Corrosion behaviour of MgO-based refractories by different existence states of manganese-containing volatile phases

Medium/high manganese steel is great interest because of its excellent energy absorption and strength. Further, it has emerged as a potential advanced high-strength steel in automobiles. In this study, the corrosion behaviour of MgO-based refractories by different existence states of manganese-containing volatile phases was systematically investigated via laboratory experiments of the vacuum treatment of medium/high manganese steel. The dissection of a magnesia crucible indicated that the manganese-containing volatile phases exist in three states: manganese vapour, liquid manganese and manganese oxides. The corrosion mechanism models of the MgO-based refractories were established as follows: (1) For the manganese vapour, the periclase was dissociated into the fine blocks when the magnesia-carbon brick was continuously exposed to the manganese vapour. (2) For the liquid manganese, a MnO layer was detected near the manganese metal, which was resulted from the MgO oxidation, and a further reaction took place to form a MnO·MgO solid solution layer at elevated temperatures, which led to the poor binding ability of the MgO crucibles. (3) For the manganese oxides, the Al₂O₃ crucibles were preferentially corroded by the molten manganese oxides, which are attributed to the superior thermodynamic driving force for the formation of MnAlO₄. Moreover, a reaction layer composed of a MnO·MgO solid solution and a low-melting-point silicic acid phase was formed at the interface between the manganese oxides and the magnesia-carbon brick along the decarburized infiltrated zone. In addition, the molten manganese oxides infiltrated along the periclase grain boundaries into the magnesia-carbon brick and to form a reactive metamorphic layer. Consequently, under the attack of physical and chemical corrosion, the refractory particles were loosened and dropped into the corrosive manganese oxides, causing the irreversible damage of the refractory. This work provides a strong theoretical basis for the qualitative evaluating damage of MgO-based refractories by manganese-containing volatile phases.     

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.     

Application of Cr3C2/C composite powders synthesized via molten-salt method in low-carbon MgO–C refractories

High-performance and low-carbon MgO–C refractories are important refractories for smelting ultra-low carbon steel and clean steel. Based on this, Cr₃C₂/C composite powders were synthesized by the molten-salt method, and used as an additive to prepare low-carbon MgO–C refractories under nitrogen atmosphere. The phase, morphology and oxidation kinetics of Cr₃C₂/C composite powders were studied. In addition, the effect of Cr₃C₂/C composite powders on the morphology, mechanical properties, thermal shock resistance, and corrosion resistance of MgO–C refractories was investigated. The results indicated that the Cr₃C₂/C composite powders exhibited superior oxidation resistance than flake graphite. Moreover, the Cr₃C₂/C composite powders were introduced into the MgO–C refractories. Compared with the sample without Cr₃C₂/C composite powders, the introduction of 1 wt% Cr₃C₂/C composite powders significantly improved the thermomechanical properties and corrosion resistance of the material, its CMOR, CCS before and CCS after thermal shock were 9.06 MPa, 50.40 MPa and 32.60 MPa, respectively, and the corrosion index was significantly reduced from 44.6% to 26.5%.     

Designing environment-friendly chromium-free Spinel-Periclase-Zirconia refractories for Ruhrstahl Heraeus degasser

Ruhrstahl Heraeus (RH) degassers are globally used to manufacture vacuum-treated steel for automotive and railroad applications. The state-of-the-art environment-friendly chromium-free alternatives for direct-bonded magnesia-chrome refractories used in RH degassers are expensive, and the scientific literature lacks direct correlation between materials chemistry, processing, and functional properties. We have designed a novel spinel-periclase-15 wt% ZrO₂ composition containing 14 wt% in situ spinel which exhibited 7.2 MPa hot modulus of rupture (1500℃), exceeding all reported Cr-free refractories for RH degasser applications. Investigation with scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) attributed this improvement to a reduction in interparticle Ca and Si content which forms low-melting phases, as supported by FactSage thermodynamic simulations. The spinel-periclase composition SP exhibited superior thermal shock resistance because thermal shock-induced cracks were stopped by fracture porosity around MgO particles, formed due to thermal expansion mismatch. SEM-EDS analysis of the SP composition corroded by RH slag at 1650°C revealed that Fe is the most corrosive element followed by Ca and Si. Contradicting the consensus, it was observed that corrosion resistance of fused MgO was better than that of ZrO₂. The cubic ZrO₂ phase reduced FeO_x penetration locally by incorporating CaO from the RH slag into a solid solution and forming a CaZrO₃ phase creating a “slag barrier”. Lastly, pore size was found to greatly exacerbate slag penetration following the Washburn percolation model.     

