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Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys exposed to air at 870 K studied by TMS, CEMS and XPS
Physica B: Physics of Condensed Matter 528 (2018) 27–36
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Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys exposed to air at 870 K studied by TMS, CEMS and XPS R. Idczak a, b, *, K. Idczak a, R. Konieczny a a b
Institute of Experimental Physics, University of Wrocław, pl. M. Borna 9, 50-204 Wrocław, Poland Institute of Low Temperatures and Structure Research, Polish Academy of Sciences, ul. Okolna 2, 50-422 Wrocław, Poland
A R T I C L E I N F O Keywords: M€ ossbauer spectroscopy X-ray photoelectron spectroscopy Fe-Cr alloys Surface segregation Oxidation
A B S T R A C T
The room temperature 57Fe M€ ossbauer and XPS spectra were measured for polycrystalline Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys which were exposed to air at 870 K. The spectra were collected using three techniques: the transmission M€ ossbauer spectroscopy (TMS), the conversion electron M€ ossbauer spectroscopy (CEMS) and the Xray photoelectron spectroscopy (XPS). The combination of these experimental methods allows to determine changes in atomic composition in the bulk, in the subsurface layer and on the surface of the studied alloys. The results obtained by XPS confirm the strong chromium segregation process on the surface of Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys. At the same time, the CEMS and TMS measurements reveal that in both samples, during the exposure to air at 870 K, iron oxides were formed. These findings lead to conclusion that the formation of a stable Cr oxide film on the surface of Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys, does not prevent atmospheric corrosion at 870 K. Moreover, the observed by CEMS and TMS, oxidation process of iron atoms is slower for Fe0.88Cr0.12 than for Fe0.85Cr0.15. This fact is in agreement with XPS results which show that Cr surface segregation process is more effective in case of Fe0.88Cr0.12.
1. Introduction This work concerns determination of changes in chemical composition and the presence of various oxides in the bulk, in the subsurface layer and on the surface of the Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys which were exposed to air at 870 K. The obtained data provide information about oxidation and surface segregation processes in studied materials. Motivation for these studies is fact, that ferritic/martensitic steels, containing up to 15 at.% of Cr, are candidate materials for advanced fission reactors (Generation IV), accelerator-driven systems using spallation neutron sources and fusion reactors [1,2]. These types of steels are able to perform reliably for long time under high irradiation levels (total dose of 50–200 displacements per atom (dpa)) and at high temperatures (up to 900 K). Moreover, high-chromium steels are known for their excellent corrosion resistant properties. These properties originate from the formation, on the alloy surface, a stable Cr oxide film, which prevents further oxidation. On the atomic scale, the necessary supply of Cr atoms on the surface should be provided by a surface segregation process. The majority of former, experimental studies on oxidation and surface segregation of iron-based alloys were performed on single crystals or under controlled oxygen exposure [3–5]. Taking into account that these
alloys are one of the most important engineering materials, it is much more appropriate to investigate a polycrystalline samples which are relatively cheap and easy to produce. For the same reason, the study of oxidation processes caused by direct contact with atmospheric oxygen more closely resembles the conditions in which that type of construction materials are utilized. More recent studies performed by surface-sensitive experimental techniques as well as theoretical calculations confirm Cr surface segregation processes in polycrystalline Fe-Cr alloys [6–9]. For example, A. Kuronen et al. [8] showed Cr segregation and precipitation in the Fe/Cr double layer and Fe0.95Cr0.05 and Fe0.85Cr0.15 alloys using hard X-ray photoelectron spectroscopy and Auger electron spectroscopy supported by ab initio (CPA-EMTO) calculations. Moreover, initial oxidation of Fe-Cr was investigated experimentally at 108 Torr pressure of the spectrometers showing intense Cr2O3 signal. In our previous paper on iron-chromium alloys [9], we used the combination of three experimental techniques: the transmission M€ ossbauer spectroscopy (TMS), the conversion electron M€ ossbauer spectroscopy (CEMS) and the X-ray photoelectron spectroscopy (XPS), to determine changes in Cr concentration and the presence of various oxides in the bulk (TMS), in the subsurface layer (CEMS) and on the surface (XPS) of polycrystalline Fe–Cr alloys. The studied samples were obtained by simple melting
* Corresponding author. Institute of Experimental Physics, University of Wrocław, pl. M. Borna 9, 50-204 Wrocław, Poland. E-mail address: [email protected] (R. Idczak). https://doi.org/10.1016/j.physb.2017.10.082 Received 31 July 2017; Received in revised form 15 October 2017; Accepted 18 October 2017 Available online 22 October 2017 0921-4526/© 2017 Elsevier B.V. All rights reserved.
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at a photoelectron take-off angle of 90 at room temperature. The sampling depth SD which is defined as the depth from which 95% of all photoelectrons are scattered by the time they reach the surface is equal:
process, annealed in the vacuum and exposed to air at room temperature for 30 days. The XPS measurements revealed the strong segregation process of Cr atoms towards the surface. In particular, the atomic concentration of Cr on the surface of prepared Fe0.88Cr0.12 alloy was almost two and a half times larger than Fe concentration. Moreover, a slightly depletion of Cr atoms in the subsurface region, observed by CEMS technique, also confirms the surface segregation process. Additionally, the XPS measurements revealed the presence of Cr and Fe oxides at the alloy surface, while CEMS as well as TMS spectra gave a clear evidence that iron atoms located in deeper parts of the alloy were not oxidised. Consequently, it can be concluded that direct contact of Fe-Cr alloys with atmospheric oxygen at room temperature leads to formation of thin oxide layer, mainly composed of Cr2O3, which prevent a further oxidation process. On the other side, the TMS investigation of Fe-Cr alloys with 3 and 6 at.% of Cr showed that exposure to air at temperatures above 570 K, leads to systematic growth of iron oxides in the bulk [10]. That result suggests that Fe0.97Cr0.03 and Fe0.94Cr0.06 alloys are not corrosion resistant at high temperatures. However, as one can notice in Ref. [10], the oxidation of iron atoms slows with increasing of Cr content. Taking the above into account, in this work, the oxidation process in two Fe-Cr alloys with much higher Cr content (12 and 15 at.%) were studied using XPS, CEMS and TMS. Having in mind that all recent experimental studies by surface-sensitive techniques were performed on Fe-Cr samples which are not oxidised at high temperatures [7–9], this time, before the XPS measurements, the alloys were exposed to air at 870 K for 2 h. In order to obtain a valuable information about the surface oxidation process at high temperature, more detailed information on the chemical state of Cr, Fe, O and C atoms was derived from the deconvoluted Fe 2p, Cr 2p, O 1s and C 1s spectra. Thanks to that, the XPS results compared with CEMS and TMS should give a better understanding of high-temperature, atmospheric corrosion of Fe-Cr alloys.
SD ¼ 3λFe ¼ 3.95 nm,
(1)
where λFe is the inelastic mean free path (IMFP) in iron [13]. Taking the above into account, the estimated surface layer thickness observed in XPS measurements is close to 4 nm. Before each measurement the oxidised samples were firstly exposed to argon ion bombardment for 210 min (in cycles) using 3.8 keV ions and an average current density of 3.1 μA/cm2. The angle of incidence was 90 to the surface. After Arþ sputtering the samples were annealed at the temperature range of 870–1270 K for 5 min. Typical pressure during sample heating grows up to 5⋅106 Pa. The temperature was measured by optical pyrometer, with an accuracy of 20 K. 2.3. Spectra analysis Each measured TMS and CEMS spectrum was analysed in terms of a sum of various number of six-line patterns (sextets) corresponding to different chemical state of 57Fe M€ ossbauer probes as well as to various hyperfine fields B at 57Fe nuclei generated by different numbers of Fe and Cr atoms located in the first two coordination shells of the probing nuclei. The fitting procedure was done under the assumption that the influence of Cr atoms on B as well as the corresponding isomer shift IS on a subspectrum, is additive and independent of the atom positions in the first two coordination shells of the nuclear probe. In other words it was accepted that for each subspectrum the quantities B and IS are linear functions of the numbers n1 and n2 of Cr atoms located, respectively, in the first and second coordination shells of 57Fe and the functions can be written as follows:
2. Experimental
B(n1,n2) ¼ B0 þ n1ΔB1 þ n2ΔB2,
2.1. Samples preparation
IS(n1,n2) ¼ IS0 þ n1ΔIS1 þ n2ΔIS2,
(2)
where ΔB1 (ΔIS1) and ΔB2 (ΔIS2) stand for the changes of B (IS) with one Cr atom in the first and the second coordination shell of the M€ ossbauer probe, respectively. For each six-line pattern, the two line area ratio I16/ I34 was constant and equal to 3/1. Taking into account that in studied samples a strong texture could be formed during preparation procedure (cold-rolling and annealing), the ratio I25/I34 was a free parameter. At the same time, we assume that the quadrupole splitting QS in a cubic lattice is equal to zero. The number of six-line patterns fitted to CEMS and TMS spectra of annealed Fe0.88Cr0.12 and Fe0.85Cr0.15 samples was 9 and 12, respectively. The relatively large number of sextets is connected with high chromium content in studied binary alloys and due to that many possible local configurations of Cr atoms in the first two coordination shells of 57Fe atom have to be taken into consideration. Theoretically the probabilities of atomic configuration for the random Fe1-xCrx b.c.c. alloy can be calculated from the formula:
Polycrystalline samples of Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys were prepared from pure elements Fe: 99.98% and Cr: 99.995%. A synthesis was carried out using an arc-melting furnace under a Ti-gettered purified argon atmosphere. Weight losses during the melting process were negligible (about 0.2%). Resulting ingots were cold-rolled to the final thickness of about 0.03 mm for TMS and 0.10 mm for CEMS and XPS measurements. In the next step, the foils were annealed in the vacuum at 1270 K for 2 h. The base pressure during the annealing procedure was lower than 104 Pa. To obtain homogeneous and defect free samples [11], after annealing process, the foils were slowly cooled to room temperature during 6 h. The annealed samples were exposed to air at 870 K for 2 h. Finally, to obtain information about diffusion of O atoms in iron matrix, the samples of Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys which were prepared for TMS measurements, were subsequently exposed to air at 870 K for additional several hours. To shorten the description, in the further parts of this article, the samples which were only annealed in the vacuum will be referred as “annealed” and the samples which were subsequently exposed to air will be referred as “oxidised”.
