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The widely used stainless steel and its wrought versions are resistant to corrosion in ambient conditions due to the passivation layer consisting of chromium oxide. Corrosion and erosion of steel is usually associated with the destruction of these layers, but rarely with the appearance of surface inhomogeneities, depending on the microscopic level. In this work, nanoscale chemical surface heterogeneity, detected by spectroscopic microscopy and chemometric analysis, unexpectedly dominates the fracture and corrosion of cold rolled cerium modified super duplex stainless steel 2507 (SDSS) during its hot deformation. Although X-ray photoelectron microscopy showed a relatively uniform coverage of the natural Cr2O3 layer, the passivation performance of the cold rolled SDSS was poor due to the local distribution of Fe3+ rich nanoislands on the Fe/Cr oxide layer. This atomic scale knowledge provides a deep understanding of stainless steel corrosion and is expected to help combat corrosion of similar high-alloy metals.
Since the invention of stainless steel, the anti-corrosion properties of ferrochrome have been attributed to chromium, which forms strong oxides/oxyhydroxides and exhibits a passivating behavior in most environments. Compared to conventional (austenitic and ferritic) stainless steels 1, 2, 3, super duplex stainless steels (SDSS) have better corrosion resistance and excellent mechanical properties. Increased mechanical strength allows for lighter and more compact designs. In contrast, the economical SDSS has high resistance to pitting and crevice corrosion, resulting in a longer service life, thereby expanding its application to pollution control, chemical containers, and the offshore oil and gas industry4. However, the narrow range of heat treatment temperatures and poor formability hinder their wide practical application. Therefore, SDSS is modified to improve the above performance. For example, the Ce modification was introduced in SDSS 2507 (Ce-2507) with a high nitrogen content6,7,8. The rare earth element (Ce) at an appropriate concentration of 0.08 wt.% has a beneficial effect on the mechanical properties of the DSS, since it improves grain refinement and grain boundary strength. Wear and corrosion resistance, tensile strength and yield strength, and hot workability are also improved9. Large amounts of nitrogen can replace expensive nickel content, making SDSS more cost-effective10.
Recently, SDSS has been plastically deformed at various temperatures (cryogenic, cold and hot) to achieve excellent mechanical properties6,7,8. However, the excellent corrosion resistance of SDSS due to the presence of a thin oxide film on the surface is affected by many factors such as inherent heterogeneity due to the presence of heterogeneous phases with different grain boundaries, unwanted precipitates and different response. deformations of austenitic and ferritic phases7. Therefore, the study of the microscopic domain properties of such films down to the level of the electronic structure becomes crucial for understanding SDSS corrosion and requires complex experimental techniques. So far, surface-sensitive methods such as Auger electron spectroscopy11 and X-ray photoelectron spectroscopy12,13,14,15 and hard X-ray photoemission microscopy (HAX-PEEM)16 have generally failed to detect chemical differences in surface layers. chemical states of the same element in different places of the nanoscale space. Several recent studies have correlated localized oxidation of chromium with the observed corrosion behavior of austenitic stainless steels17, martensitic steels18 and SDSS19,20. However, these studies have mainly focused on the effect of Cr heterogeneity (eg, Cr3+ oxidation state) on corrosion resistance. Lateral heterogeneity in the oxidation states of elements can be caused by different compounds with the same constituent elements, such as iron oxides. These compounds, which have inherited a small size as a result of thermomechanical treatment, are in close proximity to each other, but differ in composition and oxidation state16,21. Therefore, in order to detect cracking of oxide films and subsequent pitting, it is necessary to understand surface heterogeneity at the microscopic level. Despite these requirements, quantitative estimates such as lateral heterogeneity in oxidation, especially for Fe at the nano- and atomic scale, are still lacking, and its correlation with corrosion resistance remains unexplored. Until recently, the chemical state of various elements, such as Fe and Ca22, on steel samples was quantitatively characterized using soft X-ray photoelectron microscopy (X-PEEM) in nanoscale synchrotron radiation facilities. Combined with chemically sensitive X-ray absorption spectroscopy (XAS), X-PEEM enables XAS measurements with high spatial and spectral resolution, providing chemical information about the composition of elements and their chemical state with spatial resolution down to twenty-three nanometer scale. . This spectromicroscopic observation of the onset facilitates local chemical observations and can demonstrate chemical changes in the space of the iron layer that have not previously been investigated.
This study extends the advantages of PEEM in detecting chemical differences at the nanoscale and presents an insightful atomic-level surface analysis method for understanding the corrosion behavior of Ce-2507. It uses a clustered K-means24 chemometric approach to map the global chemical (hetero)homogeneity of the elements involved, whose chemical states are presented in a statistical representation. In contrast to corrosion initiated by the destruction of the chromium oxide film in the traditional case, less passivation and lower corrosion resistance are currently attributed to localized Fe3+ rich nanoislands near the Fe/Cr oxide layer, which may be protective properties. The oxide destroys the dotted film and causes corrosion.
