ¼ 1:72 108 exp
0:46±0:03½eV kT
(4)
The permeation activation energy of original SIMP steel and oxidized SIMP steel are similar and are consistence with that of other ferritic/martensitic steels [13,14]. The permeation activation energy is not affected by oxidation, which is consistent with [37,38]. Even after oxidation, the controlling process of the deuterium permeation is still volume diffusion process, which means that the controlling process is in the substrate.
Fig. 5 shows the SEM microstructure of the cross-section of the SIMP steel oxidized in air at 1073 K for 1 h and 210 h. The thickness of the oxide layer of the 1 h oxidized SIMP steel is about 1 mm and that of the 210 h oxidized SIMP steel ranges from 20 to 65 mm. Small voids are found in the oxide layer of the 1 h oxidized SIMP steel. Large quantity of voids are located at the oxide layer of the 210 h oxidized SIMP steel, and some big voids are found in the oxide layer, which indicated that small voids segregate and grow up as the oxidation time increase. Many voids in the oxide layer formed a not efficient protection layer could be the reason why with the increase of the oxidation time, there is still no obvious reduction of the increase rate of the weight gain of unite area of the SIMP steel. Spallation of the outer oxide layer is observed at some place. EDS analyses have been performed to obtain the element distribution of cross-section of the 1 h and 210 h oxidized SIMP steel. The element distribution in the cross-section of 1 h and 210 h oxidized SIMP steels is shown in Figs. 6 and 7. The element distribution in the oxide layer on 1 h and 210 h oxidized SIMP steel presents a single layer structure with mixed Fe, Cr, Si, Mn and O. Cr gathers in the oxide layer and forms a continuous layer and Fe distributes uniformly in the oxide layer with the decreased content compared with the steel substrate. The Si in the SIMP steel might be oxidized and react with FeO to form Fe2SiO4 [29], which could impede the diffusion of iron cations and reduce the iron content in the oxide layer. The Mn is collected with Si around the pores, which indicated that Si and Mn might prefer to accumulate near the pores.
XRD analyses XRD analyses have been performed to confirm the chemical composition of the oxide layer. Phase analyses of the 1 h and 210 h oxidized SIMP steels by XRD method are shown in Fig. 8. The oxidized products on the surface of 1 h oxidized SIMP steel are composed of Cr2O3 and Cr1.5Mn1.5O4. The oxidized products on the surface of 210 h oxidized SIMP steel are (Fe0.6Cr0.4)2O3 and MnFe2O4. No Si element is detected in the XRD experiments, which might relate to the limited content (1.43 wt %) in the SIMP steel. Chromium oxides can serve as a tritium permeation barrier due to its low hydrogen permeability. The steels with a layer of chromium oxide as tritium permeation barrier show a permeation reduction factor of 1e2 orders [39]. Thus the formation of the chromium oxide in the oxide layer could be the reason of the decrease of the deuterium permeability of the oxidized SIMP steel. Combined with the results of the EDS experiments, the oxide layer formed on 1 h oxidized SIMP steel is a single-layer of mixed Cr1.5Mn1.5O4 and Cr2O3. For the oxide layer formed on 210 h oxidized SIMP steel, there is a single-layer of mixed (Fe0.6Cr0.4)2O3 and MnFe2O4. The enthalpies of formation for stoichiometrically equivalent Cr2O3 and Fe2O3 are 1139.7 kJ/ mol and 824.2 kJ/mol at room temperature [40], thus Cr will preference to combine with the O to form chromium oxide and the inner side Cr will also diffuse to the surface to react with the O, which lead to the enrichment of the Cr and decrease of the content of Fe in the oxide layer. Pores located
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 8 2 6 5 e1 8 2 7 1
Fig. 5 e Cross-section morphologies of the SIMP steel oxidized in air at 1073 K for (a) 1 h and (b) (c) 210 h.
Fig. 6 e EDS images of cross-section of the SIMP steel oxidized in air at 1073 K for 1 h.
Fig. 7 e EDS images of cross-section of the SIMP steel oxidized in air at 1073 K for 210 h.
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oxidation experiments. More work should be done in the future with lower oxidation temperature to understand the influence of oxidation on the deuterium permeation behavior of the SIMP steel systematically.
