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Microstructural characteristics of oxide scales grown on stainless steel exposed to supercritical water
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Scripta Materialia 61 (2009) 996–999 www.elsevier.com/locate/scriptamat
Microstructural characteristics of oxide scales grown on stainless steel exposed to supercritical water Mingcheng Sun,a Xinqiang Wu,a,* En-Hou Hana and Jiancun Raob a
State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, People’s Republic of China b Department of Materials Science, Harbin Institute of Technology, Harbin 150001, People’s Republic of China Received 11 June 2009; revised 27 July 2009; accepted 11 August 2009 Available online 13 August 2009
The microstructural characteristics of oxide scales grown on stainless steel (SS) exposed to supercritical water (SCW) have been investigated by means of transmission electron microscopy. The oxide scales were found to have a multi-layer structure with Ni enrichment at the oxide/matrix interface and were identified as (Fe,Cr)2O3/(Fe,Cr)3O4/Cr2O3/Ni-rich steel/SS from the outer to the inner layer. The growth mechanism of the multi-layer oxides in SCW is discussed. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Stainless steel; Supercritical water (SCW); Oxides; TEM
Water is in a supercritical state at temperatures above 374.15 °C and pressures above 22.1 MPa [1]. Supercritical water (SCW) can dissolve most gases, such as oxygen and organic compounds, in all proportions. SCW acts as a dense gas whose properties, such as density, ion product and dielectric constant, can be tuned by adjusting the temperature and pressure to meet the requirements of different applications. Supercritical water oxidation (SCWO) has emerged as a promising technology for the destruction of hazardous organic waste [2–4]. The SCWO process especially containing hetero-atoms provides a severe environment for component materials. Corrosion of SCWO vessels has become one of the major problems hindering the development and industrial application of SCWO technology. Common reactor materials, like stainless steels, are widely used in industry and are probably the lowest cost materials suitable for the SCWO systems [5–8]. Relatively few oxidation studies of stainless steels have been conducted in SCW environments with high oxygen concentrations corresponding to the SCWO processes [9]. The microstructure of the oxide/alloy interface plays a key role in determining the reliability of the oxide scales formed on the alloy after SCWO [10]. Understanding the oxidation mechanism of stainless steels in SCWO
* Corresponding author. Tel.: +86 24 2384 1883; fax: +86 24 2389 4149; e-mail: [email protected]
environments is of vital importance for their industrial applications. In the present work, the microstructural characteristics of oxide scales grown on 316 stainless steel (316 SS) exposed to SCWO environments were investigated in detail by means of transmission electron microscopy (TEM). The material used in the present study was a 316 SS sheet, which had been solution annealed at 1050 °C for 60 min followed by water quenching. Its chemical composition (wt.%) is 0.03% C, 10.91% Ni, 17.42% Cr, 2.6% Mo, 0.56% Si, 1.22% Mn, 0.045% P, 0.012% S, 0.011% N, with Fe as the balance. Samples (20 mm 12.5 mm 2.5 mm) were polished with emery paper down to 1000# grit and finally mechanically finished to a 2.5 lm diamond finish. During the SCW exposure tests, the pressure was maintained at 24 MPa and the exposure time was 250 h at 500 °C. The 2.0% H2O2 feed solution, provided by the pump at a flow rate of 2.5 ml min 1, was preheated to 350 °C before entering the autoclave. Thin-foil specimens for TEM observations were prepared using a focused ion beam (FEI Quanta 200 3D) with Ga ion sputtering. A Protective W strap (20 lm 1 lm, 2 lm thickness) was first deposited on the surface of the oxide scale to protect it. The specimen was finally thinned to 50–150 nm. A Tecnai G2 F20 TEM instrument equipped with an energy-dispersive spectroscopy (EDS) system operating at 200 kV was used for selected area electron diffraction (SAED) anal-
1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2009.08.013
M. Sun et al. / Scripta Materialia 61 (2009) 996–999
