純TiならびにTi-6 mass%Al合金の高温酸化に及ぼす微量NaCl蒸気の影響

Abstract

The effect of trace amounts of NaCl vapor on the oxidation behavior of pure Ti and Ti-6 mass%Al alloy was investigated by thermogravimetry in a temperature range from 923 to 1073 K. Observation and identification of scales were performed. In addition, hardness measurement of the metal substrate beneath the scale was performed. The formation of scales consisting of periodically stacked layers was observed for pure Ti after oxidation in O2 without NaCl vapor. On the other hand, the addition of NaCl vapor resulted in the formation of scales consisting of a single layer without a continuous crack. The oxidation kinetics for pure Ti in O2 containing NaCl vapor followed a parabolic rate law, and the oxidation rate increased with increasing concentration of NaCl vapor. The marker experiment showed that the rate-determining step in this parabolic oxidation was outward diffusion of titanium ions through a rutile TiO2 layer. Therefore, NaCl vapor in O2 appears to promote the outward diffusion of titanium ions through the scale. The oxidation rate for a Ti-6 mass%Al alloy, whose kinetics followed a parabolic rate law, also increased with increasing concentration of NaCl vapor. However, in the O2 containing NaCl vapor the oxidation rate for the Ti-6 mass%Al alloy was lower than that for pure Ti. The decrease in the oxidation rate by aluminum addition to titanium seemed to be attributable to the fact that an inner layer consisting of α-Al2O3 formed at the oxide/metal interface inhibited the outward diffusion of titanium ions.