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HCM12A oxide layer investigation using scanning probe microscope
Journal of Nuclear Materials 431 (2012) 120–124
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
HCM12A oxide layer investigation using scanning probe microscope Kenji Kikuchi a,⇑, Abu Khalid Rivai b, Shigeru Saito b, Alan Michael Bolind a, Akinori Kogure c a
Frontier Research Center for Applied Atomic Sciences, Ibaraki University, 162-1, Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan JAEA, 2-4, Shirane, Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan c Shimadzu AMC, 380-1, Horiyamashita, Hadao, Kanagawa 259-1304, Japan b
a r t i c l e
i n f o
Article history: Available online 23 November 2011
a b s t r a c t The oxide layer on the 12Cr ferritic/martensitic steel, HCM12A, has been investigated using a scanning probe microscope (SPM). The oxide layer was formed in lead bismuth eutectic at 450–500 °C, during 5500 h. After EDX analyses, scanning probe microscope techniques were utilized for analyzing the oxide layers and the bulk material with a function of the potential and delayed phase measurements. Hitherto oxide formation mechanisms were studied from the points of view of the location of the original base metal surface, the path for oxygen travel, and Fe diffusion, in order to understand the stability of oxide layer, which is expected to permit the steel to have an endurable performance against lead–bismuth eutectic at high temperatures. The new findings in this research are that surface potential measurement detected the boundary between (FeCr)3O4 and Fe3O4, which is not found with topographic mode measurements. The spinel layer can be distinguished from the bulk area with lower surface potential profile, but near the boundary between the spinel and magnetite layers, the surface potential profile seems to be continuous except for the narrow path corresponding to the boundary line. The band structure penetrating the magnetite and spinel layers was found, which was not found in the topography also. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiment
The oxide layer on the ferritic martensitic (F/M) steel consists of duplex layers: magnetite Fe3O4 at the LBE side and spinel (FeCr)3O4 at the base metal side [1,2]. The original surface exists near the interface between the magnetite and spinel. An early question was how the oxide layers on the surface of the base metal grew. The oxidation reaction needs a path, the formation of which is achieved by the dissociative/perforative growth in the magnetite. The path allows the quick diffusion of oxygen to the FM/spinel interface. Oxygen cannot diffuse in the oxide lattice because its rate is insufficient for Fe–Cr spinel formation. Quick oxygen diffusion could be possible along the grain boundaries or short cut paths like cracks [3,4]. This concept is, however, not fully verified because of invisibility. In order to investigate those paths that remained in the oxidized surface layers, a scanning probe microscope has been adapted to the research on the HCM12A oxide layer using surface potential and phase delay mode measurements. Hitherto, Hosemann et al. attempted nano-scale characterization of F/M steel oxide layers, on HT-9 using atomic force microscopy (AFM) [5]. In this report, a new attempt was made to investigate the HCM12A oxide layer.
2.1. Material
⇑ Corresponding author. Tel.: +81 29 352 3238; fax: +81 29 287 7872. E-mail address: [email protected] (K. Kikuchi). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.11.020
HCM12A material was provided in a cylinder form by the developer, Sumitomo Metal Industries, Ltd. The chemical composition of the steel is shown in Table 1 [6]. The material is high chromium, ferritic steel with tempered martensite, and was developed for the use in high temperature components, at temperatures up to 620 °C. It is applicable to the usage in the core cladding of a nuclear reactor with a coolant of liquid metal, for example, lead bismuth eutectic (LBE). HCM12A is one of the leading candidate F/M steels for Generation IV systems, e.g., LFR (lead-cooled fast reactor) and SCWR (supercritical-watercooled reactor), due to its high strength and favorable corrosion resistance [2]. 2.2. Specimen preparation The material is the same as described in [7]. Samples were obtained from a specimen holder, which had a size of 48 mm in outer diameter and 230 mm in length, and was used in the LBE loop at MES, Mitsui Engineering & Ship-building Co. Ltd. [6]. The specimen holder included two channels for corrosion test pieces in the middle part of the cylinder, arrayed in the axial direction. There was a plenum area at the top and bottom parts of the cylinder. A ring
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K. Kikuchi et al. / Journal of Nuclear Materials 431 (2012) 120–124 Table 1 Chemical composition of HCM12A (mass%). Fe
Cr
Si
Mn
Mo
C
Cu
W
Ni
V
Bal.
