Mn Cr Spinel Research Articles

The role of thermal treatment and formulation on modifying the structural nature and optimizing certain physical features of coprecipitated superparamagnetic Co–Mn–Cr spinel ferrite

Examining the composition and heat treatment effects of co-precipitated Co0.7Mn0.3CrxFe2-xO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1) ferrite nanoparticles provides valuable insights into the structural, morphological, optical, magnetic, and electrical properties of these materials. X-ray diffraction (XRD) and Rietveld refinement analyses confirm the spinel cubic crystalline phase of the samples, indicating the formation of well-defined crystal structures. Transmission electron microscope micrographs manifest the nanoscale nature of the produced specimens and their narrow particle size distribution with a small standard deviation. This uniformity is often desirable in many applications because it confirms the similarity of the properties and behaviors of the nanoparticles. The Fourier transform infrared spectroscopy study suggests that the substitution of Cr3+ ions at octahedral sites influences molecular stability. Annealing causes a slight expansion in bond length and a subsequent decrease in stability. The presence of Cr3+ ions enhances the strength of the specimens, while annealing weakens them. This indicates a fine balance between composition and processing conditions in determining the strength of the materials. The estimated optical indirect bandgap undergoes a redshift by adding Cr3+ ions. Annealing at elevated temperatures reduces the bandgap due to the quantum confinement effect, indicating the tunability of optical properties through compositional and thermal control. Samples with x ≥ 0.6 exhibit nearly zero coercivity, indicating superparamagnetic behavior, which have promising applications. The preference of Cr3+ ions to occupy octahedral B-sites influences the magnetic behavior of the materials. The dielectric polarization and dielectric loss improved by adding Cr3+ ions, while the alternating current conductivity decreased. From impedance spectroscopy, the real and imaginary parts, Z’ and Z”, were increased by increasing Cr content. Furthermore, the annealing process greatly affects the electrical properties of the specimens. Overall, the study emphasizes the intricate relationship between composition, annealing conditions, and the resulting structural, magnetic, and electrical properties of Co–Mn–Cr ferrite nanoparticles, providing significant observations for the development of tailored materials for diverse applications in electronics, magnetics, and medicine.

An Expanded Model for the Pressure Effect in Metal Dusting of Mn-Containing Alloy 600 Based on Advanced Scale Characterization

Commercial alloy 600 was exposed to a metal dusting inducing gas mixture to investigate the effect of ambient and elevated pressures on the oxide scale formation behavior. The formed scales on this alloy were investigated via Raman spectroscopy, transmission electron microscopy (TEM), and atom probe tomography (APT) to reveal the differences in their morphology and chemical composition(s). Raman spectroscopy revealed graphite within the chromia scales as well as oxide grains with Mn–Cr spinel. High-pressure exposure promoted the presence of Mn within the oxide scale, and APT demonstrated these Mn-enriched areas dissolved up to 4 at.% C. This was consistent with the TEM results, which confirmed higher amounts of Mn in the scale formed under elevated pressure, as well as crystalline graphite in the scales for both conditions. The consequences for the scale breakdown, reflecting the end of the incubation time for metal dusting attack, are discussed in light of these results. A two-fragment dissociative adsorption model for CO, previously applied to catalysis reactions, is proposed when Mn is incorporated into the chromia scale.

Nanostructured Mg-doped Mn–Cr spinel oxide cathodes for solid oxide fuel cells with optimized performance

The cathode properties are critical for solid oxide fuel cells (SOFCs) performance and a major challenge for commercialization. This study employs the impregnation method to produce uniformly distributed Mg-doped manganese-chromium spinel oxide nanoparticles infiltrated into a network on the Gd0.1Ce0.9O1.95 (GDC) skeleton as SOFC cathodes. The effects of Mg doping on the performance of these nanostructured spinel oxides are investigated. Both experimental results and density functional theory (DFT) calculations demonstrate that Mg doping can optimize the electronic structure of the manganese-chromium spinel oxide, increase the oxygen vacancy concentration, and significantly enhance the electrical conductivity and oxygen reduction reaction (ORR) catalytic activity. The Mg-doped manganese-chromium spinel oxide cathode exhibits superior performance, with an area-specific polarization resistance of 0.33 Ω cm2 and a maximum power density of 976 mW cm−2 at 800 °C. Moreover, the nanostructures of the cathode barely change after 100 h of cell operation. These findings provide a new strategy for optimizing the performance of SOFC spinel oxide cathodes.

