Spinel Layer Research Articles

Analysis of oxide scale thickness and pores position of HCM12A steel in supercritical water

Abstract More attention has been paid to the exfoliation of oxide scale on high-temperature heating surface of utility boiler. The oxidation mechanism of HCM12A steel in supercritical water is proposed and the growth of oxide film is simulated. The duplex scale contains an outer magnetite layer and an inner Cr-rich spinel layer. According to the data of Backhaus and Töpfer, the diffusion coefficient values of iron in magnetite layer are discussed and the function of R V, R I {R}_{\text{I}} for oxygen activity can be used for calculation of iron diffusion coefficients in Cr-rich spinel layer. Based on Wagner’s oxidation theory, the oxidation rate constants of HCM12A are calculated at 500 and 600°C in supercritical water, compared with experimental data of the relevant literatures. The oxygen activities at the interfaces of alloy/Cr-rich spinel oxide and magnetite/supercritical water are estimated. The simulation results of weight gain are matched with the test data. The iron diffusion mechanisms inside the magnetite layer and the Cr-rich spinel layer are analyzed. The iron diffusion coefficient at the interface of Cr-rich spinel/magnetite is discontinuous, while the oxygen activity is continuous in the whole double layer. The thickness of oxide scale on inner tube walls of the final superheater coils (T91) of a 600 MW supercritical boiler is calculated by using the calculation method provided by the paper. The modeling results, the measured data, and the calculation results by the method are compared. Accurate calculation of the thickness of the inner and outer oxide scales can provide a necessary basis for predicting the stress and exfoliation of oxide scales.

Ultrathin 3 V Spinel Clothed Layered Lithium‐Rich Oxides as Heterostructured Cathode for High‐Energy and High‐Power Li‐ion Batteries†

Main observation and conclusionIn an attempt to overcome the drawbacks of high‐capacity layered lithium‐rich cathodes xLi2MnO3·(1–x) LiMO2 (0 < x < 1, M = Mn, Ni, and Co), the spinel clothed layered heterostructured materials, x’Li4Mn5O12·(1–x’) Li[Li0.2Mn0.55Ni0.15Co0.1]O2 (x’ = 0.01, 0.03, 0.05) have been proposed and synthesized as high‐performance cathode materials for high‐energy and high‐power Li‐ion batteries. Based on the characterizations of X‐ray diffraction (XRD), transmission electron microscopy (TEM), Raman scattering spectroscopy, it is indicated that ultrathin 3 V spinel Li4Mn5O12 has been successfully clothed on the layered lithium‐rich cathode. Electrochemical tests demonstrate the sample 0.01Li4Mn5O12·0.99 Li[Li0.2Mn0.55Ni0.15Co0.1]O2 with an ultrathin clothing layer of spinel phase, exhibits the highest reversible capacity of 289.4 mAh g–1 and maintains 259.8 mAh g–1 after 80 cycles at 0.1 C rate. Meanwhile, it delivers outstanding rate discharge capacities of 229.4 mAh g–1 at 1 C, 216.8 mAh g–1 at 2 C and 184.4 mAh g–1 at 5 C as well as alleviated voltage fade. It is believed the ultrathin clothing spinel layer plays a vital role in the modification of the materials kinetics, and structural and electrochemical stability of the heterostructured cathode.

Microstructure evolution and mechanical properties of Co coated AISI 441 ferritic stainless steel/ YSZ reactive air brazed joint

This study investigates the microstructure evolution and mechanical properties of bare and Co coated AISI 441 ferritic stainless steel/YSZ ceramic reactive air brazed joints achieved by Ag–CuO braze for Solid Oxide Fuel/Electrolysis Cells applications. Interfacial microstructure of steel/YSZ joints is analyzed by SEM and TEM with EDS. A thick and porous oxide layer rich in Fe, Cr, and Cu is found in bare steel/YSZ ceramic joints, which is induced by the severe oxidation and its intense reaction with CuO during the brazing process. For the coated steel/YSZ ceramic joint, a comparably dense and uniform (Co, Fe, Cu)3O4 spinel layer is formed on the steel surface, which is tightly bonded with Ag–CuO braze without visible bonding defects. Meanwhile, the oxidation of steel substrates and its interaction with CuO is significantly suppressed. Co coated steel/YSZ joints possess reliable mechanical properties with the shear strength of 51 MPa, which is 54.5% higher than that of bare steel/YSZ ceramic joints (33 MPa). Besides, the microstructure evolution of coated steel/YSZ ceramic joints during brazing is schematically illustrated by a physical model.

