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.     

Radical reaction-induced Turing pattern corrosion of alumina refractory ceramics with CaO–Al2O3–SiO2–MgO slags

Quasi-volcanic corrosion occurs at the triple-phase interface of alumina refractory ceramics and MgO-containing CaO–Al₂O₃–SiO₂ slags in the air, causing severe damage to ceramics. To address this limitation, in this study, a slag corrosion experiment is performed on alumina refractory ceramics using CaO–Al₂O₃–SiO₂–MgO slags. Various spectroscopic techniques, including electron paramagnetic resonance spectroscopy, are used to investigate the influence of slag structures with varied MgO contents on the corrosion peaks and mechanism. The results show large quantities of reactive radicals, including superoxide radicals, in the slags. Free-radical reactions between refractory ceramics and slags lead to Turing pattern corrosion. An increase in the amount of non-bridged oxygen in the slag structure decreases the amount of original superoxide radicals. Consequently, the intensity of the free-radical reactions of alumina dissolution increases, thereby increasing the height of the corrosion peaks.     

Visual measurement and characterisation of quasi-volcanic corrosion at alumina ceramic-oxides melt-air interface

Alumina ceramics undergo quasi-volcanic corrosion at the melt-air interface, which causing severe damage. However, the growth behaviour of quasi-volcanic corrosion requires further investigation. Here, high-temperature visualisation technology was used to characterise and quantify quasi-volcanic corrosion at the melt-air interface at 1600 °C. An evolution model of the corrosion peak area with changes in the melt compositions was also established. The formation and growth of the corrosion peak is closely related to the temperature, soaking time and the dissolution reaction; quasi-volcanic corrosion occurs at ≥ 1600 °C, and is visibly different from normal corrosion at a three-phase interface. The peak height exhibits competitive evolution with the full width at half maximum of the peak. The obtained evolution of the corrosion peak and relationship between the corrosion area and the C/S ratios of the melts can support for prolonging the service life of alumina ceramics.     

Thermodynamic Stability of Spinel Phase at the Interface Between Alumina Refractory and CaO–CaF2–SiO2–Al2O3–MgO–MnO Slags

The interfacial reaction between alumina refractory and CaO–CaF₂–SiO₂–Al₂O₃–MgO–MnO slag was observed at 1873 K to estimate the stability of the spinel phase using computational thermodynamics under refining conditions of Mn‐containing steels. The concentration of MnO formed by the slag–steel reaction in the CaO–CaF₂–SiO₂–Al₂O₃–MgO melts generally increased by decreasing the CaO/SiO₂ ratio of the initial melts. No intermediate compounds were formed at the refractory–slag interface when the initial CaO/SiO₂ ratio was 0.5, whereas CaAl₁₂O₁₉ (CA6) and Mg(Mn)Al₂O₄ (spinel), identified from TEM analysis using EDS mapping and SAED patterns, were observed at the refractory–slag interface when the CaO/SiO₂ ratio was 1.0 or greater. The (at.%Mg)/(at.%Mn) ratio in the spinel solution increased by increasing the CaO/SiO₂ ratio, which originated from the fact that MgO activity continuously increased as the CaO/SiO₂ ratio increased. From thermodynamic analysis considering the equilibrium constant (K_SP) and activity quotient (Q_SP) of the spinel formation reaction at the slag–refractory interface and the bulk slag phase, the precipitation–dissolution behavior of the spinel phase was predicted, which exhibited good consistency with the experimental results. Hence, the dissolutive corrosion mechanism of alumina refractory into the CaO–CaF₂–SiO₂–Al₂O₃–MgO–MnO slag was proposed.     

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.     

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%.     

Effect of slag basicity on Turing pattern corrosion of alumina refractory ceramics in the presence of free radicals

Turing pattern corrosion causes serious damage to alumina ceramics. Therefore, the quaternary slag corrosion of alumina refractory ceramics was studied with different slag basicity by adjusting relative content of MgO and Al₂O₃. The results show that reactive oxygen species (ROS) like singlet oxygen and superoxide radicals are generated in the slags. The height of corrosion peak is determined by both the original superoxide radical and the amount of non-bridging oxygen (NBO) of the slags under the change of basicity. Moreover the influence of free radicals on the corrosion peak is greater than that of NBOs. A free-radical reaction was proposed as the formation mechanism of the Turing pattern corrosion peaks based on ROS.     

