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Corrosion behavior of 14Cr ODS steel in supercritical water: The influence of substituting Y2O3 with Y2Ti2O7 nanoparticles
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Corrosion behavior of 14Cr ODS steel in supercritical water: The influence of substituting Y2O3 with Y2Ti2O7 nanoparticles Haozhi Zhaoa, Tong Liua,*, Zhonglian Baia, Linbo Wanga, Wenhua Gaob, Lefu Zhangb,* a b
Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China School of Nuclear Science and Engineering, Shanghai Jiao Tong University, No 800 Dongchuan Road, Shanghai 200240, China
A R T I C LE I N FO
A B S T R A C T
Keywords: A. ODS steel B. X-ray diffraction B. SEM C. Oxidation C. Supercritical water
14Cr-4.5Al ODS steels were prepared by adding Y2Ti2O7 and Y2O3 nanoparticles respectively. Bilayer-structure corrosion scales were formed while the Y2Ti2O7-ODS steel showed a lower mass gain of 108.1 mg/dm2 in SCW for 1500 h. Compared with Y2O3, Y2Ti2O7 with finer size and higher number density exhibited more effective hindrance impacts on the diffusion of ions. Furthermore, adding Y2Ti2O7 in the ODS steel can provide more zero valent Al to form alumina film through inhibiting the formation of coarse Y-Al-O particles. Our work demonstrates that substituting Y2O3 with Y2Ti2O7 is an effective route to improve the corrosion resistance of Al-adding ODS steels.
1. Introduction Oxide dispersion strengthened (ODS) alloys as one of promising class structure materials have been investigated for application for supercritical water reactor (SCWR) due to their superior high temperature mechanical performance and corrosion resistance [1–5]. It is well known that the corrosion behavior of Fe-based alloys strongly depends on the Cr and Al contents. Cho et al. found that Cr content ranging from 9 to 12 wt.% for Al-free ODS steels was not suitable for SCWR owing to the insufficient corrosion resistance of the materials [6]. Kimura further pointed out that the corrosion issue in SCW required Cr concentration be higher than 14 wt.% and lower than 16 wt.% to avoid aging embrittlement [7]. Thus, the Cr content must be controlled properly for the ODS steels applied under SCW condition. It is worthy to note that Al-adding ODS steels show higher corrosion resistance than Al-free ODS steels by forming protective alumina scale in SCW environment [8–12]. Hence, the addition of Al is indispensable for designing the high resistance ODS steels. Moreover, Lee reported that ODS steels with Cr content ranging from 14 to 16 wt.% should combine with a certain amount of Al (3.5–4.5 wt.%) to generate a dense and adherent alumina layer [9]. That is to say, the contents of Cr and Al have a relatively definite range for high corrosion resistance ODS steels. On the other hand, it has been found that adding Y2O3 nanoparticles (NPs) as the general dispersoids can facilitate the fast formation of αalumina and decrease the average grain size of scales in the Fe-Al steels during the oxidation process [13,14]. Thus, besides Cr and Al, the
⁎
dispersed oxide NPs play important roles on the corrosion resistance of Al-adding ODS steels. For Al-free ODS steels, Behnamian et al. investigated the corrosion behavior of 304-ODS steel in SCW by comparing with alloy 304. The excellent corrosion resistance of 304-ODS steel is due to the fact that dispersed Y2O3 can promote the refinement of grain size and trap vacancies to hinder the diffusion of Fe [15]. In addition, the diffusion of oxygen and iron along grain boundaries was considered to be critical on the corrosion of ODS steels in SCW [9]. The outward diffusion of metal elements was strongly inhibited on account of the dispersed oxide particles distributed in intergranular and intragranular of Fe-20Cr-4.5Al-0.5Y2O3 ODS steels [16]. It should be noted that for Al-adding ODS steels, Y2O3 always reacts with Al and residual oxygen to form the coarse Y-Al-O particles, such as yttrium aluminum perovskites (YAP, YAlO3) and/or yttrium aluminum garnet (YAG, Y3Al5O12). This greatly deteriorates the mechanical properties of materials and reduces the actual content of Al required to form alumina [17,18]. In fact, Y2O3 can combine with Ti to generate finer and more stable Y2Ti2O7 NPs in Al-free ODS steels [19–21]. Furthermore, the interfaces between the oxide NPs and the matrix have a significant impact on the mechanical properties of ODS steels. It is generally believed that a coherent or semi-coherent interface can effectively improve the mechanical properties of steels [22]. Mao reported that over 95% Y2Ti2O7 NPs are semi-coherent with the matrix in ODS steels [23], but the lattice coherency between Y-Al-O particles and matrix alloy was rarely detected [24]. Hence, it is preferable to produce Y2Ti2O7 NPs rather than Y-Al-O particles in the Al-adding ODS steels. In
Corresponding authors. E-mail address: [email protected] (T. Liu).
https://doi.org/10.1016/j.corsci.2019.108272 Received 30 June 2019; Received in revised form 1 October 2019; Accepted 8 October 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Haozhi Zhao, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2019.108272
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microstructure of two ODS steels after HIP at 1160 °C. There is no obvious difference in grain sizes between YTO and YO, and both of them possess equiaxed grains ranging from 0.2 to 1.2 μm with an average grain size of 0.5 μm. This indicates that different dispersed oxides in this work have little effect on the grain size of ODS steels. Fig. 2(a) and (b) distinctly illustrate the TEM images of the oxide dispersoids in two ODS steels, and Fig. 2(c) and (d) are the corresponding histograms of the oxide particle size distribution. The oxide dispersoids in both ODS steels are observed to exhibit an approximately spherical shape. The mean size of the oxide NPs in YTO is 9.2 ± 1.9 nm and the number density is 6.2 × 1023 m−3. While the mean size of the oxide NPs in YO is 14.6 ± 5.8 nm and the number density is 3.2 × 1022 m−3. These results indicate that the oxide NPs in YTO possess smaller size and higher number density than in YO. Surprisingly, the size of oxide NPs in YTO is as small as Y2Ti2O7 NPs (9.6 nm) in 14Cr Al-free ODS steel [26]. In order to further analyze the crystal structure of individual oxide NPs in the ODS steels, the HRTEM technique and FFT method were adopted. Fig. 3(a) displays a typical oxide particle in YO, which is approximately spherical and has a diameter of 23 nm. The measured interplanar distances and inter-axial angles of the oxide particle conform to the cubic structure of yttrium-aluminum-perovskite (YAP) YAlO3 (JCPDS: 38-0222, Body Centered Cubic). Meanwhile, the corresponding FFT diagram in Fig. 3(b) further confirms the formation of YAlO3 dispersoid in the matrix. Futhermore, the YAlO3 nanoparticle and α-Fe matrix do not reveal stringent coherent or semi-coherent relationship, which is in agreement with other studies on Al-adding ODS steels, such as PM2000 [24]. The HRTEM image of a representative oxide particle in YTO is shown in Fig. 3(c). This oxide particle exhibits a spherical shape with a size of 12 nm. The measured interplanar distances of the spherical oxide particle are 1.786 and 1.785 Å, with an inter-axial angle of 60.1°. These results are in great agreement with the (044 ) and (404) planes of pyrochlore Y2Ti2O7 (JCPDS: 42-0413, Space group: Fd3m). In addition, the corresponding FFT diagram in Fig. 3(d) further demonstrates that there is a lattice coherency between the Y2Ti2O7 nanoparticle and α-Fe matrix. The orientation relationship between the lattice of Y2Ti2O7 nanoparticle and that of the matrix is [1 1 1]α-Fe// [1 1 1]Y2Ti2O7 and (110) α-Fe// (440) Y2Ti2O7. The lattice misfit ε ∗ between Y2Ti2O7 and matrix corresponding to d(110)α-Fe = 2.023 Å and d (440)Y2Ti2O7 = 1.786 Å can be calculated to be 12.4%, implying a semicoherent interface between Y2Ti2O7 nanoparticle and α-Fe matrix in YTO. Similar semi-coherent orientation relationships between Y2Ti2O7 NPs and Fe matrix were also observed in the 14Cr Al-free ODS steel [27].
