HOT SALT CORROSION OF TITANIUM ALUMINIDES

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