Effect of gas composition on the oxide scale growth mechanisms in a ferritic steel for solid oxide cell interconnects

The oxidation behavior of a ferritic steel Fe-23Cr-0.5Mn-0.6 Nb-0.1Ti (at%) considered for application in solid oxide cell (SOC) stack interconnects was studied at 800 °C. The oxidation kinetics and oxide scale microstructure formed in Ar-20%O2, Ar-4%H2-4%H2O and Ar-1%CO-1%CO2 atmospheres, simulating the SOC operation environments, were investigated by thermogravimetry (TG) in conjunction with electron microscopy (SEM/TEM) and atom probe tomography (APT). In all three environments multilayered oxide scales formed, consisting of Mn-Cr spinel on top of Cr2O3 and an additional Nb-rich oxide layer at the chromia-alloy interface. The initially faster oxidation in the low pO2 gases was attributed to formation of porous chromia scales compared to a dense scale formed in the high pO2 (Ar-O2) atmosphere. APT revealed segregation of minor alloying elements (Mn, Nb and Ti) to chromia grain boundaries in all three simulated SOC environments in quantitatively similar amounts, suggesting their similar effect on the ionic transport through the oxide scale. The findings indicate that oxygen activity in the test gas plays a dominating role in governing the oxidation kinetics and the oxide scale microstructure of the studied ferritic steel.

Effect of Manganese Concentration in Cerium Oxide Dispersed Ferritic Stainless Steels on Oxidation Behavior for Metallic Interconnect Material of Solid Oxide Fuel Cell

The ferritic stainless steel is used as interconnect materials of planar solid oxide fuel cell (SOFC) stack designs. However, the formation of a Cr2O3 oxide layer on the metallic interconnect is inevitable in the SOFC cathodic environment, and the formed layer decreases its electrical conductivity. In this study, we have investigated the effect of manganese (Mn) concentration in the cerium oxide (CeO2) dispersed ferritic stainless steels on oxidation behavior of SOFC metallic interconnect. The mixed powders with various Mn concentrations of 0.5 to 2wt.% were prepared by high-energy ball milling (HEBM), and subsequently were pelletized and sintered at 1400oC in reduction atmosphere. The area specific resistance (ASR) tests were carried out for 1,000 hrs. in air at 800 oC. The material characterization and morphology analysis of the tested samples were performed. After oxidation test, the oxide scale formed on the surface of the sample with Mn was confirmed as a Mn-Cr spinel structure. The thickness of the oxide scales increased with Mn concentration of the samples. Among the various Mn compositions, the sample with 1wt.% Mn had the lowest area specific resistance. The results show that the Mn in the ferritic stainless steel has great influence on the oxidation behavior. In this talk, we will discuss these results as related the requirements of the SOFC metallic interconnect.

Effect of high-temperature oxidation on the subsurface microstructure and magnetic property of medium manganese austenitic steel

The oxidation behavior of Fe-Mn-Cr-Al-Si medium manganese austenitic steel and the effect of high-temperature oxidation on the subsurface microstructure and magnetic properties were studied. When the oxidation temperature is relatively low (800 °C and 900 °C), the oxide layer is mainly composed of Mn oxides, Fe oxides, and Mn-Cr spinel. On the other hand, at the oxidation temperature of 1000 °C, the outer oxide layer consists of Fe 2 O 3 , Mn 2 O 3 , Fe 3 O 4 , and Mn 3 O 4 , while the inner oxide layer is composed of Mn-Cr, Fe-Cr, Mn-Al, and Fe-Si spinel. A thin ferrite layer (less than 6 µm) is formed in the subsurface of the samples oxidized at 800 °C and 900 °C. However, in the subsurface of the sample oxidized at 1000 °C, a thicker martensite layer (over 100 µm) is also observed besides the ferrite layer with thickness of about 10 µm. VSM results show that the samples oxidized at 800 °C and 900 °C exhibit paramagnetic behavior attribute to the relatively thin oxide and ferrite layer. However, the sample oxidized at 1000 °C exhibits soft magnetic property due to the much thicker oxide and BCC layer. • Mn-containing oxide is the main oxidation product when the medium manganese austenitic steel is oxidized. • The ferrite is formed in the subsurface of the samples oxidized at 800 °C and 900 °C. • The ferrite and martensite are formed in the subsurface of the sample oxidized at 1000 °C. • The samples oxidized at 800 °C and 900 °C are paramagnetic, while that at 1000 °C exhibits soft magnetic.