Improvements on corrosion behaviours of MgO-spinel composite refractories by addition of ZrSiO4

Corrosion behaviours of MgO-spinel-ZrSiO₄ compositions were investigated. The influence of corrosion resistance based on the microstructural changes occurred due to the solubilities of constituents in corroded regions was examined using SEM/EDX analysis. The following observations were determined by microstructural characterisation performed at the interface of clinker-refractory: (i) the formation of ZrO₂ and Mg₂SiO₄ phases among MgO grains after sintering, (ii) the formation of CaZrO₃ phase during penetration, (iii) prevention of penetration by new phases formed making a barrier effect against clinker with an improvement in densification, and (iv) the decrease in the amount of CaO and the increase in the quantity of MgO using EDX analysis made moving from clinker towards refractory. The addition of ZrSiO₄ reduced the values of penetration and spreading areas of the corroded regions of composite refractories and improved the corrosion resistance significantly, leading to a long service life of MgO-spinel-zircon based refractories for industrial applications.     

Enhanced performance of low-carbon MgO–C refractories with nano-sized ZrO2–Al2O3 composite powder

Low-carbon MgO–C refractories are facing great challenges with severe thermal shock and slag corrosion in service. Here, a new approach, based on the incorporation of nano-sized ZrO₂–Al₂O₃ composite powder, is proposed to enhance the thermal shock resistance and slag resistance of such refractories in this work. The results showed that addition of ZrO₂–Al₂O₃ composite powder was helpful for improving their comprehensive performances. Particularly, the thermal shock resistance of the specimen containing 0.5 wt% composite powder was enhanced significantly which was related to the transformation toughening of zirconia and in-situ formation of more spinel phases in the matrix; also, the slag resistance of the corresponding specimen was significantly improved, which was attributed to the optimization of pore structure and formation of much thicker MgO dense layer.     

微孔镁尖晶石碳耐火材料的抗渣性能研究

以方镁石-尖晶石微孔陶瓷、电熔镁砂为骨料,以电熔镁砂细粉、鳞片石墨、金属铝粉为基质材料,以酚醛树脂为结合剂制备了含微孔陶瓷骨料的镁尖晶石碳试样.采用感应炉浸渍法对试样进行了抗渣试验,并对渣蚀后的试样进行了SEM和EDAX分析.结果发现:熔渣和熔钢冲刷是损毁的主要原因,熔损并不显著.显微结构分析表明,在侵蚀层和原质层之间可以发现MgO致密层的形成,MgO致密层的形成可抑制渣对试样进一步的侵蚀和渗透.渣对MSO颗粒的侵蚀主要是FeO和MnO等在方镁石中的固溶,导致MgO颗粒出现结构剥落;方镁石-尖晶石微孔陶瓷骨料的蚀损主要是尖晶石被渣中的CaO和SiO2所侵蚀,而渣对微孔骨料渗透并不严重.     

Corrosion mechanism of magnesia-chrome refractory bricks with FetO-SiO2-Cr2O3 copper converter slag

