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Oxidation behavior of FeCrAl coated Zry-4 under high temperature steam environment
Corrosion Science 149 (2019) 45–53
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Oxidation behavior of FeCrAl coated Zry-4 under high temperature steam environment ⁎
Xiaochun Han, Yu Wang, Shuming Peng , Haibin Zhang
T
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Innovation Research Team for Advanced Ceramics, Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang, 621900, China
A R T I C LE I N FO
A B S T R A C T
Keywords: FeCrAl coatings High temperature steam oxidation Zircaloy4 ATF
FeCrAl alloys have exhibited excellent oxidation resistance in high temperature steam environment. However, over the past few years, thin film FeCrAl on zircaloy substrate can only tolerate up to 1000 °C steam because of the fast diffusion between zirconium and iron. In this study, we introduce Mo as a buffer layer and significantly reduced the inter-diffusion of zirconium and iron. The 7-micron thick FeCrAl coatings have shown excellent oxidation resistance up to 1200 °C, which is the highest temperature of thin FeCrAl coatings reported in literature. A systematic study of the oxidation behavior of FeCrAl coatings at 1000, 1100, 1200 °C is presented in this paper.
1. Introduction After the severe nuclear accident of Fukyshima Dai-ichi in Japan in 2011, reactor safety issues with respect to fuel performance have again drawn great attention in the nuclear world. As a result, accident tolerant fuel (ATF) has been promoted to improve the current fuel performance in normal operating, design-basis (DB) and beyond-designbasis (BDB) accident scenarios [1]. For the improvement of the zircaloy cladding, which aims to reduce the oxidation rate of the current cladding by at least 100 times (X100 goal) at temperatures above 1200 °C [2], various candidate materials have been studied extensively, e.g., SiC [3], Cr coatings [4], FeCrAl alloys and coatings [5,6], MAX phase [7] and Mo based claddings [8]. Among all the candidate materials reported, SiC exhibits the best high temperature oxidation resistance. However, the performance of SiC under normal operating conditions has raised much concern [2,9]. Another structural alloy, FeCrAl alloy, which has been widely used as oxidation resistant materials in high temperature oxidizing environment, also shows excellent oxidation resistance in high temperature steam. However, due to the precipitation of Cr-rich? ?’-phase, FeCrAl claddings can lead to potential embrittlement problems under irradiation [2]. Replacement of zircaloy with other materials, such as FeCrAl alloy or SiC composite, is still under development which would be used in reactor cores in the future. In this condition, oxidation resistant coatings are selected as the short-term solution of ATF. Therefore, oxidation resistant coatings, such as Cr, FeCrAl, SiC, and Ti2AlC coatings, etc., have been examined extensively. Up to now, Cr coating has exhibited
⁎
the best performance under various environment and attracted much attention. Compared with chromium, FeCrAl alloys, which is aluminaforming, can tolerate steam oxidation at higher temperatures, and experimental results have shown that alumina-forming iron alloys are superior to the chromia-forming iron alloys [10,11]. In addition, autoclave test has demonstrated the excellent performance of FeCrAl alloys in normal operating conditions of light water reactor (LWR) [5]. However, to the best of our knowledge, published results in literature have shown that thin FeCrAl coatings (< 30 μm) can only tolerate 700 °C and 1000 °C in steam environment [6,12–14]. The inter-diffusion of Fe and Zr deteriorates the protective alumina at higher temperatures in steam because of the formation of a Fe-Zr eutectic phase which lead to the failure of the FeCrAl coatings [6]. Wang et al. has innovatively applied ZrO2 as a barrier layer by plasma electrolytic oxidation method, and successfully prevent the diffusion of Zr and Fe at 1000 °C [12]. However, at higher temperatures, the bi-layer coating is not protective. It has been reported that CrAl alloys [15] exhibit excellent behavior under high temperature steam environment at 1200 °C, but CrAl coatings [16] failed at even lower temperatures. The only reported FeCrAl coating which is protective under 1200 °C steam is a thick Mo-FeCrAl bi-layer coating produced by plasma spraying [9]. The thickness of the coating is about 200 μm and additional cold rolling technique was applied after film deposition. Previous studies have suggested that the coating thickness should be minimized below 30 μm due to neutronic penalty [17,18]. Thus, coatings thinner than 30 μm is preferred in reactor cores. It can be seen that great efforts have been put to fabricate coatings
Corresponding authors. E-mail addresses: [email protected] (S. Peng), [email protected] (H. Zhang).
https://doi.org/10.1016/j.corsci.2019.01.004 Received 3 November 2018; Received in revised form 4 January 2019; Accepted 5 January 2019 Available online 06 January 2019 0010-938X/ © 2019 Published by Elsevier Ltd.
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Fig. 1. Normalized 2θ-ω scans of the coatings. (a) as-grown, (b) after 1000 °C steam oxidation, (c) after 1100 °C steam oxidation, and (d) after 1200 °C steam oxidation, (e) 3D plot to show the major peak intensity evolution as a function of temperature (the phases are distinguished by the shape of the symbols, and temperature is denoted by colors). Each scan is normalized to the total area of the spectrum and the intensity scale is graduated linearly.
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Fig. 2. Plan-view of the sample surfaces before and after high temperature steam oxidation. (a) as-grown, (b) after 1000 °C steam oxidation, (c) after 1100 °C steam oxidation, (d) after 1200 °C steam oxidation, and (e) (f) higher magnification of the 1200 °C oxidized surface shown on (d). The corresponding elemental concentrations of EDS scans are given in Table 1.
2. Experimental
which could exhibit better performance than Cr coatings in the past several years. FeCrAl coatings are potentially possible but the highest temperature it can tolerate is 1000 °C with film thickness under 30 μm. In this study, we continue to apply the bi-layer structure to improve the performance of FeCrAl coatings under even higher temperatures. The total thickness of the coatings is limited to be less than 30 μm and the coating is produced by magnetron sputtering method without further treatment. The experimental procedures used to fabricate FeCrAl coatings and the methods for high temperature steam oxidation are presented in the next section. The micro-analytical techniques to characterize the response of the coatings before and after steam exposure are presented in Section 3. Effectiveness of the coatings, potential issues and further improvements are discussed in Section 4. Finally, summary statements are made in the last section.
1mm thick zircaloy-4 sheets (Zry-4) were supplied by the State Nuclear BAO TI Zirconium Industry Company, China. The Zry-4 sheets were cold rolled and stress relieved with surface orientation of ND. The weight percent of the major alloying elements are 1.28% Sn, 0.21% Fe, 0.12% Cr, 0.33% Fe + Cr, 0.12% O, 0.0094% Si, and Zr as the balance. Thin Zry-4 sheets were cut into dimension of 10 × 10 x 1 mm by Electrical Discharge Machining (EDM). One side of the samples were grinded with SiC grinding papers down to 2000 grit (1.5˜2.0 μm). Mo coatings were first deposited on the Zry-4 substrate in a magnetron sputtering chamber including three cathodes. A Mo target (99.99 wt.% purity) was installed on one cathode, which was driven by a DC power supply at 200 W for 3 h. A FeCrAl target (71 wt.% Fe, 21 wt.% Cr, 5 wt. % Al, 3 wt% Mo) installed on another cathode, was driven by a DC power supply at 200 W for 3 h. The substrate holder was heated to 300 °C with halogen lamps and negatively biased to 70 V. The 47
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cross section were characterized before and after oxidation for better understanding of the oxidation behavior.
