- Email: [email protected]
tritium permeation barrier
international journal of hydrogen energy 35 (2010) 2689–2693
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Influence of silicon on hot-dip aluminizing process and subsequent oxidation for preparing hydrogen/tritium permeation barrier Shilei Han*, Hualing Li, Shumao Wang, Lijun Jiang, Xiaopeng Liu Energy Materials and Technology Research Institute, General Research Institute for Nonferrous Metals, Beijing 100088, China
article info
abstract
Article history:
The development of the International Thermonuclear Experimental Reactor (ITER) requires
Received 7 April 2009
the production of a material capable of acting as a hydrogen/tritium permeation barrier on
Accepted 15 April 2009
low activation steel. It is well known that thin alumina layer can reduce the hydrogen
Available online 17 May 2009
permeation rate by several orders of magnitude. A technology is introduced here to form a ductile Fe/Al intermetallic layer on the steel with an alumina over-layer. This technology,
Keywords:
consisting of two main steps, hot-dip aluminizing (HDA) and subsequent oxidation
Hydrogen permeation barrier
behavior, seems to be a promising coating method to fulfill the required goals. According to
Alumina coating
the experiments that have been done in pure Al, the coatings were inhomogeneous and too
Hot-dip aluminizing
thick. Additionally, a large number of cracks and porous band could be observed. In order to solve these problems, the element silicon was added to the aluminum melt with a nominal composition. The influence of silicon on the aluminizing and following oxidation process was investigated. With the addition of silicon into the aluminum melt, the coating became thinner and more homogeneous. The effort of the silicon on the oxidation behavior was observed as well concerning the suppression of porous band and cracks. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The permeation of hydrogen and its isotopes through materials is not expected in many technological applications, which could produce embrittlement and other defects in steel. One of the main issues in the design of blankets for a future fusion reactor is the permeation of tritium through the structure material into the secondary circuit [1]. A considerable effort has been done in order to reduce the permeation rate of tritium in the blanket of fusion reactor. The development of the International Thermonuclear Experimental Reactor (ITER) requires the production of a material capable of acting as a tritium permeation barrier on low activation steel.
Previous measurements have shown that thin alumina layer can reduce the hydrogen permeation rate by several orders of magnitude [2,3]. A hot-dip aluminizing process seems to offer a good chance to produce aluminized coating by pumping liquid aluminum through a complex geometry of steel to form the required layer [4]. Subsequently, this aluminized layer could be oxidized in high temperature to form an alumina layer on the surface. Two important parameters that determine the mechanical characteristics of the aluminized steel are the thickness and the morphology of the Fe–Al interlayer between the outer aluminum coat and the substrate steel [5]. The whole coating should be kept thin enough since aluminum is an activating
* Corresponding author. Tel.: þ86 10 82241241; fax: þ86 10 82241294. E-mail address: [email protected] (S. Han). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.04.033
2690
international journal of hydrogen energy 35 (2010) 2689–2693
element and not desired in the matrix of low activation steels [6]. The experiments have been done by using a melt of pure Al were not quite successful. These coatings were inhomogeneous and almost all over 100 mm in thickness after the HDA process, too thick for the usage of them. Additionally, a large number of cracks and porous band could be observed in these coatings when the subsequent oxidation reaction was finished. It has been reported that the presence of certain level of silicon in the molten aluminum helps in reducing the thickness of the coating [7,8], but the preparing technology and the mechanism research of the role of silicon are not sufficient. In the present investigation, the material chosen as substrate is the reduced activation steels (namely RAFM steels). CLAM steels, one of the RAFM steels produced in China, is the candidate material for the first wall and structure of ITER.
2.
Experimental details
2.1.
Sample preparation
The chemical composition of the material to be aluminized is given in Table 1. Sheet specimens of 40 15 1.5 mm3 were cut by linear cutting machine. Each sample was degreased in acetone and finally cleaned ultrasonically in ethanol. A hole with its diameter of approximately 3 mm was drilled at one end of the specimen to facilitate its hanging into molten aluminum.
2.2.
Aluminizing process
The aluminum alloy was melted in an alumina crucible placed in a well-type furnace and heated up to 750 C. The temperature was measured by a NiCr-Ni thermocouple which was protected by an alumina tube and placed directly in the aluminum melt. Then, silicon was added into the Al melt with a nominal composition of 3 wt% and the melt was stirred time by time for 1 h. These specimens were fixed by a hook and a stainless steel wire and dipped into the melt. After 30 s of exposure, they were pulled out from the melt and cooled down in the air condition. For comparison, CLAM steel sheets were hot dipped in pure Al melt under the same conditions.