10.64
0.3
0.61
0.37
0.14
0.84
1.94
032
0.19
mounted in epoxy resin and subsequently polished using a 0.25 lm diamond paste as the last step. The average LBE flow rate was 0.08 m/s at the plenum area from which the specimens were retrieved. The oxide layer that formed on the inner surface of the ring sample was found to be a duplex layer of spinel and magnetite [2,7]. The oxide layer was formed during 5500 h operation in the MES loop at 450–500 °C under oxygen control, with the oxygen level maintained between 5 10 6 and 5 10 5 wt.% by controlling the flow rate of a mixed gas of hydrogen and argon [1]. 2.3. SEM, EDX and SPM analyses of oxide layer The cross section of sample A was observed by a field-emissiontype scanning electron microscope (FE-SEM), JEOL-7000. The surface structure at the oxide layer of sample B was characterized by a scanning probe microscope, SPM-9600 Shimadzu, using surface potential (SP), delayed-phase (DP) and dynamic atomic force microscope (AFM) modes. SP measurement is equivalent with KFM, Kelvin Force Microscopy. DP mode detects a delayed phase during scanning of the surface in dynamic mode, where a magnetized probe is vibrated at close to its resonant frequency. In dynamic mode AFM, a probe is vibrated to its resonant frequency. When a vibrated probe tip approaches the sample surface, the amplitude will change. A feedback control to maintain the same vibration amplitude is transformed to topography information. The probe tip radius used in this investigation was 20–30 nm, made of SiC coated with Pt/Ir.
Fig. 1. Sample cut from specimen holder at the MES loop.
3. Results and discussions specimen investigated in this study was retrieved from the bottom cylindrical part with 3 mm in thickness, as shown in Fig. 1. A ring sample was cut from the specimen holder. Arc-shaped pieces, indicated by samples A and B, were then cut from the ring. The cutting faces were normal to the circumferential direction in the sample A, but inclined in the sample B, respectively. The samples were
3.1. SEM and EDX analyses Elemental analyses were done along the line and the area, respectively. It showed the base metal, Cr-rich spinel layer, Fe-rich magnetite layer and LBE at the outer side of the sample, as shown
O
Bulk
Bulk
Fe Pb- Bi
Spot-2
1 0 µm
Cr IM G1
Spot-1
Oxide layer (Fe) oxide 10µm
(Fe- Cr) oxide Bulk
IMG1
10 µm
Fig. 2. FE–SEM observation of sample A.
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small round particles with a size of a less than 1 lm. The holes pattern in the bulk area consists of lines and cavities. A cavity size is less than 1 lm, which located at the boundary between spinel and base metal as well as near the boundary. All cavities are located in the bulk metal, while the holes patterns can be observed on both sides of the boundary, in the bulk and spinel areas. The lines that look like scratches may have been introduced during the polishing process, but the detail is unclear. 3.2. SPM analyses
Fig. 3. Detail observation of spots 1 and 2 in Fig. 2. LBE stained in spot 1 (top) and cavities were observed at the boundary but in the bulk side.
in Fig. 2. The oxide layer shows a typical duplex layer structure. The width of the Cr-rich layer is 8 lm, while the width of Fe-rich layer is 10 lm. Along a scanned line, a Pb–Bi rich part is locally observed in the Fe-rich magnetite layer, as indicated by the spot-1 area. Pb–Bi penetrated perhaps through a crack in the oxide layer. At the boundary between the spinel and bulk base metal, a pattern of holes is seen, as indicated by spot-2. Fig. 3 shows detailed observations of spot-1 and spot-2 in Fig. 2. The Pb and Bi stained area size is 2 lm in total, which consists of
Fig. 4 shows topography and SP measurement results on the surface of sample B. The scanned location (left) is the same as that for the topography image (right). The scanned area is 100 100 lm. The cross section is characterized by bulk material (B), spinel layer (S) and magnetite layer (M). A line structure can be seen, as indicated by an Arrow-1 between the B and S layers. This line structure can also be seen as a boundary between the B and S layers in the topography. Another line structure indicated by Arrow-2 can be seen in the potential mapping by a relatively lower potential value. The surface potential between the probe tip and the measuring surface is related to a difference between the work functions of the two surfaces. Importantly the line structure is not seen in the topography image. The line is located between the S and M layers, which corresponds to the front line of the spinel layer. It is a new piece of knowledge that the surface potential mapping can detect the boundary between the spinel layer of the base metal and the oxide layer formed toward the LBE layer. Arrow-3 indicates a band structure that penetrates the M and S layers. The band structure shows a higher potential value as compared to other regions, originating from the top area that corresponds to lead oxide. The area indicated by circles shows that inhomogeneous structure exits in the spinel layer, but the detail is unclear. Fig. 5 shows topography (left) and DP (right) results on the surface of sample B, which was observed at the different cross section, as shown in Fig. 4. The scanned area is 95 95 lm. The cross section is characterized by bulk material (B), spinel layer (S) and magnetite layer (M) as well as adhered LBE area (L), which was shown at the top right corner. A broken line indicates the boundary between the B and S layers. The delayed degree in the bulk area is less than that in the magnetite and spinel layers. A large delayed-phase area, however, is shown in the bulk area. The small dots shown in the DP mode are a noise in the measurement. The probe scanned the surface in the horizontal direction.