ysis and high-resolution TEM observations. Fast Fourier transformation (FFT) was carried out using a digital micrograph software package. Figure 1(a) shows the general TEM morphology of the cross-section of the oxide scales. A multi-layer oxide structure was observed to be developed on the samples exposed to the present SCWO environments. The outer layer consists of large-crystal oxide particles with a thickness of about 6.7 lm, while the inner layer was fine grained, with a thickness of about 4.1 lm. Figure 1(b–e) shows the SAED patterns corresponding to the different areas of the oxide scales. The SAED patterns from the outer to inner layer are (Fe,Cr)2O3, (Fe,Cr)3O4, Cr2O3 and Ni-rich steel, with the incident electron beam parallel to the [1 2 3], [4 1 1], [2 4 3] and [1 1 0] zone axes, respectively. The areas marked “1”–“6” were selected for quantitative element analysis (point analysis) by EDS. Table 1 shows the EDS analysis results. It was found that Ni is enriched at the oxide/metal interface by nearly thrice its bulk concentration. This may be due to mass transport processes in the oxide scales during the oxidation, including the outward diffusion of Fe and Cr. Fe and Cr possess a greater affinity to oxygen than Ni. Therefore, the rate of incorporation of Ni in the outer oxide scale will be slower than that of Fe and Cr. Ni would be expected to be left in the inner oxide scale and subsequently become concentrated at the oxide/metal interface. Areas “1”, “2” and “3” all consisted of (Fe,Cr)2O3 but contained
Figure 1. (a) A TEM micrograph of a cross-section of the oxide scales. SAED patterns with the incident electron beam parallel to (b) the [1 2 3] zone axes of the (Fe,Cr)2O3, (c) the [4 1 1] zone axes of the (Fe,Cr)3O4, (d) the [2 4 3] zone axes of the Cr2O3 and (e) the [1 1 0] zone axes of the Ni-rich steel, respectively.
997
Table 1. Chemical compositions (at.%) of the marked area in oxide scales by EDS analysis. Element
Fe Cr Ni O
(Fe,Cr)2O3
(Fe,Cr)3O4
Cr2O3
Ni-rich steel
1
2
3
4
5
6
41 2 0 57
35 4 0 61
42 8 0 50
16 16 3 65
54 15 6 25
44 11 31 14
different Cr contents in the different areas. The Cr content in (Fe,Cr)2O3 was found to increase from 2 at.% in the outer area “1” to 8 at.% in the inner area “3” (Table 1). Figure 2 shows the HRTEM image of (Fe,Cr)2O3 in area “3” with the incident electron beam parallel to the [4 7 1] direction. The lattice image of (Fe,Cr)2O3 can be taken as being constructed with (1 0 4) and (1 1 3) planes with crystal lattice spacings of 0.27 and 0.22 nm, respectively. Both a-Fe2O3 and Cr2O3 have the space group R 3c, with lattice constants of a = 0.5035 nm, c = 1.372 nm and a = 0.4958 nm, c = 1.359 nm, respectively. a-Fe2O3 and Cr2O3 have the same space group and similar lattice constants, so a-Fe2O3 could contain some Cr and become (Fe,Cr)2O3 in the outer layer. Figure 3 shows the HRTEM image of (Fe,Cr)3O4 in area “4” with the incident electron beam parallel to the [1 1 0] direction. The lattice image of (Fe,Cr)3O4 can be taken as being constructed with (1 1 1) and (1 1 1) planes with crystal lattice spacings of 0.48 and 0.48 nm, respectively. The orientation difference angles between the (1 1 1) planes in the oxide grains are small. Hence, most (Fe,Cr)3O4 crystallites in the scale are separated by small-angle grain boundaries. The grain boundaries are clean and free of secondary phases. The spinel grains are not randomly oriented. Due to
Figure 2. HRTEM image of (Fe,Cr)2O3 with the incident electron beam parallel to the [4 7 1] direction. The inset shows the corresponding FFT pattern of the image.
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M. Sun et al. / Scripta Materialia 61 (2009) 996–999
Figure 3. (a) HRTEM image of (Fe,Cr)3O4 with the incident electron beam parallel to the [1 1 0] direction. The orientation difference angles between the ( 11 1) planes in the oxide grains are small. (b) The inset shows the corresponding FFT pattern of the HRTEM image. (c) The inset shows the SAED patterns of (Fe,Cr)3O4 taken from the area with the incident electron beam parallel to the [1 1 0] direction.
an increased structural order, small-angle boundaries have lower grain boundary energies and diffusivities than high-angle boundaries [11–13], thus the diffusion of oxygen and iron through the (Fe,Cr)3O4 layer can be inhibited to a certain extent.
