Research on the corrosion behavior of low carbon mold flux on submerged nozzle refractory

In this paper, the wetting and corrosion behavior between medium and low carbon mold flux and Al₂O₃-ZrO₂-C nozzle refractory were studied through a high-temperature wetting experiment. Combined with microstructure analysis, the corrosion mechanism of low carbon mold flux on nozzle slag line material was clarified. The results show that the wettability of low-carbon mold flux and Al₂O₃-ZrO₂-C nozzle refractory is better, compared with the traditional medium carbon mold flux, the contact angle between low-carbon mold flux and Al₂O₃-ZrO₂-C material is smaller and the mold flux spreads faster on the nozzle surface at the same temperature range. According to the microstructure analysis, the corrosion degree of low-carbon protective slag on the ZrO₂-C material of the nozzle slag line is serious, and the corrosion depth is large. Due to the influence of carbon content, the wettability between low-carbon mold flux and ZrO₂-C material of the nozzle slag line is better, which provides favorable kinetic conditions for the dissolution and penetration of low-carbon mold flux into the nozzle. In addition, compared with medium carbon slag, the carbon concentration difference between low-carbon mold flux and nozzle material is larger, resulting in greater diffusion driving force of carbon atoms, which affects the wetting, dissolution, and chemical reaction of the interface between the two phases.     

Dissolution behavior of a novel Al2O3-SiC-SiO2-C composite refractory in blast furnace slag

Al₂O₃-SiC-SiO₂-C composite refractories are interesting potential blast furnace hearth lining materials that feature several advantageous properties. In this study, the corrosion resistance of a novel Al₂O₃-SiC-SiO₂-C composite refractory to blast furnace slag was investigated by adopting a rotating immersion method (25 r/min) at 1450–1550 °C and comparing it against a conventional corundum-based refractory at 1550 °C as a benchmark. The results showed that the apparent activation energy of Al₂O₃-SiC-SiO₂-C composite refractory over the dissolution process in the slag is 150.4 kJ/mol. Dissolution of the Al₂O₃ and 3Al₂O₃·2SiO₂ phases appeared to be the main cause of Al₂O₃-SiC-SiO₂-C composite refractory corrosion. High-melting-point compounds in the slag layer formed a protective layer which mitigated the corrosion. The novel Al₂O₃-SiC-SiO₂-C composite refractory is better suited to blast furnace hearth lining than the conventional corundum-based refractory, because the carbon phase and SiC phase in the material are not readily wetted by the blast furnace slag and therefore are more resistant to slag penetration. Higher melting point phases also may crystallize on the hot face of the hearth lining due to the high thermal conductivity of the Al₂O₃-SiC-SiO₂-C composite refractory, promoting a more stable protective layer.     

Effect of carbon black on corrosion resistance of Al2O3–SiC–C castables to SiO2–MgO slag

Metallurgical solid waste recycling is the shape of things to come in green development of Chinese iron and steel industry. Utilization of ironworks slag for producing mineral wool at high temperature is an important approach. However, refractory lining is seriously corroded by the SiO₂–MgO based slag at 1600 °C during the production process. Different production steps need different atmospheres, the changeable service atmospheres (air and reducing atmosphere) put forward high requirements for slag resistance. The Al₂O₃–SiC–C castables containing carbon black are usually used in iron runner, which faces high-temperature service condition of 1450 °C–1500 °C. Nevertheless, the function of carbon black in the Al₂O₃–SiC–C castables at 1600 °C is till essentially unknown. In the current study, the carbon black was introduced to tabular alumina based Al₂O₃–SiC–C castables to improve corrosion resistance to SiO₂–MgO based slag at 1600 °C. The result showed that 0.4 wt% carbon black was suitable for the castables, which the slag resistance of castables was significantly improved. The carbon black had contributed to block slag by wettability resistance. By comparison with the castables without carbon black, the corrosion index and penetration index had been reduced by 20.2% and 28.0%, respectively, under air atmosphere. And there were little corrosion or penetration under reducing atmosphere for castables with 0.4 wt% carbon black. For the mechanical properties, the Al₂O₃–SiC–C castables with 0.4 wt% carbon black could serve production process although the carbon black impaired the physical properties.     

