Cr2O3 Solid Solutions Research Articles

Microstructure evolution, mechanical characteristics and erosion behavior of Cr2O3–Al2O3–MgO–ZrO2 refractories in molten reduction slag

In order to overcome the serious corrosion problem of furnace hearth refractory materials for smelting reduction ironmaking, the Cr2O3–Al2O3–MgO–ZrO2 refractories were synthesized, and the phase component, microstructure, mechanical characteristics, and slag erosion behavior of composite refractories were investigated and evaluated. The results indicate that the composite refractories mainly consist of Mg(Al, Cr)2O4 spinel phase, (Al, Cr)2O3 solid solution and ZrO2. The content of Mg (Al, Cr)2O4 spinel significantly increases with the increasing MgO content, while the content of (Al, Cr)2O3 obviously decreases. Furthermore, the comprehensive mechanical properties of the MgO modified composite refractories can be obviously enhanced by increasing the MgO content, and sample S4 presents the best mechanical characteristics when the MgO addition is 10 wt%. The corresponding hardness, compressive strength and bending strength are 14.22 GPa, 267.14 MPa and 80.0 MPa, respectively. This is mainly because the formation of Mg(Al, Cr)2O4 significantly improves the densification of composite refractories, and a large number of high-strength Mg (Al, Cr)2O4 spinel “bridge” formed between the large-sized (Al, Cr)2O3 grains. The erosion test results indicate that increasing the MgO content can significantly improve the slag infiltration resistance of composite refractories, because the formation of Mg (Al, Cr)2O4 spinel layer significantly improves the densification of MgO reinforced composite refractories. Although the slag corrosion resistance of Mg(Al, Cr)2O4 is lower than that of (Al, Cr)2O3, the Cr2O3–Al2O3–MgO–ZrO2 refractories still presents good erosion resistance, permeability resistance, and excellent mechanical characteristics.

Utilization of industrial wastes in Al2O3‐SiC‐SiO2‐C refractories: Characterization of erosion and corrosion resistance

AbstractIndustrial waste has a high potential for reuse in the production of refractories, which can significantly lower costs. The purpose of the research is to access the role of three types of recycled industrial wastes (ferro‐titanium slag [FTS], ferhro‐chromium slag [FCS], and ferro‐vanadium slag [FVS]) utilized as aggregate in the traditional Al2O3‐SiC‐SiO2‐C refractories (ASSC). Erosion and corrosion resistance characterization was performed to determine whether these industrial waste‐containing ASSC still perform better than traditional ASSC during service. Based on the results, incorporating FVS into the ASSC was not recommended, its corrosion resistance was significantly reduced due to the presence of excessive liquid phase. Nonetheless, FTS and FCS could be added as aggregate to the ASSC without affecting their performance. This necessity of the proper amount of liquid phase in ASSC was highlighted in this research since the parameters could alter its properties. Furthermore, the Ti(C,N) transformed from TiO2 in FTS via carbothermal reduction nitridation could effectively prevent further corrosion, and Cr2O3 derived from the decomposition of (Al,Cr)2O3 solid solution in FCS could play the same role during the corrosion process, both of which may be potential raw materials to be used in refractories.

Microstructure, phase evolution, mechanical properties and slag corrosion resistance of ZrO2 reinforced Al2O3–Cr2O3 composites