Characterization of a metallic interconnect operated in stack during 40,000 hours in SOFC mode

An SOFC stack operated for 40,000 hours has been dismantled offering the opportunity to characterize the metallic interconnect. The metal plate was carefully investigated to define the evolution of the surfaces exposed to the air and to the hydrogen electrodes respectively. The observations of the surfaces reveal the stability of the layers applied on top of the rib at the air side while in the bottom of the channels the protective coating (i.e., Co-Mn base spinel oxide) shows large crystals. The cross section allowed to highlight the formation of a rather homogeneous layer of thermal grown oxide between the metal and the coating. The average thickness of the TGO is around 11 μm. The hydrogen side shows a superficial alteration (due to the interaction with the water vapour) changing from the inlet to the outlet where it seems thinner as if the TGO further reacted by forming volatile compounds. The cross section observations confirmed the presence of a porous TGO with a rather high content of manganese in a Cr-Mn spinel oxide. Several spots testifies the zones of contact with the Ni base contacting layer. The cross section corresponding to such zones highlighted the Ni diffusion in the metal substrate.

Investigating Effects of Operational Parameters on the Rate of Electrochemical Cleaning of Chromium Deposits on Strontium-Doped Lanthanum Manganite Cathodes in Solid Oxide Fuel Cells

Chromium poisoning of the cathode remains a significant obstacle to the long-term performance of solid oxide fuel cells. Previously, a quick, easy method was introduced that requires no material modifications, major thermal cycling, nor exposes the cell to any species it does not already encounter during operation. The method, electrochemical cleaning, reverses the electrochemical deposition reaction of chromium-containing species by applying a mild anodic bias. Chromium vapor species are formed, freeing active sites in the cathode. The method has been shown to completely recover cell performance via this current-voltage technique.The present work investigates the mechanism of electrochemical cleaning by assessing the effect of various operating parameters on the rate of chromium removal. Using a design of experiments method, eight cells were poisoned and then cleaned using differing combinations of the following cleaning parameters: cell temperature, air humidity, anodic current density, and fuel humidity. Regression analysis of two comparative metrics, cell performance via current voltage curves and post-test chromium content determined via Energy Dispersive Spectroscopy, demonstrated that cell temperature has the highest effect on the rate of chromium removal, followed by air humidity. A refined parameter space for future study is proposed for further optimization of the parameters. Previously, the electrochemical cleaning method was shown to be effective in the removal of Cr2O3 deposited species, but not Cr-Mn spinel deposits. The effects of operating parameters on the removal of the two different types of Cr deposits is discussed. An additional process for the removal of Cr-Mn spinel is demonstrated, which again makes use of the cell operating parameters. Together, the two methods present a facile, periodic process to greatly extend solid oxide fuel cell lifetime.

High temperature cyclic oxidation behavior of a low manganese Fe12Mn9Cr5Si4Ni-NbC shape memory stainless steels

High amounts of Mn in Fe-Mn-Si-Cr-Ni shape memory stainless steels are detrimental for oxidation resistance at high temperatures. However, in order to maintain the shape memory effect of these alloys the content of manganese must be between 12 and 19 wt%. Thus, this study evaluated an alloy with relative low Mn (12 wt%) in cyclic oxidation tests at 800, 900 and 1000 °C. Kinetic analysis demonstrated parabolic oxidation behavior with rate similar to higher Mn-containing alloys from literature. It was also observed internal oxidation, for the first time in these alloys, and oxide layers composed by Mn2O3 and Mn-Cr spinel at 800 °C as well as Mn-Cr spinel and no continuous silica at 1000 °C. The internal oxidation mechanism was related to oxide layer defects and not to partial oxygen pressure decrease, considering that the internal oxidation presented the same oxides of the external layer. These results show that low Mn content does not improve the use viability of these alloys at high temperatures, since no decreasing in oxidation rate was observed and further internal oxidation occurred.