The corrosive behavior of deformed SDSS 2507 was first evaluated using electrochemical measurements. On fig. Figure 1 shows the Nyquist and Bode curves for selected samples in an acidic (pH = 1) aqueous solution of FeCl3 at room temperature. The selected electrolyte acts as a strong oxidizing agent, characterizing the tendency of the passivation film to break down. Although the material did not undergo stable pitting at room temperature, the analysis provided insight into possible failure events and subsequent corrosion. The equivalent circuit (Fig. 1d) was used to fit the electrochemical impedance spectroscopy (EIS) spectrum, and the corresponding fitting results are shown in Table 1. Incomplete semicircles appear in solution-treated and hot-worked specimens, while compressed semicircles appear in cold-rolled counterparts (Fig. .1b). In EIS spectroscopy, the radius of the semicircle can be considered as the polarization resistance (Rp)25,26. The Rp of solution-treated runway in Table 1 is about 135 kΩ cm–2, however, the values of hot-worked and cold-rolled runway runway are much lower, 34.7 and 2.1 kΩ cm–2, respectively. This significant reduction in Rp shows the detrimental effect of plastic deformation on passivation and corrosion resistance, as shown in previous reports27,28,29,30.
a Nyquist, b, c Bode impedance and phase diagrams, and d corresponding equivalent circuit models, where RS is the electrolyte resistance, Rp is the polarization resistance, and QCPE is the oxide of the constant phase element used to model the non-ideal capacitance (n). EIS measurements are made at open circuit potential.
The simultaneous constants are shown in the Bode plot, with a plateau in the high frequency range representing the electrolyte resistance RS26. As the frequency decreases, the impedance increases and a negative phase angle is found, indicating capacitance dominance. The phase angle increases, retaining a maximum over a relatively wide frequency range, and then decreases (Fig. 1c). However, in all three cases, this maximum is still less than 90°, indicating non-ideal capacitive behavior due to capacitive dispersion. Thus, the QCPE constant phase element (CPE) is used to represent interfacial capacitance distributions arising from surface roughness or inhomogeneity, especially at the atomic scale, fractal geometry, electrode porosity, non-uniform potential, and geometry with the shape of electrodes31,32. CPE impedance:
where j is the imaginary number and ω is the angular frequency. QCPE is a frequency independent constant that is proportional to the effective open area of the electrolyte. n is a dimensionless power number describing the deviation of a capacitor from ideal capacitance, i.e. the closer n is to 1, the closer the CPE is to purely capacitive, while if n is close to zero, it appears resistive. Small deviations of n, close to 1, indicate the non-ideal capacitive behavior of the surface after polarization tests. The QCPE of cold rolled SDSS is significantly higher than its counterparts, meaning that the surface quality is less uniform.
Consistent with most corrosion resistance properties of stainless steels, the relatively high Cr content of SDSS generally results in excellent corrosion resistance of SDSS due to the presence of a passivating protective oxide film on the surface17. Such passivating films are usually rich in Cr3+ oxides and/or hydroxides, mainly in combination with Fe2+, Fe3+ oxides and/or (oxy)hydroxides33. Despite the same surface uniformity, passivating oxide layer, and no observed surface cracking according to microscopic measurements6,7, the corrosion behavior of hot-worked and cold-rolled SDSS is different, so an in-depth study of the microstructural characteristics is necessary for steel deformation.
The microstructure of deformed stainless steel was quantitatively studied using intrinsic and synchrotron high-energy X-rays (Supplementary Figures 1, 2). A detailed analysis is provided in the Supplementary Information. Although there is a general consensus on the type of major phase, differences in bulk phase fractions were found, which are listed in Supplementary Table 1. These differences may be due to inhomogeneous phase fractions at the surface and in the volume, which are affected by different X-ray diffraction (XRD) detection depths. ) with different energy sources of incident photons34. Relatively high austenite fractions in cold rolled specimens determined by XRD from a laboratory source indicate better passivation and then better corrosion resistance35, while more accurate and statistical results suggest opposite trends in phase fractions. In addition, the corrosion resistance of steel also depends on the degree of grain refinement, grain size reduction, increase in microdeformations and dislocation density that occur during thermomechanical treatment36,37,38. The hot-worked specimens showed a more grainy nature, indicative of micron-sized grains, while the smooth rings observed in the cold-rolled specimens (Supplementary Fig. 3) were indicative of significant grain refinement to nanosize in previous work. This should favor the passive film. formation and increase of corrosion resistance. Higher dislocation density is usually associated with lower resistance to pitting, which agrees well with electrochemical measurements.