Table 1 Chemical compositions of the ODS steels (wt.%). Sample ID
Cr
Al
W
Ti
Y2O3
Y2Ti2O7
Fe
YO YTO
14 14
4.5 4.5
2 2
0.35 0.35
0.6 –
– 0.6
Balance Balance
our previous studies, the more stable and finer Y2Ti2O7 NPs were added to ODS steels directly, which improved the mechanical strength obviously [25]. However, the effects of substituting Y2Ti2O7 NPs for Y2O3 on the corrosion behavior of Al-adding ODS steels in SCW are still unknown. In this work, we prepared 14Cr-4.5Al ODS steel by substituting general Y2O3 NPs with Y2Ti2O7 NPs, and the effects of Y2Ti2O7 and Y2O3 on the microstructure of ODS steels were systematically characterized. Moreover, comparative investigations were carried out on the corrosion behavior of Y2Ti2O7 ODS steel and Y2O3 ODS steel in SCW (600 °C, 25 MPa), chiefly in terms of morphology, composition and structure of the corrosion scales. 2. Experiments The nominal chemical compositions of the Y2Ti2O7-ODS steel and Y2O3-ODS steel referred to as YTO and YO are given in Table 1 respectively. For Fe-14Cr-4.5Al-2W-0.35Ti-0.6Y2Ti2O7 ODS steel, the high purity (> 99.9 wt.%) elemental metal powders and Y2Ti2O7 NPs were firstly mechanically alloyed (MA) by a planetary ball mill with a ball to powder ratio of 15:1 at a rotation speed of 280 rpm for 48 h in high purity Ar. After MA, the ODS powders were then consolidated at 1160 °C by hot isostatic pressing (HIP) under a pressure of 150 MPa for 4 h. The same fabrication process was adopted for Fe-14Cr-4.5Al-2W0.35Ti-0.6Y2O3 ODS steel by substituting Y2Ti2O7 with Y2O3 as raw oxide nanoparticle. The TEM samples (3 mm in diameter) were prepared using the Struers Tenupol-3 double jet electro-polisher at −30 °C with an electrolyte of 10% HClO4 and 90% CH3CH2OH. TEM observations of the matrix, the oxides and the oxide/matrix interface were carried out on the JEOL-JSM-2100 F TEM at an accelerating voltage of 200 k V. The high resolution TEM (HRTEM) images were analyzed by the fast Fourier transform (FFT) method. The corrosion coupons were cut from the HIP-ed samples in the form of square-sheet with the dimension of 10 × 15 × 1.5 mm3 and had a small hole for hanging in autoclave safely. The coupons were then mechanically abraded with SiC papers up to 2000 grade to dislodge the preformed oxide layer, followed by ultrasonic cleaning in alcohol for 5 min. Before the corrosion tests, the masses of all samples were measured using a balance with a resolution of 0.1 mg. The corrosion tests were conducted in the SCW corrosion loop at 600 °C under a pressure of 25 MPa for 100, 300, 600, 1000 and 1500 h. The SCW had a flow velocity of 0.8 L/h and the dissolved oxygen content was 200 ppb. After the corrosion tests, the cross-section samples were embedded in bakelite by using XQ-2B mounting press and then were ground with SiC papers up to 2000 grade. The surface and cross section of the selected specimens were analyzed by scanning electron microscopy (SEM, Camscan CS3400) at 25 k V equipped with Energy Dispersive Spectrometer (EDS) to evaluate the thickness, morphology and element distribution of the oxide scales. X-ray diffractometer (XRD, Bruker AXS D8) with monochromatic Co Kα radiation was used to determine the phases of the corrosion products on the surface of these specimens. The scanning rate was 2°/min from 20°–90°.
3.2. Corrosion kinetics The corrosion kinetics is usually adopted to evaluate the lifetime of the materials operated in corrosive environment. Fig. 4 shows the dependence of mass gain on exposure time for two ODS steels in SCW (600 °C, 25 MPa). It can be discovered that the slopes of the initial stage have a close relationship with the types of oxide dispersoids in the ODS steels. YTO with the addition of Y2Ti2O7 NPs displays a gentler slope in the curve of mass gain versus exposure time. After passing the rapid mass increasing region of about 300 h, the slopes of the curves of both YTO and YO decrease significantly and reach a nearly saturation status gradually. For YO corroded for 600 h in SCW (600 °C, 25 MPa), its mass gain is 140.1 mg/dm2, less than the mass gain of 304-ODS steel (about 150.0 mg/dm2) under the same corrosion condition [28]. It is worth to note that the mass gain of YTO corroded for 600 h in SCW (600 ºC, 25 MPa) is only 82.7 mg/dm2, which is much lower than YO. After being exposed for 1500 h in SCW, the YTO exhibits a good corrosion resistance with a very low mass gain of 108.1 mg/dm2. For comparison, the YO with the same Cr (14 wt.%) and Al (4.5 wt.%) content demonstrates severe corrosion with a large mass gain of 156.5 mg/dm2. This implies that the corrosion resistance of ODS steel is tightly associated with the types of oxide dispersoids.
3. Results and discussion 3.1. Microstructure of the ODS steels The TEM images in Fig. 1(a) and (b) display the typical 2
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Fig. 1. TEM images of the ODS steels after HIP: (a) YO and (b) YTO.
Fig. 2. TEM images of the oxide particles dispersed in the ODS steels of (a) YO and (b) YTO; the corresponding histograms of oxide particle size distribution of (c) YO and (d) YTO.
is the temperature (K). It is worth to note that YO exhibits a much larger kp value of 38.25, which is about 4 times that of YTO (9.28). On the basis of this equation, YTO possesses a higher oxidation activation energy than YO in SCW (600 °C, 25 MPa). The oxidation reaction in SCW usually occurs due to the diffusion of metal cations to the water-oxide interface, or the diffusion of oxygen anions to the oxide-metal interface, which means the diffusion of metal cations and oxygen anions through the oxide layer in YTO is more difficult to carry out than in YO. This explains why YO performs worse than YTO in SCW corrosion resistance.
The time dependence of the corrosion kinetics can be described as a power law [29,30]:
Δm = kp t n
(1)
where Δm is the mass gain per unit area in mg/dm2, kp is the effective corrosion rate constant (mg/dm2/hn), n is the rate exponent and t is the exposure time in SCW. The relevant kinetic parameters for specimens corroded in SCW (600 °C, 25 MPa) are given in Table 2. The experimental data of the specimens are in good agreement with the fitting results, and the values of n for YTO and YO samples are 0.34 and 0.20 respectively, which are much lower than the parabolic growth law (n = 0.5). This means that the oxide scales on the surfaces of both ODS steels grow very slowly. Previous studies suggested that the further oxidation depends on the diffusion mechanism, and the oxidation rate can be fitted by Arrhenius equation [31]:
kp = k 0exp(−
Q ) RT
3.3. Structure and composition of corrosion scale X-ray diffraction spectra patterns taken on the surfaces of YTO and YO exposed to SCW (600 °C, 25 MPa) for various times are shown in Fig. 5(a) and (b). With regard to YTO, the diffraction peaks of α-Fe phase (JCPDS: 85–1410, Space group: Im 3 m) are distinctly observed at 52.4° and 76.8° after an exposure time of 300 h. This indicates that the corrosion depth is very shallow since α-Fe is the main constituent of ODS steel. Besides the matrix α-Fe peaks, hematite (Fe2O3, JCPDS: 89–8103, Space group: R 3 c) is also clearly detected in the oxide scale. It is noteworthy that the characteristic peaks of magnetite (Fe3O4,
(2) 2
n
where k0 is the rate constant (mg/dm /h ), Q is the activation energy of the oxidation reaction (J/mol), R is gas constant (8.314 J/mol/K) and T 3
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Fig. 3. HRTEM images of one typical oxide nanoparticle in (a) YO and (c) YTO; the corresponding FFT patterns of (b) YO and (d) YTO.
Fig. 4. Mass gain as function of exposure time for YTO and YO in SCW (600 °C, 25 MPa).
FeCr2O4, which is the same as that of YTO corrosion layer. What is particularly intriguing is that the characteristic XRD peak intensity ratio of Fe2O3 (38.7°) to α-Fe (52.4°) is 0.55 for YO after 300 h corrosion in SCW (600 °C, 25 MPa), which is obviously larger than that of YTO (0.07) under the same conditions. It means the YTO has the thinner corrosion layer than YO. As the exposure time prolongs to 1500 h, the characteristic XRD peak intensity ratio of Fe2O3 (38.7°) to α-Fe (52.4°) increases from 0.55 to 2.21. All the results illustrate the relative mass ratio of oxide to α-Fe in YTO is much smaller than that of YO after being corroded for 1500 h, meaning the YTO have corrosion resistance than YO. This implies that Y2Ti2O7 NPs play superior role to Y2O3 NPs in improving corrosion resistance. It should be pointed out that no Al2O3 is detectable in both YTO and YO, although it has been suggested that Y can promote the selective oxidation of aluminum [33,34]. This may be attributed to the much thinner alumina layer that can not be identified by XRD.
Table 2 The kp and n values of the samples in SCW (600 °C, 25 MPa).