Arrow-3
M 650.00 [nm]
Arrow-2 Area
S
Arrow-1 50.00 µm
B 100.00 x 100.00 µm
0.22 [V]
Arrow-1 0.00
50.00 µm
100.00 x 100.00 µm
Fig. 4. Topography (left) and SP (right) results on the surface of sample B.
0.10
123
K. Kikuchi et al. / Journal of Nuclear Materials 431 (2012) 120–124
L M
Large delayed phase area
S
B
Fig. 5. Topography (left) and DP (right) results on the surface of sample B.
C-D
0.23 [V]
C 0.11
E
0.00
20.00 [µm] [µm]
A
D
G
3.21
0.22 [V]
[º]
[V]
[º]
[V]
[º]
0.10
F
B
E-F
E-F H
0.13 [V]
0.11
0.00
20.00 [µm] [µm]
1.20
50.00 µm
A-B
[V]
100.00 x 100.00 µm
0.02
0.10
0.19 [V]
G-H
A-B
0.15 [V]
G-H 0.11
0.11
0.00
20.00 [µm] [µm]
3.39
[V]
0.00
20.00 [µm] [µm]
[º] 0.00
0.07
0.03
Fig. 6. Detail surface potential measurement of sample B.
Hitherto, HCM12A materials have been studied for the condition of corrosion in LBE [7,8]. But detailed observations on the oxide layers using AFM have not been done yet. Concerns have been paid to the understanding of the formation mechanism of oxide layers in LBE. Through EDX analyses, it has been found that the oxide layer of ferritic martensitic steel HCM12A consists of a Cr-rich spinel layer and a Fe-rich magnetite layer. Magnetite and spinel layers were formed. Oxygen diffused from the LBE to these layers. The LBE stained spot area was found in the magnetite layer by the EDX analyses. High magnification observation shows round-shaped particles gathered in the stained spot area, as shown in Fig. 3. The round particles seem to be lead oxide, although EDX line analyses indicated Pb and Bi. It indicates that
there was a path for lead penetration in the magnetite layers. Apparently the magnetite layer shows a very perforated structure, especially near the surface, as shown in Fig. 2. It shows that LBE was accommodated during magnetite-layer formation, or it traveled from the surface into the layer through the paths. Cavities were observed at the boundary between the bulk material and the spinel layer. They are located on the bulk side, but not on the spinel side. Furthermore they are located on the bulk side in the area between the boundary and a couple of micron meters away from the boundary. The area corresponds to the transient space where the oxygen concentration reduced very quickly and the iron concentration recovered quickly to the bulk side state. But Cr kept its concentration through the spinel and bulk material as shown in Fig. 2. Fe diffused out from the bulk material. In the spinel layer, no
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cavities were seen. Cavities were filled with oxidation of Fe and Cr, where spinel was formed [3,4]. The AFM technique applied in this report was developed in 1980s. Hosemann reported the application of AFM, MFM [5] and C-AFM [9] techniques to ferritic/martensitic (HT-9) and austenitic (D9) steel samples corroded in LBE. C-AFM is a contact-mode electrical characterization technique between the conductive AFM tip and the sample surface, by which variations in the local electrical properties are monitored in a range of pA to lA. MFM has the same function as the delayed phase mode in this measurement. The conductive/magnetic property was characterized throughout the oxide layer. ‘‘SPM’’ (scanning probe microscope) in principle is synonymous with STM (Scanning Tunneling Microscope) and AFM, and it is often used as a generic technical word. SPM-9600 scans surface in the same way as AFM with the functions of SP and DP measurements. For detailed observation using SPM, a cross-section was enlarged in the circumferential direction in order to observe structural continuity and discontinuity among bulk, spinel and magnetite