Influence of Tantalum on the Rates of High Temperature Oxidation and Chromia Volatilization for Cast (Fe and/or Ni)-30Cr-0.4C Alloys

Four cast superalloys, Fe-base and (Fe,Ni)-base alloys, all containing 30%Cr and 0.4%C, were elaborated with addition of 3% and 6% of tantalum. Their oxidation behaviours were studied at 1000, 1100 and 1200°C during 50 hours. The oxidized surfaces of the samples were quantitatively characterized by measurements of thicknesses and surface fractions of internal oxides. The thermogravimetry files were treated according to the {m×(dm/dt) = Kp - Kv×m} equation, to obtain simultaneously the parabolic constant and the chromia volatilization constant. The internal tantalum oxides are more present in the Fe-base alloys and the carbide-free zones are less developed for the (Fe,Ni)-base alloys than for the others. The Fe-base alloys oxidize faster than the (Fe,Ni)-base and Ni-base alloys. The comparison with the corresponding Ta-free ternary alloys shows that the presence of Ta tends to accelerate the oxidation.