2.3.
Heat oxidation
The heat oxidation was carried out in a vacuum heat-treatment furnace. The aluminized samples were cleaned ultrasonically in ethanol, dried and placed in alumina crucible which was positioned in the hot zone of the furnace about 900 C in an atmosphere of O2 for 5 h.
Table 1 – Chemical composition of CLAM steel (wt%). Fe
Cr
W
V
C
Mn
Ta
Ni
N
Si
Bal.
9.0
1.5
0.2
0.1
0.5
0.07
0.02
0.02
0.01
2.4.
Analytical examination
Metallographic examination was carried out to study the influence of Si on the coating thickness, adherence and the quality of the layer. The scanning electron microscopic examination was conducted on a Hitachi S-4800 microscope. The samples were studied by Grazing Incidence X-ray Diffraction (GIXRD) on a D8-Discover X-ray diffractometer to get information on the compounds existing on the surface.
3.
Results and discussions
3.1.
Aluminized specimens
High purity aluminum (99.99%) was used for aluminizing the samples in pure aluminum without any alloying addition. Typical microstructures of HDA samples at four different temperatures are shown in Fig. 1. Three distinct regions which could be identified in these microstructures included: the outer aluminum layer, the intermetallic compound and the substrate steel. The outer layer had the composition of the aluminum bath. The intermetallic layer (about 20–50 mm thick) formed beneath the Al over-layer by Al diffusion into the steel substrate. When the immersing temperature was elevated, the thickness of the intermetallic layer increased, while the outer layer decreased. With the immersing temperature improved, the diffusion velocity of Al into the steel matrix increased and the intermetallic layer became thick. At the same time, the flowability of the Al melt became better. That is why the thickness of outer layer decreased. SEM/EDX point analyses of HDA samples have shown that the intermetallic layer mainly consisted of the brittle compound Fe2Al5. The composition found with EDX point analyses was 71–76 at.% Al, 21–24 at.% Fe and 2 at.% Cr. The precipitates in the solidified Al layer were also found to be crystals of FeAl3. Additionally, cracks could be observed in the intermetallic layer (Fig. 1(d)). When the alloy element Si was added into the melt Al, it could be easily observed that Si had a significant influence on the thickness and the morphology of the coating. As shown in Fig. 2, the thickness of the intermetallic layer was less than 10 mm and independent of the immersion temperature. The border of the intermetallic layer to the steel side was sharp, and that one to the outer layer was more irregular. The outer layer was 20–40 mm thick and its surface was smooth. Also, cavities or pores could not be observed in each layer. The coating of specimen (Fig. 2(b)). with the total thickness about 25 mm, was the thinnest of the four (Fig. 2(a–d)). The cross sections of the specimens were also analyzed by means of SEM and EDX point analyses. EDX line scans of an aluminized specimen (Fig. 2(b)) are shown in Fig. 3. The top layer (about 10 mm in thickness) mainly consisted of pure Al. The main part of the intermetallic layer corresponded to the Fe2Al5 phase, from the depth of 10 mm to 25 mm. In the transition zone between the intermetallic layer and the outer Al layer, a small band of the compound
international journal of hydrogen energy 35 (2010) 2689–2693
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Fig. 1 – Cross-sectional views of aluminized CLAM steel sheets which were hot dip in a melt pure Al at different temperatures, (a) 700 8C, (b) 750 8C, (c) 800 8C, (d) 850 8C.
Fig. 2 – Cross-sectional views of aluminized CLAM steel sheets which were hot dip in a melt of Al–Si at different temperatures (a) 700 8C, (b) 750 8C, (c) 800 8C, (d) 850 8C.
FeAl3 could be detected. In addition, in all specimens (HDA in Al–Si melt) large crystals (marked with arrows) could be observed in the intermetallic layer. These large crystals consisted of Al–Fe–Cr–Si, with Al as the main component decreased the diffusion velocity of Al into the steel matrix and inhibited the growth of Fe2Al5 during the HDA process. It is obvious that the presence of certain level of Si in the molten aluminum helped in reducing the thickness of the intermetallic layer.
3.2.