Pðn1 ; n2 ; xÞ ¼
8 n1
6 xn1 þn2 ð1 xÞ14n1 n2 ; n2
(3)
where x stands for the atomic concentration of Cr atoms. Most of the possible configurations have vanishingly small probabilities and can therefore be neglected. In practice one usually takes into account only the most probable ones. In this work, it was assumed that configurations only with P > 3% could be reliably deconvoluted form the M€ ossbauer spectra. The configurations which fulfill this condition are P(0,0), P(1,0), P(0,1), P(2,0), P(1,1), P(0,2), P(3,0), P(2,1), P(1,2) for Fe0.88Cr0.12 alloy and P(0,0), P(1,0), P(0,1), P(2,0), P(1,1), P(0,2), P(3,0), P(2,1), P(1,2), P(0,3), P(3,1) and P(2,2) for Fe0.85Cr0.15 alloy. Finally, in some spectra obtained for oxidised samples, the additional magnetic components were observed. These subspectra were attributed to iron oxides (see Section 3.2 and 3.3.). The fits obtained under these assumptions are presented
2.2. Measurements The room temperature CEMS and TMS measurements were performed by means of a constant-acceleration POLON spectrometer of standard design, using a 3.7 GBq 57Co-in-Rh standard source with a full width at half maximum (FWHM) of 0.22 mm/s. The XPS measurements were performed in ultra high vacuum apparatus (with a pressure about 4⋅108 Pa) equipped with the XPS analyzer (Phoibos 150) with Mg Kα and Al Kα X-ray sources. The XPS spectrometer was calibrated to yield the standard values of the Au 4f doublet (binding energy at 84.0 eV for 4f7/2 and 87.7 eV for 4f5/2 [12]) for the clean gold sample. All scans were taken 28
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in Figs. 1–4. The XPS analysis was used to investigate the chemical composition and bonding environment on the alloy surface. The obtained spectra were analysed using the CasaXPS program: the chosen fitting method was the Gaussian-Lorentzian (GL(30%)) and the background of spectra was subtracted using a software based on Shirley method. The atomic concentration N in the surface region was estimated by a standard calculation: Ii
Ni ¼ Pσi λiIj ⋅100%;
(4)
σ j λj
where I is a characteristic (Fe 2p3/2, Cr 2p, O 1s or C 1s) XPS peak intensity, σ is a photoionization cross section (Scofield parameter) [14] and λ is an inelastic mean free path of electrons with kinetic energy of the reference line [13]. 3. Results and discussion 3.1. XPS results Fig. 5 presents a selected XPS spectra for oxidised Fe0.88Cr0.12 (a-d) and Fe0.85Cr0.15 (e-h) alloys just after inserting into the vacuum chamber. For both oxidised alloys, the estimated surface atomic concentration is quite similar and equal to 11% of Fe, 3% of Cr, 47% of O and 39% of C atoms. For more detailed information, the selected Fe 2p, Cr 2p, O 1s and C 1s XPS spectra analysis was done. The Fe 2p high resolution core level line (see Fig. 5a and e) were fitted using Gupta-Sen (GS) multiplet peaks [15–18]. In this case five components, corresponding to various iron oxides, can be distinguished. The first peak at binding energy (BE) of 709.4 eV represents Fe2þ and Fe3þ species [13]. Second, third, fourth and fifth peaks at BE equal to: 710.5 eV, 711.7 eV, 713.0 eV and 714.4 eV respectively, correspond to Fe3þ species [18]. The Cr 2p doublet can be divided into two components for both alloys but distinguished peaks are located at the different BE. For Fe0.88Cr0.12 alloy (see Fig. 5b) positions of peaks maximum are 576.8 eV and 578.1 eV, while for Fe0.85Cr0.15 alloy (see Fig. 5f) there are 577.2 eV and 578.8 eV. According to [19] it can be
Fig. 2. The 57Fe transmission M€ ossbauer spectra for Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys measured at room temperature.
Fig. 3. The 57Fe transmission M€ ossbauer spectra for Fe0.88Cr0.12 alloy exposed to air at 870 K for several hours measured at room temperature.
suggested that these components correspond to Cr2O3. Further analysis are made for O 1s and C 1s spectra. The O 1s spectra (see Fig. 5c and g), for both samples, are composed of three components which represent signals from oxygen in oxide (O2) at BE of 530.6 eV, water at BE of 532.5 eV and carbon-oxygen bonds at BE of 534.1 eV [15,18]. Similarly, the C 1 spectra consist three components, at BE of 286.0 eV, 287.3 eV and 289.2 eV (the first one is connected with carbon contamination while the next two represent the carbon-oxygen bonds [20]).
Fig. 1. The 57Fe conversion electron M€ ossbauer spectra for the Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys measured at room temperature. 29
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is connected with the presence of water. Significant changes can be observed for C 1s spectra for both alloys (see Fig. 7d and h). The XPS signal intensity, after ion sputtering for 210 min, decreases resulting in change of carbon concentration to 6%. Due to low signal-to-noise ratio a detailed deconvolution cannot be made. However, it can be noticed that the peak maximum shifts to 285.5 eV. After Arþ sputtering, samples were annealed at various temperatures. Figs. 8 and 9 present selected XPS spectra for Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys after annealing at 890 K, 1070 K and 1270 K. The first group of spectra (see Fig. 8a) consists of Fe 2p3/2, Cr 2p, O 1s and C 1s peaks for Fe0.88Cr0.12 sample after annealing at 890 K. Atomic surface concentration and presented core level line shapes do not change significantly but some differences in selected components can be observed. In case of Fe 2p3/2 spectrum (Fig. 8a), there are six peaks at BE of: 708.1 eV, 709.4 eV, 710.7 eV, 712.1 eV, 713.6 eV and 715.6 eV. First three components correspond to the Fe2þ species in Fe3O4 [17] and simultaneously components from second to fifth correspond to the Fe3þ species in Fe3O4 and Fe2O3 [17]. Similar to results for the alloy after sputtering, the last peak represents satellite Fe2þ species. For Cr 2p and C 1s peaks a low signal-to-noise ratio does not allow to perform a reliable deconvolution. The maximum of signal intensity for presented core level lines (Fig. 8a) are located at BE of 577.3 eV for chromium and 284.7 eV for carbon. The O 1s spectrum consist of three components at the position of 530.5 eV, 531.6 eV and 533.0 eV representing various oxygen species which was observed previously. Annealing Fe0.88Cr0.12 alloy at 1070 K (Fig. 8b) caused the contamination desorption and, as a consequence, an increase of surface atomic concentration of alloy components (40% for Fe and 3% for Cr atoms). Significant changes are observed in the Fe 2p3/2 spectrum (Fig. 8b), where the new peak appears at BE of 706.8 eV, which represent the presence of metallic iron [15]. Others components correspond to Fe2O3 and Fe3O4 compound [15]. Worth noting is that in the Cr 2p spectrum (Fig. 8b) two components can be distinguish: first at the BE of 576.5 eV and second at the BE of 577.7 eV. Both of them represent the Cr3þ species from the Cr2O3 [19]. In case of O 1s and C 1s spectra (Fig. 8b), only a small decrease in signal intensities was observed. The last group of spectra presents results for Fe0.88Cr0.12 alloy surface after sample annealing at 1270 K (Fig. 8c). It can be easily seen, that Fe 2p3/2 line becomes thinner while its signal intensity decreases (instead of increasing). Contrary results can be made from Cr 2p spectrum, for which widening of core level line and increasing in signal intensity was noticed. In a consequence, Fe 2p3/2 spectrum (Fig. 8c) is composed of four components at BE of: 707.2 eV, 708.6 eV, 710.1 eV and 711.8 eV. The first represents the metallic iron while the others iron-oxide bonds. In case of Cr 2p core level line (Fig. 8c), five components have been distinguished, for which positions of peaks maximum are: 575.2 eV, 576.8 eV, 577.8 eV, 579.0 eV and 580.4 eV. All designated peaks represent Cr-O bonds, due to a multiplet splitting described by M. C. Besinger in Ref. [24]; the first four peaks correspond to Cr3þ species in Cr2O3 compound and the fifth (with the highest BE) correspond to Cr6þ species in CrO3 compound. The estimated atomic surface concentration for the alloy is: 13% of Fe, 27% of Cr, 59% of O and 1% of C atoms and the chromium to iron ratio is close to 2. That result reveals a strong chromium segregation on the surface of Fe0.88Cr0.12 alloy. Despite sample high temperature treatment, signal from the oxygen atoms in the surface region is still significant. The same as after alloy annealing at lower temperatures, O 1s spectrum (Fig. 8c) consists of three components at BE of 531.3 eV, 532.3 eV and 533.6 eV. Analysis of described peaks positions and comparison with distinguished in Fe 2p3/2 and Cr 2p spectra iron-oxide and chromium-oxide bonds allows to confirm the presence of the Cr2O3 and Fe2O3 compounds on the oxidised alloy surface. With good agreement to [24] the first two peaks represent the O-Cr bond in Cr2O3 and the remaining three peaks represent the O-Fe bond in Fe2O3. Fig. 9 presents selected XPS spectra for Fe0.85Cr0.15 alloy after annealing at 890 K, 1070 K and 1270 K. Estimated surface atomic concentration for sample after annealing at 890 K is: 24% of Fe, 9% of Cr,
Fig. 4. The 57Fe transmission M€ ossbauer spectra for Fe0.85Cr0.15 alloy exposed to air at 870 K for several hours measured at room temperature.
Fig. 6 presents the surface atomic concentration after a sequence of Ar-sputtering cycles, 0–210 min. The depth scale was calculated using the sputtering rate of 0.344 nm/min. This value was estimated using the measured average current density of 3.1 μA/cm2 and the average of sputtering rates for iron and chromium oxides which were determined experimentally in works [21–23]. Here, it is worth noting that due to rather complex surface chemical composition of the oxidised samples, the assumed sputtering rate of 0.344 nm/min can only be regarded as approximate. Despite this, it can be clearly seen that during the argon ion sputtering at room temperature the carbon contamination decreases significantly. According to that, the contribution of other elements increases. In case of the Fe0.88Cr0.12 alloy (see Fig. 6a), after sputtering for 30 min, the surface iron concentration changed from 11% to 18%, and then increased to 31% after sputtering for 210 min. Oxygen atoms contribution in a measured surface region achieved 59% and remain constant after further sputtering. Chromium concentration increased to 5%, after sputtering for 60 min and then decreased to 3% after subsequent cycles of sputtering. In case of Fe0.85Cr0.15 alloy (see Fig. 6b), iron and oxygen surface atomic contributions increased up to 29% of Fe atoms and 59% of O atoms, while chromium atomic concentration increased to 5% after surface sputtering for 90 min and remain constant for further cycles. Fig. 7 presents a high resolution selected XPS spectra for both alloys after ion sputtering for 210 min. Taking into account, the observed line shapes and the full width at half maximum (FWHM) of selected peaks, the multiple peaks analysis method was performed for detailed interpretation of the oxidation state of the alloys. The Fe 2p spectra for Fe0.88Cr0.12 (Fig. 7a–d) and Fe0.85Cr0.15 (Fig. 7e–h) reveal a nonsymmetric line shape with the FWHM about 4.5 eV. The Fe 2p3/2 peak (see Fig. 7a and e) is composed of five components which represent Fe2þ species and Fe3þ species [17] while Cr 2p core level line (see Fig. 7b and f) consists of two peaks which correspond to Cr3þ species [19]. Worth noting is that observed component in the Fe 2p3/2 spectrum at the BE of 716.0 eV represents the Fe2þ satellite peak [15]. The O 1s spectrum (see Fig. 7c and g) exhibits three components. The main peak, at BE of 530.6 eV represents the O2 species, next at BE of 531.6 eV corresponds to signal from the defective oxide [24] and the last peak at BE of 532.9 eV
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Fig. 5. XPS selected spectra (Fe 2p, Cr 2p, O 1s and C 1s) for Fe0.88Cr0.12 (a–d) and Fe0.85Cr0.15 (e–h) oxidised alloys after inserting into the UHV chamber.
