Corrosion Behavior of Engineering Ceramics in Water at High-Temperatures and Pressures

点击翻译按钮获取中文摘要

The corrosion behaviors of the promising engineering ceramics, silicon nitrides and mullites, were studied by the autoclave method at 300ﾟC and 8.6MPa for times up to 10 days. The results are summarized as follows :1. Corrosion behavior of silicon nitride ceramics :Pressureless-sintered Si_3N_4 with additives(Y_2O_3-Al_2O_3 and MgO)prepared with imide decomposed Si_3N_4 powder was severely corroded in water at high-temperature and pressure. resulting in weight loss and formation of corrosion product layer. After removal of corrosion products, pits up to about 100 mu m in diameter were found distributed over the Si_3N_4 surface. Pit features in these sintered Si_3N_4 are different depending on the species of oxide additives ; Si_3N_4 with Y_2O_3-Al_2O_3 forms deep pits with a thin protective corrosion product layer and that with MgO forms shallow pits with a thick non-protective corrosion product layer. This result in higher corrosion resistance for the former than the latter.2. Corrosion behavior of mullite ceramics :The aqueous corrosion of mullite ceramics proceeded according to the reaction, 3Al_2O_3・2SiO_2+3H_2O -> 6AlOOH+2SiO_2, with weight loss by dissolution Of SiO_2 and formation of boehmite(AIOOH)residue layer. The extent of corrosion resistance was significantly dependent on the powder preparation method and chemical composition. Seventy four wt% Al_2O_3-mullite prepared from an alkoxide-derived powder showed the highest corrosion resistance. Mullite ceramics with highcorrosion resistance was found to be attained by homogeneous mixing SiO_2 with Al_2O_3 in an alkoxide-derived or coprecipitation powder.3. Corrosive strength degradation of the ceramics :The bending strength of the corroded specimens for both silicon nitride and mullite ceramics reduced to around 2/3 of the original values in the early stage of corrosion, and then remained almost constant up to 10 days.