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.     

Role of graphite on the corrosion resistance improvement of MgO–C bricks to MnO-rich slag

In order to clarify the effect of graphite content on the corrosion behavior of MgO–C refractories immersed in MnO-rich slag, the MgO–C refractory samples bearing 5 wt%, 10 wt% and 15 wt% graphite were prepared, and exposed in the slag composed of 40 wt% CaO, 40 wt% SiO₂ and 20 wt% MnO. The results show that metallic Mn particles and (Mg,Mn)O solid solution are formed at the slag/refractories interface. Whereas, no dense layer is formed by (Mg,Mn)O solid solution at the interface. The decrease in MnO content of slag is mainly attributed to the reaction with graphite to form liquid Mn. The graphite is found in the slag, and dissolved in the form of oxidation. The poor wetting limits the contact area of graphite and slag, reducing graphite oxidation and decarburized area. The graphite does not become the passage for slag to penetrate into the refractories due to the oxidation. On the contrary, the dissolution of MgO in slag is faster than graphite, thus is mainly responsible for the degradation of refractories. As a result, MnO and MgO contents change less in the slag contacted with the refractories bearing higher graphite content.     

Effect of carbon content on the mechanical behavior of MgO–C refractories characterized by Hertzian indentation

The mechanical behavior of magnesia–carbon (MgO–C) refractory materials containing 4, 11, and 19 wt% carbon was investigated by Hertzian indentation tests. No microstructural changes were observed for the various carbon amounts. The slope during loading and the residual displacement after unloading were determined from indentation load–displacement curves. Indentation conditions were varied to study their influence on the mechanical behavior of the MgO–C. Repetitive contact fatigue tests were also conducted. The carbon content and indentation ball size ultimately affected the indentation mechanical behavior. The MgO–C containing 4 wt% carbon exhibited relatively hard and brittle behavior, while the MgO–C containing 19 wt% carbon exhibited a quasiplastic behavior, independent of ball radius indented on the MgO–C. Results indicated that carbon, in suitable amounts, provides the MgO–C with soft and ductile behavior in contact, wear, and impact environments.     

Fracture behavior of low carbon MgO–C refractories using the wedge splitting test

The fracture behavior of low carbon MgO–C refractories containing various carbon sources were investigated by means of the wedge splitting test and microscopic fractographic analysis to evaluate quantitatively their thermal shock resistance in the present work. The results showed that the addition of various nanocarbons in MgO–C specimens can lead to more tortuous crack propagation path during the wedge splitting test and much better thermal shock resistance compared to the specimen with flaky graphite as carbon source; particularly, the specimen containing carbon nanotubes had the most outstanding thermal shock resistance. Also, it was suggested from the correlation analysis that the increase of the specific fracture energy and interface crack propagation as well as the decrease of the modulus of elasticity, coefficient of thermal expansion and transgranular crack propagation can contribute to an improvement of thermal shock resistance of MgO–C refractories.     

Effect of nanocarbon sources on microstructure and mechanical properties of MgO–C refractories

A study of microstructural evolution, mechanical and thermo-mechanical properties of MgO–C refractories, based on graphite oxide nanosheets (GONs), carbon nanotubes (CNTs) and carbon black (CB), was carried out by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), three-point bending and thermal shock tests. Meanwhile, these results were compared to the conventional MgO–C refractory containing 10 wt% flaky graphite prepared under the same conditions. The results showed that higher cold modulus of rupture was obtained for the composition containing GONs, and the composition containing CNTs exhibited larger displacement after coking at 1000 °C and 1400 °C. Also, the addition of nanocarbons led to an improvement of the thermal shock resistance; in particular, both compositions containing CNTs and CB had higher residual strength ratio, approaching the thermal shock resistance of the reference composition containing 10 wt% flaky graphite, as it was associated with the presence of nanocarbons and in-situ formation of ceramic phases in the matrix.     