Al2O3–Cr2O3 refractories have excellent slag corrosion resistance and can adapt to the oxidation/reduction atmosphere in the smelting reduction ironmaking furnace. Improving the corrosion resistance of chrome corundum refractories is of great significance for extending the life length of industrial kilns and decreasing carbon emissions. In this work, the influences of ZrO2 content on the phase evolution, microstructure, physicochemical properties, mechanical properties and slag corrosion resistance of the ZrO2 reinforced Al2O3–Cr2O3 composite refractories were investigated. The results show that the ZrO2 reinforced Al2O3–Cr2O3 composites are primarily composed of (Al, Cr)2O3 solid solution and m-ZrO2. The compressive strength, flexural strength and hardness of the composite refractories can be improved by adding 2.5 wt% ZrO2. However, adding more ZrO2 will reduce its mechanical properties. This is due to ZrO2 excessive content will augement the porosity of the composite refractories, and the phase transformation of ZrO2 will also lead to the formation of microcracks during the cooling process. It is worth noting that higher ZrO2 content improves the thermal shock resistance of the composites, which is primarily put down to the microcrack toughening mechanism of ZrO2. The corrosion tests show that the thicknesses of the corrosion and infiltration layers of composite refractories are significantly decreased accompanied by ZrO2 content increase. This is because the formation of high melting point CaZr4O9 reduces the CaO content and slag alkalinity, and inhibits the dissolution of Al2O3 into the slag to a certain extent. Therefore, the slag corrosion/penetration resistance of the of the composites are obviously improved with the increase of ZrO2 addition.

Experimental Investigation of the Initial Stage of the Oxidation Mechanism of Co Coating for Solid Oxide Fuel Cell Interconnects at 650 °C

The evolution of Co coating in solid oxide fuel cells (SOFCs) under low and medium temperatures (<700 °C) is different from that under high temperatures. In this context, the oxidation corrosion of 441 ferritic stainless steel (FSS) with a Co coating in air under 650 °C was investigated. The results indicated that the Co coating was oxidized rapidly and a Co3O4 spinel layer formed in the initial exposure of 5 min, which improved the oxidation resistance of 441 steel. After oxidation at 650 °C for 120 h, a Cr2O3 layer with a thickness of 0.2–0.4 μm was observed on the surface of bare 441 steel, while the surface oxide scale of Co-coated 441 steel samples was composed of an inner Cr2O3 and Fe2O3 solid solution (0.1–0.3 μm thick), an intermediate (Fe, Co)3O4 layer and an outer Co3O4 spinel layer, respectively. The Co-coated 441 steel sample exhibited better electrical properties. After oxidation at 650 °C for 120 h, the area-specific resistance (ASR) of the Co-coated steel was 3.73 mΩ·cm2, which was 25.4% lower than that of bare 441 steel as 5 mΩ·cm2. Furthermore, the thermal growth process and protection mechanism of Co coating at 650 °C were discussed.

Mechanical properties and microstructure of Cr2O3 reinforced 3Al2O3·2SiO2 composite refractories

Chromium-containing refractories have excellent high-temperature corrosion resistance. In this work, Cr2O3 reinforced Al2O3–SiO2 composite refractories were prepared by reaction sintering process. The microstructure, phase composition, mechanical properties and strengthening mechanism of the Cr2O3 reinforced Al2O3–SiO2 composite refractories were investigated. The results show that when the Cr2O3 content is 10–20 wt%, the composites are mainly composed of Cr-mullite (Al3.94Cr0.50Si1.56O10.44), (Al, Cr)2O3, and a small amount of SiO2 phase. When the Cr2O3 content exceeds 20%, the composite refractories mainly consist of (Al, Cr)2O3 and SiO2 phases. The appropriate amount of Cr2O3 can effectively enhance the mechanical properties and compactness of the Cr2O3 reinforced Al2O3–SiO2 composites. This is mainly due to the formation of Cr doped mullite and SiO2 phases, which reduces the sintering temperature of mullite ceramics and improves its density and mechanical properties. The highest hardness (27.03 GPa), flexural strength (213.21 MPa) and compressive strength (528 MPa) values are observed at a Cr2O3 content of 20 wt%. However, excessive addition of Cr2O3 will deteriorate the mechanical properties of the composites. The formation of a large amount of (Al, Cr)2O3 solid solution phase will increase the porosity of the composites, and reduce the compactness and sintering properties of the composites.