Aluminized and subsequent oxidized specimens
The coating of the specimen hot-dip aluminized at 750 C in a melt of Al–Si was the thinnest and was chosen as the subsequent high temperature oxidized materials. Fig. 4(a) shows a cross section of an aluminized CLAM specimen which
Al
800
FeAl3
Fe2Al5
Matrix
Al Fe
Intensity
600
400
200
0 0
10
20 depth (µm)
30
40
Fig. 3 – EDX line scans were carried out on the cross section of an aluminized CLAM steel sheet which was hot dip in a melt of Al–Si at 750 8C.
was oxidized at the temperature of 900 C for 5 h in at the temperature of 900 C for 5 h. Two layers can be identified on the steel surface: an internal layer namely a-Fe(Al) and an external layer (FeAl) about 50 mm and 60 mm in thickness, respectively. Fe2Al5 layer, which formed in the hot-dip aluminized process, was not found after the oxidation treatment owning to the out ward diffusion of iron from matrix and to the inward diffusion of aluminum. Hence, the intermetallic Fe2Al5 phase transformed completely into softer phases, a-Fe(Al) and FeAl. The thickness of the internal layer was found to be dependent on the heat treatment chosen, while the thickness of the external layer was dependent on the amount of solidified Al which adhered to the surface after the hot-dip aluminizing process [9]. It has been reported that the internal and external layer are separated by a porous band. In our experiment, the border was not very clear and the adhesion between those two phases seemed to be excellent, and no cavities or pores could be observed in the layer. If there are less pores formed during the heat oxidation process, it is likely that the mechanical properties of the layer could be improved and the hydrogen/ tritium permeation rate could be reduced as well [10]. EDX point analyses have shown that there was a high concentration of Al and Oxygen near the surface of the coating (the oxygen content reached 55 at.%, while the Al content was 40 at.%). The compound Al2O3 corresponded correctly to the concentration of oxygen and Al. These specimens were analyzed by Grazing Incidence X-ray Diffraction (GIXRD) in order to make sure the existence of Al2O3. In Fig. 4(b), the XRD patterns of the specimen provided support to the existence of the out layer FeAl and the g-Al2O3 layer. As shown in Fig. 4(c), the g-Al2O3 layer could also be found about 0.8 mm in thickness. The results depend on both the heating temperature and the exposure time. In fact, if from a thermodynamic point of view the appearance of the different phases is closely linked
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international journal of hydrogen energy 35 (2010) 2689–2693
all the intermetallic Fe2Al5 to ductile phases (FeAl and a-Fe(Al)) and product g-Al2O3 over-layer.
4.
Conclusions
It was shown that CLAM steel could be aluminized by means of hot-dipping in an Al/Si melt. The thickness of the intermetallic layer of the specimen aluminized decreased significantly from 80 mm to about 10 mm compared to the experiments in pure Al bath. The thinnest coating (aluminized at 750 C in a melt of Al–Si) was uniform in thickness and composition. The existence of the large crystals (consist of Al– Fe–Cr–Si) in the intermetallic layer may be the reason for the decrease of the diffusion velocity of Al into the steel matrix during the HDA process. In the subsequent heat treatment, the intermetallic layer and the outer aluminum layer transformed into the ductile phases (FeAl and a-Fe(Al)), with the thickness of about 100 mm. The porous band between FeAl layer and a-Fe(Al) layer was suppressed and no cavities or pores could be observed in the internal or external layer. It is also interesting to observe that at the temperature 900 C the presence of g-Al2O3 was detected by XRD on the surface of the specimen at the atmosphere of O2. The oxide coating was only 0.8 mm thick. However, as far as the tritium permeation barriers are concerned, this problem appears not so important because an oxide thickness less than 1 mm is sufficient. Conclusion demonstrated that with the addition of Si into the Al melt, the coating preparation for hydrogen/tritium was improved.
references
Fig. 4 – (a)Cross-section view of hot aluminized and subsequent oxidized specimen (900 8C/5 h). (b) Grazing Incidence X-ray Diffraction of aluminized and subsequent oxidized specimen. (c) SEM microphotograph of the crosssection view of aluminized and subsequent oxidized specimen (900 8C/5 h).