Fe0.88Cr0.12 alloy (see Fig. 8a). Moreover, in contrast to analysis for Fe0.88Cr0.12 sample, there can be made a deconvolution of Cr 2p core level line. It was found that Cr 2p spectrum (Fig. 9a) consists of three peaks at BE of: 576.7 eV, 578.0 eV and 579.5 eV. All of them represent Cr3þ species in Cr2O3 compound [24]. In case of O 1s spectrum (Fig. 9a), due to FWHM decreasing, only two components at BE of 530.8 eV and 532.5 eV, which correspond to O2 species, could be distinguished. The C 1s spectrum (Fig. 9a) is composed of two peaks with maximum positions at 285.1 eV and 287.2 eV. The first one corresponds to standard C-C and the second represents C-O bonds [20]. The second group of spectra (Fig. 9b) presents results for Fe0.85Cr0.15 alloy after annealing at 1070 K. From the atomic concentration calculations (Eq. (4)) it can be easily seen that chromium segregation occurs. In the surface region the composition of elements is: 15% of Fe, 25% of Cr, 56% of O and 4% of C atoms. The Fe 2p3/2 spectrum (Fig. 9b) can be deconvoluted into five components. First peak, which position is located at 706.8 eV, corresponds to metallic iron, while the others four components at BE of 707.5 eV, 708.6 eV, 710.2 eV and 711.9 eV represent Fe2þ and Fe3þ species [17]. Signal intensity of Cr 2p spectrum (Fig. 9b) allows to distinguish four components, from which the first three (at BE of 575.3 eV, 577.0 eV, 578.1 eV) correspond to Cr3þ species [24], while the fourth one at BE of 579.5 eV is connected with Cr6þ species in CrO3 compound [24,25].
Fig. 6. Surface atomic concentration for: a) Fe0.88Cr0.12 and b) Fe0.85Cr0.15 oxidised alloys after subsequent Ar ion sputtering process.
61% of O and 6% of C atoms. In the Fe 2p3/2 spectrum (Fig. 9a), beside observed previously five components, a new peak appears on the lower binding energy side of spectrum. This new component with a BE of 707.2 eV represents metallic iron, which was not observed for
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Fig. 7. Selected XPS spectra (Fe 2p3/2, Cr 2p, O 1s and C 1s) for Fe0.88Cr0.12 (a–d) and Fe0.85Cr0.15 (e–h) oxidised alloys after Ar ion sputtering for 210 min.
3.2. CEMS results
This Cr-O bond was not detected for Fe0.88Cr0.12 alloy. The O 1s and C 1s spectra showed in Fig. 9b, reveal decrease in signal intensities and a small shift of the peaks maximum to higher BE. Further annealing at 1270 K causes a contamination desorption. Signal from carbon atoms is not detected and contribution of oxygen atoms decreases to 45% (Fig. 9c). The O 1s spectrum consists of three peaks at BE of 530.5 eV, 531.5 eV and 533.0 eV; which correspond to O-Fe and O-Cr bonds [24]. The calculated concentration of alloy elements is: 27% of Fe and 27% of Cr atoms. Fe 2p3/2 core level line (Fig. 9c) is composed of three peaks from which the main at the lowest BE (707.1 eV) represents metallic Fe while the next two correspond to various Fe – O bonds (708.4 eV and 710.2 eV). Detailed analysis of Cr 2p spectrum (Fig. 9c) allow to distinguish three components (576.5 eV, 577.6 eV and 579.0 eV) representing Cr-O bonds and an additional peak with BE of 574.8 eV which correspond to metallic chromium [26]. Summarizing, the XPS results show that in case of Fe0.85Cr0.15 alloy the chromium segregation occurs at lower temperatures then in Fe0.88Cr0.12 alloy. However, this process is more effective for alloy with less chromium content; Crtotal:Fetotal ratio for Fe0.88Cr0.12 and Fe0.85Cr0.15 samples after annealing at 1270 K are equal to 2.08 and 1.03, respectively. Moreover, the XPS spectra obtained for alloys annealed at the highest temperature reveal a different oxidation states. For the sample with lower Cr content, on the surface some amount of metallic iron were detected but none of metallic chromium (see Fig. 8c). In case of Fe0.85Cr0.15 alloy, the XPS analysis of surface region allow to distinguish in chromium and iron spectra a metallic components.
The estimated parameters of the hyperfine field B, quadrupole splitting QS and isomer shift IS obtained for Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys annealed and oxidised are listed in Table 1. As one can see on Fig. 1, the CEMS spectra measured for annealed samples are quite different from the spectra obtained for oxidised alloys. In case of annealed samples, both spectra consist only six-line patterns with hyperfine parameters similar to those obtained for single phase Fe-Cr alloy with b.c.c. structure [9,27]. For oxidised samples, the additional magnetic-split component was observed. The estimated for this subspectrum hyperfine parameters, which are listed in Table 1, correspond to α-Fe2O3 [28]. At the same time, in all measured CEMS spectra, there is no other components which could be connected with FeO, Fe3O4, FeOOH, FeCrO3 or iron carbides [29–33]. This fact gives an important information that in subsurface layer of polycrystalline Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys exposed to air at 870 K, only one type of iron oxide was formed. Moreover, the results of CEMS spectra analysis give detailed information about chemical composition of subsurface layer. The relative areas I of components for each spectrum were determined. Assuming that the relative ratio of the Lamb-M€ ossbauer factors fα-Fe: fα-Fe2O3 is equal to 1: 1.08 [34], the fraction ci of absorbing atoms corresponding to the ith component can be easily calculated using Ii and fi values: Ii f
ci ¼ Pi Ii ⋅100%: i
(5)
fi
Distributions of the hyperfine fields produced using determined ci 32
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Fig. 8. Selected XPS spectra (Fe 2p3/2, Cr 2p, C 1s and O 1s) for Fe0.88Cr0.12 oxidised alloy after annealing at: a) 890 K, b) 1070 K and c) 1270 K.
That drastic changes in M€ ossbauer probe concentration in the subsurface region could have a significant influence of the CEMS signal depth profile and due to that, the statement (correct for a pure iron) that more than 50% of the signal comes from the first 50 nm is no longer valid in case of studied oxidised alloys. Taking the above into account, it seems reasonable to assume that the obtained CEMS data show the iron oxidation process in deeper part of the material than XPS measurements. Therefore, the CEMS spectra reveal the presence of large number of unoxidised iron atoms which are located below the oxide layer.
values were presented in Fig. 10. At the same time, ci values were used to calculate c(alloy) and c(Fe2O3) parameters which are the fractions of 57Fe nuclear probes located in Fe-Cr alloy with b.c.c. structure and α-Fe2O3, respectively. As one can notice, the estimated for studied samples c(alloy) and c(Fe2O3) parameters, which are listed in Table 2, show that exposure of polycrystalline Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys to air at 870 K for 2 h results in growth of α-Fe2O3 and the iron oxidation process is faster in case of Fe0.85Cr0.15 sample. Here, it is worth noting that the probing depth of the CEMS technique for pure iron is up to 300 nm however due to the attenuation of the conversion electron beam, more than 50% of the signal comes from the first 50 nm. Taking this fact into account, it seems that the CEMS data which reveal the presence of only 16% (Fe0.88Cr0.12) and 56% (Fe0.85Cr0.15) of oxidised iron atoms in the subsurface layer of the oxidised samples do not correspond with the XPS results. In particular, the calculated surface atomic concentration for oxidised alloys after subsequent Ar ion sputtering process (Fig. 6) as well as the absence of component which corresponds to metallic iron in the Fe 2p3/2 spectra measured for oxidised samples before and after sputtering give a clear evidence that up to 70 nm, all or almost all of the iron atoms are oxidised. That effect could be explained by the fact that the surface of oxidised alloys is mainly composed of carbon as well as Cr and Fe oxides. Due to that the electron attenuation coefficient for oxidised sample should be much lower than for a pure iron and the CEMS probing depth is probably much greater than 300 nm. At the same time, as one can notice in Fig. 6, the iron concentration in the surface layer is close to 10% and increases to about 30% after ion sputtering. Moreover, below the oxide layer the iron content should be close to 88% (Fe0.88Cr0.12) and 85% (Fe0.85Cr0.15).
3.3. TMS results The analysis of the measured TMS spectra reveals existence of components which could be assigned only to single phase Fe-Cr alloy with b.c.c. structure. The estimated parameters of the hyperfine field B and isomer shift IS obtained for Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys annealed and exposed to air at 870 K for 2 h are similar to those presented in Table 1 (Fe-Cr alloy). The XPS, TMS and CEMS results obtained for the same samples suggest that the formation of a stable oxide film on the surface and subsurface layer of Fe-Cr alloys, prevents further oxidation deep into the bulk. To confirm that hypothesis, it was decided to measure TMS spectra for Fe0.88Cr0.12 and Fe0.85Cr0.15 samples which were exposed to air at 870 K for additional periods of time. The obtained TMS spectra, which are presented on Figs. 3 and 4, reveal the presence of additional magnetic-split components. The estimated for these subspectra hyperfine parameters correspond to α-Fe2O3 (B ¼ 51.7(2) T, IS ¼ 0.38(2) mm/s, QS ¼ 0.20(5) mm/s) and Fe3O4 compounds (Fe3O4 (Feþ3) - B ¼ 49.1(2) 33
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Fig. 9. Selected XPS spectra (Fe 2p3/2, Cr 2p, C 1s and O 1s) for Fe0.85Cr0.15 oxidised alloy after annealing at: a) 890 K, b) 1070 K and c) 1270 K.