Metallothermic SHS of Al2O3–Cr2O3 + TiC ceramic composite material

Ceramic composite material Al2O3–Cr2O3 + TiC was prepared by metallothermic self-propagating high-temperature synthesis (SHS) under 5 MPa of Ar pressure from green mixtures containing TiO2, Al, and C powders, and additive of CrO3 + Al blend. An increase in the mass fraction α of CrO3 + Al in the general charge was found to raise the synthesis temperature. For α < 0.2, sintered multiphase products were formed. In case of high additive content (α > 0.2), the combustion products were separated into two layers: carbide and oxide phases. Optimum synthesis conditions for forming the cast product consisting of Al2O3–Cr2O3 solid solution (“ruby”) and uniformly distributed TiC particles were found. In order to evaluate the possibility of using this material as a cutting tool, physical and mechanical properties were determined. SHS-prepared Al2O3–Cr2O3 + TiC composite after 24-h milling in a planetary mill, pressing under a pressure of 200 MPa, and sintering at 1460°С was found to possess density of 4.55 g/cm3, hardness of 20 GPa, flexural strength of 680 MPa, and crack resistance of 4.2 MPa m1/2.

Microstructures and property profiles of atmospheric plasma sprayed (Al,Cr,Ti)2O3 solid solution coatings

Ternary compositions of the Al2O3-Cr2O3-TiO2 system are expected to improve the coating properties of the binary compositions, such as Al2O3-Cr2O3. Previous studies have shown that mechanical blends of all three components are not suitable for the formation of solid solutions. Thus, experimental ternary agglomerated and sintered feedstock powders were prepared in this study, which had different Al2O3/Cr2O3 ratios but a similar content of TiO2 (about 8.5 mol%). Atmospheric plasma spraying (APS) was used to prepare coatings from these powders, as well as from commercial powders of the Al2O3-Cr2O3 system. In the experimental feedstock powders, an (Al,Cr)2O3 solid solution was formed, while significant amounts of TiO2 were found to exist separately. During spraying a ternary (Al,Cr,Ti)2O3 solid solution was formed. The deposition efficiency is positively influenced by the use of agglomeration and sintering for feedstock powder preparation and Al2O3-rich solid solutions connected with some porosity in the powder. The coatings were characterized by their microstructure, phase composition, hardness, abrasion and sliding wear resistance, corrosion against H2SO4 and specific DC resistivity. The coating properties depend on the Al/Cr-ratio. All ternary solid solution coatings show a high corrosion resistance. The addition of TiO2 creates the possibility to tailor the electrical resistance.

Synthesis of (Fe,Cr)2O3 solid solution pigment powders for ink application

AbstractIron chromite pigment was synthesized via solution combustion using iron(III) nitrate nonahydrate and chromium(III) nitrate nonahydrate as starting materials, and glycine, urea, citric acid, and ethylene glycol as fuels. The effect of postheating temperature on the structure, microstructure, and chromatic properties of the synthesized powders was also studied. X‐ray diffraction patterns showed that the as‐synthesized powders were amorphous to crystalline FeCr2O4 phases, depending on fuel type. Moreover, regardless of the fuel type, postheating led to the d‐space shift and oxidation and formation of (Fe,Cr)2O3 solid solution. Phase transformation of FeCr2O4 to (Fe,Cr)2O3 solid solution was observed at 500/750°C depending on the dominant phase present in the as‐synthesized particles. Fourier transform infrared analysis illustrated a shift in the band position of octahedral M–O and tetrahedral M–O bonds due to the movement of Fe cations and the lattice shrinkage by increasing the postheating temperature. Moreover, scanning electron micrographs showed that Fe0.7Cr1.3O3 semispherical fine particles consisted mainly of porous and spongy FeCr2O4 particles due to the oxidation and phase transformation during postheating. According to chromatic measurements, the ink prepared by using the powders synthesized in the presence of glycine and post‐heated at 500°C showed reddish‐brown color which could be considered a promising candidate for tile decoration application. Furthermore, rheology studies revealed that the prepared ink showed non‐Newtonian shear thinning behavior.