Fe3O4 and spinel FeCr2O4 have the space group Fd3 m, with lattice constants of a = 0.8396 nm and a = 0.8379 nm, respectively, which are nearly identical to each other. The addition of Cr3+ into Fe3O4 tends to transform the inverse spinel structure to the spinel structure. The (Fe,Cr)3O4 spinel is much denser than the Fe3O4, which is believed to provide better oxidation resistance than the porous Fe3O4 layer [14,15]. Figure 4 shows perspective views of the (Fe,Cr)2O3 and spinel (Fe,Cr)3O4 structures. At 24 MPa (237 atm), the equilibrium dissociation oxygen partial pressure in the steam is 1.41 10 8 atm at 500 °C, assuming that the dissociation via H2O = H2 + 1/2O2 goes to equilibrium at the metal– steam interface [16]. The decomposition of H2O2 into oxygen is almost 100% in the supercritical region after the residence time exceeds about 1–6 s [17]. The oxygen concentration from almost 100% decomposition of 2.0% H2O2 was 0.294 mol l 1. The equilibrium oxygen partial pressure in the present SCWO environments was 1.25 atm, which was high enough to form less protective Fe-rich outer layer [18]. Let us now consider the transport processes by grain boundary diffusion or molecular diffusion other than lattice diffusion, which can control or contribute to oxide growth. The outer layer allows for rapid inward oxygen diffusion through the oxide scales. The grain boundary may behave as a short circuit diffusion path in crystalline oxides. Short circuit diffusion of oxygen increases the effective diffusion coefficient [14]. The measured activation energies (172–189 kJ mol 1) for the oxidation of ferritic stainless steels support the concept of grain boundary diffusion for oxygen (167 kJ mol 1) [19]. Oxygen isotope profiles are known to be a powerful tool for determining the direction of mass transport in thin oxides. For the ferritic steel P91, the outer scale consists of Fe2O3 and Fe3O4 and the inner scale consists of Fe3O4 + (Fe,Cr)3O4 after oxidation in16O2 and H218O at 650 °C. The isotope 16O was always more abundant than 18O in the inner part of the scale, while 18O was enriched in outer layer after a long oxidation time [18]. Similarly, in the present study, the outer layer (Fe,Cr)2O3 is formed as a result of outward iron diffusion through Fe3O4 and
Figure 4. (a) The (Fe,Cr)2O3 structure in terms of polyhedra. (b) The spinel (Fe,Cr)3O4 structure in terms of polyhedra.
M. Sun et al. / Scripta Materialia 61 (2009) 996–999
reaction with oxygen at the oxide–water interface, creating large grains of (Fe,Cr)2O3 and accounting for the growth of the outer layer [20]. Iron has a higher diffusion coefficient than Cr in both the oxides and metal phases, coincident with the fact that, in our study, the Cr content in (Fe,Cr)2O3 increased from 2 at.% in the outer area to 8 at.