Corrosion resistance and anti-reaction mechanism of Al2O3-based refractory ceramic under weak static magnetic field

A slag resistance experiment of the Al₂O₃-based refractory ceramic with CaO–Al₂O₃–SiO₂ slag at 1600°C under a milli-Tesla static magnetic field was conducted. The magnetic flux density effect on the corrosion at the two- and three-phase interfaces of the Al₂O₃-based refractory ceramic, excluding the ‘electromagnetic damping’, was studied. The slag resistance of the Al₂O₃-based refractory was enhanced and quasi-volcanic corrosion at the three-phase interface was eliminated gradually with an increase in the magnetic flux density. A hypothesis and mechanism for the inhibition effect of the static magnetic field based on the free radical pair reaction model was proposed.     

Effect of partial substitution of alumina–chromium slag for Al2O3 on microstructures and properties of Al2O3–SiC–C trough castables

Alumina–chromium slag (ACS), a cheap and abundant refractory raw material comprising aluminum–chromium oxides and β-Al₂O₃, is a byproduct of ferrochrome smelting. For this reason, we investigated the relationships between composition and mechanical properties, abrasion resistance, oxidation resistance, and resistance to iron slag erosion for Al₂O₃–SiC–C trough castables in which ACS was substituted for alumina. Due to the presence of β-Al₂O₃ in ACS, the aluminum-chromium slag reacted with SiO₂ to form a low-melting phase of albite and promoted the formation of mullite, which filled the pores at high temperatures and reduced the porosity, thereby promoting densification and strengthening of the sample. The cold mechanical properties of the sample and the normal temperature wear resistance were enhanced, but the high-temperature mechanical properties and the resistance to iron slag corrosion of the sample were impaired. According to the results of the anti-oxidation experiment, the presence of β-Al₂O₃ in the ACS reduced the porosity and made the sample more dense, which remarkably improved oxidation resistance of the sample. For industrial production requirements, ACS substitution should not exceed 48?wt% due to of thermomechanical properties and anti-slag corrosion performance in Al₂O₃–SiC–C trough castables.     

Improved corrosion resistance of Al2O3-SiC-C castables through in situ carbon containing aluminate cement as binder

Slag corrosion is one of the main damage modes for refractory castables used in iron and steel metallurgy. The matrix plays vital roles for corrosion resistance of refractory castables. In the present paper, the properties of the Al₂O₃-SiC-C-based trough castables with in situ carbon containing calcium aluminate cement (CCAC) and ball pitch as carbon sources, respectively, were comparatively investigated. The microstructures of the trough castables after corrosion were observed by field-emission scanning electron microscopy. The results showed that after being corroded at 1450°C for 3 hours, the corrosion depth of Al₂O₃-SiC-C-castables with CCAC as binder (A2) was 1.2 mm, 65.71% lower than that of the trough castables with ball pitch as carbon source (A1), though the content of carbon materials in the former was much lower than that of the latter. The reasons for these observations were that the in situ carbon materials of CCAC exhibit improved distribution in the matrices of castables and excellent oxidation resistance, resulting in lower porosity within the castables, and hindered the penetration of the molten slag at high temperatures. In addition, the Al₂O₃-SiC-C-castables bonded by CCAC displayed good mechanical properties at room temperature and elevated temperature.     

Catalytic Fabrication of SiC/SiO2 coated graphite and its behaviour in Al2O3–C castable systems

SiC/SiO₂ coated graphite was prepared via a combined sol-gel coating and catalytic conversion route, using graphite flake and tetraethyl orthosilicate as the starting materials, and Fe(NO₃)₃·9H₂O as the catalyst precursor. X-ray diffraction analysis and microstructural examination revealed that a homogeneous coating comprising SiC and cristobalite (SiO₂) and covering the whole surface of graphite was formed. As prepared SiC/SiO₂ coated graphite exhibited better oxidation resistance and water wettability than its uncoated counterpart. Also, oxidation resistance and slag corrosion resistance of a model Al₂O₃–C castable using coated graphite as a carbon source were better than in the case of its counterpart using uncoated graphite.     

Nanoscale toughening of low carbon Al2O3-C refractories by means of SiCnw/Al2O3 composite reinforcement

To improve the thermal shock resistance of low carbon Al₂O₃-C refractories, SiC nanowires (SiC_nw) containing SiC_nw/Al₂O₃ composite reinforcement were introduced. The specific fracture energy of the Al₂O₃-C refractory matrix was obtained by statistical grid nano-indentation. The reinforcement mechanism of SiC_nw/Al₂O₃ on thermal shock resistance of refractories was investigated. The results revealed that the matrix-specific fracture energy of A6 (6 wt% SiC_nw/Al₂O₃ added) was 217 N/m, which was 58.4% higher than reference sample A0 (137 N/m) and 18.6% higher than MA6 (183 N/m, 6 wt% SiC/Al₂O₃ added). A6 showed the highest residual strength ratio of 49.8%, which was 114.7 % higher than A0 (23.2%) and 82.4 % higher than MA6 (27.3%). The components with different morphology in SiC_nw/Al₂O₃ cluster, especially SiC nanowires, promote the generation of microcracks, crack multi-deflection, and branching, which toughen the matrix and improve the thermal shock resistance of refractories. In comparison to the literature, A6 showed a higher rising in residual strength ratio than those with higher graphite content (4 wt% and 20 wt%), which will greatly reduce the consumption of carbon-containing refractories and contribute to the reduction of CO₂ emission.     

