Orlova et al. [5.16] reported that divalent cation additions to alkaline silicate melts decreased the dissolution temperature of mullite while increasing the intensity of dissolution in the order CdO>ZnO>BaO. Additional crystalline phases form depending on the divalent cation added: corundum (CdO), spinel (ZnO), or celsian (BaO).

There are times when the microstructure of the resultant corrosion product can offer information useful in determining the cause of the deterioration. Fig. 5.4 shows a sample taken from surface runnage on a corundum refractory that was attacked by silica in a glass furnace. The dendritic and fibrous nature of the mullite formed were indicative of crystallization from a mullite melt containing a slight excess of silica and a variable cooling rate. The sample shown had apparently been at a temperature near or slightly in excess of 1850°C (the melting point of mullite). This temperature was approximately 300°C above the normal operating temperature for this furnace. The mullite identification for the dendrites was confirmed by SEM/EDS.

Zircon

The attack of zircon by soda-lime-silicate glasses is similar to that of AZS materials in that an interface of zirconia crystals embedded in a highly viscous glass is formed. The difference is the lack of alumina, which keeps nepheline from forming and the viscous glass is now a silicious glass as opposed to an alumina-rich glass. Thomas and Brock [5.17] reported that as the sodium content of the attacking glass decreased, the thickness of the zirconia layer decreased. The attack of zircon by E-glass (a borosilicate) that contains only about 0.5% Na2O exhibited no observable alteration. Zircon has been successfully used in contact with high-temperature lithium-alumino-silicate glasses. A protective layer of zirconia crystals suspended in a very viscous glass is formed by the leaching of silica from the zircon. Because these glasses are melted at temperatures above 1700°C, it is quite possible that the zircon dissociates into zirconia and silica, with the silica then going into solution rather than the

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FIGURE 5.4 Dendritic and fibrous mullite formation caused by silica attack of a corundum refractory. Transmitted light illumination.

silica being leached from the zircon. As long as thermal cycling does not occur, this protective layer remains intact.

Zirconia

Because of the polymorphic transitions associated with zirconia, it has not been widely used as material for furnace linings except when combined with other materials, such as the AZS refractories described above, or when stabilized in the cubic phase. Stabilization of the cubic phase by incorporation of CaO, MgO,

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or Y2O3 has been successful. These oxides, however, are susceptible to leaching when in contact with liquids. Chung and Schlesinger [5.18] studied the effects of molten calcia-ferro- silicate slags upon the stability of zirconia stabilized by magnesia and calcia. Three slags were chosen: a basic, an acid, and a high iron slag. They found that low basicity slag attacked magnesia-stabilized zirconia by leaching the magnesia. This subsequently allowed the more soluble monoclinic phase to form. Higher iron content slags had lower viscosities and thus were more corrosive. High basicity slag was found to attack magnesia stabilized zirconia by direct dissolution of the cubic phase. Calcia stabilized zirconia samples were completely destroyed by these slags even after short time periods.

Carbides and Nitrides

Silicon carbide and nitride are relatively inert to most silicate liquids as long as they do not contain significant amounts of iron oxide. The reaction:

(5.1)

can occur and becomes destructive at temperatures above about 1100°C [5.19].

The dissolution of Si3N4 by glass is important not only in evaluating attack by various environments, but also for gaining an understanding of the operative mechanism in liquid phase sintering and solution/precipitation creep phenomena occurring in materials that contain a glassy bonding phase. Tsai and Raj [5.20] studied the dissolution of β-silicon nitride in a Mg–Si– O–N glass, which they reported separated into SiO2-rich, and MgOand N-rich regions. They concluded that the dissolution of β-silicon nitride into the glass at 1573–1723 K occurred in three steps, with precipitation of Si2N2O:

1.β-Si3N4 dissolved into the melt as Si and N.

2.This Si and N diffused through the melt toward Si2N2O.

3.The Si and N then attached to the growing Si2N2O.

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Ferber et al. [5.21] reported that the corrosion of a-SiC at 1175–1250°C when coated with a static layer of a basic coal slag involved at least three reaction mechanisms. These were:

1.Oxidation of SiC with the formation of silica between the slag and SiC

2.Dissolution of the silica by the slag

3.Localized formation of Fe-Ni silicides at the SiC surface due to reaction of the SiC with the slag

Which of these predominated was dependent upon the thickness of the slag layer, which in turn, determined the local partial pressure of oxygen available by diffusion through the layer. When the slag thickness was <100 µm, passive oxidation occurred with the formation of SiO2. As the slag thickness increased, the available oxygen at the surface was insufficient for SiO2 formation causing SiO to form instead. This active oxidation, forming the gaseous phases of SiO and CO, disrupted the silica layer allowing the slag to come in contact with the SiC thus forming iron and nickel silicides.

McKee and Chatterji [5.22] have reported a similar effect upon the corrosion mechanism of SiC, where a molten salt layer provided a barrier to oxygen diffusion promoting the formation of SiO gas.

Based upon the work of Deal and Grove [5.23], Ferber et al. [5.21] gave the following equation for calculating the oxygen partial pressure at the SiC/slag interface:

(5.2)

where:

R=gas constant T=temperature

C*=equilibrium concentration of oxygen in slag X1=slag layer thickness

A=constant determined from kinetics

For oxidation in air only, estimating A as 0.31 µm at 1250°C and taking C* as 0.086 mol/m3 (oxygen concentration in silica),

Copyright © 2004 by Marcel Dekker, Inc.