Immersion tests of TiB/sub 2/-based materials for aluminum processing applications

Abstract

The behavior of four types of TiB 2-based materials exposed to molten Al at 970'C was evaluated for time durations of up to 20 wk. These materials are prime candidates for cathodes in advanced Al smelters and include I) high-purity polycrystalline titanium diboride (TiB 2), 2) TiB 2-graphite (TiB 2-G) composites, 3) composites of woven ceramic fiber mesh coated with high-purity TiB 2 , and 4) composites of 50 vol% TiB 2-so val% aluminum nitride (AlN). The evaluations primarily involved optical ceramographic comparisons of samples before and after nonpolarized immersion in molten Al. The samples of TiB 2-AlN and high-purity TiB 2 exhibited superior resistance to chemical attack by molten Al; no visible microcracking or intergranular penetration by Al was observed. A reaction layer grew to 20 ~ in thickness at the surfaces of the TiB 2-AlN after exposure to Al for 20 wk; additional analyses are needed to determine the chemistry of the reaction lQ~. The TiB 2-ceramic fiber mesh samples exhibited mixed results; the TiB 2 coatings remained largely intact after 20 wk of exposure to Al, but the SiC-based fibers were attacked within 4 wk of exposure to molten Al. Fiber deterioration may be attributed to chemical reactions between molten Al and reactive constituents of the SiC-based fibers. An evaluation of composites with ceramic fibers that are less susceptible to degradation is recommended. The TiB 2-G composites exhibited poor resistance to chemical attack by molten Al. The experiments indicate that the surfaces of the composites are attacked by a three-step process: 1) removal of graphite accompanied by penetration of the TiB 2 pores by Al, 2) formation of Al-filled microcracks along TiB 2 grain boundaries, and 3) disruption of the microstructure by extension of large Al-filled cracks. Samples that were exposed for 4 to 20 wk were completely disrupted by Al-filled crack systems and TiB 2 grain boundary penetration by Al. Such catastrophic deterioration does not agree with the satisfactory performance observed with TiB 2-G cathodes during Hall-Herault cell electrolysis. Additional research is needed to elucidate the chemical reactions causing the difference in performance during polarized

Inert Electrodes Program fiscal year 1988 annual report

“…The results summarized in Table 6.1 corroborate earlier findings (Strachan et al 1988;Schilling and Graff 1988) involving long-term "soak tests" that the time period during which the electrolysis cell is off is the period of most rapid degradation of the TiB2-graphite material. From those tests, it was concluded that Al4C3 can form at the Al-graphite interface.…”

Laboratory testing of TiB/sub 2/-based cathodes for electrolytic production of aluminum

“…It is hypothesized that this process may fracture the brittle TiB 2 matrix, subsequently exposing Al to deeper regions of the composite and allowing the attack process to proceed inward from the exter ior surfaces. Supporti ng thi s hypothesi s, mol ar vol ume cal cul ati ons and solubility calculations suggest that, if all of the C present formed Al 4 c 3 during the immersion tests, the saturation solubi lity of Al 4 c 3 in Al would be exceeded by several times (Schilling and Graff 1988). As a result, dissolution rates would approach zero as the saturation solubility was reached, and…”

“…Samples contained Nicalon SiC-based fibers and produced by Nippon Carbon Co. , Tokyo, Japan. Details of these studies are reported by Schilling and Graff (1988).…”

“…Analyses by x-ray diffraction were performed on cut surfaces of an immersion test specimen , and the results suggest that the graphite is totally consumed by the formation of Al 4 c 3 (Schilling and Graff 1988). Anoxic conditions during the immersion tests are not expected to result in a mechanism for rapid removal of Al 4 c 3 from the TiB 2 pores; therefore , it may be suggested that stresses are exerted on the Ti B 2 pore walls as a result of the 3.8:1 volume expansion of Al 4 c 3 relative to graphite (Weast , Astle, and Beyer 1985 ;Schilling and Graff 1988) . It is hypothesized that this process may fracture the brittle TiB 2 matrix, subsequently exposing Al to deeper regions of the composite and allowing the attack process to proceed inward from the exter ior surfaces.…”

“…It is possible that molten Al penetrated these cracks, leading to contact with the graphite and the subsequent formation of Al 4 c 3 . The formation of Al 4 c 3 from elemental Al and C is thermodynamically favored at 97o•c; -35.1 kcal is the standard Gibbs free energy for the formation of one mole of Al 4 c 3 at 97o•c (Kubaschewski and Alcock 1979}. In addition, the 3.8:1 volume expansion of Al 4 c 3 , relative to graphite, may have contributed to the disruption of the coating leading to further penetration by Al (Schilling and Graff 1988;Weast, Astle, and Beyer 1985}. Another cause for the attack may be the chemical reactions between molten Al and Si0 2 , which is a major constituent of the coating according to the manufacturer. The reduction of Si0 2 to form Al 2 o 3 and Si metal at temperatures as low as 7oo•c has been reported elsewhere (Brondyke 1953;Standage and Gani 1967}.…”