H. Buscail et Jp. Larpin, THE INFLUENCE OF CERIA SURFACE ADDITIONS ON MANGANESE OXIDATION AT HIGH-TEMPERATURES, Oxidation of metals, 43(3-4), 1995, pp. 237-261
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.