High‐temperature corrosion was studied under multiple chemical loads on ferritic‐austenitic model alloys (Fe–13Cr, Fe–18Cr–12Ni, and Fe‐25Cr–20Ni) with KCl deposit under 0.5% SO2 ...${\text{SO}}_{2}$/99.5% Ar gas atmosphere at 560°C $560^\circ {\rm{C}}$. Postexposure characterization was done by X‐ray diffraction and scanning electron microscopy. In a pure SO2 ${\text{SO}}_{2}$/Ar environment a protective Cr2O3 ${\text{Cr}}_{2}{{\rm{O}}}_{3}$ scale was formed by all samples. The introduction of KCl deposits causes the scale to be nonprotective and multilayered, consisting of CrS, FeS, Cr2O3,Fe3O4 ${\text{Cr}}_{2}{{\rm{O}}}_{3},{\text{Fe}}_{3}{{\rm{O}}}_{4}$, and Fe2O3 ${\text{Fe}}_{2}{{\rm{O}}}_{3}$. The impact of the microstructure and alloying elements is discussed.
The effect of KCl in high temperature corrosion of highly alloyed steels in SO2 was studied, simulating biofuel environments. The effect of the alloying elements and the microstructure was determined using XRD, SEM and EDX analysis. A corrosion mechanism of highly alloyed FeCrNi steels in SO2 + KCl environments was proposed.
The prediction of structural parameters and optoelectronic properties of compound semiconductors is very important. However, calculations often neglect chemical variability and structural defects. In ...chalcopyrite type semiconductors one of the major defects are copper vacancies (VCu). The four cation neighbors of the anion determine its position in the chalcopyrite type structure expressed by the Wyckoff position 8d (x, 1/4, 1/8). Intrinsic point defects like VCu and anti‐sites may cause variations of the anion position in the middle of the cation tetrahedron, especially in the anion position parameter x. For stoichiometric chalcopyrite type compounds a formalism according to the principle of conservation of tetrahedral bonds (CTB) can be applied to calculate the anion position parameter, but it fails in the case of off‐stoichiometric chalcopyrites. This case study of chalcopyrite type CuGaS2 and Mn‐substituted GuGaS2 shows that the experimentally determined anion position parameter x deviate from values calculated by CTB approach. The systematic investigation of off‐stoichiometric CuGaS2 and Mn‐substituted GuGaS2 demonstrates the effect of copper vacancies on the average radii of the cation sites (Wyckoff positions 4a and 4b) as well as on the anion position parameter x. By applying an elaborated CTB approach implementing copper vacancies an agreement between experimental and calculated anion position parameter x can be obtained.
In the chalcopyrite‐type semiconductor CuGaS2 a copper vacancy causes four degenerated coordination tetrahedra where the anion is dragged towards the gallium atoms. As a result, the Ga‐S bond length decreases and the Cu‐S bond length increases. A newly developed enhanced formalism for the conservation of tetrahedral bonds principle is verified by applying it to off‐stoichiometric chalcopyrite‐type phases in CuGaS2 – MnS series.
Insights into early damage mechanisms during high‐temperature gas corrosion provide important aspects for the prediction of the long‐term stability of high‐temperature materials exposed to a hot and ...corrosive environment. The current work presents a real time study concerning the combined oxidation and sulfidation of ferritic model alloys in a hot SO2 containing atmosphere using energy dispersive X‐ray diffraction. The applied high temperature reactor allows the transmittance of high energetic X‐rays in order to collect X‐ray diffraction pattern of the sample surface in situ during the reaction with the reactive environment. The results revealed that the first phases crystallizing on iron with 2 wt% chromium and with 9 wt% chromium are oxides. The aging experiments at T = 650° C with 0.5% SO2 and 99.5% Ar were followed in situ and caused an external mixed oxide‐sulfide layer on top of iron with 2 wt% chromium. On the higher alloyed material the external scale consists of iron oxides and the inner scale of mixed (Fe,Cr)‐oxides and chromium‐sulfides. The oxide content continuously increases parallel to an increase of the sulfide amount. Thus, the initially formed (Fe,Cr)‐oxides do not have a protective character and support the transport of sulfur through the growing oxide scale.