Acknowledgments
Fig. 8 e XRD patterns of the SIMP steels oxidized in air at 1073 K for 1 h and 210 h. in the oxide layers of 1 h and 210 h oxidized SIMP steel are defects in the oxide layers, which can act as the defects described in the Area-Defect model [32,41]. Even though the oxide layer thickness of 210 h oxidized SIMP steel is much thicker than the 1 h oxidized SIMP steel, there are more defects such as voids in the oxide layer, which lead to the similar permeation reduction factor in these two samples.
Conclusions In this study, the deuterium permeability of the SIMP steel has been first obtained. The permeation behavior of the original SIMP steel, oxidized SIMP steel and thermal aging SIMP steel has been investigated and compared. After oxidation in the air for 1 h and 210 h, the deuterium permeability of the oxidized SIMP steel is 4 and 8 times lower than that of original SIMP steel. Heat treatment process in our condition has limited influence on the deuterium permeability of the SIMP steel. Chromium oxide is detected to be the major constituent in the oxide layers. Large amount of pores are observed in the oxide layer, and with the increase of the oxidation time, the pores gather and grow. The activation energy of deuterium permeation in gas form for oxidized SIMP steel is similar to that of original SIMP steel. The diffusion process of oxidized and original SIMP steel can be assumed to be controlled by the bulk diffusion, which fits well with the permeation behavior described in the Area-Defect Model. According to our experimental results, the hydrogen permeability of steel could be several times lower after oxidation in air at high temperature. Although the thickness of the oxide layer could be larger if the oxidation time is enlarged based on the weight gain curves, we believe that the decrease of the hydrogen permeability will be limited as the oxide layer is porous in this oxidation environment. Thus, it can be inferred that it is not sufficient to rely on thermal oxidation of RAFM steels at high temperature to reduce the hydrogen permeability. Besides, it should be noted that in our work, the oxidation temperature is higher than the temperature of the final heat treatment and in the real working condition, the atmosphere is different with what employed in this
This work is supported by National Key R&D Program of China (No. 2017YFE0301502, National MCF Energy R&D Program of China under contract number of 2018YFE0303103, the National Magnetic Confinement Fusion Science Program of China (No. 2015GB109001), National Postdoctoral Program for Innovative Talents (BX201700248), China Postdoctoral Science Foundation (No. 2017M622035), the National Natural Science Foundation of China (No. 1150523, 11505233, 11875287 and 11575242).
references
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Effects of oxidation on the deuterium permeation behavior of the SIMP steel Yi-Ming Lyu a,b, Yu-Ping Xu a,*, Hao-Dong Liu a,b, Xin-Dong Pan a,b, Hai-Shan Zhou a, Xiao-Chun Li a, Yanfen Li c, Yiyin Shan c, Qian Xu a, Zhong-Shi Yang a, Guang-Nan Luo a,b a
Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, China Science Island Branch of Graduate, University of Science & Technology of China, Hefei, 230031, China c Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China b
article info
abstract
Article history:
Reduced activation ferritic/martensitic (RAFM) steels are the primary candidate structural
Received 21 February 2019
materials for the blanket in future fusion power plants. In the recent years, a novel 9e12%
Received in revised form
Cr modified F/M steel named SIMP steel has been developed. The deuterium permeation
5 May 2019
behavior of the SIMP steel with and without oxide layer has been evaluated using a gas
Accepted 7 May 2019
driven permeation (GDP) device. The permeability of original and oxidized SIMP steel has
Available online 13 June 2019
been derived and compared. In order to characterize the oxide layer, X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS)
Keywords:
experiments have been carried out. GDP results show that the SIMP steel has similar
RAFM steel
deuterium permeability with the CLF-1 steel. The deuterium permeability of the oxidized
Oxidation
SIMP steel is lower than that of the original SIMP steel. Heat treatment during the oxi-
Permeation
dization process has limited influence on the deuterium permeability of the bare SIMP
Hydrogen isotope
