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Corrosion behavior of an Al added high-Cr ODS steel in supercritical water at 600 °C
Applied Surface Science 480 (2019) 969–978
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Corrosion behavior of an Al added high-Cr ODS steel in supercritical water at 600 °C
T
Jian Rena, Liming Yua, , Yongchang Liua, , Zongqing Maa, Chenxi Liua, Huijun Lia, Jiefeng Wub ⁎
a b
⁎
State Key Lab of Hydraulic Engineering Simulation and Safety, Tianjin key Lab of Composite and Functional Materials, Tianjin University, Tianjin 300072, China Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
ARTICLE INFO
ABSTRACT
Keywords: ODS steel Supercritical water reactor High temperature corrosion Microstructure observations
Corrosion behavior in the supercritical water (SCW) of an Al added high-Cr oxide dispersion strengthened (ODS) steel was investigated in this study. The steel was exposed to SCW at 600 °C and 25 MPa for various exposure times up to 1000 h, and the resultant oxide layers were characterized by using weight change measurements, Xray diffraction (XRD), X-ray photoelectron spectroscopies (XPS), scanning/transmission electron microscopy (SEM/TEM), and energy dispersive spectroscopy (EDS). The weight gain after exposure for 1000 h was 6.24 mg/ dm2, indicating the superior corrosion resistance of 16Cr-3Al ODS steel to SCW. Chemical analysis of the surface oxides showed that the content of Cr-rich oxides increased with the increase of exposure time in SCW. Crosssectional observation of the oxide scales revealed that an outer layer composed of (Cr, Fe)2O3 and an inner layer composed of Al2O3 formed on the sample surface. The growth of both outer and inner oxide layers followed the parabolic law with the increase of exposure times. The corrosion mechanism was discussed in detail based on the microstructure evolution process with exposure time in SCW.
1. Introduction As a kind of promising Generation IV advanced nuclear reactors, supercritical water reactors (SCWRs) can provide much higher thermal efficiency and have lower construct cost than current light water reactors [1–3]. They are designed to produce core outlet temperatures as high as 600 °C and system pressures up to 25 MPa [4]. At a temperature above of 374.15 °C and a pressure above of 22.1 MPa, the supercritical water (SCW) acts as a nonpolar solvent that can dissolve gases like oxygen to complete miscibility, which dramatically aggravates the corrosion of structural materials commonly used in nuclear reactors and fossil power plants. Therefore, fuel cladding materials with appropriate corrosion resistance are urgently required in the design process of SCWRs. Oxide dispersion strengthened (ODS) steels have been considered as candidate structure materials for nuclear fusion reactors due to their comprehensive mechanical properties and superior irradiation resistance [5–7]. Nano-sized yttria particles in the matrix can act as obstacles to pin the movement of dislocations and the migration of grain boundaries. Besides, homogenous distribution of the nano-sized yttria particles can play the role of sinking irradiation induced defects. However, in the application of ODS steels for SCWRs, their corrosion resistance should be taken into consideration. Compared to the 9Cr⁎
ODS steels, high-Cr (14–20 wt%) ODS steels show higher corrosion resistance due to the higher Cr content [8,9]. In order to obtain superior corrosion resistance of ODS steels in SCW, it may be a feasible way to add Al into the matrix during the fabrication processes. However, Al addition is easy to introduce large Y-Al-O oxides due to its high affinity with oxygen, thus deteriorating the mechanical performance of ODS steels [10–12]. Fortunately, it is found that Zr addition can refine the precipitates due to the substitution of Y-Al-O oxides by fine Y-Zr-O nanoparticles in the matrix [10–13]. According to the evaluation on the mechanical performance of the Al added high-Cr ODS steel with Zr addition, its application prospect in SCWRs is worth of further research. Furthermore, although several works have been conducted to investigate the corrosion behavior of high-Cr ODS steels in SCW [9,14–19], few focused on Al added high-Cr ODS steels and their corrosion mechanism in SCW. In this work, the corrosion behavior of an Al added high-Cr ODS steel was studied in SCW at 600 °C and 25 MPa for various exposure times. Post-exposure surface analysis was performed by using X-ray photoelectron spectroscopies (XPS). The oxidation behavior, the microstructure and composition of the oxides scale were investigated by using weight change measurement, X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and scanning transmission electron microscope (STEM). Based on these
Corresponding authors. E-mail addresses: [email protected] (L. Yu), [email protected] (Y. Liu).
https://doi.org/10.1016/j.apsusc.2019.03.019 Received 14 January 2019; Received in revised form 23 February 2019; Accepted 1 March 2019 Available online 08 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 480 (2019) 969–978
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exposure in SCW. To minimize the error in measurements, every weight gain measurement was conducted five times. The surface chemistry of the oxide films was examined by XPS (EscaLab 250Xi, Thermo Fisher). The surface morphologies and phase identification of the oxide films were characterized by SEM (S4800, Hitachi) and XRD (D8 Advanced, Bruker), respectively. The detailed microstructure characterization of the cross-sectional oxide films was conducted by a TEM (Tecnai G2 F20, FEI) equipped with an energy dispersive spectroscopy (EDS) detector system and a high angle annular dark-field (HAADF) detector. The preparation of specimens for cross-sectional TEM observation can be found in Ref. [20]. A so-called ‘Tripod polishing method’ was used to prepare cross-sectional TEM specimens. To this end, two slices cut from the same specimen were glued together face-to-face (i.e. the oxide film to the oxide film). The thus-obtained sandwich was again cut into smaller sandwiches (2 mm × 2 mm × 0.5 mm), and both sides of a sandwich were plane-polished perpendicular to the glue plane (i.e., perpendicular to the oxide film) until the thickness of the specimen reached to 30 μm. Then the specimen was glued with copper collar (i.e. copper collar plane parallel to the oxide film) followed by soaking in acetone solvent for 30 min. Finally, ion-milling was performed using an Ar-ion polishing system (Gatan, PIPS695) at 3 kV (13 mA) with an incident beam angle of about 9°, until a hole was formed. Thus, the edges of the ion-milled hole could act as electron transparent areas for crosssectional TEM analysis.