3.4. Microstructure of the corrosion scale
Sample ID
kp
n
Error of n
Correlation coefficient (R)
YO YTO
38.25 9.28
0.20 0.34
0.04 0.03
0.90 0.98
The surface morphology observation of corrosion scale can contribute a lot to comprehending the corrosion behavior of ODS steel. Fig. 6(a) and (b) display the SEM images of the surface of YTO and YO after being corroded for 300 h in SCW (600 °C, 25 MPa). There is no scale spallation or cracks detected on the corrosion scale. It can be seen from Fig. 6(a) that the oxide nodules on the surface of YTO possess regularly polygonal shapes with sizes ranging from 1 to 3 μm. The EDS analysis of a typical big oxide nodule (about 3 μm) at the position of P1 demonstrates that it is enriched with Fe, Al and O elements while poor in Cr (3.47 wt.%), see Fig. 7(a). The measured contents of Cr and Fe of another small oxide nodule (about 1 μm) at P2 position are significantly higher than that of the big oxide nodule at P1 position, while the small oxide nodule possesses less Al and O content than the big oxide nodule, see Fig. 7(b). This indicates that the bigger oxide nodules are subjected to more severe oxidation for YTO after 300 h corrosion, and the homogeneous oxide layer is not generated. In fact, Fe as the main constituent in ODS steel tends to react with oxygen and form the Fe-rich
JCPDS: 75-0449, Space group: Fd 3 m) and the spinel type oxide FeCr2O4 (JCPDS: 89–2618, Space group: Fd3m) are almost overlapped at 35.4°, 41.8°, 51.2°, 67.1° and 75.0°, making it so hard to tell the two phases apart. The similar phenomenon was also reported by Novotný and coworkers for commercial ODS steels MA956 and PM2000 in SCW (650 °C, 25 MPa) [32]. For YTO, as the exposure time in SCW (600 °C, 25 MPa) prolongs from 300 h to 1500 h, the characteristic XRD peak intensity ratio of Fe2O3 (38.7°) to α-Fe (52.4°) increases from 0.07 to 0.99. This indicates the relative mass ratio of oxide to α-Fe increases gradually while the phases of oxide do not change. As shown in Fig. 5(b), the phase composition of YO corrosion layer is hematite (Fe2O3), magnetite (Fe3O4) and the spinel type oxide 4
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Fig. 5. XRD patterns of (a) YTO and (b) YO corroded in SCW (600 °C, 25 MPa) for 300, 600, 1000 and 1500 h.
and the elements distribution inside the oxide scale. Fig. 9 displays the SEM image and element mappings of YTO after being corroded for 300 h in SCW (600 °C, 25 MPa). The corrosion scale is too thin to be distinguished, and the Fe, Cr, Al and O elements distribute uniformly. While the XRD result of YTO for 300 h in Fig. 5(a) illustrates a corrosion layer composed of hematite (Fe2O3), magnetite (Fe3O4) and the spinel type oxide FeCr2O4. It is worthy to note that a dual corrosion layer is generated on the surface of YTO after a long-term exposure of 1500 h, see Fig. 10. The outer layer (about 5 μm) is enriched with Fe and O elements, suggesting that it consists of Fe2O3 and Fe3O4 on the basis of XRD results. A continuous and quite thin Al-containing film coupled with Cr occurs in the same area, implying that the inner layer (about 3 μm) is composed of FeCr2O4 and Al2O3, although the Al2O3 phase is not identified by XRD in virtue of its low concentration. Meanwhile, neither cracks nor pores are observed at the oxide scale/substrate interface of YTO, indicating the excellent adherence of the oxide scale. Fig. 11 shows the SEM image and EDS mappings of YO cross-section after being corroded for 300 h in SCW (600 °C, 25 MPa). Like YTO, there is no obvious corrosion scale discovered on the surface of YO due to the short exposure time. After 1500 h corrosion, the scale on the surface of YO divides into two layers, see Fig. 12. The outer layer of YO exhibits a much larger thickness of 8 μm than that of YTO (5 μm), which is rich in Fe and O elements, illustrating that it has the same composition of Fe2O3 and Fe3O4 with the outer layer of YTO. The Al element in the corrosion scale of YO (4.5 Al wt.%) is hardly detected in the outer layer by EDS point analysis, which is probably attributed to the thick outer layer. Furthermore, the Cr element is deficient in the outer oxide layer, but enriched in the inner oxide layer, implying that the outward diffusion of Cr is not suppressed efficiently. The thin inner layer (about 3 μm) is mainly composed of Al and Cr elements, which can be identified as FeCr2O4 and Al2O3 on the basis of XRD results. Compared with YTO, the inner layer of YO exhibits the same phase composition and similar thickness. The corrosion schematic diagrams of YO and YTO are shown in Fig. 13(a) and (b) respectively. The thickness of the oxide layers is
oxide during the initial corrosion period. According to the EDS results, it can be deduced that the formation of oxide nodule with big size is largely due to the rapid oxidation of Al. It has been reported that the activation energy of oxygen in alumina is higher than that in magnetite or spinel [35,36], thus Al has a stronger affinity for oxygen than Cr. In addition, Lee found that the diffusion of Al occurred prior to the diffusion of Cr for ODS steels corroded in SCW environment [9]. The corrosion scale of YO after exposed in SCW for 300 h is adherent with the substrate as well, see Fig. 6(b). However, the oxide nodules of YO obviously grow up to about 5 μm in diameter, which are much larger than that of YTO. It indicates that YO has suffered more severe corrosion, which is in good agreement with mass gain curve. The EDS analyses at P3 and P4 positions in Fig. 6(b) illustrate that the Cr content of the big oxide nodule (P3, about 7 μm) is 1.72 wt.%, while the small oxide nodule (P4, about 2 μm) contains nearly twice the Cr content (4.33 wt.%), see Fig. 7(c) and Fig. 7(d). It is interesting to find that there is no detectable Al on the surface of YO, although 4.5 wt.% Al was added in YO. Compared with the EDS results of YTO, it can be concluded that the available Al concentration in substrate to form alumina has significant effect on the composition and size of the surface oxide. Moreover, we inferred that through inhibiting the formation of coarse Y-Al-O particles in ODS steel, the addition of Y2Ti2O7 NPs can provide more zero valent Al to form alumina film. This probably explains why the kp of YO is much larger than that of YTO. After 1500 h corrosion in SCW (600 °C, 25 MPa), the oxide scale of YTO become slightly rough without any spallation, as shown in Fig. 8(a). The size of oxide nodules generally increases as the corrosion time prolongs. Surprisingly, the majority of the oxide nodules on the surface of YTO are still below 5 μm in diameter. It is obvious that the YO sample have suffered more serious corrosion, as shown in Fig. 8(b) highlighted by red arrow, the oxide scale and the substrate no longer adhere tightly together owing to the partial spallation, and then the freshly exposed substrate will be further eroded in SCW. The SEM and EDS analyses for the cross-section of the ODS steels were conducted to further investigate the thickness of the oxide scale
Fig. 6. SEM images of samples exposed for 300 h in SCW (600 °C, 25 MPa): (a) YTO, (b) YO. 5
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Fig. 7. Compositional point analyses on the surfaces of YTO and YO after 300 h corrosion in SCW (600 °C, 25 MPa): (a) P1, (b) P2, (c) P3 and (d) P4; the sites of P1-P4 are indicated in Fig. 6.
drawn in proportion to the experimental results above. For ODS steels corroded in SCW (600 °C, 25 MPa), not only grain boundary diffusion, but also volume diffusion is a vital factor in controlling the diffusion rate [29]. In this work, two ODS steels were prepared by adding the same content of oxide NPs, which are almost spherical. The radius of the ith oxide dispersoid is considered as ri, and the total number of oxide dispersoids in ODS steel is n. The total volume of oxide dispersoids in ODS steel is illustrated by the following equation: nYO
VYO =
∑
VYTO = nY TO
(7)
VYO = VYTO
( )3 3
(6)
Owing to the same amount of oxide NPs added to the two ODS steels and the similar density between Y2O3 (5.03 g/cm3) and Y2Ti2O7 (4.98 g/cm3), the total volume of all oxide dispersoids in each ODS steel are almost equal.
4 π rYOi
i= 1
¯ )3 4π(rYTO 3
On the basis of Eqs. (5)–(7), the number ratio of the oxide dispersoids in YO (nYO ) to those in YTO (nYTO ) can be expressed as:
(3)
3
nYTO
VYTO =
∑ i= 1
(
nYO r = ⎛ YTO ⎞ nYTO ⎝ rYO ⎠ ⎜
)
4 π rYTOi 3
3
(4)
⎟
The total cross-sectional area of all oxide dispersoids in ODS steel (S) can be calculated by the following equation:
We presume that the oxide dispersoids in each ODS steel are monodisperse, and they possess the same radius of r , then the Eq. (3) and (4) can be described as:
n
S=
∑ π(ri )2 i=1
VYO
¯ )3 4π(rYO = nYO 3
(8)
(9)
According to the above assumption of the monodispersion of oxide dispersoids in ODS steel, the Eq. (9) can be described as:
(5)
Fig. 8. SEM images of samples exposed for 1500 h in SCW (600 °C, 25 MPa): (a) YTO, (b) YO. 6
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Fig. 9. SEM image and EDS element mappings on cross section of YTO sample corroded for 300 h in SCW (600 °C, 25 MPa).
Fig. 10. SEM image and EDS element mappings on cross section of YTO sample corroded for 1500 h in SCW (600 °C, 25 MPa).
Fig. 11. SEM image and EDS element mappings on cross section of YO sample corroded for 300 h in SCW (600 °C, 25 MPa). 2 π S= nπ ⎛ , r ⎞ 4 ⎝ ⎠
nYO π SYO = SYTO nYTO π
(10)
So the total area ratio of all oxide dispersoids in YO (SYO) to those in YTO (SYTO) can be expressed as: 7
( (
2 π ,r 4 YO
) )
π ,r 4 YTO
2
=
(rYTO )3 (rYO )2 r = YTO (rYO )3 (rYTO )2 rYO
(11)
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Fig. 12. SEM image and EDS element mappings on cross section of YO sample corroded for 1500 h in SCW (600 °C, 25 MPa).
Fig. 13. Corrosion schematic diagrams of (a) YO and (b) YTO.
in YO to form alumina film and improve the corrosion resistance of ODS steel. At the same time, the more stable Y2Ti2O7 NPs are propitious to enhance the adhesion between the corrosion layer and the substrate via releasing internal stress and reducing interface defects [16,33]. Thus, adding Y2Ti2O7 NPs to substitute Y2O3 NPs in the preparation of Aladding ODS steels is an effective route to improve their corrosion resistance in SCW while ensuring high mechanical performance.