steel. The cross-section SEM pictures present that with the increase of the oxidation time, the thickness of the oxide layer is increased. The oxide layers are observed to be porous. The results of XRD and EDS show that the oxide layer mainly consists of chromium oxide and accumulation of chromium can be detected in the surface region. The formation of the chromium oxide on the surface of the SIMP steel could be the reason of the decrease of the deuterium permeability of the oxidized SIMP steel. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen isotopes permeation in the structural material of fusion blanket will cause uncontrolled tritium (T) recycling, resulting in T safety and T economy problems [1]. Therefore, the proper confinement of the T is one of the most important
requirements for the safe and economic operation of the fusion facility [2]. For materials working in T environment, the permeation behavior of T through these materials should be noticed for the safely control of T [3e12]. Reduced activation ferritic/martensitic (RAFM) steels are the primary candidate structural materials for the blanket in future fusion power plants and lots of work have been done to study the
* Corresponding author. E-mail address: [email protected] (Y.-P. Xu). https://doi.org/10.1016/j.ijhydene.2019.05.046 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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permeation behavior of hydrogen isotopes through the RAFM steels [13e16]. In real fusion reactor environment, RAFM steels can react with oxygen gas and water during manufacture and installation. Besides, in the solid breeding blankets, tritiated water can be released by the breeding ceramic pebbles [17], and RAFM steel can be oxidized by the high temperature tritiated water [18]. Oxide layer on the steels might influence the hydrogen isotopes permeation behavior significantly. Many researches have been performed to investigate the oxidation behavior of steels or alloys [19e24], but the influence of the oxidation on the hydrogen isotopes permeation behavior of the RAFM steel is still unsettled. A recently developed 9e12% Cr modified F/M steel named SIMP steel has been developed by Institute of Metal Research, Chinese Academy of Sciences [25e28]. Using lower activation Ta and W elements to substitute the higher activation Nb and Mo elements, the concentration of Ni and Mo in the SIMP steel is reduced to lessen the activation. In addition, the SIMP steel has excellent resistance to corrosion [29]. In this study, the deuterium permeation behavior of the original SIMP steel, oxidized SIMP steel and thermal aging SIMP steel has been first investigated mainly employing a gas driven permeation (GDP) device. The oxide layer formed on the SIMP steel has been characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and EnergyDispersive X-ray Spectroscopy (EDS). The mechanism of the effect of oxidation on the deuterium permeation behavior of the SIMP steel has been discussed.
Experimental Material preparation The studied material chosen in this experiment was the SIMP steel. The detailed chemical composition of the SIMP steel is shown in Table 1. Disks with a diameter of 20 mm and a thickness of 0.85 mm were cut and mechanically polished by SiC papers up to 2500 grade, then polished to mirror face using the diamond polishing powder and polishing cloth for the GDP experiments. Samples with the dimensions of 10 10 0.85 mm3 have been prepared for the XRD and SEM/ EDS experiments. After these processing steps, the disks were ultrasonically cleaned in acetone and alcohol. The sample after electro-polished in 10 wt % HCLO4 alcoholic solution at ~253 K was obtained for the surface characterization using SEM method. The precipitates are found accumulating on the surface in Fig. 1. For the cross-section SEM observation, the samples were fixed using resin and mechanical polished to obtain a primary cross-section. Then the samples were put into the Leica EM TIC 3X equipment to obtain a clean and smooth cross section. In order to study the oxidation behavior of the SIMP steel under extreme service environment, 1073 K
was chosen as the oxidation temperature. The isothermal oxidation was conducted for 1 h and 210 h under air condition. Curves of weight gain versus oxidation time at 1073 K in air for up to 210 h are shown in Fig. 2. The weight gain of unite area of oxidized SIMP steel is increased with the increase of the oxidation time. The increase rate of the weight gain of unite area of the SIMP steel is decreased slightly with the increase of oxidation time. But until 210 h, the increase of the weight is not saturated. In order to evaluate the influence of heat treatment process on the deuterium permeation behavior of the SIMP steel excluding the influence of oxide layer, the same heat treatment process in the vacuum (thermal aging) has been performed for the SIMP steel.
