Abstract
The effect of pre-oxidation in O2 containing trace amounts of NaCl vapor on high-temperature oxidation of TiAl was examined by thermogravimetry, and analyses of the pre-oxidation scale and the scale formed on the pre-oxidized specimen after the oxidation test. Particular attention was given to the influence of NaCl vapor concentration on the structure of the pre-oxidation scale. The pre-oxidation at 1173 and 1273 K in O2 containing 1.4∼32.2 vol ppm NaCl vapor largely decreased the oxidation rate of the TiAl specimen in pure O2 at 1273 K. The magnitude of the decrease in the oxidation rate increased with increasing NaCl vapor concentration. For the specimens pre-oxidized in O2 containing 1.4 and 7.2 vol ppm NaCl vapor, a new scale formed on the pre-oxidation scale and internal oxidation in the metal below the pre-oxidation scale were observed, whereas for the specimen pre-oxidized in O2 containing 32.2 vol ppm NaCl vapor a slight growth of the pre-oxidation scale without forming a new scale was observed. The scale formed by pre-oxidation in O2 containing 32.2 vol ppm NaCl vapor was dense and adhesive to underlying metal, consisting mainly of Al2O3. Consequently, the inhibition effect of pre-oxidation in O2 containing trace amounts of NaCl vapor on high-temperature oxidation of TiAl resulted from the fact that pre-oxidation scale acted as a barrier against the outward diffusion of metal ions.