Table 1 Atomic concentration of the major elements at the coating surface before and after high temperature steam oxidation at tested temperatures. C(at.%) Element
As Grown
1000 °C
1100 °C
1200 °C
Al O Fe Cr Mo
11.1 – 66.0 20.8 2.1
27.4 47.0 17.7 6.8 1.1
35.2 50.0 9.2 4.2 1.5
31.0 59.7 2.7 5.7 0.7
3. Results Specular 2θ-ω scans from XRD measurements have shown that the coating is mainly composed of α-FeCr phase (space group Im-3 m, No. 229), which is denoted by the three major peaks in Fig. 1(a). One minor peak at 34.8° is identified to be α-Al9Cr4 phase (space group I-43 m, No. 217). After oxidation in steam at 1000 °C, the major phase remains stable, and other phases, i.e., α-Al2O3 phase (space group R-3c, No. 167) and χ-Mo5Cr6Fe18 phase (space group I-43 m, No. 217) emerge. At 1100 °C, however, the α-FeCr phase diminishes significantly. On the other hand, the α-Al2O3 and χ-Mo5Cr6Fe18 grow to be the major phases on the surface of the coating. As temperature increasing to 1200 °C, the α-FeCr phase disappears which is replaced by the Fe0.54Mo0.73. The intensity of α-Al2O3 keeps increasing and new oxide of Fe1.89Mo4.11O7 (space group Imma, No. 74) emerges. For convenience, the evolution of the major peak intensities is represented as a 3D plot, as shown in Fig. 1(e). Surface morphologies of the coated samples before and after high temperature steam oxidation are shown in Fig. 2. Typical accelerating voltage used for SEM is 15 kV and the secondary electrons are collected for imaging. The escape depth of the secondary electrons is within 10 nm. The as-grown one does not exhibit specific features but the surface is uniform. EDS point scan indicates that it is composed of FeCrAlMo alloy with composition consistent with that of the sputtering target, as shown in Table 1. After oxidation at 1000 °C, the surface features change to evenly distributed platelets because of the transition from metallic phase to aluminum oxide, as illustrated in Table 1. The surface feature changes to evenly distributed granular structures at
negatively biased substrate could induce more positive ion-bombardment which would improve the coating-substrate adhesion properties. The base pressure before deposition was 8.31 × 10−4 Pa, and the pressure with flowing 10 sccm Ar was 0.50 ± 0.03 Pa during sputtering deposition. High temperature steam oxidation tests were conducted in a thermogravimetric analyzer (TGA, SETARAM SETSYS, France) equipment. The coated samples were exposed to a continuous flow of 40 sccm water vapor (62.5% RH at 50 °C) and 10 sccm Ar gas. Temperatures were ramped up from 40 °C to designated temperatures at a rate of 50 °C/ min. The temperature was then kept for 60 min, finally reduced to 60 °C after the oxidation test. High temperature steam was supplied to the oxidation chamber during the entire heating procedure. X-ray diffraction (XRD) measurements were performed using a Philips X’Pert diffractometer with a Cu-K?? source (λ = 1.540598 Å). Surface morphologies were characterized by a ZEISS high resolution field emission scanning electron microscope (SEM). Elemental compositions were measured by energy-dispersive X-ray spectrometry (EDS) which was equipped within the SEM system. Both the plan-view and
Fig. 3. (a) Cross-section view of the SEM image of the as-grown sample; (b) The corresponding EDS scan along the white line in (a); (c) EDS spectrum obtained on the surface of the as-grown sample (Fig. 2a); (d) zoomed in plot near the surface in Fig. 3(b). 48
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Fig. 4. (a) Cross-section view of the SEM image of the sample oxidized at 1000 °C for 60 min; (b) The corresponding EDS scan along the white line in (a); (c) EDS spectrum obtained on the surface of sample oxidized at 1000 °C (Fig. 2b); (d) zoomed in plot near the surface in Fig. 4(b).
becomes thicker (4.0 μm). The diffusion between Zr and Mo is apparent but not significant. The intensities of Fe and Cr peaks on the sample surface continue to decrease, as shown in Fig. 5(c), which is consistent with the EDS line scan near the surface (Fig. 5d). At 1200 °C, the diffusion between the Mo layer and the Zry-4 substrate is more apparent, as shown in Fig. 6. The surface is uniformly covered with a dense 1.5 μm thick Al2O3 layer. Underneath the alumina layer, the coating exhibits some depleted zone which forms separated voids, as shown in Fig. 6(a). No zirconium oxide is found at the surface which means that the Zry-4 substrate is well protected by the bi-layer Mo-FeCrAl coating under 1200 °C steam oxidation. Again, the intensity of the Al peak is much higher than that of the Fe and Cr peaks (Fig. 5c), which agrees with the EDS line scan near the sample surface (Fig. 5d). Cross-section view of the un-coated side of the Zry-4 sample after oxidation is shown in Fig. 7. The oxidation rate on the un-coated side is fast and the substrate is oxidized to ZrO2 with the thickness of 57.4, 112.3, and 200.0 μm at temperatures of 1000, 1100, and 1200 °C, respectively. Conventionally, weight gain is used to determine the oxidation rate. However, in the current study, the Zry-4 substrates are oneside coated and drilled with holes for hanging samples in the oxidation chamber. And the edges are partially covered with coatings which makes the determination of the weight gain more complicated. Therefore, we use the thickness of the ZrO2 oxide layer, which is confirmed by EDS and XRD analysis, to determine the oxidation rate. Following the approach of Sawarn et al. [19], the growth rate of the oxide layer (ZrO2) is expressed by an equation of the formξ= kp t0.5, whereξis the thickness of the oxide layer, t is oxidation time, and kp is the rate constant. Using the thickness of the oxide layer at different temperatures measured above, the rate constants determined in the current study are kp = 0.96, 1.87, and 3.33 μm/s0.5 at temperatures of 1000, 1100, and 1200 °C, respectively. The corresponding rate
1100 °C. EDS scan shows that these granules are mostly aluminum oxide. As temperature increases to 1200 °C, the entire surface becomes rough with wave like structure (with peaks and valleys). EDS scan indicates that the surface is mainly composed of aluminum and oxygen with concentrations close to the stoichiometric Al2O3, as shown in Table 1. The cross-section of the as-grown coating on Zry-4 substrate is shown in Fig. 3(a). The thickness of the Mo layer is 10.6 μm, and the FeCrAl layer is 6.6 μm. EDS line scan was performed along the crosssectional area and the depth profile is given in Fig. 3(b), which indicates the film is uniform and well-adhered to the Zry-4 substrate. Due to the current data processing software we use, Y axis can only be plotted as the counts per second. But relative concentration variation across the coatings can be revealed. EDS spectrum on the surface of the coating is given in Fig. 3(c) to show the elemental information. And a zoom-in plot near the open surface side is also given in Fig. 3(d) to show the concentration variation of the minor elements. Cross-section view of the coating after high temperature steam oxidation at 1000 °C is shown in Fig. 4(a). It can be seen that the surface is uniformly covered with a thin layer of Al2O3 which is about 0.7 μm. EDS line scan indicates Al and O peaks emerge on the surface of the coating. Mo and Fe diffuse significantly which forms an inter-metallic layer with thickness of 2.4 μm. However, there is no apparent diffusion between Zry-4 substrate and Mo layer. In Fig. 4(c), the intensity of the Al peak increases and that of the Fe, Cr peaks decreases as compared to Fig. 3(c). In the zoomed in plot near the surface, as shown in Fig. 4(d), Fe and Cr is still within the alumina layer, but the intensity is low. Cross-section view after oxidation at 1100 °C and the corresponding depth profile are presented in Fig. 5(a) and (b), respectively. The Al2O3 layer becomes thicker (greater than 1.5 μm) and non-uniform. Mo diffuses to the entire FeClAl coating and the inter-metallic layer of MoCrFe 49
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Fig. 5. (a) Cross-section view of the SEM image of the sample oxidized at 1100 °C for 60 min; (b) The corresponding EDS scan along the white line in (a); (c) EDS spectrum obtained on the surface of sample oxidized at 1100 °C (Fig. 2c); (d) zoomed in plot near the surface in Fig. 5(b).