Deuterium GDP experiments Deuterium GDP experiments have been performed for the original SIMP steel, oxidized SIMP steel and thermal aging SIMP steel at 670e831 K. The D2 gas driving pressure was set between 104-105 Pa. Before the permeation experiments, all the samples were pre-heated to 773 K for 10 h in the high vacuum to degas the sample. Detailed description of the GDP setup and experimental procedure has been described in our previous papers [30,31]. In order to obtain a quantitative data of the permeation signal, the QMS was calibrated by a D2 standard leak [19].
Sample characterization The chemical states of the oxide layers were confirmed by XRD using an installation coupled with CueKa radiation source, and scans were performed at a range of 0.06 /s in the 2q interval ranging from 30 to 80 . The microstructure and morphology of the oxide layer on the SIMP steel were investigated using SEM, the chemical composition of the coatings was identified using EDS.
Results and discussion Deuterium gas-driven permeation Shown in Fig. 3 is the correlation between the steady-state permeation flux and the square root of upstream deuterium gas pressure. According to the studies [32,33], the steady-state permeation flux (J∞ ) is related to the square root of driving gas pressure (p1/2) through P J∞ ¼ p1=2 d
(1)
where P means the permeability of deuterium in material; d represents the thickness of the sample. As the thickness of all the samples is same in this study, the phenomenon that the slope of fitting line is steeper with a higher permeation
Table 1 e Detailed chemical composition of the SIMP steel [29] samples. Element
Cr
Nb
W
Ta
V
C
Mn
Si
Fe
SIMP wt%
10.8
0.01
1.2
0.11
0.19
0.25
0.54
1.43
Balance
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Fig. 1 e Surface morphology of the SIMP steel after electropolished in 10 wt % HCLO4 alcoholic solution at ~253 K.
Fig. 2 e Weight gain curves vs. time of the SIMP steel in air at 1073 K for up to 210 h.
Fig. 3 e (a) Relationship between steady state permeation flux and the square root of upstream deuterium gas pressure of original SIMP steel and (b) the comparison of the steady state permeation flux of original SIMP steel, 1 h oxidized SIMP steel and 210 h oxidized SIMP steel in different upstream deuterium gas pressure at 831 K.
temperature can be corresponded to the faster deuterium permeation rate at the higher temperature. Good linear fittings are obtained for all the samples during different permeation temperatures, indicating that the volume diffusion process is the controlling process of the deuterium permeation during the deuterium GDP through the original SIMP steel, 1 h and 210 h oxidized SIMP steel [34]. Fig. 3(b) shows that the steady-state deuterium permeation flux is decreased with the increase of oxidation time. The slight change of background pressure in the GDP device owing to temperature change might be the reason why the fitted lines of the oxidized samples do not go through the origin. According to the measured steady-state permeation flux and the Eq. (1), the temperature dependence of permeability of deuterium can be obtained. Fig. 4 shows the permeability of deuterium in original SIMP steel, 1 h and 210 h oxidized SIMP steel and thermal aging SIMP steel vs. the reciprocal temperature compared with data of the CLF-1 steel [30], JLF1 steel [35], JLF-1 steel oxidized at 973 K in reduced-pressure atmosphere for 2 h [35], SS-316 steel [36], and SS-316 steel oxidized at 673 K in Ar gas with 1000 ppm oxygen for 50 h [19]. Clearly, the values of permeability of the SIMP steel, the
CLF-1 steel and the JLF-1 steel are similar, and they are higher than the permeability of the austenitic steel SS-316. The deuterium permeability of 1 h oxidized SIMP steel is about 4 times less than that of original SIMP steel, and for 210 h oxidized SIMP steel, the permeability is about 8 times less compared with that of the original SIMP steel. The oxidation temperature is lower than the phase transition temperature of the ferrite to martensite, but the heat treatment process might change the microstructure of the SIMP steel. Thus GDP experiments have been performed for the SIMP steel with same heat treatment process in the vacuum. The deuterium permeability of the thermal aging SIMP steel shows few difference with the original SIMP steel, which indicated that heat treatment in this condition has limited influence on the deuterium permeability of the SIMP steel. There are usually an original oxide layer on the steel surface, and impurities such as HDO in the D2 which is introduced in the GDP experiments may also oxidize the SIMP steel. Taking these conditions into consideration, the permeation reduction factor could be larger than 4e8 times. Tanaka et al. have studied the oxidation behavior of reduced activation ferritic steel JLF-1, and their deuterium
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Cross-section morphology of oxidized SIMP steel
Fig. 4 e Temperature dependence of permeability of deuterium in original SIMP steel, 1 h and 210 h oxidized SIMP steels and thermal aging SIMP steel compared with the CLF-1 steel [30], JLF-1 steel [35], JLF-1 steel oxidized at 973 K in reduced-pressure atmosphere for 2 h [35], SS-316 steel [36], SS-316 steel oxidized at 673 K in Ar gas with 1000 ppm oxygen for 50 h [19].