layers. SP mode measures the surface potential in a range of mV. It is found that SP mode can detect the boundary between spinel and magnetite layers, which was not seen in the topographic mode. This is the first finding that the surface potential measurement can detect the boundary between S and M layers because of discontinuity of the potential between the conductive probe tip and the sample surface. A high potential path penetrating the magnetite and spinel layers has been detected as shown in Fig. 4. The path is linked to the perforated area of the magnetite layer, as shown in Fig. 2. This structural continuity is suggesting possible LBE travel from the surface into the inner structure. The width of the high potential path in the spinel layer is narrower than that in the magnetite area near the surface. Fig. 6 shows a surface potential described by voltage at crossing lines A–B and C–D at the band paths, E–F at the S and M interface, and G–H at the B and S interface, as shown in Fig. 4. The differential voltage at A–B and C–D is 0.07 V and 0.1 V, respectively. The width of the band path is 3–4 lm. The differential voltage at E–F drops by 0.02 V from the surrounding area and the width of the boundary line is 1.2 lm. The potential in the magnetite is slightly higher than that in the spinel. The surface potential profile from bulk material to spinel layer along the G-
H line indicates that the potential in the spinel is 0.03 V larger than in the bulk material. 4. Conclusions Scanning probe microscopy with a function of surface potential measurement and delayed phase modes was adapted to investigate the oxide layer of HCM12A. The surface potential measurement detected the boundary between (FeCr)3O4 and Fe3O4, which is not found in the topographic mode measurement. The spinel layer can be distinguished from the bulk area with lower surface potential profile, but near the boundary between the spinel and magnetite layers the surface potential profile seems to be continuous except for the narrow path corresponding to the boundary line. The band structure penetrating the magnetite and spinel layers was found, which was not found in the topography image. The understanding of the results in the delayed phase mode needs further investigation. Acknowledgements Authors appreciate Dr. Yoshiatsu SAWARAGI of SUMITOMO Metal Technology Ltd. for providing HCM12A materials, and also appreciate Dr. Mikinori ONO of MES Ltd. for using the materials as sample holder in the LBE loop. References [1] K. Kikuchi, K. Kamata, M. Ono, T. Kitano, K. Hayashi, H. Oigawa, J. Nucl. Mater. 377 (2008) 232–242. [2] L. Tan, M.T. Machut, K. Sridharan, T.R. Allen, J. Nucl. Mater. 371 (2007) 161–170. [3] L. Martinelli, F. Balbaud-Célérier, A. Terlain, S. Delpech, G. Santarini, J. Favergeon, G. Moulin, M. Tabarant, G. Picard, Corros. Sci. 50 (2008) 2523–2536. [4] L. Martinelli, F. Balbaud-Célérier, A. Terlain, S. Bosonnet, G. Picard, G. Santarini, Corros. Sci. 50 (2008) 2537–2548. [5] P. Hosemann, M. Hawley, G. Mori, N. Li, S.A. Maloy, J. Nucl. Mater. 376 (2008) 289–292. [6] F. Masuyama, New developments in steels for power generation boilers, in: R. Viswanathan, J. Nutting (Eds.), Advanced Heat Resistant Steels for Power Generation, Conference Proceedings, San Sebastian, Spain, 27 April, 1998. [7] E. Yamaki, K. Kikuchi, J. Nucl. Mater. 398 (2010) 153–159. [8] Tomohiro Furukawa, Georg Müller, Gustav Schumacher, Alfons Weisenburger, Annette Heinzel, Frank Zimmermann, Kazumi Aoto, J. Nucl. Sci. Technol. 41 (2004) 265–270. [9] P. Hosemann, M.E. Hawley, D. Koury, D. Welch, A.J. Johson, G. Mori, N. Li, S.A. Maloy, J. Nucl. Mater. 381 (2008) 211–215.