Changes in the chemical state of the microdomains of the main elements were systematically studied using X-PEEM. Although there are more alloying elements, Cr, Fe, Ni and Ce39 are chosen here, since Cr is the key element for forming the passive film, Fe is the main element for steel, and Ni enhances passivation and balances the ferrite-austenitic phase. Structure and modification is the purpose of Ce. By tuning the synchrotron beam energy, XAS captured the main characteristics of Cr (L2.3 edge), Fe (L2.3 edge), Ni (L2.3 edge), and Ce (M4.5 edge) from the surface. -2507 SDSS. Appropriate data analysis was performed by including energy calibration with published data (eg XAS on Fe L2, 3 ribs40,41).
On fig. Figure 2 shows X-PEEM images of hot-worked (Fig. 2a) and cold-rolled (Fig. 2d) Ce-2507 SDSS and corresponding XAS Cr and Fe L2,3 edges at individually marked positions. The L2,3 XAS edge explores the unoccupied 3d states of electrons after photoexcitation at the 2p3/2 (L3 edge) and 2p1/2 (L2 edge) spin-orbit splitting levels. Information about the valence state of Cr was obtained from X-ray diffraction analysis of the L2,3 edge in Fig. 2b,d. Link comparison. 42, 43 showed that four peaks A (578.3 eV), B (579.5 eV), C (580.4 eV), and D (582.2 eV) were observed near the L3 edge, reflecting octahedral Cr3+ ions, corresponding Cr2O3. The experimental spectra are in agreement with theoretical calculations, as shown in panels b and e, obtained from multiple crystal field calculations at the Cr L2.3 interface using a crystal field of 2.0 eV44. Both surfaces of hot-worked and cold-rolled SDSS are coated with a relatively uniform layer of Cr2O3.
a Thermal image of X-PEEM hot-formed SDSS corresponding to edge b Cr L2.3 and edge c Fe L2.3, d Thermal image X-PEEM of cold-rolled SDSS corresponding to edge e Cr L2.3 and f Fe L2.3 of side (e) . The XAS spectra plotted at various spatial positions marked on the thermal images (a, d) by the orange dotted lines in (b) and (e) represent simulated XAS spectra of Cr3+ with a crystal field value of 2.0 eV. For X-PEEM images, a thermal palette is used to improve image readability, where colors from blue to red are proportional to the intensity of X-ray absorption (from low to high).
Regardless of the chemical environment of these metallic elements, the chemical state of the additions of Ni and Ce alloying elements for both samples remained the same. Additional drawing. On fig. 5-9 show X-PEEM images and corresponding XAS spectra for Ni and Ce at various positions on the surface of hot-worked and cold-rolled specimens. Ni XAS shows the oxidation state of Ni2+ over the entire measured surface of hot-worked and cold-rolled specimens (Supplementary Discussion). It is noteworthy that in the case of hot-worked specimens, the XAS signal of Ce is not observed, while the spectrum of Ce3+ of cold-rolled specimens is observed at one point. The observation of Ce spots in cold-rolled samples showed that Ce mainly exists in the form of precipitates.
In thermally deformed SDSS, no local structural change in XAS was observed at the Fe L2.3 edge (Fig. 2c). However, as shown in fig. 2f, the Fe matrix microscopically changes its chemical state at seven randomly selected points in the cold rolled SDSS. In addition, in order to get an accurate idea of the changes in the state of Fe at the selected locations in Fig. 2f, local surface studies were performed (Fig. 3 and Supplementary Fig. 10) in which smaller circular regions were selected. The XAS spectra of the Fe L2,3 edge of α-Fe2O3 systems and Fe2+ octahedral oxides were modeled using multiplet crystal field calculations using crystal fields of 1.0 (Fe2+) and 1.0 (Fe3+)44. We note that α-Fe2O3 and γ-Fe2O3 have different local symmetries45,46, Fe3O4 has combination of both Fe2+ & Fe3+,47, and FeO45 as a formally divalent Fe2+ oxide (3d6). We note that α-Fe2O3 and γ-Fe2O3 have different local symmetries45,46, Fe3O4 has a combination of both Fe2+ & Fe3+,47, and FeO45 as a formally divalent Fe2+ oxide (3d6). Note that α-Fe2O3 and γ-Fe2O3 have different local symmetries45,46, Fe3O4 combines both Fe2+ and Fe3+,47 and FeO45 in the form of formally divalent oxide Fe2+ (3d6). Note that α-Fe2O3 and γ-Fe2O3 have different local symmetries45,46, Fe3O4 has combinations of Fe2+ and Fe3+,47 and FeO45 acts as a formal divalent Fe2+ oxide (3d6). All Fe3+ ions in α-Fe2O3 have only Oh positions, while γ-Fe2O3 is usually expressed as Fe3+ t2g [Fe3+5/3V1/3]eg O4 spinel with vacancies in eg positions. Therefore, the Fe3+ ions in γ-Fe2O3 have both Td and Oh positions. As mentioned in the previous work, although the intensity ratios of the two are different, their intensity ratio eg/t2g is ≈1, while in this case the observed intensity ratio eg/t2g is about 1. This rules out the possibility of only Fe3+ being present in this case. Considering the case of Fe3O4 with combinations of Fe2+ and Fe3+, it is known that a weaker (strong) first feature in the L3 edge of Fe indicates a smaller (greater) unoccupancy in the t2g state. This applies to Fe2+ (Fe3+), which indicates an increase in the first sign indicating an increase in the content of Fe2+47. These results show that Fe2+ and γ-Fe2O3, α-Fe2O3 and/or Fe3O4 predominate on the cold-rolled surfaces of the composites.