Experimental observation of spin-phonon coupling in Cr2O3 and Fe2O3 solid solution and bandgap engineering

In this work, we present experimental evidence of spin-phonon coupling at room temperature in the solid solutions of Cr2O3 and Fe2O3. The bulk ceramics of Cr2-xFexO3 were fabricated by sol–gel method. The corundum crystal structure with R-3c space group confirmed by Reitveld refinement. Spin-flop transitions observed in the 100–400 K temperature ranges from M−H studies were observed in the lower doping range. The magnetic transition observed from the temperature dependent magnetic studies showed the transition is consistent with the previous reports. The spin-phonon coupling in the ceramics is observed from the two phonon modes in the materials originating from Eg and A1g modes in the Raman spectroscopy. Eg mode shifts towards lower frequency, while A1g mode shifts towards high frequency. UV–vis spectral studies revealed the narrowing of bandgap originating from the Cr-O-Fe hybridisation. Impedance spectroscopy analysis shows that relaxation peaks shift from 90 K to 240 K towards high temperatures. The activation energy value is 0.32 – 0.21 eV, indicating the polaron hopping conductivity in the bulk ceramics.

Phase transformation of FeCr2O4 to (Fe,Cr)2O3 solid solution pigment powders: Effect of post‐heating temperature

AbstractIron chromite powders were synthesized via solution combustion route using iron(III) nitrate nonahydrate and chromium(III) nitrate nonahydrate as starting materials, as well as glycine–urea, glycine–citric acid, and glycine–ethylene glycol mixtures as fuels. The effect of postheating at different temperatures on the structure, molecular, microstructure, and chromatic properties of powders and tiles colored by in‐glaze powders was studied. The X‐ray diffraction patterns showed that as‐synthesized powders were obtained in crystalline FeCr2O4 phases moreover, postheating of the powders led to d‐space shift and oxidation and formation of (Fe,Cr)2O3 solid solution phase regardless of fuel type. Phase transformation of FeCr2O4 to (Fe,Cr)2O3 solid solution was observed at 500/750°C depending on the dominant phase of as‐synthesized particles. Fourier transform infrared analysis illustrated that the band positions of octahedral M–O and tetrahedral M–O bonds were shifted due to Fe cations movement from their position and lattice shrinkage by increasing of post‐heating temperature. Moreover, scanning electron micrographs showed that Fe0.7Cr1.3O3 semispherical fine particles were formed from porous spongy FeCr2O4 particles due to oxidation and phase transformation during the postheating. Furthermore, chromatic properties of the samples were represented. The color properties of the pigments showed that the formation of brown pigments is provided with the phase transformation from FeCr2O4 to (Fe,Cr)2O3 at a temperature of up to 750°C. Moreover, increasing the color purity to this temperature is related to the removal of residual carbonaceous matters. The chromatic properties of the glazed tiles colored using the pigments showed that postheating between 250 and 500°C led to more brown appearance.

Microstructure and corrosion behavior of Al2O3–Cr2O3 composites with various Cr2O3 content in Al2O3–SiO2–CaO–FeO slag

The corrosion of refractory materials by high-temperature slag has caused severe technical challenges and economic losses to the metallurgical industry. In this study, the microstructure and mechanical properties of Al2O3–Cr2O3 composites were investigated. The static crucible method was used to investigate the corrosion and infiltration behavior of a typical smelting reduction ironmaking slag for the Al2O3–Cr2O3 refractory. Corrosion test results indicate that the corrosion region of Cr2O3–Al2O3 composites is mainly composed of a corrosion and infiltration layer, and the thickness of the corrosion layer decreases significantly with increasing Cr2O3 content in the sample, whereas the thickness of the infiltration layer increases gradually. The dissolution of Al2O3 into the slag is the main reason for the severe erosion of Al2O3–Cr2O3 composites with a low Cr2O3 content. Increasing the amount of Cr2O3 can significantly improve the slag corrosion resistance of Cr2O3–Al2O3 composites, however will have a negative effect on the slag infiltration resistance, which is mainly owing to the increase in porosity of the composite refractory. The formation of the (Al, Cr)2O3 solid solution effectively inhibited the dissolution of alumina in the slag. In addition, excessive Cr2O3 content can also consume Al2O3 in the slag and form an (Al, Cr)2O3 solid solution, which improves the slag corrosion resistance of the aluminum chromium composite refractory.