permeability has been evaluated [35]. The deuterium permeability of the JLF-1 steel oxidized at 973 K in reducedpressure atmosphere for 2 h is about 2 orders of magnitude lower than that of the original JLF-1 steel. The deuterium permeation reduction factor of the oxidized JLF-1 steel is much higher than that of the oxidized SIMP steel, which might relate to the different oxidation condition, such as the reduced-pressure atmosphere and lower oxidation temperature [35]. Oya et al. have studied the deuterium permeation behavior of the SS-316 steel oxidized at 673 K in Ar gas with 1000 ppm oxygen for 50 h, the deuterium permeation reduction factor of the oxidized SS-316 steel is about 1 orders [19]. Chromium content in SS-316 steel is much higher than the SIMP steel, which leads to the easier formation of chromium oxide. The temperature dependence of obtained permeability (mol/m/s/Pa1/2) of deuterium in the SIMP steel, 1 h and 210 h oxidized SIMP steel can be presented by 0:48±0:03½eV PSIMP ¼ 1:58 107 exp kT
(2)
0:48±0:01½eV PSIMP1 hoxidized ¼ 3:22 108 exp kT
(3)
PSIMP210 hoxidized ¼ 1:72 108 exp
0:46±0:03½eV kT
(4)
The permeation activation energy of original SIMP steel and oxidized SIMP steel are similar and are consistence with that of other ferritic/martensitic steels [13,14]. The permeation activation energy is not affected by oxidation, which is consistent with [37,38]. Even after oxidation, the controlling process of the deuterium permeation is still volume diffusion process, which means that the controlling process is in the substrate.
Fig. 5 shows the SEM microstructure of the cross-section of the SIMP steel oxidized in air at 1073 K for 1 h and 210 h. The thickness of the oxide layer of the 1 h oxidized SIMP steel is about 1 mm and that of the 210 h oxidized SIMP steel ranges from 20 to 65 mm. Small voids are found in the oxide layer of the 1 h oxidized SIMP steel. Large quantity of voids are located at the oxide layer of the 210 h oxidized SIMP steel, and some big voids are found in the oxide layer, which indicated that small voids segregate and grow up as the oxidation time increase. Many voids in the oxide layer formed a not efficient protection layer could be the reason why with the increase of the oxidation time, there is still no obvious reduction of the increase rate of the weight gain of unite area of the SIMP steel. Spallation of the outer oxide layer is observed at some place. EDS analyses have been performed to obtain the element distribution of cross-section of the 1 h and 210 h oxidized SIMP steel. The element distribution in the cross-section of 1 h and 210 h oxidized SIMP steels is shown in Figs. 6 and 7. The element distribution in the oxide layer on 1 h and 210 h oxidized SIMP steel presents a single layer structure with mixed Fe, Cr, Si, Mn and O. Cr gathers in the oxide layer and forms a continuous layer and Fe distributes uniformly in the oxide layer with the decreased content compared with the steel substrate. The Si in the SIMP steel might be oxidized and react with FeO to form Fe2SiO4 [29], which could impede the diffusion of iron cations and reduce the iron content in the oxide layer. The Mn is collected with Si around the pores, which indicated that Si and Mn might prefer to accumulate near the pores.