The SYO/SYTO is calculated as 0.63 (< 1) from the Eq. (11) since rYTO (9.2 nm) is finer than rYO (14.6 nm). Thus, it can be inferred that for one diffusion channel in the ODS steels, the total cross-sectional area of oxide dispersoids in YTO sample (SYTO) is larger than that in YO sample (SYO). So the YTO sample is supposed to own a much smaller corresponding cross-sectional area of volume diffusion for ions than YO sample. Therefore, Y2Ti2O7 NPs in YTO exhibit more effective hindrance impacts than Y-Al-O particles in YO on the outward diffusion of metal cations. Furthermore, the Y2Ti2O7 nanoparticle in YTO shows a semi-coherent orientation relationship with the matrix compared to the incoherent coarse Y-Al-O particle in YO, meaning that the Y2Ti2O7 nanoparticle has a lower lattice misfit with the Fe matrix. That is to say, metal cations will become tougher to diffuse outward along the interface of oxide/matrix since there are fewer defects in the semi-coherent interface of YTO. Bischoff and co-workers found that the Cr element and Y-rich oxides tended to segregate along the grain boundary during the diffusion process, and hindered the diffusion of both metal cations and oxygen anions [37]. In this work, for the two ODS steels with similar grain sizes, since Y2Ti2O7 NPs (semi-coherent with the matrix) are distributed at grain boundaries of YTO with higher number density than Y-Al-O NPs in YO, they play better roles as obstructions. It should be mentioned that the addition of Y2Ti2O7 NPs can inhibit the reaction of Al with oxide dispersoids for YTO sample, while Y2O3 will combine with Al to form coarse Y-Al-O particles in YO sample. Consequently, there will be more zero valent Al remaining in YTO than
4. Conclusions By substituting Y2Ti2O7 for Y2O3, the oxide dispersoids in the ODS steels changed from coarse YAlO3 to fine Y2Ti2O7 (semi-coherent with the matrix). Compared with YO, YTO exhibited better corrosion resistance with a lower mass gain of 108.1 mg/dm2 in SCW (600 °C, 25 MPa) for 1500 h. The corrosion scales with bilayer structure were generated on both ODS steels. The outer layer was composed of Fe2O3 and Fe3O4, while the inner layer consisted of FeCr2O4 and Al2O3. The diffusion of ions is better inhibited by the finer Y2Ti2O7 NPs in YTO than Y-Al-O NPs in YO. Furthermore, adding stable Y2Ti2O7 NPs can provide more zero valent Al to form alumina film to improve the corrosion resistance of ODS steel. For Al-adding ODS steels, adding Y2Ti2O7 NPs to substitute Y2O3 NPs is an effective route to improve the corrosion resistance while ensuring high mechanical properties.
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Data availability
[14] W.J. Quadakkers, Growth mechanisms of oxide scales on ODS alloys in the temperature range 1000–1100°C, Mater. Corros. 41 (1990) 659–668. [15] Y. Behnamian, A. Mostafaei, A. Kohandehghan, B.S. Amirkhiz, R. Zahiri, W.Y. Zheng, D. Guzonas, M. Chmielus, W.X. Chen, J.L. Luo, A comparative study on the oxidation of austenitic alloys 304 and 304-oxide dispersion strengthened steel in supercritical water at 650 °C, J. Supercrit. Fluids 119 (2017) 245–260. [16] T. Liu, L.B. Wang, C.X. Wang, H.L. Shen, Effect of Al content on the oxidation behavior of Y2Ti2O7-dispersed Fe-14Cr ferritic alloys, Corros. Sci. 104 (2016) 17–25. [17] L.L. Hsiung, M.J. Fluss, S.J. Tumey, B.W. Choi, Y. Serruys, F. Willaime, A. Kimura, Formation mechanism and the role of nanoparticles in Fe-Cr ODS steels developed for radiation tolerance, Phys. Rev. B 82 (2010) 184103. [18] M.F. Hupalo, M. Terada, A.M. Kliauga, A.F. Padilha, Microstructural characterization of INCOLOY alloy MA 956, Mater. Werkst. 34 (2003) 505–508. [19] A.J. London, S. Santra, S. Amirthapandian, B.K. Panigrahi, R.M. Sarguna, S. Balaji, R. Vijay, C.S. Sundar, S. Lozano-Perez, C.R.M. Grovenor, Effect of Ti and Cr on dispersion, structure and composition of oxide nano-particles in model ODS alloys, Acta Mater. 97 (2015) 223–233. [20] M.J. Alinger, G.R. Odette, D.T. Hoelzer, On the role of alloy composition and processing parameters in nanocluster formation and dispersion strengthening in nanostuctured ferritic alloys, Acta Mater. 57 (2009) 392–406. [21] T. Liu, H.L. Shen, C.X. Wang, W.S. Chou, Structure evolution of Y2O3 nanoparticle/ Fe composite during mechanical milling and annealing, Prog. Nat. Sci. 23 (2013) 434–439. [22] A. Hirata, T. Fujita, Y.R. Wen, J.H. Schneibel, C.T. Liu, M.W. Chen, Atomic structure of nanoclusters in oxide-dispersion-strengthened steels, Nat. Mater. 10 (2011) 922–926. [23] X.D. Mao, K.H. Oh, S.H. Kang, T.K. Kim, J.S. Jang, On the coherency of Y2Ti2O7 particles with austenitic matrix of oxide dispersion strengthened steel, Acta Mater. 89 (2015) 141–152. [24] M. Klimiankou, R. Lindau, A. Möslang, J. Schröder, TEM study of PM 2000 steel, Powder Metall. 48 (2005) 277–287. [25] T. Liu, L.B. Wang, C.X. Wang, H.L. Shen, H.T. Zhang, Feasibility of using Y2Ti2O7 nanoparticles to fabricate high strength oxide dispersion strengthened Fe-Cr-Al steels, Mater. Des. 88 (2015) 862–870. [26] G.M. Zhang, Z.J. Zhou, K. Mo, Y.B. Miao, S.F. Li, X. Liu, M. Wang, J.S. Park, J. Almer, J.F. Stubbins, The comparison of microstructures and mechanical properties between 14Cr-Al and 14Cr-Ti ferritic ODS alloys, Mater. Des. 98 (2016) 61–67. [27] J. Ribis, Y. de Carlan, Interfacial strained structure and orientation relationships of the nanosized oxide particles deduced from elasticity-driven morphology in oxide dispersion strengthened materials, Acta Mater. 60 (2012) 238–252. [28] S.F. Li, Z.J. Zhou, L.F. Zhang, L.W. Zhang, H.L. Hu, M. Wang, G.M. Zhang, Corrosion behavior of a 304-oxide dispersion strengthened austenitic stainless steel in supercritical water, Mater. Corros. 67 (2016) 264–270. [29] X. Ren, K. Sridharan, T.R. Allen, Corrosion of ferritic-martensitic steel HT9 in supercritical water, J. Nucl. Mater. 358 (2006) 227–234. [30] Y. Chen, K. Sridharan, S. Ukai, T.R. Allen, Oxidation of 9Cr oxide dispersion strengthened steel exposed in supercritical water, J. Nucl. Mater. 371 (2007) 118–128. [31] P. Ampornrat, G.S. Was, Oxidation of ferritic-martensitic alloys T91, HCM12A and HT-9 in supercritical water, J. Nucl. Mater. 371 (2007) 1–17. [32] R. Novotný, P. Janík, S. Penttilä, P. Hähner, J. Macák, J. Siegl, P. Haušild, High Cr ODS steels performance under supercritical water environment, J. Supercrit. Fluids 81 (2013) 147–156. [33] F.A. Golightly, F.H. Stott, G.C. Wood, The influence of yttrium additions on the oxide-scale adhesion to an iron-chromium-aluminum alloy, Oxid. Met. 10 (1976) 163–187. [34] F.H. Stott, G.C. Wood, Growth and adhesion of oxide scales on Al2O3-forming alloys and coatings, Mater. Sci. Eng. 87 (1987) 267–274. [35] G.S. Was, P. Ampornrat, G. Gupta, S. Teysseyre, E.A. West, T.R. Allen, K. Sridharan, L. Tan, Y. Chen, X. Ren, Corrosion and stress corrosion cracking in supercritical water, J. Nucl. Mater. 371 (2007) 176–201. [36] A.H. Heuer, Oxygen and aluminum diffusion in α-Al2O3: how much do we really understand, J. Eur. Ceram. Soc. 28 (2008) 1495–1507. [37] J. Bischoff, A.T. Motta, Oxidation behavior of ferritic-martensitic and ODS steels in supercritical water, J. Nucl. Mater. 424 (2012) 261–276.
