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Oxide scales formed on Fe–Cr–Al-based model alloys exposed to oxygen containing molten lead
Journal of Nuclear Materials 437 (2013) 282–292
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Oxide scales formed on Fe–Cr–Al-based model alloys exposed to oxygen containing molten lead A. Weisenburger, A. Jianu ⇑, S. Doyle, M. Bruns, R. Fetzer, A. Heinzel, M. DelGiacco, W. An, G. Müller Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
a r t i c l e
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Article history: Received 17 December 2012 Accepted 18 February 2013 Available online 26 February 2013
a b s t r a c t Based on the state of the art oxide maps concerning oxidation behavior of Fe–Cr–Al model alloys at 800 and 1000 °C in oxygen atmosphere, ten compositions, belonging to this alloy system, were designed in order to tap the borders of the alumina stability domain, during their exposure to oxygen (10 6 wt.%) containing lead, at 400, 500 and 600 °C. Eight alloys, Fe-6Cr-6Al, Fe-8Cr-6Al, Fe-10Cr-5Al, Fe-14Cr-4Al, Fe-16Cr-4Al, Fe-6Cr-8Al, Fe-10Cr-7Al and Fe-12Cr-5Al, were found to be protected against corrosion in oxygen containing lead, either by a duplex layer (Fe3O4 + (Fe1 x yCrxAly)3O4) or by (Fe1 x yCrxAly)3O4, depending on the temperature at which they were exposed. Two alloys namely Fe-12Cr-7Al and Fe-16Cr-6Al were found to form transient aluminas, j-Al2O3 (at 400 and 500 °C) and h-Al2O3 (at 600 °C), as protective oxide scale against corrosion in oxygen containing lead. An oxide map illustrating the stability domain of alumina, grown on Fe–Cr–Al alloys when exposed to molten, oxygen containing lead, was drawn. The map includes also additional points, extracted from literature and corresponding to alumina forming alloys, when exposed to HLMs, which fit very well with our findings. Chromium and aluminium contents of 12.5–17 wt.% and 6–7.5 wt.%, respectively, are high enough to obtain thin, stable and protective alumina scales on Fe–Cr–Al-based alloys exposed to oxygen containing lead at 400, 500 and 600 °C. For the temperature range and exposure times used during the current evaluation, the growth rate of the alumina scale was low. No area with detached scale was observed and no trace of a-Al2O3 was detected. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The use of heavy liquid metals (HLMs), namely lead or lead-based alloys, in energy-related applications is currently under consideration because of their beneficial thermal and neutronic properties [1–3]. However, HLM compatibility with structural steels, in terms of corrosion and mechanical resistance, causes considerable concern [4–18]. The improvement of corrosion resistance by alloying the steels with strong oxide-forming elements (e.g. Al, Si) has been attempted [19–21]. Aluminium addition to the steel composition has shown to be beneficial for protecting steel surfaces in contact with lead alloys (containing small amounts of oxygen) against corrosion attack and severe oxidation, by forming an Al-rich oxide scale [5,20–22]. A thin, continuous, dense, stable, adherent Al2O3 layer provides an effective diffusion barrier, which protects the Al-containing steels from corrosion attack in HLM. The development of such a layer requires minimal Al levels, since selective ⇑ Corresponding author. Tel.: +49 721 608 28519; fax: +49 721 608 22256. E-mail address: [email protected] (A. Jianu). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.02.044
Al oxidation occurs [21,23–25]. Unfortunately, such minimal Al contents could negatively affect the mechanical properties of the steel. This is the reason why Al-containing coatings or steel surface alloying with Al (instead of steel bulk alloying) were proposed [5]. Fe–Cr–Al alloys are of large interest for practical applications at high-temperatures in reactive atmospheres [26–29]. Cr was introduced because it was found to reduce the Al content required to form a protective Al2O3 layer (third-element effect – TEE) [30,31]. Up to now, the alumina stability domain was defined for Fe–Cr–Al alloy systems (low Al content) after exposure in gas atmosphere at temperatures higher than 800 °C [27,32]. Few experimental data concerning the oxidation behaviour of Fe–Cr–Al alloy system at lower temperatures, during exposures in reactive atmosphere [33], are also available. The phenomena occurring during the Fe–Cr–Al alloys exposure to HLMs, containing very small amounts of dissolved oxygen, is scarcely documented [4,12,17,34,35]. No systematic investigation of the corrosion behaviour of the Fe–Cr–Al system, when exposed to HLM was performed, in order to find the minimum Al content for the formation of an alumina scale.
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The current paper presents the results of a systematic study concerning the corrosion behaviour of Fe–Cr–Al-based model alloys during their exposure to oxygen-containing (10 6 wt.%) lead in the temperature range 400–600 °C. In this vein a map of the alumina stability domain for this temperature range has been drawn in Fe–Cr–Al ternary diagrams and the type of alumina polymorph, formed at different temperatures, was determined. 2. Experimental Ten Fe–Cr–Al alloys were prepared as ingots by arc-melting in argon atmosphere starting from high purity elements. Their nominal compositions are: Fe-6Cr-6Al, Fe-8Cr-6Al, Fe-10Cr-5Al, Fe-12Cr-5Al, Fe-14Cr-4Al, Fe-16Cr-4Al, Fe-6Cr-8Al, Fe-10Cr-7Al, Fe-12Cr-7Al, Fe-16Cr-6Al (numbers in wt.%). The distribution of these compositions in the Fe–Cr–Al ternary diagram is shown in Fig. 1, together with two reference borderlines, corresponding to the oxide-maps drawn after oxidation experiments performed on iron-rich Fe–Cr–Al-based alloys in oxygen, at 800 °C [27] and 1000 °C [32]. The actual compositions of the alloys were determined by energy dispersive X-ray spectroscopy (EDX) and are shown in Table 1. The ingots were cut into discs with around 1.2 mm thickness. All specimens were mechanically grinded on successively finer abrasive papers down to 4200 grit. They were finally cleaned with water, acetone and ethanol in an ultrasonic bath and dried immediately before the experiments. After preparation, the specimens were exposed to stagnant oxygen-containing liquid lead, in the COSTA facility [5]. The lead was
Fig. 1. Ternary phase diagram with compositions of prepared alloys and the oxidemaps drawn at 800 °C [27] and 1000 °C [32].
Table 1 Actual composition of ternary Fe–Cr–Al alloys, analyzed with EDX. Alloy
Nominal composition
Cr (wt.%)
Al (wt.%)
Fe
P1 P2 P3 P4 P5 P6 P7 P8 P9 P10
Fe–6Cr–6Al Fe–8Cr–6Al Fe–10Cr–5Al Fe–14Cr–4Al Fe–16Cr–4Al Fe–6Cr–8Al Fe–10Cr–7Al Fe–12Cr–7Al Fe–16Cr–6Al Fe–12Cr–5Al
6.2 8.4 10.8 14.5 16.4 6.4 10.4 12.4 16.9 12.3
6.8 6.6 5.6 4.2 4.7 8.8 7.6 7.5 6.4 5.8
Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance
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placed in alumina crucibles, which were loaded into the quartz tubes of the furnaces and then conditioned at the selected oxygen content and temperatures. The oxygen content of the lead was 10 6 wt.% and kept constant. This was obtained by a continuous flow of Ar and Ar + H2 through a water bath, for a precisely controlled H2/H2O ratio in the gas atmosphere above the melted lead. Once the oxygen content and temperature were reached, the specimens were placed inside the conditioned lead. The experiments were performed at the following lead temperatures/exposure times: 400 °C/840 h, 500 °C/930 h and 600 °C/1830 h. After the extraction from molten lead, the specimens were cooled in air and then cleaned with a solution of ethanol, acetic acid and hydrogen peroxide (1:1:1) to remove the remaining adherent lead. The evaluation of the specimens was performed using the following characterization techniques: scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction using conventional and synchrotron radiation (XRD), and for some selected samples, X-ray photoelectron spectroscopy (XPS). SEM–EDX examination was performed with Philips XL40 equipped with a SAMx–EDX system. The phase composition of the majority of grown oxides was analysed using a Seyfert C3000 powder diffractometer (Cu Ka radiation) with h 2h conventional geometry. For a batch of selected samples the evaluation of the grown oxide was made with grazing incidence-X-ray diffraction (GI-XRD). In the latter case, the measurements were made using synchrotron radiation at the ANKA synchrotron [36], PDIFF beamline, with the wave length k = 0.117945 nm. Incidence angles between 1° and 3° were used. XPS measurements were performed on P9 samples using a KAlpha XPS spectrometer (ThermoFisher Scientific). Data acquisition and processing using the Thermo Avantage software is described elsewhere [37]. The samples were analyzed using a microfocused, monochromated Al Ka X-ray source (200 lm spot size). The KAlpha charge compensation system was employed during analysis, using electrons of 8 eV energy, and low-energy argon ions to prevent any localized charge build-up. The spectra were fitted with one or more Voigt profiles (BE uncertainty: ±0.2 eV). The analyzer transmission function, Scofield sensitivity factors [38] and effective attenuation lengths (EALs) for photoelectrons were applied for quantification. EALs were calculated using the standard TPP-2M formalism [39]. All spectra were referenced to the C1s peak of hydrocarbon at 285.0 eV binding energy, controlled by means of the well-known photoelectron peaks of metallic Au, Ag and Cu. XPS sputter depth profiling for evaluation of the composition of the grown oxides was performed using a 2 keV Ar+ ion beam at a raster size of 1 2 mm2 and a 30° angle of incidence. The depth of sputtering was measured at the end of the evaluation by profilometry (DektakXT Stylus Profilometer, Bruker AXS). 3. Results Prior to corrosion testing, the microstructure of the alloys was evaluated using light microscopy (LM). The original grain size of all alloys was in the range of 0.5–2 mm, so that a possible influence of grain boundary diffusion on the formation of oxide scales can be considered very low. Fig. 2 reveals the microstructure of the samples in the original state. The grains are preferentially oriented due to solidification conditions. 3.1. Exposure at 400 °C Fig. 3 shows typical surface views of the Fe–Cr–Al-based alloys exposed to oxygen-containing molten lead at 400 °C (after chemical removal of the adherent lead). No dissolution attacks were
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Fig. 2. Light microscope images of original microstructure of P1 (Fe-6Cr-6Al), P2 (Fe-8Cr-6Al), P3 (Fe-10Cr-5Al), P4 (Fe-14Cr-4Al), P5 (Fe-16Cr-4Al), P6 (Fe-6Cr-8Al), P7 (Fe-10Cr-7Al), P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al) and P10 (Fe-12Cr-5Al) samples. Large grains in the range of 0.5–2 mm and a slight texture characterized all samples.