constants reported by Sawarn et al. are 0.83, 1.93, and 3.19 μm/s0.5. Following the same approach, we have calculated the rate constants from the oxide thickness reported in literature, which are 1.11 μm/s0.5 at 1000 °C (McHugh et al. [20]), 1.71 μm/s0.5 at 1100 °C (Brachet et al. [21]), 3.18 μm/s0.5 at 1200 °C (Brachet et al. [21]), 2.24 μm/s0.5 at 1200 °C (Park, Kim, et al. [4,22]). Overall, the rate constants determined in the current study are consistent with the reported values except the one measured by Park and Kim, which are lower than expected.
as shown in the zoomed in plot in Fig. 4(d). Just beneath the alumina layer, the film is mainly composed of Fe and Cr. The XRD results show that the coating is mainly composed of α-Al2O3, α-FeCr, and χMo5Cr6Fe18 phases, as shown in Fig. 1(b). Because the penetration depth of 8 keV X-ray (Cu-K??) in Al2O3 is about 3.5 μm at 40° (90% attenuation), which is larger than the thickness of the alumina layer (0.7 μm). Therefore, phases containing Fe, Cr and Mo can be detected by X-ray. For EDS point analysis on the surface, the major elements are O, Al, Fe and Cr, as shown in Table 1. Since the analysis depth of 15 keV electron is 1.5 μm in the alumina layer (0.7 μm), elemental information just underneath the alumina layer can also be detected by EDS. Therefore, concentrations of Fe and Cr are high in the EDS point analysis. As temperature increases to 1100 °C, the Al2O3 layer on the surface of the coating becomes thicker (> 1.5 μm) and Mo diffuses to the entire FeCrAl coating, as shown in Fig. 5. As a result, reflections from both of the Al2O3 and Mo5Cr6Fe18 phases are strong, as indicated in Fig. 1(c). EDS point analysis indicates the surface is mainly composed of Al2O3 phase. Since at 1100 °C, the thickness of the Al2O3 layer is greater than the analysis depth of 15 keV electrons, the elemental information is mostly within the alumina layer. In addition, the alumina layer is nonuniform compared to that at 1000 °C, which means the effect of fast diffusion path, e.g., grain boundary, is apparent at 1100 °C. And the aluminum element also segregates to the Mo-FeCrAl interface, as indicated by the aluminum peak at 7 μm in Fig. 5(b). At 1200 °C, most of the aluminum diffuses to the surface and forms a dense layer of Al2O3. The inter-diffusion between Mo and FeCrAl reaches an equilibrium state and forms a new layer of FeCrMo, as shown in Fig. 6(b). X-ray reflections indicate that the major phase is composed of Fe0.54Mo0.73, which is consistent with the EDS line scan. The Cr element probably forms solid solution in the Fe site, which
4. Discussion Mo and FeCrAl bi-layer coatings deposited on Zry-4 substrate have successfully prevented the high temperature steam oxidation up to 1200 °C, which is the highest temperature for thin FeCrAl coatings reported in literature. As temperature increasing from 1000 °C to 1200 °C, more Al2O3 grow on the surface of the FeCrAl coating, which is indicated by the increasing intensity of alumina reflection from the XRD results, as shown in Fig. 1(e). No chromium oxide is found, which means it is the Al2O3 film that effectively prevented the inward-diffusion of oxygen, thus protected the underlying substrate. The diffusion rate between Zr and Mo is slow, as shown in Fig. 4(b) with a sharp interface between Zr and Mo. And the thermal expansion coefficients of the two materials are close (5.7 × 10−6/K and 5.0 × 10−6/K). These two factors make Mo as an ideal buffer layer for preventing diffusion and with adequate film-substrate adhesion. The diffusion between Mo and FeCrAl is relatively fast which forms an intermetallic layer between the Mo and FeCrAl coatings. During high temperature oxidation at 1000 °C, aluminum diffuses to the surface and forms a protective alumina layer. The EDS line analysis indicates that small amount of Fe and Cr are within the alumina layer, 50
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Fig. 6. (a) Cross-section view of the SEM image of the sample oxidized at 1200 °C for 60 min; (b) The corresponding EDS scan along the white line in (a); (c) EDS spectrum obtained on the surface of sample oxidized at 1200 °C (Fig. 2d); (d) zoomed in plot near the surface in Fig. 6(b).
Fig. 7. Cross-section view of the un-coated Zry-4 substrate after oxidation at tested temperatures. (a) after 1000 °C steam oxidation; (b) after 1100 °C steam oxidation; (c) after 1200 °C steam oxidation.
interesting phenomena have been found: first, the thickness of the intermetallic layer between the FeCrAl coating and the Mo coating increases as temperature increases, and the interface between FeCrAl and Mo moves towards the free surface; second, the fraction of voids in the FeCrAl coating increases considerably as temperature increases, especially at 1200 °C. These phenomena can be well explained based on the Kirkendall effect: the diffusion coefficient of FeCr into Mo is faster than that of Mo into FeCr. Therefore, as shown in Fig. 8(II), flux of FeCr is larger than the flux of Mo, which results in a net flux towards the Mo coating. Consequently, inter-metallic layer of FeCrMo forms and the original interface between the FeCrAl and Mo moves to the free surface side. In the meantime, flux of vacancies is in the opposite direction of JFeCr, which accumulate at the interface and form small voids, as shown in Fig. 8(III). The Al element diffuses out and forms alumina layer on the free surface, and leaves vacancy sites migrate to the sinks (voids,
cannot be distinguished by X-ray. The diffusion between Zr and Mo becomes apparent but not significant, which enhance the adhesion force without deteriorating the protection of the Mo-FeCrAl coating. Because of the severe diffusion and formation of new phases, some voids emerge underneath the Al2O3 layer, as shown in Fig. 6(a). This phenomenon can be well explained by the Kirkendall effect, which will be discussed in detail in the next paragraph. However, unlike the diffusion of Zr element into the coating which is detrimental to the protectiveness of the Al2O3 layer, the formation of the FeCrMo phase seems to stabilize the outer Al2O3 layer since addition of Mo is known to improve the density of alumina coatings [23] and stabilize alloys at elevated temperatures [24]. The existence of separated voids underneath the Al2O3 layer can also explain the wavy structure on the surface morphology, as shown in Fig. 2(d). After examine the cross-section morphology in Figs. 3–6, two 51
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Fig. 8. Schematic representations of the voids and inter-metallic layer formation during high temperature oxidation: (I) the coatings in the as-grown state; (II) fluxes across the interfaces at high temperatures; (III) voids and inter-metallic layer formation as a result of diffusion at high temperature; (IV) cross sectional morphology after oxidation at higher temperatures.
studied in the future.
interface, etc.). At higher temperatures, diffusion becomes faster and the flux is larger, which results in a thicker inter-metallic layer and larger voids formed, as shown in Fig. 8(IV). From Figs. 3–6, the thickness of the inter-metallic layer is 2.4, 4.0 and 4.7 μm at temperatures of 1000, 1100, 1200 °C, respectively. As a result, the original interface between FeCrAl and Mo coatings moves toward the free surface side. The size and number density of the voids also increase as temperature increases, which agrees well with the model described above. The Kirkendall effect also occurs between the Zr and Mo interface, however, the diffusion rate is much lower, thus, this effect is not apparent as that of the FeCrAl coating. For engineering applications, the Kirkendall effect should be suppressed to prevent void formation, which is detrimental to the mechanical properties.