Enlarged photoemission electron thermal images of the (a, c) and (b, d) XAS spectra across the Fe L2,3 edge at various spatial positions within selected regions 2 and E in Figs. 2d.
The obtained experimental data (Fig. 4a and Supplementary Fig. 11) were plotted and compared with those of pure compounds 40, 41, 48. Basically, three different types of experimentally observed Fe L-edge XAS spectra (XAS-1, XAS-2 and XAS-3: Fig. 4a) were observed at spatially different locations. In particular, a spectrum similar to 2-a (denoted as XAS-1) in Fig. 3b was observed over the entire region of interest, followed by a 2-b spectrum (labeled XAS-2), while a spectrum similar to E-3 was observed in fig. 3d (referred to as XAS-3) has been observed in certain localized locations. Usually, four parameters are used to identify the valence states present in a probe sample: (1) L3 and L2 spectral features, (2) energy positions of L3 and L2 features, (3) L3-L2 energy difference, (4) L2 intensity ratio /L3. According to visual observations (Fig. 4a), all three Fe components, namely Fe0, Fe2+, and Fe3+, are present on the surface of the studied SDSS. The calculated intensity ratio L2/L3 also indicated the presence of all three components.
a Observed different three experimental data (solid lines XAS-1, XAS-2 and XAS-3 correspond to 2-a, 2-b and E-3 in Fig. 2 and Fig. 3) compared to simulated XAS Comparison spectra, octahedrons Fe2+, Fe3+, crystal field values of 1.0 eV and 1.5 eV, respectively, b–d Measured experimental data (XAS-1, XAS-2, XAS-3) and corresponding optimized LCF data (solid black line), and comparison XAS-3 spectra with Fe3O4 (mixed state of Fe) and Fe2O3 (pure Fe3+) standards.
A linear combination (LCF) fit of the three standards40,41,48 was used to quantify the composition of iron oxide. LCF was implemented for three selected Fe L-edge XAS spectra showing the highest contrast, namely XAS-1, XAS-2 and XAS-3, as shown in Fig. 4b–d. For LCF fittings, 10% Fe0 was considered in all cases due to the small ledge we observed in all data and the fact that ferrous metal is the main component of steel. Indeed, the probation depth of X-PEEM for Fe (~6 nm)49 is bigger than the estimated oxidation layer thickness (slightly > 4 nm), allowing detection of signal from the iron matrix (Fe0) beneath the passivation layer. Indeed, the probation depth of X-PEEM for Fe (~6 nm)49 is bigger than the estimated oxidation layer thickness (slightly > 4 nm), allowing detection of signal from the iron matrix (Fe0) beneath the passivation layer. Действительно, пробная глубина X-PEEM для Fe (~ 6 нм)49 больше, чем предполагаемая толщина слоя окисления (немного > 4 нм), что позволяет обнаружить сигнал от железной матрицы (Fe0) под пассивирующим слоем. Indeed, the probe X-PEEM depth for Fe (~6 nm)49 is greater than the assumed thickness of the oxidation layer (slightly >4 nm), which makes it possible to detect the signal from the iron matrix (Fe0) under the passivation layer. In fact, X-PEEM detects Fe (~6 nm)49 deeper than the expected thickness of the oxide layer (just over 4 nm), allowing detection of signals from the iron matrix (Fe0) below the passivation layer. Various combinations of Fe2+ and Fe3+ were performed to find the best possible solution for the observed experimental data. On fig. Figure 4b shows the combination of Fe2+ and Fe3+ in the XAS-1 spectrum, where the proportions of Fe2+ and Fe3+ are close, about 45%, which indicates a mixed oxidation state of Fe. Whereas for the XAS-2 spectrum, the percentage of Fe2+ and Fe3+ becomes ~30% and 60%, respectively. The content of Fe2+ is lower than that of Fe3+. The Fe2+ to Fe3 ratio of 1:2 means that Fe3O4 can be formed at the same ratio of Fe ions. In addition, for the XAS-3 spectrum, the percentages of Fe2+ and Fe3+ changed to ~10% and 80%, indicating a higher conversion of Fe2+ to Fe3+. As mentioned above, Fe3+ can come from α-Fe2O3, γ-Fe2O3 or Fe3O4. To understand the most likely source of Fe3+, XAS-3 spectra are plotted along with various Fe3+ standards in Fig. 4e showing similarity with all two standards when Peak B is considered. However, the intensity of the shoulder (A: from Fe2+) and the intensity ratio B/A indicate that the spectrum of XAS-3 is close to but not the same as that of γ-Fe2O3. Compared to bulk γ-Fe2O3, the Fe 2p XAS intensity of the A SDSS peak is slightly higher (Fig. 4e), which indicates a higher Fe2+ intensity. Although the spectrum of XAS-3 is similar to that of γ-Fe2O3, where Fe3+ is present in both the Oh and Td positions, the identification of different valence states and coordination only by the L2,3 edge or the L2/L3 intensity ratio is still a problem. a recurring topic of discussion due to the complexity of the various factors involved in the final spectrum41.
