Effects of Sodium Silicate Exposure at High Temperature on Sintered α‐Silicon Carbide and Siliconized Silicon Carbide

Abstract

Sintered α‐silicon carbide and siliconized silicon carbide were exposed to combustion off‐gas containing sodium silicate vapors and particulates in a combustion test facility for 24 to 373 h at 900° to 1050°C. Degradation was evaluated by measuring dimensional changes, by measuring loss in strength due to changes in flaw population, and by evaluating surface corrosion morphology. It is suggested that passive oxidation and dissolution of the silica oxidation scale play an important role in the corrosion process. These mechanisms were enhanced by the continuous removal and replenishment of corrosive material by the high‐velocity gas. These degradation phenomena caused surface pitting and an approximately 50% reduction in strength for both materials after long‐term exposure (>100 h). Morphological evaluation suggested that the grain boundaries in the α‐silicon carbide were oxidized more rapidly than the grains, while for the case of the siliconized silicon carbide the silicon phase was oxidized rapidly along with preferential oxidation of the silicon carbide grains parallel to the {0001} plains.

Coal-fired high performance power generating system

“…Tests of exposure to alkali salts (halides and sulfides) have been extensive (e.g., Butt et al, 1989;Fox et al, 1990;Federer, 1990).…”

“…The hypothesis was that the free silicon was attacked more readily than SiC, resulting in a higher corrosion rate. Butt et al (1989) exposed siliconized SiC and _-SiC in a burner facility in which sodium silicate was injected into the burner.…”

Coal-fired high performance power generating system. Quarterly progress report

“…Data ofButt et al (1989) c.,=,uFigure 20. Strength of silicon carbide as a function of time exposed to sodium silicate at 960°C…”

Coal-fired high performance power generating system. Final report

“…Previous work on the corrosion of silicon carbide at high temperatures has looked at a wide range of environments. Tests of exposure to alkali salts (halides and sulfides) have been extensive (e.g., Butt et al, 1989;Fox et al, 1990;Federer, 1990). A number of tests have been conducted on the interactions between coal ash and silicon carbide (Ward et al, 1980;Ferber and Tennery, 1981;Ferber and Tennery, 1982;Easler et ai., 1985).…”

“…The hypothesis was that the free silicon was attacked more readily than Sic, resulting in a higher corrosion rate. Butt et al (1989) exposed siliconized Sic and a-Sic in a burner facility in which sodium silicate was injected into the burner. Volatile and particulate sodium was delivered to the samples at avelocity of 30 m/s.…”