Effect of boron on the oxidation behavior of NiCrAlYHfTi in H2O and CO2 environments

Cast NiCrAl alloys, co-doped with Y, Hf, Ti, and/or B, were evaluated at 1100 °C and 1150 °C in dry air, wet air (10 or 50% H2O), and CO2-10% H2O, in order to study the effect of boron additions on alumina scale growth and adhesion. After 200, 1-h cycles at 1100 °C, all of the alloys with Y and Hf showed good scale adhesion. By increasing the test temperature to 1150 °C, the addition of ~ 0.3 at.% (0.07 wt.%) B was shown to improve alumina scale adhesion during 1-h cycles in air with 10% H2O. Analytical transmission electron microscopy showed no effect of B on the alumina scale microstructure and only minor effects on the depth of internal oxidation at 1100 °C. Combined Ti and B additions did not produce an additional benefit and Cr-B precipitates were detected in both B-doped alloys. Exposures in O2-buffered CO2-H2O did not result in any detrimental effect at 1100 °C.