WUSTITE TEXTURE AND MICROSTRUCTURE MODIFICATION BY CERIA SURFACE ADDITION DURING LOW-PRESSURE OXIDATION OF PURE IRON AT HIGH-TEMPERATURES

Pure iron oxidation is studied in the oxygen partial pressure ranging between p(O-2) = 2.10(-4) Pa and p(O-2) = 4.10(-2) Pa. These pressures were chosen in order to insure wustite formation in the temperature d omain 700-950 degrees C. Textural analysis with pole figure plotting i s performed on specimens oxidized at 800 and 950 degrees C in oxygen ( partial pressure of p(O-2)= 2.10(-3) Pa). We have demonstrated that ce rium coatings modify the microstructure and texture of a wustite scale grown on pure iron. All the results indicate that the main effect of the coating is to prevent the epitaxial relations between iron and the wustite scale. Then, the whole surface is covered by a fast growing s cale. The pole figures show that the (200) planes of the wustite struc ture are mainly oriented parallel to the specimen surface. The [200] o rientation is always associated to the faster growing scale. On blank specimens, the slow kinetic rate is attributed to the fact that the ma in part of the iron surface is covered by a wustite scale with orienta tion distinct from [200]. A good scale adherence was observed since th e coating was sufficiently thin and regular to permit the (CeFeO3) cer ium orthoferrite formation from the ceria coating. A perowskite type o xide appears as a good candidate for an homogeneous cerium introductio n into oxide scales. Its formation is favoured by the iron external di ffusion which looks like easy in the proceeding conditions.