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.     

The effect of increasing MgO content in dendromass on ash fusibility and corrosion of corundum refractory castable

Low melting temperatures of the biomass ash at the intensive combustion can generate the slag with a portion of unburned fuel. The portion of imperfectly burned fuel reduces the efficiency of its use. An alternative route to solve the problem without changing the combustion condition is a modification of the chemical composition of the ash with the addition of an agent increasing the melting temperature of the ash/slag. In this paper, we report on the impact of adding the magnesite waste sludge to ash on their melting. Three types of ashes with different SiO₂ and K₂O content were selected for the tests. The melting temperatures of ash and ash mixtures with magnesite sludge (mass ratio 0.5–2:1) have been determined. The addition of magnesite sludge to ash increased the temperature of fusibility index of the mixtures by 50–100 °C. The ash fusibility temperature rose with a decrease of the SiO₂ content in the ash. Subsequently, the interaction of the ash/ash mixtures with the refractory corundum castable was monitored at a temperature of 1450 °C using a static crucible corrosion test. The addition of magnesite sludge to ash reduced the aggression of the slags to the refractory corundum materials. The Al₂O₃ concentration in the post mortem slags of the ash mixtures with the magnesite sludge (2:1) was about 20–23 wt% lower in comparison to the slag without magnesite. The evaluation of the effect of magnesite sludge addition to ash and the elements (K, Ca, Mg, Fe, Si) penetration on the extent of corrosion is the subject of further work aimed at analyses of the corrosion interface of the slag – corundum refractory material.     

Corrosion behavior of Monofrax K-3 refractory in borosilicate-based model low activity waste glass melts

Owing to its good chemical and thermal durabilities at high temperatures, Monofrax K-3 refractory is widely used in nuclear waste vitrification as a lining material in melting vessels. However, the corrosion of K-3 refractory during the vitrification of nuclear waste is a serious problem because it affects the melter's safety, performance, and lifetime. Therefore, in the present study, we have focused on unearthing the impact of glass network formers, such as SiO₂, B₂O₃, and Al₂O₃, in a model nuclear waste glass composition on the corrosion of Monofrax K-3 refractory. The corrosion tests have been performed per ASTM C621 at 1150°C for 5 days. The dimensional measurements on corroded K-3 refractory suggest that Al₂O₃ and SiO₂ tend to reduce the refractory corrosion (neck loss), with the effect of Al₂O₃ being significant. A corroded region on the K-3 refractory at the melt–refractory interface is observed. The corrosion occurs via a coupling of the melt infiltration induced by a capillary effect and the dissolution of Al, Mg, and Fe components from K-3 into the melt through chemical reactions. A Cr-rich layer is retained on the glass contact surface of the corroded K-3 refractory.     

Static and dynamic corrosion behavior of SnO2-doped AZS slip cast refractory in SLS glass

The corrosion behavior of a tin IV oxide-doped AZS-refractory, subject to static and dynamic corrosion testing at 1370˚C in soda-lime-silica glass, was studied considering the effect of the microstructural features on corrosion. The refractory was synthesized by slip cast methods through reaction sintering of alumina and zircon raw materials using SnO₂ as a sintering agent. SnO₂ had a considerable influence in the enhanced alumina/zircon reaction sintering and the subsequently evolved microstructures of an interlocked Zr_(1-x)Sn_(x)O₂ solid solution reinforced alumina-mullite composite. The process kinetics of the refractory corrosion followed reasonably well the predicted dependence on the square root of angular velocity under forced convection corrosion. Glass chemical corrosion and erosion of the refractory, under static and dynamic glass conditions, respectively, revealed the Zr_(1-x)Sn_(x)O₂ solid solution-rich mullite matrix as providing the most corrosion resistance and glass compatibility.     

A comparative study on the slag resistance of dense corundum-spinel refractory and lightweight corundum-spinel refractory with density gradient

Lightweight corundum-spinel refractory with a density gradient structure from exterior to interior was fabricated. Slag resistance of lightweight and dense corundum-spinel refractory is investigated by X-ray diffraction, scanning electron microscopy, energy dispersive analysis, mercury intrusion porosimetry and Factsage. The results show that lightweight corundum-spinel refractory with high apparent porosity exhibits comparable slag resistance to dense corundum-spinel refractory, especially with superior slag penetration resistance. The dense exterior with small pore size in the lightweight corundum-spinel refractory can effectively hinder slag penetration. Corrosion product phases (C₂S, CA₂, CA₆, and C₂M₂A₁₄) with high melting point and inconsistent melting temperature, most of Fe and Mn elements in steel slag solubilizing in spinel, especially strip CA₆ around corundum aggregate, prevent the refractory from further slag penetration and corrosion.     