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
HCM12A oxide layer investigation using scanning probe microscope Kenji Kikuchi a,⇑, Abu Khalid Rivai b, Shigeru Saito b, Alan Michael Bolind a, Akinori Kogure c a
Frontier Research Center for Applied Atomic Sciences, Ibaraki University, 162-1, Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1106, Japan JAEA, 2-4, Shirane, Shirakata, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan c Shimadzu AMC, 380-1, Horiyamashita, Hadao, Kanagawa 259-1304, Japan b
a r t i c l e
i n f o
Article history: Available online 23 November 2011
a b s t r a c t The oxide layer on the 12Cr ferritic/martensitic steel, HCM12A, has been investigated using a scanning probe microscope (SPM). The oxide layer was formed in lead bismuth eutectic at 450–500 °C, during 5500 h. After EDX analyses, scanning probe microscope techniques were utilized for analyzing the oxide layers and the bulk material with a function of the potential and delayed phase measurements. Hitherto oxide formation mechanisms were studied from the points of view of the location of the original base metal surface, the path for oxygen travel, and Fe diffusion, in order to understand the stability of oxide layer, which is expected to permit the steel to have an endurable performance against lead–bismuth eutectic at high temperatures. The new findings in this research are that surface potential measurement detected the boundary between (FeCr)3O4 and Fe3O4, which is not found with topographic mode measurements. The spinel layer can be distinguished from the bulk area with lower surface potential profile, but near the boundary between the spinel and magnetite layers, the surface potential profile seems to be continuous except for the narrow path corresponding to the boundary line. The band structure penetrating the magnetite and spinel layers was found, which was not found in the topography also. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiment
The oxide layer on the ferritic martensitic (F/M) steel consists of duplex layers: magnetite Fe3O4 at the LBE side and spinel (FeCr)3O4 at the base metal side [1,2]. The original surface exists near the interface between the magnetite and spinel. An early question was how the oxide layers on the surface of the base metal grew. The oxidation reaction needs a path, the formation of which is achieved by the dissociative/perforative growth in the magnetite. The path allows the quick diffusion of oxygen to the FM/spinel interface. Oxygen cannot diffuse in the oxide lattice because its rate is insufficient for Fe–Cr spinel formation. Quick oxygen diffusion could be possible along the grain boundaries or short cut paths like cracks [3,4]. This concept is, however, not fully verified because of invisibility. In order to investigate those paths that remained in the oxidized surface layers, a scanning probe microscope has been adapted to the research on the HCM12A oxide layer using surface potential and phase delay mode measurements. Hitherto, Hosemann et al. attempted nano-scale characterization of F/M steel oxide layers, on HT-9 using atomic force microscopy (AFM) [5]. In this report, a new attempt was made to investigate the HCM12A oxide layer.
2.1. Material
⇑ Corresponding author. Tel.: +81 29 352 3238; fax: +81 29 287 7872. E-mail address: [email protected] (K. Kikuchi). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.11.020
HCM12A material was provided in a cylinder form by the developer, Sumitomo Metal Industries, Ltd. The chemical composition of the steel is shown in Table 1 [6]. The material is high chromium, ferritic steel with tempered martensite, and was developed for the use in high temperature components, at temperatures up to 620 °C. It is applicable to the usage in the core cladding of a nuclear reactor with a coolant of liquid metal, for example, lead bismuth eutectic (LBE). HCM12A is one of the leading candidate F/M steels for Generation IV systems, e.g., LFR (lead-cooled fast reactor) and SCWR (supercritical-watercooled reactor), due to its high strength and favorable corrosion resistance [2]. 2.2. Specimen preparation The material is the same as described in [7]. Samples were obtained from a specimen holder, which had a size of 48 mm in outer diameter and 230 mm in length, and was used in the LBE loop at MES, Mitsui Engineering & Ship-building Co. Ltd. [6]. The specimen holder included two channels for corrosion test pieces in the middle part of the cylinder, arrayed in the axial direction. There was a plenum area at the top and bottom parts of the cylinder. A ring
121
K. Kikuchi et al. / Journal of Nuclear Materials 431 (2012) 120–124 Table 1 Chemical composition of HCM12A (mass%). Fe
Cr
Si
Mn
Mo
C
Cu
W
Ni
V
Bal.