Data availability The raw data required to reproduce these findings are available to download from [DOI: https://doi.org/10.17632/mccxtfmdjn.1]. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grants No. 91326102 and 51532009), and the Science and Technology Development Foundation of China Academy of Engineering Physics (Grant No. 2013A0301012). Haibin Zhang is grateful to the foundation by the Recruitment Program of Global Youth Experts and the Youth Hundred Talents Project of Sichuan Province. Assistance of Canhui Xu (magnetron sputtering and thermalgravimetric measurements), Lili Luo (SEM/EDS) and Xingquan Zhang (SEM/EDS) are all gratefully acknowledged.
5. Conclusion FeCrAl coatings with Mo buffer layer has been applied on Zry-4 substrate to prevent high temperature steam oxidation up to 1200 °C. A systematic study of the surface morphology, elemental composition and phase evolution have been conducted at temperatures of 1000, 1100 and 1200 °C. The total thickness of the FeCrAl-Mo bi-layer coating is less than 30 μm and it successfully protected the underlying Zry-4 substrate which is not oxidized at all tested temperatures. A gradual evolution of surface morphologies as oxidation temperature increases has been found. Strong diffusion between Mo and FeCrAl layer occurs and finally forms a new layer composed of FeCrMo at 1200 °C. The formation of the FeCrMo layer, the interface migration, and substantial amount of voids formed at high temperatures are modeled based on the Kirkendall effect. Improvement of the composition and structure of the coatings, and the oxidation behavior at higher temperatures will be
References [1] S.J. Zinkle, K.A. Terrani, J.C. Gehin, L.J. Ott, L.L. Snead, Accident tolerant fuels for LWRs: a perspective, J. Nucl. Mater. 448 (1) (2014) 374–379 2014/05/01/. [2] B.A. Pint, K.A. Terrani, Y. Yamamoto, L.L. Snead, Material selection for accident tolerant fuel cladding, Metall. Mater. Trans. E 2 (3) (2015) 190–196 2015/09/01. [3] J.-Y. Park, I.-H. Kim, Y.-I. Jung, H.-G. Kim, D.-J. Park, W.-J. Kim, Long-term corrosion behavior of CVD SiC in 360°C water and 400°C steam, J. Nucl. Mater. 443 (1) (2013) 603–607 2013/11/01/. [4] J.-H. Park, H.-G. Kim, J.-y. Park, Y.-I. Jung, D.-J. Park, Y.-H. Koo, High temperature steam-oxidation behavior of arc ion plated Cr coatings for accident tolerant fuel claddings, Surf. Coat. Technol. 280 (2015) 256–259 2015/10/25/. [5] D.J. Park, H.G. Kim, J.Y. Park, Y.I. Jung, J.H. Park, Y.H. Koo, A study of the oxidation of FeCrAl alloy in pressurized water and high-temperature steam
52
Corrosion Science 149 (2019) 45–53
X. Han et al.
environment, Corros. Sci. 94 (2015) 459–465 2015/05/01/. [6] W. Zhong, P.A. Mouche, X. Han, B.J. Heuser, K.K. Mandapaka, G.S. Was, Performance of iron–chromium–aluminum alloy surface coatings on Zircaloy 2 under high-temperature steam and normal BWR operating conditions, J. Nucl. Mater. 470 (2016) 327–338 2016/03/01/. [7] B.R. Maier, B.L. Garcia-Diaz, B. Hauch, L.C. Olson, R.L. Sindelar, K. Sridharan, Cold spray deposition of Ti2AlC coatings for improved nuclear fuel cladding, J. Nucl. Mater. 466 (2015) 712–717 2015/11/01/. [8] B. Cheng, Y.-J. Kim, P. Chou, Improving accident tolerance of nuclear fuel with coated Mo-alloy cladding, Nucl. Eng. Technol. 48 (1) (2016) 16–25 2016/02/01/. [9] D.J. Park, H.G. Kim, Y.I. Jung, J.H. Park, J.H. Yang, Y.H. Koo, Behavior of an improved Zr fuel cladding with oxidation resistant coating under loss-of-coolant accident conditions, J. Nucl. Mater. 482 (2016) 75–82 2016/12/15/. [10] K.A. Terrani, C.M. Parish, D. Shin, B.A. Pint, Protection of zirconium by aluminaand chromia-forming iron alloys under high-temperature steam exposure, J. Nucl. Mater. 438 (1) (2013) 64–71 2013/07/01/. [11] C. Tang, M. Stueber, H.J. Seifert, M. Steinbrueck, Protective coatings on zirconiumbased alloys as accident-tolerant fuel (ATF) claddings, Corros. Rev. 35 (3) (2017). [12] Y. Wang, et al., Oxidation resistance improvement of Zr-4 alloy in 1000 °C steam environment using ZrO2/FeCrAl bilayer coating, Surf. Coat. Technol. 349 (2018) 807–815 2018/09/15/. [13] W. Zhong, P.A. Mouche, B.J. Heuser, Response of Cr and Cr-Al coatings on Zircaloy2 to high temperature steam, J. Nucl. Mater. 498 (2018) 137–148 2018/01/01/. [14] D. Park, P.A. Mouche, W. Zhong, K.K. Mandapaka, G.S. Was, B.J. Heuser, TEM/ STEM study of Zircaloy-2 with protective FeAl(Cr) layers under simulated BWR environment and high-temperature steam exposure, J. Nucl. Mater. 502 (2018) 95–105 2018/04/15/. [15] H. G. Kim, "Chromium-aluminum binary alloy having excellent corrosion resistance
[16] [17] [18]
[19]
[20]
[21]
[22]
[23]
[24]
53
and mehtod of manufacturing thereof," United States Patent US 20160108507A1, 2016. J.-M. Kim, T.-H. Ha, I.-H. Kim, H.-G. Kim, Microstructure and Oxidation Behavior of CrAl Laser-Coated Zircaloy-4 Alloy, Metals 7 (2) (2017) 59. I. Younker, M. Fratoni, Neutronic evaluation of coating and cladding materials for accident tolerant fuels, Prog. Nucl. Energy 88 (2016) 10–18 2016/04/01/. X. Wu, T. Kozlowski, J.D. Hales, Neutronics and fuel performance evaluation of accident tolerant FeCrAl cladding under normal operation conditions, Ann. Nucl. Energy 85 (2015) 763–775 2015/11/01/. T.K. Sawarn, S. Banerjee, A. Samanta, B.N. Rath, S. Kumar, Study of oxide and α-Zr (O) growth kinetics from high temperature steam oxidation of Zircaloy-4 cladding, J. Nucl. Mater. 467 (2015) 820–831 2015/12/01/. K.M. Mchugh, J.E. Garnier, S. Rashkeev, M.V. Glazoff, G.W. Griffith, S.M. BraggSitton, High temperature steam corrosion of cladding for nuclear applications: experimental, Office of Scientific & Technical Information Technical Reports (2013) 149–160. J.-C. Brachet, et al., ON-GOING STUDIES AT CEA ON CHROMIUM COATED ZIRCONIUM BASED NUCLEAR FUEL CLADDINGS FOR ENHANCED ACCIDENT TOLERANT LWRS FUEL, TopFuel, Switzerland, (2015). H.-G. Kim, I.-H. Kim, Y.-I. Jung, D.-J. Park, J.-Y. Park, Y.-H. Koo, Adhesion property and high-temperature oxidation behavior of Cr-coated Zircaloy-4 cladding tube prepared by 3D laser coating, J. Nucl. Mater. 465 (2015) 531–539 2015/10/01/. J. Cheng, et al., Improvement of thermal stability in the solar selective absorbing Mo–Al2O3 coating, Sol. Energy Mater. Sol. Cells 109 (2013) 204–208 2013/02/ 01/. K. Liu, H. Ma, X.G. Chen, Enhanced elevated-temperature properties via Mo addition in Al-Mn-Mg 3004 alloy, J. Alloys. Compd. 694 (2017) 354–365 2017/02/15/.
