TiAlの溶融NaCl塗布下での高温腐食挙動

抄録

The hot corrosion behavior of TiAl intermetallic compound due to molten NaCl has been examined by measuring the mass change of the specimen, observation and analyses of scales formed and electrochemical measurement. The corrosion mass loss of TiAl under a coating of molten NaCl was far larger than those under whole immersion in molten NaCl and under coating of molten Na₂SO₄. Such a hot corrosion of the TiAl under coating of molten NaCl was due to accelerated oxidation during the initial period. Mass gain of the TiAl due to this accelerated oxidation increased with increasing mass of coating NaCl, O₂ concentration in the atmosphere and test temperature. Scales formed after the accelerated oxidation consisted of three layers; an outer layer consisting of rutile-TiO₂, a middle layer consisting of α-Al₂O₃ and an inner layer consisting of a mixture of rutile-TiO₂ and Ti₂O. In these oxide layers NaCl was detected, showing that molten NaCl penetrated into the scale. The period, during which NaCl was contained in the scale, agreed with that when the accelerated oxidation occurred. This suggests that the molten NaCl contained in the scale contributed to the accelerated oxidation. In molten NaCl, the anodic dissolution reaction and the cathodic reduction reaction of dissolved O₂ were active. On the basis of these experimental results, a mechanism for accelerated oxidation in TiAl is presented, in which the growth of the scale proceeds through electrochemical steps at metal/oxide and oxide/molten salt interfaces and a diffusion step for metal and oxide ions in molten NaCl penetrates in the scale.