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Murphy et al. [5.47] reported that reaction with water liberated oxygen, formed Y2BaCuO5 and CuO, in addition to barium hydroxide, and was a function of temperature and surface area. This is similar to the leaching of barium from perovskites in aqueous solutions when the pH is less than 11.5 reported by Myhra et al. [5.48].

Titanates and Titania

A crystalline titanate mineral assemblage called SYNROC* has been under investigation for many years as a possible encapsulant for high-level radioactive wastes. The titanates are commonly a mixture of perovskites, CaTiO3 and BaTiO3, zirconolite, CaZrTi2O7, and hollandite, BaAl2Ti6O16. In a study of the dissolution of these titanates in CO2 enriched (4 ppm) deionized water (pH=5–6) at 300 and 350°C and 500 bars, Myhra et al. [5.48] reported the following reactions:

(5.11)

(5.12)

(5.13)

(5.14)

(5.15)

(5.16)

*	SYNROCK was developed in 1978 at the Australian National University

by Prof. T.Ringwood. Several compositional variants have been developed, called SYNROCK-C, -D, or -F. Additional information can be obtained from the website www.uic.com.au) of the Uranium Information Centre, Ltd. in Melbourne, Australia.

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The dissolution mechanism proposed for these titanates was one involving initial selective leaching of the alkaline earth ions along with hydration of the titanate surface. This first step was rather rapid, but then overall dissolution slowed as the solution became saturated. When the solubility product was exceeded, precipitation and equilibration with CO2 occurred. As the precipitate concentration increased, the dissolution rate decreased. Thus the overall dissolution of these titanates was dependent upon the solubility of the alteration products in the solution. In contrast, Kastrissios et al. [5.49] proposed that the calcium was not selectively leached from the perovskite but instead, the perovskite dissolved congruently forming an amorphous titanium-rich surface layer from which TiO2 precipitated. This titania layer was not continuous and therefore did not protect the underlying material from continued corrosion.

Buykx et al. [5.50] gave a diagram of relative phase stability for various titanium-containing compounds, among others, for dissolution in water at 150°C for 3 days. No alteration was found for zirconolite-zirkelite (CaZrTi2O7). Some alteration and precipitation of TiO2 was found for hollandite (BaAl2Ti6O16), loveringite-landauite (FeTi3O7), pseudobrookite (Fe2TiO5), and rutile (TiO2). Extensive replacement by TiO2 was found for perovskite (CaTiO3) and freudenbergite (Na2Ti6Fe2O14(OH)4). Complete and rapid dissolution was found for any glassy phases. The stoichiometries given above are only approximate, the complete analyzed stoichiometries for the compounds investigated are given in the original paper.

Titania was investigated by Bright and Readey [5.51] as the least complex titanate to evaluate the quantitative dependence of kinetics upon ambient conditions. Powdered anatase (~0.54 µm agglomerate size and ~0.13 µm crystallite size) was added to acid solutions of HF-HCl and stirred for several hours at temperatures ranging from 37.5 to 95.0°C. Although very little is known about the titanium species in HF-HC1 solutions, it was believed that the most predominant complex was (TiF6)2-.

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The rate-controlling step in the kinetics of dissolution was concluded to be the removal of the highly charged cations from kink sites on the surface. The average calculated initial (for the first hour) dissolution rate was 59.0 wt.% TiO2 dissolved per hour.

Slightly reduced titania has been investigated for its use in electricity generation and for water decomposition [5.52]. In these applications, n-type semiconducting titania is used as a photoanode in an aqueous solution of 0.5 M H2SO4. The photogenerated positive holes in the valence band of illuminated n-type titania reacts with the solution according to the following equation:

(5.17)

The (SO4)- that forms is an active species that reacts with titania forming etch pits. This phenomenon is called photoelectrochemical aging.

Transition Metal Oxides

The use of transition metal oxides (RuO2, NiO, MoO2, Mo4O11, Mo8O23, Mo9O26, and WO2) as fuel cell electrocatalysts requires that they be stable in aqueous solution of 1 N H2SO4. These oxides are relatively stable in acid solutions but undergo redox reactions in the region of pH=7. Horkans and Shafer [5.53] reported that Mo4O11 exhibited anodic dissolution but that WO2 did not; however, it did form a layer of WO3 on its surface. They reported that MoO3 was more soluble than WO3 in acid solutions, whereas MoO2 was more stable.