Cr7C3: A potential antioxidant for low carbon MgO–C refractories

Magnesia carbon (MgO–C) refractory, one of the most commonly used refractories in the steelmaking system, relies on graphite to improve the thermal shock resistance and slag corrosion resistance. The oxidation of graphite carbon in a MgO–C brick usually leads to the destruction of the carbon network in the brick, which causes the structure of the brick to become loose and easily eroded. At present, metal powders, carbides, and borides are used as antioxidants to prevent the oxidation of carbon in MgO–C bricks. The metal carbide Cr₇C₃ can be prepared from aluminum chromium slag through a simple synthetic process and at a low cost. In this work, we investigated the oxidation resistance of low carbon MgO–C refractories with different amounts of Cr₇C₃ powder (1, 2, 3, and 4 wt%). The refractories with 3 wt% Cr₇C₃ powder showed optimal resistance to oxidation. The microstructure indicated that oxygen reacts with Cr₇C₃ preferentially over carbon to form chromium oxide and magnesium chromium spinel, blocking the pores and hindering oxygen diffusion. Carbon arising from the reduction of carbon monoxide by Cr₇C₃ can act as a supplementary carbon source. The better oxidation resistance also contributed to the improvements in slag corrosion and thermal shock resistance of the refractories.     

Formation of hollow MgO-rich spinel whiskers in low carbon MgO–C refractories with Al additives

MgO–C refractories with different carbon contents have been developed to meet the requirement of steel-making technologies. Actually, the carbon content in the refractories will affect their microstructure. In the present work, the phase compositions and microstructure of low carbon MgO–C refractories (1 wt% graphite) were investigated in comparison with those of 10 wt% and 20 wt% graphite, respectively. The results showed that Al₄C₃ whiskers and MgAl₂O₄ particles formed for all the specimens fired at 1000 °C. With the temperature up to 1400 °C, more MgAl₂O₄ particles were detected in the matrix and AlN whiskers occurred locally for high carbon MgO–C specimens (10 wt% and 20 wt% graphite). However, the hollow MgO-rich spinel whiskers began to form locally at 1200 °C and grew dramatically at 1400 °C in low carbon MgO–C refractories, whose growth mechanism was dominated by the capillary transportation from liquid Al at these temperatures.     

Effect of applied voltage on wetting and corrosion of corundum refractory by CaO–SiO2–MgO molten slag

The wetting and corrosion behavior of the corundum substrate anode by CaO–SiO₂–MgO molten slag was investigated via the joint application of the sessile drop method with applied voltage and SEM-EDS technique. The slag drop exhibited a good wettability on the corundum substrate. The apparent contact angle at zero voltage slightly exceeded that at a 1 V applied voltage but was lower than those at 1.5 V and 2 V ones. Low applied voltage of 1 V had little effect on the corundum substrate's direct dissolution corrosion processes; high ones of not less than 1.5 V caused the electrode reaction to occur. The stirring effect of O₂ bubbles from the anode reaction aggravated the substrate's direct dissolution and physical stripping. It was found that the applied voltage could inhibit the slag penetration, and the apparent contact angle had no obvious relation with the direct dissolution thickness and penetration depth. A thin but almost continuous MgO?Al₂O₃ (MA) layer could form at the slag/substrate interface at the applied voltage of 1.5 V. These results indicate the positive effect of applied voltage on the distribution of interfacial products and the molten slag penetration in reducing the corrosion of corundum anode under certain conditions. However, when the applied voltage was too high, the vigorous electrode reaction could aggravate the direct dissolution and physical stripping of the corundum anode, and damage the continuation of the formed interface product layer with a high melting point.     