High-strength and corrosion-resistant Al2O3 ceramics with excellent closed-cell structure

As a lightweight refractory, porous Al₂O₃ ceramics are advocated in the iron and steel smelting industry because of their excellent resource-saving and low heat loss. However, the severely poor slag corrosion resistance and low mechanical strength caused by open pores shorten their service life. To solve this problem, Al₂O₃ ceramics with excellent closed-cell structure were fabricated by combining β-SiC pore-foaming and gel-casting techniques, and their pore structure and properties were tailored by tuning the content of β-SiC and sintering temperature. It is noted that the closed pores introduced in the dense Al₂O₃ matrix play a pivotal role in improving the corrosion resistance and mechanical strength while maintaining the lightweight. And the sample with closed porosity of 20.6% exhibited compressive strength of 640 MPa and flexural strength of 272 MPa. Meanwhile, its corrosion and penetration indices were at a low level, 6.3% and 54.8%, respectively.     

Comparison study on effect of nano-sized Al2O3 addition on the corrosion resistance of microporous magnesia aggregates against tundish slag

The microporous magnesia aggregates show a promising application prospect as tundish lining, due to the excellent thermal insulation. In this study, the effect of nano-sized Al₂O₃ addition on the corrosion resistance of microporous magnesia aggregates against tundish slag is explored. The results show that the addition of nano-sized Al₂O₃ deteriorates the slag resistance of microporous magnesia aggregates, which is mainly because that the apparent porosity of aggregates increases with the addition of nano-sized Al₂O₃. Furthermore, MgO·Al₂O₃ spinel is formed in situ at the grain boundaries of Al₂O₃-bearing aggregates and the dissolution of MgO·Al₂O₃ spinel into molten slag damages the structure of aggregates. For the Al₂O₃-free microporous magnesia aggregates, as expected, the penetration of high basicity slag (CaO/SiO₂ = 9, mass ratio) into refractory is slighter than that of low basicity slag (CaO/SiO₂ = 4, mass ratio). But, for the Al₂O₃-bearing microporous magnesia aggregates, the corrosion of refractory by high basicity slag is severer. This is mainly because that MgO·Al₂O₃ spinel is more unstable in high basicity slag. Therefore, it is not suitable to add nano-sized Al₂O₃ for the preparation of microporous magnesia as tundish lining.     

Key features of alumina/magnesia/graphite refractories for steel ladle lining

The corrosion resistance of resin bonded alumina/magnesia/graphite refractories containing different kinds of aggregates were investigated when submitted to the action of slags of several CaO/SiO₂ ratios. The laboratory testing was performed by means of the rotary slag attack test. Specifically evaluated was the influence of alumina/carbon ratio and magnesia and silica contents on the refractories corrosion resistance. It was found that this property could be improved by increasing the refractory Al₂O₃/SiO₂ ratio as well as by choosing the appropriate Al₂O₃/C ratio.     

Oxidation behavior and kinetics of Al2O3-SiC-SiO2-C refractories in CO2 atmosphere

Al₂O₃-SiC-SiO₂-C refractories are widely used as blast furnace hearth lining but have low oxidation resistance in the presence of CO₂. Carbon composite brick is a newly developed Al₂O₃-SiC-SiO₂-C refractory that may function better in the blast furnace hearth due to improved erosion-resistance and high thermal conductivity. In this study, the oxidation behavior and kinetics of carbon composite brick, carbon brick, and corundum brick were investigated in CO₂ atmosphere. The results show the oxidation resistance of carbon composite brick was better than that of carbon brick but worse than that of corundum brick. SiC functions as an antioxidant to play an important role in the oxidation process. For carbon composite brick, the oxidation activation energy was 141.72 kJ/mol at the step controlled by the interface reaction and 161.24 kJ/mol at the step controlled by gas diffusion. Carbon composite brick exhibits improved thermal conductivity and oxidation resistance relative to carbon brick and corundum brick that should allow improved performance in blast furnace hearth lining.     

