Effects of Ti and La on High Temperature Oxidation Behavior of 20Cr-5Al Steel

Abstract

An investigation has been undertaken into the oxidation behavior of foils of 20Cr-5Al steel and similar steels containing titanium and lanthanum additions, in air at 1373 and 1423K.
Oxides scales on the 20Cr-5Al steel spalled when the specimen was cooled and breakaway took place immediately due to depletion of aluminium in the metal during oxidation. Titanium lowered the depletion rate of aluminium in the metal. The oxide scales on the steel containing titanium maintained relatively good contact with the specimen and breakaway behavior was retarded compared with that of the base steel. The steel with lanthanum and titanium additions showed excellent oxide scales adherence and the protective oxide scales were retained over long periods.
In addition to depletion of aluminium, the morphology of metal-oxide scales interface influenced the oxidation behavior of the steels. Voids formed at the interface during oxidation. Due to depletion of aluminium, convoluted Cr₂O₃ scales developed in the voids following breakaway. The 20Cr-5Al steel formed voids in the early stages. However, lanthanum and titanium additions were effective in reducing void formation. These effects can be associated with a cation transport mechanism of oxidation.