EFFECTS OF CERIA COATINGS ON PURE IRON OXIDATION - COMPARISON WITH EXTRA LOW-CARBON STEELS

A comparison of results obtained on blank and coated specimens shows t hat ceria coatings strongly modify the microstructure and texture of t he wustite scale formed during low pressure oxidation of pure iron. On coated specimens, all the scale is composed of fast growing (200) ori ented wustite grains; crystallographic orientations (different from (2 00)) are no longer present in the wustite scale. The linear oxidation rates are always higher in the case of surface coated specimens due to the lack of epitaxial relationships with the metallic substrate. Ceri um is located in the wustite matrix as a CeFeO3 phase which dissolves in FeO in time. Carbon and manganese, present as major impurities in a n Extra Low Carbon Steel, have a strong influence on oxidation kinetic s and on the rare-earth element location in oxide scales. Carbon, diff using slowly from the matrix bulk to the metal-scale interface, promot es spallation of the wustite scale, but carbon does not hinder CeFeO3 formation. On the other hand, the manganese impurity prevents the form ation of CeFeO3 due to the lack of contact between the reducing FeO ge rms and the oxidant CeO2. This lack of contact is due to the high vapo ur pressure of manganese oxides, which evaporate at the beginning of t he reaction, and promotes immediate CeO2 coating spallation.