The paper investigates the effect of Cr₂O₃ on the resistance of magnesia-chrome refractory bricks to copper converter slag. The static crucible method was employed to carry out the slag resistance experiment. The corrosion of magnesia-chrome refractory bricks under the action of Fe_tO-SiO₂-xCr₂O₃ (x = 0–5 wt%) slag at 1300 °C was discussed. The microstructure of the corroded sample was analyzed by XRD and SEM-EDS to elucidate the corrosion mechanisms of magnesia-chrome refractory bricks with Fe_tO-SiO₂-Cr₂O₃ slag. The results indicated that the permeability index of the slag-resistant samples gradually decreased with increasing Cr₂O₃ content in the Fe_tO-SiO₂-Cr₂O₃ slag. Combined with SEM and XRD characterization, the MgO in the refractory reacted with FeO and SiO₂ in the molten slag, leading to dissolution and reaction corrosion of the refractories. Meanwhile, forming a (Mg, Fe)O solid solution layer in corroded samples can prevent further chemical reactions and high-temperature dissolution between the Fe_tO-SiO₂-Cr₂O₃ slag and refractories. With the addition of Cr₂O₃ in the Fe_tO-SiO₂-Cr₂O₃ slag, the corrosion effect of slag on refractories was weakened, and the (Mg, Fe)O solid solution layer became thinner. The magnesia-chrome refractory bricks showed excellent slag resistance when the Cr₂O₃ content in the copper converter slag was 5 wt%.     

Electromagnetic field effects on the formation of MgO dense layer in low carbon MgOC refractories

Electromagnetic field (EMF) would speed up the corrosion of low carbon MgOC refractories. Its influence mechanism will be investigated in this paper. The slag-resistance experiments of low carbon MgOC refractories were carried out under the condition of an induction furnace and a resistance furnace, respectively. Low carbon MgOC refractories with carbon of 6% (in mass) and a slag with the basicity (CaO/SiO₂) of around 0.87 were used in the experiments. The slag line of MgOC refractories corroded by the slag under the different conditions were analyzed by X-ray diffractometer (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The results show that in the induction furnace with an electromagnetic field (EMF), no MgO dense layer exists in the interface. However, MgO dense layer could be formed in the interface without any EMF. The formation mechanism of MgO dense layer indicates that the EMF could enhance the solution of MgO and power of Mg (g) discharge. As a result, EMF restrains the formation of MgO dense layer.     

Interface behaviour and corrosion behaviour of magnesia refractory by alkaline slag in an alkali recovery furnace

The adhesion of Na₂CO₃ slag to the surface of refractories in an alkali recovery furnace can cause corrosion and spall. Magnesia refractories can be used as linings in alkali recovery furnaces owing to their strong corrosion resistance to alkali slag. However, the permeability resistance of magnesia refractories is relatively poor. Hence, the interface and corrosion behaviours of slag cladding on magnesia refractories were studied using sessile drop and static crucible tests. The experimental results showed that an increase in the heating rate positively affected the cladding of the molten column on the refractory surface. The microstructure, element changes, and chemical composition changes of the corroded refractories were analysed using SEM-EDS and XRD. Thermodynamic simulation of the reaction between the slag and refractory was performed using Factsage 7.3. The results indicate that the generated forsterite filled the pores of the magnesia refractories. The microstructure of dense slag-refractory interface layer was formed, which prevented the infiltration of slag phases and alleviated the corrosion of refractories by the slag.     

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.     

Corrosion modeling of magnesia aggregates in contact with CaO–MgO–SiO2 slags

Ladle refining is an efficient process for improvement of quality of steel on secondary metallurgy under harsh conditions. Magnesia refractories with high purity are important raw materials for ladle lining in high-quality steel production. However, the penetration by CaO–MgO–SiO₂ slags damages magnesia refractories, which considerably limits their service life. Abundant grain boundaries in magnesia create channels for slag penetration and lead to the destruction of the structure. The effect of the microstructure on the slag corrosion behavior of magnesia aggregates requires further systematic investigation. In this study, a corrosion model was established to describe the slag penetration process of magnesia aggregates. The effects of the grain-boundary size and slag CaO/SiO₂ mass ratio (C/S ratio) on slag penetration were investigated, and the possibility of the microstructure optimization of magnesia aggregates was discussed. The results indicated that magnesia aggregates exhibited excellent slag resistance for slag with a C/S ratio above 1.5 or even 2.0. When the slag C/S ratio was lower than 1.5, the dissolution rate of magnesia decreased more rapidly with the increase in the slag C/S ratio. In addition, the much smaller grain-boundary size increased the slag penetration resistance by promoting the formation of a dense isolation layer and inhibiting further penetration processes. The calculation results agreed well with the experimental results, suggesting that the corrosion model is promising for predicting slag corrosion.     

