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Addition of ceramics materials to improve the corrosion resistance of alumina refractories

Abstract

This review shows that adding ceramic materials such as ZrO2, ZrSiO4, TiO2, SiO2, Cr2O3, etc., to abase alumina refractory clay, improves the corrosion resistance under the presence of acid (H2SO4, HCl, HNO3, etc) and/or alkaline (NaOH, KOH, Na2O, Ca(OH)2, etc) solutions. Characterization techniques, such as SEM, XRD, ICP-MS analysis, and weight loss show individual or correlated properties between apparent porosity, bulk density, mass loss, total number of eluted ions from alumina refractories, and the degree of corrosion resistance obtained by sintering. Static and dynamic (ASTM C-874) slag cup tests are performed by immersion or exposure to a continuous flow stream in acid and/or alkaline solutions. Results show that adding 81.6 mol% Cr2O3, 4 mol% TiO2 to the Al2O3 base reduces corrosion damage depth by SiO2–CaO–Al2O3 molten oxide. Sintered mullite matrix increases its corrosion resistance against alkaline vapors by adding 16% of ZrSiO4. SiO2–Al2O3 refractory ceramics with mixtures of 67–23, 56–38, 25–71%, respectively, have 98% resistance against acid solutions. However, their resistance to alkaline solutions is 58% with 23% alumina and rises up to 88% with 71% of alumina. It is concluded that higher content of corundum (α-Al2O3) and mullite phases on refractory ceramic improves its resistance to acid and alkaline solutions and that the attack of degrading solution is preferential over impurities in the phases present near the grain boundaries.

Introduction

The industry (energy or petrochemical) has the need to incinerate corrosive gases product of waste, which cannot be discharged to the atmosphere for ecological reasons. One of the major challenges of the industry is to increase the corrosion resistance of refractory materials and to improve the durability of combustion chambers in power plants used for waste incineration.

By increasing corrosion resistance of refractory materials, the durability of combustion chambers in energy plants used for waste incineration is improved. Ceramic materials such as Al2O3 have been used as refractories in combustion chambers, despite the severe corrosion that occurs by fusing gases. Al2O3 is always the first option as a refractory because it is less expensive, easy to handle and produce. Latest developments in non-oxide refractory ceramics are SiC y MgO-C (magnesia-carbon refractory), but these materials cannot be used in highly oxidizing atmospheres, including combustion chambers, due to poor thermal shock resistance of MgO. Although Cr2O3 may be the first candidate as a compound for Al2O3, diffusion rate of the Cr ion is smaller than any other metal ions; however, pure Cr2O3 reacts with the basicity of the slag, expelling toxic Cr6+ into the sintered body or slag [1].

Refractory materials in operation are subjected to three types of loads: thermal, mechanical, and chemical. Chemical resistance of ceramics is defined as the ability to withstand destructive action in an aggressive environment. This resistance depends on the properties of the aggressive medium, chemical composition and structure of the ceramic, and the conditions of the corrosion process, especially the point of contact between the ceramic and the aggressive medium [2]. The life cycle of a refractory material is usually determined by the degree of corrosion it can withstand [3]. Corrosion may occur by a gas (inert or chemically active) or by a liquid (acid solutions, bases, salts, molten salts, glass, slag, metals, fresh or sea water, etc) [2].

A lot of scientific papers have been published on corrosion in alumina under several conditions, where corrosion behavior of highly pure alumina at room temperature for periods of 10 days, in a solution of H2SO4 at different concentrations (2 wt%, 10 wt% and 20 wt%), shows that impurities (MgO, CaO, SiO, Na2O, etc.) present in the grain boundaries are the first cause of corrosion in high-purity alumina ceramics [4, 5].

Multiple processes contribute to the corrosion of refractory materials, but these processes are always based on the physicochemical properties of the corrosive agent and on the intrinsic properties of the refractory materials, such interconnected porosity [6] and the presence of mullite phase 3Al2O3 2SiO2 [7]. A high interconnected porosity allows the access of external agents and increases the surface of reaction, which favors material interaction and corrosion. Also, refractories are multiphase materials that enable preferential or selective attack [6].

For example, mullite phase [7] is the only stable crystalline phase in the aluminosilicate system under normal atmospheric pressure. The high levels of functional properties of aluminosilicate ceramics depend on the general content of mullite on its structural and morphological states [7], including resistance to acid and alkaline solutions [5]. Therefore, the industrial methods for producing aluminosilicate ceramics are based on the highest formations of mullite quantity. This is achieved by using the refractory clays with high amount of SiO2 and alumina Al2O3 and SiO2 [7].