Flash oxidation tests to evidence the segregation of boron at the grain boundaries of superalloy Inconel 718

In this study very short oxidation tests, called flash oxidation tests, were carried out at high temperature on samples of superalloy Inconel 718 containing some boron. Following these tests, a specific oxidation pattern was evidenced: spots were observed on the oxide film, these spots being discontinuously distributed along the grain boundaries. Whether outside or inside the spots, the oxide film exhibited the same three oxide layers which were likely rutile oxide (Cr,Nb,Ti)O2, chromia oxide Cr2O3 and spinel oxide (Ni,Fe)(Cr,Fe)2O4. However, inside the spots the oxide film was characterized by thicker oxide layers, a slightly different chemical composition of the spinel layer, and vitreous oxides at the top. Moreover, all the intergranular (Nb,Mo)2CrB2 borides of the microstructure were involved in this specific oxidation pattern, thus leading to the suspicion of high boron concentrations within the spots. FEG-EPMA analyses confirmed that, during the oxidation process, boron atoms diffused from the matrix to the oxide film, especially within the spots. Actually, the spots revealed areas where boron was highly concentrated just before oxidation. Such areas originated from the non-equilibrium segregation of boron near grain boundaries, which is a classical phenomenon when cooling and/or reheating a boron-containing alloy. Thus, the spots of the oxide film highlighted the grain boundary segments where boron segregated the most, confirming that the segregation of boron was not uniform within the grain boundary network.

Influence of Gd deposition on the oxidation behavior and electrical properties of a layered system consisting of Crofer 22 APU and MnCo2O4 spinel

In the study, the surface of the Crofer 22APU ferritic steel was modified with gadolinium oxide nanoparticles and a protective-conducting layer consisting of the MnCo2O4 spinel. This system was studied in the context of application in IT-SOFC interconnects. Four types of samples were studied: unmodified steel, steel coated with a manganese-cobalt spinel layer, steel modified with gadolinium oxide nanoparticles, and a system consisting of steel after both modifications. Steel samples with Gd2O3 nanoparticles were obtained via dip-coating, while the spinel coatings were deposited electrophoretically. All samples were oxidized for 1000 and 2260 h in air at 1073 K. Microstructural and oxidation kinetics studies revealed that gadolinium segregation at grain boundaries in the Cr2O3 scale between the steel and the spinel layer reduces the oxidation rate for the steel/coating system, thereby improving the adhesion of the scale to the substrate. The applied modification decreased the chromium evaporation rate and improved the electrical properties of the interconnect material.

Inexpensive and robust iron-based electrode substrates for water electrolysis and energy storage

The instability of iron under anodic conditions makes iron-based electrode substrates unsuitable for alkaline electrolyzers and rechargeable alkaline batteries. Therefore, significantly more expensive substrates such as nickel foam or sintered nickel are used. Nickel adds a significant cost to electrolysis and energy storage systems. We show that iron substrates can be stabilized using a unique protective thermal coating. These coatings can also yield some of the most electrocatalytically active electrodes in addition to showing no notable change in performance even after 1500 h of anodic polarization. Besides sintered iron, low-carbon steel mesh can be stabilized similarly. Low-carbon steel protected by a thin layer of lithium-doped cobalt spinel was found to be an excellent current collector for positive nickel hydroxide electrodes in alkaline batteries. Thus, surface-modified iron substrates, 40 times less expensive than nickel, are promising for lowering the material costs of alkaline water electrolyzers and rechargeable alkaline batteries.

Uniform corrosion behavior of FeCrAl alloys in borated and lithiated high temperature water

The uniform corrosion behavior of model FeCrAl alloys as well as commercial Zircaloy-4 as reference material were investigated in 360 °C borated and lithiated water. The results reveal that FeCrAl alloys exhibited better corrosion resistance than Zircaloy-4. It is found that the oxide films formed on the FeCrAl alloys are composed of outer Fe3O4 layer, inner layer consisting of compact spinel layer and porous spinel layer, and transition layer containing Al-Cr-rich oxides and matrix enriched with Fe°. The spinel oxides in the inner layer are FeFe2-x-y-zCrxAlyMozO4. The corrosion mechanism of FeCrAl alloys in high temperature water is discussed.