to the temperature (for example, FeAl and a-Fe(Al)), the complete transformation from one phase to another (for example, from Fe2Al5 to FeAl and a-Fe(Al)) is more or less the combined result of thermodynamic and kinetic factors, such as temperature and time [11]. The combined effect of these two parameters (at 900 C for 5 h) was sufficient to transform
[1] Tanabe T. On the estimation of tritium retention and permeation in metallic first wall. Fusion Engineering Design 1989;10:325–30. [2] Perujo A, Forcey KS, Sample T. Reduction of deuterium permeation through DIN 1.4914 stainless steel (MANET) by plasma-spray deposited aluminum. Nuclear Material 1993; 207:86–91. [3] Glasbrenner H, Perujo A, Serra E. Hydrogen permeation behavior of hot-dip aluminized MANET steel. Fusion Technology 1995;28:1159–64. [4] Benamati G, Chabrol C, Perujo A. Development of tritium permeation barriers on Al base in Europe. Journal of Nuclear Materials 1999;271:391–5. [5] Gul Hameed Awan, Faiz ul Hasan. The morphology of coating/substrate interface in hot-dip-aluminized steels. Materials Science and Engineering 2008;472:157–165. [6] Glasbrenner H, Wedemeyer O. Comparison of hot dip aluminized F28-mod. steel after different subsequent heat treatments. Journal of Nuclear Materials 1998;257: 274–81. [7] Akdeniz MV, Mekhrabov AO, Yilmaz T. The role of Si addition on the interfacial interaction in Fe–Al diffusion layer. Scripta Metall Mater 1994;31:1723. [8] Kobayashi S, Yakou T. Control of intermetallic compound layers at interface between steel and aluminum by diffusiontreatment. Materials Science and Engineering 2002;338:44–53. [9] Glasbrenner H, Stein-Fechner K, Konys J. Development of a tritium permeation barrier on F28H-mod. sheets and on
international journal of hydrogen energy 35 (2010) 2689–2693
MANET tubes by hot dip aluminizing an subsequent heat treatment. Report FZK 6079, Karlsruhe; 1998. [10] Glasbrenner H, Stein-Fechner K, Konys J. Scale structure of aluminized Manet steel after HIP treatment. Journal of Nuclear Materials 2000;283:1302–5.
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[11] Benamati G, Buttol p, Casagrande A, Fazio C. Effects of heat treatment and cooling rate on microstructural and mechanical characteristics of aluminide coatings on MANET steel. Journal of Nuclear Materials 1996;230: 214–8.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Influence of silicon on hot-dip aluminizing process and subsequent oxidation for preparing hydrogen/tritium permeation barrier Shilei Han*, Hualing Li, Shumao Wang, Lijun Jiang, Xiaopeng Liu Energy Materials and Technology Research Institute, General Research Institute for Nonferrous Metals, Beijing 100088, China
article info
abstract
Article history:
The development of the International Thermonuclear Experimental Reactor (ITER) requires
Received 7 April 2009
the production of a material capable of acting as a hydrogen/tritium permeation barrier on
Accepted 15 April 2009
low activation steel. It is well known that thin alumina layer can reduce the hydrogen
Available online 17 May 2009
permeation rate by several orders of magnitude. A technology is introduced here to form a ductile Fe/Al intermetallic layer on the steel with an alumina over-layer. This technology,
Keywords:
consisting of two main steps, hot-dip aluminizing (HDA) and subsequent oxidation
Hydrogen permeation barrier
behavior, seems to be a promising coating method to fulfill the required goals. According to
Alumina coating
the experiments that have been done in pure Al, the coatings were inhomogeneous and too
Hot-dip aluminizing
thick. Additionally, a large number of cracks and porous band could be observed. In order to solve these problems, the element silicon was added to the aluminum melt with a nominal composition. The influence of silicon on the aluminizing and following oxidation process was investigated. With the addition of silicon into the aluminum melt, the coating became thinner and more homogeneous. The effort of the silicon on the oxidation behavior was observed as well concerning the suppression of porous band and cracks. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The permeation of hydrogen and its isotopes through materials is not expected in many technological applications, which could produce embrittlement and other defects in steel. One of the main issues in the design of blankets for a future fusion reactor is the permeation of tritium through the structure material into the secondary circuit [1]. A considerable effort has been done in order to reduce the permeation rate of tritium in the blanket of fusion reactor. The development of the International Thermonuclear Experimental Reactor (ITER) requires the production of a material capable of acting as a tritium permeation barrier on low activation steel.