Table 1 Element compositions (wt%) of the 16Cr-3Al ODS steel. Cr
W
Al
C
N
Y2O3
Zr
Fe
15.89
1.45
2.64
0.081
0.053
0.29
0.42
Bal.
results, the formation process of oxide films was analyzed and the corrosion mechanism was discussed. 2. Material and methods Chemical composition (wt%) of the Al added high-Cr ODS steel (referred to as 16Cr-3Al ODS hereon) used in this study was listed in Table 1. The fabrication processes for the 16Cr-3Al ODS are presented as follow: the pre-alloyed powders，yttria (Y2O3) and Zr powders were mechanically alloyed by high energy ball milling in a pure argon atmosphere for 30 h, then the powders were hot isostatic pressed (HIPed) at 1150 °C for 3 h under a pressure of 150 MPa. The grain size and microstructure of 16Cr-3Al ODS steel were examined by SEM (Quanta 650F, FEI) equipped with electron backscattered diffraction (EBSD) (HKL Channel 5) and TEM (Tecnai G2 F20, FEI), respectively. Fig. 1 shows the microstructure of the 16Cr-3Al ODS steel. The mean grain size can be determined as 0.88 μm from Fig. 1a and the precipitates can be observed in Fig. 1b. Coupon specimens with dimensions of 20 mm × 10 mm × 2 mm were fabricated from the 16Cr-3Al ODS HIPed bulk alloy. To prepare samples for SCW experiment, the specimens were grounded using diamond lapping films down to 2000 grit and then polished using a soft felt pad wet by colloidal silica suspension with a nominal grain size of 0.05 μm. After polishing, the specimens were washed with ethyl alcohol in an ultrasonic bath in sequence. Corrosion tests were conducted in SCW at 600 °C/25 MPa with dissolve oxygen (DO) of 200 ppb by mass. Fig. 2a shows the schematic of the SCW loop facility. The sketch of the coupon specimens is shown in Fig. 2b. The exposure time was set as 200 h, 400 h, 600 h, 800 h and 1000 h, respectively. The inlet conductivity of the water was measured to be 0.1 μS/cm and the flow rate was 1.8–2 L/h. After each SCW exposure period, the test coupons were dried in air immediately. All samples were weighted using METTER-TOLDO MS204 with an accuracy of 0.1 mg to record the weight change before and after
3. Results and discussion 3.1. Weight gain The weight gain data can be fitted using the follow equation [21]: (1)
w = kt n 2
where Δw represents weight gain (mg/dm ), k is a rate constant mg/ dm2·h, t is the exposure time (h), and n is an exponent describing the time dependence of the oxide growth. Table 2 gives the weight gain data and the fitted curves are plotted in Fig. 3 as a function of exposure time in SCW at 600 °C and 25 MPa. For comparison, coupon specimens of the 9Cr ODS steel were put into SCW together with the 16Cr-3Al ODS steel. The weight gain data of the 304, 316 stainless steel and the 304 ODS steel in Fig. 3 can be found in Ref. [22], where the exposure
Fig. 1. (a) Inverse pole figure (IPF) and grain boundary maps taken from the HIPed 16Cr-3Al ODS steel and (b) TEM bright-field micrographs of the unexposed 16Cr3Al ODS steel. 970
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Fig. 2. (a) The schematic of the SCW loop facility and (b) sketch of the dimension of the coupon specimens subjected to the system. Table 2 Weight gain data of the 9Cr ODS steel and the 16Cr-3Al ODS steel in SCW as a function of exposure time. Alloy
9Cr-ODS 16Cr-3Al ODS
Weight gain at exposure time (mg/dm2)
k(mg/dm2/h)
200 h
400h
600 h
800 h
1000 h
255.90 2.50
342.49 4.62
402.36 5.04
461.20 5.72
498.73 6.24
27.94 0.229
n
R2
0.41 0.473
0.9992 0.9999
Fig. 3. (a) Weight gain curves for the 16Cr-3Al ODS and 9Cr ODS steels in SCW, (b) Y-axis magnified figure of the weight gain curve for the 16Cr-3Al ODS steel.
conditions were similar to those in this research. As can be seen from Fig. 3, the weight gain data of different steels all follow the parabolic law, and the weight gain of the 16Cr-3Al ODS steel is extremely small compared to that of other steels. This illustrates that the 16Cr-3Al ODS steel exhibits higher corrosion resistance during exposure in SCW.
show a slightly increase in peak intensity with the increase of exposure time. 3.3. Morphology and chemical composition of the surface oxides Fig. 5 shows the surface morphology of the 16Cr-3Al ODS steel after exposure in SCW for different times. It can be observed that uniform oxide crystals are formed on the surface of the 16Cr-3Al ODS steel with the increase of exposure time. As the exposure time extends, the shape and size of the oxide crystals change synchronously. After exposure for 200 h (Fig. 5a), ultrafine rod-shaped oxide crystals with a length of about 500 nm can be observed. When the exposure time increases to
3.2. XRD measurement of the oxide scales Fig. 4 shows the XRD patterns of the specimens after exposure to SCW with various exposure times. As can be seen, there are three major diffraction peaks corresponding to the ferrite matrix during exposure. In addition, weak diffraction peaks of Cr2O3 can be also observed and 971
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Fig. 4. XRD results of the 16Cr-3Al ODS steel after exposure to SCW with various exposure times.
400 h (Fig. 5b) and 600 h (Fig. 5c), the rod-shaped oxide crystals change into the granular shape, indicating the growth of oxide crystals. Eventually, after exposure for 800 h (Fig. 5d) and 1000 h (Fig. 5e), relatively large prismatic oxide crystals are formed. The diameter of the oxide crystals is small and most of the oxide crystals are below 1 μm even after exposure for 1000 h. Besides, no spallation can be found in the outer oxides surface, which is different from that observed in the austenitic alloy 304 and the T91 steel exposed to SCW [22,23]. This indicates that the surface oxides of the 16Cr-3Al ODS steel are stable during exposure. The excellent stability can be attributed to the formation of relatively fine oxide crystals at the surface, which exhibit good uniformity and compaction. Fig. 6 shows the XPS survey scans of the surface of the 16Cr-3Al ODS steel exposed to SCW for 200, 400, 600, 800 and 1000 h,
Fig. 6. XPS results for the surface chemistry of the 16Cr-3Al ODS steel exposed to SCW for (a) 200 h, (b) 400 h, (c) 600 h, (d) 800 h and (e) 1000 h, respectively.
Fig. 5. SEM micrographs of the outer surface of the 16Cr-3Al ODS steel after exposure to SCW for (a) 200 h, (b) 400 h, (c) 600 h, (d) 800 h and (e) 1000 h, respectively.
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Fig. 7. XPS spectra of (a) Fe 2p3/2 (b) Cr 2p3/2 and (c) Al 2p core level for the surface oxides formed on the 16Cr-3Al ODS steel in SCW.
respectively. The results confirm the existence of Cr in addition to Fe, Al, O and C on the sample surface. The full spectra of the surface oxides for different exposure times show a similar chemical element types. To identify the chemical states of the elements of Fe, Cr and Al, the highresolution spectra (obtained at a binding energy intervals of 0.06 eV) for different exposure times are presented in Fig. 7a–c, respectively. For the spectra of Fe element in Fig. 7a, negligible differences can be observed, indicating the stable chemical state of Fe on the outer oxide surface during exposure. Fig. 7b displays the narrow scans of Cr 2p3/2. As can be seen, the peak intensity of Cr 2p3/2 is enhanced with increase of exposure time, indicating the increase of the content of Cr-containing oxides at the outer surface with the increase of exposure time. Fig. 7c shows the Al 2p core level spectra. The intensity of Al 2p increases with exposure time up to 600 h, but decreases suddenly after exposure for
800 h and 1000 h. At the early stage of the oxidation process up to 600 h, due to relatively higher affinity with O, Al atoms can diffuse quickly to the interface and produce Al-containing oxides. However, with further oxidation of the outer surface, the Al-containing oxides may be covered by relatively thick Fe or Cr containing oxides. The detection depth of XPS technique is about several nanometers [24], and the depth of Al-containing oxides may exceed the detection limit of XPS, thus leading to the decrease of the peak intensity of Al 2p spectra. 3.4. Cross-sectional characterization of the oxide scales 3.4.1. Exposure to SCW for 200 h Fig. 8 shows the STEM-HAADF image of the cross-sectional oxide film of the 16Cr-3Al ODS steel after exposure to SCW for 200 h and its
Fig. 8. (a) STEM-HAADF image of the cross-sectional oxide film on the 16Cr-3Al ODS steel after exposure to SCW for 200 h and the corresponding EDS elemental maps for (b) O, (c) Fe, (d) Cr and (e) Al, respectively.