In addition to the spectral discrimination of the chemical states of the selected regions of interest described above, the global chemical heterogeneity of the key elements Cr and Fe was assessed by classifying all XAS spectra obtained on the sample surface using the K-means clustering method. The edge profiles Cr L were set in such a way as to form two optimal clusters spatially distributed in the hot-worked and cold-rolled specimens shown in Figs. 5. It is clear that no local structural changes were observed, since the two centroids of the XAS Cr spectra are very similar. These spectral shapes of the two clusters are almost identical to those corresponding to Cr2O342, which means that the Cr2O3 layers are relatively uniformly distributed over the SDSS.
a cluster of K-means L-edge Cr regions, b corresponding XAS centroids. Results of K-means X-PEEM comparison of cold-rolled SDSS: c clusters of K-means edge regions of Cr L2,3 and d corresponding XAS centroids.
To illustrate a more complex FeL edge map, four and five optimized clusters and their associated centroids (spectral distributions) are used for hot-worked and cold-rolled specimens, respectively. Therefore, the percentage (%) of Fe2+ and Fe3+ can be obtained by adjusting the LCF shown in Fig.4. The pseudoelectrode potential Epseudo as a function of Fe0 was used to reveal the microchemical inhomogeneity of the surface oxide film. Epseudo is roughly estimated by the mixing rule,
where \(\rm{E}_{\rm{Fe}/\rm{Fe}^{2 + (3 + )}}\) equals \(\rm{Fe} + 2e^ – \to\rm { Fe}^{2 + (3 + )}\), which is 0.440 and 0.036 V, respectively. Areas with a lower potential have a higher content of Fe3+ compounds. The potential distribution in a thermally deformed sample has a layered character with a maximum change of about 0.119 V (Fig. 6a,b). This potential distribution is closely related to the surface topography (Fig. 6a). No other position-related changes were observed in the underlying lamellar interior (Fig. 6b). On the contrary, for the combination of different oxides with different contents of Fe2+ and Fe3+ in cold-rolled SDSS, a non-uniform nature of the pseudopotential can be observed (Fig. 6c, d). Fe3+ oxides and/or (oxy)hydroxides are the main components of corrosion in steel and are permeable to oxygen and water50. In this case, it can be seen that the islands rich in Fe3+ are distributed locally and can be considered as corrosion areas. In this case, the gradient in the potential field, rather than the absolute value of the potential, can be considered as an indicator for the localization of active corrosion regions51. This inhomogeneous distribution of Fe2+ and Fe3+ on the surface of the cold rolled SDSS can change the local chemical properties and provide a more effective surface area in oxide film cracking and corrosion reactions, thereby allowing the underlying metal matrix to continuously corrode, resulting in internal inhomogeneity. and reduce the protective characteristics of the passivating layer.
K-mean clusters of Fe L2,3 edge regions and corresponding XAS centroids for a–c hot-worked X-PEEM and d–f cold-rolled SDSS. a, d K-means cluster plot overlaid on the X-PEEM image. Estimated pseudoelectrode potentials (epseudo) are mentioned along with K-means cluster diagrams. The brightness of an X-PEEM image such as the color in Fig. 2 is directly proportional to the X-ray absorption intensity.
Relatively uniform Cr but different chemical state of Fe leads to different origin of oxide film cracking and corrosion patterns in hot-rolled and cold-rolled Ce-2507. This property of cold rolled Ce-2507 is well known. With regard to the formation of oxides and hydroxides of Fe in atmospheric air, the following reactions are closed in this work as neutral reactions:
Based on the measurement of X-PEEM, the above reaction occurred in the following cases. A small shoulder corresponding to Fe0 is associated with the underlying metallic iron. The reaction of metallic Fe with the environment leads to the formation of an Fe(OH)2 layer (equation (5)), which amplifies the Fe2+ signal in the XAS of the L edge of Fe. Prolonged exposure to air will result in the formation of Fe3O4 and/or Fe2O3 oxides after Fe(OH)252,53. Two types of stable Fe, Fe3O4 and Fe2O3, can also form in a Cr3+ rich protective layer, where Fe3O4 prefers a uniform and cohesive structure. The presence of both results in mixed oxidation states (XAS-1 spectrum). The XAS-2 spectrum mainly corresponds to Fe3O4. Whereas the XAS-3 spectra observed at several positions indicated complete conversion to γ-Fe2O3. Since unwrapped X-rays have a penetration depth of approximately 50 nm, the signal from the underlying layer results in a higher intensity of the A peak.
