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Preferential leaching of the TiB2 from the surface was reported to be the cause of decreasing weight loss with time. After about 100 hr, weight loss was stopped for the aqua regia and HF/ HNO3 solutions, whereas it took approximately 250 hr in the 50% NaOH solution.

5.1.3 Attack by Molten Salts

Oxides

The importance of molten salt reactions is well known in alumina reduction cells for the production of aluminum metal (Hall-Heroult process). In this process, the electrolyte consists of a solution of alumina (<10 wt.%) dissolved into molten cryolite (Na3AlF6) [5.61]. Pure molten cryolite contains , , F-, and Na+ ions. When alumina is added, the complex ion (x=3–5) forms in addition to the others. In a study of the cryolite-mullite and cryolite/sodium fluoride-mullite systems, Siljan and Seltveit [5.62] reported that materials with high Si/Al ratios experienced high weight losses when in contact with NaF-cryolite eutectic melts because of the formation of gaseous SiF4. They reported that mullite readily dissolved into cryolite and cryolite-NaF melts, and that NaF reacted with alumina to form β-alumina. Allaire [5.63] studied the resistance of 15 different commercial refractories with varying Al2O3/SiO2 ratios in a simulated alumina reduction cell and found that the resistance to attack by molten metallic sodium and NaF increased with alumina content. He reported that above 700°C, an Al2O3/

SiO2 ratio greater than 0.90 should be used.

The corrosion of fused silica by molten sodium sulfate in atmospheres containing either 1% SO2/O2 or pure oxygen at 700 and 1000°C has been described by Lawson et al. [5.64] to take place according to the ease of sodium diffusion in the various phases that form. Sodium diffuses into the fused silica, leading to the nucleation of cristobalite. Once a continuous layer of cristobalite formed, sodium diffusion was minimized. The sodium at the cristobalite/fused silica interface then diffused

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further into the fused silica. The basicity of the reaction determined whether or not a cristobalite layer formed, with less cristobalite forming as the reaction became more acidic. Cristobalite globules, however, were reported to precipitate from the salt solution. Low partial pressures of SO3 were reported to promote the fluxing action of the molten sulfate by increasing the activity of Na2O.*

In the evaluation of cathode materials for molten carbonate fuel cells, Baumgartner [5.65] reported solubility data for NiO, CuO, ZnO, LiFeO2, and LaNiO3 in a molten binary carbonate of Li/K (62/38 molar ratio) between 823 and 1223 K. Both NiO and CuO exhibited dissolution (CuO being more soluble than NiO) into the molten carbonate and diffusion toward the anode until the local partial pressure of oxygen was sufficiently low for metal precipitation. At temperatures exceeding 1123 K, LaNiO3 decomposed to La2NiO4 and NiO, which dissolved and reduced to metallic Ni. A similar situation was found for LaCoO3, which decomposed to La2CoO4 and CoO at temperatures exceeding 1073 K. Dissolution of LiFeO2 into the molten carbonate resulted in reduction at the anode to LiFe5O8, while ZnO at the anode became nonstoichiometric. The solubilities of these oxides were in the order

LaNiO3<NiO<LiFeO2<CuO<ZnO below 1023 K. Above this temperature, the relative solubilities of CuO and ZnO were reversed.

An example of when corrosion becomes beneficial is the removal of ceramic cores in the investment casting process. The new process of directional solidification and the new alloys involved (NiTaC) require contact between the molten metal and the core material for times up to 20 hr at temperatures as high as 1800°C [5.66]. The requirement that the core material must withstand these conditions and then be chemically removed is a contradiction in stability. Core removal requires high dissolution rates at low temperatures. Potential core materials

* Compare with the discussion in the next section involving equations 5.20 through 5.24 and the relationship of soda activity with SO3 levels.

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are Al2O3, Y2O3, Y3Al5O12, LaAlO3, and MgAl2O4, which all possess satisfactory resistance to the casting conditions as

reported by Huseby and Klug [5.66]. These materials, except for Y2O3, are insoluble in aqueous acids or bases. The solvents used must be aggressive toward the core material but not toward the alloy. Borom et al. [5.67] reported the weakly basic or amphoteric oxides of Al2O3, Y2O3, and La2O3 can be dissolved by molten M3AlF6, M3AlF6+MF, M3AlF6+M’F2, or M3AlF6+MCl, where M=Li, Na, or K, and M’=Mg, Ca, Ba, or Sr. The more acidic core materials, such as ZrO2 or ThO2, required alkali or alkaline earth oxide additions to make the molten salt more basic.

Another field of study where the solution in molten salts is beneficial is that of crystal growth. The solubility of Be2SiO4 and ZrSiO4 in various solvents was studied by Ballman and Laudise [5.68]. Solvents studied included alkali vanadates and molybdates. Because of solvent volatility problems (more important for molybdates than vanadates), most of their data contained substantial error at higher temperatures. The reported ion solubilities were greater for Be2SiO4 ranging from 3 to 5 1/2 mol% in the 900–1000°C range than for ZrSiO4, which ranged around 1 mol% in the 900–1400°C region.

Except for the solution by Na2O3V2O5, which reached as much as 6 mol% at 1400°C, the vanadates were more corrosive than the corresponding molybdates of those studied. The greater solubility of Be2SiO4 over that of ZrSiO4 can also be predicted from acid/base theory, because BeO is a stronger base than ZrO2.

