Oxide ridges formed during the transient stage oxidation of the scale evolution in iron alloys containing 22 wt % Cr that were held at 800°C in dry air. The surface oxidation process was imaged in situ through a confocal scanning laser microscope, and the results were correlated with postexperiment characterization through scanning electron microscopy and the DualBeam system ͑focus ion beam and electron beam͒ 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 Cr oxide scale. 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 La ͑120 and 290 ppm͒ and Ce ͑270 and 610 ppm͒ additions during melt-stage processing were also investigated.Solid oxide fuel cells ͑SOFCs͒ are considered to be promising energy conversion devices that can electrochemically convert a range of fuels including syngas and methane into electricity. Among the different types of fuel cell designs, the flat plate ͑planar͒ design is now more favored due to the ease of processing and manufacturing the cell components. 1 This planar design requires the use of interconnectors in the SOFC stack that provide mechanical support to the cell components as well as the electrical contact between single cells and that separate the fuel on the anode side from the oxidant on the cathode side. Ceramic oxides based on doped lanthanum chromite and on relatively expensive high Cr superalloys have been used as interconnector materials. 2 However, lowering the operational temperature of SOFCs from around 1000 to 800°C offers the possibility of using less expensive alloys.Because the scales formed at SOFC operating temperatures must be electrically conductive for interconnector applications, alumina or silica formers are not suitable. Fe-Cr-based alloys ͑less than 24 wt % Cr͒ are considered the favorable candidates for metallic interconnectors in planar cells 2 because there is a good balance between the slow growth rate of the oxide scale and the electrical conductivity of the oxide scale. Moreover, they have an appropriate thermal expansion coefficient that closely matches that of the other components of the cell. 2,3 Finally, they also offer good mechanical support for the entire cell structure. A major problem with metallic interconnectors, such as those based on Fe-Cr alloys, is their reactivity with the anode and/or cathode side service environments at operating temperatures. 2 The resulting high temperature corrosion or oxidation can lead to the failure of the cell. Also, volatile CrO 3 and/or CrO 2 ͑OH͒ 2 can form at the oxide scale/oxidant interface, migrate to the cathode/electrolyte interface, and form chromia deposits, which can poison the cathode reaction and reduce SOFC efficiencies. 2 The oxidation of metals and...

It is known that additions of reactive elements such as Ce, La or Y improve the properties of protective oxide-scales on Ni and Fe based alloys [ - ] by increasing oxide adhesion, decreasing the transient time until a continuous Cr2O3 layer is formed and decreasing the parabolic rate constant. Nevertheless, the precise roles played by these reactive elements to improve scales and the precise mechanisms by which they are incorporated into the scale during the surface treatment processes are unknown. Although they are believed to be associated with transport properties in the scale, it is not clear how this occurs or why it improves oxidation resistance. This project is aimed to gain understanding of the scale evolution in Fe-22 wt.% Cr alloys at 800 oC in dry air during the transient stage after 15 minutes of oxidation. The effect of La (120 and 290 ppm) and Ce (270 and 610 ppm) additions added during melt-stage processing are investigated. The surface oxidation process was imaged in-situ through a Confocal Scanning Laser Microscope (CSLM) and the results were correlated with post-experiment characterization through FEG-SEM and FIB-SEM combined with 3D reconstruction. The roles of rare-earth oxide particles on nucleation of Cr2O3 and blockage of short-circuit diffusion paths in the oxide scale and underlying metal are discussed.

In this study, the effects of Ce (270 ppm) and La (120 ppm) mischmetal additions on the transient oxidation of an Fe-22Cr alloy were investigated. The oxidation process was imaged in situ using a confocal scanning laser microscope. The oxidation microstructures were studied by scanning electron microscopy, energy dispersive X-ray analysis, and transmission electron microscopy with the help of focused ion beam in situ lift-out specimen preparation. The Ce and La, referred to as reactive elements, were found in nonmetallic inclusion particles in the forms of oxides, sulfides, and phosphates. An affected zone formed around rare earth (RE)-containing inclusion particles at the alloy free surface during the transient oxidation. This zone consisted of an internal Cr-oxide formed beneath the particle as well as a thinner external oxide scale on the surface compared with the surroundings. The relation of this microstructure to oxidation kinetics is discussed. With time, the RE elements diffused into the scale from the RE particles on the alloy surface during the high-temperature exposure. A diffusion mechanism is presented to describe these observations.