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Oxidation behavior of FeCrAl coated Zry-4 under high temperature steam environment ⁎
Xiaochun Han, Yu Wang, Shuming Peng , Haibin Zhang
T
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Innovation Research Team for Advanced Ceramics, Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang, 621900, China
A R T I C LE I N FO
A B S T R A C T
Keywords: FeCrAl coatings High temperature steam oxidation Zircaloy4 ATF
FeCrAl alloys have exhibited excellent oxidation resistance in high temperature steam environment. However, over the past few years, thin film FeCrAl on zircaloy substrate can only tolerate up to 1000 °C steam because of the fast diffusion between zirconium and iron. In this study, we introduce Mo as a buffer layer and significantly reduced the inter-diffusion of zirconium and iron. The 7-micron thick FeCrAl coatings have shown excellent oxidation resistance up to 1200 °C, which is the highest temperature of thin FeCrAl coatings reported in literature. A systematic study of the oxidation behavior of FeCrAl coatings at 1000, 1100, 1200 °C is presented in this paper.
1. Introduction After the severe nuclear accident of Fukyshima Dai-ichi in Japan in 2011, reactor safety issues with respect to fuel performance have again drawn great attention in the nuclear world. As a result, accident tolerant fuel (ATF) has been promoted to improve the current fuel performance in normal operating, design-basis (DB) and beyond-designbasis (BDB) accident scenarios [1]. For the improvement of the zircaloy cladding, which aims to reduce the oxidation rate of the current cladding by at least 100 times (X100 goal) at temperatures above 1200 °C [2], various candidate materials have been studied extensively, e.g., SiC [3], Cr coatings [4], FeCrAl alloys and coatings [5,6], MAX phase [7] and Mo based claddings [8]. Among all the candidate materials reported, SiC exhibits the best high temperature oxidation resistance. However, the performance of SiC under normal operating conditions has raised much concern [2,9]. Another structural alloy, FeCrAl alloy, which has been widely used as oxidation resistant materials in high temperature oxidizing environment, also shows excellent oxidation resistance in high temperature steam. However, due to the precipitation of Cr-rich? ?’-phase, FeCrAl claddings can lead to potential embrittlement problems under irradiation [2]. Replacement of zircaloy with other materials, such as FeCrAl alloy or SiC composite, is still under development which would be used in reactor cores in the future. In this condition, oxidation resistant coatings are selected as the short-term solution of ATF. Therefore, oxidation resistant coatings, such as Cr, FeCrAl, SiC, and Ti2AlC coatings, etc., have been examined extensively. Up to now, Cr coating has exhibited
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the best performance under various environment and attracted much attention. Compared with chromium, FeCrAl alloys, which is aluminaforming, can tolerate steam oxidation at higher temperatures, and experimental results have shown that alumina-forming iron alloys are superior to the chromia-forming iron alloys [10,11]. In addition, autoclave test has demonstrated the excellent performance of FeCrAl alloys in normal operating conditions of light water reactor (LWR) [5]. However, to the best of our knowledge, published results in literature have shown that thin FeCrAl coatings (< 30 μm) can only tolerate 700 °C and 1000 °C in steam environment [6,12–14]. The inter-diffusion of Fe and Zr deteriorates the protective alumina at higher temperatures in steam because of the formation of a Fe-Zr eutectic phase which lead to the failure of the FeCrAl coatings [6]. Wang et al. has innovatively applied ZrO2 as a barrier layer by plasma electrolytic oxidation method, and successfully prevent the diffusion of Zr and Fe at 1000 °C [12]. However, at higher temperatures, the bi-layer coating is not protective. It has been reported that CrAl alloys [15] exhibit excellent behavior under high temperature steam environment at 1200 °C, but CrAl coatings [16] failed at even lower temperatures. The only reported FeCrAl coating which is protective under 1200 °C steam is a thick Mo-FeCrAl bi-layer coating produced by plasma spraying [9]. The thickness of the coating is about 200 μm and additional cold rolling technique was applied after film deposition. Previous studies have suggested that the coating thickness should be minimized below 30 μm due to neutronic penalty [17,18]. Thus, coatings thinner than 30 μm is preferred in reactor cores. It can be seen that great efforts have been put to fabricate coatings
Corresponding authors. E-mail addresses: [email protected] (S. Peng), [email protected] (H. Zhang).
https://doi.org/10.1016/j.corsci.2019.01.004 Received 3 November 2018; Received in revised form 4 January 2019; Accepted 5 January 2019 Available online 06 January 2019 0010-938X/ © 2019 Published by Elsevier Ltd.
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Fig. 1. Normalized 2θ-ω scans of the coatings. (a) as-grown, (b) after 1000 °C steam oxidation, (c) after 1100 °C steam oxidation, and (d) after 1200 °C steam oxidation, (e) 3D plot to show the major peak intensity evolution as a function of temperature (the phases are distinguished by the shape of the symbols, and temperature is denoted by colors). Each scan is normalized to the total area of the spectrum and the intensity scale is graduated linearly.
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Fig. 2. Plan-view of the sample surfaces before and after high temperature steam oxidation. (a) as-grown, (b) after 1000 °C steam oxidation, (c) after 1100 °C steam oxidation, (d) after 1200 °C steam oxidation, and (e) (f) higher magnification of the 1200 °C oxidized surface shown on (d). The corresponding elemental concentrations of EDS scans are given in Table 1.
2. Experimental
which could exhibit better performance than Cr coatings in the past several years. FeCrAl coatings are potentially possible but the highest temperature it can tolerate is 1000 °C with film thickness under 30 μm. In this study, we continue to apply the bi-layer structure to improve the performance of FeCrAl coatings under even higher temperatures. The total thickness of the coatings is limited to be less than 30 μm and the coating is produced by magnetron sputtering method without further treatment. The experimental procedures used to fabricate FeCrAl coatings and the methods for high temperature steam oxidation are presented in the next section. The micro-analytical techniques to characterize the response of the coatings before and after steam exposure are presented in Section 3. Effectiveness of the coatings, potential issues and further improvements are discussed in Section 4. Finally, summary statements are made in the last section.
1mm thick zircaloy-4 sheets (Zry-4) were supplied by the State Nuclear BAO TI Zirconium Industry Company, China. The Zry-4 sheets were cold rolled and stress relieved with surface orientation of ND. The weight percent of the major alloying elements are 1.28% Sn, 0.21% Fe, 0.12% Cr, 0.33% Fe + Cr, 0.12% O, 0.0094% Si, and Zr as the balance. Thin Zry-4 sheets were cut into dimension of 10 × 10 x 1 mm by Electrical Discharge Machining (EDM). One side of the samples were grinded with SiC grinding papers down to 2000 grit (1.5˜2.0 μm). Mo coatings were first deposited on the Zry-4 substrate in a magnetron sputtering chamber including three cathodes. A Mo target (99.99 wt.% purity) was installed on one cathode, which was driven by a DC power supply at 200 W for 3 h. A FeCrAl target (71 wt.% Fe, 21 wt.% Cr, 5 wt. % Al, 3 wt% Mo) installed on another cathode, was driven by a DC power supply at 200 W for 3 h. The substrate holder was heated to 300 °C with halogen lamps and negatively biased to 70 V. The 47
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cross section were characterized before and after oxidation for better understanding of the oxidation behavior.
