The effect of ΔT (oxidizing temperature minus cooling temperature) on oxide spallation

Abstract

Several alloys (one iron base and five nickel base) were cyclically oxidized in a series of tests in which the higher temperature (1100 or 1200° C) of the cycle was fixed at a level to allow ample oxidation in reasonable time and the lower temperature was variable to allow cycle temperature differences (ΔT _s ) of up to 1400°C. The alloys oxidized included those which formed simple oxides such as Al₂O₃ or Cr₂O₃, as well as those which formed complex scales. Cooling rates were relatively low to minimize thermal shock effects. Each cycle consisted of 1 hr at the higher temperature and 1/2 hr at the lower temperature. Samples were tested up to 370 cycles. The extent of attack was determined by specific weight change which was continuously monitored. For all nickel alloys, as ΔT increased the extent of spallation increased. This effect was attributed to thermal expansion mismatches between the oxide and the nickel substrate. The FeCrAl alloy was not sensitive to ΔTand resisted spalling at ΔT levels to 1400°C. FeCrAl, and the Al₂O₃ scale which forms on it, have thermal expansion coefficients which are substantially more alike than any of the other oxide-metal combinations tested.