Solid-state cold spraying of FeCoCrNiMn high-entropy alloy: an insight into microstructure evolution and oxidation behavior at 700-900 °C

About 3 mm thick five-element equimolar high-entropy alloy (HEA) FeCoCrNiMn was successfully deposited by solid-state cold spraying (CS). The high-temperature oxidation behavior of the CSed HEA was investigated at 700-900 °C. Heat treatment was performed on the CSed HEA before oxidation to heal the incomplete interfaces between the deposited particles. Results show that the microstructure of the CSed HEA is characterized by grain refinement and abundant interparticle incomplete interfaces. Post-spray heat treatment promotes recrystallization and grain growth in the CSed HEA. After oxidation testing, the oxide scales are composed of multi-layers: a Mn2O3 (or Mn3O4) outer layer, a Mn-Cr spinel intermediate layer and a Cr2O3 inner layer. The CSed HEA exhibits higher parabolic rate constants and more favorable internal oxidation than the bulk HEAs that have similar compositions in the literature. Such a discrepancy becomes pronounced at higher temperatures. The grain refinement and numerous particle boundaries are responsible for such a distinctive performance of the CSed HEA.

High Voltage Mg-Ion Battery Cathode via a Solid Solution Cr–Mn Spinel Oxide

Lattice Mg2+ in a tailored solid solution spinel, MgCrMnO4, is electrochemically utilized at high Mn-redox potentials in a nonaqueous electrolyte. Complementary evidence from experimental and theor...

Analysis of oxide scales on oxidised EN 1.4509 ferritic stainless steel catalyst support by scanning electron microscopy

This study aims to investigate the oxidation behaviour at high temperature of a commercial ferritic stainless steel EN 1.4509. Tests of high temperature oxidation in air were carried out under isothermal condition (950 ºC) with different exposure times in order to obtain the oxidation kinetics law associated to this material. The surface and cross-section were analysed by scanning electron microscopy and energy-dispersive X-ray spectrometry (SEM/EDS), where the oxide scales proved to be formed by an external Mn-Cr spinel and an inner chromia layer. These oxides were generated by the migration of metallic cations from the stainless steel to the atmosphere. The size of the generated spinel crystals increased when increasing the exposure time as well as the cross-section thickness did. Moreover, the surface modification of ferritic stainless steel EN1.4509 was studied when platinum is deposited on its surface. The feature of this surface-modified ferritic stainless steel against oxidative atmosphere was evaluated by SEM/EDS. This platinum deposition led to reach a different surface morphology in terms of crystal size and nature.

Dissimilar Welding Between 1.4742 Ferritic and 310S Austenitic Stainless Steels: Assessment of Oxidation Behaviour

In the present work, the isothermal and cyclic oxidation behaviour of the 1.4742 ferritic and 310S austenitic stainless steels together with their dissimilar metal welds was investigated in the air with the humidity of 30 ± 3%. The welding was performed by the gas tungsten arc welding process utilizing ER310, ERNiCr-3, and ER446 filler metals. In the isothermal conditions, all the tested specimens followed the parabolic rate law. The maximum and minimum kp values were related to the 310S-BM, 1.89 × 10−1 mg2 cm−4 h−1, and the 1.4742-BM, 0.03 × 10−2 mg2 cm−4 h−1, respectively. Due to the formation of a dense and thin mono-layer of Cr–Fe–Al spinel ((Cr,Fe,Al)3O4) on the surface of the 1.4742 stainless steel alongside the lowest coefficients of the thermal expansion (CTE), it exhibits a highly satisfying oxidation resistance in both isothermal and cyclic conditions. In the cyclic conditions, except for the 1.4742-BM, the other specimens encountered the oxide scale cracking and spallation. The large CTE mismatch between the underlying 310S-BM, -BMS, and ER310-WM with the multi-layer chromia (Cr2O3) and Cr–Mn spinel ((Cr,Mn)3O4) oxide scale caused extensive spallation and mass loss in cyclic conditions. The ERNiCr-3-W in comparison with the two other weldments, by reducing the thermal expansion mismatch between the adjacent underlying metals and the better oxidation resistance of the nickel-base ERNiCr-3-WM, showed better oxidation resistance in both isothermal and cyclic conditions.