Fig. 3. SEM micrographs of the oxide scale grown on the surface of Fe–Cr–Al alloys exposed to oxygen containing liquid lead for 840 h at 400 °C: P1 (Fe-6Cr-6Al), P2 (Fe-8Cr-6Al), P3 (Fe-10Cr-5Al), P4 (Fe-14Cr-4Al), P5 (Fe-16Cr-4Al), P6 (Fe-6Cr-8Al), P7 (Fe-10Cr-7Al), P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al) and P10 (Fe-12Cr-5Al).
Fig. 4. Cross sections of samples made of Fe–Cr–Al-based alloys exposed to oxygen containing liquid lead for 840 h at 400 °C: P1 (Fe-6Cr-6Al), P2 (Fe-8Cr-6Al), P3 (Fe-10Cr-5Al), P4 (Fe-14Cr-4Al), P5 (Fe-16Cr-4Al), P6 (Fe-6Cr-8Al), P7 (Fe-10Cr-7Al), P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al) and P10 (Fe-12Cr-5Al). P1–P7 and P10 are covered by duplex scale (Fe3O4 + (Fe1 x yCrxAly)3O4). P8 is covered by a mixture of duplex and thin Al-rich oxide layers, while a thin Al-rich scale is grown on P9 (SE: secondary electrons image; BSE: back-scattered electrons image).
observed. Based on the oxide morphologies formed on the surfaces, the samples were classified into two categories: samples with ‘‘rough-granular surfaces’’ and samples with generally ‘‘smooth surfaces’’. The first category comprises samples of P1–P7 and P10 alloys, while the second category contains P8 and P9 samples. The oxide scales formed on the first category exhibit grains protruded above the original surface and have a wrinkled appearance with porosities, as shown in Fig. 3. EDX microanalysis indicated that these grains are rather pure iron oxide, with the atomic ratio Fe:O = (40–45):(55–60), matching the stoichiometry of magnetite Fe3O4.
The samples with generally smooth surface are covered by a green-yellow oxide scale, seemingly transparent, showing also some spots with rough-granular morphology. The area fraction covered by granular scale varied from around 20%, in case of P8, to <2%, for P9 sample. The protrusions, which are, similarly to the first category, pure iron oxide, formed preferentially along grain boundaries and around some material defects (e.g. alumina inclusions observed in ‘‘as prepared’’ alloys). The cross sections shown in Fig. 4 correspond to the samples exposed at 400 °C. Samples having a rough-granular surface aspect (made out of P1–P7 and P10 alloys) formed a duplex oxide scale with thickness in the range of 2–4 lm. The outer layer is a pure
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Fig. 5. EDX line scan showing a duplex oxide scale formed on P4 (Fe-14Cr-4Al) sample (a) and EDX line scan of the very thin Al-rich oxide scale formed on the P9 (Fe-16Cr-6Al) alloy (b) during the exposure at 400 °C, for 840 h to molten lead containing 10 6 wt.% oxygen.
Fig. 6. XRD patterns of P7 (a) and P8 (b) after exposure in oxygen containing liquid lead for 840 h at 400 °C. P7 is covered by a duplex layer (Fe3O4 + Fe(Cr, Al)2O4), while on P8 the protective scale is j-Al2O3 alumina polymorph.
iron oxide with pores (some with trapped lead), while the inner layer is a Fe–Cr–Al-mixed oxide, as concluded from EDX line scan analysis. An example, corresponding to the P4 sample, is given in Fig. 5a. The measured oxygen concentration on the inner layer was around 55 at.%, indicating a (Fe1 x yCrxAly)3O4 spinel-type oxide. The samples with smooth surfaces (P8 and P9) formed an almost invisible oxide scale (Fig. 4). The thin scale, grown on both alloys, was an Al-rich oxide, as established by EDX line scan analysis. The pattern obtained on the P9 sample is shown in Fig. 5b, as an example. Few thick oxide nodules were also observed mainly on the P8 sample, confirming the findings during surface evaluation. These nodules consist of an external layer of almost pure iron oxide,
followed by a Fe–Cr–Al-based oxide, similar to the oxide scale grown on P1–P7 and P10 alloys. The phase composition of the oxidised samples showing a ‘‘rough-granular surface’’ (P1–P7 and P10) was examined by XRD (h 2h). The XRD patterns clearly evidence the presence of the spinel-type structure (space group Fd-3 m), corresponding to the oxide scale and identified as magnetite Fe3O4 (PDF-card no. 890688) and chromite Fe(Cr, Al)2O4 (PDF-card no. 34-0140), and also the presence of the cubic structure a-Fe(Cr) (PDF-card no. 340396), which corresponds to the substrate. Fig. 6a depicts, as an example, the XRD pattern of the P7 (Fe-10Cr-7Al) sample. The structure of the oxide grown on the samples showing a generally ‘‘smooth surface’’ was investigated by GI-XRD. Both P8 and P9 samples were covered by j-Al2O3 transient alumina
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Fig. 7. XPS sputter depth profile of P9 (Fe-16Cr-6Al) sample, exposed for 840 h to melted lead (with 10 6 wt.% oxygen) at 400 °C, obtained after 7500 s sputter times (a) and a detailed view of the same graph (b). The average thickness of Al2O3 scale, estimated from FWHM of the AlOx peak, is 35 nm.
(PDF card no. 26-31), as can be concluded from the example shown in Fig. 6b (P8: Fe-12Cr-7Al). GI-XRD patterns of P8 and P9 also contain peaks of magnetite, which, in accordance with SEM investigation results, is located at the grain boundaries or around surface defects, such as alumina inclusions (see Fig. 3, micrographs P8 and P9). Fig. 7 shows the XPS compositional sputter depth profile of the P9 sample. It was possible to evaluate the thickness of the Al2O3 layer grown on P9 (Fe-16Cr-6Al) sample by de-convoluting the Al 2p photoelectron emission signal into two peaks, corresponding to Al° (metallic, 72.4 eV) and AlOx (oxidised, 75.4 eV) [40], and by extracting the contribution of the Cr 3s line overlapping on Al 2p (Cr 3s: 75.0 eV). This approach is semi-quantitative due to lower energy resolution in the sputter depth profile mode. Based on such considerations, the full width at half-maximum (FWHM) of the AlOx peak, shown in Fig. 7, was estimated to be 35 nm corresponding to the average thickness of the alumina layer. The same procedure was applied in the case of Fe and Cr, by considering the corresponding energy values for metallic and oxidised states (Fe 2p3/2: 706.8 eV/Fe°, 709.9 eV/Fe3+ and Cr 2p3/2: 574.2 eV/Cr°, 576.9 eV/Cr3+) [41]. Oxidised Fe and Cr, detected at the surface of the sample, might be related (i) to the residuum remained on the sample surface after Pb chemical etching or (ii) to some nodules with duplex oxide (Fe3O4/(Fe1 x yCrxAly)3O4) caught in the XPS investigated area.