Table 1 Atomic concentration of the major elements at the coating surface before and after high temperature steam oxidation at tested temperatures. C(at.%) Element
As Grown
1000 °C
1100 °C
1200 °C
Al O Fe Cr Mo
11.1 – 66.0 20.8 2.1
27.4 47.0 17.7 6.8 1.1
35.2 50.0 9.2 4.2 1.5
31.0 59.7 2.7 5.7 0.7
3. Results Specular 2θ-ω scans from XRD measurements have shown that the coating is mainly composed of α-FeCr phase (space group Im-3 m, No. 229), which is denoted by the three major peaks in Fig. 1(a). One minor peak at 34.8° is identified to be α-Al9Cr4 phase (space group I-43 m, No. 217). After oxidation in steam at 1000 °C, the major phase remains stable, and other phases, i.e., α-Al2O3 phase (space group R-3c, No. 167) and χ-Mo5Cr6Fe18 phase (space group I-43 m, No. 217) emerge. At 1100 °C, however, the α-FeCr phase diminishes significantly. On the other hand, the α-Al2O3 and χ-Mo5Cr6Fe18 grow to be the major phases on the surface of the coating. As temperature increasing to 1200 °C, the α-FeCr phase disappears which is replaced by the Fe0.54Mo0.73. The intensity of α-Al2O3 keeps increasing and new oxide of Fe1.89Mo4.11O7 (space group Imma, No. 74) emerges. For convenience, the evolution of the major peak intensities is represented as a 3D plot, as shown in Fig. 1(e). Surface morphologies of the coated samples before and after high temperature steam oxidation are shown in Fig. 2. Typical accelerating voltage used for SEM is 15 kV and the secondary electrons are collected for imaging. The escape depth of the secondary electrons is within 10 nm. The as-grown one does not exhibit specific features but the surface is uniform. EDS point scan indicates that it is composed of FeCrAlMo alloy with composition consistent with that of the sputtering target, as shown in Table 1. After oxidation at 1000 °C, the surface features change to evenly distributed platelets because of the transition from metallic phase to aluminum oxide, as illustrated in Table 1. The surface feature changes to evenly distributed granular structures at
negatively biased substrate could induce more positive ion-bombardment which would improve the coating-substrate adhesion properties. The base pressure before deposition was 8.31 × 10−4 Pa, and the pressure with flowing 10 sccm Ar was 0.50 ± 0.03 Pa during sputtering deposition. High temperature steam oxidation tests were conducted in a thermogravimetric analyzer (TGA, SETARAM SETSYS, France) equipment. The coated samples were exposed to a continuous flow of 40 sccm water vapor (62.5% RH at 50 °C) and 10 sccm Ar gas. Temperatures were ramped up from 40 °C to designated temperatures at a rate of 50 °C/ min. The temperature was then kept for 60 min, finally reduced to 60 °C after the oxidation test. High temperature steam was supplied to the oxidation chamber during the entire heating procedure. X-ray diffraction (XRD) measurements were performed using a Philips X’Pert diffractometer with a Cu-K?? source (λ = 1.540598 Å). Surface morphologies were characterized by a ZEISS high resolution field emission scanning electron microscope (SEM). Elemental compositions were measured by energy-dispersive X-ray spectrometry (EDS) which was equipped within the SEM system. Both the plan-view and
Fig. 3. (a) Cross-section view of the SEM image of the as-grown sample; (b) The corresponding EDS scan along the white line in (a); (c) EDS spectrum obtained on the surface of the as-grown sample (Fig. 2a); (d) zoomed in plot near the surface in Fig. 3(b). 48
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Fig. 4. (a) Cross-section view of the SEM image of the sample oxidized at 1000 °C for 60 min; (b) The corresponding EDS scan along the white line in (a); (c) EDS spectrum obtained on the surface of sample oxidized at 1000 °C (Fig. 2b); (d) zoomed in plot near the surface in Fig. 4(b).
becomes thicker (4.0 μm). The diffusion between Zr and Mo is apparent but not significant. The intensities of Fe and Cr peaks on the sample surface continue to decrease, as shown in Fig. 5(c), which is consistent with the EDS line scan near the surface (Fig. 5d). At 1200 °C, the diffusion between the Mo layer and the Zry-4 substrate is more apparent, as shown in Fig. 6. The surface is uniformly covered with a dense 1.5 μm thick Al2O3 layer. Underneath the alumina layer, the coating exhibits some depleted zone which forms separated voids, as shown in Fig. 6(a). No zirconium oxide is found at the surface which means that the Zry-4 substrate is well protected by the bi-layer Mo-FeCrAl coating under 1200 °C steam oxidation. Again, the intensity of the Al peak is much higher than that of the Fe and Cr peaks (Fig. 5c), which agrees with the EDS line scan near the sample surface (Fig. 5d). Cross-section view of the un-coated side of the Zry-4 sample after oxidation is shown in Fig. 7. The oxidation rate on the un-coated side is fast and the substrate is oxidized to ZrO2 with the thickness of 57.4, 112.3, and 200.0 μm at temperatures of 1000, 1100, and 1200 °C, respectively. Conventionally, weight gain is used to determine the oxidation rate. However, in the current study, the Zry-4 substrates are oneside coated and drilled with holes for hanging samples in the oxidation chamber. And the edges are partially covered with coatings which makes the determination of the weight gain more complicated. Therefore, we use the thickness of the ZrO2 oxide layer, which is confirmed by EDS and XRD analysis, to determine the oxidation rate. Following the approach of Sawarn et al. [19], the growth rate of the oxide layer (ZrO2) is expressed by an equation of the formξ= kp t0.5, whereξis the thickness of the oxide layer, t is oxidation time, and kp is the rate constant. Using the thickness of the oxide layer at different temperatures measured above, the rate constants determined in the current study are kp = 0.96, 1.87, and 3.33 μm/s0.5 at temperatures of 1000, 1100, and 1200 °C, respectively. The corresponding rate
1100 °C. EDS scan shows that these granules are mostly aluminum oxide. As temperature increases to 1200 °C, the entire surface becomes rough with wave like structure (with peaks and valleys). EDS scan indicates that the surface is mainly composed of aluminum and oxygen with concentrations close to the stoichiometric Al2O3, as shown in Table 1. The cross-section of the as-grown coating on Zry-4 substrate is shown in Fig. 3(a). The thickness of the Mo layer is 10.6 μm, and the FeCrAl layer is 6.6 μm. EDS line scan was performed along the crosssectional area and the depth profile is given in Fig. 3(b), which indicates the film is uniform and well-adhered to the Zry-4 substrate. Due to the current data processing software we use, Y axis can only be plotted as the counts per second. But relative concentration variation across the coatings can be revealed. EDS spectrum on the surface of the coating is given in Fig. 3(c) to show the elemental information. And a zoom-in plot near the open surface side is also given in Fig. 3(d) to show the concentration variation of the minor elements. Cross-section view of the coating after high temperature steam oxidation at 1000 °C is shown in Fig. 4(a). It can be seen that the surface is uniformly covered with a thin layer of Al2O3 which is about 0.7 μm. EDS line scan indicates Al and O peaks emerge on the surface of the coating. Mo and Fe diffuse significantly which forms an inter-metallic layer with thickness of 2.4 μm. However, there is no apparent diffusion between Zry-4 substrate and Mo layer. In Fig. 4(c), the intensity of the Al peak increases and that of the Fe, Cr peaks decreases as compared to Fig. 3(c). In the zoomed in plot near the surface, as shown in Fig. 4(d), Fe and Cr is still within the alumina layer, but the intensity is low. Cross-section view after oxidation at 1100 °C and the corresponding depth profile are presented in Fig. 5(a) and (b), respectively. The Al2O3 layer becomes thicker (greater than 1.5 μm) and non-uniform. Mo diffuses to the entire FeClAl coating and the inter-metallic layer of MoCrFe 49
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Fig. 5. (a) Cross-section view of the SEM image of the sample oxidized at 1100 °C for 60 min; (b) The corresponding EDS scan along the white line in (a); (c) EDS spectrum obtained on the surface of sample oxidized at 1100 °C (Fig. 2c); (d) zoomed in plot near the surface in Fig. 5(b).