Materials and methods

Eight of the most abundant elements in Earth are presented in Table 1. Silicon is never found in its elemental form, but in form of silicate minerals, and their structures depend on the composition and formation conditions of the place where they are found. They can be contained in relatively pure mineral deposits or in deposits made up of one or more mineral species [8].

Table 1 Chemical composition of crust of Earth [8]

It is very common for alumino silicates to be referred with the term “clay,” but dozens of minerals fall under the classification of clay, and a single clay deposit may contain a variety of individual clay minerals along with impurities. Therefore, clay minerals are classified as phyllo silicates, due to their layered structure. The most common clay mineral is kaolinite Al2Si2O5(OH)4, although others such as montmorillonite (NaCa)0.33 (Al,Mg)2Si4O10 (OH)2nH2O, and vermiculite [(Si3.04 Al0.92 Ti0.04) (Al0.11\({\text{Fe}}_{0.35}^{3 + }\)\({\text{Fe}}_{0.07}^{2 + }\)Mg2.41Mn0.003) O10(OH)2]Ca0.21K0.05Na0.10 are also abundant [8].

The type and proportion of clay minerals in individual sediments are related to sediments of source rocks, weather conditions, and transport mechanisms that occur in the material according to Biscaye [9], Liangbiao and Liu [10]. Also in the study by Kurkura Kabeto [11], sediments of the clay show the presence of kaolinite as main mineral, over a base of kaolin and other minerals associated with clay. These can be divided in to four types: kaolinite, microcline KAlSi3O8, kaolinite 3Al2O3 2SiO2 2H2O, muscovite KAl2Si3AlO10(OH)2–microcline KAlSi3O8–kaolinite 3Al2O32SiO2 2H2O. However, in their study, clays show other associated minerals such as SiO2 quartz, vermiculite Mg0,7(Mg,Fe,Al)6(Si,Al)8O20(OH)4·8H2O, albite NaSi3AlO8, calcite CaCO3 and calcite magnesia in less quantity [11].

Clay minerals from different regions are added in certain compositions to formulate corrosion resistant alumina refractories. Mineral extraction is carried out in open sky. It is subsequently cracked to homogenization (ASTM C-702) and dried at 110 °C for up to 24 h. [12, 13]. Samples are reduced to microns (40–74 μm), using in some cases mixtures of different grain sizes [3, 7, 14] and using ball mills to produce superficial and structural changes to the solid material [15]. The ball and load ratio for ball mills varies between 10, 15 and 20–1, respectively, with rotation speeds between 250 and 480 rpm [16, 17]. They were sieved to obtain homogeneous particle size (ASTM C-136). Samples that are subjected to corrosion tests are formed into cylinders [3, 13], rectangles [14] or squares, mostly with variable dimensions by cold isostatic compaction using a hydraulic press at 10.12, 22, 40, 50, 75 and even 150 Mpa [3].

They are sintered an oven at heating rates of 2–10 °C/min [7] and temperatures of 1600–1700 °C. The soaking times are ranging from a few hours up to few days, depending on the composition of the mixture and phases to be obtained [3]. The cooling is carried out inside the oven off. Static [18] or dynamic [13, 19] testing for corrosion resistance is made by chemical reaction, simulating conditions of a real application, at certain temperature, atmospheric pressure or vacuum, and with acid or alkaline substances, through methods such as cup slag test, drip slag test (ASTM C-768), gradient slag test, rotary slag test (ASTM C-874) and dip and spin test.

Result and discussion

A diffusion of ions (atoms) is common between a ceramic material and the aggressive medium. But it is not necessary to consider this interaction as destruction and subsequent loss of ceramic material. In ceramic corrosion, there are two main types of chemical resistance, which are acid and alkaline. Table 2 shows chemical resistance for alumina oxide, through mass loss under different reactive media [2].

Table 2 Chemical resistance of aluminum oxide ceramics in different acids and alkalis [2]

Booklet of the Friedrichsfeld Company (Germany) reported the mass loss (Table 3) to measure chemical resistance of ceramics with 96.0 and 99.5% of Al2O3 subjected to several acid and alkaline media [2].

Table 3 Chemical resistance of corundum ceramics in different acids and alkalis [2]

In a ceramic mixture of silica and alumina, corrosion resistance is higher under acidic than alkaline media, and mass loss under acidic is lower than alkaline medium, Table 4.