Performance and In-Situ Densification of Mn-Co Spinel Coating on SUS430/441 Interconnect by Spraying Method

The oxidation of interconnect in the SOFC stacks results in the degradation of cell performance, and thus the protective coating is needed. In this work, manganese oxide and cobalt oxide with molar ratio of 1:1 were deposited on the interconnect surface by spraying method to form the protective coating, whilst the interconnect materials were SUS430 and SUS441. First, the coated interconnects were heated at 800 oC in air for 1000 h. The results suggest that the two oxides were converted to not dense Mn-Cr spinel, yet the weight-gain and surface chromium content of the coated interconnects were not efficiently inhibited. Next, the coated interconnects were pretreated in 5 % H2/95 % N2 atmosphere for 10 h at 800 oC and the coatings were reduced into MnO and metallic Co. The diffusion sintering of metallic Co during the subsequent oxidation treatment facilitated the formation of dense Mn-Co-Cr spinel, resulting in excellent oxidation resistance, where the oxidation rate constant was an order of magnitude lower and surface chromium content was remarkably reduced. In addition, the phenomena described above for SUS430 and SUS441 were similar. As a result, the low-cost coating by spraying method with H2 pretreated was efficient, which significantly enhanced the oxidation resistance of SUS430 and SUS441 due to the formation of dense spinel.

Optimization of the formation of coatings based on SiAlOC glasses via structural, microstructural and electrochemical studies

The main objective of this work was to determine the tightness and wet corrosion resistance of SiAlOC glasses in the form of coatings on a steel substrate. The coatings were optimized in terms of the formation process, along with their resulting structure and microstructure. The coating material was obtained using sol-gel-synthesized, Al-doped, ladder-like polysilsesquioxanes as glasses' precursors. The layers were deposited by means of the dip-coating technique, for which the influence of both surface preparation (grinding/polishing) and withdrawal speed (5, 15, and 30 cm/min) was investigated. After the deposition, a two-stage thermal treatment (curing −70 °C/air and pyrolysis −800 °C/Ar) was applied. A series of structural (XRD, XPS, Raman) studies confirmed the formation of SiAlOC glass coatings, with a noticeable presence of an additional inner layer, most probably of MnCr2O4 spinel. Furthermore, the direct correlation between the increasing content of aluminum and the decreasing amount of carbon inward the coating can be suggested. Additionally, spectroscopic studies revealed the ongoing amorphization process of the free carbon phase. The result was that more crystalline species were present at the surface of the coating and the amorphous phases were near the inner MnCr spinel layer. Moreover, a set of microstructural (SEM, EDS, Confocal) studies was performed to evaluate the influence of the investigated deposition parameters on the coatings’ compactness, homogeneity, and thickness. The coating with the best characteristics obtained on the ground substrate when using the lowest withdrawal speed was chosen for the final examination of the tightness via electrochemical corrosion studies in a wet environment of chloride ions. The wet corrosion resistance was nearly 60 times higher for the coated specimen in comparison with the bare steel substrate. Additionally, an uncoated specimen after the same thermal treatment was investigated with the use of the same electrochemical studies. It exhibited significantly higher resistance to the corrosion in the wet environment than the rest of the samples due to the pre-formation of a mixed scale containing highly resistant Cr2O3.