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Fig. 9. (a) Bright field TEM micrograph of the cross-sectional oxide scale of the 16Cr-3Al ODS steel after exposure to SCW for 200 h, (b) HRTEM image of the out oxide grain marked by square dashed line in (a), and (c) FFT image corresponding to the area marked in (b).
characterized by HRTEM. Fig. 9a shows the bright field TEM micrographs of the outer oxide scale. The HRTEM image of the selected area and the corresponding FFT image are provided in Fig. 9b and c, respectively. Based on these results, the phase composition of the outer oxide scales is identified as hexagonal (Cr, Fe)2O3. The solid reaction between Cr2O3 and Fe2O3 in Fe-Cr-Al alloy can promote the substitution of Cr atoms in Cr2O3 by Fe atoms since the two oxides have the similar crystal structure [25]. The formation of (Cr, Fe)2O3 crystals in this study may also due to the substitution of Cr atoms by Fe in the oxide scale. 3.4.2. Exposure to SCW for 600 h Fig. 10a shows the HAADF image of cross-sectional oxide film on the 16Cr-3Al ODS steel after exposure to SCW for 600 h. After exposure, the average thickness of the oxide scale calculated from the HADDF image grows up to 350 nm. Fig. 10b–e show the EDS element maps for O, Fe, Cr and Al corresponding to marked area in Fig. 10a. The results show that the outer oxide layer mainly consists of Fe, Cr and O and that the inner oxide layer contains Al and O. Fig. 10f shows the EDS compositional profiles of O, Fe, Cr and Al by scanning of the red line marked in Fig. 10a. The peaks of Cr and Al indicate the enrichment of Cr in the outer oxide layer and the enrichment of Al in the inner oxide layer, and the atomic fraction of Cr in the outer oxide layer is much higher than that of Fe. The standard Gibbs free energy ΔGf0 of Cr2O3 and Fe2O3 formation at 600 °C are −602.2 and −393.8 kJ mol−1, respectively [26]. Hence, the formation of Cr2O3 is thermodynamically driven prior to that of Fe oxides, which leads to the selective oxidation of Cr. As a result, the outer oxide layer can be considered to be composed of Crrich oxides. Fig. 11a shows the bright field TEM micrograph of the oxide scale of the 16Cr-3Al ODS steel after exposure to SCW for 600 h. Fig. 11b shows the HRTEM image of the outer layer interface area marked in Fig. 11a. The enlarge image of the crystal lattice is shown in Fig. 11c. The phase is also identified as hexagonal (Cr, Fe)2O3 through the FFT image (Fig. 11d) corresponding to the area marked in Fig. 11b. Phase identification of the inner oxide layer was conducted as shown in Fig. 12. Fig. 12b shows the HRTEM image of the marked area in Fig. 12a. Three different regions in the HRTEM image are chosen for further research by FFT images as shown in Fig. 12c–e. All the three regions are identified as Al2O3 crystals but with different grain orientations. Compared to the relatively large (Cr, Fe)2O3 crystals in the outer oxide layer (Fig. 11), Al2O3 nanocrystals are formed in the inner oxide layer due to the rapid crystallization process [25]. At the early stage of oxidation, Al atoms are oxidized quickly to form Al2O3 nanocrystals and (Cr, Fe)2O3 crystals are formed above the Al2O3 nanocrystals oxide layer synchronously. As the corrosion process extends, (Cr, Fe)2O3 crystals may grow faster than Al2O3 crystals due to the
Fig. 10. (a) STEM-HAADF image of the cross-sectional oxide film on the 16Cr3Al ODS steel after exposure to SCW for 600 h and the corresponding EDS elemental maps for (b) O, (c) Fe, (d) Cr and (e) Al, respectively, (f) EDS compositional profile of O, Fe, Cr and Al by scanning of the red line marked in (a). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
corresponding EDS elemental maps. It can be observed from the elemental maps that the oxide scale could be divided into two regions. The outer layer mainly consists of Fe, Cr and O and the inner layer is mainly composed of Al and O. After exposure to SCW for 200 h, the average thickness of the oxide scale evaluated from the HADDF image is 250 nm. The detailed crystal structure of the outer oxide scale was 974
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Fig. 11. (a) Bright field TEM micrograph of the cross-sectional oxide scale of the 16Cr-3Al ODS steel after exposure to SCW at 600 °C and 25 MPa for 600 h, (b) HRTEM image of the outer oxide grain marked by square dashed line in (a), (c) enlarged image of the red squared area in (b) and (d) FFT image corresponding to the marked area in (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 12. (a) Bright field TEM micrograph of the cross-sectional oxide scale of the 16Cr-3Al ODS steel after exposure to SCW at 600 °C and 25 MPa for 600 h, (b) HRTEM image of the inner oxide layer marked by square dashed line in (a), (c, d and e) corresponding to the marked areas in (b) and their FFT images.
direct contact with O on the oxide surface. As the outer oxide layer grows thicker, the partial pressure of oxygen at the interface of outer and inner oxide layer decreases remarkably and the growth of Al2O3 crystals is restricted.
morphologies can be distinguished from the image. Fig. 14c shows the TEM image of an individual grain in the outer oxide layer as marked in Fig. 14b and the SAED pattern of the grain is shown in Fig. 14d. The phase composition of the grain is identified as hexagonal (Cr, Fe)2O3. In this regard, (Cr, Fe)2O3 crystals are formed on the outer oxide layer of the 16Cr-3Al ODS steel after exposure for different times (200 h, 600 h and 1000 h). Besides, with the exposure time prolonged, the (Cr, Fe)2O3 oxides grow big on the outer oxide layer with a diameter of 150 nm with exposure of 1000 h (see Fig. 14b).