Table 1 Some parameters of the assumed model fitted to the room temperature CEMS spectra measured for Fe0.88Cr0.12 and Fe0.85Cr0.15. Values of the isomer shifts IS0 and IS are reported relative to the corresponding value for α-Fe at room temperature. Sample
Fe0.88Cr0.12 annealed Fe0.88Cr0.12 oxidised Fe0.85Cr0.15 annealed Fe0.85Cr0.15 oxidised
α-Fe2O3
Fe-Cr alloy B0 [T]
IS0 [mm/s]
ΔB1 [T]
ΔIS1 [mm/s]
ΔB2 [T]
ΔIS2 [mm/s]
B [T]
IS [mm/s]
QS [mm/s]
34.1(1)
0.01(1)
3.3(1)
0.02(1)
2.2(1)
0.01(1)
–
–
–
34.0(1)
0.00(1)
3.4(1)
0.02(1)
2.3(1)
0.01(1)
51.4(5)
0.37(4)
0.19(6)
33.9(1)
0.01(1)
3.6(3)
0.02(1)
2.4(2)
0.01(1)
–
–
–
33.9(1)
0.00(1)
3.3(1)
0.01(1)
2.2(1)
0.02(1)
51.9(1)
0.37(1)
0.17(2)
T, IS ¼ 0.32(2) mm/s, QS ¼ 0.04(4) mm/s and Fe3O4 (Feþ2,þ3) B ¼ 45.8(2) T, IS ¼ 0.61(2) mm/s, QS ¼ 0.02(5) mm/s) [28,30]. The presence of iron oxides in bulk of Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys which were exposed to air at 870 K for 4, 6 and 12 h proves that the formation of a stable oxide film on the surface of those alloys does not prevent further oxidation of iron atoms located in deeper parts of studied materials. Finally, the calculated c(alloy), c(Fe2O3) and c(Fe3O4) parameters, presented in Fig. 11, shows that the iron oxidation process is faster in case of Fe0.85Cr0.15 sample. This finding is in agreement with CEMS results and could be explained by the surface segregation process of Cr atoms which was observed in XPS measurements. In both studied alloys, the XPS results give a clear evidence that Cr segregation occurs. If we assume that in Fe-Cr alloys this process leads to formation of a
passivation layer, which significantly reduce the oxygen diffusion deep into the bulk of material then in case of Fe0.88Cr0.12, for which the segregation process is more effective, the oxidation of iron atoms should be slower. 4. Conclusions The combination of three experimental techniques allows to determine changes in atomic composition and the presence of various oxides in the bulk (TMS), in the subsurface layer (CEMS) and on the surface (XPS) of the studied Fe-Cr alloys. As one can expect, the XPS measurements give a clear evidence that the surfaces of the oxidised samples are composed mainly of oxygen and carbon. After Ar ion sputtering and
34
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Physica B: Physics of Condensed Matter 528 (2018) 27–36
Fig. 10. Hyperfine field distributions obtained from CEMS spectra measured for Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys after annealing as well as after exposure to air at 870 K for 2 h.
Fe0.88Cr0.12 than for Fe0.85Cr0.15. This finding could be connected with XPS observations which reveal that the Cr surface segregation process is more effective in case of Fe0.88Cr0.12.
Table 2 The c(alloy) and c(Fe2O3) parameters estimated from CEMS spectra measured for Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys. Sample Fe0.88Cr0.12 annealed Fe0.88Cr0.12 oxidised Fe0.85Cr0.15 annealed Fe0.85Cr0.15 oxidised
c(alloy) [%]
c(Fe2O3) [%]
100
0
84(1)
16(1)
100
0
44(1)
56(1)
Acknowledgement This work was supported by the Polish Ministry of Science and Higher Education under the “Iuventus Plus” programme in the years 2015-17, project number IP2014 015573. References [1] L.K. Mansur, A.F. Rowcliffe, R.K. Nanstad, S.J. Zinkle, W.R. Corwin, R.E. Stoller, Materials needs for fusion, Generation IV fission reactors and spallation neutron sources – similarities and differences, J. Nucl. Mater. 329–333 (2004) 166–172. [2] I. Cook, Materials research for fusion energy, Nat. Mater. 5 (2006) 77–80. [3] C. Leygraf, G. Hultquist, S. Ekelund, J.C. Eriksson, Surface composition studies of the (100) and (110) faces of monocrystalline Fe0.84Cr0.16, Surf. Sci. 46 (1974) 157–176. [4] P.A. Dowben, M. Grunze, D. Wright, Surface segregation of chromium in a Fe72Cr28(110) crystal, Surf. Sci. 134 (1983) L524–L528. [5] J.R. Lince, S.V. Didziulis, D.K. Shuh, T.D. Durbin, J.A. Yarmoff, Interaction of O2 with the Fe0.84Cr0.16(001) surface studied by photoelectron spectroscopy, Surf. Sci. 277 (1992) 43–63. [6] W.T. Geng, Cr segregation at the Fe-Cr surface: a first-principles GGA investigation, Phys. Rev. B 68 (2003) 233402. [7] K. Kokko, S. Granroth, M.H. Heinonen, R.E. Per€al€a, T. Kilpi, E. Kukk, M.P.J. Punkkinen, E. Nurmi, M. Ropo, A. Kuronen, L. Vitos, Atomistic study of surfaces and interfaces of Fe-Cr and Fe-Cr-Al alloys, Mater. Sci. Forum 762 (2013) 728–733. [8] A. Kuronen, S. Granroth, M.H. Heinonen, R.E. Perala, T. Kilpi, P. Laukkanen, J. Lang, J. Dahl, M.P.J. Punkkinen, K. Kokko, M. Ropo, B. Johansson, L. Vitos, Segregation, precipitation, and α-α’ phase separation in Fe-Cr alloys, Phys. Rev. B 92 (2015) 214113. [9] R. Idczak, K. Idczak, R. Konieczny, Oxidation and surface segregation of chromium in Fe–Cr alloys studied by M€ ossbauer and X-ray photoelectron spectroscopy, J. Nucl. Mater. 452 (2014) 141–146. [10] R. Idczak, Internal oxidation process in diluted Fe–Cr alloys: a transmission M€ ossbauer spectroscopy study, Appl. Phys. A 122 (2016) 1009. [11] R. Idczak, R. Konieczny, J. Chojcan, A study of defects in iron-based binary alloys by the M€ ossbauer and positron annihilation spectroscopies, J. Appl. Phys. 115 (2014) 103513. [12] B.V. Crist, Handbooks of Monochromatic XPS Spectra, vol. 1 the Elements and Native Oxides, XPS Interantional Inc., California, USA, 1999. [13] S. Tanuma, C.J. Powell, D.R. Penn, Calculations of electron inelastic mean free paths. V. Data for 14 organic compounds over the 50–2000 eV range, Surf. Interface Anal. 21 (1994) 165–176. [14] J.H. Scofield, Hartree-Slater subshell photojonization cross-sections at 1254 and 1487 eV, J. Electron. Spectrosc. Relat. Phenom. 8 (1976) 129–137. [15] K. Idczak, R. Idczak, R. Konieczny, An investigation of the corrosion of polycrystalline iron by XPS, TMS CEMS, Phys. B 491 (2016) 37–45. [16] R.P. Gupta, S.K. Sen, Calculation of multiplet structure of core p-vacancy levels, Phys. Rev. B 10 (1974) 71–77. [17] A.P. Grosvenor, B.A. Kobe, M.C. Biesinger, N.S. McIntyre, Investigation of multiplet splitting of the Fe 2p XPS spectra and bonding in iron compounds, Surf. Interface Anal. 36 (2004) 1564–1574.
Fig. 11. The c(alloy), c(Fe2O3) and c(Fe3O4) parameters estimated from TMS spectra measured for a) Fe0.88Cr0.12 and b) Fe0.85Cr0.15 alloys in function of time of exposure to air at 870 K.
annealing in vacuum chamber, the carbon contamination was almost removed but the surface concentration of oxygen do not change significantly. This means that during exposure to air at 870 K for 2 h, on the surface of studied alloys a stable thin oxide layer is formed which is composed mainly of Fe2O3, Fe3O4, Cr2O3 and CrO3 compounds. The XPS measurements also shown that during annealing in vacuum, many iron atoms located in the surface of oxidised samples are reduced from oxidation state þ3 to þ2. This fact suggests that during annealing process Fe2O3 transform to Fe3O4 oxide. Finally, one of the most interesting result obtained by XPS is that in both studied alloys Cr surface segregation process occurs. At the same time, the CEMS and TMS results reveal that in both samples during the exposure to air at 870 K for several hours, iron oxides were formed. This fact leads to conclusion that the formation of a stable Cr oxide film on the surface of Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys, does not prevent atmospheric corrosion at 870 K. Moreover, the observed by CEMS and TMS, oxidation process of iron atoms is slower for 35
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Physica B: Physics of Condensed Matter 528 (2018) 27–36 [25] M. Aronniemi, J. Sainio, J. Lahtinen, Chemical state quantification of iron and chromium oxides using XPS: the effect of the background subtraction method, Surf. Sci. 578 (2005) 108–123. [26] A. Idhil, C.N. Borca, A.-C. Uldry, N. Zemba, S. Turchini, D. Catone, A. Foelske, D. Grolimund, M. Samaras, The influence of Cr-composition on the lokal magnetic structure of FeCr alloys, Nucl. Instrum. Methods Phys. Res. B 284 (2012) 1–5. [27] R. Idczak, R. Konieczny, J. Chojcan, Atomic short-range order in Fe1-xCrx alloys studied by 57Fe M€ ossbauer spectroscopy, J. Phys. Chem. Solid. 73 (2012) 1095–1098. [28] R. Zboril, M. Mashlan, D. Petridis, Iron(III) oxides from thermal processess synthesis, structural and magnetic properties, M€ ossbauer Spectrosc. Char. Appl., Chem. Mater. 24 (2002) 969–982. [29] L.F. Checherskaya, V.P. Romanov, P.A. Tatsienko, M€ ossbauer effect in wüstite, Phys. Stat. Sol. (a) 19 (1973) K177–K182. [30] B.J. Evans, S.S. Hafner, 57Fe hyperfine fields in magnetite (Fe3O4), J. Appl. Phys. 40 (1969) 1411–1413. [31] F. Van der Woude, A.J. Dekker, M€ ossbauer effect in α-FeOOH, Phys. Stat. Sol 13 (1966) 181–193. [32] A. Yogi, D. Varshney, Magnetic and structural properties of pure and Cr-doped haematite: α-Fe2-xCrxO3 (0 x 1), J. Adv. Ceram. 2 (4) (2013) 360–369. [33] B. Kniep, A. Constantinescu, D. Niemeier, K.D. Becke, An in-situ m€ ossbauer study of the formation of cementite, Fe3C, Z. Anorg. Allg. Chem. 629 (2003) 1795–1804. [34] D.A. Channing, M.J. Graham, A study of iron oxidation processes by m€ ossbauer spectroscopy, Corrosion Sci. 12 (1972) 271–289.
