Hot salt corrosion of titanium aluminides

There is considerable interest in the use of gamma-TiAl within advanced gas turbines where they offer greater high temperature capability over conventional titanium at reduced weight. These factors would enable engines with greater thrust to weight ratio to be developed. Thus gamma-TiAl offers the potential of replacing nickel based alloys within the high pressure compressor and potentially the fourth stage turbine. Service in both of these locations would require that the gamma-TiAl be resistant, not only to oxidation, but hot salt corrosion. This paper presents a study of the hot salt corrosion resistance of gamma-TiAl over the temperature range 500-700 degrees C. At 700 degrees C, laboratory tests have shown that corrosion rates in a salt ladened environment are some 20x that of equivalent oxidation for exposures out to 100 h. The morphology of attack is consistent with a corrosion mechanism involving the vapour phase transport of aluminium from within the alloy and formation of a non protective oxide scale. A model for the corrosion mechanisms is presented, involving intermediate chloride phases. The mechanism is believed to be self sustaining, requires little chloride present, and leads to the observed accelerated oxidation rates.