Oxidation and electrical properties of chromium\u2013iron alloys in a corrosive molten electrolyte environment

Chromium–iron (CrFe) binary alloys have recently been proposed to serve as the “inert” anode for molten oxide electrolysis (MOE). Herein, the effects of anodic polarization on physical and functional properties of CrFe anodes in the corrosive environment of MOE are studied via empirical observations and theoretical calculations. The findings indicate that the alloys form an inner chromia–alumina solid-solution covered by an MgCr2O4 spinel layer. A survey into the electrical properties of the detected oxides suggests that the layered oxide scale function as an efficient conductor of electricity at elevated temperature. The formation mechanism of the oxides is also investigated.

Mitigation of Chromium Poisoning of Ferritic Interconnect from Annealed Spinel of CuFe2O4

Low-temperature solid oxide fuel cells permit the possibility of metallic interconnects over conventional ceramic interconnects. Among various metallic interconnects, the ferritic interconnects are the most promising. However, chromium poisoning in them adversely affects their performance. To resolve this issue, various coatings and pretreatment methods have been studied. Herein, this article encloses the coating of CuFe2O4 spinel over two prominent ferritic interconnects (Crofer 22 APU and SUS 430). The CuFe2O4 spinel layer coating has been developed by the dip-coating of both samples in CuFe2O4 slurry, followed by heat treatment at 800 °C in a reducing environment (5% hydrogen and 95% nitrogen). Additionally, both samples were annealed to further enhance their spinel coating structure. The morphological and crystallinity analysis confirmed that the spinel coating formed multiple layers of protection while annealing further reduced the thickness and improved the densities. Moreover, the area-specific resistance (ASR) and weight gain rate (WGR) of both samples before and after annealing was calculated using mathematical modeling, which matches with the experimental data. It has been noted that CuFe2O4 spinel coating improved the ASR and WGR of both samples which were further improved after annealing. This research reveals that the CuFe2O4 spinel is the promising protective layer for ferritic interconnects and annealing is the better processing technique for achieving the preferred properties.

Hydrogen production by ethanol steam reforming: A study of Co- and Ce-based catalysts over FeCrAlloy monoliths

Co- and Ce-based structured catalysts deposited on FeCrAlloy monoliths have been prepared. A new two-step strategy for coating the monolith is used: (i) first, a MgAl2O4 spinel layer is generated on the FeCrAlloy substrate, and (ii) then, Co and Ce are incorporated in two different molar ratios by the conventional wet impregnation method. The spinel layer is formed from a solution of colloidal alumina and Mg(NO3)2, with an apparent viscosity of around 3300 mPa s. The results indicate that a homogeneous spinel coating with excellent adherence is obtained after two immersions and a calcination at 700 °C. Both structured catalysts are active in the steam reforming of ethanol at 650 °C. The system with a Co/Ce molar ratio of 3.7 exhibits the best performance with a high stability. A complete ethanol conversion and a hydrogen selectivity of around 95% are obtained in two reaction cycles of 36 h each with intermediate regeneration.

In situ investigation of atmospheric plasma-sprayed Mn\u2013Co\u2013Fe\u2013O by synchrotron X-ray nano-tomography

Applying atmospherically plasma-sprayed (APS) Mn1.0Co1.9Fe0.1O4 (MCF) protective coatings on interconnector steels minimized the chromium-related degradation within solid oxide fuel cell stack-tests successfully. Post-test characterization of the coatings disclosed a severe microstructural and phase evolution. A self-healing of micro-cracks, the formation and agglomeration of small pores, the occurrence of a dense spinel layer at the surface and a strong elemental de-mixing were reported in ex situ experiments. In the present publication, we prove for the first time these mechanisms by tracking the microstructure in situ at a single APS coating using synchrotron X-ray nano-tomography at the European Synchrotron Radiation Facility. Therefore, a 100-µm-long cylindrical sample with a diameter of 123 µm was cut from an APS-MCF free-standing layer and measured within a high-temperature furnace. All microstructural changes mentioned above could be verified. Porosity measurements reveal a decrease in the porosity from 9 to 3% during the annealing, which is in good accordance with the literature. Additionally, a partial detachment of an approximately 5-µm-thick layer at the sample surface is observed. The layer is dense and does not exhibit any cracks which are penetrating the layer. This kind of shell is assumed to be gastight and thus protecting the bulk from further oxidation.