Effect of bimodal microstructure on high-temperature properties of nano-reinforced Al2O3–MgO–C refractories

The beneficial effects of adding nanostructured expandable graphite (EG) hybridized yttrium aluminium garnet (EG\YAG) powder as a composite reinforcement in improving the oxidation resistance, hot-strength, and microstructure development in Al₂O₃–MgO–C refractories were studied. The refractory components reinforced with EG\YAG exhibited more than 60% of oxidation resistance enhancement and as high as 200% increase in hot-strength performance over the standard refractories, formulated without EG\YAG. Correlating the damage parameter (D_E) calculations based on ultrasonic measurements with residual strength data (R_c, R_b) showed that there was a progressive increase in R_c and R_b values with consistent reduction in the oxidative damage of EG\YAG reinforced refractories. Analysis indicated that these beneficial features were majorly ascribed to the in-situ development of bimodal microstructure with EG\YAG sintered framework throughout the refractory interior in these new class of reinforced systems. Additionally, the mechanism of toughening and implications of these results to materials design are discussed.     

Enhancement of thermal shock and slag corrosion resistance of MgO–ZrO2 ceramics by doping CeO2

The purpose of this paper was to develop ceramics materials with high thermal shock resistance and corrosion resistance for preparing gas blowing components. In this paper, MgO-rich MgO–ZrO₂ ceramics were obtained by using MgO powder and ZrO₂ powder as starting materials and CeO₂ as an additive. Changes in the properties in terms of thermal shock resistance, mechanical properties, and slag corrosion-resistance with chemical compositions were examined correlated to microstructure and phase changes. Especially, the effect of doping CeO₂ on phase transition of zirconia in MgO-rich system was discussed. The results showed that doping amount of CeO₂ significantly improved properties of MgO–ZrO₂ ceramics. Especially when doping amount of CeO₂ was 2 wt%, residual strength ratio was enhanced over 100% after thermal shock testing. In samples doped with CeO₂, ZrO₂ was stable in cubic or tetragonal form due to complete solution of CeO₂, which was important reason for the improvement of various properties of MgO–ZrO₂ ceramics.     

Comparative study of B4C,Mg2B2O5, and ZrB2 powder additions on the mechanical properties,oxidation, and slag corrosion resistance of MgO–C refractories

Boron-containing additives are used to improve the oxidation resistance of carbon-containing refractories; however, their effects on the mechanical properties and slag corrosion resistance of the refractories have rarely been studied. In this work, B₄C, Mg₂B₂O₅, and ZrB₂ powders were incorporated into low-carbon MgO–C refractories to study their effects on the mechanical properties, oxidation resistance, and slag corrosion resistance of the refractories. The relationships between these properties and the microstructure and phase evolution were also studied. The results show that the flexural strengths of the MgO–C refractories at high temperatures are closely related to the apparent porosity and formation of an Mg₃B₂O₆ phase. The oxidation resistances are greatly improved after the introduction of boron-containing additives into the MgO–C refractories in terms of both thermodynamical aspects and the filling of voids and pores. The most effective antioxidant is B₄C, followed by the ZrB₂ and Mg₂B₂O₅ powders. The mechanisms through which the vanadium-containing slag attacks the MgO–C refractories mainly include the dissolution of magnesia to form melting phases, penetration through pores, and redox reaction with carbon.     

Effect of chloride concentration range on the corrosion resistance of Cu–xNi alloys

The anodic behaviour of Cu–xNi alloys and Cu and Ni metals was studied in slightly alkaline solutions containing Cl–-ions in the concentration range from 0.01 to 2.0moldm–3. The morphology and composition of the surface films formed by anodic polarization were analysed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). On the basis of quasi-potentiodynamic polarization data, Ec against cNaCl diagrams were constructed, where Ec is the critical pitting potential. These diagrams allow the determination of areas where the materials are susceptible to localized pitting attack. A critical chloride concentration (ccrit) exists below which the resistance to localized corrosion increases with decreasing nickel content and above which it increases with increasing nickel content. This effect is connected with the change in the corrosion resistance observed for the pure metal constituents, i.e., copper and nickel as a function of chloride concentration. The kinetic parameters of pitting corrosion of Cu–xNi alloys reflect the specific properties of both elements and suggest that an increase in Ni content above 40% would not have a significant effect on the corrosion resistance of Cu–xNi alloys.     

