EFFECTS OF CHROMIA COATINGS ON THE HIGH-TEMPERATURE BEHAVIOR OF F17TISTAINLESS-STEEL IN AIR - ANALYTICAL STUDIES OF THE EFFECT OF RARE-EARTH-ELEMENT OXIDES

The MOCVD deposition of neodymium oxide and/or chromium oxide provided beneficial effects both on isothermal- and cyclic high-temperature be havior of commercial F17Ti stainless steel. Fracture cross sections pr ovided information about the morphology of the oxide scales formed on bare steel and coated specimens. The chromia scales developed small eq uiaxed grains on the Nd2O3-coated samples and columnar grains on the u ncoated ones. Neodymium segregated within a surface layer composed of Mn1.5Cr1.5O4 spinel oxide. A complex phase (close to the structure of CeTi21O38) was identified in this zone. it could act as a source of ne odymium ions, which could segregate to the grain boundaries of the chr omia scale. Polished cross sections associated with X-ray mapping stud ies confirmed the scale structure and the location of the rare-earth e lement in the outer part of the oxide layer.