The XRD spectrum shows that the Fe component in the oxide film has a layered structure, which is combined with the Cr oxide layer. In contrast to the passivation characteristic of corrosion due to local inhomogeneity of Cr2O317, despite the uniform layer of Cr2O3 in this study, low corrosion resistance was observed in this case, especially for cold-rolled samples. The observed behavior can be understood as the heterogeneity of the chemical oxidation state of the top layer (Fe) affecting the corrosion performance. The slow transfer of metal or oxygen ions in the lattice due to the same stoichiometry of the upper (Fe oxide) and lower layers (Cr oxide)52,53 leads to better interaction (adhesion) between them. This, in turn, improves corrosion resistance. Therefore, continuous stoichiometry, i.e. one oxidation state of Fe, is preferable to abrupt stoichiometric changes. Thermally deformed SDSS has a more uniform surface and a denser protective layer, which provides better corrosion resistance. However, for cold-rolled SDSS, the presence of Fe3+-rich islands under the protective layer destroys the integrity of the surface and causes galvanic corrosion of the nearby substrate, which leads to a decrease in Rp (Table 1) in the EIS spectra and its corrosion. resistance. Therefore, locally distributed islands rich in Fe3+ due to plastic deformation mainly influence the corrosion resistance performance, which is a breakthrough in this work. Therefore, this study presents spectromicrographs of the reduction in corrosion resistance due to plastic deformation of the studied SDSS samples.
Furthermore, while rare earth alloying in dual phase steels performs better, the interaction of this added element with the individual steel matrix in terms of corrosion behavior remains elusive based on spectroscopic microscopy observations. The Ce signal (along the XAS M-edge) appears only at a few positions during cold rolling, but disappears during hot deformation of the SDSS, indicating local deposition of Ce in the steel matrix instead of homogeneous alloying. Although the mechanical properties of SDSS are not improved6,7, the presence of REE reduces the size of the inclusions and is thought to suppress pitting at the origin54.
In conclusion, this work discloses the effect of surface heterogeneity on the corrosion of 2507 SDSS modified with cerium by quantifying the chemical content of nanoscale components. We answered the question of why stainless steel corrodes even when coated with a protective oxide layer by quantitatively studying the microstructure, chemical state of surface features and signal processing using K-means clustering. It has been established that Fe3+-rich islands, including their octahedral and tetrahedral coordination throughout the structure of mixed Fe2+/Fe3+, are a source of oxide film destruction and a source of corrosion of cold-rolled SDSS. Nanoislands dominated by Fe3+ lead to poor corrosion resistance even in the presence of a sufficient stoichiometric Cr2O3 passivating layer. In addition to the methodological advances made in determining the effect of nanoscale chemical heterogeneity on corrosion, the present work is expected to inspire engineering processes to improve the corrosion resistance of stainless steels during steelmaking.
To prepare the Ce-2507 SDSS ingots used in this study, the mixed components, including the Fe-Ce master alloy sealed with pure iron tubes, were melted in a 150 kg medium frequency induction furnace to produce molten steel and poured into casting molds. The measured chemical compositions (wt %) are listed in Supplementary Table 2. The ingot is first hot formed into blocks. Then the steel was annealed at 1050°C for 60 minutes to a solid solution, and then quenched in water to room temperature. The studied samples were studied in detail using TEM and DOE to study the phases, grain size and morphology. More detailed information about samples and production process can be found in other sources6,7.
Process cylindrical samples (φ10 mm × 15 mm) for hot pressing with the axis of the cylinder parallel to the deformation direction of the block. High-temperature compression was performed at a constant strain rate in the range of 0.01-10 s-1 at various temperatures in the range of 1000-1150°C using a Gleeble-3800 thermal simulator. Before deformation, the samples were heated at the selected temperature at a rate of 10 °C s-1 for 2 min to eliminate the temperature gradient. After achieving temperature uniformity, the samples were deformed to a true strain value of 0.7. After deformation, it is immediately quenched with water to maintain the deformed structure. Then the hardened specimens were cut parallel to the direction of compression. For this particular study, we chose a specimen thermally deformed at 1050°C, 10 s-1 due to a higher observed microhardness than other specimens7.