% in the inner area. In another study [21], the (Cr,Mn)3O4 spinel and Cr2O3 scales on Fe–15Cr steels were thermally grown at 800 °C first in H216O and subsequently in H218O water vapour. The surface oxygen was more than twice as rich in 16O as in 18O. The experiments demonstrated that (Cr,Mn)3O4 spinel and Cr2O3 scales forming in water vapour grow by inward transport of OH species. The oxide plasticity/adhesion is frequently improved when the oxide is formed in water vapour, and this is believed to be the result of the increased inward flux of oxygen [21,22]. The magnetite layer is highly porous while the spinel has a denser structure that may act as a protective oxide layer. The formation of (Fe,Cr)3O4 spinel and Cr2O3 layer in the present SCWO environments may also be due to inward diffusion of oxygen to the oxide–metal interface. In summary, a multi-layer oxide structure with Ni enrichment at the oxide–metal interface was observed on the samples exposed to the SCWO environments, which was identified as (Fe,Cr)2O3/(Fe,Cr)3O4/Cr2O3/Ni-rich steel/316 SS from the outer to the inner layer. The Cr content in (Fe,Cr)2O3 increased from 2 at.% in the outer area of the oxide scales to 8 at.% in the inner area. The addition of Cr3+ to the Fe3O4 tends to transform the inverse spinel structure into the spinel structure. The (Fe,Cr)3O4 spinel is much denser and is believed to provide better oxidation resistance than the Fe3O4 inverse spinel. This study was jointly supported by the Science and Technology Foundation of China (50871113), the Special Funds for the Major State Basic Research Projects (2006CB605001) and the Innovation Fund of Insti-
999
tute of Metal Research (IMR), Chinese Academy of Sciences (CAS). [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
W. Wagner, A. Kruse, Springer-Verlag, Berlin, 1998. M. Modell, US Patent. 4,388,199 (6 July 1982). P. Kritzer, J. Supercrit. Fluids 29 (2004) 1. D.D. Macdonald, J. Supercrit. Fluids 30 (2004) 375. T.M. Hayward, I.M. Svishchev, R.C. Makhija, J. Supercrit. Fluids 27 (2003) 275. N. Saito, Y. Tsuchiya, Y. Akai, H. Omura, T. Takada, N. Hara, Corrosion 62 (2006) 383. M. Sun, X. Wu, Z. Zhang, E.-H. Han, Corros. Sci. 51 (2009) 1069. T. Hirose, K. Shiba, M. Enoeda, M. Akiba, J. Nucl. Mater. 367–370 (2007) 1185. W.K. Boyd, H.A. Pray, Corrosion (1957) 375t. Y. Chen, K. Sridharan, T.R. Allen, S. Ukai, J. Nucl. Mater. 359 (2006) 50. G. Gupta, P. Ampornrat, X. Ren, K. Sridharan, T.R. Allen, G.S. Was, J. Nucl. Mater. 361 (2007) 160. L. Tan, K. Sridharan, T.R. Allen, J. Nucl. Mater. 348 (2006) 263. S. Yang, U. Krupp, H.-J. Christ, V.B. Trindade, Adv. Eng. Mater. 7 (2005) 723. P. Ampornrat, G.S. Was, J. Nucl. Mater. 371 (2007) 1. G.S. Was, S. Teysseyre, J. McKinley, Corrosion 2004, Paper No. 04492. I.G. Wright, B.A. Pint, Corrosion 2002, Paper No. 02377. E. Croiset, S.F. Rice, R.G. Hanush, AIChE J. 43 (1997) 2343. J. Ehlers, D.J. Young, E.J. Smaardijk, A.K. Tyagi, H.J. Penkalla, L. Singheiser, W.J. Quadakkers, Corros. Sci. 48 (2006) 3428. A.G. Crouch, J. Robertson, Acta Metall. Mater. 38 (1990) 2567. X. Ren, K. Sridharan, T.R. Allen, J. Nucl. Mater. 358 (2006) 227. G. Bamba, Y. Wouters, A. Galerie, G. Borchardt, S. Shimada, O. Heintz, S. Chevalier, Scripta Mater. 57 (2007) 671. S.R.J. Saunders, M. Monteiro, F. Rizzo, Prog. Mater. Sci. 53 (2008) 775.