3.2. Exposure at 500 °C One sample, namely P1 (Fe-6Cr-6Al), has shown partially rough-granular, partially smooth surface morphology, after exposure to liquid lead (with 10 6 wt.% oxygen) at 500 °C for 930 h (Fig. 8). Some spots with rough-granular aspect were also observed on the P2 (Fe-8Cr-6Al) sample. Element point EDX microanalysis, performed on the rough area, indicated the presence of almost pure iron oxide with a stoichiometry matching the magnetite Fe3O4. The surface aspect of all the other specimens was smooth, as can be seen in the SEM micrographs depicted in Fig. 8. No dissolution attacks were observed. However, cracks and spots with spalled areas were found on some samples (e.g. P4: Fe-14Cr-4Al) which presumably occurred during the cooling period, after extraction from molten lead. The appearance of cracks and spalled areas indicates the presence of thicker oxide layers formed at the specimen surface. The samples with smooth morphology differ in the color of the scales. The samples P2–P7 and P10 were covered by a dark-brown oxide scale. Sample P8 (Fe-12Cr-7Al) showed an alternation between regions with dark-brown (40%) and regions with greenyellow oxide scale, while P9 (Fe-16Cr-6Al) was covered, on more than 95% of the surface, by a green-yellow oxide scale (the remainder being made up of dark-brown spots).
Fig. 8. SEM micrographs of the oxide scale grown on the surface of Fe–Cr–Al-base alloys exposed to oxygen containing liquid lead for 930 h at 500 °C: P1 (Fe-6Cr-6Al), P2 (Fe-8Cr-6Al), P3 (Fe-10Cr-5Al), P4 (Fe-14Cr-4Al), P5 (Fe-16Cr-4Al), P6 (Fe-6Cr-8Al), P7 (Fe-10Cr-7Al), P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al) and P10 (Fe-12Cr-5Al). Magnetite was observed on the surface of P1 and P2 samples. All the other samples showed smooth surfaces. Cracks and exfoliated scale like the ones depicted for P4 were also observed.
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Fig. 9. Cross sections of samples made of Fe–Cr–Al-based alloys exposed to oxygen containing liquid lead for 930 h at 500 °C: P1 (Fe-6Cr-6Al), P2 (Fe-8Cr-6Al), P3 (Fe-10Cr-5Al), P4 (Fe-14Cr-4Al), P5 (Fe-16Cr-4Al), P6 (Fe-6Cr-8Al), P7 (Fe-10Cr-7Al), P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al), P10 (Fe-12Cr-5Al). Magnetite was observed on P1 sample and IOZ was found in case of P1–P3 samples. P2–P7 and P10 are covered by (Fe1 x yCrxAly)3O4. P8 is covered by a mixture of duplex and thin Al-rich layers, while a thin Al-rich scale is grown on P9.
Fig. 10. EDX line scan showing (Fe1 x yCrxAly)3O4 scale formed on P7 (Fe-10Cr-7Al) sample (a) and EDX line scan of Al-rich oxide scale formed on the P9 (Fe-16Cr-6Al) alloy (b) during the exposure at 500 °C, for 930 h to molten lead containing 10 6 wt.% oxygen.
To check the surface composition of the scale showing smooth morphology, element point EDX analyses were performed at lower electron energy (10 keV). The samples P2–P7 and P10 have shown an enrichment of Cr at the surface, pointing out the presence of Fe–Cr–Al-based oxide. In case of the P8 sample, the amount of chromium was found to be increased in some surface regions, while in others the aluminium concentration was increased. This was interpreted as an alternation between Al-rich oxide and Fe–Cr–Al-based oxide on the P8 surface. In case of the P9 sample, only aluminium and oxygen enrichment at the surface covered by a green-yellow oxide scale was measured. The results of surface evaluation were confirmed by the crosssectional examinations. The samples P1–P7 and P10 are protected by an oxide scale with the thickness of around 5 lm. EDX line-scans (an example given in Fig. 10a) and element point analyses revealed that this scale is (Fe1 x yCrxAly)3O4 spinel-type oxide. Spots of pure iron oxide were observed in case of P1 (Fig. 9) on top of the spinel-type oxide.
In case of P1–P3 samples an internal oxidation zone (IOZ) was observed underneath the oxide layer, which was not present beneath the oxide scale of the other alloys. Cross sectional evaluation of the P8 sample (Fe-12Cr-7Al) confirmed the assumption made during surface observation, namely: a very thin, almost invisible Al-rich oxide scale alternating with 2–3 lm thick (Fe1 x yCrxAly)3O4 spinel-type oxide (Fig. 9). The P9 (Fe-16Cr-6Al) sample was almost entirely covered by a very thin oxide scale. EDX cross sectional line scan of this scale, depicted in Fig 10b, revealed the presence of Al-rich oxide on the sample surface. Peaks of XRD patterns, obtained on P1–P7 and P10, correspond to chromite Fe(Cr, Al)2O4 (PDF-card no. 34-0140) – the scale – and to a-Fe(Cr) (PDF-card no. 34-0396) – the substrate. The XRD pattern obtained on the P1 (Fe-6Cr-6Al) sample, is given in Fig. 11a as an example. GI-XRD pattern of the P9 sample (Fig. 11b) evidences the presence of the transient j-Al2O3 phase.
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Fig. 11. XRD patterns of P1 P1: Fe-6Cr-6Al - (a) and P9: Fe-16Cr-6Al - (b) taken after exposure in oxygen containing liquid lead for 930 h at 500 °C. A duplex layer (Fe3O4 + Fe(Cr, Al)2O4) is grown on P1 sample, while on P9 the scale is j-Al2O3 alumina polymorph.
alumina layer, grown on the P9 sample at 500 °C, was estimated to be 84 nm. 3.3. Exposure at 600 °C
Fig. 12. XPS sputter depth profile of P9 (Fe-16Cr-6Al) sample, exposed for 930 h to melted lead (with 10 6 wt.% oxygen) at 500 °C. The average thickness of Al2O3 scale, estimated from FWHM of the AlOx peak, is 84 nm.
The XPS compositional sputter depth profile of the P9 sample is shown in Fig. 12. Following the same semi-quantitative procedure described previously in Section 3.1, the average thickness of the
For none of the specimens tested at this temperature, dissolution attack was observed. The surfaces of P1–P7 and P10 samples have shown a rough-granular morphology (Fig. 13). Elemental point EDX analysis results matched the stoichiometry of the magnetite Fe3O4. Samples of the P8 and P9 alloys were covered by an opaque, gray-green smooth oxide scale (Fig. 13). Protrusions of magnetite were also observed, covering a cumulative area of the total sample surfaces of around 20%, in case of P8, and of less than 2%, in case of P9. SEM images, obtained during the investigation of the cross sections of the samples exposed for 1830 h in lead, containing 10 6 wt.% oxygen, at 600 °C, are displayed in Fig. 14. Samples P1–P7 and P10 are covered by a duplex oxide scale with thickness ranging from 2 lm (P5: Fe-16Cr-4Al) to 15 lm (P6: Fe-6Cr-8Al). Based on EDX line scan and element point analyses performed on the scales it was concluded that the outer layer is magnetite Fe3O4, while the inner layer is (Fe1 x yCrxAly)3O4 spinel-type oxide. The samples having an opaque, gray-green oxide scale (P8, P9) are covered by a thin Al-rich oxide scale, as the patterns of EDX line scan analysis indicated (Fig. 15). In case of P9 (b) the Cr peak, well
Fig. 13. Cross sections of samples made of Fe–Cr–Al-base alloys exposed to oxygen containing liquid lead for 1830 h at 600 °C: P1 (Fe-6Cr-6Al), P2 (Fe-8Cr-6Al), P3 (Fe-10Cr-5Al), P4 (Fe-14Cr-4Al), P5 (Fe-16Cr-4Al), P6 (Fe-6Cr-8Al), P7 (Fe-10Cr-7Al), P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al), P10 (Fe-12Cr-5Al). Magnetite was observed in case of P1–P4, P6, P7 and P10 samples (100% of the surface), P5 (20%), P8 (20%) and P9 (2%).
