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https://doi.org/10.1007/s11837-018-2951-8 Ó 2018 The Minerals, Metals & Materials Society

NUCLEAR MATERIALS, OXIDATION, SUPERCRITICAL CO2, AND CORROSION BEHAVIOR

Transmission Electron Microscopy (TEM) Study of the Oxide Layers Formed on Fe-12Cr-4Al Ferritic Alloy in an Oxygenated Pb-Bi Environment at 800°C M.P. POPOVIC,1,4 Y. YANG,1 A.M. BOLIND,1 V.B. OZDOL,2 D.L. OLMSTED,3 M. ASTA,3 and P. HOSEMANN1 1.—Department of Nuclear Engineering, University of California, Berkeley, CA 94720, USA. 2.—National Center for Electron Microscopy, The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA, USA. 3.—Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA. 4.—e-mail: [email protected]

Liquid lead–bismuth eutectic (LBE) can serve as a heat transfer fluid for advanced nuclear applications as well as concentrated solar power but poses corrosion challenges for the structural materials at elevated temperatures. Oxide passivation of the surfaces of these materials during exposure to liquid LBE can inhibit such material degradation. In this study, transmission electron microscopy of oxides formed on Fe-Cr-Al alloy during exposure to lowoxygenated LBE at 800°C has been performed. A complex structure of the oxide film has been revealed, consisting of a homogeneous inner layer of mostly Al2O3 and a heterogeneous outer layer.

INTRODUCTION Efficient and nonreactive heat transfer fluids are essential in many high-power-density applications, such as nuclear power systems (fast neutron reactors), neutron spallation sources, and concentrated solar power (CSP) systems. Lead–bismuth eutectic (LBE) is a heavy liquid metal (HLM) coolant characterized by many favorable qualities.1–6 However, a significant drawback of the use of LBE is that it causes issues on the sutural materials deployed, including liquid metal embrittlement,7 liquid metal enhanced creep,8 and liquid metal corrosion.4,5,9–11 The main mechanism of the liquid metal corrosion is the solubility of the structural material’s alloying elements in the liquid LBE.3,4 The common strategy to mitigate the corrosion issues is founded on active oxygen control coupled with a good understanding of passive layer formation on the structural material.12–14 A significant number of studies on the formation of oxide layers in LBE have been performed on a variety of different steels, leading to corrosion models.11,15–17 These studies utilized scanning electron microscopy (SEM) with energy-dispersive x-ray spectroscopy (EDS),18–20 scanning probe microscopy,21–23 Raman

spectroscopy,24,25 x-ray photoelectron spectroscopy (XPS),19 x-ray diffraction (XRD),21,25 and nanoindentation.22,24 Numerous studies addressed the degradation of austenitic (Fe-Cr-Ni) steels, which cannot be used at temperatures above 550°C because of the high solubility of Ni in LBE. The improvement of corrosion resistance in LBE by alloying the steels with strong oxide-forming elements has been found to be a viable route to enhancing their corrosion performance.26,27 Aluminum as an alloying element is especially beneficial in mitigating corrosion and excessive oxidation.26,27 Studies have shown that a thin but adherent Al2O3 oxide layer provides an effective diffusion barrier to protect the steel from attack by heavy liquid metal. Aluminum concentrations of 4– 7 wt.% in addition to 12.5–17 wt.% Cr are found to be sufficient to form an alumina layer20,28 on ferritic Fe-Cr-Al alloys at temperatures of 400–600°C. The higher the Cr content present, the lower the Al content that is acceptable. Lower Al concentrations lead to the formation of Fe-based spinel-type oxides: a single layer of (Fe1 x yCrxAly)3O4 at 400–500°C (2–5 lm) and a double layer of Fe3O4 + (Fe1 x yCrxAly)3O4 at 600°C (up to 15 lm total).28 The alumina scales grow as transient polymorphs,