Previous measurements have shown that thin alumina layer can reduce the hydrogen permeation rate by several orders of magnitude [2,3]. A hot-dip aluminizing process seems to offer a good chance to produce aluminized coating by pumping liquid aluminum through a complex geometry of steel to form the required layer [4]. Subsequently, this aluminized layer could be oxidized in high temperature to form an alumina layer on the surface. Two important parameters that determine the mechanical characteristics of the aluminized steel are the thickness and the morphology of the Fe–Al interlayer between the outer aluminum coat and the substrate steel [5]. The whole coating should be kept thin enough since aluminum is an activating
* Corresponding author. Tel.: þ86 10 82241241; fax: þ86 10 82241294. E-mail address: [email protected] (S. Han). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.04.033
2690
international journal of hydrogen energy 35 (2010) 2689–2693
element and not desired in the matrix of low activation steels [6]. The experiments have been done by using a melt of pure Al were not quite successful. These coatings were inhomogeneous and almost all over 100 mm in thickness after the HDA process, too thick for the usage of them. Additionally, a large number of cracks and porous band could be observed in these coatings when the subsequent oxidation reaction was finished. It has been reported that the presence of certain level of silicon in the molten aluminum helps in reducing the thickness of the coating [7,8], but the preparing technology and the mechanism research of the role of silicon are not sufficient. In the present investigation, the material chosen as substrate is the reduced activation steels (namely RAFM steels). CLAM steels, one of the RAFM steels produced in China, is the candidate material for the first wall and structure of ITER.
2.
Experimental details
2.1.
Sample preparation
The chemical composition of the material to be aluminized is given in Table 1. Sheet specimens of 40 15 1.5 mm3 were cut by linear cutting machine. Each sample was degreased in acetone and finally cleaned ultrasonically in ethanol. A hole with its diameter of approximately 3 mm was drilled at one end of the specimen to facilitate its hanging into molten aluminum.
2.2.
Aluminizing process
The aluminum alloy was melted in an alumina crucible placed in a well-type furnace and heated up to 750 C. The temperature was measured by a NiCr-Ni thermocouple which was protected by an alumina tube and placed directly in the aluminum melt. Then, silicon was added into the Al melt with a nominal composition of 3 wt% and the melt was stirred time by time for 1 h. These specimens were fixed by a hook and a stainless steel wire and dipped into the melt. After 30 s of exposure, they were pulled out from the melt and cooled down in the air condition. For comparison, CLAM steel sheets were hot dipped in pure Al melt under the same conditions.
2.3.
Heat oxidation
The heat oxidation was carried out in a vacuum heat-treatment furnace. The aluminized samples were cleaned ultrasonically in ethanol, dried and placed in alumina crucible which was positioned in the hot zone of the furnace about 900 C in an atmosphere of O2 for 5 h.
Table 1 – Chemical composition of CLAM steel (wt%). Fe
Cr
W
V
C
Mn
Ta
Ni
N
Si
Bal.
9.0
1.5
0.2
0.1
0.5
0.07
0.02
0.02
0.01
2.4.
Analytical examination
Metallographic examination was carried out to study the influence of Si on the coating thickness, adherence and the quality of the layer. The scanning electron microscopic examination was conducted on a Hitachi S-4800 microscope. The samples were studied by Grazing Incidence X-ray Diffraction (GIXRD) on a D8-Discover X-ray diffractometer to get information on the compounds existing on the surface.
3.
Results and discussions
3.1.