Oxide properties of deformed Alloy 690TT in caustic solution at high temperature

Oxide properties and growth mechanism of deformed Alloy 690 T T SG tube in caustic solution at high temperature were investigated. The results indicated that the oxide film was comprised of the surface oxide layer and the inner oxide layer. The surface oxide layer exhibited a double layer structure with semi-coherent surface Ni-rich phases and subsurface Fe-rich phase. The inner oxide layer exhibited a multilayered structure: outermost large-sized NiO particle layer, transitional mixed Cr-rich spinel, NiO and Ni layer and inner Cr2O3 layer. Furthermore, some preferential oxidation channels extended along the grain boundaries and deformation bands inward respectively.

Enhanced Structural Stability of Boron-Doped Layered@Spinel@Carbon Heterostructured Lithium-Rich Manganese-Based Cathode Materials

Layered Li-rich 3d-transition-metal cathode materials, xLi2MnO3·(1–x)LiMO2, have increasingly triggered immense interest for their use in Li-ion batteries due to their advantages in terms of energy density. Nevertheless, poor cycle and rate performances cause limitations in practical commercial applications. We modified the material with boron bulk doping and carbon surface modification to form a B-doped layered@spinel@carbon heterostructure. Herein, B-doping can increase the lattice spacing favorable for Li+ insertion/extraction and inhibit oxygen loss successfully. The spinel layer and carbon on the surface can protect the material from corrosion due to electrolyte decomposition, which can accelerate Li+ and electron conduction and lessen the phase transition. The co-modified material reveals outstanding cycle and rate capability. Especially, it not only shows superior thermal stability at the high temperature of 45 °C, with a capacity retention rate of 83.3%, but also shows a higher discharge capacity of 108.9 mAh g–1 at the low temperature of −20 °C. Furthermore, the mechanism of the Li-rich cathode material with improved performance was also detected systematically. The proposed facile synthesis and co-modification of the boron-doped layered@spinel@carbon heterostructure can shed light on the design direction for cathode materials of lithium-ion batteries to solve the problem of electrochemical performance degradation caused by structural instability.

Growth mechanism of surface scales on Ni–Fe–Cr alloys at 960 °C in air

The oxidation mechanism of Ni–Fe–Cr alloys was studied at 960 °C in air. The results showed that an outward-growing (Ni,Fe)3O4/Fe2O3 layer and a Cr internal oxidation zone were initially formed on Ni–Fe–10Cr alloy. With increasing oxidation time, a continuous Cr2O3 inner layer was developed between internal oxidation zone and substrate, and subsequently the internal oxidation zone was further oxidized into (Ni,Fe,Cr)3O4 spinel layer. For the Ni–Fe–15Cr alloy, a thin Cr2O3 layer was firstly formed and broken down after 5 h. Then the development process of scale was similar to that on Ni–Fe–10Cr alloy.

Researh of Influence of Technological Factors of Formation of Plasma Coatings on their Thermal Technical Properties

The article considers issues related to the choice of abrasive material and technology of its use for the formation of necessary roughness on the base material and necessary heat resistance of the heat-protective coating during plasma spraying. The influence of the shape and size of the abrasive on final roughness of the treated surface was studied at various angles of contact between the sprayed abrasive and the surface (angle of attack). From the condition of maximum heat resistance, the composition of the heat-resistant composite coating of the spinel type was determined, which consists of 3 layers: the 1st layer (sublayer) material is a chromium-aluminum composite and the 2nd layer of a transitional spinel-based aluminum and chromium oxide + chromium-aluminum composite and the 3rd a layer of spinel based on aluminum and chromium oxides.

Ni/NiFe2 dual-layer coating for SOFC steel interconnects application

Ni/NiFe2 dual-layer coating is fabricated on SUS 430 stainless steel by magnetron sputtering for solid oxide fuel cells (SOFCs) interconnect application. Ni/NiFe2 coated steels are exposed to air at 800 °C for duration of up to 10 weeks in comparison with NiFe2 coated steels. Ni/NiFe2 coating is thermally converted to an oxide structure with an inner NiO layer and an outer NiFe2O4 spinel layer. NiO/NiFe2O4 oxide layer is more effective in blocking Cr migration and reducing oxidation rate during long-term exposure than a single NiFe2O4 layer converted from NiFe2 coating. Enhancements on oxidation resistance and Cr-blocking capability are attributed to NiO layer serving as a diffusion barrier between Cr2O3 and NiFe2O4.