3.4.3. Exposure to SCW for 1000 h Fig. 13a shows the HAADF image taken from cross-section of the oxide film on the 16Cr-3Al ODS steel after exposure to SCW for 1000 h. After exposure, the average thickness of the oxide scale calculated from the HADDF image grows up to 400 nm. Fig. 13b–e show the corresponding EDS elemental maps for O, Fe, Cr and Al, respectively. The elemental concentration regions are consistent with those in the EDS maps for 200 h and 600 h. However, the growth of the outer oxide layer at 1000 h is obvious, indicating more Cr atoms diffuse into the outer oxide layer and are oxidized as (Cr, Fe)2O3. Besides, Al element is concentrated in the inner layer after exposure for 1000 h rather than partially existing in the outer oxide layers after exposure for 200 h and 600 h. The inner oxide layer is thick and compact, which can significantly enhance the corrosion resistance of the 16Cr-3Al ODS steel. Fig. 14a shows the TEM micrograph of the cross-sectional oxide scale of the 16Cr-3Al ODS steel after exposure to SCW for 1000 h. The oxide layer can be observed along with the fine ferrite grains in the matrix. Fig. 14b shows the TEM image with higher magnification of the marked area in Fig. 14a. It can be seen that two oxide layers with different
3.5. Corrosion mechanism Based on the cross-sectional TEM analysis, the outer oxide layer is mainly composed of (Cr, Fe)2O3 while the inner oxide layer is mainly composed of Al2O3 after exposure in SCW. To study the evolution of the oxide layer, the thickness of the outer and inner oxide layer is calculated through the cross-sectional HADDF images (see Figs. 8, 10 and 13). The detailed thickness data are listed in Table 3. Fig. 15 illustrates the evolution of the oxide scale thickness with the increase of exposure time from 200 h to 1000 h. As can be seen, the growth kinetics of both the outer and inner oxide layers follows the parabolic law, and the growth rate of the outer oxide layer is faster than that of the inner layer. The corrosion behavior of the 16Cr-3Al ODS steel under supercritical condition is similar to that in gaseous environment. Oxidation in 975
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Fig. 13. STEM-HAADF image of the cross-sectional of the oxide film on the 16Cr-3Al ODS steel after exposure to SCW at 600 °C and 25 MPa for 1000 h and the corresponding EDS elemental maps for (b) O, (c) Fe, (d) Cr and (e) Al, respectively.
Fig. 14. (a) Bright field TEM micrograph of the cross-sectional oxide scale of the 16Cr-3Al ODS steel after exposure to SCW at 600 °C and 25 MPa for 1000 h, (b) high magnification TEM image of the red squared area in (b), (c) an individual grain with a diameter of about 120 nm in the outer oxide layer in (b), (d) selected area electron diffraction (SAED) pattern of the red squared area in (b).
of Al2O3 oxides is higher than that of (Cr, Fe)2O3. Thus, as illustrated in Fig. 16, a large number of Al2O3 formed on the sample surface. Whereas only a small amount of (Cr, Fe)2O3 crystals are formed at the early stage. As the corrosion process extends, a continuous oxide layer composed of Al2O3 and (Cr, Fe)2O3 is formed. The inner oxide layer is mainly composed of Al2O3 and the outer layer mainly consists (Cr, Fe)2O3. Then the growth of the oxide scale is controlled by the diffusion stage. With the increase of exposure time from 200 h to 600 h, since the content of Fe and Cr is higher than that of Al in the matrix. The diffusion of Cr3+ and Fe3+ can be driven by chemical gradient, and diffuse outward across the inner oxide layer to form (Cr, Fe)-containing oxides in the outer oxide layer. Besides, the outer oxide layer is closer to SCW, and it is easier for Fe and Cr combine with O, thus promoting the growth of the oxide layer. This diffusion mechanism leads to the formation of thicker outer oxide layer than inner layer. With the increase of exposure time in SCW from 200 h to 600 h (Figs. 8 and 10), the growth of both the outer and inner oxide layers can be observed, and
Table 3 Oxide scale thickness of the 16Cr-3Al ODS steel exposed to SCW at 600 °C and 25 MPa for 200 h, 600 h and 1000 h, respectively. Exposure times
200 h
600 h
1000 h
Inner layer (nm) Outer layer (nm)
63 ± 10 187 ± 12
78 ± 11 272 ± 13
89 ± 12 311 ± 15
gaseous environments is dominated by the molecular processes and the diffusion rates of anions and cations in the oxide are the rate-determining steps [27,28]. Fig. 16 illustrates the evolution of the oxide scale with the increase of exposure times in SCW. At the early stage of the corrosion, the rapid uptake of O atoms converts the surface layer. Since the standard Gibbs free energy ΔGf0 of Al2O3 formation is −934.6 kJ mol−1, which is smaller than that of Cr2O3 and Fe2O3 formation (−602.2 and −393.8 kJ mol−1) [26]. As the result, the formation rate 976
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the protection of the compact inner Al2O3 oxide layers. 4. Conclusions The oxidation behavior of the 16Cr-3Al ODS steel in SCW at 600 °C/ 25 MPa with dissolved oxygen of 200 ppb by mass was investigated in this study. The following conclusions can be summarized: 1) The 16Cr-3Al ODS steel exhibited superior corrosion resistance to SCW. The weight gain after exposure for 1000 h was 6.24 mg/dm2, which was much lower than that of the 9Cr ODS and austenitic stainless steel. 2) Duplex oxide layers were formed after exposure in SCW. The outer oxide layer was mainly composed of (Cr, Fe)2O3 and the inner layer was mainly composed of Al2O3. 3) The growth of both the inner and outer oxide layer followed the parabolic law, and the outer oxide layer grew faster than the inner layer. As the inner Al2O3 layer became compact, the growth of the oxide layer was restricted by the protection of compact Al2O3, and the corrosion process became steady. The formation of compact Al2O3 was the control step for the decrease of corrosion rate of the steel in SCW.
Fig. 15. Thickness of the oxide scales with the increase of exposure time in SCW from 200 h to 1000 h and their fitting results.
the outer oxide layer grows faster than the inner layer. The growth rate of the inner oxide layer is restricted due to the limited inward diffusion O2– by the outer oxide layer. However, the growth of the outer oxide layer can be driven by the direct contact with O2– and the outward diffusion of Cr3+ and Fe3+. With the increase of exposure time from 600 to 1000 h (Fig. 10 and Fig. 13), relatively compact inner oxide layer composed of Al2O3 is formed. Since the diffusion coefficients of Fe and Cr in Al2O3 are quite low [29], the outward diffusion of Cr3+ and Fe3+ is significantly restricted and the growth of the outer oxide layer slows down. In this case, the oxidation process would be steady due to
Acknowledgments The authors are grateful to the International Thermonuclear Experimental Reactor (ITER) Program Special Project (No. 2015GB107003 and 2015GB119001), the National Natural Science Foundation of China (No. 11672200, 51674175 and U1660201), and the Science and Technology Program of Tianjin (No. 18YFZCGX00070) for grant and financial support. We thank Dr. Xiaoming Zhang from ZKKF (Beijing) Science & Technology Co., Ltd for TEM observations.