Influence of bonding carbon on low carbon Al2O3-C refractory composites

The carbonized behavior of binders (carbores pitch, mesophase pitch, high temperature pitch and phenolic resins) was investigated. Meanwhile, the influence of binders, Ni-catalyst and heat treatment on structure and properties of low carbon Al₂O₃-C materials was researched. Mesophase pitch and high molecular weight resin provided higher carbon yield. The synergistic interaction of pitch and phenolic resin provided the higher carbon yield than that of single component. Ni-catalyst changed structure and morphology of bonded carbon promoting the transition of amorphous carbon to crystalline carbon increasing carbon yield and degree of graphitization. High molecular weight phenolic resins, mesophase or carbores pitch and Ni-catalyst worked together providing low carbon Al₂O₃-C samples of excellent strength performance in a wide temperature range and high resistance of thermal shock. Embedding coke in N₂ could promote the generating of carbon fibers which obviously enhanced thermal shock resistance of low carbon (6% flake graphite) Al₂O₃-C materials after heat treatment.     

Corrosion of refractory alumina plates used in the sliding gate system of steelmaking ladle: Chemical experiment

The aim of this study is to investigate the corrosion process of the refractory Al₂O₃-ZrO_2-C slide gate plate caused by chemical interactions with secondary refining slag. The plate is part of the steelmaking ladle slide gate system, therefore its lifespan extension and integrity are of concern. Post mortem plates with slag adhered on their channel surface were analysed and static cup corrosion test experiments were performed in a controlled atmosphere furnace for 1 h at 1600 °C. The corrosion product phases Gehlenite (Al₂Ca₂O₇Si) and Spinel (MgAl₂O₄) identified by DRX were formed after the experiment, showing that the slag is able to chemically corrode the Al₂O₃-ZrO₂-C plates. In addition, a comparison of the interface region between the slag and the post mortem plate and the interface region between the slag and the static cup corrosion test specimens showed that the corrosion products are swept away by the fluid flow. Even though chemical corrosion by slag has been identified as one of the causes of the plate channel degradation, the study suggests that it acts synergistically with other mechanisms of degradation.     

Towards chrome-free of high-temperature solid waste gasifier through in-situ SiC whisker enhanced silica sol bonded SiC castable

Refractory containing Cr₂O₃ was widely used in solid waste gasifier due to its excellent slag resistance. However, hexavalent chrome compounds (formed during the preparation and the use of refractory containing Cr₂O₃) will give rise to detrimental effect on environment and human's health. In addition, the Al₂O₃-Cr₂O₃ materials acted as the lining materials. Serious exfoliation occurred after about twenty days. For the purpose of chrome-free and service longevity of lining materials for solid waste gasifier, in-situ SiC whisker enhanced SiC castable and spinel castable containing 20%wt of Cr₂O₃ were prepared. The mechanical properties and corrosion resistance after heat treatment in different temperature of the castables were determined. The strength of SiC castable rised with the increasing of the temperature. And the nano SiC/SiO₂ core-shell whiskers was formed at 1500 °C. In comparison to the spinel castable containing 20 wt% of Cr₂O₃, the better volume stability and the reinforcement of the nano whiskers led to excellent resistance to crack propagation at high temperature. In addition, SiC castable showed lower apparent porosity because of the forming of SiO₂ through the oxidation of SiC over 1300 °C, the viscosity of slag increased since that the SiO₂ dissolve into the slag, which caused excellent penetration resistance of SiC castable compared with spinel-Cr₂O₃ castable. Excellent mechanical properties and slag resistance at high temperature indicated that SiC castable had the application prospect for high-temperature solid waste gasifier.     

Corrosion resistance of Al–Cr-slag containing chromium–corundum refractories to slags with different basicity

Al–Cr slag is the solid waste generated by the smelting of Cr metal. It presents a range of environmental hazards. This study addressed the corrosion resistance of Al–Cr slag containing chromium–corundum refractories to slags with different basicity. Herein, we provide suggestions for the use of Cr–corundum of different basicity in kilns. Al–Cr slag, brown fused Al₂O₃, and chrome green were used as the raw materials, with pure calcium aluminate cement being used as a binder. The brick samples, prepared using different blends of chrome green and corundum, were fired at 1600?°C, and subsequently subjected to a slag corrosion test. After corrosion by slag of different basicity, the phase composition and microstructure of the sample were analyzed by X-ray diffraction, energy dispersive spectrometer and scanning electron microscopy. There were two major findings. First, Cr–corundum brick made from Al–Cr slag has a better slag corrosion resistance than that made from Cr₂O₃ and brown fused Al₂O₃. Second, Cr–corundum brick made from Al–Cr slag has superior corrosion resistance to slag with a CaO:SiO₂ ratio of 2:1.     

