The effect of pre-oxidation with ceria on the carburisation of Incoloy 800HT

摘要

Superficial ceria layers applied by dip-coating and electrophoresis, retard the oxidation of Incoloy 800HT in air. A small ceria addition incorporates in the on-growing oxide layer and makes it thin with a good adherence to the metal. Thicker ceria layers remain at the surface as a separate phase and the oxide layer grows beneath them. Ceria that integrates in the oxide layer during a pre-oxidation treatment, retards weight gain during subsequent carburisation. For Incoloy 800HT in H{sub}2-3%CH{sub}4 at 1000°C, the reaction rate decreases by a factor 2-3 compared to samples without ceria. The carburised zone is thin after 24 hours and the oxide layer is maintained at the surface. Thick porous ceria layers that remain on the surface, accelerate carburisation. A chromium carbide layer replaces the chromia layer, and carbides form on the grain boundaries and in the alloy grains. Thermodynamic calculations show that the difference is most likely caused by ceria's ability to catalyse the dissociation of CH{sub}4.