constants reported by Sawarn et al. are 0.83, 1.93, and 3.19 μm/s0.5. Following the same approach, we have calculated the rate constants from the oxide thickness reported in literature, which are 1.11 μm/s0.5 at 1000 °C (McHugh et al. [20]), 1.71 μm/s0.5 at 1100 °C (Brachet et al. [21]), 3.18 μm/s0.5 at 1200 °C (Brachet et al. [21]), 2.24 μm/s0.5 at 1200 °C (Park, Kim, et al. [4,22]). Overall, the rate constants determined in the current study are consistent with the reported values except the one measured by Park and Kim, which are lower than expected.
as shown in the zoomed in plot in Fig. 4(d). Just beneath the alumina layer, the film is mainly composed of Fe and Cr. The XRD results show that the coating is mainly composed of α-Al2O3, α-FeCr, and χMo5Cr6Fe18 phases, as shown in Fig. 1(b). Because the penetration depth of 8 keV X-ray (Cu-K??) in Al2O3 is about 3.5 μm at 40° (90% attenuation), which is larger than the thickness of the alumina layer (0.7 μm). Therefore, phases containing Fe, Cr and Mo can be detected by X-ray. For EDS point analysis on the surface, the major elements are O, Al, Fe and Cr, as shown in Table 1. Since the analysis depth of 15 keV electron is 1.5 μm in the alumina layer (0.7 μm), elemental information just underneath the alumina layer can also be detected by EDS. Therefore, concentrations of Fe and Cr are high in the EDS point analysis. As temperature increases to 1100 °C, the Al2O3 layer on the surface of the coating becomes thicker (> 1.5 μm) and Mo diffuses to the entire FeCrAl coating, as shown in Fig. 5. As a result, reflections from both of the Al2O3 and Mo5Cr6Fe18 phases are strong, as indicated in Fig. 1(c). EDS point analysis indicates the surface is mainly composed of Al2O3 phase. Since at 1100 °C, the thickness of the Al2O3 layer is greater than the analysis depth of 15 keV electrons, the elemental information is mostly within the alumina layer. In addition, the alumina layer is nonuniform compared to that at 1000 °C, which means the effect of fast diffusion path, e.g., grain boundary, is apparent at 1100 °C. And the aluminum element also segregates to the Mo-FeCrAl interface, as indicated by the aluminum peak at 7 μm in Fig. 5(b). At 1200 °C, most of the aluminum diffuses to the surface and forms a dense layer of Al2O3. The inter-diffusion between Mo and FeCrAl reaches an equilibrium state and forms a new layer of FeCrMo, as shown in Fig. 6(b). X-ray reflections indicate that the major phase is composed of Fe0.54Mo0.73, which is consistent with the EDS line scan. The Cr element probably forms solid solution in the Fe site, which
4. Discussion Mo and FeCrAl bi-layer coatings deposited on Zry-4 substrate have successfully prevented the high temperature steam oxidation up to 1200 °C, which is the highest temperature for thin FeCrAl coatings reported in literature. As temperature increasing from 1000 °C to 1200 °C, more Al2O3 grow on the surface of the FeCrAl coating, which is indicated by the increasing intensity of alumina reflection from the XRD results, as shown in Fig. 1(e). No chromium oxide is found, which means it is the Al2O3 film that effectively prevented the inward-diffusion of oxygen, thus protected the underlying substrate. The diffusion rate between Zr and Mo is slow, as shown in Fig. 4(b) with a sharp interface between Zr and Mo. And the thermal expansion coefficients of the two materials are close (5.7 × 10−6/K and 5.0 × 10−6/K). These two factors make Mo as an ideal buffer layer for preventing diffusion and with adequate film-substrate adhesion. The diffusion between Mo and FeCrAl is relatively fast which forms an intermetallic layer between the Mo and FeCrAl coatings. During high temperature oxidation at 1000 °C, aluminum diffuses to the surface and forms a protective alumina layer. The EDS line analysis indicates that small amount of Fe and Cr are within the alumina layer, 50
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Fig. 6. (a) Cross-section view of the SEM image of the sample oxidized at 1200 °C for 60 min; (b) The corresponding EDS scan along the white line in (a); (c) EDS spectrum obtained on the surface of sample oxidized at 1200 °C (Fig. 2d); (d) zoomed in plot near the surface in Fig. 6(b).
Fig. 7. Cross-section view of the un-coated Zry-4 substrate after oxidation at tested temperatures. (a) after 1000 °C steam oxidation; (b) after 1100 °C steam oxidation; (c) after 1200 °C steam oxidation.
interesting phenomena have been found: first, the thickness of the intermetallic layer between the FeCrAl coating and the Mo coating increases as temperature increases, and the interface between FeCrAl and Mo moves towards the free surface; second, the fraction of voids in the FeCrAl coating increases considerably as temperature increases, especially at 1200 °C. These phenomena can be well explained based on the Kirkendall effect: the diffusion coefficient of FeCr into Mo is faster than that of Mo into FeCr. Therefore, as shown in Fig. 8(II), flux of FeCr is larger than the flux of Mo, which results in a net flux towards the Mo coating. Consequently, inter-metallic layer of FeCrMo forms and the original interface between the FeCrAl and Mo moves to the free surface side. In the meantime, flux of vacancies is in the opposite direction of JFeCr, which accumulate at the interface and form small voids, as shown in Fig. 8(III). The Al element diffuses out and forms alumina layer on the free surface, and leaves vacancy sites migrate to the sinks (voids,
cannot be distinguished by X-ray. The diffusion between Zr and Mo becomes apparent but not significant, which enhance the adhesion force without deteriorating the protection of the Mo-FeCrAl coating. Because of the severe diffusion and formation of new phases, some voids emerge underneath the Al2O3 layer, as shown in Fig. 6(a). This phenomenon can be well explained by the Kirkendall effect, which will be discussed in detail in the next paragraph. However, unlike the diffusion of Zr element into the coating which is detrimental to the protectiveness of the Al2O3 layer, the formation of the FeCrMo phase seems to stabilize the outer Al2O3 layer since addition of Mo is known to improve the density of alumina coatings [23] and stabilize alloys at elevated temperatures [24]. The existence of separated voids underneath the Al2O3 layer can also explain the wavy structure on the surface morphology, as shown in Fig. 2(d). After examine the cross-section morphology in Figs. 3–6, two 51
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Fig. 8. Schematic representations of the voids and inter-metallic layer formation during high temperature oxidation: (I) the coatings in the as-grown state; (II) fluxes across the interfaces at high temperatures; (III) voids and inter-metallic layer formation as a result of diffusion at high temperature; (IV) cross sectional morphology after oxidation at higher temperatures.
studied in the future.
interface, etc.). At higher temperatures, diffusion becomes faster and the flux is larger, which results in a thicker inter-metallic layer and larger voids formed, as shown in Fig. 8(IV). From Figs. 3–6, the thickness of the inter-metallic layer is 2.4, 4.0 and 4.7 μm at temperatures of 1000, 1100, 1200 °C, respectively. As a result, the original interface between FeCrAl and Mo coatings moves toward the free surface side. The size and number density of the voids also increase as temperature increases, which agrees well with the model described above. The Kirkendall effect also occurs between the Zr and Mo interface, however, the diffusion rate is much lower, thus, this effect is not apparent as that of the FeCrAl coating. For engineering applications, the Kirkendall effect should be suppressed to prevent void formation, which is detrimental to the mechanical properties.