[18] A.P. Grosvenor, B.A. Kobe, N.S. McIntyre, Examination of the oxidation of iron by oxygen using X-ray photoelectron spectroscopy and QUASESTM, Surf. Sci. 565 (2004) 151–162. [19] R. Steiberger, J. Walter, T. Greunz, J. Duchoslav, M. Arndt, S. Molodtsov, D.C. Meyer, D. Stifter, XPS study of the effects of long-term Arþ ion and Ar cluster sputtering on the chemical degradation of hydrozincite and iron oxide, Corrosion Sci. 99 (2015) 66–75. [20] F. Bonnet, F. Ropital, P. Lecour, D. Espinat, Y. Huiban, L. Gengembre, Y. Berthier, P. Marcus, Study of the oxide/carbide transition on iron surfaces during catalytic coke formation, Surf. Interface Anal. 34 (2002) 418–422. [21] R.L. Tapping, R.D. Davidson, T.E. Jackman, Calibration of argon ion sputter rates using nuclear microanalysis and auger spectroscopy, Surf. Interface Anal. 7 (1985) 105–108. [22] R.L. Tapping, R.D. Davidson, T.E. Jackman, J.A. Davies, Effect of analytical method on thickness measurements of thin oxide films, Surf. Interface Anal. 11 (1988) 441–446. [23] H. Viefbaus, K. Hennesen, M. Lucas, E.M. Miiller-Loreaz, H.J. Grabke, Ion sputter rates and yields for iron, chromium and aluminium oxide layers, Surf. Interface Anal. 21 (1994) 665–672. [24] M.C. Biesinger, B.P. Payne, A.P. Grosvenor, L.W.M. Lau, A.R. Gerson, R. St. C. Smart, Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and Ni, Appl. Surf. Sci. 257 (2011) 2717–2730.
36
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Physica B: Physics of Condensed Matter journal homepage: www.elsevier.com/locate/physb
Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys exposed to air at 870 K studied by TMS, CEMS and XPS R. Idczak a, b, *, K. Idczak a, R. Konieczny a a b
Institute of Experimental Physics, University of Wrocław, pl. M. Borna 9, 50-204 Wrocław, Poland Institute of Low Temperatures and Structure Research, Polish Academy of Sciences, ul. Okolna 2, 50-422 Wrocław, Poland
A R T I C L E I N F O Keywords: M€ ossbauer spectroscopy X-ray photoelectron spectroscopy Fe-Cr alloys Surface segregation Oxidation
A B S T R A C T
The room temperature 57Fe M€ ossbauer and XPS spectra were measured for polycrystalline Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys which were exposed to air at 870 K. The spectra were collected using three techniques: the transmission M€ ossbauer spectroscopy (TMS), the conversion electron M€ ossbauer spectroscopy (CEMS) and the Xray photoelectron spectroscopy (XPS). The combination of these experimental methods allows to determine changes in atomic composition in the bulk, in the subsurface layer and on the surface of the studied alloys. The results obtained by XPS confirm the strong chromium segregation process on the surface of Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys. At the same time, the CEMS and TMS measurements reveal that in both samples, during the exposure to air at 870 K, iron oxides were formed. These findings lead to conclusion that the formation of a stable Cr oxide film on the surface of Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys, does not prevent atmospheric corrosion at 870 K. Moreover, the observed by CEMS and TMS, oxidation process of iron atoms is slower for Fe0.88Cr0.12 than for Fe0.85Cr0.15. This fact is in agreement with XPS results which show that Cr surface segregation process is more effective in case of Fe0.88Cr0.12.
1. Introduction This work concerns determination of changes in chemical composition and the presence of various oxides in the bulk, in the subsurface layer and on the surface of the Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys which were exposed to air at 870 K. The obtained data provide information about oxidation and surface segregation processes in studied materials. Motivation for these studies is fact, that ferritic/martensitic steels, containing up to 15 at.% of Cr, are candidate materials for advanced fission reactors (Generation IV), accelerator-driven systems using spallation neutron sources and fusion reactors [1,2]. These types of steels are able to perform reliably for long time under high irradiation levels (total dose of 50–200 displacements per atom (dpa)) and at high temperatures (up to 900 K). Moreover, high-chromium steels are known for their excellent corrosion resistant properties. These properties originate from the formation, on the alloy surface, a stable Cr oxide film, which prevents further oxidation. On the atomic scale, the necessary supply of Cr atoms on the surface should be provided by a surface segregation process. The majority of former, experimental studies on oxidation and surface segregation of iron-based alloys were performed on single crystals or under controlled oxygen exposure [3–5]. Taking into account that these
alloys are one of the most important engineering materials, it is much more appropriate to investigate a polycrystalline samples which are relatively cheap and easy to produce. For the same reason, the study of oxidation processes caused by direct contact with atmospheric oxygen more closely resembles the conditions in which that type of construction materials are utilized. More recent studies performed by surface-sensitive experimental techniques as well as theoretical calculations confirm Cr surface segregation processes in polycrystalline Fe-Cr alloys [6–9]. For example, A. Kuronen et al. [8] showed Cr segregation and precipitation in the Fe/Cr double layer and Fe0.95Cr0.05 and Fe0.85Cr0.15 alloys using hard X-ray photoelectron spectroscopy and Auger electron spectroscopy supported by ab initio (CPA-EMTO) calculations. Moreover, initial oxidation of Fe-Cr was investigated experimentally at 108 Torr pressure of the spectrometers showing intense Cr2O3 signal. In our previous paper on iron-chromium alloys [9], we used the combination of three experimental techniques: the transmission M€ ossbauer spectroscopy (TMS), the conversion electron M€ ossbauer spectroscopy (CEMS) and the X-ray photoelectron spectroscopy (XPS), to determine changes in Cr concentration and the presence of various oxides in the bulk (TMS), in the subsurface layer (CEMS) and on the surface (XPS) of polycrystalline Fe–Cr alloys. The studied samples were obtained by simple melting
* Corresponding author. Institute of Experimental Physics, University of Wrocław, pl. M. Borna 9, 50-204 Wrocław, Poland. E-mail address: [email protected] (R. Idczak). https://doi.org/10.1016/j.physb.2017.10.082 Received 31 July 2017; Received in revised form 15 October 2017; Accepted 18 October 2017 Available online 22 October 2017 0921-4526/© 2017 Elsevier B.V. All rights reserved.
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at a photoelectron take-off angle of 90 at room temperature. The sampling depth SD which is defined as the depth from which 95% of all photoelectrons are scattered by the time they reach the surface is equal:
process, annealed in the vacuum and exposed to air at room temperature for 30 days. The XPS measurements revealed the strong segregation process of Cr atoms towards the surface. In particular, the atomic concentration of Cr on the surface of prepared Fe0.88Cr0.12 alloy was almost two and a half times larger than Fe concentration. Moreover, a slightly depletion of Cr atoms in the subsurface region, observed by CEMS technique, also confirms the surface segregation process. Additionally, the XPS measurements revealed the presence of Cr and Fe oxides at the alloy surface, while CEMS as well as TMS spectra gave a clear evidence that iron atoms located in deeper parts of the alloy were not oxidised. Consequently, it can be concluded that direct contact of Fe-Cr alloys with atmospheric oxygen at room temperature leads to formation of thin oxide layer, mainly composed of Cr2O3, which prevent a further oxidation process. On the other side, the TMS investigation of Fe-Cr alloys with 3 and 6 at.% of Cr showed that exposure to air at temperatures above 570 K, leads to systematic growth of iron oxides in the bulk [10]. That result suggests that Fe0.97Cr0.03 and Fe0.94Cr0.06 alloys are not corrosion resistant at high temperatures. However, as one can notice in Ref. [10], the oxidation of iron atoms slows with increasing of Cr content. Taking the above into account, in this work, the oxidation process in two Fe-Cr alloys with much higher Cr content (12 and 15 at.%) were studied using XPS, CEMS and TMS. Having in mind that all recent experimental studies by surface-sensitive techniques were performed on Fe-Cr samples which are not oxidised at high temperatures [7–9], this time, before the XPS measurements, the alloys were exposed to air at 870 K for 2 h. In order to obtain a valuable information about the surface oxidation process at high temperature, more detailed information on the chemical state of Cr, Fe, O and C atoms was derived from the deconvoluted Fe 2p, Cr 2p, O 1s and C 1s spectra. Thanks to that, the XPS results compared with CEMS and TMS should give a better understanding of high-temperature, atmospheric corrosion of Fe-Cr alloys.
SD ¼ 3λFe ¼ 3.95 nm,
(1)
where λFe is the inelastic mean free path (IMFP) in iron [13]. Taking the above into account, the estimated surface layer thickness observed in XPS measurements is close to 4 nm. Before each measurement the oxidised samples were firstly exposed to argon ion bombardment for 210 min (in cycles) using 3.8 keV ions and an average current density of 3.1 μA/cm2. The angle of incidence was 90 to the surface. After Arþ sputtering the samples were annealed at the temperature range of 870–1270 K for 5 min. Typical pressure during sample heating grows up to 5⋅106 Pa. The temperature was measured by optical pyrometer, with an accuracy of 20 K. 2.3. Spectra analysis Each measured TMS and CEMS spectrum was analysed in terms of a sum of various number of six-line patterns (sextets) corresponding to different chemical state of 57Fe M€ ossbauer probes as well as to various hyperfine fields B at 57Fe nuclei generated by different numbers of Fe and Cr atoms located in the first two coordination shells of the probing nuclei. The fitting procedure was done under the assumption that the influence of Cr atoms on B as well as the corresponding isomer shift IS on a subspectrum, is additive and independent of the atom positions in the first two coordination shells of the nuclear probe. In other words it was accepted that for each subspectrum the quantities B and IS are linear functions of the numbers n1 and n2 of Cr atoms located, respectively, in the first and second coordination shells of 57Fe and the functions can be written as follows:
2. Experimental
B(n1,n2) ¼ B0 þ n1ΔB1 þ n2ΔB2,
2.1. Samples preparation
IS(n1,n2) ¼ IS0 þ n1ΔIS1 þ n2ΔIS2,
(2)
where ΔB1 (ΔIS1) and ΔB2 (ΔIS2) stand for the changes of B (IS) with one Cr atom in the first and the second coordination shell of the M€ ossbauer probe, respectively. For each six-line pattern, the two line area ratio I16/ I34 was constant and equal to 3/1. Taking into account that in studied samples a strong texture could be formed during preparation procedure (cold-rolling and annealing), the ratio I25/I34 was a free parameter. At the same time, we assume that the quadrupole splitting QS in a cubic lattice is equal to zero. The number of six-line patterns fitted to CEMS and TMS spectra of annealed Fe0.88Cr0.12 and Fe0.85Cr0.15 samples was 9 and 12, respectively. The relatively large number of sextets is connected with high chromium content in studied binary alloys and due to that many possible local configurations of Cr atoms in the first two coordination shells of 57Fe atom have to be taken into consideration. Theoretically the probabilities of atomic configuration for the random Fe1-xCrx b.c.c. alloy can be calculated from the formula:
Polycrystalline samples of Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys were prepared from pure elements Fe: 99.98% and Cr: 99.995%. A synthesis was carried out using an arc-melting furnace under a Ti-gettered purified argon atmosphere. Weight losses during the melting process were negligible (about 0.2%). Resulting ingots were cold-rolled to the final thickness of about 0.03 mm for TMS and 0.10 mm for CEMS and XPS measurements. In the next step, the foils were annealed in the vacuum at 1270 K for 2 h. The base pressure during the annealing procedure was lower than 104 Pa. To obtain homogeneous and defect free samples [11], after annealing process, the foils were slowly cooled to room temperature during 6 h. The annealed samples were exposed to air at 870 K for 2 h. Finally, to obtain information about diffusion of O atoms in iron matrix, the samples of Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys which were prepared for TMS measurements, were subsequently exposed to air at 870 K for additional several hours. To shorten the description, in the further parts of this article, the samples which were only annealed in the vacuum will be referred as “annealed” and the samples which were subsequently exposed to air will be referred as “oxidised”.