In situ studies of material interactions with hot and strongly corrosive gases render information of fundamental mechanisms influencing the lifetime of a material. A technique is introduced to study the early stage of high temperature corrosion in real time. First results of the aging mechanism of two model alloys and the potential for the real time analysis of high temperature corrosion by EDXRD are presented.
Mixed ionic electronic conducting ceramics Nd6−yWO12−δ (δ is the oxygen deficiency) provide excellent stability in harsh environments containing strongly reactive gases such as CO2, CO, H2, H2O or ...H2S. Due to this chemical stability, they are promising and cost‐efficient candidate materials for gas separation, catalytic membrane reactors and protonic ceramic fuel cell technologies. As in La6−yWO12−δ, the ionic/electronic transport mechanism in Nd6−yWO12−δ is expected to be largely controlled by the crystal structure, the conclusive determination of which is still lacking. This work presents a crystallographic study of Nd5.8WO12−δ and molybdenum‐substituted Nd5.7W0.75Mo0.25O12−δ prepared by the citrate complexation route. High‐resolution synchrotron and neutron powder diffraction data were used in combined Rietveld refinements to unravel the crystal structure of Nd5.8WO12−δ and Nd5.7W0.75Mo0.25O12−δ. Both investigated samples crystallize in a defect fluorite crystal structure with space group Fm3m and doubled unit‐cell parameter due to cation ordering. Mo replaces W at both Wyckoff sites 4a and 48h and is evenly distributed, in contrast with La6−yWO12−δ. X‐ray absorption spectroscopy as a function of partial pressure pO2 in the near‐edge regions excludes oxidation state changes of Nd (Nd3+) and W (W6+) in reducing conditions: the enhanced hydrogen permeation, i.e. ambipolar conduction, observed in Mo‐substituted Nd6−yWO12−δ is therefore explained by the higher Mo reducibility and the creation of additional – disordered – oxygen vacancies.
The crystal structures of non‐substituted and Mo‐substituted neodymium tungstates are described in detail through neutron diffraction and high‐resolution X‐ray diffraction. Combined X‐ray and neutron diffraction refinements and electron probe micro‐analysis were employed to locate Mo atoms in the crystal structure of Nd6−yW1−zMozO12−δ (z = 0, 0.25), while X‐ray absorption spectroscopy in the near‐edge regions confirmed no changes in the oxidation states of Nd and W.
The precise analysis of cation diffusion profiles through corrosion scales is an important aspect to evaluate corrosion phenomena under multicomponent chemical load, as during high‐temperature ...corrosion under deposits and salts. The present study shows a comprehensive analysis of cation diffusion profiles by electron microprobe analysis and microbeam X‐ray absorption near edge structure (µ‐XANES) spectroscopy in mixed oxide/sulfide scales grown on Fe–Cr model alloys after exposing them to 0.5% SO2. The results presented here correspond to depth‐dependent phase identification of oxides and sulfides in the corrosion scales by µ‐XANES and the description of oxidation‐state‐dependent diffusion profiles. Scales grown on low‐ and high‐alloyed materials show both a well‐pronounced diffusion profile with a high concentration of Fe3+ at the gas and a high concentration of Fe2+ at the alloy interface. The distribution of the cations within a close‐packed oxide lattice is strongly influencing the lattice diffusion phenomena due to their different oxidation states and therefore different crystal‐field preference energies. This issue is discussed based on the results obtained by µ‐XANES analysis.
Microbeam X‐ray absorption near edge structure spectroscopy was applied to the mixed oxide/sulfide scales grown on Fe–Cr model alloys. Quantitative analysis of corrosion products and oxidation state depth profiling was performed to determine the diffusion length of cations and gas components. A cation diffusion model through close‐packed oxide lattices is discussed based on the presented results.
•Chromia growth can be related to microstructural properties.•Sulphur was discovered within chromia by ToF-SIMS.•Sulphidation was independent on the alloy microstructure.•Grain refinement is ...beneficial to reduce sulphidation.