XRD analyses XRD analyses have been performed to confirm the chemical composition of the oxide layer. Phase analyses of the 1 h and 210 h oxidized SIMP steels by XRD method are shown in Fig. 8. The oxidized products on the surface of 1 h oxidized SIMP steel are composed of Cr2O3 and Cr1.5Mn1.5O4. The oxidized products on the surface of 210 h oxidized SIMP steel are (Fe0.6Cr0.4)2O3 and MnFe2O4. No Si element is detected in the XRD experiments, which might relate to the limited content (1.43 wt %) in the SIMP steel. Chromium oxides can serve as a tritium permeation barrier due to its low hydrogen permeability. The steels with a layer of chromium oxide as tritium permeation barrier show a permeation reduction factor of 1e2 orders [39]. Thus the formation of the chromium oxide in the oxide layer could be the reason of the decrease of the deuterium permeability of the oxidized SIMP steel. Combined with the results of the EDS experiments, the oxide layer formed on 1 h oxidized SIMP steel is a single-layer of mixed Cr1.5Mn1.5O4 and Cr2O3. For the oxide layer formed on 210 h oxidized SIMP steel, there is a single-layer of mixed (Fe0.6Cr0.4)2O3 and MnFe2O4. The enthalpies of formation for stoichiometrically equivalent Cr2O3 and Fe2O3 are 1139.7 kJ/ mol and 824.2 kJ/mol at room temperature [40], thus Cr will preference to combine with the O to form chromium oxide and the inner side Cr will also diffuse to the surface to react with the O, which lead to the enrichment of the Cr and decrease of the content of Fe in the oxide layer. Pores located
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 8 2 6 5 e1 8 2 7 1
Fig. 5 e Cross-section morphologies of the SIMP steel oxidized in air at 1073 K for (a) 1 h and (b) (c) 210 h.
Fig. 6 e EDS images of cross-section of the SIMP steel oxidized in air at 1073 K for 1 h.
Fig. 7 e EDS images of cross-section of the SIMP steel oxidized in air at 1073 K for 210 h.
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oxidation experiments. More work should be done in the future with lower oxidation temperature to understand the influence of oxidation on the deuterium permeation behavior of the SIMP steel systematically.
Acknowledgments
Fig. 8 e XRD patterns of the SIMP steels oxidized in air at 1073 K for 1 h and 210 h. in the oxide layers of 1 h and 210 h oxidized SIMP steel are defects in the oxide layers, which can act as the defects described in the Area-Defect model [32,41]. Even though the oxide layer thickness of 210 h oxidized SIMP steel is much thicker than the 1 h oxidized SIMP steel, there are more defects such as voids in the oxide layer, which lead to the similar permeation reduction factor in these two samples.
Conclusions In this study, the deuterium permeability of the SIMP steel has been first obtained. The permeation behavior of the original SIMP steel, oxidized SIMP steel and thermal aging SIMP steel has been investigated and compared. After oxidation in the air for 1 h and 210 h, the deuterium permeability of the oxidized SIMP steel is 4 and 8 times lower than that of original SIMP steel. Heat treatment process in our condition has limited influence on the deuterium permeability of the SIMP steel. Chromium oxide is detected to be the major constituent in the oxide layers. Large amount of pores are observed in the oxide layer, and with the increase of the oxidation time, the pores gather and grow. The activation energy of deuterium permeation in gas form for oxidized SIMP steel is similar to that of original SIMP steel. The diffusion process of oxidized and original SIMP steel can be assumed to be controlled by the bulk diffusion, which fits well with the permeation behavior described in the Area-Defect Model. According to our experimental results, the hydrogen permeability of steel could be several times lower after oxidation in air at high temperature. Although the thickness of the oxide layer could be larger if the oxidation time is enlarged based on the weight gain curves, we believe that the decrease of the hydrogen permeability will be limited as the oxide layer is porous in this oxidation environment. Thus, it can be inferred that it is not sufficient to rely on thermal oxidation of RAFM steels at high temperature to reduce the hydrogen permeability. Besides, it should be noted that in our work, the oxidation temperature is higher than the temperature of the final heat treatment and in the real working condition, the atmosphere is different with what employed in this
This work is supported by National Key R&D Program of China (No. 2017YFE0301502, National MCF Energy R&D Program of China under contract number of 2018YFE0303103, the National Magnetic Confinement Fusion Science Program of China (No. 2015GB109001), National Postdoctoral Program for Innovative Talents (BX201700248), China Postdoctoral Science Foundation (No. 2017M622035), the National Natural Science Foundation of China (No. 1150523, 11505233, 11875287 and 11575242).
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