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Fig. 14. Cross sections of samples made of Fe–Cr–Al-based alloys exposed to oxygen containing liquid lead for 1830 h at 600 °C: P1 (Fe-6Cr-6Al), P2 (Fe-8Cr-6Al), P3 (Fe-10Cr-5Al), P4 (Fe-14Cr-4Al), P5 (Fe-16Cr-4Al), P6 (Fe-6Cr-8Al), P7 (Fe-10Cr-7Al), P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al), P10 (Fe-12Cr-5Al). P1–P4, P6, P7 and P10 are covered by duplex scale (Fe3O4+(Fe1 x yCrxAly)3O4). P5 is covered by thin (Fe1 x yCrxAly)3O4 layer. P8 and P9 are covered by thin Al-rich scale.
Fig. 15. EDX line scan of P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al) samples: Al-rich oxide scale formed on both samples during their exposure to oxygen containing liquid lead for 1930 h at 600 °C. In case of P9 (b) the Cr peak, well defined beneath Al peak, might indicate the existence of Cr2O3 sub-layer.
Fig. 16. XRD patterns of P6 (Fe-6Cr-8Al) showing the presence of magnetite (a) and of P9 (Fe-16Cr-6Al) showing peaks corresponding to transient alumina h-Al2O3 phase (PDF card no. 86-1410) and Cr2O3 (PDF card no. 84-1616) (b).
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Fig. 18. The evolution of the thickness of the (Fe1 exposure temperatures.
x yCrxAly)3O4
sub-layer, with the
Fig. 17. XPS sputter depth profile of P9 (Fe-16Cr-6Al) sample, exposed for 1830 h to melted lead (with 10 6 wt.% oxygen) at 600 °C.
defined beneath the Al peak, might indicate the existence of a Cr2O3 sub-layer. XRD analyses, of which an example is given in Fig. 16a, confirmed the conclusions obtained from SEM examination, in case of P1–P7 and P10 samples: the peaks correspond to Fe-containing oxides: Fe(Cr, Al)2O4 in case of the P5 sample and magnetite Fe3O4 in case of P1–P4, P6 and P7. The GI-XRD pattern of the P9 sample (Fig. 16b) evidences the presence of two oxides at the sample surface: transient alumina h-Al2O3 phase (PDF card no. 86-1410) and Cr2O3 (PDF card no. 84-1616). This information, corroborated with the information from the EDX line scan, where the Cr peak appears immediately beneath the Al peak (Fig. 15b), drive to the conclusion that a chromia layer, formed during the initial oxidation, was ‘‘buried’’ inside of a half micron thick Al2O3 layer (estimated from SEM cross section), during oxide scale growth, due to the outward aluminium cation diffusion [42,43]. Fig. 17 shows the XPS compositional sputter depth profile of the P9 sample. Al and O enrichment was measured at the sample surface. The evaluation, using profilometry, of the footprint left by the sputtering process reveals an uneven area due to an inhomogeneous ion milling process. This is the reason why, in this case, the average thickness of the oxide scale was not possible to be evaluated from XPS compositional sputter depth profile. 4. Discussion There were some important observations made during these experiments. The first of them refers to the fact that all model alloys developed protective oxide scales when exposed to oxygencontaining liquid lead at 400, 500 and 600 °C. While eight alloys (P1–P7 and P10) were covered by a Fe-containing oxide scale (Fe3 O4 + (Fe1 x yCrxAly)3O4), two others (P8 and P9) formed transient alumina (j-Al2O3 at 400 and 500 °C and h-Al2O3 at 600 °C) as a protective scale. The thickness of the duplex scale, consisting of Fe-containing oxides, increases slowly from 2 to 4 lm, at 400 °C, to around 5 lm at 500 °C and reaches a maximum of approximately 15 lm (P6: Fe-6Cr-8Al) after exposure at 600 °C. It should be mentioned that the exposure time at 600 °C was twice as high as those at lower temperatures. Fig. 18 shows the thickness of the (Fe1 x yCrx Aly)3O4 sub-layer, formed on P1–P7 and P10 samples during exposure to molten lead (with 10 6 wt.% oxygen), at 400 °C, 500 °C and
Fig. 19. Oxide ‘‘map’’ for the oxidation of Fe–Cr–Al-based model alloys exposed to oxygen containing molten lead, in the temperature range 400–600 °C, containing also data from literature: line at 800 °C [27] and line at 1000 °C [32] (in oxygen atmosphere), R1s [34], R2 [35] and R3 [44] (in lead and lead–bismuth, respectively).
600 °C. As can be observed, the thickness of this sub-layer is lower above certain Cr content (e.g. P5: Fe-16Cr-4Al). One of the two samples forming an alumina scale, namely P8 (Fe-12Cr-7Al), was found to be also covered by a duplex layer (magnetite + spinel) on a significant area fraction (20–40%). This behaviour could be attributed to small reduction in that region of Al concentration below a critical level, where a selective oxidation of aluminium occurs. Based on the above described results, the stability domain of alumina has been estimated for the Fe–Cr–Al alloy system, exposed to molten lead containing 10 6 wt.% oxygen, over 400– 600 °C temperature range (Fig. 19). The P9 (Fe-16Cr-6Al) sample was included in the alumina stability domain, while the P8 sample was located on the boundary between the alumina stability domain and the Fe–Cr-based oxide domain. As can be seen, the Al concentration, critical in terms of a selective oxidation to alumina, must be increased when the exposure to oxidising environment is performed at lower temperatures. This is also in
A. Weisenburger et al. / Journal of Nuclear Materials 437 (2013) 282–292
concordance with the findings reported in literature for exposures at 800 °C [27] and 1000 °C [32] to oxygen atmosphere. Another observation is related to the Cr concentration: higher Cr content leads to the alumina formation at lower Al concentration (TEE). The ternary diagram in Fig. 19 shows additional points R1s, R2 and R3 reported in literature, [34,44,35] respectively. Alumina forming R1s coatings, as well as R2 (ODS) and R3 (Kanthal AF) alloys, when exposed to molten, oxygen containing Pb and Pb–Bi-eutectic, respectively, belong to the Fe–Cr–Al-RE system. RE is the so called reactive element. The exposure temperatures for these three references were 400–550 °C (R1s), 650–700 °C (R2 for 10.000 h) and 500–750 °C (R3). The results are consistent with our findings: R1s and R3 alloys are inside of alumina stability domain and R2 is located between the border lines of the alumina stability domain at 800 °C [27] and 600 °C (our results). RE elements (yttrium in case of R1s, hafnium in case of R2 and silicon in case of R3) are added in small quantities (<1 wt.%) because, as experimentally proved, they reduce the scale growth rate and improve the scale/substrate adherence [45]. Whether the addition of reactive elements, either individually or in combination may influence the morphology, the microstructure of the alumina scale, the growth rate, or may facilitate the nucleation of alumina scale, when Fe–Cr–Al-RE alloys are exposed to oxygen containing HLMs, at temperature relevant for applications, has to be further addressed. The alumina scales, grown on P8 and P9 samples, are transient polymorphs j-Al2O3 (at 400 and 500 °C) and h-Al2O3 (at 600 °C). These metastable alumina types are known to be formed in the early stages of the oxidation process, during exposure in oxygencontaining gas atmosphere around 900 °C, and have a higher growth rate than a-Al2O3. Later on, these polymorphs transform into stable alumina [46]. Also it is known that the transformation of c-Al2O3 into a-Al2O3 implies a volumetric change of around 14% [47]. These phenomena, accompanying the formation, growth and transformation of transient alumina, are detrimental due to the accumulation of stresses and defects in the oxide scale, which can weaken its adherence to the underlying substrate, leading to spallation. For the temperature range, oxygen concentration in the molten lead and exposure time used during the current evaluation, the growth rate of the alumina scale was low. It seems that a limited metal oxidation occurs in these experimental conditions. No area with detached scale was observed and no trace of a-Al2O3 was detected. However, to assess the stability and properties of the alumina scale formed on Fe–Cr–Al-based model alloys, exposed to oxygen containing lead, experiments with longer exposure time (e.g. 5000 h) are underway.
5. Conclusions 1. Chromium and aluminium contents of 12.5–17 wt.% and 6– 7.5 wt.%, respectively, are high enough to obtain thin, stable and protective alumina scales on Fe–Cr–Al-based alloys exposed to oxygen containing lead at 400, 500 and 600 °C. 2. Lower aluminium concentrations leads to the formation of Febased spinel-type oxides protrusions on extended area. However, this can also happen at high aluminium concentration (7.6 wt.%) if the chromium content is lower than 10.4 wt.%. 3. The alumina stability domain border shifts with lower temperatures to higher chromium and aluminium concentrations. 4. The influence of the model alloys grain size, of the addition of reactive elements and of longer exposure time on the alumina scale polymorphs type, growth, stability and properties is currently under evaluation.
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Acknowledgments Financial support by GETMAT (FP7-212175) and LEADER (FP7249668) within the EU-7th Framework program is gratefully acknowledged. The authors wish to tank Prof. Victor Geanta (University Politechnica of Bucharest) for providing high quality alloys.