Performance evaluation of a new Fe-Cr-Mn alloy in the reducing atmosphere of solid oxide fuel cells

A newly designed Fe-Cr-Mn alloy, considered as a candidate intermediate-temperature solid oxide fuel cell (SOFC) interconnect material, was cyclically oxidized at 650 °C, 750 °C, and 850 °C in a reducing atmosphere consisting of N2+3.75 vol% H2+25 vol% H2O(g) and corresponding to the SOFC anode environment. Results indicate that this alloy possesses good oxidation resistance and acceptable electrical conductivity at 750 °C in the reducing atmosphere. During the 1000 h oxidation, the oxide scale formed on the substrate at this temperature consists of an inner layer of Cr2O3 doped with Ti, a Mn-rich middle layer of Mn-Cr spinel and Mn2TiO4, and a top layer of MnO, with an overall thickness of around 3 μm. The oxidation kinetics at 750 °C obey a parabolic law with rate constants of 1.37 × 10−13 g2 cm4 s−1 from 0 to 130 h, and of 8.84 × 10−14 g2 cm−4 s−1 from 130 to 1000 h. The area specific resistance (ASR) of the oxide scale measured at 750 °C is 16.3 mΩcm2, which is higher than that formed in the oxidizing atmosphere but is still within the maximum limit of 100 mΩcm2, required for metallic interconnects. The increase in oxidizing temperature from 650 °C to 850 °C promoted the outward diffusion of Mn and Ti ions, thus changing the composition and morphology of the formed oxides; in addition, the oxidation rate of the alloy increased dramatically, leading to an increase of both the oxide scale thickness and the ASR values.

Formation Process and Stability of Co-W Oxide from an Electroplated Co-W Alloy Coating at 1000 °C for Cr-Based Steels

1. Introduction. Stainless steels are known as corrosion-resistant iron-based alloys which have wide range of applications in the modern society because of their good durability, relatively high electronic conductivity, good mechanical properties and low costing. Ferritic stainless steel is one kind of stainless steel which consists primarily of iron and chromium. The so-called passive film formed on the surface of ferritic stainless steel which is a very thin and self-healing film formed by Cr oxide provides the corrosion prevent property in high temperature environment. However, there is a critical issue related to the use of Cr based-steels which is the evaporation of the Cr, the volatile species of Cr are CrO3 and CrO2(OH)2 at high temperature. Some researchers reported that heating process would reduce the Chromium level which leads to the loss of corrosion resistance and transformation of microstructure. The evaporation of Cr results to the limitation for ferritic application. The evaporated Cr would poison the cathode materials in SOFC, and the acute oral toxicity for Cr (VI) ranges between only 50 and 150 µg/kg while the chromium(III) compounds were known to be carcinogenic. For the extensive application of ferritic stainless steels in high temperature environment such as the tubes in automobile exhaust system or boiler and stable the performance of these stainless steels, many efforts have been made. The use of Mn containing materials to form a Mn-Cr spinel oxide which can reduce the chromium evaporation to 30-40%. But the evaporation still occurred. Recently, researchers showed a tendency to prepare a coating on the stainless steels using various coating methods such as PVD, thermal spraying or electroplating. In our previous work, we have found the Co-W alloy coating formed by electroplating spontaneously formed a Co-W oxide layer at 800 °C, and this layer performed as a Cr barrier to stop the Cr outside penetration completely. In this research, we focus on the stability and barrier property of Co-W oxide layer at 1000 °C. Co-W alloy with 5 at% W was prepared by electroplating method on type 430 stainless steels. And the products after 1000 °C oxidation test in air were characterized in detail in order to elucidate the Cr barrier property offered by this coating. 2. Experimental. A commercially used Type 430 was set as the substrate whose composition was 16.2Cr–1.0Mn–0.7Si–0.12C–0.02S–0.04P (mass%). The steel was cut into small coupons. Thin Co-W alloy coating was deposited on the coupons to obtain a 10-µm-thick Co–W layer (85 g m−2). All solutions were prepared with analytical grade reagents and deionized water. The composition of Co-W electroplating was measured with an energy dispersive X-ray fluorescent analyzer (XRF) to confirm the composition to be Co-5 at%W. In the following paragraph, we called this alloy coating as Co-5W. After plating, the samples were oxidized in air with 2 kinds of heating processes. Process 1 (P1) is to heat the sample to 1000 °C directly and keep the temperature until 25 hours. Process 2 (P2) is to do the pre-treatment in air at 800 °C for 3 hours to form a Co-W oxide layer and then raise the temperature to 1000 °C. After the oxidation test, the characterization was made with 3D-digital scanner, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDXS). 3. Results and Conclusion. The oxidation behaviors at 1000 °C of type 430 stainless steels, with electroplating Co-W alloy coatings, was studied. The results are summarized as follows: After heated directly at 1000 °C, the coating layer showed reticulate cracks. While with a pretreatment at 800 °C, the surface showed no cracks.The formation of the cracks may lead to discontinuous Co-W protective layer and could provide a route for Cr and Fe outwards penetration.The crystal structure has been observed with TEM analysis. The Co-W oxide would grow to large cells while the Cr oxide remained small size approximately amorphous.The initial oxidation behaviors have been checked to confirm the order of the compounds to be oxidized. Higher oxygen potential resulted to the oxidation of Cr in early stage of oxidation.As shown in figure(A)(B)(C)(D) below, Co-W oxide continuous layer was not formed when oxidation directly at 1000 °C. However, with a pretreatment at 800 °C, the formed Co-W oxide can be stable at 1000 °C and it can block Cr outwards penetration at 1000 °C, as figure(E)(F)(G)(H). And the Co-W coating provided an oxidation resistance at 1000 °C as well. Figure 1