This study elucidates the impact of the microstructure of Fe-16Cr-0.2C on oxide layer formation at 650 °C in Ar-0.5 % SO2. A cold-rolled and two heat-treated states of the alloy were exposed for up to 1000 h. The samples were characterised in detail from microstructural and chemical perspectives using scanning electron microscopy (SEM), X-ray diffraction (XRD) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The microstructural modification of the alloy by heat-treatment was advantageous. It was found that Cr-carbides support chromia formation and reduce sulphidation when their area fraction is low and diameter is small.
Abstract
High‐temperature corrosion was studied under multiple chemical loads on ferritic‐austenitic model alloys (Fe–13Cr, Fe–18Cr–12Ni, and Fe‐25Cr–20Ni) with KCl deposit under 0.5% /99.5% Ar gas ...atmosphere at . Postexposure characterization was done by X‐ray diffraction and scanning electron microscopy. In a pure /Ar environment a protective scale was formed by all samples. The introduction of KCl deposits causes the scale to be nonprotective and multilayered, consisting of CrS, FeS, , and . The impact of the microstructure and alloying elements is discussed.
Surface degradation phenomena of two model equiatomic alloys from the CrMnFeCoNi alloy system were investigated in 2% O
2
and 10% H
2
O (
p
O2
= 0.02 and 10
−7
atm, respectively) at 800 °C for times ...up to 96 h. The crystallographic structures, morphologies, and chemical compositions of the corrosion layers developing on CrMnFeCoNi and CrCoNi were comparatively analyzed by mass gain analysis, X-ray diffraction, and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy and electron backscatter diffraction. The oxidation resistance of CrMnFeCoNi is relatively poor due to the fast growth of porous Mn-oxide(s). CrCoNi forms an external chromia layer that is dense and continuous in a dry 2% O
2
atmosphere. This layer buckles and spalls off after exposure to 10% H
2
O atmosphere. Beneath the chromia layer, a Cr-depleted zone forms in the CrCoNi alloy in both environments. As the oxide scale spalls off in the H
2
O-containing atmosphere, a secondary chromia layer was observed and correspondingly enlarges the Cr-depleted zone. In contrast, as the chromia layer remains without significant spallation when CrCoNi is exposed to a dry oxidizing atmosphere, the region depleted in Cr is narrower.
Graphic Abstract
Sulfur and water have a fundamental impact on the corrosion rate and potential failure of materials. It is therefore necessary to understand the mechanisms, rates, and potential means of transport, ...as well as the reactions of these elements with an alloy. This paper investigates the effect of water vapor in the initial stages of SO
2
corrosion of an Fe-9Cr-0.5Mn model alloy at 650°C in situ under laboratory conditions using energy-dispersive x-ray diffraction analysis. Two separate experiments were run, one with a 99.5% Ar + 0.5% SO
2
atmosphere and one with a 69.5% Ar + 0.5% SO
2
+ 30% H
2
O atmosphere. With a wet atmosphere, the alloy formed a scale with decreasing oxygen content towards the scale–alloy interface. Sulfides were identified above and below a (Fe, Cr)
3
O
4
layer in the inner corrosion zone. In contrast to this, the overall scale growth was slower in a dry SO
2
atmosphere.
Interpreting high-temperature corrosion induced by mixed-gas atmospheres is challenging due to the different contributions of oxidizing gases. Here, a comprehensive study on the combined ...oxidation/sulfidation using label molecules is presented. Fe-Cr model alloys with 2 wt% and 9 wt% Cr were isothermally exposed using a volumetric mixture of 0.5%S16O2/27%H218O and 0.5%S16O2/7%H218O at 650 °C for 5 h and then characterized by secondary ion mass spectroscopy (SIMS). Additionally, the reactions were followed in-situ utilizing energy dispersive X-ray diffraction. The study showed that both S16O2 and H218O contribute to the oxidation of the alloys but to different extents depending on the Cr-content.
•S16O2 an H218O contribute both to oxidation.•Grown oxides are deficient in cations (Fe and Cr).•Cr content in the alloy affected the extent of 18 O incorporation.•Oxidation/sulfidation was followed in-situ.•Differences between in-situ and ex-situ observed reaction products were demonstrated.
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