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Oxide scales formed on Fe–Cr–Al-based model alloys exposed to oxygen containing molten lead A. Weisenburger, A. Jianu ⇑, S. Doyle, M. Bruns, R. Fetzer, A. Heinzel, M. DelGiacco, W. An, G. Müller Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
a r t i c l e
i n f o
Article history: Received 17 December 2012 Accepted 18 February 2013 Available online 26 February 2013
a b s t r a c t Based on the state of the art oxide maps concerning oxidation behavior of Fe–Cr–Al model alloys at 800 and 1000 °C in oxygen atmosphere, ten compositions, belonging to this alloy system, were designed in order to tap the borders of the alumina stability domain, during their exposure to oxygen (10 6 wt.%) containing lead, at 400, 500 and 600 °C. Eight alloys, Fe-6Cr-6Al, Fe-8Cr-6Al, Fe-10Cr-5Al, Fe-14Cr-4Al, Fe-16Cr-4Al, Fe-6Cr-8Al, Fe-10Cr-7Al and Fe-12Cr-5Al, were found to be protected against corrosion in oxygen containing lead, either by a duplex layer (Fe3O4 + (Fe1 x yCrxAly)3O4) or by (Fe1 x yCrxAly)3O4, depending on the temperature at which they were exposed. Two alloys namely Fe-12Cr-7Al and Fe-16Cr-6Al were found to form transient aluminas, j-Al2O3 (at 400 and 500 °C) and h-Al2O3 (at 600 °C), as protective oxide scale against corrosion in oxygen containing lead. An oxide map illustrating the stability domain of alumina, grown on Fe–Cr–Al alloys when exposed to molten, oxygen containing lead, was drawn. The map includes also additional points, extracted from literature and corresponding to alumina forming alloys, when exposed to HLMs, which fit very well with our findings. Chromium and aluminium contents of 12.5–17 wt.% and 6–7.5 wt.%, respectively, are high enough to obtain thin, stable and protective alumina scales on Fe–Cr–Al-based alloys exposed to oxygen containing lead at 400, 500 and 600 °C. For the temperature range and exposure times used during the current evaluation, the growth rate of the alumina scale was low. No area with detached scale was observed and no trace of a-Al2O3 was detected. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The use of heavy liquid metals (HLMs), namely lead or lead-based alloys, in energy-related applications is currently under consideration because of their beneficial thermal and neutronic properties [1–3]. However, HLM compatibility with structural steels, in terms of corrosion and mechanical resistance, causes considerable concern [4–18]. The improvement of corrosion resistance by alloying the steels with strong oxide-forming elements (e.g. Al, Si) has been attempted [19–21]. Aluminium addition to the steel composition has shown to be beneficial for protecting steel surfaces in contact with lead alloys (containing small amounts of oxygen) against corrosion attack and severe oxidation, by forming an Al-rich oxide scale [5,20–22]. A thin, continuous, dense, stable, adherent Al2O3 layer provides an effective diffusion barrier, which protects the Al-containing steels from corrosion attack in HLM. The development of such a layer requires minimal Al levels, since selective ⇑ Corresponding author. Tel.: +49 721 608 28519; fax: +49 721 608 22256. E-mail address: [email protected] (A. Jianu). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.02.044
Al oxidation occurs [21,23–25]. Unfortunately, such minimal Al contents could negatively affect the mechanical properties of the steel. This is the reason why Al-containing coatings or steel surface alloying with Al (instead of steel bulk alloying) were proposed [5]. Fe–Cr–Al alloys are of large interest for practical applications at high-temperatures in reactive atmospheres [26–29]. Cr was introduced because it was found to reduce the Al content required to form a protective Al2O3 layer (third-element effect – TEE) [30,31]. Up to now, the alumina stability domain was defined for Fe–Cr–Al alloy systems (low Al content) after exposure in gas atmosphere at temperatures higher than 800 °C [27,32]. Few experimental data concerning the oxidation behaviour of Fe–Cr–Al alloy system at lower temperatures, during exposures in reactive atmosphere [33], are also available. The phenomena occurring during the Fe–Cr–Al alloys exposure to HLMs, containing very small amounts of dissolved oxygen, is scarcely documented [4,12,17,34,35]. No systematic investigation of the corrosion behaviour of the Fe–Cr–Al system, when exposed to HLM was performed, in order to find the minimum Al content for the formation of an alumina scale.
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The current paper presents the results of a systematic study concerning the corrosion behaviour of Fe–Cr–Al-based model alloys during their exposure to oxygen-containing (10 6 wt.%) lead in the temperature range 400–600 °C. In this vein a map of the alumina stability domain for this temperature range has been drawn in Fe–Cr–Al ternary diagrams and the type of alumina polymorph, formed at different temperatures, was determined. 2. Experimental Ten Fe–Cr–Al alloys were prepared as ingots by arc-melting in argon atmosphere starting from high purity elements. Their nominal compositions are: Fe-6Cr-6Al, Fe-8Cr-6Al, Fe-10Cr-5Al, Fe-12Cr-5Al, Fe-14Cr-4Al, Fe-16Cr-4Al, Fe-6Cr-8Al, Fe-10Cr-7Al, Fe-12Cr-7Al, Fe-16Cr-6Al (numbers in wt.%). The distribution of these compositions in the Fe–Cr–Al ternary diagram is shown in Fig. 1, together with two reference borderlines, corresponding to the oxide-maps drawn after oxidation experiments performed on iron-rich Fe–Cr–Al-based alloys in oxygen, at 800 °C [27] and 1000 °C [32]. The actual compositions of the alloys were determined by energy dispersive X-ray spectroscopy (EDX) and are shown in Table 1. The ingots were cut into discs with around 1.2 mm thickness. All specimens were mechanically grinded on successively finer abrasive papers down to 4200 grit. They were finally cleaned with water, acetone and ethanol in an ultrasonic bath and dried immediately before the experiments. After preparation, the specimens were exposed to stagnant oxygen-containing liquid lead, in the COSTA facility [5]. The lead was
Fig. 1. Ternary phase diagram with compositions of prepared alloys and the oxidemaps drawn at 800 °C [27] and 1000 °C [32].
Table 1 Actual composition of ternary Fe–Cr–Al alloys, analyzed with EDX. Alloy
Nominal composition
Cr (wt.%)
Al (wt.%)
Fe
P1 P2 P3 P4 P5 P6 P7 P8 P9 P10
Fe–6Cr–6Al Fe–8Cr–6Al Fe–10Cr–5Al Fe–14Cr–4Al Fe–16Cr–4Al Fe–6Cr–8Al Fe–10Cr–7Al Fe–12Cr–7Al Fe–16Cr–6Al Fe–12Cr–5Al
6.2 8.4 10.8 14.5 16.4 6.4 10.4 12.4 16.9 12.3
6.8 6.6 5.6 4.2 4.7 8.8 7.6 7.5 6.4 5.8
Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance
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placed in alumina crucibles, which were loaded into the quartz tubes of the furnaces and then conditioned at the selected oxygen content and temperatures. The oxygen content of the lead was 10 6 wt.% and kept constant. This was obtained by a continuous flow of Ar and Ar + H2 through a water bath, for a precisely controlled H2/H2O ratio in the gas atmosphere above the melted lead. Once the oxygen content and temperature were reached, the specimens were placed inside the conditioned lead. The experiments were performed at the following lead temperatures/exposure times: 400 °C/840 h, 500 °C/930 h and 600 °C/1830 h. After the extraction from molten lead, the specimens were cooled in air and then cleaned with a solution of ethanol, acetic acid and hydrogen peroxide (1:1:1) to remove the remaining adherent lead. The evaluation of the specimens was performed using the following characterization techniques: scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction using conventional and synchrotron radiation (XRD), and for some selected samples, X-ray photoelectron spectroscopy (XPS). SEM–EDX examination was performed with Philips XL40 equipped with a SAMx–EDX system. The phase composition of the majority of grown oxides was analysed using a Seyfert C3000 powder diffractometer (Cu Ka radiation) with h 2h conventional geometry. For a batch of selected samples the evaluation of the grown oxide was made with grazing incidence-X-ray diffraction (GI-XRD). In the latter case, the measurements were made using synchrotron radiation at the ANKA synchrotron [36], PDIFF beamline, with the wave length k = 0.117945 nm. Incidence angles between 1° and 3° were used. XPS measurements were performed on P9 samples using a KAlpha XPS spectrometer (ThermoFisher Scientific). Data acquisition and processing using the Thermo Avantage software is described elsewhere [37]. The samples were analyzed using a microfocused, monochromated Al Ka X-ray source (200 lm spot size). The KAlpha charge compensation system was employed during analysis, using electrons of 8 eV energy, and low-energy argon ions to prevent any localized charge build-up. The spectra were fitted with one or more Voigt profiles (BE uncertainty: ±0.2 eV). The analyzer transmission function, Scofield sensitivity factors [38] and effective attenuation lengths (EALs) for photoelectrons were applied for quantification. EALs were calculated using the standard TPP-2M formalism [39]. All spectra were referenced to the C1s peak of hydrocarbon at 285.0 eV binding energy, controlled by means of the well-known photoelectron peaks of metallic Au, Ag and Cu. XPS sputter depth profiling for evaluation of the composition of the grown oxides was performed using a 2 keV Ar+ ion beam at a raster size of 1 2 mm2 and a 30° angle of incidence. The depth of sputtering was measured at the end of the evaluation by profilometry (DektakXT Stylus Profilometer, Bruker AXS). 3. Results Prior to corrosion testing, the microstructure of the alloys was evaluated using light microscopy (LM). The original grain size of all alloys was in the range of 0.5–2 mm, so that a possible influence of grain boundary diffusion on the formation of oxide scales can be considered very low. Fig. 2 reveals the microstructure of the samples in the original state. The grains are preferentially oriented due to solidification conditions. 3.1. Exposure at 400 °C Fig. 3 shows typical surface views of the Fe–Cr–Al-based alloys exposed to oxygen-containing molten lead at 400 °C (after chemical removal of the adherent lead). No dissolution attacks were
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Fig. 2. Light microscope images of original microstructure of P1 (Fe-6Cr-6Al), P2 (Fe-8Cr-6Al), P3 (Fe-10Cr-5Al), P4 (Fe-14Cr-4Al), P5 (Fe-16Cr-4Al), P6 (Fe-6Cr-8Al), P7 (Fe-10Cr-7Al), P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al) and P10 (Fe-12Cr-5Al) samples. Large grains in the range of 0.5–2 mm and a slight texture characterized all samples.