Bulk (80 × 10 × 17 mm3) samples of the Ce-2507 solid solution were tested on a three-phase asynchronous two-roll deformation machine LG-300, which provided the best mechanical properties among all other deformation classes6. The strain rate and thickness reduction were 0.2 m·s-1 and 5% for each path, respectively.
An Autolab PGSTAT128N electrochemical workstation was used to electrochemically measure SDSS after cold rolling to 90% thickness reduction (1.0 equivalent true strain) and hot pressing to 0.7 true strain at 1050 oC and 10 s-1. The workstation has a three-electrode cell with a saturated calomel electrode as the reference electrode, a graphite counter electrode, and an SDSS sample as the working electrode. The samples were cut into cylinders with a diameter of 11.3 mm, to the sides of which copper wires were soldered. Then the sample was poured with epoxy resin, leaving a working open area of 1 cm2 as a working electrode (the lower surface of the cylindrical sample). Use care during curing of the epoxy and during subsequent sanding and polishing to avoid cracking. The working surface is lapped and polished with a diamond polishing suspension with a particle size of 1 micron, cleaned with distilled water and ethanol and dried in cold air. Before electrochemical measurements, the polished samples were exposed to air for several days to form a natural oxide film. An aqueous solution of FeCl3 (6.0 wt.%), stabilized with HCl to pH = 1.0 ± 0.01, has been used to accelerate the corrosion of stainless steel55, since it is found in aggressive environments where chloride ions are present with strong oxidizing power and low pH as specified by ASTM. Proposed standards are G48 and A923. The samples were immersed in the test solution for 1 hour before any measurements were taken in order to reach a state close to stationary. For solid solution, hot-worked and cold-rolled specimens, the impedance measurement frequency range was 1 × 105 ~ 0.1 Hz, and the open-circuit potential (OPS) was 5 mV, which was 0.39, 0.33, and 0.25 VSCE, respectively. Each electrochemical test of any sample was repeated at least three times under the same conditions to ensure data reproducibility.
For HE-SXRD measurements, 1 × 1 × 1.5 mm3 rectangular duplex steel blocks were measured on a high-energy Brockhouse wiggler line at CLS, Canada to quantify the phase composition56. Data collection was carried out at room temperature in Debye-Scherrer geometry or transport geometry. The wavelength of X-rays calibrated to the LaB6 calibrant is 0.212561 Å, which corresponds to 58 keV, which is much higher than that of Cu Kα (8 keV) commonly used as a laboratory X-ray source. The sample is placed at a distance of 740 mm from the detector. The detection volume of each sample is 0.2 × 0.3 × 1.5 mm3, which is determined by the beam size and sample thickness. Each of these data was collected using a Perkin Elmer area detector, flat panel X-ray detector, 200 µm pixels, 40 × 40 cm2, using an exposure time of 0.3 seconds and 120 frames.
X-PEEM measurements of two selected model systems were carried out at the PEEM end station of the Beamline MAXPEEM line in the MAX IV laboratory (Lund, Sweden). Samples were prepared in the same way as for electrochemical measurements. The prepared samples were kept in air for several days and degassed in an ultrahigh vacuum chamber before being irradiated with synchrotron photons. The energy resolution of the beam is obtained by measuring the ion output spectrum from N 1 s to 1\(\pi _g^ \ast\) of the excitation region with hv = 401 eV in N2 and the dependence of the photon energy on E3/2.57. Spectral fit gave ΔE (spectral linewidth) ~0.3 eV over the measured energy range. Therefore, the beamline energy resolution was estimated to be E/∆E = 700 eV/0.3 eV > 2000 and flux ≈1012 ph/s by utilizing a modified SX-700 monochromator with a Si 1200-line mm−1 grating for the Fe 2p L2,3 edge, Cr 2p L2,3 edge, Ni 2p L2,3 edge, and Ce M4,5 edge. Therefore, the beamline energy resolution was estimated to be E/∆E = 700 eV/0.3 eV > 2000 and flux ≈1012 ph/s by utilizing a modified SX-700 monochromator with a Si 1200-line mm−1 grating for the Fe 2p L2.3 edge, Cr 2p L2.3 edge, Ni 2p L2.3 edge, and Ce M4.5 edge. Таким образом, энергетическое разрешение канала пучка было оценено как E/∆E = 700 эВ/0,3 эВ > 2000 и поток ≈1012 ф/с при использовании модифицированного монохроматора SX-700 с решеткой Si 1200 штрихов/мм для Fe кромка 2p L2,3, кромка Cr 2p L2,3, кромка Ni 2p L2,3 и кромка Ce M4,5. Thus, the energy resolution of the beam channel was estimated as E/∆E = 700 eV/0.3 eV > 2000 and flux ≈1012 f/s using a modified SX-700 monochromator with a Si grating of 1200 lines/mm for Fe edge 2p L2 ,3, Cr edge 2p L2.3, Ni edge 2p L2.3, and Ce edge M4.5.因此，光束线能量分辨率估计为E/ΔE = 700 eV/0.3 eV > 2000 和通量≈1012 ph/s 通过使用改进的SX-700 单色器和Si 1200 线mm−1 光栅用于Fe 2p L2,3 边缘、Cr 2p L2,3 边缘、Ni 2p L2,3 边缘和Ce M4,5 边缘。