Effect of SiC on the corrosion resistance of electroless Cu–P–SiC composite coating

In this work, Cu–P–SiC composite coatings were deposited via electroless plating with the addition of sodium hypophoshite (NaH₂PO₂) as a reducing agent. The coating compositions deposited were determined by using energy dispersive X-ray spectroscopy (EDX). The surface morphology of the coatings that were analyzed using scanning electron microscopy (SEM) showed that SiC particles were uniformly distributed by virtue of surfactant addition and mechanical stirring. The anti-corrosion properties of Cu–P and Cu–P–SiC coatings in NaCl and HCl solutions were investigated by the weight loss and potentiodynamic polarization techniques. The results showed that the corrosion resistance of Cu–P–SiC coatings was superior to that of electroless Cu–P coatings and carbon steel substrates in various concentrations of NaCl and HCl solutions.     

Analysis on microcrystalline graphite and properties of MgO–C refractories with microcrystalline graphite

The composition and microstructure of microcrystalline graphite were studied by X-ray diffraction, differential thermal analysis–thermogravimetry, SEM and energy dispersive analysis in this paper. The chemical composition of ash in microcrystalline graphite was also analysed in the study. The microcrystalline graphite was introduced in MgO–C refractories fabrication to investigate the influence of microcrystalline graphite on the main properties of MgO–C refractories. The oxidation resistance, thermal shock resistance, hot bending strength and expansion rate of MgO–C samples with microcrystalline graphite and flake graphite were investigated in this study. It is indicated that the proper addition amount of microcrystalline graphite in MgO–C refractories should be no more than 4?wt-%.     

Properties of MgO–Fe–C refractories as linings of vanadium-extraction converter

For the purpose to extend the service life of MgO–C bricks used as linings of vanadium-extraction converters, MgO–Fe–C bricks with different carbon content were designed and the properties of this novel refractory were investigated by comparing to the traditional MgO–C bricks. The results showed that the poor service life of MgO–C bricks was due to the poor sinterability of the oxidized layer at 1400 °C, whereas the oxidized layer of MgO–Fe–C brick was well sintered due to the oxidation of Fe particles in the oxidized layer and formation of MgO–FeO_ss in air atmosphere. Excellent oxidation resistance and corrosion resistance against vanadium containing slag were also obtained due to the increase of compactness of oxidized layer and concentration of FeO in the oxidized layer compared to MgO–C bricks, and it is considered that MgO–Fe–C brick is a favorable substitute of MgO–C refractory to be used as linings of vanadium-extraction converters.     

Slag resistance mechanism of MgO–Mg2SiO4–SiC–C refractories containing porous multiphase aggregates

The slag resistance of MgO–SiC–C (MSC) refractories should be improved because of the mismatch in the thermal expansion coefficient between the aggregates and matrix, as well as the defects caused by the affinity between periclase and slag. In this study, MgO–Mg₂SiO₄–SiC–C (MMSC) refractories were prepared using porous multiphase MgO–Mg₂SiO₄ (M-M₂S) aggregates to replace dense fused magnesia aggregates. Compared to MSC, the slag penetration index of MMSC decreased by 43.5%. The structure of the porous aggregates increased the surface roughness, and the multiphase composition of the aggregates decreased the mismatch of the thermal expansion coefficient with the matrix, thus reducing debonding between the aggregates and matrix. The aggregates and matrix in the MMSC formed an interlocking structure, which bound them more tightly to improve the slag resistance. The slag viscosity at different depths from the initial slag/refractory interface was calculated using the Ribond model. The M-M₂S aggregates increased Si_xO_y^z− in the slag, which increased the slag polymerization and slag viscosity. The aggregates and matrix in the MMSC reacted with the slag to form high melting point phases, which reduced the channel of the slag. In addition, the penetration depth and velocity derived from the Washburn Equation were modified for the CaO–SiO₂–Al₂O₃–MgO–FeO slag and magnesia based refractory to accurately evaluate slag penetration.     

