Effects of Ceria Coatings on Pure Iron Oxidation. Comparison with Extra Low Carbon Steels

Abstract
A comparison of results obtained on blank and coated specimens shows that ceria coatings strongly modify the microstructure and texture of the wüstite scale formed during low pressure oxidation of pure iron. On coated specimens, all the scale is composed of fast growing (200) oriented wüstite grains ; crystallographic orientations (different from (200)) are no longer present in the wüstite 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. Cerium is located in the wüstite matrix as a CeFeO₃ phase which dissolves in FeO in time. Carbon and manganese, present as major impurities in an Extra Low Carbon Steel, have a strong influence on oxidation kinetics and on the rare-earth element location in oxide scales. Carbon, diffusing slowly from the matrix bulk to the metal-scale interface, promotes spallation of the wüstite scale, but carbon does not hinder CeFeO₃ formation. On the other hand, the manganese impurity prevents the formation of CeFeO₃ due to the lack of contact between the reducing FeO germs and the oxidant CeO₂. This lack of contact is due to the high vapour pressure of manganese oxides, which evaporate at the beginning of the reaction, and promotes immediate CeO₂ coating spallation.