Scripta Materialia 61 (2009) 996–999 www.elsevier.com/locate/scriptamat
Microstructural characteristics of oxide scales grown on stainless steel exposed to supercritical water Mingcheng Sun,a Xinqiang Wu,a,* En-Hou Hana and Jiancun Raob a
State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, People’s Republic of China b Department of Materials Science, Harbin Institute of Technology, Harbin 150001, People’s Republic of China Received 11 June 2009; revised 27 July 2009; accepted 11 August 2009 Available online 13 August 2009
The microstructural characteristics of oxide scales grown on stainless steel (SS) exposed to supercritical water (SCW) have been investigated by means of transmission electron microscopy. The oxide scales were found to have a multi-layer structure with Ni enrichment at the oxide/matrix interface and were identified as (Fe,Cr)2O3/(Fe,Cr)3O4/Cr2O3/Ni-rich steel/SS from the outer to the inner layer. The growth mechanism of the multi-layer oxides in SCW is discussed. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Stainless steel; Supercritical water (SCW); Oxides; TEM
Water is in a supercritical state at temperatures above 374.15 °C and pressures above 22.1 MPa [1]. Supercritical water (SCW) can dissolve most gases, such as oxygen and organic compounds, in all proportions. SCW acts as a dense gas whose properties, such as density, ion product and dielectric constant, can be tuned by adjusting the temperature and pressure to meet the requirements of different applications. Supercritical water oxidation (SCWO) has emerged as a promising technology for the destruction of hazardous organic waste [2–4]. The SCWO process especially containing hetero-atoms provides a severe environment for component materials. Corrosion of SCWO vessels has become one of the major problems hindering the development and industrial application of SCWO technology. Common reactor materials, like stainless steels, are widely used in industry and are probably the lowest cost materials suitable for the SCWO systems [5–8]. Relatively few oxidation studies of stainless steels have been conducted in SCW environments with high oxygen concentrations corresponding to the SCWO processes [9]. The microstructure of the oxide/alloy interface plays a key role in determining the reliability of the oxide scales formed on the alloy after SCWO [10]. Understanding the oxidation mechanism of stainless steels in SCWO
* Corresponding author. Tel.: +86 24 2384 1883; fax: +86 24 2389 4149; e-mail: [email protected]
environments is of vital importance for their industrial applications. In the present work, the microstructural characteristics of oxide scales grown on 316 stainless steel (316 SS) exposed to SCWO environments were investigated in detail by means of transmission electron microscopy (TEM). The material used in the present study was a 316 SS sheet, which had been solution annealed at 1050 °C for 60 min followed by water quenching. Its chemical composition (wt.%) is 0.03% C, 10.91% Ni, 17.42% Cr, 2.6% Mo, 0.56% Si, 1.22% Mn, 0.045% P, 0.012% S, 0.011% N, with Fe as the balance. Samples (20 mm 12.5 mm 2.5 mm) were polished with emery paper down to 1000# grit and finally mechanically finished to a 2.5 lm diamond finish. During the SCW exposure tests, the pressure was maintained at 24 MPa and the exposure time was 250 h at 500 °C. The 2.0% H2O2 feed solution, provided by the pump at a flow rate of 2.5 ml min 1, was preheated to 350 °C before entering the autoclave. Thin-foil specimens for TEM observations were prepared using a focused ion beam (FEI Quanta 200 3D) with Ga ion sputtering. A Protective W strap (20 lm 1 lm, 2 lm thickness) was first deposited on the surface of the oxide scale to protect it. The specimen was finally thinned to 50–150 nm. A Tecnai G2 F20 TEM instrument equipped with an energy-dispersive spectroscopy (EDS) system operating at 200 kV was used for selected area electron diffraction (SAED) anal-
1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2009.08.013
M. Sun et al. / Scripta Materialia 61 (2009) 996–999
ysis and high-resolution TEM observations. Fast Fourier transformation (FFT) was carried out using a digital micrograph software package. Figure 1(a) shows the general TEM morphology of the cross-section of the oxide scales. A multi-layer oxide structure was observed to be developed on the samples exposed to the present SCWO environments. The outer layer consists of large-crystal oxide particles with a thickness of about 6.7 lm, while the inner layer was fine grained, with a thickness of about 4.1 lm. Figure 1(b–e) shows the SAED patterns corresponding to the different areas of the oxide scales. The SAED patterns from the outer to inner layer are (Fe,Cr)2O3, (Fe,Cr)3O4, Cr2O3 and Ni-rich steel, with the incident electron beam parallel to the [1 2 3], [4 1 1], [2 4 3] and [1 1 0] zone axes, respectively. The areas marked “1”–“6” were selected for quantitative element analysis (point analysis) by EDS. Table 1 shows the EDS analysis results. It was found that Ni is enriched at the oxide/metal interface by nearly thrice its bulk concentration. This may be due to mass transport processes in the oxide scales during the oxidation, including the outward diffusion of Fe and Cr. Fe and Cr possess a greater affinity to oxygen than Ni. Therefore, the rate of incorporation of Ni in the outer oxide scale will be slower than that of Fe and Cr. Ni would be expected to be left in the inner oxide scale and subsequently become concentrated at the oxide/metal interface. Areas “1”, “2” and “3” all consisted of (Fe,Cr)2O3 but contained
Figure 1. (a) A TEM micrograph of a cross-section of the oxide scales. SAED patterns with the incident electron beam parallel to (b) the [1 2 3] zone axes of the (Fe,Cr)2O3, (c) the [4 1 1] zone axes of the (Fe,Cr)3O4, (d) the [2 4 3] zone axes of the Cr2O3 and (e) the [1 1 0] zone axes of the Ni-rich steel, respectively.