Preparation of Al2O3–Cr2O3 Solid Solutions as Buoyancy Markers and Their High Pressure Synchrotron X-ray Diffraction Analysis

Preparation of Al2O3–Cr2O3 Solid Solutions as Buoyancy Markers and Their High Pressure Synchrotron X-ray Diffraction Analysis

In-situ improved corrosion resistance of corundum-mullite refractory for the incineration of hazardous spent high-salt organic liquor by Cr2O3: Interfacial anti-erosion mechanism

Existing refractory-life-extending technologies had concentrated on the preparation of homogeneous high-performance refractory materials in advance. In this paper, an in situ anti-erosion approach of conventional corundum-mullite refractory material was proposed for the incineration of hazardous spent high-salt organic liquor, in which the powdery additives were directly added into the spent liquor. The phase transformation, interfacial morphology evolution, and pore distribution of the corundum-mullite refractory with and without Cr2O3/ZrO2 addition were compared, and the in situ anti-erosion mechanism was determined via the XRD, SEM, XPS, mercury intrusion porosimetry, and thermodynamic analyses. In conventional corrosion, molten Na2SO4, the primary phase of incinerating slag, directly entered the interior of the corundum-mullite refractory through pores on the interface. The Na2SO4 strongly reacted with alumina and silica at 1000–1300 °C to produce liquid albite and nepheline. Upon the addition of Cr2O3 into the spent liquor, the Cr2O3 powder was suspended in the spent liquor and deposited at the refractory interface to form a protective (Al,Cr)2O3 solid solution layer with Al2O3. The dense corrosion-resistant layer of (Al,Cr)2O3 with a depth of 0–3 mm on the surface of the firebricks seldom reacted with molten Na2SO4. This protective layer can effectively slow down the penetration and erosion of incinerating slag. The environmental risks of Cr(VI) contamination associated with the Cr2O3 addition in the incineration process were also evaluated in this work. Eventually, preliminary trial operation results showed that the service life of the corundum-mullite firebrick was prolonged after 5% Cr2O3 addition into the spent liquor.

Influence of simultaneous Cr2O3 and TiO2 additions on the microstructure and properties of APS alumina coatings

Alumina is an important material for thermal spray coating solutions. For improving the coating properties, additions of TiO2 or Cr2O3 are common, but their simultaneous addition has not been studied so far. One alumina-rich ternary feedstock powder composition (81 wt% Al2O3−12.5 wt% Cr2O3–6.5 wt% TiO2) was prepared in three different ways: 1) blending of commercial fused and crushed plain oxide powders; 2) blending of two commercial binary jointly fused and crushed feedstock powders (Al2O3–13 wt% TiO2 and Al2O3–25 wt% Cr2O3); 3) preparing an experimental agglomerated and sintered powder. Plain oxide powders and the two binary powders were used for comparison. The feedstock powders were studied by a standard procedure, including particle size distribution by laser diffraction, the morphology and microstructure by SEM and EDS, and phase composition by XRD. Coatings were prepared by APS with an Ar-H2 plasma gas mixture, using a single spray parameter set optimized for the plain Al2O3 powder. The influence of the feedstock powder properties on coating deposition was evaluated by the average thickness per layer and coating roughness. The coating microstructures were studied by optical microscopy and SEM, including the investigation of local compositions by EDS. The phase compositions and lattice parameters of the alumina phases in the powders and coatings were investigated by XRD. The coatings were characterized for their microhardness HV0.3, and tribological (unidirectional dry sliding wear resistance), corrosion (exposure testing against H2SO4) and electrical (resistivity, dielectric breakdown field strength and impedance) properties. Chromia additions, existing as a (Al,Cr)2O3 solid solution in the feedstock powder, were found to have a positive effect on the stabilization of the corundum phase for APS coatings, while titania additions were found to have no influence on the corundum phase stability. The corrosion resistance was found to be positively influenced by the joint addition of chromia and titania compared to plain alumina-

In situ detoxification and mechanical properties of Al2O3-Cr2O3-CaO castables with zeolite