Aluminized specimens
High purity aluminum (99.99%) was used for aluminizing the samples in pure aluminum without any alloying addition. Typical microstructures of HDA samples at four different temperatures are shown in Fig. 1. Three distinct regions which could be identified in these microstructures included: the outer aluminum layer, the intermetallic compound and the substrate steel. The outer layer had the composition of the aluminum bath. The intermetallic layer (about 20–50 mm thick) formed beneath the Al over-layer by Al diffusion into the steel substrate. When the immersing temperature was elevated, the thickness of the intermetallic layer increased, while the outer layer decreased. With the immersing temperature improved, the diffusion velocity of Al into the steel matrix increased and the intermetallic layer became thick. At the same time, the flowability of the Al melt became better. That is why the thickness of outer layer decreased. SEM/EDX point analyses of HDA samples have shown that the intermetallic layer mainly consisted of the brittle compound Fe2Al5. The composition found with EDX point analyses was 71–76 at.% Al, 21–24 at.% Fe and 2 at.% Cr. The precipitates in the solidified Al layer were also found to be crystals of FeAl3. Additionally, cracks could be observed in the intermetallic layer (Fig. 1(d)). When the alloy element Si was added into the melt Al, it could be easily observed that Si had a significant influence on the thickness and the morphology of the coating. As shown in Fig. 2, the thickness of the intermetallic layer was less than 10 mm and independent of the immersion temperature. The border of the intermetallic layer to the steel side was sharp, and that one to the outer layer was more irregular. The outer layer was 20–40 mm thick and its surface was smooth. Also, cavities or pores could not be observed in each layer. The coating of specimen (Fig. 2(b)). with the total thickness about 25 mm, was the thinnest of the four (Fig. 2(a–d)). The cross sections of the specimens were also analyzed by means of SEM and EDX point analyses. EDX line scans of an aluminized specimen (Fig. 2(b)) are shown in Fig. 3. The top layer (about 10 mm in thickness) mainly consisted of pure Al. The main part of the intermetallic layer corresponded to the Fe2Al5 phase, from the depth of 10 mm to 25 mm. In the transition zone between the intermetallic layer and the outer Al layer, a small band of the compound
international journal of hydrogen energy 35 (2010) 2689–2693
2691
Fig. 1 – Cross-sectional views of aluminized CLAM steel sheets which were hot dip in a melt pure Al at different temperatures, (a) 700 8C, (b) 750 8C, (c) 800 8C, (d) 850 8C.
Fig. 2 – Cross-sectional views of aluminized CLAM steel sheets which were hot dip in a melt of Al–Si at different temperatures (a) 700 8C, (b) 750 8C, (c) 800 8C, (d) 850 8C.
FeAl3 could be detected. In addition, in all specimens (HDA in Al–Si melt) large crystals (marked with arrows) could be observed in the intermetallic layer. These large crystals consisted of Al–Fe–Cr–Si, with Al as the main component decreased the diffusion velocity of Al into the steel matrix and inhibited the growth of Fe2Al5 during the HDA process. It is obvious that the presence of certain level of Si in the molten aluminum helped in reducing the thickness of the intermetallic layer.
3.2.
Aluminized and subsequent oxidized specimens
The coating of the specimen hot-dip aluminized at 750 C in a melt of Al–Si was the thinnest and was chosen as the subsequent high temperature oxidized materials. Fig. 4(a) shows a cross section of an aluminized CLAM specimen which
Al
800
FeAl3
Fe2Al5
Matrix
Al Fe
Intensity
600
400
200
0 0
10
20 depth (µm)
30
40
Fig. 3 – EDX line scans were carried out on the cross section of an aluminized CLAM steel sheet which was hot dip in a melt of Al–Si at 750 8C.
was oxidized at the temperature of 900 C for 5 h in at the temperature of 900 C for 5 h. Two layers can be identified on the steel surface: an internal layer namely a-Fe(Al) and an external layer (FeAl) about 50 mm and 60 mm in thickness, respectively. Fe2Al5 layer, which formed in the hot-dip aluminized process, was not found after the oxidation treatment owning to the out ward diffusion of iron from matrix and to the inward diffusion of aluminum. Hence, the intermetallic Fe2Al5 phase transformed completely into softer phases, a-Fe(Al) and FeAl. The thickness of the internal layer was found to be dependent on the heat treatment chosen, while the thickness of the external layer was dependent on the amount of solidified Al which adhered to the surface after the hot-dip aluminizing process [9]. It has been reported that the internal and external layer are separated by a porous band. In our experiment, the border was not very clear and the adhesion between those two phases seemed to be excellent, and no cavities or pores could be observed in the layer. If there are less pores formed during the heat oxidation process, it is likely that the mechanical properties of the layer could be improved and the hydrogen/ tritium permeation rate could be reduced as well [10]. EDX point analyses have shown that there was a high concentration of Al and Oxygen near the surface of the coating (the oxygen content reached 55 at.%, while the Al content was 40 at.%). The compound Al2O3 corresponded correctly to the concentration of oxygen and Al. These specimens were analyzed by Grazing Incidence X-ray Diffraction (GIXRD) in order to make sure the existence of Al2O3. In Fig. 4(b), the XRD patterns of the specimen provided support to the existence of the out layer FeAl and the g-Al2O3 layer. As shown in Fig. 4(c), the g-Al2O3 layer could also be found about 0.8 mm in thickness. The results depend on both the heating temperature and the exposure time. In fact, if from a thermodynamic point of view the appearance of the different phases is closely linked
2692
international journal of hydrogen energy 35 (2010) 2689–2693
all the intermetallic Fe2Al5 to ductile phases (FeAl and a-Fe(Al)) and product g-Al2O3 over-layer.