Carbides and Nitrides

The normally protective layer of SiO2 that forms on SiC and Si3N4 can exhibit accelerated corrosion when various molten salts are present. McKee and Chatterji [5.22] described several different modes of behavior of SiC in contact with gas-salt mixed environments relating to the formation of various interfacial reaction layers. Salt mixtures containing Na2SO4 and Na2CO3,

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Na2O, NaNO3, Na2S, or graphite were tested. McKee and Chatterji found that a SiO2 protective layer corroded in a basic salt solution, but not in an acid salt solution. With low oxygen pressures, active corrosion took place by formation of SiO gas.

The activity of Na2O has been shown to be an important parameter in the action of molten sodium salts by Jacobson and coworkers [5.69–5.74]. The higher this activity, the greater the potential reaction with silica. The relationship of soda activity and SO3 partial pressure can be obtained from the following equation:

(5.20)

where the equilibrium constant k (which can be written in terms of concentrations, activities, or partial pressures) is given by:

(5.21)

Therefore, the highest Na2O activity is related to the lowest partial pressure of SO3. Jacobson [5.71] reported that at partial pressures of SO3 greater than 0.1 Pa, no reaction occurred between SiC and Na2SO4 at 1000°C for at least up to 20 hr. It is assumed, as always, that Na2O and Na2SO4 are chemically pure stoichiometric compounds and that SO3 acts as an ideal gas. Experimentally, the Na2O activity can be set by the appropriate partial pressure of SO3.

The decomposition of sodium sulfate by reaction (5.20) is not something that takes place readily. Sodium sulfate melts at 884°C, and is relatively nonreactive toward silica, even at temperatures as high as 1400°C. To increase the reactivity, the sulfate must be reduced to some lower oxide. This has been known for many years by the manufacturers of soda-lime-silica glass. Sodium sulfate has been used not only as a source of sodium but also as a fining agent to remove the bubbles from the glass melt during processing. If the sulfate is not reduced, it either floats on the surface or forms lenticlular immiscible inclusions in the finished product. References in the old glass

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literature refer to blocking the furnace, a term used to describe the process of adding wooden blocks (i.e., carbon) to pools of nonreactive sodium sulfate floating on the surface of the molten glass. The carbon from the burning wood reduced the sulfate to a form reactive or at least miscible with the molten glass. This reaction, shown below:

(5.22)

is controlled more scientifically in modern glass manufacture through the use of coal as a batch ingredient and precise control of the combustion system to control the partial pressure of oxygen above the melt, which in turn controls the SO3 equilibrium through:

(5.23)

and subsequently the soda (or some sodium sulfur containing compound) activity. The reaction of Na2SO3 with silica according to:

(5.24)

is the one of importance in the dissolution of silica in the manufacture of glass and is most likely the one of major importance in the corrosion of the silica layer formed on SiC or Si3N4. Continued reduction* of the Na2SO3 to Na2S, although still reactive with silica is not necessary for excessive dissolution of carbides and nitrides.

Jacobson and Smialek [5.69] found that the partial decomposition of Na2SO4 enhanced the oxidation of SiC, forming a layer of tridymite, a sodium silicate glass, and some Na2SO4. Any free carbon in the SiC enhanced the corrosion, because it aided in the reduction of the sulfate. This enhanced corrosion

* The more reduced forms of the sodium-sulfur compounds are the basis of the amber color formed in the manufacture of brown bottles.

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resulted from the ease of diffusion of oxygen through the predominantly sodium silicate amorphous layer compared to that of a crystalline silica layer. A somewhat different mechanism has been proposed for the corrosion of SiC by potassium sulfate, although details of the behavior have not yet been reported [5.75]. In the case of the potassium salt, no silica layer formed on the carbide, because it was immediately dissolved by the sulfate according to:

(5.25)

presumably due to dissolution being faster than oxidation. Cree and Amateau [5.76], in their studies of the behavior of

SiC in molten lithium salts, found that fracture strengths were reduced by a factor of 2 when tested above 600°C. In the mixed salts of LiS/Li/LiF and LiCl/Li/LiF (i.e., nonoxide), the corrosion was via grain boundary penetration, whereas in LiSO4/Li/LiF the penetration was uniform. The decrease in fracture strength above 600°C was attributed to the large volume increase when lithia reacted with the silica surface coating on the SiC to form Li2SiO3.

The corrosion of hot-pressed silicon nitride (HPSN), reaction bonded silicon nitride (RBSN), and silicon carbide by molten sodium sulfate, sodium chloride and the eutectic composition between these two salts at temperatures from 670 to 1000°C for up to 120 hr was reported by Tressler et al. [5.77]. Molten sodium sulfate was the most corrosive, the eutectic composition was intermediate, and sodium chloride was the least effective in dissolving the silica surface layer present on these materials. HPSN was the most resistant, whereas silicon carbide completely dissolved in sodium sulfate at 1000°C within 20 min. The lower reactivity of Si3N4 compared to SiC with molten Na2SO4 was reported by Fox and Jacobson [5.72] to be a result of the formation of an inner protective layer of silica that stopped the continued reaction of Si3N4. The formation of this inner protective layer was highly dependent upon whether oxidation or dissolution

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