Dissolution kinetics of alumina in molten CaO–Al2O3–FetO–MgO–SiO2 oxide representing the RH slag in steelmaking process

In order to effectively remove alumina inclusions suspending in ultra-low C steel during RH process, the dissolution kinetics of alumina in molten CaO–Al₂O₃–Fe_tO–MgO–SiO₂ oxide was investigated. A crucible dissolution technique was used where the alumina crucible was allowed to dissolve in the slag of various conditions ((% CaO)/(% Al₂O₃), (% Fe_tO), temperature). The obtained data were interpreted using a kinetic mass transport equation to obtain the mass transport coefficient (k_m) in each condition. Increasing (% CaO)/(% Al₂O₃), (% Fe_tO), and temperature increased the dissolution rate as well as the k_m provided that the slag composition is not close to its saturation composition by alumina. In order to simulate the dissolution of alumina inclusion in the RH slag, which cannot be measured by a confocal scanning laser microscopy (CSLM) at present due to the opaqueness of the slag, the modified invariant interface approximation was employed. Along with the obtained k_m, the viscosity of slag, and a reference experiment using the CSLM, the dissolution kinetics of alumina inclusion in the Fe_tO-containing RH slag was predicted. The time required for the dissolution of alumina inclusions from liquid steel to RH slag was discussed.     

Analysis of corrosion of corundum refractory castables in relation to increased MgO content in dendromass ashes

Biomass ash has a significantly lower proportion of Al₂O₃ and higher proportions of K₂O, CaO and SiO₂ than coal ash. Biomass combustion in power plants increases demands on refractory linings and metal fittings in boilers. Grates and linings of combustion chambers in furnaces and boilers are more susceptible to clogging and are degraded by bio-ash slag. The results of this study suggest that the addition of approximately 2% of powder magnesite waste to the wood-chip fuel can significantly mitigate ash slagging and also corrosion of the corundum refractory material. With regard to the resulting increased MgO content in the dendromass ash, the corrosion of corundum refractory material was studied. The MgO content in the ash was increased by adding powder magnesite waste to ash samples. The results of corrosion tests (1450 °C/7 h) showed that ash slag with MgCO₃ addition corroded the corundum material less. Analyses of the post-mortem slag and corrosion interface confirmed: (i) higher K₂O concentration in the ash caused increased corundum material corrosion by both vapours (g-s) and melt (l-s); (ii) K₂O reacted with Al₂O₃ at the corrosion interface and also penetrated intensively into the fine-grain matrix by surface diffusion; (iii) MgO remained in the slag; (iv) increased Al₂O₃ content in the molten slag initiated a liquation of MA-spinel. These results (especially MA spinel liquation from the slag and K₂O diffusion through the matrix followed by micro-grain dissolution) indicate that replacing the mullite binder phase in the matrix with MgO and/or spinel could lead to improved resistance of the refractory material to biomass ash slag.     

Corrosion in high alumina refractory serviced in a bench-scale entrained flow gasifier

Corroded microstructure of high alumina refractory serviced in a bench-scale entrained flow gasifier was investigated. The bench-scale gasifier established a relatively real environment: 0.1 MPa, 1400–1450 °C and reducing atmosphere. By analyzing the two stages of the slag accumulation and slag further corrosion in pore structures, it was found that the refractory in the pore structures was dissolved and formed low temperature eutectics with the slag penetrating. Moreover, the compositions of the slag infiltrated around the burner plane was compared and the results indicated that the slag infiltrated into the refractory above the burner plane had a higher solubility to the refractory. Thermodynamic equilibrium calculation for the mixture of refractory and slag was used to predict the changes of solid phase and liquid phase. Additionally, as the slag penetrated and the temperature decreased, anorthite (Ca₂Al₂Si₂O₈) was the first to precipitate.     

