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various interfacial reaction products that may form as a result of attack by gases for several selected materials have been listed in Table 5.2. The interested reader should examine the original articles to determine the experimental conditions under which the various reaction products formed, and also to determine the exact nature of the ceramic tested. In the sections below, several selected materials are described in more detail.

Although oxidation is generally the most prevalent form of attack by gases, hydrogen reactions are becoming more important as a result of the development of miniature fuel cells for applications such as cell phones. Historically, hydrogen fuel cells were the realm of the space industry. According to Nelson [5.87], the corrosion of a material by hydrogen is dependent on the following parameters:

(a)ease of transport of H into the material

(b)type of reactions available between H and material

(c)design of the structure

Corrosion of ceramics by hydrogen is generally not a problem below about 1100°C, where the molecular form exists. Thermal dissociation begins to become significant above 1100°C, and transport rates to material surfaces increases rapidly.

5.2.1 Oxides

Alumina

In the ceramics community, alumina is considered one of the most inert materials toward a large number of environments. For this reason, alumina that is produced as a 95–100% Al2O3 material is used in many furnace applications. The one area where its reactivity is often overlooked is its application in laboratory furnaces. Most high-temperature laboratory furnaces use alumina as the standard lining. When the materials that are under investigation react to form gaseous species, and especially when the furnace atmosphere contains a low partial pressure of oxygen, one should be aware of the possible reactions that may occur with alumina.

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The lining of a laboratory furnace can receive a much more severe usage than an industrial furnace. Generally, this is a result of repeated thermal cycling and to the investigation of a wide variety of materials that produce a wide range of corrosive environments. An example of the corrosion of sample crucible setter tiles is given below.

The test environment of a vertical tube furnace that used an alumina tube and horizontal alumina disks” on which to place alumina sample crucibles included an input atmosphere of hydrogen and methane that gave an oxygen partial pressure of 10-14 Pa at the test temperature of 1300°C. Samples being tested were various silicon nitride samples containing small quantities of MgO, Y2O3, Fe2O3, ZrO2, Al2O3, and CaO. A discussion of the furnace set-up and operation can be found in Ref. [5.88].

The alumina disks in the upper portion of the furnace tube above the silicon nitride samples, along a temperature gradient that ranged from 1250 to 1185°C, exhibited a glazed surface layer of silicate glass containing crystals of cordierite and cristobalite (see Fig. 5.7). The formation of cordierite was caused by the active oxidation of the silicon nitride and the vaporization of magnesia contained within the nitride samples that subsequently reacted with the alumina to form cordierite.

The following equations describe the reaction:

(5.30)

(5.31)

(5.32)

(5.33)

Although Anderson [5.89] reported that oxygen was necessary for the formation of potassium β-alumina from sapphire used in vapor arc lamps, van Hoek et al. [5.90] showed that potassium

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FIGURE 5.7 Cordierite and cristobalite formation on alumina. Reflected light, differential interference contrast micrograph (magnification 500×).

vapors (at 1 MPa) were able to reduce alumina at 1373 K in the absence of oxygen by the following reaction:

(5.34)

forming a potassium β-alumina and metallic aluminum. Although approximately 3 wt.% aluminum should form, they detected no metallic aluminum by XRD in their samples. They suggested that the presence of metallic aluminum was confirmed by the blackening of samples due to the formation of finely divided metallic aluminum. They found that the β-alumina formed with the crystallographic c-axis parallel to the substrate surface. This created an easy diffusion path perpendicular to the surface for diffusion of potassium and continued corrosion. They also suggested that this oriented growth was not the result of epitaxial growth, because the starting alumina was a

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polycrystalline material. It is unfortunate that many references can be found in the literature that refer to oriented growth on polycrystalline substrates as epitaxy. Although localized crystallographic matching may occur on a grain-to-grain basis, forming an oriented polycrystalline layer on an oriented polycrystalline substrate, this was not the original meaning of epitaxy. It is enlightening that van Hoek et al. suggested another reason for oriented growth—the faster growth direction (easy diffusion path) being perpendicular to the reaction surface.

In a simulated industrial furnace atmosphere cycling between 8–10% combustibles (reducing) and 6–10% excess oxygen (oxidizing) at temperatures of 1260 and 1400°C, Mayberry et al. [5.91] showed that refractories containing chrome ore developed a permanent expansion and a loss in strength. This was the result of spinel solution into periclase, and then exsolution with the accompanying phase redistribution, recrystallization, and pore development during cycling. This example shows how a material may experience degradation as a result of atmosphere effects although it does not exhibit any signs of classic corrosion (weight gain or loss, reaction product surface layer formation, etc.).

In a study of UF6 -fueled gas-core reactor systems, Wang et al. [5.92] investigated the effects of UF6 gas (at pressures of 20.0–22.7 kPa) upon alumina at temperatures of 973, 1073, 1273, and 1473 K for times up to 4 hr. At the three lower temperatures, the following reaction was suggested to represent their findings:

(5.35)

The AlF3 formed on the alumina surface, whereas the UF4 was found on the colder portions of the furnace chamber. At 1473 K, several oxides of uranium were found in the surface scale, and no AlF3 was found because of its high vapor pressure (30.6 kPa) at that temperature. Weight gain was reported for the lower temperatures; however, at 1473 K, a large weight loss was exhibited as a result of vaporization of the AlF3.

Copyright © 2004 by Marcel Dekker, Inc.