因此 ， 光束线 能量 分辨率 为 为 为 为 δe = 700 EV/0.3 EV> 2000 和 ≈1012 PH/S 通过 改进 的 SX-700 单色器 和 SI 1200 线 mm-1 光栅 于 Fe 2P 2P 2P L2.3 边缘、Cr 2p L2.3 边缘、Ni 2p L2.3 边缘和Ce M4.5 边缘。 Thus, when using a modified SX-700 monochromator and a 1200 line Si grating. 3, Cr edge 2p L2.3, Ni edge 2p L2.3 and Ce edge M4.5. Expand the photon energy in 0.2 eV steps. At each energy, PEEM images were recorded using a TVIPS F-216 CMOS detector with a 2 x 2 binning fiber optic connection providing 1024 × 1024 pixels in a 20 µm field of view. The exposure time of the images is 0.2 seconds, averaging 16 frames. The photoelectron image energy is chosen in such a way as to provide the maximum secondary electron signal. All measurements are performed at normal incidence of a linearly polarized photon beam. For more information on measurements, see a previous study58. After studying the total electron yield (TEY)59 detection mode and its application in X-PEEM, the detection depth of this method is estimated at ~4–5 nm for the Cr signal and ~6 nm for the Fe signal. The Cr depth is very close to the oxide film thickness (~4 nm)60,61 while the Fe depth is larger than the oxide film thickness. The XAS collected near the Fe L edge is a mixture of iron oxide XAS and FeO from the matrix. In the first case, the intensity of the emitted electrons is due to all possible types of electrons contributing to TEY. However, a pure iron signal requires higher kinetic energy for the electrons to pass through the oxide layer, reach the surface, and be collected by the analyzer. In this case, the Fe0 signal is mainly due to LVV Auger electrons and secondary electrons emitted by them. In addition, the TEY intensity contributed by these electrons decays during the electron escape path49 further reducing the spectral signature of Fe0 in the iron XAS map.
Integrating data mining into data cubes (X-PEEM data) is a key step in extracting relevant information (chemical or physical properties) in a multidimensional way. K-means clustering is widely used in several areas, including machine vision, image processing, unsupervised pattern recognition, artificial intelligence, and classificatory analysis24. For example, K-means clustering is well applied to clustering hyperspectral image data62. In principle, for multi-object data, the K-means algorithm can easily group them according to information about their attributes (photon energy characteristics). K-means clustering is an iterative algorithm for partitioning data into K non-overlapping groups (clusters), where each pixel belongs to a specific cluster depending on the spatial distribution of chemical inhomogeneity in the steel microstructural composition. The K-means algorithm consists of two steps: the first step calculates the K centroids, and the second step assigns each point to a cluster with neighboring centroids. The center of gravity of a cluster is defined as the arithmetic mean of the data points (XAS spectra) of that cluster. There are different distances to define neighboring centroids as Euclidean distances. For an input image of px,y (x and y are resolution in pixels), CK is the center of gravity of the cluster; this image can then be segmented (clustered) into K clusters using K-means63. The final steps of the K-means clustering algorithm are:
Step 2. Calculate the degree of membership of all pixels according to the current centroid. For example, it is calculated from the Euclidean distance d between the center and each pixel:
Step 3 Assign each pixel to the nearest centroid. Then recalculate the K centroid positions as follows:
Step 4. Repeat the process (equations (7) and (8)) until the centroids converge. The final cluster quality results are highly correlated with the optimal choice of initial centroids63. For the PEEM data structure of steel images, typically X (x × y × λ) is a cube of 3D array data, while the x and y axes represent spatial information (pixel resolution) and the λ axis corresponds to the energy spectral mode of photons . The K-means algorithm was used to explore regions of interest in X-PEEM data by separating pixels (clusters or sub-blocks) according to their spectral characteristics and extracting the best centroid (XAS spectral curve) for each analyte (cluster). It is used to study spatial distribution, local spectral changes, oxidation behavior and chemical state. For example, the K-means clustering algorithm was used for Fe L-edge and Cr L-edge regions in hot-worked and cold-rolled X-PEEM. Various numbers of K-clusters (microstructural regions) were tested to find the best clusters and centroids. When the graph is displayed, the pixels are reassigned to the correct cluster centroids. Each color distribution corresponds to the center of the cluster, showing the spatial arrangement of chemical or physical objects. The extracted centroids are linear combinations of pure spectra.
Data supporting the results of this study are available from the respective WC author upon reasonable request.
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