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the support of this work by the National Science and Technology Major Project (2017-VI-0008-0078), the Joint Funds of the National Natural Science Foundation of China (No. U1560106), the Aeronautical Science Foundation of China (No. 2016ZF51050) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. References [1] P.D. Edmondson, C.M. Parish, Q. Li, M.K. Miller, Thermal stability of nanoscale helium bubbles in a 14YWT nanostructured ferritic alloy, J. Nucl. Mater. 445 (2014) 84–90. [2] Z.B. Kang, H.L. Shen, Z.L. Bai, H.Z. Zhao, T. Liu, Influences of different hydride nanoparticles on microstructure and mechanical properties of 14Cr-3Al ferritic ODS steels, Powder Technol. 343 (2019) 137–144. [3] Z.L. Bai, L.B. Wang, C.X. Wang, W.H. Gao, L.F. Zhang, T. Liu, Corrosion behavior of ferritic ODS steel prepared by adding YH2 nanoparticles in supercritical water at 600 degrees C, Prog. Nat. Sci. 28 (2018) 505–509. [4] A. Hojna, H. Hadraba, F. Di Gabriele, R. Husak, Behaviour of pre-stressed T91 and ODS steels exposed to liquid lead-bismuth eutectic, Corros. Sci. 131 (2018) 264–277. [5] S. Ukai, R. Miyata, S. Kasai, N. Oono, S. Hayashi, T. Azuma, R. Kayano, E. Maeda, S. Ohtsuka, Super high-temperature strength in hot rolled steels dispersing nanosized oxide particles, Mater. Lett. 209 (2017) 581–584. [6] H.S. Cho, A. Kimura, S. Ukai, M. Fujiwara, Corrosion properties of oxide dispersion strengthened steels in super-critical water environment, J. Nucl. Mater. 329 (2004) 387–391. [7] H.S. Cho, A. Kimura, Corrosion resistance of high-Cr oxide dispersion strengthened ferritic steels in super-critical pressurized water, J. Nucl. Mater. 367 (2007) 1180–1184. [8] J. Isselin, R. Kasada, A. Kimura, Corrosion behaviour of 16%Cr-4%Al and 16%Cr ODS ferritic steels under different metallurgical conditions in a supercritical water environment, Corros. Sci. 52 (2010) 3266–3270. [9] J.H. Lee, R. Kasada, A. Kimura, T. Okuda, M. Inoue, S. Ukai, S. Ohnuki, T. Fujisawa, F. Abe, Influence of alloy composition and temperature on corrosion behavior of ODS ferritic steels, J. Nucl. Mater. 417 (2011) 1225–1228. [10] S. Ukai, M. Harada, H. Okada, M. Inoue, S. Nomura, S. Shikakura, T. Nishida, M. Fujiwara, K. Asabe, Tube manufacturing and mechanical properties of oxide dispersion strengthened ferritic steel, J. Nucl. Mater. 204 (1993) 74–80. [11] T. Liu, C.X. Wang, H.L. Shen, W.S. Chou, N.Y. Iwata, A. Kimura, The effects of Cr and Al concentrations on the oxidation behavior of oxide dispersion strengthened ferritic alloys, Corros. Sci. 76 (2013) 310–316. [12] X.L. Guo, K. Chen, W.H. Gao, Z. Zhao, L.F. Zhang, Corrosion behavior of aluminaforming and oxide dispersion strengthened austenitic 316 stainless steel in supercritical water, Corros. Sci. 138 (2018) 297–306. [13] M.A. Montealegre, J.L. González-Carrasco, Influence of the yttria content on the oxidation behaviour of the intermetallic Fe40Al Alloy, Intermetallics 11 (2003) 169–175.
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Corrosion behavior of 14Cr ODS steel in supercritical water: The influence of substituting Y2O3 with Y2Ti2O7 nanoparticles Haozhi Zhaoa, Tong Liua,*, Zhonglian Baia, Linbo Wanga, Wenhua Gaob, Lefu Zhangb,* a b
Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China School of Nuclear Science and Engineering, Shanghai Jiao Tong University, No 800 Dongchuan Road, Shanghai 200240, China
A R T I C LE I N FO
A B S T R A C T
Keywords: A. ODS steel B. X-ray diffraction B. SEM C. Oxidation C. Supercritical water
14Cr-4.5Al ODS steels were prepared by adding Y2Ti2O7 and Y2O3 nanoparticles respectively. Bilayer-structure corrosion scales were formed while the Y2Ti2O7-ODS steel showed a lower mass gain of 108.1 mg/dm2 in SCW for 1500 h. Compared with Y2O3, Y2Ti2O7 with finer size and higher number density exhibited more effective hindrance impacts on the diffusion of ions. Furthermore, adding Y2Ti2O7 in the ODS steel can provide more zero valent Al to form alumina film through inhibiting the formation of coarse Y-Al-O particles. Our work demonstrates that substituting Y2O3 with Y2Ti2O7 is an effective route to improve the corrosion resistance of Al-adding ODS steels.
1. Introduction Oxide dispersion strengthened (ODS) alloys as one of promising class structure materials have been investigated for application for supercritical water reactor (SCWR) due to their superior high temperature mechanical performance and corrosion resistance [1–5]. It is well known that the corrosion behavior of Fe-based alloys strongly depends on the Cr and Al contents. Cho et al. found that Cr content ranging from 9 to 12 wt.% for Al-free ODS steels was not suitable for SCWR owing to the insufficient corrosion resistance of the materials [6]. Kimura further pointed out that the corrosion issue in SCW required Cr concentration be higher than 14 wt.% and lower than 16 wt.% to avoid aging embrittlement [7]. Thus, the Cr content must be controlled properly for the ODS steels applied under SCW condition. It is worthy to note that Al-adding ODS steels show higher corrosion resistance than Al-free ODS steels by forming protective alumina scale in SCW environment [8–12]. Hence, the addition of Al is indispensable for designing the high resistance ODS steels. Moreover, Lee reported that ODS steels with Cr content ranging from 14 to 16 wt.% should combine with a certain amount of Al (3.5–4.5 wt.%) to generate a dense and adherent alumina layer [9]. That is to say, the contents of Cr and Al have a relatively definite range for high corrosion resistance ODS steels. On the other hand, it has been found that adding Y2O3 nanoparticles (NPs) as the general dispersoids can facilitate the fast formation of αalumina and decrease the average grain size of scales in the Fe-Al steels during the oxidation process [13,14]. Thus, besides Cr and Al, the
⁎
dispersed oxide NPs play important roles on the corrosion resistance of Al-adding ODS steels. For Al-free ODS steels, Behnamian et al. investigated the corrosion behavior of 304-ODS steel in SCW by comparing with alloy 304. The excellent corrosion resistance of 304-ODS steel is due to the fact that dispersed Y2O3 can promote the refinement of grain size and trap vacancies to hinder the diffusion of Fe [15]. In addition, the diffusion of oxygen and iron along grain boundaries was considered to be critical on the corrosion of ODS steels in SCW [9]. The outward diffusion of metal elements was strongly inhibited on account of the dispersed oxide particles distributed in intergranular and intragranular of Fe-20Cr-4.5Al-0.5Y2O3 ODS steels [16]. It should be noted that for Al-adding ODS steels, Y2O3 always reacts with Al and residual oxygen to form the coarse Y-Al-O particles, such as yttrium aluminum perovskites (YAP, YAlO3) and/or yttrium aluminum garnet (YAG, Y3Al5O12). This greatly deteriorates the mechanical properties of materials and reduces the actual content of Al required to form alumina [17,18]. In fact, Y2O3 can combine with Ti to generate finer and more stable Y2Ti2O7 NPs in Al-free ODS steels [19–21]. Furthermore, the interfaces between the oxide NPs and the matrix have a significant impact on the mechanical properties of ODS steels. It is generally believed that a coherent or semi-coherent interface can effectively improve the mechanical properties of steels [22]. Mao reported that over 95% Y2Ti2O7 NPs are semi-coherent with the matrix in ODS steels [23], but the lattice coherency between Y-Al-O particles and matrix alloy was rarely detected [24]. Hence, it is preferable to produce Y2Ti2O7 NPs rather than Y-Al-O particles in the Al-adding ODS steels. In
Corresponding authors. E-mail address: [email protected] (T. Liu).
