Fundamental Studies on Controlling Scale Growth in Iron-Chromium Alloys through Additions of Reactive Elements

To commercialize solid oxide fuel cell (SOFC) technologies, Fe-Cr based alloys (less than 24 wt pct Cr) are favored candidate material for the metallic interconnector in planar design cells. This study focused on documenting the oxidation process and products as well as the behaviors of rare earth (RE) elements added to the alloy by two methods: misch-metal RE addition to the bulk alloy and superficial enrichment of RE oxide, aiming to provide fundamental understanding of the effects of RE on the oxidation resistance of the alloys.;Oxide ridges formed during the transient stage oxidation of the scale evolution in the alloys that were held at 800°C in dry air. The surface oxidation process was imaged in situ through a CSLM, and the results were correlated with postoxidation characterization through scanning electron microscopy and the DualBeam system (focus ion beam and electron beam) analysis combined with three-dimensional reconstruction. The oxide ridges that formed on top of the Cr oxide scale overlapped the intersections of the underlying alloy grain boundaries with the scale. Ridge size was also correlated to the disorientation of the underlying alloy grain boundaries that was calculated from the orientation map by EBSD technique. Ridges were generally very small on grain boundaries with disorientation angles of less than 15°, and it was suggested that the boundaries of the surface grains in the alloy may serve as bottlenecks for the transport of scale-forming elements. The effects of two levels of RE additions during melt-stage processing were also investigated, and it seemed the ridge size decreased with increasing RE content in the alloy.;The behavior of RE inclusion particles resulting from misch-metal addition during transient oxidation at 800°C in dry air was studied, and the evolution of the scale and RE-containing features was elucidated. The formation of "affected zones" around the RE particles was investigated and their possible relation to oxidation kinetics was discussed. RE elements were found to diffuse into the scale from the RE particles on the alloy surface during the high temperature exposure. The formation of affected zone may be responsible for shortening of the transient stage of oxidation. Based on the location of RE elements in the scale, it was proposed that the RE diffused into the scale by both diffusion along the metal/scale and scale/gas interfaces and oxide grain boundaries, which seemed to be supporting evidence for both the oxide grain boundary segregation theory and the poisoned interface mechanism.;The oxidation of the base alloy with ceria stripe coating during transient and extended high temperature exposures at 800°C in dry air were studied. The inversion of scale growth direction and the formation of the two-layered scale were investigated and their possible relation to oxidation kinetics was discussed. Ce4+ ions was found to promote the Mn-Cr spinel to form a continuous layer, also Ce4+ diffused into the inward grown scale layers along the scale growth direction from the ceria coating in the well-coated area starting in the early stage of the high temperature exposure. Based on the understandings of the defect structure and reaction of ceria available in the literature, a two-stage effect of ceria coating on the oxidation of the alloy was proposed, and the mechanisms of reduction of oxidation rate with the misch-metal additions and with surface enrichment were compared; and similarities as well as differences were discussed. Also, effects from amorphous and crystalline ceria coating were compared. It was found that amorphous ceria contributed to larger outward flux of metal cations than crystalline ceria coating and was not as effective as crystalline ceria in prohibiting outward metal diffusion in extended exposure due to their grain structure that crystallized during the oxidation process. The thickness of the coating also seemed to have an effect on blocking the outward metal cation flux. (Abstract shortened by UMI.);Two sets of study alloys were prepared to investigate both transient and steady-state oxidation. Laboratory scale oxidation experiments were carried out to elucidate the evolution of the RE-containing features during the oxidation process.