Fig. 3. SEM micrographs of the oxide scale grown on the surface of Fe–Cr–Al alloys exposed to oxygen containing liquid lead for 840 h at 400 °C: P1 (Fe-6Cr-6Al), P2 (Fe-8Cr-6Al), P3 (Fe-10Cr-5Al), P4 (Fe-14Cr-4Al), P5 (Fe-16Cr-4Al), P6 (Fe-6Cr-8Al), P7 (Fe-10Cr-7Al), P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al) and P10 (Fe-12Cr-5Al).
Fig. 4. Cross sections of samples made of Fe–Cr–Al-based alloys exposed to oxygen containing liquid lead for 840 h at 400 °C: P1 (Fe-6Cr-6Al), P2 (Fe-8Cr-6Al), P3 (Fe-10Cr-5Al), P4 (Fe-14Cr-4Al), P5 (Fe-16Cr-4Al), P6 (Fe-6Cr-8Al), P7 (Fe-10Cr-7Al), P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al) and P10 (Fe-12Cr-5Al). P1–P7 and P10 are covered by duplex scale (Fe3O4 + (Fe1 x yCrxAly)3O4). P8 is covered by a mixture of duplex and thin Al-rich oxide layers, while a thin Al-rich scale is grown on P9 (SE: secondary electrons image; BSE: back-scattered electrons image).
observed. Based on the oxide morphologies formed on the surfaces, the samples were classified into two categories: samples with ‘‘rough-granular surfaces’’ and samples with generally ‘‘smooth surfaces’’. The first category comprises samples of P1–P7 and P10 alloys, while the second category contains P8 and P9 samples. The oxide scales formed on the first category exhibit grains protruded above the original surface and have a wrinkled appearance with porosities, as shown in Fig. 3. EDX microanalysis indicated that these grains are rather pure iron oxide, with the atomic ratio Fe:O = (40–45):(55–60), matching the stoichiometry of magnetite Fe3O4.
The samples with generally smooth surface are covered by a green-yellow oxide scale, seemingly transparent, showing also some spots with rough-granular morphology. The area fraction covered by granular scale varied from around 20%, in case of P8, to <2%, for P9 sample. The protrusions, which are, similarly to the first category, pure iron oxide, formed preferentially along grain boundaries and around some material defects (e.g. alumina inclusions observed in ‘‘as prepared’’ alloys). The cross sections shown in Fig. 4 correspond to the samples exposed at 400 °C. Samples having a rough-granular surface aspect (made out of P1–P7 and P10 alloys) formed a duplex oxide scale with thickness in the range of 2–4 lm. The outer layer is a pure
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Fig. 5. EDX line scan showing a duplex oxide scale formed on P4 (Fe-14Cr-4Al) sample (a) and EDX line scan of the very thin Al-rich oxide scale formed on the P9 (Fe-16Cr-6Al) alloy (b) during the exposure at 400 °C, for 840 h to molten lead containing 10 6 wt.% oxygen.
Fig. 6. XRD patterns of P7 (a) and P8 (b) after exposure in oxygen containing liquid lead for 840 h at 400 °C. P7 is covered by a duplex layer (Fe3O4 + Fe(Cr, Al)2O4), while on P8 the protective scale is j-Al2O3 alumina polymorph.
iron oxide with pores (some with trapped lead), while the inner layer is a Fe–Cr–Al-mixed oxide, as concluded from EDX line scan analysis. An example, corresponding to the P4 sample, is given in Fig. 5a. The measured oxygen concentration on the inner layer was around 55 at.%, indicating a (Fe1 x yCrxAly)3O4 spinel-type oxide. The samples with smooth surfaces (P8 and P9) formed an almost invisible oxide scale (Fig. 4). The thin scale, grown on both alloys, was an Al-rich oxide, as established by EDX line scan analysis. The pattern obtained on the P9 sample is shown in Fig. 5b, as an example. Few thick oxide nodules were also observed mainly on the P8 sample, confirming the findings during surface evaluation. These nodules consist of an external layer of almost pure iron oxide,
followed by a Fe–Cr–Al-based oxide, similar to the oxide scale grown on P1–P7 and P10 alloys. The phase composition of the oxidised samples showing a ‘‘rough-granular surface’’ (P1–P7 and P10) was examined by XRD (h 2h). The XRD patterns clearly evidence the presence of the spinel-type structure (space group Fd-3 m), corresponding to the oxide scale and identified as magnetite Fe3O4 (PDF-card no. 890688) and chromite Fe(Cr, Al)2O4 (PDF-card no. 34-0140), and also the presence of the cubic structure a-Fe(Cr) (PDF-card no. 340396), which corresponds to the substrate. Fig. 6a depicts, as an example, the XRD pattern of the P7 (Fe-10Cr-7Al) sample. The structure of the oxide grown on the samples showing a generally ‘‘smooth surface’’ was investigated by GI-XRD. Both P8 and P9 samples were covered by j-Al2O3 transient alumina
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Fig. 7. XPS sputter depth profile of P9 (Fe-16Cr-6Al) sample, exposed for 840 h to melted lead (with 10 6 wt.% oxygen) at 400 °C, obtained after 7500 s sputter times (a) and a detailed view of the same graph (b). The average thickness of Al2O3 scale, estimated from FWHM of the AlOx peak, is 35 nm.
(PDF card no. 26-31), as can be concluded from the example shown in Fig. 6b (P8: Fe-12Cr-7Al). GI-XRD patterns of P8 and P9 also contain peaks of magnetite, which, in accordance with SEM investigation results, is located at the grain boundaries or around surface defects, such as alumina inclusions (see Fig. 3, micrographs P8 and P9). Fig. 7 shows the XPS compositional sputter depth profile of the P9 sample. It was possible to evaluate the thickness of the Al2O3 layer grown on P9 (Fe-16Cr-6Al) sample by de-convoluting the Al 2p photoelectron emission signal into two peaks, corresponding to Al° (metallic, 72.4 eV) and AlOx (oxidised, 75.4 eV) [40], and by extracting the contribution of the Cr 3s line overlapping on Al 2p (Cr 3s: 75.0 eV). This approach is semi-quantitative due to lower energy resolution in the sputter depth profile mode. Based on such considerations, the full width at half-maximum (FWHM) of the AlOx peak, shown in Fig. 7, was estimated to be 35 nm corresponding to the average thickness of the alumina layer. The same procedure was applied in the case of Fe and Cr, by considering the corresponding energy values for metallic and oxidised states (Fe 2p3/2: 706.8 eV/Fe°, 709.9 eV/Fe3+ and Cr 2p3/2: 574.2 eV/Cr°, 576.9 eV/Cr3+) [41]. Oxidised Fe and Cr, detected at the surface of the sample, might be related (i) to the residuum remained on the sample surface after Pb chemical etching or (ii) to some nodules with duplex oxide (Fe3O4/(Fe1 x yCrxAly)3O4) caught in the XPS investigated area.