Fig. 16. Schematic representation of microstructural evolution of the oxide layer at early stage in SCW and after exposure for 200 h, 600 h and 1000 h, respectively.
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Corrosion behavior of an Al added high-Cr ODS steel in supercritical water at 600 °C
T
Jian Rena, Liming Yua, , Yongchang Liua, , Zongqing Maa, Chenxi Liua, Huijun Lia, Jiefeng Wub ⁎
a b
⁎
State Key Lab of Hydraulic Engineering Simulation and Safety, Tianjin key Lab of Composite and Functional Materials, Tianjin University, Tianjin 300072, China Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
ARTICLE INFO
ABSTRACT
Keywords: ODS steel Supercritical water reactor High temperature corrosion Microstructure observations
Corrosion behavior in the supercritical water (SCW) of an Al added high-Cr oxide dispersion strengthened (ODS) steel was investigated in this study. The steel was exposed to SCW at 600 °C and 25 MPa for various exposure times up to 1000 h, and the resultant oxide layers were characterized by using weight change measurements, Xray diffraction (XRD), X-ray photoelectron spectroscopies (XPS), scanning/transmission electron microscopy (SEM/TEM), and energy dispersive spectroscopy (EDS). The weight gain after exposure for 1000 h was 6.24 mg/ dm2, indicating the superior corrosion resistance of 16Cr-3Al ODS steel to SCW. Chemical analysis of the surface oxides showed that the content of Cr-rich oxides increased with the increase of exposure time in SCW. Crosssectional observation of the oxide scales revealed that an outer layer composed of (Cr, Fe)2O3 and an inner layer composed of Al2O3 formed on the sample surface. The growth of both outer and inner oxide layers followed the parabolic law with the increase of exposure times. The corrosion mechanism was discussed in detail based on the microstructure evolution process with exposure time in SCW.
1. Introduction As a kind of promising Generation IV advanced nuclear reactors, supercritical water reactors (SCWRs) can provide much higher thermal efficiency and have lower construct cost than current light water reactors [1–3]. They are designed to produce core outlet temperatures as high as 600 °C and system pressures up to 25 MPa [4]. At a temperature above of 374.15 °C and a pressure above of 22.1 MPa, the supercritical water (SCW) acts as a nonpolar solvent that can dissolve gases like oxygen to complete miscibility, which dramatically aggravates the corrosion of structural materials commonly used in nuclear reactors and fossil power plants. Therefore, fuel cladding materials with appropriate corrosion resistance are urgently required in the design process of SCWRs. Oxide dispersion strengthened (ODS) steels have been considered as candidate structure materials for nuclear fusion reactors due to their comprehensive mechanical properties and superior irradiation resistance [5–7]. Nano-sized yttria particles in the matrix can act as obstacles to pin the movement of dislocations and the migration of grain boundaries. Besides, homogenous distribution of the nano-sized yttria particles can play the role of sinking irradiation induced defects. However, in the application of ODS steels for SCWRs, their corrosion resistance should be taken into consideration. Compared to the 9Cr⁎
ODS steels, high-Cr (14–20 wt%) ODS steels show higher corrosion resistance due to the higher Cr content [8,9]. In order to obtain superior corrosion resistance of ODS steels in SCW, it may be a feasible way to add Al into the matrix during the fabrication processes. However, Al addition is easy to introduce large Y-Al-O oxides due to its high affinity with oxygen, thus deteriorating the mechanical performance of ODS steels [10–12]. Fortunately, it is found that Zr addition can refine the precipitates due to the substitution of Y-Al-O oxides by fine Y-Zr-O nanoparticles in the matrix [10–13]. According to the evaluation on the mechanical performance of the Al added high-Cr ODS steel with Zr addition, its application prospect in SCWRs is worth of further research. Furthermore, although several works have been conducted to investigate the corrosion behavior of high-Cr ODS steels in SCW [9,14–19], few focused on Al added high-Cr ODS steels and their corrosion mechanism in SCW. In this work, the corrosion behavior of an Al added high-Cr ODS steel was studied in SCW at 600 °C and 25 MPa for various exposure times. Post-exposure surface analysis was performed by using X-ray photoelectron spectroscopies (XPS). The oxidation behavior, the microstructure and composition of the oxides scale were investigated by using weight change measurement, X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and scanning transmission electron microscope (STEM). Based on these
Corresponding authors. E-mail addresses: [email protected] (L. Yu), [email protected] (Y. Liu).
https://doi.org/10.1016/j.apsusc.2019.03.019 Received 14 January 2019; Received in revised form 23 February 2019; Accepted 1 March 2019 Available online 08 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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exposure in SCW. To minimize the error in measurements, every weight gain measurement was conducted five times. The surface chemistry of the oxide films was examined by XPS (EscaLab 250Xi, Thermo Fisher). The surface morphologies and phase identification of the oxide films were characterized by SEM (S4800, Hitachi) and XRD (D8 Advanced, Bruker), respectively. The detailed microstructure characterization of the cross-sectional oxide films was conducted by a TEM (Tecnai G2 F20, FEI) equipped with an energy dispersive spectroscopy (EDS) detector system and a high angle annular dark-field (HAADF) detector. The preparation of specimens for cross-sectional TEM observation can be found in Ref. [20]. A so-called ‘Tripod polishing method’ was used to prepare cross-sectional TEM specimens. To this end, two slices cut from the same specimen were glued together face-to-face (i.e. the oxide film to the oxide film). The thus-obtained sandwich was again cut into smaller sandwiches (2 mm × 2 mm × 0.5 mm), and both sides of a sandwich were plane-polished perpendicular to the glue plane (i.e., perpendicular to the oxide film) until the thickness of the specimen reached to 30 μm. Then the specimen was glued with copper collar (i.e. copper collar plane parallel to the oxide film) followed by soaking in acetone solvent for 30 min. Finally, ion-milling was performed using an Ar-ion polishing system (Gatan, PIPS695) at 3 kV (13 mA) with an incident beam angle of about 9°, until a hole was formed. Thus, the edges of the ion-milled hole could act as electron transparent areas for crosssectional TEM analysis.