Slag conditioning effects on MgO–C refractory corrosion performance

Slag conditioning practice provides important benefits to the steelmaking processes including ladle refractory life improvement, lower flux cost/additions, better control of metal recovery, among others. Furthermore, with the advent of thermodynamic and mass balance calculations it is possible to predict the slag saturation and define the most suitable conditioner (oxide components) that should be added during the operational process. Thus, the aim of this work was to analyze the interaction of industrial slags and a MgO–C refractory composition by thermodynamic simulations (FactSage^™). Based on the attained results, a conditioned slag was designed and corrosion tests in the laboratory were carried out (cup-tests and induction furnace), for two MgO–C materials containing distinct magnesia sources. According to the calculated slag penetration area and refractory corrosion rate, a remarkable enhancement of the MgO–C bricks wear resistance was observed when such materials were placed in contact with the conditioned molten slag. Therefore, proper slag engineering can be a suitable replacement for purchased synthetic slags and the conditioning practices can directly improve refractory life, with more competitive costs for both, refractory producers and end-users.     

Atmosphere and carbon effects on microstructure and phase analysis of in situ spinel formation in MgO–C refractories matrix

The effects of carbon, air and reducing atmospheres on microstructure and phase evolution of in situ MgAl₂O₄ spinel (S) formation in the matrix of MgO–C refractories were investigated by X-ray diffraction powder analysis (XRD) and scanning electron microscopy (SEM)/energy-dispersive spectroscopy (EDS) techniques. The formation of spinel started under 1000 °C in both air and reducing atmospheres. The morphology of in situ spinel and its formation mechanism were however different and dependent upon the atmosphere. The solid-state reaction was clarified to be the dominant mechanism of spinel formation in oxide atmosphere, while the gas–solid reaction was found to play a vital role in reducing atmosphere. Reaction of MgO and C in reducing atmosphere led to the formation of Mg_(g) which was found to be partially controlling the in situ spinel formation in the carbon containing samples fired in reducing environment. The results which were necessary are explained with emphasis on MgO–C refractories applications.     

Improved thermal shock stability and oxidation resistance of low-carbon MgO–C refractories with introduction of SiC whiskers

This research focuses on the utilization of SiC whiskers synthesized from rice husk powders in low-carbon magnesia–carbon (MgO–C) refractories, and attempts to reduce the flake graphite content in refractories by adding synthesized SiC whiskers. The effect of the addition amount of SiC whiskers on the microstructure, mechanical properties, thermal shock stability and oxidation resistance of MgO–C refractories with different graphite content was studied. The results indicated that the introduction of SiC whiskers facilitated the generation and growth of ceramic phases in MgO–C refractories. By adding 1 wt% SiC whiskers, the graphite content could be reasonably reduced (from 5 wt% to 4 wt%), and the strength, thermal shock stability and oxidation resistance of refractories were enhanced by the synergistic effect of the introduced SiC whiskers and the generated ceramic phases, the CMOR, CCS, residual CCS, and oxidation resistance were increased by 44, 6, 12 and 27% respectively.     

Basic slag corrosion of alumina–magnesia–carbon refractory bricks containing Al and Si antioxidants

The slag-corrosion behavior of two alumina–magnesia–carbon refractories (AMC) with different antioxidants (Si and Al) has been comparatively studied by means of thermodynamic simulation and laboratory tests. A cup test (static) at 1723 K and 1873 K and dipping test (dynamic) at 1873 K were carried out using a steelmaking ladle slag. An iterative method that considers the change of the liquid's composition while it penetrates the refractory was employed for the thermodynamic calculation. The simulation as well as the static tests showed similar performance between both refractories (5% of wear at 1723 K and 6% at 1873 K). In spite of the type of antioxidant, the material with smaller particles of magnesia exhibited better performance in the dynamic test, with 55% less wear than the other AMC brick. This behavior was attributed to the faster MgAl₂O₄ spinel formation, which increased the material's cohesion, especially the matrix, during the heating stage. The Si antioxidant improved oxidation resistance, although it did not have a positive effect on the corrosion resistance of the material in any of the performed tests.