Reducing Effect of a Slight Amount of NaCl Vapor on Pest Oxidation of Ta–75at%Al at High Temperature

Abstract

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.