Preparation of in situ grown silicon carbide whiskers onto graphite for application in Al2O3-C refractories

C-SiC composite powders were prepared by salt-assisted synthesis from Si powders, graphite, and a molten salt medium (NaCl and NaF) with the molar ratio of Si/C =?1/2 at 1300?°C for 3?h. After the C-SiC composite powders part and complete replacement of the graphite, the mechanical properties, oxidation resistance and slag-corrosion resistance of the Al₂O₃-C materials were studied by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), as well as with dedicated equipment. The results indicated that SiC whiskers, with lengths of 10–50?nm, formed on the surface of the flake graphite, and the activation energy of oxidation of the C-SiC composite powder increased by 45.72?kJ?mol^?1 as compared to that of flake graphite. Furthermore, the decarburization area and slag erosion area of the Al₂O₃-C material decreased after 3?wt% of C-SiC composite powder was substituted for the flake graphite. Meanwhile, the cold modulus of rupture was maintained when 3?wt% of C-SiC composite powder was added. This improved both the oxidation and slag resistance of the Al₂O₃-C materials.     

Investigation of slag-refractory interactions for the Ruhrstahl Heraeus (RH) vacuum degassing process in steelmaking

In order to determine the effect of slag composition during the RH process on refractory wear, magnesia–carbon and magnesia–chromite refractories were immersed for 10 min at 1600 °C in a ladle slag, two FeO-rich slags (20 and 40 wt% FeO) and two CaO–Al₂O₃ slags. Corrosion of magnesia–carbon refractory by the ladle and CaO–Al₂O₃ slags was limited as the refractory carbon phase efficiently prevented slag infiltration. Severe degradation was observed in contact with FeO-rich slags. FeO oxidized the carbon phase with formation of Fe droplets at the hot face. Regarding magnesia–chromite refractory, the corrosion mechanism consisted of severe slag infiltration, high temperature inactivation of the secondary chromite and primary chromite dissolution in the infiltrating slag. The FeO-rich slags seem to have generated more severe conditions as the infiltrating slag pushed apart the periclase grains, leading to severe refractory erosion. The degradation mechanisms are discussed by combining experimental results and thermodynamic calculations.     

Enhanced thermal shock resistance of low-carbon Al2O3-C refractories with direct CVD synthesis of nano carbon decorated oxides

Novel low carbon Al₂O₃-C refractories were prepared through adopting chemical vapour deposition (CVD) synthesized nano carbon decorated Al₂O₃ powder. The phase compositions, microstructures, mechanical properties and thermal shock resistance of Al₂O₃-C refractories were characterized and evaluated. The results show that the morphologies of nano carbon composites are mainly dominated by the concentration of catalyst. Specifically, the growth of MWCNTs is preferred with a Ni²⁺ concentration at 0.1?mol/L, while higher concentrations e.g. 0.3?mol/L would stimulate the formation of nano-onion like carbon. With the introduction of nano carbon decorated Al₂O₃ additives, the residual strength after thermal shock can reach 12.4?MPa, which is much higher than the 2?wt% nano carbon black containing specimens (6.4?MPa). The enhanced thermal shock resistance should be attributed to that the nano onion-like carbon reduces the cohesion between the matrix and the Al₂O₃ particles and decreases the thermal expansion coefficient.     

Designing low-carbon MgO–Al2O3–La2O3–C refractories with balanced performance for ladle furnaces

In order to achieve high-quality and stable production of special steel, the performance of low-carbon MgO-C refractories needs to be further optimized. For this purpose, low-carbon MgO–Al₂O₃–La₂O₃–C refractories with enhanced thermal shock resistance and slag resistance were designed and successfully prepared by introducing Al₂O₃ as a reinforcer and La₂O₃ as a modifier. The results showed that the refractory samples with additives show better overall performance than those without additives. When 10 wt% of Al₂O₃ and La₂O₃ were added, the oxidation resistance, thermal shock resistance and slag resistance of the refractory samples coked at 1400 °C are increased by 13.57%, 17.75% and 43.09%, respectively. The analysis found that this can be mainly attributed to the formation of MgAl₂O₄, Mg₂SiO₄, and 2CaO·4La₂O₃·6SiO₂ and the consequent volume expansion effect and intergranular phase enhancement effect. Therefore, a low-cost and enforceable reinforcement strategy for low-carbon MgO-C refractories is proposed, which is expected to be applied in steelmaking.