Table 1 Element compositions (wt%) of the 16Cr-3Al ODS steel. Cr
W
Al
C
N
Y2O3
Zr
Fe
15.89
1.45
2.64
0.081
0.053
0.29
0.42
Bal.
results, the formation process of oxide films was analyzed and the corrosion mechanism was discussed. 2. Material and methods Chemical composition (wt%) of the Al added high-Cr ODS steel (referred to as 16Cr-3Al ODS hereon) used in this study was listed in Table 1. The fabrication processes for the 16Cr-3Al ODS are presented as follow: the pre-alloyed powders，yttria (Y2O3) and Zr powders were mechanically alloyed by high energy ball milling in a pure argon atmosphere for 30 h, then the powders were hot isostatic pressed (HIPed) at 1150 °C for 3 h under a pressure of 150 MPa. The grain size and microstructure of 16Cr-3Al ODS steel were examined by SEM (Quanta 650F, FEI) equipped with electron backscattered diffraction (EBSD) (HKL Channel 5) and TEM (Tecnai G2 F20, FEI), respectively. Fig. 1 shows the microstructure of the 16Cr-3Al ODS steel. The mean grain size can be determined as 0.88 μm from Fig. 1a and the precipitates can be observed in Fig. 1b. Coupon specimens with dimensions of 20 mm × 10 mm × 2 mm were fabricated from the 16Cr-3Al ODS HIPed bulk alloy. To prepare samples for SCW experiment, the specimens were grounded using diamond lapping films down to 2000 grit and then polished using a soft felt pad wet by colloidal silica suspension with a nominal grain size of 0.05 μm. After polishing, the specimens were washed with ethyl alcohol in an ultrasonic bath in sequence. Corrosion tests were conducted in SCW at 600 °C/25 MPa with dissolve oxygen (DO) of 200 ppb by mass. Fig. 2a shows the schematic of the SCW loop facility. The sketch of the coupon specimens is shown in Fig. 2b. The exposure time was set as 200 h, 400 h, 600 h, 800 h and 1000 h, respectively. The inlet conductivity of the water was measured to be 0.1 μS/cm and the flow rate was 1.8–2 L/h. After each SCW exposure period, the test coupons were dried in air immediately. All samples were weighted using METTER-TOLDO MS204 with an accuracy of 0.1 mg to record the weight change before and after
3. Results and discussion 3.1. Weight gain The weight gain data can be fitted using the follow equation [21]: (1)
w = kt n 2
where Δw represents weight gain (mg/dm ), k is a rate constant mg/ dm2·h, t is the exposure time (h), and n is an exponent describing the time dependence of the oxide growth. Table 2 gives the weight gain data and the fitted curves are plotted in Fig. 3 as a function of exposure time in SCW at 600 °C and 25 MPa. For comparison, coupon specimens of the 9Cr ODS steel were put into SCW together with the 16Cr-3Al ODS steel. The weight gain data of the 304, 316 stainless steel and the 304 ODS steel in Fig. 3 can be found in Ref. [22], where the exposure
Fig. 1. (a) Inverse pole figure (IPF) and grain boundary maps taken from the HIPed 16Cr-3Al ODS steel and (b) TEM bright-field micrographs of the unexposed 16Cr3Al ODS steel. 970
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Fig. 2. (a) The schematic of the SCW loop facility and (b) sketch of the dimension of the coupon specimens subjected to the system. Table 2 Weight gain data of the 9Cr ODS steel and the 16Cr-3Al ODS steel in SCW as a function of exposure time. Alloy
9Cr-ODS 16Cr-3Al ODS
Weight gain at exposure time (mg/dm2)
k(mg/dm2/h)
200 h
400h
600 h
800 h
1000 h
255.90 2.50
342.49 4.62
402.36 5.04
461.20 5.72
498.73 6.24
27.94 0.229
n
R2
0.41 0.473
0.9992 0.9999
Fig. 3. (a) Weight gain curves for the 16Cr-3Al ODS and 9Cr ODS steels in SCW, (b) Y-axis magnified figure of the weight gain curve for the 16Cr-3Al ODS steel.
conditions were similar to those in this research. As can be seen from Fig. 3, the weight gain data of different steels all follow the parabolic law, and the weight gain of the 16Cr-3Al ODS steel is extremely small compared to that of other steels. This illustrates that the 16Cr-3Al ODS steel exhibits higher corrosion resistance during exposure in SCW.
show a slightly increase in peak intensity with the increase of exposure time. 3.3. Morphology and chemical composition of the surface oxides Fig. 5 shows the surface morphology of the 16Cr-3Al ODS steel after exposure in SCW for different times. It can be observed that uniform oxide crystals are formed on the surface of the 16Cr-3Al ODS steel with the increase of exposure time. As the exposure time extends, the shape and size of the oxide crystals change synchronously. After exposure for 200 h (Fig. 5a), ultrafine rod-shaped oxide crystals with a length of about 500 nm can be observed. When the exposure time increases to
3.2. XRD measurement of the oxide scales Fig. 4 shows the XRD patterns of the specimens after exposure to SCW with various exposure times. As can be seen, there are three major diffraction peaks corresponding to the ferrite matrix during exposure. In addition, weak diffraction peaks of Cr2O3 can be also observed and 971
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Fig. 4. XRD results of the 16Cr-3Al ODS steel after exposure to SCW with various exposure times.
400 h (Fig. 5b) and 600 h (Fig. 5c), the rod-shaped oxide crystals change into the granular shape, indicating the growth of oxide crystals. Eventually, after exposure for 800 h (Fig. 5d) and 1000 h (Fig. 5e), relatively large prismatic oxide crystals are formed. The diameter of the oxide crystals is small and most of the oxide crystals are below 1 μm even after exposure for 1000 h. Besides, no spallation can be found in the outer oxides surface, which is different from that observed in the austenitic alloy 304 and the T91 steel exposed to SCW [22,23]. This indicates that the surface oxides of the 16Cr-3Al ODS steel are stable during exposure. The excellent stability can be attributed to the formation of relatively fine oxide crystals at the surface, which exhibit good uniformity and compaction. Fig. 6 shows the XPS survey scans of the surface of the 16Cr-3Al ODS steel exposed to SCW for 200, 400, 600, 800 and 1000 h,
Fig. 6. XPS results for the surface chemistry of the 16Cr-3Al ODS steel exposed to SCW for (a) 200 h, (b) 400 h, (c) 600 h, (d) 800 h and (e) 1000 h, respectively.
Fig. 5. SEM micrographs of the outer surface of the 16Cr-3Al ODS steel after exposure to SCW for (a) 200 h, (b) 400 h, (c) 600 h, (d) 800 h and (e) 1000 h, respectively.
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Fig. 7. XPS spectra of (a) Fe 2p3/2 (b) Cr 2p3/2 and (c) Al 2p core level for the surface oxides formed on the 16Cr-3Al ODS steel in SCW.
respectively. The results confirm the existence of Cr in addition to Fe, Al, O and C on the sample surface. The full spectra of the surface oxides for different exposure times show a similar chemical element types. To identify the chemical states of the elements of Fe, Cr and Al, the highresolution spectra (obtained at a binding energy intervals of 0.06 eV) for different exposure times are presented in Fig. 7a–c, respectively. For the spectra of Fe element in Fig. 7a, negligible differences can be observed, indicating the stable chemical state of Fe on the outer oxide surface during exposure. Fig. 7b displays the narrow scans of Cr 2p3/2. As can be seen, the peak intensity of Cr 2p3/2 is enhanced with increase of exposure time, indicating the increase of the content of Cr-containing oxides at the outer surface with the increase of exposure time. Fig. 7c shows the Al 2p core level spectra. The intensity of Al 2p increases with exposure time up to 600 h, but decreases suddenly after exposure for
800 h and 1000 h. At the early stage of the oxidation process up to 600 h, due to relatively higher affinity with O, Al atoms can diffuse quickly to the interface and produce Al-containing oxides. However, with further oxidation of the outer surface, the Al-containing oxides may be covered by relatively thick Fe or Cr containing oxides. The detection depth of XPS technique is about several nanometers [24], and the depth of Al-containing oxides may exceed the detection limit of XPS, thus leading to the decrease of the peak intensity of Al 2p spectra. 3.4. Cross-sectional characterization of the oxide scales 3.4.1. Exposure to SCW for 200 h Fig. 8 shows the STEM-HAADF image of the cross-sectional oxide film of the 16Cr-3Al ODS steel after exposure to SCW for 200 h and its
Fig. 8. (a) STEM-HAADF image of the cross-sectional oxide film on the 16Cr-3Al ODS steel after exposure to SCW for 200 h and the corresponding EDS elemental maps for (b) O, (c) Fe, (d) Cr and (e) Al, respectively.
