THE INFLUENCE OF CERIA SURFACE ADDITIONS ON MANGANESE OXIDATION AT HIGH-TEMPERATURES

The kinetics, scale composition, and growth mechanism of ceria-coated and blank specimens of manganese oxidation in air were examined. The s cale growth obeys the parabolic rate law at 700 degrees C for all spec imens. Lower parabolic rate constants for coated specimens are attribu ted to the presence of a CeO2 external scale. It constitutes a limitin g factor of the oxygen activity at the gas-oxide interface. This lower -oxygen activity leads to a less-metal-deficient state of the scale. D ue to this, the inner-MnO scale becomes more adherent to the substrate . Preheating at 700 degrees C, in hydrogen (P-O2 = 10(-24) atm), was p erformed in older to be placed in the MnO stability domain and try to introduce cerium in the manganese-oxide scale. This pretreatment promo tes macroscopic bonding in the layer for med during subsequent oxidati on in air. It ensures a better scale adherence. A new diffusional-tran sport mechanism in manganosite is proposed in accordance with all expe rimental observations of the literature and with the cerium-manganese- oxygen system studied in the present work. This model considers the hi gh Mn3+ stability in octahedral sites of the MnO oxygen ion body, Low- oxygen partial pressure conditions permit the formation of an adherent inner-MnO scale on coated specimens. A CeO2 scale formed above the Mn O scale; MnO is present as a minor component in this scale and it is l ocated mainly at the internal interface. The difficulties in forming t he cerium-orthomanganite are attributed to the very high stability of MnO related to this wide range of nonstoichiometry and to the low mang anese diffusivity through the cerium-containing scale.