3.2. Exposure at 500 °C One sample, namely P1 (Fe-6Cr-6Al), has shown partially rough-granular, partially smooth surface morphology, after exposure to liquid lead (with 10 6 wt.% oxygen) at 500 °C for 930 h (Fig. 8). Some spots with rough-granular aspect were also observed on the P2 (Fe-8Cr-6Al) sample. Element point EDX microanalysis, performed on the rough area, indicated the presence of almost pure iron oxide with a stoichiometry matching the magnetite Fe3O4. The surface aspect of all the other specimens was smooth, as can be seen in the SEM micrographs depicted in Fig. 8. No dissolution attacks were observed. However, cracks and spots with spalled areas were found on some samples (e.g. P4: Fe-14Cr-4Al) which presumably occurred during the cooling period, after extraction from molten lead. The appearance of cracks and spalled areas indicates the presence of thicker oxide layers formed at the specimen surface. The samples with smooth morphology differ in the color of the scales. The samples P2–P7 and P10 were covered by a dark-brown oxide scale. Sample P8 (Fe-12Cr-7Al) showed an alternation between regions with dark-brown (40%) and regions with greenyellow oxide scale, while P9 (Fe-16Cr-6Al) was covered, on more than 95% of the surface, by a green-yellow oxide scale (the remainder being made up of dark-brown spots).
Fig. 8. SEM micrographs of the oxide scale grown on the surface of Fe–Cr–Al-base alloys exposed to oxygen containing liquid lead for 930 h at 500 °C: P1 (Fe-6Cr-6Al), P2 (Fe-8Cr-6Al), P3 (Fe-10Cr-5Al), P4 (Fe-14Cr-4Al), P5 (Fe-16Cr-4Al), P6 (Fe-6Cr-8Al), P7 (Fe-10Cr-7Al), P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al) and P10 (Fe-12Cr-5Al). Magnetite was observed on the surface of P1 and P2 samples. All the other samples showed smooth surfaces. Cracks and exfoliated scale like the ones depicted for P4 were also observed.
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Fig. 9. Cross sections of samples made of Fe–Cr–Al-based alloys exposed to oxygen containing liquid lead for 930 h at 500 °C: P1 (Fe-6Cr-6Al), P2 (Fe-8Cr-6Al), P3 (Fe-10Cr-5Al), P4 (Fe-14Cr-4Al), P5 (Fe-16Cr-4Al), P6 (Fe-6Cr-8Al), P7 (Fe-10Cr-7Al), P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al), P10 (Fe-12Cr-5Al). Magnetite was observed on P1 sample and IOZ was found in case of P1–P3 samples. P2–P7 and P10 are covered by (Fe1 x yCrxAly)3O4. P8 is covered by a mixture of duplex and thin Al-rich layers, while a thin Al-rich scale is grown on P9.
Fig. 10. EDX line scan showing (Fe1 x yCrxAly)3O4 scale formed on P7 (Fe-10Cr-7Al) sample (a) and EDX line scan of Al-rich oxide scale formed on the P9 (Fe-16Cr-6Al) alloy (b) during the exposure at 500 °C, for 930 h to molten lead containing 10 6 wt.% oxygen.
To check the surface composition of the scale showing smooth morphology, element point EDX analyses were performed at lower electron energy (10 keV). The samples P2–P7 and P10 have shown an enrichment of Cr at the surface, pointing out the presence of Fe–Cr–Al-based oxide. In case of the P8 sample, the amount of chromium was found to be increased in some surface regions, while in others the aluminium concentration was increased. This was interpreted as an alternation between Al-rich oxide and Fe–Cr–Al-based oxide on the P8 surface. In case of the P9 sample, only aluminium and oxygen enrichment at the surface covered by a green-yellow oxide scale was measured. The results of surface evaluation were confirmed by the crosssectional examinations. The samples P1–P7 and P10 are protected by an oxide scale with the thickness of around 5 lm. EDX line-scans (an example given in Fig. 10a) and element point analyses revealed that this scale is (Fe1 x yCrxAly)3O4 spinel-type oxide. Spots of pure iron oxide were observed in case of P1 (Fig. 9) on top of the spinel-type oxide.
In case of P1–P3 samples an internal oxidation zone (IOZ) was observed underneath the oxide layer, which was not present beneath the oxide scale of the other alloys. Cross sectional evaluation of the P8 sample (Fe-12Cr-7Al) confirmed the assumption made during surface observation, namely: a very thin, almost invisible Al-rich oxide scale alternating with 2–3 lm thick (Fe1 x yCrxAly)3O4 spinel-type oxide (Fig. 9). The P9 (Fe-16Cr-6Al) sample was almost entirely covered by a very thin oxide scale. EDX cross sectional line scan of this scale, depicted in Fig 10b, revealed the presence of Al-rich oxide on the sample surface. Peaks of XRD patterns, obtained on P1–P7 and P10, correspond to chromite Fe(Cr, Al)2O4 (PDF-card no. 34-0140) – the scale – and to a-Fe(Cr) (PDF-card no. 34-0396) – the substrate. The XRD pattern obtained on the P1 (Fe-6Cr-6Al) sample, is given in Fig. 11a as an example. GI-XRD pattern of the P9 sample (Fig. 11b) evidences the presence of the transient j-Al2O3 phase.
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Fig. 11. XRD patterns of P1 P1: Fe-6Cr-6Al - (a) and P9: Fe-16Cr-6Al - (b) taken after exposure in oxygen containing liquid lead for 930 h at 500 °C. A duplex layer (Fe3O4 + Fe(Cr, Al)2O4) is grown on P1 sample, while on P9 the scale is j-Al2O3 alumina polymorph.
alumina layer, grown on the P9 sample at 500 °C, was estimated to be 84 nm. 3.3. Exposure at 600 °C
Fig. 12. XPS sputter depth profile of P9 (Fe-16Cr-6Al) sample, exposed for 930 h to melted lead (with 10 6 wt.% oxygen) at 500 °C. The average thickness of Al2O3 scale, estimated from FWHM of the AlOx peak, is 84 nm.
The XPS compositional sputter depth profile of the P9 sample is shown in Fig. 12. Following the same semi-quantitative procedure described previously in Section 3.1, the average thickness of the
For none of the specimens tested at this temperature, dissolution attack was observed. The surfaces of P1–P7 and P10 samples have shown a rough-granular morphology (Fig. 13). Elemental point EDX analysis results matched the stoichiometry of the magnetite Fe3O4. Samples of the P8 and P9 alloys were covered by an opaque, gray-green smooth oxide scale (Fig. 13). Protrusions of magnetite were also observed, covering a cumulative area of the total sample surfaces of around 20%, in case of P8, and of less than 2%, in case of P9. SEM images, obtained during the investigation of the cross sections of the samples exposed for 1830 h in lead, containing 10 6 wt.% oxygen, at 600 °C, are displayed in Fig. 14. Samples P1–P7 and P10 are covered by a duplex oxide scale with thickness ranging from 2 lm (P5: Fe-16Cr-4Al) to 15 lm (P6: Fe-6Cr-8Al). Based on EDX line scan and element point analyses performed on the scales it was concluded that the outer layer is magnetite Fe3O4, while the inner layer is (Fe1 x yCrxAly)3O4 spinel-type oxide. The samples having an opaque, gray-green oxide scale (P8, P9) are covered by a thin Al-rich oxide scale, as the patterns of EDX line scan analysis indicated (Fig. 15). In case of P9 (b) the Cr peak, well
Fig. 13. Cross sections of samples made of Fe–Cr–Al-base alloys exposed to oxygen containing liquid lead for 1830 h at 600 °C: P1 (Fe-6Cr-6Al), P2 (Fe-8Cr-6Al), P3 (Fe-10Cr-5Al), P4 (Fe-14Cr-4Al), P5 (Fe-16Cr-4Al), P6 (Fe-6Cr-8Al), P7 (Fe-10Cr-7Al), P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al), P10 (Fe-12Cr-5Al). Magnetite was observed in case of P1–P4, P6, P7 and P10 samples (100% of the surface), P5 (20%), P8 (20%) and P9 (2%).
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Fig. 14. Cross sections of samples made of Fe–Cr–Al-based alloys exposed to oxygen containing liquid lead for 1830 h at 600 °C: P1 (Fe-6Cr-6Al), P2 (Fe-8Cr-6Al), P3 (Fe-10Cr-5Al), P4 (Fe-14Cr-4Al), P5 (Fe-16Cr-4Al), P6 (Fe-6Cr-8Al), P7 (Fe-10Cr-7Al), P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al), P10 (Fe-12Cr-5Al). P1–P4, P6, P7 and P10 are covered by duplex scale (Fe3O4+(Fe1 x yCrxAly)3O4). P5 is covered by thin (Fe1 x yCrxAly)3O4 layer. P8 and P9 are covered by thin Al-rich scale.
