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was the faster mechanism. Sato et al. [5.75] reported that this inner protective layer of silica formed on pressureless sintered Si3N4 containing 5 wt.% Y2O3 and Al2O3 in contact with molten potassium sulfate at 1200°C, when the tests were conducted in air but not when conducted in nitrogen. This same situation was not true for attack by molten potassium carbonate at 1013°C. In this case, attack occurred in both nitrogen and air, with air causing a greater degree of reaction. Compared to studies performed in molten sodium and lithium sulfate and carbonates, Sato et al. found that the corrosion rate, in a nitrogen atmosphere, was independent of the alkali present, with the sulfates yielding an activation energy of 430 kJ/mol compared to that of the carbonates of 106 kJ/mol.

Superconductors

In an investigation of the molten-salt synthesis of YBa2Cu3O7-x (123), Raeder and Knorr [5.78] reported the stability of 123 against decomposition toward several molten salts at 1173 K. They concluded that 123 was not stable in molten LiCl or the dichlorides of Cu, Ca, Mg, or Ba, or their mixtures. However, minimum decomposition was found in the NaCl-KCl system.

The mechanism of decomposition was postulated as being one of selective dissolution of the barium in the 123 forming BaCl2 and causing the 123 to decompose into several oxide phases that were consistent with the phase diagram reported by Lee and Lee [5.79]. These oxide phases were generally CuO and Y2Cu2O5, or CuO and Y2BaCuO5 depending upon the amount of barium in the initial mixture.

5.1.4 Attack by Molten Metals

The application of ceramics to withstand the attack by molten metals is a very large part of the ceramic industry. Refractories are used to line furnaces for the manufacture of steel and the nonferrous metals of which aluminum and copper are probably the most important. The steel and nonferrous metals industries

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consume approximately 60% of all refractories manufactured today. Thus an understanding of the potential problems one may encounter from metal attack is quite important.

The attack by molten metals generally involves mechanisms of corrosion other than those by liquids in combination with liquid attack. The actual process that occurs in a blast furnace, for example, is truly a combination of corrosion mechanisms. In many cases, small amounts of metal become oxidized and the corrosion is through essentially a molten slag process. An example of this is shown in Fig. 5.6, which illustrates the corrosion of a 60% MgO magnesite-chrome refractory from an electric furnace that was in contact with a high iron oxide content slag. Diffusion of the iron oxide into the refractory and reaction with the magnesia and chrome-containing spinel formed an interface of large iron-rich mixed spinel crystals. Diffusion of iron into the magnesia caused precipitation of smaller iron-

FIGURE 5.6 Corrosion interface between iron oxide-rich slag and a 60% MgO magnesite-chromite refractory (magnification 150×). Brightest regions are an iron-rich mixed spinel. (Courtesy of HarbisonWalker.)

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rich mixed spinel crystals within the magnesia and at the grain boundaries.

The reaction of silica-containing refractories with molten iron containing dissolved manganese has been known to be very deleterious. This reaction, however, is not only a reaction with a molten metal but also with an oxide of manganese (i.e., MnO). The initial reaction between SiO2 and Mn forms MnO and Si metal. Although this reaction is thermodynamically unlikely, it has been reported by Kim et al. [5.80] to occur at 1600°C under an argon atmosphere. The subsequent reaction of MnO and silica can form one of two intermediate compounds, but more importantly, can form a eutectic liquid with a solidus temperature of 1250°C.

Many steel plant refractories are carbon-containing from manufacturing processes involving pitch or tar, organic resins, or graphite. Solid-solid corrosion through reduction by carbon takes place, or if the carbon becomes oxidized corrosion by molten metals is then very seldom a simple reaction involving only solid ceramic and molten metal.

The attack of molten aluminum upon materials containing silica follows the reaction:

(5.26)

which should be expected from examination of the free energy vs. temperature data of an Ellingham diagram. The alumina that forms in many cases provides an adherent protective layer against further corrosion [5.81]. This reaction is accompanied by a volume decrease of about 26% according to Siljan et al. [5.82]. Although many previous investigators have attributed the spalling of refractory linings to the volume expansion of this reaction, Siljan et al. have attributed any expansive spalling that may occur to the growth of corundum along thermal gradients and/or any contraction due to the volume decrease.

The action of molten aluminum upon any β-alumina contained in materials such as high alumina (70%) refractories produces metallic sodium [5.83]. The metallic sodium present can then lead to reduction of silica, and if oxidized, it can lead to the

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formation of NaAlO2. The formation of NaAlO2 is enhanced in the presence of a reducing atmosphere containing nitrogen by the intermediate formation of aluminum nitride according to the following reactions:

(5.27)

(5.28)

The difference in densities between alumina and sodium aluminate (3.96 vs. 2.69 g/cm3) implies that a considerable volume expansion can take place during conversion of the original protective layer to a nonprotective aluminate, thus leading to continued corrosion.

According to Lindsay et al. [5.84], mullite, when attacked by molten aluminum, converted to silicon metal and alumina; when attacked by molten magnesium-containing aluminum alloys, mullite converted to spinel and magnesia. Brondyke [5.81] has shown that molten aluminum will reduce silica in all forms contained in alumino-silicate refractories.

According to Cornie et al. [5.85], the attack of SiC by molten aluminum at temperatures greater than 700°C can be represented by the following equation:

(5.29)

Nickel-base eutectic alloys such as NiTaC provide very severe temperature (as high as 1800°C) requirements upon their containers. Huseby and Klug [5.66] studied the reactions of many oxides in contact with NiTaC-13 at 1700 and 1800°C, and found that only Al2O3, Y3Al5O12, and LaAlO3 formed no interfacial reaction layers.

In a study to evaluate sialon* crucible materials as candidates for containing molten silicon, Wills et al. [5.86] found that the

* Sialon is an acronym used to represent solid solutions of silicon, aluminum, oxygen, and nitrogen.

Copyright © 2004 by Marcel Dekker, Inc.