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Fig. 9. (a) Bright field TEM micrograph of the cross-sectional oxide scale of the 16Cr-3Al ODS steel after exposure to SCW for 200 h, (b) HRTEM image of the out oxide grain marked by square dashed line in (a), and (c) FFT image corresponding to the area marked in (b).
characterized by HRTEM. Fig. 9a shows the bright field TEM micrographs of the outer oxide scale. The HRTEM image of the selected area and the corresponding FFT image are provided in Fig. 9b and c, respectively. Based on these results, the phase composition of the outer oxide scales is identified as hexagonal (Cr, Fe)2O3. The solid reaction between Cr2O3 and Fe2O3 in Fe-Cr-Al alloy can promote the substitution of Cr atoms in Cr2O3 by Fe atoms since the two oxides have the similar crystal structure [25]. The formation of (Cr, Fe)2O3 crystals in this study may also due to the substitution of Cr atoms by Fe in the oxide scale. 3.4.2. Exposure to SCW for 600 h Fig. 10a shows the HAADF image of cross-sectional oxide film on the 16Cr-3Al ODS steel after exposure to SCW for 600 h. After exposure, the average thickness of the oxide scale calculated from the HADDF image grows up to 350 nm. Fig. 10b–e show the EDS element maps for O, Fe, Cr and Al corresponding to marked area in Fig. 10a. The results show that the outer oxide layer mainly consists of Fe, Cr and O and that the inner oxide layer contains Al and O. Fig. 10f shows the EDS compositional profiles of O, Fe, Cr and Al by scanning of the red line marked in Fig. 10a. The peaks of Cr and Al indicate the enrichment of Cr in the outer oxide layer and the enrichment of Al in the inner oxide layer, and the atomic fraction of Cr in the outer oxide layer is much higher than that of Fe. The standard Gibbs free energy ΔGf0 of Cr2O3 and Fe2O3 formation at 600 °C are −602.2 and −393.8 kJ mol−1, respectively [26]. Hence, the formation of Cr2O3 is thermodynamically driven prior to that of Fe oxides, which leads to the selective oxidation of Cr. As a result, the outer oxide layer can be considered to be composed of Crrich oxides. Fig. 11a shows the bright field TEM micrograph of the oxide scale of the 16Cr-3Al ODS steel after exposure to SCW for 600 h. Fig. 11b shows the HRTEM image of the outer layer interface area marked in Fig. 11a. The enlarge image of the crystal lattice is shown in Fig. 11c. The phase is also identified as hexagonal (Cr, Fe)2O3 through the FFT image (Fig. 11d) corresponding to the area marked in Fig. 11b. Phase identification of the inner oxide layer was conducted as shown in Fig. 12. Fig. 12b shows the HRTEM image of the marked area in Fig. 12a. Three different regions in the HRTEM image are chosen for further research by FFT images as shown in Fig. 12c–e. All the three regions are identified as Al2O3 crystals but with different grain orientations. Compared to the relatively large (Cr, Fe)2O3 crystals in the outer oxide layer (Fig. 11), Al2O3 nanocrystals are formed in the inner oxide layer due to the rapid crystallization process [25]. At the early stage of oxidation, Al atoms are oxidized quickly to form Al2O3 nanocrystals and (Cr, Fe)2O3 crystals are formed above the Al2O3 nanocrystals oxide layer synchronously. As the corrosion process extends, (Cr, Fe)2O3 crystals may grow faster than Al2O3 crystals due to the
Fig. 10. (a) STEM-HAADF image of the cross-sectional oxide film on the 16Cr3Al ODS steel after exposure to SCW for 600 h and the corresponding EDS elemental maps for (b) O, (c) Fe, (d) Cr and (e) Al, respectively, (f) EDS compositional profile of O, Fe, Cr and Al by scanning of the red line marked in (a). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
corresponding EDS elemental maps. It can be observed from the elemental maps that the oxide scale could be divided into two regions. The outer layer mainly consists of Fe, Cr and O and the inner layer is mainly composed of Al and O. After exposure to SCW for 200 h, the average thickness of the oxide scale evaluated from the HADDF image is 250 nm. The detailed crystal structure of the outer oxide scale was 974
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Fig. 11. (a) Bright field TEM micrograph of the cross-sectional oxide scale of the 16Cr-3Al ODS steel after exposure to SCW at 600 °C and 25 MPa for 600 h, (b) HRTEM image of the outer oxide grain marked by square dashed line in (a), (c) enlarged image of the red squared area in (b) and (d) FFT image corresponding to the marked area in (b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 12. (a) Bright field TEM micrograph of the cross-sectional oxide scale of the 16Cr-3Al ODS steel after exposure to SCW at 600 °C and 25 MPa for 600 h, (b) HRTEM image of the inner oxide layer marked by square dashed line in (a), (c, d and e) corresponding to the marked areas in (b) and their FFT images.
direct contact with O on the oxide surface. As the outer oxide layer grows thicker, the partial pressure of oxygen at the interface of outer and inner oxide layer decreases remarkably and the growth of Al2O3 crystals is restricted.
morphologies can be distinguished from the image. Fig. 14c shows the TEM image of an individual grain in the outer oxide layer as marked in Fig. 14b and the SAED pattern of the grain is shown in Fig. 14d. The phase composition of the grain is identified as hexagonal (Cr, Fe)2O3. In this regard, (Cr, Fe)2O3 crystals are formed on the outer oxide layer of the 16Cr-3Al ODS steel after exposure for different times (200 h, 600 h and 1000 h). Besides, with the exposure time prolonged, the (Cr, Fe)2O3 oxides grow big on the outer oxide layer with a diameter of 150 nm with exposure of 1000 h (see Fig. 14b).