Interfacial reaction between magnesia refractory and “FeO”-rich slag: Formation of magnesiowüstite layer

This study investigated the reaction between CaO-SiO₂-Al₂O₃-xFeO-MgO-MnO (CaO/SiO₂?=?1.2, x?=?20–50?wt%) slag and magnesia refractory. Using SEM-EDS analysis, we confirmed the formation of a (Mg,Fe)O_{ss(solid_solution)}, called magesiowüstite (MW), intermediate layer at the slag-refractory interface. MgO dissolved from refractory and reacted with the bulk slag to form MW layer at the interface. Simultaneously, slag penetrated through micro-pores and reacted with the refractory to form MW layer. In other words, the MW layer built up in both directions from initial refractory-slag interface. The thickness of the MW layer increased as the FeO content in the slag increased, and using EDS line scanning, a Mg and Fe concentration gradient was confirmed within the MW layer. The slag, which penetrated into the refractory, had a chemical composition of the CaO-SiO₂-Al₂O₃-MgO system without FeO, indicating that FeO was consumed by forming a MW layer at the refractory hot face. The slag-refractory interfacial reaction was simulated using thermochemical software, FactSage?7.0. The results predicted a MW monoxide composed of MgO and FeO. A spinel phase was formed when FeO was greater than 40?wt%. These thermochemical computations were comparable to our experimental findings.     

Formation of ferrospinel layer at the corroded interface between Al2O3-spinel refractory and molten steel in RH refining ladle

Refining ladle is generally the last inclusion-removal vessel before the continuous casting of steel, so, it has a vital effect on the final steel quality. In this work, degradation process of a cement-free Al₂O₃-MgAl₂O₄ refractory in contact with molten steel/slag in the metal bath area of a Ruhrstahl Heraeus (RH) refining ladle was investigated. A reaction product layer with the formation of ferrospinel (hercynite) and Fe-rich phases was observed, suggesting that the interactions/reactions between the refractory lining and the molten steel should also be considered to have a better understanding of overall degradation mechanism of the refractory served under the RH refining conditions. The two types of alumina grains in the refractory, sintered and fused alumina, were attacked in an active way and a passive way, respectively. The effect of the crack generation and steel infiltration on degradation of the refractory was also discussed in detail, based on the microstructural characterizations.     

Slag Resistance of Al2O3–MgO Refractory Castables in Different Environmental Conditions

Although the corrosion performance of spinel‐containing castables has been extensively investigated in recent years, no previous studies accessed the different conditions present in the ladle bottom. In this region, strong variations in the atmospheric environment are often detected, which could drastically change the interactions between refractory and molten slag. In the present work, the main corrosion mechanisms of an alumina–magnesia castable in two environmental conditions (oxidizing – pO₂ = 0.21 atm—or reducing – pO₂ = 10^?15 atm— atmosphere) were evaluated by means of scanning electron microscopy and EDS analyses of the corroded samples and thermodynamic simulations. The attained results showed that the slag penetration was suppressed in the presence of oxygen due to the precipitation of a great amount of calcium monoaluminate (CA) crystals as the refractory interacted with slag. Conversely, the CA phase was not stable under reducing conditions and, therefore, many more refractory components (Al₂O₃, MgO, and MgAl₂O₄) had to be dissolved to precipitate calcium dialuminate (CA₂) by reacting with infiltrating slag. Thus, besides providing a suitable and more realistic understanding of the castable performance in service conditions, the results also indicated that the prediction of the environmental conditions is of utmost importance for the design of high performance refractories.     

A basic gunning material interfaces study

Corrosion mechanisms of various basic gunning materials were investigated using crucible tests. The low corrosion resistance of olivine-containing materials was shown to be related to melted phases formation which occurs, at the gunning material/slag interface, at high temperature. MgO-rich materials are more resistant to basic slag attack. Moreover, increasing the refractory material CaO content by means of dolomite addition rises the slag penetration resistance and lowers the bulk modification resulting from slag interaction. In these materials, the corrosion mechanism involves a slag reaction with the starting binder, (NaPO₃)_n, and dolomite initially contained in the gunning material. A dolomite-containing material was projected on slag-covered MgO-C bricks, using a laboratory test equipment. Investigations carried out at the gunned material/slag/bricks interface show that the suitable silicophosphate bond still forms, at interface, despite slag interaction.     

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.