Oxidation behavior of cerium and tungsten-containing ferritic stainless steels at 1200 ºC in air

The high temperature oxidation behavior of medium chromium ferritic stainless steels containing different contents of alloying elements, cerium (Ce) and tungsten (W) was explored at 1200 ºC for 2 h in air. Under this oxidation condition, breakaway oxidation did not occur in all the specimens. A multilayer oxide film formed on all of the four steels, showing spinel oxide (Cr, Mn)3O4 on top of the Cr2O3 layer. A discontinuous SiO2 layer existed at oxide/metal interface. The top layer Cr-Mn spinel oxide exerts significant influence on the oxidation behavior of Fe-Cr alloys. The addition of cerium and tungsten has little influence on the oxidation behavior of ferritic stainless steels.

Investigation of Anomalous Oxidation Behavior of SUS430 Alloy in Solid Oxide Fuel Cell Dual Atmosphere

The oxidation behavior of SUS430 metallic interconnects for solid oxide fuel cell (SOFC) is investigated. Different reductive atmospheres are applied on one side of the interconnect with air on the other side to simulate the environment in a SOFC stack. The oxidation behavior under dual-atmosphere differs from that in a single air atmosphere. A thicker Mn-Cr spinel layer is formed on CH4/air dual atmosphere sample while Fe2O3 nodules are found on 5%H2/air sample, and a continual Fe2O3 layer is formed for H2/air sample. It can be concluded that the anomalous oxidation behavior of the interconnect is induced by a high concentration of H2 at the fuel side, which is caused by hydrogen diffusion from the fuel side to the air side across the interconnect. This results in the formation of cationic vacancies which accelerate the diffusivity of iron cations. For the fuel side of the dual atmosphere, the oxides morphologies on the interconnect surface are related to the oxygen partial pressure. Fibrous shape oxides are formed at low oxygen partial pressure and fine grain shape at high oxygen partial pressure. The anomalous oxidation processes in dual atmosphere are further investigated by means of marker experiments using an inert Au marker.

Initial Stage Oxidation and Surface Morphology of Modified 9Cr–1Mo Steel at High Temperatures in Air

Modified 9Cr–1Mo steel was oxidized in air at 550 and 750 °C for 25, 100, 250 and 500 h and the oxide scales formed were analysed. The surface morphology and the chemical state of the oxide scales were evaluated using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. The different exposure temperatures and time showed significant variations on the surface morphologies, the nature of oxide scale, and oxide constituents. The energy dispersive X-ray spectroscopic (EDS) analysis revealed the segregation of Mn at 750 °C even for short exposure time. Grazing incidence X-ray diffraction (GIXRD) patterns revealed the scales to be enriched with haematite and less intense magnetite peaks. Detailed XPS characterization indicated the presence of mixed oxides of iron (Fe), chromium (Cr) and manganese (Mn) in the oxide scales. The Fe–Cr spinel in the oxide scale offered resistance to oxidation of the steel, whereas Mn–Cr spinel was deleterious in nature as it promoted cracking and formation of blisters.