https://doi.org/10.1016/j.corsci.2019.108272 Received 30 June 2019; Received in revised form 1 October 2019; Accepted 8 October 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Haozhi Zhao, et al., Corrosion Science, https://doi.org/10.1016/j.corsci.2019.108272
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microstructure of two ODS steels after HIP at 1160 °C. There is no obvious difference in grain sizes between YTO and YO, and both of them possess equiaxed grains ranging from 0.2 to 1.2 μm with an average grain size of 0.5 μm. This indicates that different dispersed oxides in this work have little effect on the grain size of ODS steels. Fig. 2(a) and (b) distinctly illustrate the TEM images of the oxide dispersoids in two ODS steels, and Fig. 2(c) and (d) are the corresponding histograms of the oxide particle size distribution. The oxide dispersoids in both ODS steels are observed to exhibit an approximately spherical shape. The mean size of the oxide NPs in YTO is 9.2 ± 1.9 nm and the number density is 6.2 × 1023 m−3. While the mean size of the oxide NPs in YO is 14.6 ± 5.8 nm and the number density is 3.2 × 1022 m−3. These results indicate that the oxide NPs in YTO possess smaller size and higher number density than in YO. Surprisingly, the size of oxide NPs in YTO is as small as Y2Ti2O7 NPs (9.6 nm) in 14Cr Al-free ODS steel [26]. In order to further analyze the crystal structure of individual oxide NPs in the ODS steels, the HRTEM technique and FFT method were adopted. Fig. 3(a) displays a typical oxide particle in YO, which is approximately spherical and has a diameter of 23 nm. The measured interplanar distances and inter-axial angles of the oxide particle conform to the cubic structure of yttrium-aluminum-perovskite (YAP) YAlO3 (JCPDS: 38-0222, Body Centered Cubic). Meanwhile, the corresponding FFT diagram in Fig. 3(b) further confirms the formation of YAlO3 dispersoid in the matrix. Futhermore, the YAlO3 nanoparticle and α-Fe matrix do not reveal stringent coherent or semi-coherent relationship, which is in agreement with other studies on Al-adding ODS steels, such as PM2000 [24]. The HRTEM image of a representative oxide particle in YTO is shown in Fig. 3(c). This oxide particle exhibits a spherical shape with a size of 12 nm. The measured interplanar distances of the spherical oxide particle are 1.786 and 1.785 Å, with an inter-axial angle of 60.1°. These results are in great agreement with the (044 ) and (404) planes of pyrochlore Y2Ti2O7 (JCPDS: 42-0413, Space group: Fd3m). In addition, the corresponding FFT diagram in Fig. 3(d) further demonstrates that there is a lattice coherency between the Y2Ti2O7 nanoparticle and α-Fe matrix. The orientation relationship between the lattice of Y2Ti2O7 nanoparticle and that of the matrix is [1 1 1]α-Fe// [1 1 1]Y2Ti2O7 and (110) α-Fe// (440) Y2Ti2O7. The lattice misfit ε ∗ between Y2Ti2O7 and matrix corresponding to d(110)α-Fe = 2.023 Å and d (440)Y2Ti2O7 = 1.786 Å can be calculated to be 12.4%, implying a semicoherent interface between Y2Ti2O7 nanoparticle and α-Fe matrix in YTO. Similar semi-coherent orientation relationships between Y2Ti2O7 NPs and Fe matrix were also observed in the 14Cr Al-free ODS steel [27].
Table 1 Chemical compositions of the ODS steels (wt.%). Sample ID
Cr
Al
W
Ti
Y2O3
Y2Ti2O7
Fe
YO YTO
14 14
4.5 4.5
2 2
0.35 0.35
0.6 –
– 0.6
Balance Balance
our previous studies, the more stable and finer Y2Ti2O7 NPs were added to ODS steels directly, which improved the mechanical strength obviously [25]. However, the effects of substituting Y2Ti2O7 NPs for Y2O3 on the corrosion behavior of Al-adding ODS steels in SCW are still unknown. In this work, we prepared 14Cr-4.5Al ODS steel by substituting general Y2O3 NPs with Y2Ti2O7 NPs, and the effects of Y2Ti2O7 and Y2O3 on the microstructure of ODS steels were systematically characterized. Moreover, comparative investigations were carried out on the corrosion behavior of Y2Ti2O7 ODS steel and Y2O3 ODS steel in SCW (600 °C, 25 MPa), chiefly in terms of morphology, composition and structure of the corrosion scales. 2. Experiments The nominal chemical compositions of the Y2Ti2O7-ODS steel and Y2O3-ODS steel referred to as YTO and YO are given in Table 1 respectively. For Fe-14Cr-4.5Al-2W-0.35Ti-0.6Y2Ti2O7 ODS steel, the high purity (> 99.9 wt.%) elemental metal powders and Y2Ti2O7 NPs were firstly mechanically alloyed (MA) by a planetary ball mill with a ball to powder ratio of 15:1 at a rotation speed of 280 rpm for 48 h in high purity Ar. After MA, the ODS powders were then consolidated at 1160 °C by hot isostatic pressing (HIP) under a pressure of 150 MPa for 4 h. The same fabrication process was adopted for Fe-14Cr-4.5Al-2W0.35Ti-0.6Y2O3 ODS steel by substituting Y2Ti2O7 with Y2O3 as raw oxide nanoparticle. The TEM samples (3 mm in diameter) were prepared using the Struers Tenupol-3 double jet electro-polisher at −30 °C with an electrolyte of 10% HClO4 and 90% CH3CH2OH. TEM observations of the matrix, the oxides and the oxide/matrix interface were carried out on the JEOL-JSM-2100 F TEM at an accelerating voltage of 200 k V. The high resolution TEM (HRTEM) images were analyzed by the fast Fourier transform (FFT) method. The corrosion coupons were cut from the HIP-ed samples in the form of square-sheet with the dimension of 10 × 15 × 1.5 mm3 and had a small hole for hanging in autoclave safely. The coupons were then mechanically abraded with SiC papers up to 2000 grade to dislodge the preformed oxide layer, followed by ultrasonic cleaning in alcohol for 5 min. Before the corrosion tests, the masses of all samples were measured using a balance with a resolution of 0.1 mg. The corrosion tests were conducted in the SCW corrosion loop at 600 °C under a pressure of 25 MPa for 100, 300, 600, 1000 and 1500 h. The SCW had a flow velocity of 0.8 L/h and the dissolved oxygen content was 200 ppb. After the corrosion tests, the cross-section samples were embedded in bakelite by using XQ-2B mounting press and then were ground with SiC papers up to 2000 grade. The surface and cross section of the selected specimens were analyzed by scanning electron microscopy (SEM, Camscan CS3400) at 25 k V equipped with Energy Dispersive Spectrometer (EDS) to evaluate the thickness, morphology and element distribution of the oxide scales. X-ray diffractometer (XRD, Bruker AXS D8) with monochromatic Co Kα radiation was used to determine the phases of the corrosion products on the surface of these specimens. The scanning rate was 2°/min from 20°–90°.
3.2. Corrosion kinetics The corrosion kinetics is usually adopted to evaluate the lifetime of the materials operated in corrosive environment. Fig. 4 shows the dependence of mass gain on exposure time for two ODS steels in SCW (600 °C, 25 MPa). It can be discovered that the slopes of the initial stage have a close relationship with the types of oxide dispersoids in the ODS steels. YTO with the addition of Y2Ti2O7 NPs displays a gentler slope in the curve of mass gain versus exposure time. After passing the rapid mass increasing region of about 300 h, the slopes of the curves of both YTO and YO decrease significantly and reach a nearly saturation status gradually. For YO corroded for 600 h in SCW (600 °C, 25 MPa), its mass gain is 140.1 mg/dm2, less than the mass gain of 304-ODS steel (about 150.0 mg/dm2) under the same corrosion condition [28]. It is worth to note that the mass gain of YTO corroded for 600 h in SCW (600 ºC, 25 MPa) is only 82.7 mg/dm2, which is much lower than YO. After being exposed for 1500 h in SCW, the YTO exhibits a good corrosion resistance with a very low mass gain of 108.1 mg/dm2. For comparison, the YO with the same Cr (14 wt.%) and Al (4.5 wt.%) content demonstrates severe corrosion with a large mass gain of 156.5 mg/dm2. This implies that the corrosion resistance of ODS steel is tightly associated with the types of oxide dispersoids.
3. Results and discussion 3.1. Microstructure of the ODS steels The TEM images in Fig. 1(a) and (b) display the typical 2
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Fig. 1. TEM images of the ODS steels after HIP: (a) YO and (b) YTO.
Fig. 2. TEM images of the oxide particles dispersed in the ODS steels of (a) YO and (b) YTO; the corresponding histograms of oxide particle size distribution of (c) YO and (d) YTO.
is the temperature (K). It is worth to note that YO exhibits a much larger kp value of 38.25, which is about 4 times that of YTO (9.28). On the basis of this equation, YTO possesses a higher oxidation activation energy than YO in SCW (600 °C, 25 MPa). The oxidation reaction in SCW usually occurs due to the diffusion of metal cations to the water-oxide interface, or the diffusion of oxygen anions to the oxide-metal interface, which means the diffusion of metal cations and oxygen anions through the oxide layer in YTO is more difficult to carry out than in YO. This explains why YO performs worse than YTO in SCW corrosion resistance.
The time dependence of the corrosion kinetics can be described as a power law [29,30]:
Δm = kp t n
(1)
where Δm is the mass gain per unit area in mg/dm2, kp is the effective corrosion rate constant (mg/dm2/hn), n is the rate exponent and t is the exposure time in SCW. The relevant kinetic parameters for specimens corroded in SCW (600 °C, 25 MPa) are given in Table 2. The experimental data of the specimens are in good agreement with the fitting results, and the values of n for YTO and YO samples are 0.34 and 0.20 respectively, which are much lower than the parabolic growth law (n = 0.5). This means that the oxide scales on the surfaces of both ODS steels grow very slowly. Previous studies suggested that the further oxidation depends on the diffusion mechanism, and the oxidation rate can be fitted by Arrhenius equation [31]:
kp = k 0exp(−
Q ) RT
3.3. Structure and composition of corrosion scale X-ray diffraction spectra patterns taken on the surfaces of YTO and YO exposed to SCW (600 °C, 25 MPa) for various times are shown in Fig. 5(a) and (b). With regard to YTO, the diffraction peaks of α-Fe phase (JCPDS: 85–1410, Space group: Im 3 m) are distinctly observed at 52.4° and 76.8° after an exposure time of 300 h. This indicates that the corrosion depth is very shallow since α-Fe is the main constituent of ODS steel. Besides the matrix α-Fe peaks, hematite (Fe2O3, JCPDS: 89–8103, Space group: R 3 c) is also clearly detected in the oxide scale. It is noteworthy that the characteristic peaks of magnetite (Fe3O4,
(2) 2
n
where k0 is the rate constant (mg/dm /h ), Q is the activation energy of the oxidation reaction (J/mol), R is gas constant (8.314 J/mol/K) and T 3
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Fig. 3. HRTEM images of one typical oxide nanoparticle in (a) YO and (c) YTO; the corresponding FFT patterns of (b) YO and (d) YTO.