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Table 1. Chemical compositions (at.%) of the marked area in oxide scales by EDS analysis. Element
Fe Cr Ni O
(Fe,Cr)2O3
(Fe,Cr)3O4
Cr2O3
Ni-rich steel
1
2
3
4
5
6
41 2 0 57
35 4 0 61
42 8 0 50
16 16 3 65
54 15 6 25
44 11 31 14
different Cr contents in the different areas. The Cr content in (Fe,Cr)2O3 was found to increase from 2 at.% in the outer area “1” to 8 at.% in the inner area “3” (Table 1). Figure 2 shows the HRTEM image of (Fe,Cr)2O3 in area “3” with the incident electron beam parallel to the [4 7 1] direction. The lattice image of (Fe,Cr)2O3 can be taken as being constructed with (1 0 4) and (1 1 3) planes with crystal lattice spacings of 0.27 and 0.22 nm, respectively. Both a-Fe2O3 and Cr2O3 have the space group R 3c, with lattice constants of a = 0.5035 nm, c = 1.372 nm and a = 0.4958 nm, c = 1.359 nm, respectively. a-Fe2O3 and Cr2O3 have the same space group and similar lattice constants, so a-Fe2O3 could contain some Cr and become (Fe,Cr)2O3 in the outer layer. Figure 3 shows the HRTEM image of (Fe,Cr)3O4 in area “4” with the incident electron beam parallel to the [1 1 0] direction. The lattice image of (Fe,Cr)3O4 can be taken as being constructed with (1 1 1) and (1 1 1) planes with crystal lattice spacings of 0.48 and 0.48 nm, respectively. The orientation difference angles between the (1 1 1) planes in the oxide grains are small. Hence, most (Fe,Cr)3O4 crystallites in the scale are separated by small-angle grain boundaries. The grain boundaries are clean and free of secondary phases. The spinel grains are not randomly oriented. Due to
Figure 2. HRTEM image of (Fe,Cr)2O3 with the incident electron beam parallel to the [4 7 1] direction. The inset shows the corresponding FFT pattern of the image.
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M. Sun et al. / Scripta Materialia 61 (2009) 996–999
Figure 3. (a) HRTEM image of (Fe,Cr)3O4 with the incident electron beam parallel to the [1 1 0] direction. The orientation difference angles between the ( 11 1) planes in the oxide grains are small. (b) The inset shows the corresponding FFT pattern of the HRTEM image. (c) The inset shows the SAED patterns of (Fe,Cr)3O4 taken from the area with the incident electron beam parallel to the [1 1 0] direction.
an increased structural order, small-angle boundaries have lower grain boundary energies and diffusivities than high-angle boundaries [11–13], thus the diffusion of oxygen and iron through the (Fe,Cr)3O4 layer can be inhibited to a certain extent.