Pðn1 ; n2 ; xÞ ¼
8 n1
6 xn1 þn2 ð1 xÞ14n1 n2 ; n2
(3)
where x stands for the atomic concentration of Cr atoms. Most of the possible configurations have vanishingly small probabilities and can therefore be neglected. In practice one usually takes into account only the most probable ones. In this work, it was assumed that configurations only with P > 3% could be reliably deconvoluted form the M€ ossbauer spectra. The configurations which fulfill this condition are P(0,0), P(1,0), P(0,1), P(2,0), P(1,1), P(0,2), P(3,0), P(2,1), P(1,2) for Fe0.88Cr0.12 alloy and P(0,0), P(1,0), P(0,1), P(2,0), P(1,1), P(0,2), P(3,0), P(2,1), P(1,2), P(0,3), P(3,1) and P(2,2) for Fe0.85Cr0.15 alloy. Finally, in some spectra obtained for oxidised samples, the additional magnetic components were observed. These subspectra were attributed to iron oxides (see Section 3.2 and 3.3.). The fits obtained under these assumptions are presented
2.2. Measurements The room temperature CEMS and TMS measurements were performed by means of a constant-acceleration POLON spectrometer of standard design, using a 3.7 GBq 57Co-in-Rh standard source with a full width at half maximum (FWHM) of 0.22 mm/s. The XPS measurements were performed in ultra high vacuum apparatus (with a pressure about 4⋅108 Pa) equipped with the XPS analyzer (Phoibos 150) with Mg Kα and Al Kα X-ray sources. The XPS spectrometer was calibrated to yield the standard values of the Au 4f doublet (binding energy at 84.0 eV for 4f7/2 and 87.7 eV for 4f5/2 [12]) for the clean gold sample. All scans were taken 28
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in Figs. 1–4. The XPS analysis was used to investigate the chemical composition and bonding environment on the alloy surface. The obtained spectra were analysed using the CasaXPS program: the chosen fitting method was the Gaussian-Lorentzian (GL(30%)) and the background of spectra was subtracted using a software based on Shirley method. The atomic concentration N in the surface region was estimated by a standard calculation: Ii
Ni ¼ Pσi λiIj ⋅100%;
(4)
σ j λj
where I is a characteristic (Fe 2p3/2, Cr 2p, O 1s or C 1s) XPS peak intensity, σ is a photoionization cross section (Scofield parameter) [14] and λ is an inelastic mean free path of electrons with kinetic energy of the reference line [13]. 3. Results and discussion 3.1. XPS results Fig. 5 presents a selected XPS spectra for oxidised Fe0.88Cr0.12 (a-d) and Fe0.85Cr0.15 (e-h) alloys just after inserting into the vacuum chamber. For both oxidised alloys, the estimated surface atomic concentration is quite similar and equal to 11% of Fe, 3% of Cr, 47% of O and 39% of C atoms. For more detailed information, the selected Fe 2p, Cr 2p, O 1s and C 1s XPS spectra analysis was done. The Fe 2p high resolution core level line (see Fig. 5a and e) were fitted using Gupta-Sen (GS) multiplet peaks [15–18]. In this case five components, corresponding to various iron oxides, can be distinguished. The first peak at binding energy (BE) of 709.4 eV represents Fe2þ and Fe3þ species [13]. Second, third, fourth and fifth peaks at BE equal to: 710.5 eV, 711.7 eV, 713.0 eV and 714.4 eV respectively, correspond to Fe3þ species [18]. The Cr 2p doublet can be divided into two components for both alloys but distinguished peaks are located at the different BE. For Fe0.88Cr0.12 alloy (see Fig. 5b) positions of peaks maximum are 576.8 eV and 578.1 eV, while for Fe0.85Cr0.15 alloy (see Fig. 5f) there are 577.2 eV and 578.8 eV. According to [19] it can be
Fig. 2. The 57Fe transmission M€ ossbauer spectra for Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys measured at room temperature.
Fig. 3. The 57Fe transmission M€ ossbauer spectra for Fe0.88Cr0.12 alloy exposed to air at 870 K for several hours measured at room temperature.
suggested that these components correspond to Cr2O3. Further analysis are made for O 1s and C 1s spectra. The O 1s spectra (see Fig. 5c and g), for both samples, are composed of three components which represent signals from oxygen in oxide (O2) at BE of 530.6 eV, water at BE of 532.5 eV and carbon-oxygen bonds at BE of 534.1 eV [15,18]. Similarly, the C 1 spectra consist three components, at BE of 286.0 eV, 287.3 eV and 289.2 eV (the first one is connected with carbon contamination while the next two represent the carbon-oxygen bonds [20]).
Fig. 1. The 57Fe conversion electron M€ ossbauer spectra for the Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys measured at room temperature. 29
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is connected with the presence of water. Significant changes can be observed for C 1s spectra for both alloys (see Fig. 7d and h). The XPS signal intensity, after ion sputtering for 210 min, decreases resulting in change of carbon concentration to 6%. Due to low signal-to-noise ratio a detailed deconvolution cannot be made. However, it can be noticed that the peak maximum shifts to 285.5 eV. After Arþ sputtering, samples were annealed at various temperatures. Figs. 8 and 9 present selected XPS spectra for Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys after annealing at 890 K, 1070 K and 1270 K. The first group of spectra (see Fig. 8a) consists of Fe 2p3/2, Cr 2p, O 1s and C 1s peaks for Fe0.88Cr0.12 sample after annealing at 890 K. Atomic surface concentration and presented core level line shapes do not change significantly but some differences in selected components can be observed. In case of Fe 2p3/2 spectrum (Fig. 8a), there are six peaks at BE of: 708.1 eV, 709.4 eV, 710.7 eV, 712.1 eV, 713.6 eV and 715.6 eV. First three components correspond to the Fe2þ species in Fe3O4 [17] and simultaneously components from second to fifth correspond to the Fe3þ species in Fe3O4 and Fe2O3 [17]. Similar to results for the alloy after sputtering, the last peak represents satellite Fe2þ species. For Cr 2p and C 1s peaks a low signal-to-noise ratio does not allow to perform a reliable deconvolution. The maximum of signal intensity for presented core level lines (Fig. 8a) are located at BE of 577.3 eV for chromium and 284.7 eV for carbon. The O 1s spectrum consist of three components at the position of 530.5 eV, 531.6 eV and 533.0 eV representing various oxygen species which was observed previously. Annealing Fe0.88Cr0.12 alloy at 1070 K (Fig. 8b) caused the contamination desorption and, as a consequence, an increase of surface atomic concentration of alloy components (40% for Fe and 3% for Cr atoms). Significant changes are observed in the Fe 2p3/2 spectrum (Fig. 8b), where the new peak appears at BE of 706.8 eV, which represent the presence of metallic iron [15]. Others components correspond to Fe2O3 and Fe3O4 compound [15]. Worth noting is that in the Cr 2p spectrum (Fig. 8b) two components can be distinguish: first at the BE of 576.5 eV and second at the BE of 577.7 eV. Both of them represent the Cr3þ species from the Cr2O3 [19]. In case of O 1s and C 1s spectra (Fig. 8b), only a small decrease in signal intensities was observed. The last group of spectra presents results for Fe0.88Cr0.12 alloy surface after sample annealing at 1270 K (Fig. 8c). It can be easily seen, that Fe 2p3/2 line becomes thinner while its signal intensity decreases (instead of increasing). Contrary results can be made from Cr 2p spectrum, for which widening of core level line and increasing in signal intensity was noticed. In a consequence, Fe 2p3/2 spectrum (Fig. 8c) is composed of four components at BE of: 707.2 eV, 708.6 eV, 710.1 eV and 711.8 eV. The first represents the metallic iron while the others iron-oxide bonds. In case of Cr 2p core level line (Fig. 8c), five components have been distinguished, for which positions of peaks maximum are: 575.2 eV, 576.8 eV, 577.8 eV, 579.0 eV and 580.4 eV. All designated peaks represent Cr-O bonds, due to a multiplet splitting described by M. C. Besinger in Ref. [24]; the first four peaks correspond to Cr3þ species in Cr2O3 compound and the fifth (with the highest BE) correspond to Cr6þ species in CrO3 compound. The estimated atomic surface concentration for the alloy is: 13% of Fe, 27% of Cr, 59% of O and 1% of C atoms and the chromium to iron ratio is close to 2. That result reveals a strong chromium segregation on the surface of Fe0.88Cr0.12 alloy. Despite sample high temperature treatment, signal from the oxygen atoms in the surface region is still significant. The same as after alloy annealing at lower temperatures, O 1s spectrum (Fig. 8c) consists of three components at BE of 531.3 eV, 532.3 eV and 533.6 eV. Analysis of described peaks positions and comparison with distinguished in Fe 2p3/2 and Cr 2p spectra iron-oxide and chromium-oxide bonds allows to confirm the presence of the Cr2O3 and Fe2O3 compounds on the oxidised alloy surface. With good agreement to [24] the first two peaks represent the O-Cr bond in Cr2O3 and the remaining three peaks represent the O-Fe bond in Fe2O3. Fig. 9 presents selected XPS spectra for Fe0.85Cr0.15 alloy after annealing at 890 K, 1070 K and 1270 K. Estimated surface atomic concentration for sample after annealing at 890 K is: 24% of Fe, 9% of Cr,
Fig. 4. The 57Fe transmission M€ ossbauer spectra for Fe0.85Cr0.15 alloy exposed to air at 870 K for several hours measured at room temperature.