A comparative study on the microstructure evolution of conventional and nanostructured MCrAlY powders at high-temperature

Previous studies have revealed that nanostructured MCrAlY alloys with nano-oxide dispersoids show promising potential for high-temperature applications. In an effort to provide a fundamental understanding of microstructure-oxidation performance derived insights, the influence of nano-scaled microstructure and oxide dispersion on the oxidation behavior of MCrAlY powder material is investigated in this study. The microstructural evolution within the conventional and nanostructured powders was investigated during thermal exposure at 1000 °C for up to 100 h. The results indicated that a pure alumina layer was formed onto the surface of conventional powder and the thickness of Al2O3 layer increased gradually with the oxidation time. However, a duplex-structure comprising an outer spinel layer and an inner alumina layer was formed onto the surface of the nanostructured powder. The different oxidation mechanisms of the two powders were discussed.

Early stages of iron anodic oxidation: Defective growth and density increase of oxide layer

The early stages of anodic oxidation in the potentiostatic condition at the interface between a borate buffer solution at pH 8.4 and Fe(100), (110), and (111) surfaces are investigated by sub-second resolution real-time x-ray reflectometry. The growth process is composed of three stages. The electron density of the inner-oxide layer for Fe(111) after a potential step from $\ensuremath{-}0.8$ to $+0.7$ V versus Ag/AgCl in the steady-state growth, stage III (1.65 s $lt$, where $t$ denotes the time after the potential step), is 1.52 electrons/${\AA{}}^{3}$, which is close to the density of magnetite. The growth rate is proportional to ${t}^{\ensuremath{-}1}$, which follows the direct logarithmic law or point-defect model. In an earlier stage of the oxide film growth, stage II $(0.4lt\ensuremath{\le}1.65$ s), the growth rate is proportional to ${t}^{\ensuremath{-}1.5}$, while the electron density maintains the same value as that in stage III. At the first stage that can be discriminated by our time resolution, stage I $(t\ensuremath{\le}0.4$ s), the growth rate is the same as that for stage II, i.e., proportional to ${t}^{\ensuremath{-}1.5}$, while the electron density significantly decreases. The suggested structure is a highly defective spinel oxide, and defects are filled by the end of stage I; there is no signature of the growth of other phases, such as hydroxide, as the inner layer. This behavior indicates that the early stage of the oxide film growth is faster than the formation of a complete spinel layer at the iron-oxide interface. The deviation of the growth rate from the point-defect model in stages I and II is caused by the strong electric field at the iron-oxide interface.

High temperature oxidation behavior of Co-20Re-25Cr-2Si alloy at different oxygen pressures

The oxidation of the quaternary Co-20Re-25Cr-2Si alloy has been studied at 1000−1300 °C for 24 h. On the whole, at high oxygen pressure of 0.1 MPa the alloy shows mass loss while at low oxygen pressure of 3.0×10−5 Pa, however, the alloy shows mass gain. Compared with the irregular kinetics under the oxygen pressure of 0.1 MPa, the kinetics approximately conform to parabolic law under the oxygen pressure of 3.0×10−5 Pa, with the value of oxidation activation energy equal to 262.7 kJ/mol. The scales formed under the oxygen pressure of 0.1 MPa at 1000−1300 °C are very similar. The outermost layer is CoO, followed by a mixed oxide layer probably composed of CoO and CoCr2O4. The innermost layer is probably composed mainly of a thin discontinuous layer of Cr2O3, accounting for the continuous mass loss due to the evaporation of Re element in the form of ReO3. However, the sales formed under the oxygen pressure of 3.0×10−5 Pa are probably composed of an outer thin layer of CoCr2O4 spinel and an inner dense and continuous layer of Cr2O3, which provides the good oxidation resistance for the alloy. The combination of the addition of Si and low oxygen pressure has shown a synergistic effect on the formation of a protective chromia layer.

Initial Oxidation Behavior of Ferritic Stainless Steel Interconnect with Sputtered NiFe2 Alloy Coating

Initial oxidation behavior of ferritic stainless steel with a sputtered NiFe2 coating for solid oxide fuel cells interconnect application was investigated in air at 800 °C to understand the transformation process from the NiFe2 alloy coating to NiFe2O4 spinel layer. The results indicated that the NiFe2 coating was initially converted to a surface scale with a layered structure consisting of a top Fe2O3 layer followed by NiFe2O4 mid-layer and an NiO inner layer. Cr oxide started to form at scale/steel interface before the coating was completely oxidized. The preferential growth orientation of Fe2O3 on the surface varied with time during the initial oxidation stage. Steel preoxidation prior to coating not only accelerated the oxidation of NiFe2 coating and the growth of NiFe2O4 layer, but also suppressed the orientated growth of Fe2O3. The surface scales on the coated steels were electrically conductive. The oxidation mechanism of the coated steels is discussed.