3.4.3. Exposure to SCW for 1000 h Fig. 13a shows the HAADF image taken from cross-section of the oxide film on the 16Cr-3Al ODS steel after exposure to SCW for 1000 h. After exposure, the average thickness of the oxide scale calculated from the HADDF image grows up to 400 nm. Fig. 13b–e show the corresponding EDS elemental maps for O, Fe, Cr and Al, respectively. The elemental concentration regions are consistent with those in the EDS maps for 200 h and 600 h. However, the growth of the outer oxide layer at 1000 h is obvious, indicating more Cr atoms diffuse into the outer oxide layer and are oxidized as (Cr, Fe)2O3. Besides, Al element is concentrated in the inner layer after exposure for 1000 h rather than partially existing in the outer oxide layers after exposure for 200 h and 600 h. The inner oxide layer is thick and compact, which can significantly enhance the corrosion resistance of the 16Cr-3Al ODS steel. Fig. 14a shows the TEM micrograph of the cross-sectional oxide scale of the 16Cr-3Al ODS steel after exposure to SCW for 1000 h. The oxide layer can be observed along with the fine ferrite grains in the matrix. Fig. 14b shows the TEM image with higher magnification of the marked area in Fig. 14a. It can be seen that two oxide layers with different
3.5. Corrosion mechanism Based on the cross-sectional TEM analysis, the outer oxide layer is mainly composed of (Cr, Fe)2O3 while the inner oxide layer is mainly composed of Al2O3 after exposure in SCW. To study the evolution of the oxide layer, the thickness of the outer and inner oxide layer is calculated through the cross-sectional HADDF images (see Figs. 8, 10 and 13). The detailed thickness data are listed in Table 3. Fig. 15 illustrates the evolution of the oxide scale thickness with the increase of exposure time from 200 h to 1000 h. As can be seen, the growth kinetics of both the outer and inner oxide layers follows the parabolic law, and the growth rate of the outer oxide layer is faster than that of the inner layer. The corrosion behavior of the 16Cr-3Al ODS steel under supercritical condition is similar to that in gaseous environment. Oxidation in 975
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Fig. 13. STEM-HAADF image of the cross-sectional of the oxide film on the 16Cr-3Al ODS steel after exposure to SCW at 600 °C and 25 MPa for 1000 h and the corresponding EDS elemental maps for (b) O, (c) Fe, (d) Cr and (e) Al, respectively.
Fig. 14. (a) Bright field TEM micrograph of the cross-sectional oxide scale of the 16Cr-3Al ODS steel after exposure to SCW at 600 °C and 25 MPa for 1000 h, (b) high magnification TEM image of the red squared area in (b), (c) an individual grain with a diameter of about 120 nm in the outer oxide layer in (b), (d) selected area electron diffraction (SAED) pattern of the red squared area in (b).
of Al2O3 oxides is higher than that of (Cr, Fe)2O3. Thus, as illustrated in Fig. 16, a large number of Al2O3 formed on the sample surface. Whereas only a small amount of (Cr, Fe)2O3 crystals are formed at the early stage. As the corrosion process extends, a continuous oxide layer composed of Al2O3 and (Cr, Fe)2O3 is formed. The inner oxide layer is mainly composed of Al2O3 and the outer layer mainly consists (Cr, Fe)2O3. Then the growth of the oxide scale is controlled by the diffusion stage. With the increase of exposure time from 200 h to 600 h, since the content of Fe and Cr is higher than that of Al in the matrix. The diffusion of Cr3+ and Fe3+ can be driven by chemical gradient, and diffuse outward across the inner oxide layer to form (Cr, Fe)-containing oxides in the outer oxide layer. Besides, the outer oxide layer is closer to SCW, and it is easier for Fe and Cr combine with O, thus promoting the growth of the oxide layer. This diffusion mechanism leads to the formation of thicker outer oxide layer than inner layer. With the increase of exposure time in SCW from 200 h to 600 h (Figs. 8 and 10), the growth of both the outer and inner oxide layers can be observed, and
Table 3 Oxide scale thickness of the 16Cr-3Al ODS steel exposed to SCW at 600 °C and 25 MPa for 200 h, 600 h and 1000 h, respectively. Exposure times
200 h
600 h
1000 h
Inner layer (nm) Outer layer (nm)
63 ± 10 187 ± 12
78 ± 11 272 ± 13
89 ± 12 311 ± 15
gaseous environments is dominated by the molecular processes and the diffusion rates of anions and cations in the oxide are the rate-determining steps [27,28]. Fig. 16 illustrates the evolution of the oxide scale with the increase of exposure times in SCW. At the early stage of the corrosion, the rapid uptake of O atoms converts the surface layer. Since the standard Gibbs free energy ΔGf0 of Al2O3 formation is −934.6 kJ mol−1, which is smaller than that of Cr2O3 and Fe2O3 formation (−602.2 and −393.8 kJ mol−1) [26]. As the result, the formation rate 976
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the protection of the compact inner Al2O3 oxide layers. 4. Conclusions The oxidation behavior of the 16Cr-3Al ODS steel in SCW at 600 °C/ 25 MPa with dissolved oxygen of 200 ppb by mass was investigated in this study. The following conclusions can be summarized: 1) The 16Cr-3Al ODS steel exhibited superior corrosion resistance to SCW. The weight gain after exposure for 1000 h was 6.24 mg/dm2, which was much lower than that of the 9Cr ODS and austenitic stainless steel. 2) Duplex oxide layers were formed after exposure in SCW. The outer oxide layer was mainly composed of (Cr, Fe)2O3 and the inner layer was mainly composed of Al2O3. 3) The growth of both the inner and outer oxide layer followed the parabolic law, and the outer oxide layer grew faster than the inner layer. As the inner Al2O3 layer became compact, the growth of the oxide layer was restricted by the protection of compact Al2O3, and the corrosion process became steady. The formation of compact Al2O3 was the control step for the decrease of corrosion rate of the steel in SCW.
Fig. 15. Thickness of the oxide scales with the increase of exposure time in SCW from 200 h to 1000 h and their fitting results.
the outer oxide layer grows faster than the inner layer. The growth rate of the inner oxide layer is restricted due to the limited inward diffusion O2– by the outer oxide layer. However, the growth of the outer oxide layer can be driven by the direct contact with O2– and the outward diffusion of Cr3+ and Fe3+. With the increase of exposure time from 600 to 1000 h (Fig. 10 and Fig. 13), relatively compact inner oxide layer composed of Al2O3 is formed. Since the diffusion coefficients of Fe and Cr in Al2O3 are quite low [29], the outward diffusion of Cr3+ and Fe3+ is significantly restricted and the growth of the outer oxide layer slows down. In this case, the oxidation process would be steady due to
Acknowledgments The authors are grateful to the International Thermonuclear Experimental Reactor (ITER) Program Special Project (No. 2015GB107003 and 2015GB119001), the National Natural Science Foundation of China (No. 11672200, 51674175 and U1660201), and the Science and Technology Program of Tianjin (No. 18YFZCGX00070) for grant and financial support. We thank Dr. Xiaoming Zhang from ZKKF (Beijing) Science & Technology Co., Ltd for TEM observations.
Fig. 16. Schematic representation of microstructural evolution of the oxide layer at early stage in SCW and after exposure for 200 h, 600 h and 1000 h, respectively.
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