Popovic, Yang, Bolind, Ozdol, Olmsted, Asta, and Hosemann

but gradually transform into stable alumina aAl2O329 with a volumetric change of around 14%. Magnetite24,30 as well as Al2O330,31 can spall off the steel substrate. However, despite the significant amount of work performed on Fe-based materials exposed to LBE to this date, high-resolution microscopy studies investigating the detailed microstructure of the materials are limited4,32 but needed. In this work, a Fe-Cr-Al steel exposed to 800°C LBE was analyzed by transmission electron microscopy (TEM) to understand the structure of the oxide layers formed. The goal of this study is a better understanding of the mechanisms of the formation and growth of the oxide on the Fe-Cr-Al ferritic material in LBE at temperatures higher than reported in the literature previously EXPERIMENTAL A rectangular 20 9 40 9 1-mm specimen of Alkrothal 720 (ALK) was exposed to liquid lead– bismuth eutectic (LBE) at (800 ± 5)°C in static (non-flowing) conditions for (360 ± 1) h in an inhouse designed experimental setup maintaining a controlled oxygen atmosphere of  (5.0 ± 0.5) 9 10 6 wt.% oxygen in LBE (monitored by oxygen sensor), according to the test procedure described in previous work.25 Alkrothal 720 is a ferritic iron– chromium–aluminum BCC alloy by weight: 12–14% Cr, 4.0% Al (nominal), 0.7% Mn (max.), 0.7% Si (max.), 0.25% Ti (max.), and 0.08% C (max.) with the balance being Fe. The system was built with commercially available standard stainless-steel vacuum components and Swagelok tube fittings. The oxygen content was measured using commercially available oxygen sensors (purchased from

Australian Oxytrol Systems) typically used for measuring oxygen in exhaust gas and smoke stacks. Their measurement principle is based on the electromotive force (EMF) signal between a reference (Pt–air) on the inside of the sensor and the outside of the unknown liquid.25 The Pt wire on the sensor was replaced by W wire because of the high solubility of Pt in Pb-Bi liquid at 800°C. The output EMF signal from the sensor was recorded with a Keithley 181 Nanovoltmeter and together with the thermocouple signal was fed into an Omega DAQ board.25 Experimental parameters (EMF, temperature, and time) were recorded and the cover gas above the corrosion setup controlled by operating an automated valve (switching between Ar/3%O2 and Ar/5%H2 mixtures). After the corrosion test, the sample was extracted from the liquid LBE (before solidification at  200°C) while the test system was cooling down to room temperature, mounted in epoxy resin, and subsequently cut in cross-section in the epoxy. The cross-sectioned ALK sample was polished first down to 1200 grid by SiC paper followed by 1.0 lm and 0.3 lm Al2O3 suspension. A TEM sample was manufactured from the aforementioned sample cross-section using a focused ion beam (FIB) instrument (FEI Quanta 3D FEG dual beam FIB). The foil was cut perpendicular to the bulk-oxide interface plane to capture all the oxide layers across the steel/LBE interface, as depicted in the sketch (Fig. 1a). An STEM image of the as-manufactured sample can be seen in Fig. 1b. Final cleaning with a 50 pA, 10 keV electron beam was conducted on the sample to minimize damaging.33 A 200-kV Phillips CM200/FEG microscope equipped with energy-dispersive x-ray spectroscopy (EDS), a 300-kV JEOL 3010 microscope for

Fig. 1. (a) Position and orientation of the TEM foil. (b) STEM dark-field image of the foil (x–y plane), with the structures denoted.

Transmission Electron Microscopy (TEM) Study of the Oxide Layers Formed on Fe-12Cr-4Al Ferritic Alloy in an Oxygenated Pb-Bi Environment at 800°C

Fig. 2. (a) HAADF STEM images showing the inner layer, and the three sub-structures in the outer oxide layer. (b) HAADF image segment. (c and d) High-resolution EDS map scans of the area enclosed by the green rectangle in (b) (Color figure online).

obtaining electron diffraction patterns, and a FEI TitanX 60–300 kV microscope for high-resolution EDS map scan were used in this research. RESULTS The TEM analysis revealed the existence of the two morphologically distinguishable different structures visible in the dark-field STEM image (Fig. 1b). The oxide can be categorized into an inner area, next to the substrate bulk, and an outer area, toward the liquid phase (represented as epoxy). The inner layer (Layer 1) is a 1.0–1.5-lm-thick layer

that appears morphologically more homogeneous and darker in contrast, while the outer layer (Layer 2) appears more heterogeneous with a fine-grain structure and bright-contrasted islands. Most of the bright spots in the outer layer indicate the presence of higher-density phases, such as metallic iron (Fe) and chromium (Cr), in the oxide, while the brightest spots indicate metallic lead (Pb) and bismuth (Bi). The high-angle annular dark field (HAADF) STEM imaging mode shows the existence of three distinguishable sub-zones in the outer oxide layer (Fig. 2a and b). The sub-layer next to the inner layer, Sub-layer 1, retains Al2O3 as the dominant