10.64
0.3
0.61
0.37
0.14
0.84
1.94
032
0.19
mounted in epoxy resin and subsequently polished using a 0.25 lm diamond paste as the last step. The average LBE flow rate was 0.08 m/s at the plenum area from which the specimens were retrieved. The oxide layer that formed on the inner surface of the ring sample was found to be a duplex layer of spinel and magnetite [2,7]. The oxide layer was formed during 5500 h operation in the MES loop at 450–500 °C under oxygen control, with the oxygen level maintained between 5 10 6 and 5 10 5 wt.% by controlling the flow rate of a mixed gas of hydrogen and argon [1]. 2.3. SEM, EDX and SPM analyses of oxide layer The cross section of sample A was observed by a field-emissiontype scanning electron microscope (FE-SEM), JEOL-7000. The surface structure at the oxide layer of sample B was characterized by a scanning probe microscope, SPM-9600 Shimadzu, using surface potential (SP), delayed-phase (DP) and dynamic atomic force microscope (AFM) modes. SP measurement is equivalent with KFM, Kelvin Force Microscopy. DP mode detects a delayed phase during scanning of the surface in dynamic mode, where a magnetized probe is vibrated at close to its resonant frequency. In dynamic mode AFM, a probe is vibrated to its resonant frequency. When a vibrated probe tip approaches the sample surface, the amplitude will change. A feedback control to maintain the same vibration amplitude is transformed to topography information. The probe tip radius used in this investigation was 20–30 nm, made of SiC coated with Pt/Ir.
Fig. 1. Sample cut from specimen holder at the MES loop.
3. Results and discussions specimen investigated in this study was retrieved from the bottom cylindrical part with 3 mm in thickness, as shown in Fig. 1. A ring sample was cut from the specimen holder. Arc-shaped pieces, indicated by samples A and B, were then cut from the ring. The cutting faces were normal to the circumferential direction in the sample A, but inclined in the sample B, respectively. The samples were
3.1. SEM and EDX analyses Elemental analyses were done along the line and the area, respectively. It showed the base metal, Cr-rich spinel layer, Fe-rich magnetite layer and LBE at the outer side of the sample, as shown
O
Bulk
Bulk
Fe Pb- Bi
Spot-2
1 0 µm
Cr IM G1
Spot-1
Oxide layer (Fe) oxide 10µm
(Fe- Cr) oxide Bulk
IMG1
10 µm
Fig. 2. FE–SEM observation of sample A.
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K. Kikuchi et al. / Journal of Nuclear Materials 431 (2012) 120–124
small round particles with a size of a less than 1 lm. The holes pattern in the bulk area consists of lines and cavities. A cavity size is less than 1 lm, which located at the boundary between spinel and base metal as well as near the boundary. All cavities are located in the bulk metal, while the holes patterns can be observed on both sides of the boundary, in the bulk and spinel areas. The lines that look like scratches may have been introduced during the polishing process, but the detail is unclear. 3.2. SPM analyses
Fig. 3. Detail observation of spots 1 and 2 in Fig. 2. LBE stained in spot 1 (top) and cavities were observed at the boundary but in the bulk side.
in Fig. 2. The oxide layer shows a typical duplex layer structure. The width of the Cr-rich layer is 8 lm, while the width of Fe-rich layer is 10 lm. Along a scanned line, a Pb–Bi rich part is locally observed in the Fe-rich magnetite layer, as indicated by the spot-1 area. Pb–Bi penetrated perhaps through a crack in the oxide layer. At the boundary between the spinel and bulk base metal, a pattern of holes is seen, as indicated by spot-2. Fig. 3 shows detailed observations of spot-1 and spot-2 in Fig. 2. The Pb and Bi stained area size is 2 lm in total, which consists of
Fig. 4 shows topography and SP measurement results on the surface of sample B. The scanned location (left) is the same as that for the topography image (right). The scanned area is 100 100 lm. The cross section is characterized by bulk material (B), spinel layer (S) and magnetite layer (M). A line structure can be seen, as indicated by an Arrow-1 between the B and S layers. This line structure can also be seen as a boundary between the B and S layers in the topography. Another line structure indicated by Arrow-2 can be seen in the potential mapping by a relatively lower potential value. The surface potential between the probe tip and the measuring surface is related to a difference between the work functions of the two surfaces. Importantly the line structure is not seen in the topography image. The line is located between the S and M layers, which corresponds to the front line of the spinel layer. It is a new piece of knowledge that the surface potential mapping can detect the boundary between the spinel layer of the base metal and the oxide layer formed toward the LBE layer. Arrow-3 indicates a band structure that penetrates the M and S layers. The band structure shows a higher potential value as compared to other regions, originating from the top area that corresponds to lead oxide. The area indicated by circles shows that inhomogeneous structure exits in the spinel layer, but the detail is unclear. Fig. 5 shows topography (left) and DP (right) results on the surface of sample B, which was observed at the different cross section, as shown in Fig. 4. The scanned area is 95 95 lm. The cross section is characterized by bulk material (B), spinel layer (S) and magnetite layer (M) as well as adhered LBE area (L), which was shown at the top right corner. A broken line indicates the boundary between the B and S layers. The delayed degree in the bulk area is less than that in the magnetite and spinel layers. A large delayed-phase area, however, is shown in the bulk area. The small dots shown in the DP mode are a noise in the measurement. The probe scanned the surface in the horizontal direction.