Al2O3-Cr2O3-CaO castables have been regarded as ideal linings for various furnaces owing to sufficient strength and excellent corrosion resistance. However, formation of toxic Cr(VI) is the common failing. The present work aimed at achieving in situ detoxification by introducing zeolite. The influence of zeolite on phase evolutions, mechanical properties and Cr(VI) formation were investigated. The results showed that leaching Cr(VI) at 110 °C was lowered by zeolite due to absorption, while chemical reduction of CaCrO4 and Ca4Al6CrO16 occurred at 500−1100 °C and 900−1300 °C, respectively. At 1300−1500 °C, zeolite promoted formation of (Al,Cr)2O3 solid solution (+3 state). Besides, after substituting partial cement with zeolite, CMOR of castables at 110 °C just decreased slightly due to Ca2(Al2SiO6)(OH)2 formation while at 1100−1500 °C, CMOR increased obviously because of enhanced densification. A comparable CMOR up to 68 MPa and a remarkable Cr(VI) detoxification rate of 96.3 % could be achieved for castables with zeolite.

Coherency strain induced hardening in bulk polycrystalline ceramics: A case study with MgO‐based “ceramic alloys”

AbstractThe role of coherency strain at the matrix/precipitate interface toward hardening of bulk polycrystalline “ceramic alloys” has been established here. Formation of “near ideal” bulk polycrystalline ceramic microstructure characterized by the presence of uniformly distributed coherent “ultra‐fine” MgCr2O4 particles (size: ~25 nm) within matrix (MgO) grains was achieved via solid‐state precipitation during aging treatment of bulk supersaturated MgO–Cr2O3 solid solutions (formed during pressureless sintering in air, followed by fast cooling). The as‐aged MgO–MgCr2O4 “ceramic alloys” exhibited hardness increment by ~73% over that of phase pure bulk MgO upon aging for just 10 hours at 1000°C in air. Evidences toward the presence of significant coherency strains across the MgO/MgCr2O4 coherent interfaces were obtained with transmission electron microscopy. Analysis based on hardening mechanisms and comparisons with MgO–MgFe2O4 system, having lesser hardening due to lower misfit strain at MgO/MgFe2O4 coherent interfaces (despite greater content of second‐phase particles), confirm the dominant role of coherency strains toward hardness enhancement in “ceramic alloys.”

Structure, mechanical characteristics and thermal stability of high speed physical vapor deposition (Al,Cr)2O3 coatings

In recent years (Al,Cr)2O3 coatings manufactured by physical vapor deposition (PVD) gained great interest as alternative to α-Al2O3 coatings deposited by chemical vapor deposition (CVD), due to their potential to form corundum-type structure at lower process temperatures, T ≤ 700 °C. In this context, also metastable tetragonal (Al,Cr)2O3 solid solution and semi-crystalline α-(Al,Cr)2O3 coatings were developed by high speed (HS)-PVD technology. The chemical analysis of the coatings in this paper confirmed, that the deposition of semi-crystalline α-(Al,Cr)2O3 coatings with high aluminum content, xAl ≥ 70 At.%, is possible at moderate substrate temperature, Tsub = 570 °C. With regard to the application on cutting and molding tools, the thermal stability of the developed coatings is of great interest. Therefore, the thermal stability of the deposited tetragonal and α-(Al,Cr)2O3 phases was investigated by annealing tests in vacuum up to T = 1300 °C. X-ray diffraction (XRD) measurements after the annealing tests in vacuum revealed a phase stability of semi-crystalline α-(Al,Cr)2O3 and metastable tetragonal (Al,Cr)2O3 coating up to T = 600 °C and T = 800 °C, respectively. Diffusion processes during annealing at high temperatures in vacuum lead to significant structural changes, showing detrimental effects on the mechanical properties of the coatings. Due to thermal induced structural, chemical and mechanical changes, the evaluations confirmed a thermal stability of the (Al,Cr)2O3 coatings up to a temperature of T = 800 °C in vacuum.