Comparative evaluation investigation of slag corrosion on Al2O3 and MgO-Al2O3 refractories via experiments and thermodynamic simulations

Al₂O₃- and MgO-based refractories are widely used in the steel industry as lining materials for many metallurgical reactors. Due to their direct contact with slag and steel, they suffer corrosion and degradation, especially in the slag-line position, which limits their service performance. The purpose of this article is to obtain a better understanding of the corrosion behavior of the two refractories with different compositions of virtual steelmaking slags (wt%CaO/wt%SiO₂ = 3.0–7.0, Al₂O₃: 18–35 wt%) using laboratory experiments and FactSage thermodynamic modeling. Pure Al₂O₃ and MgO-Al₂O₃ crucibles were adopted to simulate the two refractories, respectively, during the experiment. The results show that the degree of corrosion of both crucibles increases with an increase in slag basicity and a decrease in Al₂O₃ content in the slag. The Al₂O₃ crucible is more susceptible to corrosion than the MgO-Al₂O₃ crucible, which is attributed to the effect of the slag penetrating through the Al₂O₃ crucible matrix and substituting part of its matrix. For the MgO-Al₂O₃ crucible, there was no obvious slag substitution, but a transition layer was found in the contact region between the crucible and the slag. The Al₂O₃ in the crucible matrix reacts with slag to produce calcium alumina (CaAl₁₂O₁₉, CaAl₄O₇) and other complex oxides, while the MgO particles at the MgO-Al₂O₃ crucible-slag interface were only surrounded by liquid slag without an obvious chemical reaction between them. The mechanism of corrosion was studied by experiments combined with thermodynamic calculations and with the establishment of a new corrosion model. This study is expected to provide a guide for the design of related refractories and slags in industrial applications.     

Low‐Carbon Magnesia‐Carbon Refractory: Use of N220 Nanocarbon Black

Addition of nanocarbon black up to 3 wt% was done in MgO‐C refractories, containing fixed 3 and 5 wt% of graphite, to study the effect on various refractory properties. Uniform distribution of carbon particles even at 1 wt% of nanocarbon black with 3 and 5 wt% of graphite was found to improve the refractory properties. The coked strength, hot strength, corrosion resistance, and oxidation resistance were found to be improved for nanocarbon‐containing MgO‐C refractory compared with the conventional refractory due to in situ formation of Al₄C₃. Higher amount of nanocarbon black was found to deteriorate the refractory properties.     

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.     

Thermodynamic calculation and microstructure characterization of spinel formation in MgO–Al2O3–C refractories

In this paper, by simulating the gas phase conditions inside the MgO–Al₂O₃–C refractories during continuous casting process and combining with thermodynamic analysis, as well as SEM analysis, the gas-gas and gas-solid formation of MA spinel were clarified in carbon containing refractories. Thermodynamic calculations showed that gas partial pressure of CO, O₂ and Mg could meet the formation and stable existence conditions of MA spinel in MgO–Al₂O₃–C refractories under service environment, and nitrogen could not affect the formation of MA spinel at 1550 °C in the thermodynamic condition. The formation processes of MA spinel were analyzed experimentally under embedding carbon atmosphere. The carbon-coated alumina powders in MgO–Al₂O₃–C refractories prevented the direct contact between magnesia and alumina. Mg gas was formed by carbon thermal reaction, then reacted with alumina (gas-solid) and gas containing aluminum (gas-gas) to generate MA spinel. Through gas-gas or gas-solid reaction, the formation of MA spinel was effectively controlled. By means of SEM analysis, a two-layer structure with dense outer spinel layer and loose inner layer was formed in MgO–Al₂O₃–C refractories.     