Popovic, Yang, Bolind, Ozdol, Olmsted, Asta, and Hosemann

Fig. 3. (a) HAADF image of the inner oxide layer and steel bulk. The locations of EDS scans: a map scan (green square), two line scans, and a higher-resolution map scan of the section in the inner layer (red square) are shown. (b and d) Map reveals Al oxide as the by far most dominant oxide in the inner layer, with some Ti and TiO2 near the steel-oxide interface. (e) Al oxide, according to HRTEM, corresponds to the alumina (Al2O3) structure. (f) Clusters of Cr oxide and Ti oxide found in the Al-oxide matrix. (g and h) Line scans again reveal Al2O3 as the most dominant structure in the inner oxide, with some Ti and TiO2 near the steel-oxide interface (zero position on the EDS line scans corresponds to the red line’s end closer to the attributed number, on a) (Color figure online).

structure from the inner layer, with the 0.3–1.0-lmdiameter grains. Sub-layer 2 appears more heterogeneous, with smaller-size grain features of various morphologic appearances dispersed in the same matrix structure as in Sub-layer 1. In Sub-layer 3, the island-like formations from Sub-layer 2 are structured as continuous bands, intercepted with zones of pure matrix (as from the inner layer) and of structures very similar to those in Sub-layer 1. An EDS map of the overall oxide scale confirmed the dominant Al oxide and some Ti oxide in the inner layer and various elements in the outer layer (Fig. 2c and d). There is evidence of LBE penetration into the outer layer, but not into the inner one or into bulk. More detailed TEM analysis of the layers was performed at the locations indicated by the rectangles in Fig. 2a. First, Fig. 3 presents a segment of the inner oxide layer, which is considered the most important oxide layer because of its density and performance as a diffusion barrier. In Fig. 3a, one can clearly see the grain structure of this layer. The EDX map and line scans performed on different spots in the inner oxide layer (Fig. 3b–d and f–h) showed the presence of Al, O, and Ti (and rarely some traces of Cr), with a very small amount of Bi and Pb, from LBE deeper penetration. The appearance of darker and lighter contrasting islands might be related to the presence of greater or lesser trace amounts of Ti and Cr (Fig. 3f) immersed in Al-oxide matrix. Occasionally, some Ti oxide is detected near the steel-oxide interface, as small islands

surrounded by Al-rich oxide (Fig. 3g and h). Little occurrence of lead and iron, and almost no bismuth and chromium, is detected by EDS scanning in this area. High-resolution TEM (HR-TEM) of this area has been performed on several spots from the inner layer (one of them presented in Fig. 3e), with subsequent fast Fourier transformation (FFT) by using the software tool ‘‘Digital MicrographTM.’’ The ˚ corresponds measured lattice spacing of  4.79 A well to the hexagonal R3c lattice spacing of a˚ ),34,35 which indicated that the alumina (a = 4.76 A Al-rich oxide is Al2O3 in the inner oxide. An EDS map of the inner oxide layer near the interface with the steel (Fig. 4) shows that titanium concentrates at the interface. In some areas of the interface, the Ti is incorporated into the predominant aluminum oxide (Al2O3). Based on the thermodynamics at 800°C (Gibbs free energy of oxides formation, see Ellingham diagrams36,37) for the given concentration of oxygen dissolved in LBE and the Ti content in ALK, it may be assumed that there is a Ti oxide in some of the analyzed spots, but due to the size of the embedded particles its exact structure could not be determined here. In other areas of the interface, the Ti is in the form of elemental metal. Besides the lack of oxygen signal in the EDS map in these areas, the metallic form was confirmed by the TEM’s electron-diffractionimaging mode analyzing a spot in the inner oxide layer near the interface with the steel (Fig. 4a and b). An inverse-Fourier transform of the diffraction pattern (Fig. 4c) has indicated that the crystal

Transmission Electron Microscopy (TEM) Study of the Oxide Layers Formed on Fe-12Cr-4Al Ferritic Alloy in an Oxygenated Pb-Bi Environment at 800°C

Fig. 4. (a) TEM image of the interface between the steel bulk and inner oxide layer, and (b) the micro-diffraction pattern from the red spot in image (a) found to correspond to the [211] hcp crystal structure of elementary titanium (Ti). (c and d) EDS map on the green marked square from (a), revealing the alumina (Al2O3) as a predominant oxide with some TiO2 and titanium (Ti) precipitates in the steel-oxide interface zone (Color figure online).

