Abstract
High temperature oxidation behavior of an intermetallic compound of NbAl3 was investigated at 1073 and 1173 K, in oxygen containing a trace amount of NaCl vapor. Particular attention has been given to the effect of NaCl vapor concentration on the depression of the pesting oxidation. In oxygen without NaCl vapor, mass gain of NbAl3 was quite large and the pesting oxidation occurred. On the other hand, in oxygen with several tens of volume ppm NaCl vapor, mass gain was reduced within the experimental time region, down to negligible value in oxygen with 123 ppm NaCl vapor. Moreover, the pesting phenomenon disappeared when the mass gain was lower than 0.05 kg m−2. X-ray diffraction and SEM observation revealed that specimens without the pesting were covered with a relatively dense Al2O3 film, whereas the specimens that occurred pesting, formed the scale consisting of mainly AlNbO4. From the thermodynamic discussion, it was considered that NaCl vapor produces a small amount of chlorine gas, inducing chlorination of Al selectively under lower potential of oxygen at the alloy/scale interface, subsequently, produced AlCl3 is oxidized to form a protective Al2O3 film. This Al2O3 formation mechanism was proposed as the pesting depression model. Consequently, from all the result, an ability to prevent pesting oxidation by adding a trace amount of NaCl vapor to oxygen became apparent.