Fig. 15. EDX line scan of P8 (Fe-12Cr-7Al), P9 (Fe-16Cr-6Al) samples: Al-rich oxide scale formed on both samples during their exposure to oxygen containing liquid lead for 1930 h at 600 °C. In case of P9 (b) the Cr peak, well defined beneath Al peak, might indicate the existence of Cr2O3 sub-layer.
Fig. 16. XRD patterns of P6 (Fe-6Cr-8Al) showing the presence of magnetite (a) and of P9 (Fe-16Cr-6Al) showing peaks corresponding to transient alumina h-Al2O3 phase (PDF card no. 86-1410) and Cr2O3 (PDF card no. 84-1616) (b).
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Fig. 18. The evolution of the thickness of the (Fe1 exposure temperatures.
x yCrxAly)3O4
sub-layer, with the
Fig. 17. XPS sputter depth profile of P9 (Fe-16Cr-6Al) sample, exposed for 1830 h to melted lead (with 10 6 wt.% oxygen) at 600 °C.
defined beneath the Al peak, might indicate the existence of a Cr2O3 sub-layer. XRD analyses, of which an example is given in Fig. 16a, confirmed the conclusions obtained from SEM examination, in case of P1–P7 and P10 samples: the peaks correspond to Fe-containing oxides: Fe(Cr, Al)2O4 in case of the P5 sample and magnetite Fe3O4 in case of P1–P4, P6 and P7. The GI-XRD pattern of the P9 sample (Fig. 16b) evidences the presence of two oxides at the sample surface: transient alumina h-Al2O3 phase (PDF card no. 86-1410) and Cr2O3 (PDF card no. 84-1616). This information, corroborated with the information from the EDX line scan, where the Cr peak appears immediately beneath the Al peak (Fig. 15b), drive to the conclusion that a chromia layer, formed during the initial oxidation, was ‘‘buried’’ inside of a half micron thick Al2O3 layer (estimated from SEM cross section), during oxide scale growth, due to the outward aluminium cation diffusion [42,43]. Fig. 17 shows the XPS compositional sputter depth profile of the P9 sample. Al and O enrichment was measured at the sample surface. The evaluation, using profilometry, of the footprint left by the sputtering process reveals an uneven area due to an inhomogeneous ion milling process. This is the reason why, in this case, the average thickness of the oxide scale was not possible to be evaluated from XPS compositional sputter depth profile. 4. Discussion There were some important observations made during these experiments. The first of them refers to the fact that all model alloys developed protective oxide scales when exposed to oxygencontaining liquid lead at 400, 500 and 600 °C. While eight alloys (P1–P7 and P10) were covered by a Fe-containing oxide scale (Fe3 O4 + (Fe1 x yCrxAly)3O4), two others (P8 and P9) formed transient alumina (j-Al2O3 at 400 and 500 °C and h-Al2O3 at 600 °C) as a protective scale. The thickness of the duplex scale, consisting of Fe-containing oxides, increases slowly from 2 to 4 lm, at 400 °C, to around 5 lm at 500 °C and reaches a maximum of approximately 15 lm (P6: Fe-6Cr-8Al) after exposure at 600 °C. It should be mentioned that the exposure time at 600 °C was twice as high as those at lower temperatures. Fig. 18 shows the thickness of the (Fe1 x yCrx Aly)3O4 sub-layer, formed on P1–P7 and P10 samples during exposure to molten lead (with 10 6 wt.% oxygen), at 400 °C, 500 °C and
Fig. 19. Oxide ‘‘map’’ for the oxidation of Fe–Cr–Al-based model alloys exposed to oxygen containing molten lead, in the temperature range 400–600 °C, containing also data from literature: line at 800 °C [27] and line at 1000 °C [32] (in oxygen atmosphere), R1s [34], R2 [35] and R3 [44] (in lead and lead–bismuth, respectively).
600 °C. As can be observed, the thickness of this sub-layer is lower above certain Cr content (e.g. P5: Fe-16Cr-4Al). One of the two samples forming an alumina scale, namely P8 (Fe-12Cr-7Al), was found to be also covered by a duplex layer (magnetite + spinel) on a significant area fraction (20–40%). This behaviour could be attributed to small reduction in that region of Al concentration below a critical level, where a selective oxidation of aluminium occurs. Based on the above described results, the stability domain of alumina has been estimated for the Fe–Cr–Al alloy system, exposed to molten lead containing 10 6 wt.% oxygen, over 400– 600 °C temperature range (Fig. 19). The P9 (Fe-16Cr-6Al) sample was included in the alumina stability domain, while the P8 sample was located on the boundary between the alumina stability domain and the Fe–Cr-based oxide domain. As can be seen, the Al concentration, critical in terms of a selective oxidation to alumina, must be increased when the exposure to oxidising environment is performed at lower temperatures. This is also in
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concordance with the findings reported in literature for exposures at 800 °C [27] and 1000 °C [32] to oxygen atmosphere. Another observation is related to the Cr concentration: higher Cr content leads to the alumina formation at lower Al concentration (TEE). The ternary diagram in Fig. 19 shows additional points R1s, R2 and R3 reported in literature, [34,44,35] respectively. Alumina forming R1s coatings, as well as R2 (ODS) and R3 (Kanthal AF) alloys, when exposed to molten, oxygen containing Pb and Pb–Bi-eutectic, respectively, belong to the Fe–Cr–Al-RE system. RE is the so called reactive element. The exposure temperatures for these three references were 400–550 °C (R1s), 650–700 °C (R2 for 10.000 h) and 500–750 °C (R3). The results are consistent with our findings: R1s and R3 alloys are inside of alumina stability domain and R2 is located between the border lines of the alumina stability domain at 800 °C [27] and 600 °C (our results). RE elements (yttrium in case of R1s, hafnium in case of R2 and silicon in case of R3) are added in small quantities (<1 wt.%) because, as experimentally proved, they reduce the scale growth rate and improve the scale/substrate adherence [45]. Whether the addition of reactive elements, either individually or in combination may influence the morphology, the microstructure of the alumina scale, the growth rate, or may facilitate the nucleation of alumina scale, when Fe–Cr–Al-RE alloys are exposed to oxygen containing HLMs, at temperature relevant for applications, has to be further addressed. The alumina scales, grown on P8 and P9 samples, are transient polymorphs j-Al2O3 (at 400 and 500 °C) and h-Al2O3 (at 600 °C). These metastable alumina types are known to be formed in the early stages of the oxidation process, during exposure in oxygencontaining gas atmosphere around 900 °C, and have a higher growth rate than a-Al2O3. Later on, these polymorphs transform into stable alumina [46]. Also it is known that the transformation of c-Al2O3 into a-Al2O3 implies a volumetric change of around 14% [47]. These phenomena, accompanying the formation, growth and transformation of transient alumina, are detrimental due to the accumulation of stresses and defects in the oxide scale, which can weaken its adherence to the underlying substrate, leading to spallation. For the temperature range, oxygen concentration in the molten lead and exposure time used during the current evaluation, the growth rate of the alumina scale was low. It seems that a limited metal oxidation occurs in these experimental conditions. No area with detached scale was observed and no trace of a-Al2O3 was detected. However, to assess the stability and properties of the alumina scale formed on Fe–Cr–Al-based model alloys, exposed to oxygen containing lead, experiments with longer exposure time (e.g. 5000 h) are underway.
5. Conclusions 1. Chromium and aluminium contents of 12.5–17 wt.% and 6– 7.5 wt.%, respectively, are high enough to obtain thin, stable and protective alumina scales on Fe–Cr–Al-based alloys exposed to oxygen containing lead at 400, 500 and 600 °C. 2. Lower aluminium concentrations leads to the formation of Febased spinel-type oxides protrusions on extended area. However, this can also happen at high aluminium concentration (7.6 wt.%) if the chromium content is lower than 10.4 wt.%. 3. The alumina stability domain border shifts with lower temperatures to higher chromium and aluminium concentrations. 4. The influence of the model alloys grain size, of the addition of reactive elements and of longer exposure time on the alumina scale polymorphs type, growth, stability and properties is currently under evaluation.
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Acknowledgments Financial support by GETMAT (FP7-212175) and LEADER (FP7249668) within the EU-7th Framework program is gratefully acknowledged. The authors wish to tank Prof. Victor Geanta (University Politechnica of Bucharest) for providing high quality alloys.
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