Fe3O4 and spinel FeCr2O4 have the space group Fd3 m, with lattice constants of a = 0.8396 nm and a = 0.8379 nm, respectively, which are nearly identical to each other. The addition of Cr3+ into Fe3O4 tends to transform the inverse spinel structure to the spinel structure. The (Fe,Cr)3O4 spinel is much denser than the Fe3O4, which is believed to provide better oxidation resistance than the porous Fe3O4 layer [14,15]. Figure 4 shows perspective views of the (Fe,Cr)2O3 and spinel (Fe,Cr)3O4 structures. At 24 MPa (237 atm), the equilibrium dissociation oxygen partial pressure in the steam is 1.41 10 8 atm at 500 °C, assuming that the dissociation via H2O = H2 + 1/2O2 goes to equilibrium at the metal– steam interface [16]. The decomposition of H2O2 into oxygen is almost 100% in the supercritical region after the residence time exceeds about 1–6 s [17]. The oxygen concentration from almost 100% decomposition of 2.0% H2O2 was 0.294 mol l 1. The equilibrium oxygen partial pressure in the present SCWO environments was 1.25 atm, which was high enough to form less protective Fe-rich outer layer [18]. Let us now consider the transport processes by grain boundary diffusion or molecular diffusion other than lattice diffusion, which can control or contribute to oxide growth. The outer layer allows for rapid inward oxygen diffusion through the oxide scales. The grain boundary may behave as a short circuit diffusion path in crystalline oxides. Short circuit diffusion of oxygen increases the effective diffusion coefficient [14]. The measured activation energies (172–189 kJ mol 1) for the oxidation of ferritic stainless steels support the concept of grain boundary diffusion for oxygen (167 kJ mol 1) [19]. Oxygen isotope profiles are known to be a powerful tool for determining the direction of mass transport in thin oxides. For the ferritic steel P91, the outer scale consists of Fe2O3 and Fe3O4 and the inner scale consists of Fe3O4 + (Fe,Cr)3O4 after oxidation in16O2 and H218O at 650 °C. The isotope 16O was always more abundant than 18O in the inner part of the scale, while 18O was enriched in outer layer after a long oxidation time [18]. Similarly, in the present study, the outer layer (Fe,Cr)2O3 is formed as a result of outward iron diffusion through Fe3O4 and
Figure 4. (a) The (Fe,Cr)2O3 structure in terms of polyhedra. (b) The spinel (Fe,Cr)3O4 structure in terms of polyhedra.
M. Sun et al. / Scripta Materialia 61 (2009) 996–999
reaction with oxygen at the oxide–water interface, creating large grains of (Fe,Cr)2O3 and accounting for the growth of the outer layer [20]. Iron has a higher diffusion coefficient than Cr in both the oxides and metal phases, coincident with the fact that, in our study, the Cr content in (Fe,Cr)2O3 increased from 2 at.% in the outer area to 8 at.% in the inner area. In another study [21], the (Cr,Mn)3O4 spinel and Cr2O3 scales on Fe–15Cr steels were thermally grown at 800 °C first in H216O and subsequently in H218O water vapour. The surface oxygen was more than twice as rich in 16O as in 18O. The experiments demonstrated that (Cr,Mn)3O4 spinel and Cr2O3 scales forming in water vapour grow by inward transport of OH species. The oxide plasticity/adhesion is frequently improved when the oxide is formed in water vapour, and this is believed to be the result of the increased inward flux of oxygen [21,22]. The magnetite layer is highly porous while the spinel has a denser structure that may act as a protective oxide layer. The formation of (Fe,Cr)3O4 spinel and Cr2O3 layer in the present SCWO environments may also be due to inward diffusion of oxygen to the oxide–metal interface. In summary, a multi-layer oxide structure with Ni enrichment at the oxide–metal interface was observed on the samples exposed to the SCWO environments, which was identified as (Fe,Cr)2O3/(Fe,Cr)3O4/Cr2O3/Ni-rich steel/316 SS from the outer to the inner layer. The Cr content in (Fe,Cr)2O3 increased from 2 at.% in the outer area of the oxide scales to 8 at.% in the inner area. The addition of Cr3+ to the Fe3O4 tends to transform the inverse spinel structure into the spinel structure. The (Fe,Cr)3O4 spinel is much denser and is believed to provide better oxidation resistance than the Fe3O4 inverse spinel. This study was jointly supported by the Science and Technology Foundation of China (50871113), the Special Funds for the Major State Basic Research Projects (2006CB605001) and the Innovation Fund of Insti-
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tute of Metal Research (IMR), Chinese Academy of Sciences (CAS). [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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