Arrow-3
M 650.00 [nm]
Arrow-2 Area
S
Arrow-1 50.00 µm
B 100.00 x 100.00 µm
0.22 [V]
Arrow-1 0.00
50.00 µm
100.00 x 100.00 µm
Fig. 4. Topography (left) and SP (right) results on the surface of sample B.
0.10
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K. Kikuchi et al. / Journal of Nuclear Materials 431 (2012) 120–124
L M
Large delayed phase area
S
B
Fig. 5. Topography (left) and DP (right) results on the surface of sample B.
C-D
0.23 [V]
C 0.11
E
0.00
20.00 [µm] [µm]
A
D
G
3.21
0.22 [V]
[º]
[V]
[º]
[V]
[º]
0.10
F
B
E-F
E-F H
0.13 [V]
0.11
0.00
20.00 [µm] [µm]
1.20
50.00 µm
A-B
[V]
100.00 x 100.00 µm
0.02
0.10
0.19 [V]
G-H
A-B
0.15 [V]
G-H 0.11
0.11
0.00
20.00 [µm] [µm]
3.39
[V]
0.00
20.00 [µm] [µm]
[º] 0.00
0.07
0.03
Fig. 6. Detail surface potential measurement of sample B.
Hitherto, HCM12A materials have been studied for the condition of corrosion in LBE [7,8]. But detailed observations on the oxide layers using AFM have not been done yet. Concerns have been paid to the understanding of the formation mechanism of oxide layers in LBE. Through EDX analyses, it has been found that the oxide layer of ferritic martensitic steel HCM12A consists of a Cr-rich spinel layer and a Fe-rich magnetite layer. Magnetite and spinel layers were formed. Oxygen diffused from the LBE to these layers. The LBE stained spot area was found in the magnetite layer by the EDX analyses. High magnification observation shows round-shaped particles gathered in the stained spot area, as shown in Fig. 3. The round particles seem to be lead oxide, although EDX line analyses indicated Pb and Bi. It indicates that
there was a path for lead penetration in the magnetite layers. Apparently the magnetite layer shows a very perforated structure, especially near the surface, as shown in Fig. 2. It shows that LBE was accommodated during magnetite-layer formation, or it traveled from the surface into the layer through the paths. Cavities were observed at the boundary between the bulk material and the spinel layer. They are located on the bulk side, but not on the spinel side. Furthermore they are located on the bulk side in the area between the boundary and a couple of micron meters away from the boundary. The area corresponds to the transient space where the oxygen concentration reduced very quickly and the iron concentration recovered quickly to the bulk side state. But Cr kept its concentration through the spinel and bulk material as shown in Fig. 2. Fe diffused out from the bulk material. In the spinel layer, no
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cavities were seen. Cavities were filled with oxidation of Fe and Cr, where spinel was formed [3,4]. The AFM technique applied in this report was developed in 1980s. Hosemann reported the application of AFM, MFM [5] and C-AFM [9] techniques to ferritic/martensitic (HT-9) and austenitic (D9) steel samples corroded in LBE. C-AFM is a contact-mode electrical characterization technique between the conductive AFM tip and the sample surface, by which variations in the local electrical properties are monitored in a range of pA to lA. MFM has the same function as the delayed phase mode in this measurement. The conductive/magnetic property was characterized throughout the oxide layer. ‘‘SPM’’ (scanning probe microscope) in principle is synonymous with STM (Scanning Tunneling Microscope) and AFM, and it is often used as a generic technical word. SPM-9600 scans surface in the same way as AFM with the functions of SP and DP measurements. For detailed observation using SPM, a cross-section was enlarged in the circumferential direction in order to observe structural continuity and discontinuity among bulk, spinel