Data availability The raw data required to reproduce these findings are available to download from [DOI: https://doi.org/10.17632/mccxtfmdjn.1]. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grants No. 91326102 and 51532009), and the Science and Technology Development Foundation of China Academy of Engineering Physics (Grant No. 2013A0301012). Haibin Zhang is grateful to the foundation by the Recruitment Program of Global Youth Experts and the Youth Hundred Talents Project of Sichuan Province. Assistance of Canhui Xu (magnetron sputtering and thermalgravimetric measurements), Lili Luo (SEM/EDS) and Xingquan Zhang (SEM/EDS) are all gratefully acknowledged.
5. Conclusion FeCrAl coatings with Mo buffer layer has been applied on Zry-4 substrate to prevent high temperature steam oxidation up to 1200 °C. A systematic study of the surface morphology, elemental composition and phase evolution have been conducted at temperatures of 1000, 1100 and 1200 °C. The total thickness of the FeCrAl-Mo bi-layer coating is less than 30 μm and it successfully protected the underlying Zry-4 substrate which is not oxidized at all tested temperatures. A gradual evolution of surface morphologies as oxidation temperature increases has been found. Strong diffusion between Mo and FeCrAl layer occurs and finally forms a new layer composed of FeCrMo at 1200 °C. The formation of the FeCrMo layer, the interface migration, and substantial amount of voids formed at high temperatures are modeled based on the Kirkendall effect. Improvement of the composition and structure of the coatings, and the oxidation behavior at higher temperatures will be
References [1] S.J. Zinkle, K.A. Terrani, J.C. Gehin, L.J. Ott, L.L. Snead, Accident tolerant fuels for LWRs: a perspective, J. Nucl. Mater. 448 (1) (2014) 374–379 2014/05/01/. [2] B.A. Pint, K.A. Terrani, Y. Yamamoto, L.L. Snead, Material selection for accident tolerant fuel cladding, Metall. Mater. Trans. E 2 (3) (2015) 190–196 2015/09/01. [3] J.-Y. Park, I.-H. Kim, Y.-I. Jung, H.-G. Kim, D.-J. Park, W.-J. Kim, Long-term corrosion behavior of CVD SiC in 360°C water and 400°C steam, J. Nucl. Mater. 443 (1) (2013) 603–607 2013/11/01/. [4] J.-H. Park, H.-G. Kim, J.-y. Park, Y.-I. Jung, D.-J. Park, Y.-H. Koo, High temperature steam-oxidation behavior of arc ion plated Cr coatings for accident tolerant fuel claddings, Surf. Coat. Technol. 280 (2015) 256–259 2015/10/25/. [5] D.J. Park, H.G. Kim, J.Y. Park, Y.I. Jung, J.H. Park, Y.H. Koo, A study of the oxidation of FeCrAl alloy in pressurized water and high-temperature steam
52
Corrosion Science 149 (2019) 45–53
X. Han et al.
environment, Corros. Sci. 94 (2015) 459–465 2015/05/01/. [6] W. Zhong, P.A. Mouche, X. Han, B.J. Heuser, K.K. Mandapaka, G.S. Was, Performance of iron–chromium–aluminum alloy surface coatings on Zircaloy 2 under high-temperature steam and normal BWR operating conditions, J. Nucl. Mater. 470 (2016) 327–338 2016/03/01/. [7] B.R. Maier, B.L. Garcia-Diaz, B. Hauch, L.C. Olson, R.L. Sindelar, K. Sridharan, Cold spray deposition of Ti2AlC coatings for improved nuclear fuel cladding, J. Nucl. Mater. 466 (2015) 712–717 2015/11/01/. [8] B. Cheng, Y.-J. Kim, P. Chou, Improving accident tolerance of nuclear fuel with coated Mo-alloy cladding, Nucl. Eng. Technol. 48 (1) (2016) 16–25 2016/02/01/. [9] D.J. Park, H.G. Kim, Y.I. Jung, J.H. Park, J.H. Yang, Y.H. Koo, Behavior of an improved Zr fuel cladding with oxidation resistant coating under loss-of-coolant accident conditions, J. Nucl. Mater. 482 (2016) 75–82 2016/12/15/. [10] K.A. Terrani, C.M. Parish, D. Shin, B.A. Pint, Protection of zirconium by aluminaand chromia-forming iron alloys under high-temperature steam exposure, J. Nucl. Mater. 438 (1) (2013) 64–71 2013/07/01/. [11] C. Tang, M. Stueber, H.J. Seifert, M. Steinbrueck, Protective coatings on zirconiumbased alloys as accident-tolerant fuel (ATF) claddings, Corros. Rev. 35 (3) (2017). [12] Y. Wang, et al., Oxidation resistance improvement of Zr-4 alloy in 1000 °C steam environment using ZrO2/FeCrAl bilayer coating, Surf. Coat. Technol. 349 (2018) 807–815 2018/09/15/. [13] W. Zhong, P.A. Mouche, B.J. Heuser, Response of Cr and Cr-Al coatings on Zircaloy2 to high temperature steam, J. Nucl. Mater. 498 (2018) 137–148 2018/01/01/. [14] D. Park, P.A. Mouche, W. Zhong, K.K. Mandapaka, G.S. Was, B.J. Heuser, TEM/ STEM study of Zircaloy-2 with protective FeAl(Cr) layers under simulated BWR environment and high-temperature steam exposure, J. Nucl. Mater. 502 (2018) 95–105 2018/04/15/. [15] H. G. Kim, "Chromium-aluminum binary alloy having excellent corrosion resistance
[16] [17] [18]
[19]
[20]
[21]
[22]
[23]
[24]
53
and mehtod of manufacturing thereof," United States Patent US 20160108507A1, 2016. J.-M. Kim, T.-H. Ha, I.-H. Kim, H.-G. Kim, Microstructure and Oxidation Behavior of CrAl Laser-Coated Zircaloy-4 Alloy, Metals 7 (2) (2017) 59. I. Younker, M. Fratoni, Neutronic evaluation of coating and cladding materials for accident tolerant fuels, Prog. Nucl. Energy 88 (2016) 10–18 2016/04/01/. X. Wu, T. Kozlowski, J.D. Hales, Neutronics and fuel performance evaluation of accident tolerant FeCrAl cladding under normal operation conditions, Ann. Nucl. Energy 85 (2015) 763–775 2015/11/01/. T.K. Sawarn, S. Banerjee, A. Samanta, B.N. Rath, S. Kumar, Study of oxide and α-Zr (O) growth kinetics from high temperature steam oxidation of Zircaloy-4 cladding, J. Nucl. Mater. 467 (2015) 820–831 2015/12/01/. K.M. Mchugh, J.E. Garnier, S. Rashkeev, M.V. Glazoff, G.W. Griffith, S.M. BraggSitton, High temperature steam corrosion of cladding for nuclear applications: experimental, Office of Scientific & Technical Information Technical Reports (2013) 149–160. J.-C. Brachet, et al., ON-GOING STUDIES AT CEA ON CHROMIUM COATED ZIRCONIUM BASED NUCLEAR FUEL CLADDINGS FOR ENHANCED ACCIDENT TOLERANT LWRS FUEL, TopFuel, Switzerland, (2015). H.-G. Kim, I.-H. Kim, Y.-I. Jung, D.-J. Park, J.-Y. Park, Y.-H. Koo, Adhesion property and high-temperature oxidation behavior of Cr-coated Zircaloy-4 cladding tube prepared by 3D laser coating, J. Nucl. Mater. 465 (2015) 531–539 2015/10/01/. J. Cheng, et al., Improvement of thermal stability in the solar selective absorbing Mo–Al2O3 coating, Sol. Energy Mater. Sol. Cells 109 (2013) 204–208 2013/02/ 01/. K. Liu, H. Ma, X.G. Chen, Enhanced elevated-temperature properties via Mo addition in Al-Mn-Mg 3004 alloy, J. Alloys. Compd. 694 (2017) 354–365 2017/02/15/.