Fig. 6 presents the surface atomic concentration after a sequence of Ar-sputtering cycles, 0–210 min. The depth scale was calculated using the sputtering rate of 0.344 nm/min. This value was estimated using the measured average current density of 3.1 μA/cm2 and the average of sputtering rates for iron and chromium oxides which were determined experimentally in works [21–23]. Here, it is worth noting that due to rather complex surface chemical composition of the oxidised samples, the assumed sputtering rate of 0.344 nm/min can only be regarded as approximate. Despite this, it can be clearly seen that during the argon ion sputtering at room temperature the carbon contamination decreases significantly. According to that, the contribution of other elements increases. In case of the Fe0.88Cr0.12 alloy (see Fig. 6a), after sputtering for 30 min, the surface iron concentration changed from 11% to 18%, and then increased to 31% after sputtering for 210 min. Oxygen atoms contribution in a measured surface region achieved 59% and remain constant after further sputtering. Chromium concentration increased to 5%, after sputtering for 60 min and then decreased to 3% after subsequent cycles of sputtering. In case of Fe0.85Cr0.15 alloy (see Fig. 6b), iron and oxygen surface atomic contributions increased up to 29% of Fe atoms and 59% of O atoms, while chromium atomic concentration increased to 5% after surface sputtering for 90 min and remain constant for further cycles. Fig. 7 presents a high resolution selected XPS spectra for both alloys after ion sputtering for 210 min. Taking into account, the observed line shapes and the full width at half maximum (FWHM) of selected peaks, the multiple peaks analysis method was performed for detailed interpretation of the oxidation state of the alloys. The Fe 2p spectra for Fe0.88Cr0.12 (Fig. 7a–d) and Fe0.85Cr0.15 (Fig. 7e–h) reveal a nonsymmetric line shape with the FWHM about 4.5 eV. The Fe 2p3/2 peak (see Fig. 7a and e) is composed of five components which represent Fe2þ species and Fe3þ species [17] while Cr 2p core level line (see Fig. 7b and f) consists of two peaks which correspond to Cr3þ species [19]. Worth noting is that observed component in the Fe 2p3/2 spectrum at the BE of 716.0 eV represents the Fe2þ satellite peak [15]. The O 1s spectrum (see Fig. 7c and g) exhibits three components. The main peak, at BE of 530.6 eV represents the O2 species, next at BE of 531.6 eV corresponds to signal from the defective oxide [24] and the last peak at BE of 532.9 eV
30
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Physica B: Physics of Condensed Matter 528 (2018) 27–36
Fig. 5. XPS selected spectra (Fe 2p, Cr 2p, O 1s and C 1s) for Fe0.88Cr0.12 (a–d) and Fe0.85Cr0.15 (e–h) oxidised alloys after inserting into the UHV chamber.
Fe0.88Cr0.12 alloy (see Fig. 8a). Moreover, in contrast to analysis for Fe0.88Cr0.12 sample, there can be made a deconvolution of Cr 2p core level line. It was found that Cr 2p spectrum (Fig. 9a) consists of three peaks at BE of: 576.7 eV, 578.0 eV and 579.5 eV. All of them represent Cr3þ species in Cr2O3 compound [24]. In case of O 1s spectrum (Fig. 9a), due to FWHM decreasing, only two components at BE of 530.8 eV and 532.5 eV, which correspond to O2 species, could be distinguished. The C 1s spectrum (Fig. 9a) is composed of two peaks with maximum positions at 285.1 eV and 287.2 eV. The first one corresponds to standard C-C and the second represents C-O bonds [20]. The second group of spectra (Fig. 9b) presents results for Fe0.85Cr0.15 alloy after annealing at 1070 K. From the atomic concentration calculations (Eq. (4)) it can be easily seen that chromium segregation occurs. In the surface region the composition of elements is: 15% of Fe, 25% of Cr, 56% of O and 4% of C atoms. The Fe 2p3/2 spectrum (Fig. 9b) can be deconvoluted into five components. First peak, which position is located at 706.8 eV, corresponds to metallic iron, while the others four components at BE of 707.5 eV, 708.6 eV, 710.2 eV and 711.9 eV represent Fe2þ and Fe3þ species [17]. Signal intensity of Cr 2p spectrum (Fig. 9b) allows to distinguish four components, from which the first three (at BE of 575.3 eV, 577.0 eV, 578.1 eV) correspond to Cr3þ species [24], while the fourth one at BE of 579.5 eV is connected with Cr6þ species in CrO3 compound [24,25].
Fig. 6. Surface atomic concentration for: a) Fe0.88Cr0.12 and b) Fe0.85Cr0.15 oxidised alloys after subsequent Ar ion sputtering process.
61% of O and 6% of C atoms. In the Fe 2p3/2 spectrum (Fig. 9a), beside observed previously five components, a new peak appears on the lower binding energy side of spectrum. This new component with a BE of 707.2 eV represents metallic iron, which was not observed for
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Physica B: Physics of Condensed Matter 528 (2018) 27–36
Fig. 7. Selected XPS spectra (Fe 2p3/2, Cr 2p, O 1s and C 1s) for Fe0.88Cr0.12 (a–d) and Fe0.85Cr0.15 (e–h) oxidised alloys after Ar ion sputtering for 210 min.
3.2. CEMS results
This Cr-O bond was not detected for Fe0.88Cr0.12 alloy. The O 1s and C 1s spectra showed in Fig. 9b, reveal decrease in signal intensities and a small shift of the peaks maximum to higher BE. Further annealing at 1270 K causes a contamination desorption. Signal from carbon atoms is not detected and contribution of oxygen atoms decreases to 45% (Fig. 9c). The O 1s spectrum consists of three peaks at BE of 530.5 eV, 531.5 eV and 533.0 eV; which correspond to O-Fe and O-Cr bonds [24]. The calculated concentration of alloy elements is: 27% of Fe and 27% of Cr atoms. Fe 2p3/2 core level line (Fig. 9c) is composed of three peaks from which the main at the lowest BE (707.1 eV) represents metallic Fe while the next two correspond to various Fe – O bonds (708.4 eV and 710.2 eV). Detailed analysis of Cr 2p spectrum (Fig. 9c) allow to distinguish three components (576.5 eV, 577.6 eV and 579.0 eV) representing Cr-O bonds and an additional peak with BE of 574.8 eV which correspond to metallic chromium [26]. Summarizing, the XPS results show that in case of Fe0.85Cr0.15 alloy the chromium segregation occurs at lower temperatures then in Fe0.88Cr0.12 alloy. However, this process is more effective for alloy with less chromium content; Crtotal:Fetotal ratio for Fe0.88Cr0.12 and Fe0.85Cr0.15 samples after annealing at 1270 K are equal to 2.08 and 1.03, respectively. Moreover, the XPS spectra obtained for alloys annealed at the highest temperature reveal a different oxidation states. For the sample with lower Cr content, on the surface some amount of metallic iron were detected but none of metallic chromium (see Fig. 8c). In case of Fe0.85Cr0.15 alloy, the XPS analysis of surface region allow to distinguish in chromium and iron spectra a metallic components.
The estimated parameters of the hyperfine field B, quadrupole splitting QS and isomer shift IS obtained for Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys annealed and oxidised are listed in Table 1. As one can see on Fig. 1, the CEMS spectra measured for annealed samples are quite different from the spectra obtained for oxidised alloys. In case of annealed samples, both spectra consist only six-line patterns with hyperfine parameters similar to those obtained for single phase Fe-Cr alloy with b.c.c. structure [9,27]. For oxidised samples, the additional magnetic-split component was observed. The estimated for this subspectrum hyperfine parameters, which are listed in Table 1, correspond to α-Fe2O3 [28]. At the same time, in all measured CEMS spectra, there is no other components which could be connected with FeO, Fe3O4, FeOOH, FeCrO3 or iron carbides [29–33]. This fact gives an important information that in subsurface layer of polycrystalline Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys exposed to air at 870 K, only one type of iron oxide was formed. Moreover, the results of CEMS spectra analysis give detailed information about chemical composition of subsurface layer. The relative areas I of components for each spectrum were determined. Assuming that the relative ratio of the Lamb-M€ ossbauer factors fα-Fe: fα-Fe2O3 is equal to 1: 1.08 [34], the fraction ci of absorbing atoms corresponding to the ith component can be easily calculated using Ii and fi values: Ii f
ci ¼ Pi Ii ⋅100%: i
(5)
fi
Distributions of the hyperfine fields produced using determined ci 32
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Physica B: Physics of Condensed Matter 528 (2018) 27–36
Fig. 8. Selected XPS spectra (Fe 2p3/2, Cr 2p, C 1s and O 1s) for Fe0.88Cr0.12 oxidised alloy after annealing at: a) 890 K, b) 1070 K and c) 1270 K.
That drastic changes in M€ ossbauer probe concentration in the subsurface region could have a significant influence of the CEMS signal depth profile and due to that, the statement (correct for a pure iron) that more than 50% of the signal comes from the first 50 nm is no longer valid in case of studied oxidised alloys. Taking the above into account, it seems reasonable to assume that the obtained CEMS data show the iron oxidation process in deeper part of the material than XPS measurements. Therefore, the CEMS spectra reveal the presence of large number of unoxidised iron atoms which are located below the oxide layer.
values were presented in Fig. 10. At the same time, ci values were used to calculate c(alloy) and c(Fe2O3) parameters which are the fractions of 57Fe nuclear probes located in Fe-Cr alloy with b.c.c. structure and α-Fe2O3, respectively. As one can notice, the estimated for studied samples c(alloy) and c(Fe2O3) parameters, which are listed in Table 2, show that exposure of polycrystalline Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys to air at 870 K for 2 h results in growth of α-Fe2O3 and the iron oxidation process is faster in case of Fe0.85Cr0.15 sample. Here, it is worth noting that the probing depth of the CEMS technique for pure iron is up to 300 nm however due to the attenuation of the conversion electron beam, more than 50% of the signal comes from the first 50 nm. Taking this fact into account, it seems that the CEMS data which reveal the presence of only 16% (Fe0.88Cr0.12) and 56% (Fe0.85Cr0.15) of oxidised iron atoms in the subsurface layer of the oxidised samples do not correspond with the XPS results. In particular, the calculated surface atomic concentration for oxidised alloys after subsequent Ar ion sputtering process (Fig. 6) as well as the absence of component which corresponds to metallic iron in the Fe 2p3/2 spectra measured for oxidised samples before and after sputtering give a clear evidence that up to 70 nm, all or almost all of the iron atoms are oxidised. That effect could be explained by the fact that the surface of oxidised alloys is mainly composed of carbon as well as Cr and Fe oxides. Due to that the electron attenuation coefficient for oxidised sample should be much lower than for a pure iron and the CEMS probing depth is probably much greater than 300 nm. At the same time, as one can notice in Fig. 6, the iron concentration in the surface layer is close to 10% and increases to about 30% after ion sputtering. Moreover, below the oxide layer the iron content should be close to 88% (Fe0.88Cr0.12) and 85% (Fe0.85Cr0.15).
