On the Relation Between Oxide Ridge Evolution and Alloy Surface Grain Boundary Disorientation in Fe–22 wt % Cr Alloys

Abstract

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...

Effect of pre-oxidation on the oxidation resistance of Crofer 22 APU

“…Thus, the alloy is not depleted of Mn after 1000 h oxidation at 800 °C and the (Mn,Cr) 3 O 4 layer is expected to remain at the alloy/scale interface for a long period of oxidation at 800 °C.The subscale spinel nodules were observed to form preferentially at the alloy grain boundaries, which indicates that grain boundaries are fast diffusion paths for Mn. Zhu et al[40] estimated that the bulk diffusion coefficient of Mn is twice that of Fe and Cr in ferrite at 800 °C and hypothesized that the difference in diffusivity should be even greater along the grain boundaries. Thorning and Sridhar…”

The impact of alloy chemistry on the formation of a silicon-rich subscale on two classes of ferritic steels

“…The major features of the oxide scale, i.e., the Cr/Mn-rich outer and Cr-rich inner scales, have been well reported on in the literature [22,25,29,34,36e38]. Present understanding is that initially Cr 2 O 3 forms on the surface of the alloy after exposure times as short as 15 min [39], perhaps sooner. With additional exposure (or quick diffusion pathways such as alloy grain boundaries) Mn from the alloy diffuses to the top of the oxide scale and combines with the Cr to form a Cr/Mn-rich scale on the outer surface.…”

Study on Stability of Mechanical Properties for Porous Fe-Cr-Al Alloys after Long-Term Aging

“…The grain boundaries can be the channels for the diffusion of O, thus resulting in the inner oxidation of grain boundaries [ 29 , 30 ]. To suppress such abnormal oxidation, grain-boundary engineering methods such as alloying with reactive elements (boron and RE metals) are usually employed [ 31 , 32 ]. In brief, the design of porous Fe-Cr-Al alloys with improved mechanical stability should consider their corrosion resistance at elevated temperatures seriously.…”