Horkans and Shafer [5.54] reported that the oxidized surface layers that formed were generally less conductive than the bulk reduced phase, that they were generally of a wide range of compositions, and that the actual composition of the reaction surface layer was highly dependent upon the electrode potential. They also found that MO2 (M=Mo, Ru, W, Re, Os, and Ir) was

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substantially more stable in acid solutions than was indicated by their Pourbaix diagrams.

In a study of the corrosion of nuclear fuels, Clayton [5.55] investigated the effects of 633 K flowing water upon various thoriaand urania-containing materials. He found the following order of corrosion resistance: ThO2>ThO2-UO2>ZrO2- UO2>ZrO2-CaO-UO2>UO2. Corrosion in highly oxidative conditions (caused by the fission fragment radiolysis of the water) was attributed to the oxidation of uranium from the four-valent to the six-valent state. UO2 corroded easily even in low-oxygen (5 ppm) water. The test conditions of pH, sample preparation, and compact attributes had no effect on corrosion resistance.

Carbides and Nitrides

The transition metal carbides and nitrides are chemically stable at room temperature, but exhibit some attack by concentrated acid solutions. The one exception to this is VC, which slowly oxidizes at room temperature.

Bowen et al. [5.56] reported the formation of Al(OH)3 (bayerite) on AlN powder after 16 hr in contact with deionized water at 25°C. In the first 8 hr, growth of an amorphous hydrated layer occurred with a chemistry very close to AlOOH, while the pH of the solution drifted from 7 to 10 after 5.5 hr. The kinetics indicated a linear rate controlled by the surface reaction. Anyone involved in the aqueous processing of materials such as AlN should be aware of the potential reactions that can take place with the incorporation of oxygen into their product through the formation of hydrated surface layers.

The behavior of sintered SiC in 0.045 M Na2SO4+0.005 M H2SO4 and 0.1 M LiOH aqueous solutions at 290°C was studied by Hirayama et al. [5.57]. They examined weight losses for up to 200 hr in both oxygenated and deoxygenated solutions. Weight losses increased with increasing pH and were greater for oxygenated solutions. No surface silica layers were found,

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with dissolution progressing through SiC hydrolysis. The proposed reaction follows:

(5.18)

and

(5.19)

where the Si(OH)4 sol that forms immediately dissolves. The dissolution of the Si(OH)4 in acidic solutions (pH=4) was slower than that in alkaline solutions, and provided a small degree of protection, leading to a rate law that was approximately parabolic. In alkaline solutions (pH=10), the rate law was linear.

The corrosive effect of HCl aqueous solutions at 70°C upon Si3N4 has been shown by Sato et al. [5.58] to be dependent upon the sintering aid used, or more specifically, the grain boundary phase present in hot isostatically pressed materials. In solutions of <1 M HCl, the corrosion was surface reaction controlled, whereas in solutions of >5 M HCl, the corrosion was controlled by diffusion through the interfacial reaction layer that formed (assumed to be silica). Corrosion occurred through dissolution of the Al and Y ions (Y ion dissolution was about twice that of Al) contained in the grain boundary phase, with dissolution decreasing as the degree of crystallinity increased for this phase. Negligible dissolution of silicon ions was reported. In contrast to the above dissolution, the corrosive effect of 0.1– 10 M aqueous HF solutions between 50 and 80°C for HIP or hot-pressed Si3N4 containing Y2O3, Al2O3, and AlN additives has been shown by Sato et al. [5.59] to involve the selective dissolution of Si and Al ions but not Y ions. The Y ions instead formed insoluble YF3.

Seshadri and Srinivasan [5.60] investigated the corrosion of a titanium diboride particulate reinforced silicon carbide at room temperature in several aqueous solutions (aqua regia, NaOH, and HF/HNO3) for up to 500 hr. Aqua regia was the most corrosive and a 50% NaOH solution was the least.

Copyright © 2004 by Marcel Dekker, Inc.