structure at the spot is that of the [211] hcp structure of metallic Ti and not that of the fcc structure of metallic Al. DISCUSSION The TEM image in Fig. 1 reveals the multilayer oxide structure formed on ALK, a commercial FeCr-Al alloy, after exposure to oxygenated liquid LBE at 800°C. STEM and dark-field HAADF imaging identifies two distinctly different oxide layers, with the inner layer being more compact and homogeneous than the outer layer. The findings of the two oxide layers, with the inner consisting dominantly of alumina and the outer consisting of several sublayers containing Fe, Cr, Al, and O, are consistent with early speculation by one of the co-authors concerning Alkrothal Fe-Cr-Al alloy exposed to 800°C LBE.25 The analysis is focused on the inner oxide layer since it is the dense layer and therefore considered the actual protective layer for the steel. The inner oxide layer showed that it consists mostly of alumina (Al2O3), with some traces of Ti oxide (possibly TiO2) near the steel-oxide interface. It is

known that the formation of aluminia is thermodynamically more favored than magnetite formation (see the Ellingham diagram in Ref. 38). According to the oxide map for the ternary Fe-Cr-Al system at 800°C, based on experimental results obtained by Tomaszewich and Wallwork,31 we find that ALK at 800°C lies in the area of Al2O3 formation but near the phase boundary, making it possible to also form Fe oxides and Cr oxide. The finding of Ti and TiO2 in the inner oxide layer, near the alloy-oxide interface and surrounded by alumina (Al2O3), is in accordance with the theory, first reported by Young,39 that Ti may foster the formation of the a-Al2O3 layer on ferritic steels at higher temperatures. It has been shown that Ti4+ can accelerate the formation of a-Al2O3 and that concentrations as low as 0.1 at.% Ti in the alloy are enough to cause this effect.40 Previous work also suggests that a reduced Cr content and enhanced Al content provide the basis for the formation of a stable and protective Al-oxide layer.28,31 With 10–15 wt.% Cr in Fe-Cr-Al, aluminum content as low as 3 wt.% is sufficient to enable the alumina formation. It is

Popovic, Yang, Bolind, Ozdol, Olmsted, Asta, and Hosemann

hypothesized25,38 that the presence of an (Al, Cr)oxide scale formation reduces the activity and diffusion of the elements at the alloy/oxide interface and causes Al to form a dense, slow-growing, and protective Al2O3 phase at the interface. This mechanism was initially observed for oxidation in gaseous environments31 but can be expected for the static LBE environment as well with proper oxygen control. The finger-like morphologic structure of the inner Al2O3 oxide that penetrates  5 lm into the bulk (see Figs. 1b, 2b, and 3a) indicates the presence of the internal oxidation of aluminum, possibly along the grain boundaries (as faster diffusion paths for oxygen diffusion than the bulk41) at the same time as the formation of whole alumina of the inner layer (atop the bulk) occurs. CONCLUSION The structure of the oxide scale formed on an Alkrothal 720 (ALK), a ferritic Fe-Cr-Al alloy, that was exposed to LBE with 5 9 10 6 wt.% oxygen content at 800°C for 360 h was found to be a complex mixture of multiple oxide layers. The inner oxide layer is a dense and compact structure that is adherent to the bulk alloy substrate and consists mainly of a-Al2O3 with some Ti oxide and, more rarely, Cr oxide. Therefore, it is considered the protective oxide layer. No Fe oxides or evidence of Pb/Bi penetration was found. The Ti concentration at the alloy/oxide interface indicates the diffusion of titanium and its oxidation. The Ti oxide probably acts as a precursor or stabilizer for the formation of the a-Al2O3 inner layer. Based on the Cr content in ALK, internal oxidation of aluminum is possible, enabled by intergranular oxygen diffusion. ACKNOWLEDGEMENTS Funding for this research was provided by the US Department of Energy (DOE) SunShot program (Award No. DE-EE0005941). The authors thank the California Institute for Quantitative Biosciences (QB3) and Biomolecular Nanotechnology Center (BNC) at UC Berkeley for making the Quanta 3D FEG DualBeam SEM available. Work at NCEM as a part of Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under Contract No. DE-AC02-05CH11231.

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