and magnetite layers. SP mode measures the surface potential in a range of mV. It is found that SP mode can detect the boundary between spinel and magnetite layers, which was not seen in the topographic mode. This is the first finding that the surface potential measurement can detect the boundary between S and M layers because of discontinuity of the potential between the conductive probe tip and the sample surface. A high potential path penetrating the magnetite and spinel layers has been detected as shown in Fig. 4. The path is linked to the perforated area of the magnetite layer, as shown in Fig. 2. This structural continuity is suggesting possible LBE travel from the surface into the inner structure. The width of the high potential path in the spinel layer is narrower than that in the magnetite area near the surface. Fig. 6 shows a surface potential described by voltage at crossing lines A–B and C–D at the band paths, E–F at the S and M interface, and G–H at the B and S interface, as shown in Fig. 4. The differential voltage at A–B and C–D is 0.07 V and 0.1 V, respectively. The width of the band path is 3–4 lm. The differential voltage at E–F drops by 0.02 V from the surrounding area and the width of the boundary line is 1.2 lm. The potential in the magnetite is slightly higher than that in the spinel. The surface potential profile from bulk material to spinel layer along the G-
H line indicates that the potential in the spinel is 0.03 V larger than in the bulk material. 4. Conclusions Scanning probe microscopy with a function of surface potential measurement and delayed phase modes was adapted to investigate the oxide layer of HCM12A. The surface potential measurement detected the boundary between (FeCr)3O4 and Fe3O4, which is not found in the topographic mode measurement. The spinel layer can be distinguished from the bulk area with lower surface potential profile, but near the boundary between the spinel and magnetite layers the surface potential profile seems to be continuous except for the narrow path corresponding to the boundary line. The band structure penetrating the magnetite and spinel layers was found, which was not found in the topography image. The understanding of the results in the delayed phase mode needs further investigation. Acknowledgements Authors appreciate Dr. Yoshiatsu SAWARAGI of SUMITOMO Metal Technology Ltd. for providing HCM12A materials, and also appreciate Dr. Mikinori ONO of MES Ltd. for using the materials as sample holder in the LBE loop. References [1] K. Kikuchi, K. Kamata, M. Ono, T. Kitano, K. Hayashi, H. Oigawa, J. Nucl. Mater. 377 (2008) 232–242. [2] L. Tan, M.T. Machut, K. Sridharan, T.R. Allen, J. Nucl. Mater. 371 (2007) 161–170. [3] L. Martinelli, F. Balbaud-Célérier, A. Terlain, S. Delpech, G. Santarini, J. Favergeon, G. Moulin, M. Tabarant, G. Picard, Corros. Sci. 50 (2008) 2523–2536. [4] L. Martinelli, F. Balbaud-Célérier, A. Terlain, S. Bosonnet, G. Picard, G. Santarini, Corros. Sci. 50 (2008) 2537–2548. [5] P. Hosemann, M. Hawley, G. Mori, N. Li, S.A. Maloy, J. Nucl. Mater. 376 (2008) 289–292. [6] F. Masuyama, New developments in steels for power generation boilers, in: R. Viswanathan, J. Nutting (Eds.), Advanced Heat Resistant Steels for Power Generation, Conference Proceedings, San Sebastian, Spain, 27 April, 1998. [7] E. Yamaki, K. Kikuchi, J. Nucl. Mater. 398 (2010) 153–159. [8] Tomohiro Furukawa, Georg Müller, Gustav Schumacher, Alfons Weisenburger, Annette Heinzel, Frank Zimmermann, Kazumi Aoto, J. Nucl. Sci. Technol. 41 (2004) 265–270. [9] P. Hosemann, M.E. Hawley, D. Koury, D. Welch, A.J. Johson, G. Mori, N. Li, S.A. Maloy, J. Nucl. Mater. 381 (2008) 211–215.