Fig. 4. Mass gain as function of exposure time for YTO and YO in SCW (600 °C, 25 MPa).
FeCr2O4, which is the same as that of YTO corrosion layer. What is particularly intriguing is that the characteristic XRD peak intensity ratio of Fe2O3 (38.7°) to α-Fe (52.4°) is 0.55 for YO after 300 h corrosion in SCW (600 °C, 25 MPa), which is obviously larger than that of YTO (0.07) under the same conditions. It means the YTO has the thinner corrosion layer than YO. As the exposure time prolongs to 1500 h, the characteristic XRD peak intensity ratio of Fe2O3 (38.7°) to α-Fe (52.4°) increases from 0.55 to 2.21. All the results illustrate the relative mass ratio of oxide to α-Fe in YTO is much smaller than that of YO after being corroded for 1500 h, meaning the YTO have corrosion resistance than YO. This implies that Y2Ti2O7 NPs play superior role to Y2O3 NPs in improving corrosion resistance. It should be pointed out that no Al2O3 is detectable in both YTO and YO, although it has been suggested that Y can promote the selective oxidation of aluminum [33,34]. This may be attributed to the much thinner alumina layer that can not be identified by XRD.
Table 2 The kp and n values of the samples in SCW (600 °C, 25 MPa).
3.4. Microstructure of the corrosion scale
Sample ID
kp
n
Error of n
Correlation coefficient (R)
YO YTO
38.25 9.28
0.20 0.34
0.04 0.03
0.90 0.98
The surface morphology observation of corrosion scale can contribute a lot to comprehending the corrosion behavior of ODS steel. Fig. 6(a) and (b) display the SEM images of the surface of YTO and YO after being corroded for 300 h in SCW (600 °C, 25 MPa). There is no scale spallation or cracks detected on the corrosion scale. It can be seen from Fig. 6(a) that the oxide nodules on the surface of YTO possess regularly polygonal shapes with sizes ranging from 1 to 3 μm. The EDS analysis of a typical big oxide nodule (about 3 μm) at the position of P1 demonstrates that it is enriched with Fe, Al and O elements while poor in Cr (3.47 wt.%), see Fig. 7(a). The measured contents of Cr and Fe of another small oxide nodule (about 1 μm) at P2 position are significantly higher than that of the big oxide nodule at P1 position, while the small oxide nodule possesses less Al and O content than the big oxide nodule, see Fig. 7(b). This indicates that the bigger oxide nodules are subjected to more severe oxidation for YTO after 300 h corrosion, and the homogeneous oxide layer is not generated. In fact, Fe as the main constituent in ODS steel tends to react with oxygen and form the Fe-rich
JCPDS: 75-0449, Space group: Fd 3 m) and the spinel type oxide FeCr2O4 (JCPDS: 89–2618, Space group: Fd3m) are almost overlapped at 35.4°, 41.8°, 51.2°, 67.1° and 75.0°, making it so hard to tell the two phases apart. The similar phenomenon was also reported by Novotný and coworkers for commercial ODS steels MA956 and PM2000 in SCW (650 °C, 25 MPa) [32]. For YTO, as the exposure time in SCW (600 °C, 25 MPa) prolongs from 300 h to 1500 h, the characteristic XRD peak intensity ratio of Fe2O3 (38.7°) to α-Fe (52.4°) increases from 0.07 to 0.99. This indicates the relative mass ratio of oxide to α-Fe increases gradually while the phases of oxide do not change. As shown in Fig. 5(b), the phase composition of YO corrosion layer is hematite (Fe2O3), magnetite (Fe3O4) and the spinel type oxide 4
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Fig. 5. XRD patterns of (a) YTO and (b) YO corroded in SCW (600 °C, 25 MPa) for 300, 600, 1000 and 1500 h.
and the elements distribution inside the oxide scale. Fig. 9 displays the SEM image and element mappings of YTO after being corroded for 300 h in SCW (600 °C, 25 MPa). The corrosion scale is too thin to be distinguished, and the Fe, Cr, Al and O elements distribute uniformly. While the XRD result of YTO for 300 h in Fig. 5(a) illustrates a corrosion layer composed of hematite (Fe2O3), magnetite (Fe3O4) and the spinel type oxide FeCr2O4. It is worthy to note that a dual corrosion layer is generated on the surface of YTO after a long-term exposure of 1500 h, see Fig. 10. The outer layer (about 5 μm) is enriched with Fe and O elements, suggesting that it consists of Fe2O3 and Fe3O4 on the basis of XRD results. A continuous and quite thin Al-containing film coupled with Cr occurs in the same area, implying that the inner layer (about 3 μm) is composed of FeCr2O4 and Al2O3, although the Al2O3 phase is not identified by XRD in virtue of its low concentration. Meanwhile, neither cracks nor pores are observed at the oxide scale/substrate interface of YTO, indicating the excellent adherence of the oxide scale. Fig. 11 shows the SEM image and EDS mappings of YO cross-section after being corroded for 300 h in SCW (600 °C, 25 MPa). Like YTO, there is no obvious corrosion scale discovered on the surface of YO due to the short exposure time. After 1500 h corrosion, the scale on the surface of YO divides into two layers, see Fig. 12. The outer layer of YO exhibits a much larger thickness of 8 μm than that of YTO (5 μm), which is rich in Fe and O elements, illustrating that it has the same composition of Fe2O3 and Fe3O4 with the outer layer of YTO. The Al element in the corrosion scale of YO (4.5 Al wt.%) is hardly detected in the outer layer by EDS point analysis, which is probably attributed to the thick outer layer. Furthermore, the Cr element is deficient in the outer oxide layer, but enriched in the inner oxide layer, implying that the outward diffusion of Cr is not suppressed efficiently. The thin inner layer (about 3 μm) is mainly composed of Al and Cr elements, which can be identified as FeCr2O4 and Al2O3 on the basis of XRD results. Compared with YTO, the inner layer of YO exhibits the same phase composition and similar thickness. The corrosion schematic diagrams of YO and YTO are shown in Fig. 13(a) and (b) respectively. The thickness of the oxide layers is
oxide during the initial corrosion period. According to the EDS results, it can be deduced that the formation of oxide nodule with big size is largely due to the rapid oxidation of Al. It has been reported that the activation energy of oxygen in alumina is higher than that in magnetite or spinel [35,36], thus Al has a stronger affinity for oxygen than Cr. In addition, Lee found that the diffusion of Al occurred prior to the diffusion of Cr for ODS steels corroded in SCW environment [9]. The corrosion scale of YO after exposed in SCW for 300 h is adherent with the substrate as well, see Fig. 6(b). However, the oxide nodules of YO obviously grow up to about 5 μm in diameter, which are much larger than that of YTO. It indicates that YO has suffered more severe corrosion, which is in good agreement with mass gain curve. The EDS analyses at P3 and P4 positions in Fig. 6(b) illustrate that the Cr content of the big oxide nodule (P3, about 7 μm) is 1.72 wt.%, while the small oxide nodule (P4, about 2 μm) contains nearly twice the Cr content (4.33 wt.%), see Fig. 7(c) and Fig. 7(d). It is interesting to find that there is no detectable Al on the surface of YO, although 4.5 wt.% Al was added in YO. Compared with the EDS results of YTO, it can be concluded that the available Al concentration in substrate to form alumina has significant effect on the composition and size of the surface oxide. Moreover, we inferred that through inhibiting the formation of coarse Y-Al-O particles in ODS steel, the addition of Y2Ti2O7 NPs can provide more zero valent Al to form alumina film. This probably explains why the kp of YO is much larger than that of YTO. After 1500 h corrosion in SCW (600 °C, 25 MPa), the oxide scale of YTO become slightly rough without any spallation, as shown in Fig. 8(a). The size of oxide nodules generally increases as the corrosion time prolongs. Surprisingly, the majority of the oxide nodules on the surface of YTO are still below 5 μm in diameter. It is obvious that the YO sample have suffered more serious corrosion, as shown in Fig. 8(b) highlighted by red arrow, the oxide scale and the substrate no longer adhere tightly together owing to the partial spallation, and then the freshly exposed substrate will be further eroded in SCW. The SEM and EDS analyses for the cross-section of the ODS steels were conducted to further investigate the thickness of the oxide scale
Fig. 6. SEM images of samples exposed for 300 h in SCW (600 °C, 25 MPa): (a) YTO, (b) YO. 5
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Fig. 7. Compositional point analyses on the surfaces of YTO and YO after 300 h corrosion in SCW (600 °C, 25 MPa): (a) P1, (b) P2, (c) P3 and (d) P4; the sites of P1-P4 are indicated in Fig. 6.
drawn in proportion to the experimental results above. For ODS steels corroded in SCW (600 °C, 25 MPa), not only grain boundary diffusion, but also volume diffusion is a vital factor in controlling the diffusion rate [29]. In this work, two ODS steels were prepared by adding the same content of oxide NPs, which are almost spherical. The radius of the ith oxide dispersoid is considered as ri, and the total number of oxide dispersoids in ODS steel is n. The total volume of oxide dispersoids in ODS steel is illustrated by the following equation: nYO
VYO =
∑
VYTO = nY TO
(7)
VYO = VYTO
( )3 3
(6)
Owing to the same amount of oxide NPs added to the two ODS steels and the similar density between Y2O3 (5.03 g/cm3) and Y2Ti2O7 (4.98 g/cm3), the total volume of all oxide dispersoids in each ODS steel are almost equal.