Table 4 Acid and alkalis resistance of ceramics and porcelain [2]

Long Huang, Luo and Li [20] studied Al2O3 alumina ceramics under acid and alkaline corrosion, sieved, uniaxially packed at 200 MPa and sintered at 1450 °C for 2 h. Then, he calculated mass loss in solutions of 50 ml HCl and NaOH. Huang et al. determined through the sample morphology that corrosion occurs mainly in grain boundaries of alumina ceramic materials (Al2O3) Fig. 1 [20] and that mass loss increases with increasing temperature (Fig. 2) [20].

Fig. 1
figure1

SEM image of Al2O3 before and after NaOH corrosion [20]

Fig. 2
figure2

Mass loss of Al2O3 with different solution temperatures (0.5 M NaOH), exposed for 5 h [20]

Interaction between CaO–Al2O3–SiO2 leads to the formation of phases with low melting point temperatures, such as gehlenite or anorthite. Nanomaterials in shaped refractories raise the surface energy making them more reactive. Thus, their resistance to corrosion and thermal shock can be improved if the interaction of nano-phases with the other particles is favorably adapted [12].

Maitra, Das and Sen [12] studied the effect of tiny titanium oxide particles on densification of molten refractory cement Al2O3–MgO. They observed that the addition of TiO2 results in a positive influence in densification, spinel formation and reduction in grain sizes of spinel formed at high temperatures in this system, modifying physical properties such as contraction in cooking, apparent density, apparent porosity and actual density. Besides, the microstructure of the bodies with added TiO2 became more uniform and with less content of vitreous phases. Thus, it was suggested that the addition of TiO2 leads to densification and phase formation, having a direct influence in the increase in their properties [12].

Takehiko Hirataet al. [1] reported that corrosion depth in Al2O3 ceramics is reduced by adding Cr2O3, reaching the lowest corrosión value with 81.6% Cr2O3 mol-14.4 mol% Al2O3-4 mol TiO in the mixture. This is caused by diffusion difference in Cr2O3, because diffusion rate for the Cr ion is rather slow compared to other metal ions (Fig. 3). It was also reported that corrosion depth increased by increasing CaO ratio contained in the slag, on the ceramic matrix formed by CrO2–Al2O3 (Fig. 3) [1]. Since solubility and viscosity properties change in the slag as CaO content changes [21]. The dissolution process in ceramics can be controlled by the diffusion of oxide components, through the limiting layer of slag existing in the front side of the oxide surface, or by dissolution rate of oxides in the slag [1].

Fig. 3
figure3

a The relationship between the depth of corrosion and CaO content in the CaO–SiO2–Al2O3-based molten oxide. b Content of CaO [1]

Liu et al. [18] studied corrosion resistance by adding ZrO2 to a refractory Al2O3-C. Pointing that, adding ZrO2 ion gives anticorrosive characteristics to Al2O3-C base, especially in FeO castings with concentrations higher than 6%, where carbon graphite Al2O3-C content is converted in reducing agent of iron oxide, which is present in the casting. Figure 4 shows the relationship between FeO iron concentration and degree of corrosion [18].

Fig. 4
figure4

FeO content in melts and the corrosion rate of the refractories, test temperature 1773°K. [18]

M Serhat and Kara [3] reported that the addition of zirconia ZrSiO4 to the mullite sintered matrix 3Al2O3 2SiO2 improves mechanical and corrosive properties upon alkaline vapors (Na2O). Even considering that alkaline vapor corrosion is very severe for alumina-silicate refractories, since new phases produce stress and cracking. This is because of densification in mullite by adding zirconia to the mixture. Densification in the mixture reduces infiltration of alkaline vapors with reduction in porosity in the refractory material. Thus, when zirconia is added to the alumina-silica SiO2 mixture (Fig. 5), it is possible to reduce apparent porosity to 8.9% increasing its corrosion resistance to alkaline vapors [3]. Porosity depends on the distribution of grain size for the sintered mixture. A structural ceramic material should have a porosity < 2%, whereas for a refractory it should be above 20% and between 50 and 70% for refractory insulation [22].

Fig. 5
figure5

Outer surface of the test cups after alkali attack test [3]

An EDX analysis in the densified area shows that the chemical composition at the reaction zone is different from that of the green material, where atoms of sodium, aluminum, silicon and oxygen were found in the reaction areas, and sodium was not found in the affected part [3].