Corrosion resistance and microstructural stability of austenitic Fe–Cr–Al–Ni model alloys exposed to oxygen-containing molten lead

The goal of the study was to determine the critical concentrations of Al, Cr and Ni, at which the quaternary Fe–Cr–Al–Ni model alloys, exposed to oxygen-containing molten Pb up to 600 °C, are corrosion resistant, while preserving the austenite structure of the alloy matrix.Twelve alloys were designed to meet the above mentioned requirements; six of them showed corrosion resistance and preserved the austenite phase in the alloy bulk, during the exposure at 550 °C and 600 °C for 1000 h to molten Pb containing 10−6 wt% oxygen. Based on experimental results a general formula was substantiated as follows: Fe-(20–29)Ni-(15.2–16.5)Cr-(2.3–4.3)Al (wt.%). In case of temperatures below 550 °C, the critical Cr content was 14.4 wt%.Two corundum-type crystalline structures were identified as the constituent phases of the passivating scales, one being Cr2O3 and the other Al2O3–Cr2O3 solid solution. The average amount of Cr2O3 in the Al2O3–Cr2O3 solid solution, found in the passivating scales of the Fe–Cr–Al–Ni model alloys, was estimated at ≈ 40 wt% at 550 °C and ≈35 wt% at 600 °C.A transitional layer, consisting of Fe- and Ni-enriched austenitic matrix and exhibiting randomly distributed intermetallic B2-(Ni,Fe)Al, was formed below the oxide scale up to a depth of two microns.The austenite, as matrix, and Ni3(Al,Fe) as precipitates are the microstructural phases of the bulk alloys after exposure for 1000 h at 600 °C to oxygen-containing molten Pb.

Phase evolution with temperature in chromium-containing refractory castables used for waste melting furnaces and Cr(VI) leachability

Highly corrosion resistant Cr2O3 based refractory linings are required for waste melting furnaces due to the complex melting environment. However, these linings are susceptible to Cr(VI) generation during service. In this study Cr2O3 based castables were prepared by varying the Cr2O3 content (4, 8 & 12 wt%) and heated in the temperature range of 500–1500 °C in the air atmosphere to simulate the service conditions. The formations of various phases were studied using XRD and SEM-EDS. The Cr(VI)-containing phase, CaCrO4 formed at 500–700 °C, while Ca4Al6CrO16 formed in the intermediate temperature regime (900–1300 °C). The Cr(III) containing (Al,Cr)2O3 solid solution phase started to form at 1100 °C which predominantly formed at 1500 °C. The oxidation states of chromium in solid state transferred in the order Cr(III)→Cr(VI)→Cr(III) with the increase in temperature. The specimens treated at 500–1300 °C contain ~ 474–5646 mg/kg of Cr(VI) whereas specimens at 1500 °C exhibit only ~ 63–69 mg/kg of Cr(VI).

On the phase evolution of arc evaporated Al-Cr-based intermetallics and oxides

(Al,Cr)2O3 solid solutions in corundum-structure are of great potential for protective applications facing severe mechanical and thermal conditions. In order to elucidate the influence of cathode composition and reactive gas flow on the phase formation of (Al1‐xCrx)2O3, Al0.75Cr0.25, Al0.70Cr0.30, Al0.50Cr0.50, and Al0.25Cr0.75 cathodes were utilised to synthesise coatings with and without oxygen reactive gas. Transmission electron microscopy studies reveal successive microstructural modifications from intermetallic coatings towards oxides with M2O3 stoichiometry. The transition from substoichiometric (suggesting intermetallic γ2-Al8Cr5, AlCr2, and/or Cr structures with distorted lattices due to oxygen incorporation) to stoichiometric oxides is connected with the formation of metastable cubic (Al,Cr)2O3 phases for Al-rich compositions. With decreasing Al-content of the cathodes, the critical oxygen flow for this transformation increases (i.e., 380sccm for Al0.75Cr0.25, 400sccm for Al0.70Cr0.30, and 420sccm for Al0.50Cr0.50). Only when using the high-Cr containing Al0.25Cr0.75 cathode, the coatings directly crystallised with corundum-type (Al,Cr)2O3-solid solutions for O2 flow rates above ~570sccm.