4 π rYOi
i= 1
¯ )3 4π(rYTO 3
On the basis of Eqs. (5)–(7), the number ratio of the oxide dispersoids in YO (nYO ) to those in YTO (nYTO ) can be expressed as:
(3)
3
nYTO
VYTO =
∑ i= 1
(
nYO r = ⎛ YTO ⎞ nYTO ⎝ rYO ⎠ ⎜
)
4 π rYTOi 3
3
(4)
⎟
The total cross-sectional area of all oxide dispersoids in ODS steel (S) can be calculated by the following equation:
We presume that the oxide dispersoids in each ODS steel are monodisperse, and they possess the same radius of r , then the Eq. (3) and (4) can be described as:
n
S=
∑ π(ri )2 i=1
VYO
¯ )3 4π(rYO = nYO 3
(8)
(9)
According to the above assumption of the monodispersion of oxide dispersoids in ODS steel, the Eq. (9) can be described as:
(5)
Fig. 8. SEM images of samples exposed for 1500 h in SCW (600 °C, 25 MPa): (a) YTO, (b) YO. 6
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Fig. 9. SEM image and EDS element mappings on cross section of YTO sample corroded for 300 h in SCW (600 °C, 25 MPa).
Fig. 10. SEM image and EDS element mappings on cross section of YTO sample corroded for 1500 h in SCW (600 °C, 25 MPa).
Fig. 11. SEM image and EDS element mappings on cross section of YO sample corroded for 300 h in SCW (600 °C, 25 MPa). 2 π S= nπ ⎛ , r ⎞ 4 ⎝ ⎠
nYO π SYO = SYTO nYTO π
(10)
So the total area ratio of all oxide dispersoids in YO (SYO) to those in YTO (SYTO) can be expressed as: 7
( (
2 π ,r 4 YO
) )
π ,r 4 YTO
2
=
(rYTO )3 (rYO )2 r = YTO (rYO )3 (rYTO )2 rYO
(11)
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Fig. 12. SEM image and EDS element mappings on cross section of YO sample corroded for 1500 h in SCW (600 °C, 25 MPa).
Fig. 13. Corrosion schematic diagrams of (a) YO and (b) YTO.
in YO to form alumina film and improve the corrosion resistance of ODS steel. At the same time, the more stable Y2Ti2O7 NPs are propitious to enhance the adhesion between the corrosion layer and the substrate via releasing internal stress and reducing interface defects [16,33]. Thus, adding Y2Ti2O7 NPs to substitute Y2O3 NPs in the preparation of Aladding ODS steels is an effective route to improve their corrosion resistance in SCW while ensuring high mechanical performance.
The SYO/SYTO is calculated as 0.63 (< 1) from the Eq. (11) since rYTO (9.2 nm) is finer than rYO (14.6 nm). Thus, it can be inferred that for one diffusion channel in the ODS steels, the total cross-sectional area of oxide dispersoids in YTO sample (SYTO) is larger than that in YO sample (SYO). So the YTO sample is supposed to own a much smaller corresponding cross-sectional area of volume diffusion for ions than YO sample. Therefore, Y2Ti2O7 NPs in YTO exhibit more effective hindrance impacts than Y-Al-O particles in YO on the outward diffusion of metal cations. Furthermore, the Y2Ti2O7 nanoparticle in YTO shows a semi-coherent orientation relationship with the matrix compared to the incoherent coarse Y-Al-O particle in YO, meaning that the Y2Ti2O7 nanoparticle has a lower lattice misfit with the Fe matrix. That is to say, metal cations will become tougher to diffuse outward along the interface of oxide/matrix since there are fewer defects in the semi-coherent interface of YTO. Bischoff and co-workers found that the Cr element and Y-rich oxides tended to segregate along the grain boundary during the diffusion process, and hindered the diffusion of both metal cations and oxygen anions [37]. In this work, for the two ODS steels with similar grain sizes, since Y2Ti2O7 NPs (semi-coherent with the matrix) are distributed at grain boundaries of YTO with higher number density than Y-Al-O NPs in YO, they play better roles as obstructions. It should be mentioned that the addition of Y2Ti2O7 NPs can inhibit the reaction of Al with oxide dispersoids for YTO sample, while Y2O3 will combine with Al to form coarse Y-Al-O particles in YO sample. Consequently, there will be more zero valent Al remaining in YTO than
4. Conclusions By substituting Y2Ti2O7 for Y2O3, the oxide dispersoids in the ODS steels changed from coarse YAlO3 to fine Y2Ti2O7 (semi-coherent with the matrix). Compared with YO, YTO exhibited better corrosion resistance with a lower mass gain of 108.1 mg/dm2 in SCW (600 °C, 25 MPa) for 1500 h. The corrosion scales with bilayer structure were generated on both ODS steels. The outer layer was composed of Fe2O3 and Fe3O4, while the inner layer consisted of FeCr2O4 and Al2O3. The diffusion of ions is better inhibited by the finer Y2Ti2O7 NPs in YTO than Y-Al-O NPs in YO. Furthermore, adding stable Y2Ti2O7 NPs can provide more zero valent Al to form alumina film to improve the corrosion resistance of ODS steel. For Al-adding ODS steels, adding Y2Ti2O7 NPs to substitute Y2O3 NPs is an effective route to improve the corrosion resistance while ensuring high mechanical properties.
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Data availability
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The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the support of this work by the National Science and Technology Major Project (2017-VI-0008-0078), the Joint Funds of the National Natural Science Foundation of China (No. U1560106), the Aeronautical Science Foundation of China (No. 2016ZF51050) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. References [1] P.D. Edmondson, C.M. Parish, Q. Li, M.K. Miller, Thermal stability of nanoscale helium bubbles in a 14YWT nanostructured ferritic alloy, J. Nucl. Mater. 445 (2014) 84–90. [2] Z.B. Kang, H.L. Shen, Z.L. Bai, H.Z. Zhao, T. Liu, Influences of different hydride nanoparticles on microstructure and mechanical properties of 14Cr-3Al ferritic ODS steels, Powder Technol. 343 (2019) 137–144. [3] Z.L. Bai, L.B. Wang, C.X. Wang, W.H. Gao, L.F. Zhang, T. Liu, Corrosion behavior of ferritic ODS steel prepared by adding YH2 nanoparticles in supercritical water at 600 degrees C, Prog. Nat. Sci. 28 (2018) 505–509. [4] A. Hojna, H. Hadraba, F. Di Gabriele, R. Husak, Behaviour of pre-stressed T91 and ODS steels exposed to liquid lead-bismuth eutectic, Corros. Sci. 131 (2018) 264–277. [5] S. Ukai, R. Miyata, S. Kasai, N. Oono, S. Hayashi, T. Azuma, R. Kayano, E. Maeda, S. Ohtsuka, Super high-temperature strength in hot rolled steels dispersing nanosized oxide particles, Mater. Lett. 209 (2017) 581–584. [6] H.S. Cho, A. Kimura, S. Ukai, M. Fujiwara, Corrosion properties of oxide dispersion strengthened steels in super-critical water environment, J. Nucl. Mater. 329 (2004) 387–391. [7] H.S. Cho, A. Kimura, Corrosion resistance of high-Cr oxide dispersion strengthened ferritic steels in super-critical pressurized water, J. Nucl. Mater. 367 (2007) 1180–1184. [8] J. Isselin, R. Kasada, A. Kimura, Corrosion behaviour of 16%Cr-4%Al and 16%Cr ODS ferritic steels under different metallurgical conditions in a supercritical water environment, Corros. Sci. 52 (2010) 3266–3270. [9] J.H. Lee, R. Kasada, A. Kimura, T. Okuda, M. Inoue, S. Ukai, S. Ohnuki, T. Fujisawa, F. Abe, Influence of alloy composition and temperature on corrosion behavior of ODS ferritic steels, J. Nucl. Mater. 417 (2011) 1225–1228. [10] S. Ukai, M. Harada, H. Okada, M. Inoue, S. Nomura, S. Shikakura, T. Nishida, M. Fujiwara, K. Asabe, Tube manufacturing and mechanical properties of oxide dispersion strengthened ferritic steel, J. Nucl. Mater. 204 (1993) 74–80. [11] T. Liu, C.X. Wang, H.L. Shen, W.S. Chou, N.Y. Iwata, A. Kimura, The effects of Cr and Al concentrations on the oxidation behavior of oxide dispersion strengthened ferritic alloys, Corros. Sci. 76 (2013) 310–316. [12] X.L. Guo, K. Chen, W.H. Gao, Z. Zhao, L.F. Zhang, Corrosion behavior of aluminaforming and oxide dispersion strengthened austenitic 316 stainless steel in supercritical water, Corros. Sci. 138 (2018) 297–306. [13] M.A. Montealegre, J.L. González-Carrasco, Influence of the yttria content on the oxidation behaviour of the intermetallic Fe40Al Alloy, Intermetallics 11 (2003) 169–175.
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