LidijaĆurković et al. [4] determined the correlation between the number of ions eluted from Al2O3 ceramics and time of immersion at different concentrations of H2SO4. This ratio shows the number of ions dissolved after corrosion process containing ions of de Al3+, Mg2+, Ca2+, Na+, Si4+ and Fe3+. This shows that impurities play an important role in corrosion process of ceramic alumina, since corrosion degree in Al3+, Mg2+, Ca2+, Na+, Si4+ and Fe3+ increases with corrosion time. Kinetic corrosion of high-purity Al2O3 ceramics at different concentrations (2, 10 and 20%) of H2SO4 at room temperature showed high ion separation within the first 24 h, and for prolonged periods above 250 h, the effect remains stable Figure 6 [4]. Chang Woo Lee, Kang, Chang and Hahm [23] performed similar work for a porous alumina membrane subjected to HCl, HNO3, H2SO4 acid solutions and NaOH, Ca(OH)2 alkaline solutions independently highlighting the influence of temperature raise (20, 30 and 40 °C), concentration (0.001, 0.01, 0.1 N) and exposure time (30–180 min) in these solutions [23].

Fig. 6
figure6

Total amount of eluted ions (Al3+, Mg2+, Ca2+, Na+, Si4+ and Fe3+)from Al2O3 ceramics in different concentrations of H2SO4 solution [4]

In 1981, Irwin and Amónof Westinghouse Electric Corporation [19] identified structural materials from an acid vaporizer to assess corrosion compatibility with the process stream. They subjected ceramic materials to sulfuric acid (H2SO4), under temperatures of 361–452 °C, and increments of 250–1000 h. It was found that silicon and materials containing significant amounts of silicon, such as silicon carbide and silicon nitride, had the highest resistance to boiling sulfuric acid. In another corrosion study carried out by Fernanda Coen-Porisini [19], ceramic samples were subjected to 800 °C in two different vapors containing certain decomposition products of sulfuric acid. Both tests did not show significant changes for Al2O3 alumina, 3Al2O3 2SiO2 mullite, and ZrSiO4 zirconia, only small deposits on the surface. Tiegs [19] identified silicon carbide as the best ceramic exposed to temperatures between 1000 and 1225 °C in sulfuric acid, simulating a decomposition environment. In 2004, Shintaro Ishiyama [19] applied for a patent of a high-temperature corrosion resistant ceramic; his results show a percentage change in weight and corrosion rate for the samples after 100 h of exposure to boiling sulfuric acid under high pressure. Based on these studies, we can say that SiC silicon carbide, Si3N4 silicon nitride and Al2O3 alumina are the most suitable to withstand corrosion testing [19].

Conclusions

The corrosion resistance of high-purity alumina ceramics immersed in acid solutions (H2SO4, HCl, HNO3) and alkaline solutions (NaOH, KOH, Na2O, Ca(OH)2) is determined by the degree of solubility of impurities contained within the grain limits (MgO, CaO, SiO, FeO and Na2O). Due to the corrosion mechanism produced is a type of intergranular corrosion with preferential attack at the grain boundaries, where most of its impurities are found.

Also concentration of acid or alkaline solutions and the exposure temperature of either acid ceramic (SiO2, Al2O3) or alkaline ceramics (MgO, CaO, Cr2O3) are important factors to determine the corrosion resistance.

The addition of zirconium dioxide (ZrO2), zirconium silicate (ZrSiO4), titanium oxide (TiO2), chromium oxide (Cr2O3), or even micro-silica (SiO2), to alumina ceramics, improves corrosion resistance, even more than by only exposing high-purity alumina ceramics to acidic or alkaline solutions. Due to this, sintering has structure such as Al2O3–MgO refractory cement with higher densification and reduction in grain size than spinel phase alumina (Al2O3). The high produced densification causes less infiltration of corrosive medium due to low porosity. Depth corrosion is even reduced by adding amounts of Cr2O3–Al2O3 ceramics, because of delayed speed with which the slag diffuses through the material.

The phases that offer greater resistance to acid or alkaline solutions are achieved by sintering the alumina base ceramic material trying to obtain the higher content of corundum α-Al2O3 or mullite 3Al2O32SiO2 phases.

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Acknowledgements

Authors are greatly grateful to the Universidad Autónoma de Baja California and CINVESTAV-Saltillo for facilitating access and use of their facilities and equipment to carry out this research.

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Valenzuela-Gutiérrez, A., López-Cuevas, J., González-Ángeles, A. et al. Addition of ceramics materials to improve the corrosion resistance of alumina refractories. SN Appl. Sci. 1, 784 (2019). https://doi.org/10.1007/s42452-019-0789-5

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Keywords

  • Refractory ceramics
  • Acid and alkaline resistance
  • Mullite
  • Corundum