文摘
High temperature oxidation behavior of Ta–75at%Al was investigated by thermogravimetry at 800 and 900 °C in oxygen with NaCl vapor of 7.2, 32.1 and 123 volume ppm. Without NaCl vapor, pest oxidation took place and the aluminide crushed into particles at both temperatures from earlier stage. In contrast, with a slight NaCl vapor, the pest oxidation was effectively reduced to achieve very small mass gain values. For samples without pest oxidation, there formed a thin Al2O3 outer filmed scale and Al2O3 scale along the grain boundaries inside the alloy. The reducing mechanism of pest oxidation by NaCl vapor was considered from the viewpoint of thermodynamics. Namely, it was considered that NaCl vapor reacts with oxygen in the atmosphere at high temperatures to produce chlorine gas, and this Cl2 selectively chlorinates Al than Ta, which produces Al2O3 on the gas/scale interface where the oxygen potential is higher. This Al2O3 was thought to construct a protective film. Keywords Aluminide Chlorination Intermetallic compound Thermodynamics Hot corrosion