3.3. TMS results The analysis of the measured TMS spectra reveals existence of components which could be assigned only to single phase Fe-Cr alloy with b.c.c. structure. The estimated parameters of the hyperfine field B and isomer shift IS obtained for Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys annealed and exposed to air at 870 K for 2 h are similar to those presented in Table 1 (Fe-Cr alloy). The XPS, TMS and CEMS results obtained for the same samples suggest that the formation of a stable oxide film on the surface and subsurface layer of Fe-Cr alloys, prevents further oxidation deep into the bulk. To confirm that hypothesis, it was decided to measure TMS spectra for Fe0.88Cr0.12 and Fe0.85Cr0.15 samples which were exposed to air at 870 K for additional periods of time. The obtained TMS spectra, which are presented on Figs. 3 and 4, reveal the presence of additional magnetic-split components. The estimated for these subspectra hyperfine parameters correspond to α-Fe2O3 (B ¼ 51.7(2) T, IS ¼ 0.38(2) mm/s, QS ¼ 0.20(5) mm/s) and Fe3O4 compounds (Fe3O4 (Feþ3) - B ¼ 49.1(2) 33
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Fig. 9. Selected XPS spectra (Fe 2p3/2, Cr 2p, C 1s and O 1s) for Fe0.85Cr0.15 oxidised alloy after annealing at: a) 890 K, b) 1070 K and c) 1270 K.
Table 1 Some parameters of the assumed model fitted to the room temperature CEMS spectra measured for Fe0.88Cr0.12 and Fe0.85Cr0.15. Values of the isomer shifts IS0 and IS are reported relative to the corresponding value for α-Fe at room temperature. Sample
Fe0.88Cr0.12 annealed Fe0.88Cr0.12 oxidised Fe0.85Cr0.15 annealed Fe0.85Cr0.15 oxidised
α-Fe2O3
Fe-Cr alloy B0 [T]
IS0 [mm/s]
ΔB1 [T]
ΔIS1 [mm/s]
ΔB2 [T]
ΔIS2 [mm/s]
B [T]
IS [mm/s]
QS [mm/s]
34.1(1)
0.01(1)
3.3(1)
0.02(1)
2.2(1)
0.01(1)
–
–
–
34.0(1)
0.00(1)
3.4(1)
0.02(1)
2.3(1)
0.01(1)
51.4(5)
0.37(4)
0.19(6)
33.9(1)
0.01(1)
3.6(3)
0.02(1)
2.4(2)
0.01(1)
–
–
–
33.9(1)
0.00(1)
3.3(1)
0.01(1)
2.2(1)
0.02(1)
51.9(1)
0.37(1)
0.17(2)
T, IS ¼ 0.32(2) mm/s, QS ¼ 0.04(4) mm/s and Fe3O4 (Feþ2,þ3) B ¼ 45.8(2) T, IS ¼ 0.61(2) mm/s, QS ¼ 0.02(5) mm/s) [28,30]. The presence of iron oxides in bulk of Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys which were exposed to air at 870 K for 4, 6 and 12 h proves that the formation of a stable oxide film on the surface of those alloys does not prevent further oxidation of iron atoms located in deeper parts of studied materials. Finally, the calculated c(alloy), c(Fe2O3) and c(Fe3O4) parameters, presented in Fig. 11, shows that the iron oxidation process is faster in case of Fe0.85Cr0.15 sample. This finding is in agreement with CEMS results and could be explained by the surface segregation process of Cr atoms which was observed in XPS measurements. In both studied alloys, the XPS results give a clear evidence that Cr segregation occurs. If we assume that in Fe-Cr alloys this process leads to formation of a
passivation layer, which significantly reduce the oxygen diffusion deep into the bulk of material then in case of Fe0.88Cr0.12, for which the segregation process is more effective, the oxidation of iron atoms should be slower. 4. Conclusions The combination of three experimental techniques allows to determine changes in atomic composition and the presence of various oxides in the bulk (TMS), in the subsurface layer (CEMS) and on the surface (XPS) of the studied Fe-Cr alloys. As one can expect, the XPS measurements give a clear evidence that the surfaces of the oxidised samples are composed mainly of oxygen and carbon. After Ar ion sputtering and
34
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Physica B: Physics of Condensed Matter 528 (2018) 27–36
Fig. 10. Hyperfine field distributions obtained from CEMS spectra measured for Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys after annealing as well as after exposure to air at 870 K for 2 h.
Fe0.88Cr0.12 than for Fe0.85Cr0.15. This finding could be connected with XPS observations which reveal that the Cr surface segregation process is more effective in case of Fe0.88Cr0.12.
Table 2 The c(alloy) and c(Fe2O3) parameters estimated from CEMS spectra measured for Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys. Sample Fe0.88Cr0.12 annealed Fe0.88Cr0.12 oxidised Fe0.85Cr0.15 annealed Fe0.85Cr0.15 oxidised
c(alloy) [%]
c(Fe2O3) [%]
100
0
84(1)
16(1)
100
0
44(1)
56(1)
Acknowledgement This work was supported by the Polish Ministry of Science and Higher Education under the “Iuventus Plus” programme in the years 2015-17, project number IP2014 015573. References [1] L.K. Mansur, A.F. Rowcliffe, R.K. Nanstad, S.J. Zinkle, W.R. Corwin, R.E. Stoller, Materials needs for fusion, Generation IV fission reactors and spallation neutron sources – similarities and differences, J. Nucl. Mater. 329–333 (2004) 166–172. [2] I. Cook, Materials research for fusion energy, Nat. Mater. 5 (2006) 77–80. [3] C. Leygraf, G. Hultquist, S. Ekelund, J.C. Eriksson, Surface composition studies of the (100) and (110) faces of monocrystalline Fe0.84Cr0.16, Surf. Sci. 46 (1974) 157–176. [4] P.A. Dowben, M. Grunze, D. Wright, Surface segregation of chromium in a Fe72Cr28(110) crystal, Surf. Sci. 134 (1983) L524–L528. [5] J.R. Lince, S.V. Didziulis, D.K. Shuh, T.D. Durbin, J.A. Yarmoff, Interaction of O2 with the Fe0.84Cr0.16(001) surface studied by photoelectron spectroscopy, Surf. Sci. 277 (1992) 43–63. [6] W.T. Geng, Cr segregation at the Fe-Cr surface: a first-principles GGA investigation, Phys. Rev. B 68 (2003) 233402. [7] K. Kokko, S. Granroth, M.H. Heinonen, R.E. Per€al€a, T. Kilpi, E. Kukk, M.P.J. Punkkinen, E. Nurmi, M. Ropo, A. Kuronen, L. Vitos, Atomistic study of surfaces and interfaces of Fe-Cr and Fe-Cr-Al alloys, Mater. Sci. Forum 762 (2013) 728–733. [8] A. Kuronen, S. Granroth, M.H. Heinonen, R.E. Perala, T. Kilpi, P. Laukkanen, J. Lang, J. Dahl, M.P.J. Punkkinen, K. Kokko, M. Ropo, B. Johansson, L. Vitos, Segregation, precipitation, and α-α’ phase separation in Fe-Cr alloys, Phys. Rev. B 92 (2015) 214113. [9] R. Idczak, K. Idczak, R. Konieczny, Oxidation and surface segregation of chromium in Fe–Cr alloys studied by M€ ossbauer and X-ray photoelectron spectroscopy, J. Nucl. Mater. 452 (2014) 141–146. [10] R. Idczak, Internal oxidation process in diluted Fe–Cr alloys: a transmission M€ ossbauer spectroscopy study, Appl. Phys. A 122 (2016) 1009. [11] R. Idczak, R. Konieczny, J. Chojcan, A study of defects in iron-based binary alloys by the M€ ossbauer and positron annihilation spectroscopies, J. Appl. Phys. 115 (2014) 103513. [12] B.V. Crist, Handbooks of Monochromatic XPS Spectra, vol. 1 the Elements and Native Oxides, XPS Interantional Inc., California, USA, 1999. [13] S. Tanuma, C.J. Powell, D.R. Penn, Calculations of electron inelastic mean free paths. V. Data for 14 organic compounds over the 50–2000 eV range, Surf. Interface Anal. 21 (1994) 165–176. [14] J.H. Scofield, Hartree-Slater subshell photojonization cross-sections at 1254 and 1487 eV, J. Electron. Spectrosc. Relat. Phenom. 8 (1976) 129–137. [15] K. Idczak, R. Idczak, R. Konieczny, An investigation of the corrosion of polycrystalline iron by XPS, TMS CEMS, Phys. B 491 (2016) 37–45. [16] R.P. Gupta, S.K. Sen, Calculation of multiplet structure of core p-vacancy levels, Phys. Rev. B 10 (1974) 71–77. [17] A.P. Grosvenor, B.A. Kobe, M.C. Biesinger, N.S. McIntyre, Investigation of multiplet splitting of the Fe 2p XPS spectra and bonding in iron compounds, Surf. Interface Anal. 36 (2004) 1564–1574.
Fig. 11. The c(alloy), c(Fe2O3) and c(Fe3O4) parameters estimated from TMS spectra measured for a) Fe0.88Cr0.12 and b) Fe0.85Cr0.15 alloys in function of time of exposure to air at 870 K.
annealing in vacuum chamber, the carbon contamination was almost removed but the surface concentration of oxygen do not change significantly. This means that during exposure to air at 870 K for 2 h, on the surface of studied alloys a stable thin oxide layer is formed which is composed mainly of Fe2O3, Fe3O4, Cr2O3 and CrO3 compounds. The XPS measurements also shown that during annealing in vacuum, many iron atoms located in the surface of oxidised samples are reduced from oxidation state þ3 to þ2. This fact suggests that during annealing process Fe2O3 transform to Fe3O4 oxide. Finally, one of the most interesting result obtained by XPS is that in both studied alloys Cr surface segregation process occurs. At the same time, the CEMS and TMS results reveal that in both samples during the exposure to air at 870 K for several hours, iron oxides were formed. This fact leads to conclusion that the formation of a stable Cr oxide film on the surface of Fe0.88Cr0.12 and Fe0.85Cr0.15 alloys, does not prevent atmospheric corrosion at 870 K. Moreover, the observed by CEMS and TMS, oxidation process of iron atoms is slower for 35
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