Effect of electromagnetic field on slag corrosion resistance of low carbon MgO–C refractories

Using MgO–C refractories containing 6% carbon and the slag with a basicity (CaO/SiO₂) of around 0.8, the melting slag resistance experiments of low carbon MgO–C refractories were carried out in induction furnace and resistance furnace, respectively. The microstructure of low carbon MgO–C refractories corroded by slag under the different conditions was analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDAX). The results show that in induction furnace having electromagnetic field (EMF), there are MgFe₂O₄ spinel with a little of Mn ions generated in the interfacial layer. Part of the solid solution is monticellite [CaMgSiO₄] containing a little MnO and FeO. While under the condition of EMF free, there is not MgFe₂O₄ spinel in the interfacial layer and the solid solution is monticellite (CaMgSiO₄). At a high temperature, EMF increases the diffusion coefficient of Fe^2+/3+ ions, which displaces Mg²⁺ and forms MgFe₂O₄ with a little of Mn ions. There are MgAl₂O₄ spinel in the penetration layers under the conditions of both EMF and EMF free. EMF speeds up corrosion of low carbon MgO–C refractories.     

The optimization of the microstructure and phase assemblage of high chromia refractories

The effect of Al₂O₃ and ZrO₂ addition in chromia-based refractories was investigated. The strength of ZrO₂-bearing chromia refractories was greatly enhanced by 4–8 wt% Al₂O₃ additive. In the range of 0–12 wt% ZrO₂ addition, 6 wt% ZrO₂ was most beneficial to the improvement in thermal shock resistance. Corrosion resistance was compared by exposing to three kinds of coal slag with various CaO contents. As the substitution of ZrO₂ for Cr₂O₃ increased, slag penetration increased, particularly in the case of the slag containing ∼16 wt% CaO. Considering the trade-offs between corrosion resistance, thermal shock resistance and mechanical properties, the optimum phase assemblage of high chromia refractories consists of the large granular Cr₂O₃ grains bonded by annular (Cr, Al)₂O₃ grains and well-distributed fine ZrO₂ grains.     

Densification and properties of ZrO2 nanoparticles added magnesia–doloma refractories

In this work the effect of nano- and microZrO₂ addition on the densification and hydration resistance of MgO–CaO refractories was investigated. 0, 2, 4, 6 and 8 wt% ZrO₂ was added to MgO–CaO refractories that contain 35 wt% CaO. The crystalline phases and microstructure characteristics of specimens sintered at 1650 °C for 5 h in an electric furnace were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The physical properties are reported in terms of bulk density, apparent porosity and hydration resistance. Results show that with addition of ZrO₂ the bulk density and hydration resistance of the samples increased while apparent porosity decreased. Also the hydration resistance of the samples was appreciably improved by the addition of ZrO₂ due to its effect on decreasing the amount of free CaO in the refractories, promotion of densification as well as modification of the microstructure. Also it revealed that the nanoZrO₂ addition was more effective than microZrO₂ due to its higher activity.     

Cathodoluminescence imaging for rapid identification of low-melting CaO–MgO–SiO2 phases in MgO-based refractories involving the steelmaking process

For MgO–C refractories used in the steelmaking process, identifying low-melting CaO–MgO–SiO₂ phases is crucial because they accelerate the corrosion of the refractories. However, electron probe microanalysis, a conventional method for identifying such phases, is time-consuming. Herein, cathodoluminescence (CL) imaging is proposed for the rapid identification of low-melting CaO–MgO–SiO₂ phases at the reaction interface between MgO-based refractory and steelmaking slag. Monticellite, merwinite, and melilite were identified as the low-melting phases, emitting green, red, and violet luminescence, respectively, in the CL images. Other mineral phases emitted luminescence whose colors differed from those of the low-melting phases (3CaO·2SiO₂ and 2CaO·SiO₂) or no luminescence (magnesiowüstite, MgO·Al₂O₃, Ca₂Fe₂O₅, 3CaO·SiO₂, MnS, and FeS). The CL images (area: 0.5 ×0.3 mm²) were obtained in 30 s. Therefore, CL imaging is effective for the rapid identification of mineral phases, which limit the service life of MgO–C refractories during steelmaking.