4.
Conclusions
It was shown that CLAM steel could be aluminized by means of hot-dipping in an Al/Si melt. The thickness of the intermetallic layer of the specimen aluminized decreased significantly from 80 mm to about 10 mm compared to the experiments in pure Al bath. The thinnest coating (aluminized at 750 C in a melt of Al–Si) was uniform in thickness and composition. The existence of the large crystals (consist of Al– Fe–Cr–Si) in the intermetallic layer may be the reason for the decrease of the diffusion velocity of Al into the steel matrix during the HDA process. In the subsequent heat treatment, the intermetallic layer and the outer aluminum layer transformed into the ductile phases (FeAl and a-Fe(Al)), with the thickness of about 100 mm. The porous band between FeAl layer and a-Fe(Al) layer was suppressed and no cavities or pores could be observed in the internal or external layer. It is also interesting to observe that at the temperature 900 C the presence of g-Al2O3 was detected by XRD on the surface of the specimen at the atmosphere of O2. The oxide coating was only 0.8 mm thick. However, as far as the tritium permeation barriers are concerned, this problem appears not so important because an oxide thickness less than 1 mm is sufficient. Conclusion demonstrated that with the addition of Si into the Al melt, the coating preparation for hydrogen/tritium was improved.
references
Fig. 4 – (a)Cross-section view of hot aluminized and subsequent oxidized specimen (900 8C/5 h). (b) Grazing Incidence X-ray Diffraction of aluminized and subsequent oxidized specimen. (c) SEM microphotograph of the crosssection view of aluminized and subsequent oxidized specimen (900 8C/5 h).
to the temperature (for example, FeAl and a-Fe(Al)), the complete transformation from one phase to another (for example, from Fe2Al5 to FeAl and a-Fe(Al)) is more or less the combined result of thermodynamic and kinetic factors, such as temperature and time [11]. The combined effect of these two parameters (at 900 C for 5 h) was sufficient to transform
[1] Tanabe T. On the estimation of tritium retention and permeation in metallic first wall. Fusion Engineering Design 1989;10:325–30. [2] Perujo A, Forcey KS, Sample T. Reduction of deuterium permeation through DIN 1.4914 stainless steel (MANET) by plasma-spray deposited aluminum. Nuclear Material 1993; 207:86–91. [3] Glasbrenner H, Perujo A, Serra E. Hydrogen permeation behavior of hot-dip aluminized MANET steel. Fusion Technology 1995;28:1159–64. [4] Benamati G, Chabrol C, Perujo A. Development of tritium permeation barriers on Al base in Europe. Journal of Nuclear Materials 1999;271:391–5. [5] Gul Hameed Awan, Faiz ul Hasan. The morphology of coating/substrate interface in hot-dip-aluminized steels. Materials Science and Engineering 2008;472:157–165. [6] Glasbrenner H, Wedemeyer O. Comparison of hot dip aluminized F28-mod. steel after different subsequent heat treatments. Journal of Nuclear Materials 1998;257: 274–81. [7] Akdeniz MV, Mekhrabov AO, Yilmaz T. The role of Si addition on the interfacial interaction in Fe–Al diffusion layer. Scripta Metall Mater 1994;31:1723. [8] Kobayashi S, Yakou T. Control of intermetallic compound layers at interface between steel and aluminum by diffusiontreatment. Materials Science and Engineering 2002;338:44–53. [9] Glasbrenner H, Stein-Fechner K, Konys J. Development of a tritium permeation barrier on F28H-mod. sheets and on
international journal of hydrogen energy 35 (2010) 2689–2693
MANET tubes by hot dip aluminizing an subsequent heat treatment. Report FZK 6079, Karlsruhe; 1998. [10] Glasbrenner H, Stein-Fechner K, Konys J. Scale structure of aluminized Manet steel after HIP treatment. Journal of Nuclear Materials 2000;283:1302–5.
2693
[11] Benamati G, Buttol p, Casagrande A, Fazio C. Effects of heat treatment and cooling rate on microstructural and mechanical characteristics of aluminide coatings on MANET steel. Journal of Nuclear Materials 1996;230: 214–8.