Neal Magdefrau

Oxidation Kinetics of Mn1.5Co1.5O4-Coated Haynes 230 and Crofer 22 APU for Solid Oxide Fuel Cell Interconnects

The oxidation behavior of Mn 1.5 Co 1.5 O 4 (MCO)-coated Haynes 230 (H230) and Crofer 22 APU was investigated between 700 and 900°C. The oxidation kinetics of the coated alloys was compared with that of base alloys at 800°C. An apparent two-stage kinetics behavior of the MCO-coated Crofer 22 APU was observed. The coating effectively reduced the oxidation rate constants of Crofer 22 APU by 5.5 times, whereas it did not seem to affect the oxidation kinetics of H230. The oxidation activation energies of the coated alloys suggest distinctly different oxidation mechanisms between the coated H230 and Crofer 22 APU. A Cr-modified spinel was observed in the interface region between the metal oxide scale and the spinel coating after long-term oxidation for both alloys. A semiquantitative model of oxidation kinetics was developed to explain the different behaviors observed. Apparently, the Cr-modified spinel may play a more important role on H230 during long-term oxidation. The heat-treatment of H230 in the reducing environment used for the MCO coating application processes appeared to debit oxidation resistance of the base alloy. The optimization of the MCO application process is expected to benefit oxidation resistance as well as chromia containment on H230.

Microstructural effects of the reduction step in reactive consolidation of manganese cobaltite coatings on Crofer 22 APU

Abstract The microstructural effects of the reduction step in the reactive consolidation of slurry processed Mn1·5Co1·5O4 coated Crofer 22 APU were studied using cross-sectional scanning and transmission electron microscopies. Alloy samples were coated with a Mn1·5Co1·5O4 slurry and then reduced in moist H2 based forming gas at 850°C for 4 h. The reduced coating contained particles of MnO with the NaCl structure and Co with the face centred cubic (FCC) structure. The interface exhibited a thin dense chromia layer with a thicker porous MnCr2O4 overlayer with needle-like protrusions into the reduced coating. Evidence for inter-reactions during the concurrent coating reduction and substrate oxidation include Co rich metallic inclusions in the spinel layer and an enrichment of the spinel in Mn with distance from the chromia layer. The consequences of these observations for the complex microstructural variation during subsequent reoxidation and long term thermal exposure are discussed.

ALCHEMI studies of site occupancies in Cr-, Ni-, and Fe-substituted manganese cobaltite spinels

The effects of Cr, Ni, and Fe substitution into manganese cobaltite (MCO) spinels are of great interest due to the roles that the diffusion of these cations play in reaction layer development during high temperature exposure of MCO-coated alloys. Here we report a study on a series of model Cr-, Ni-, and Fe-substituted MCO spinel ceramics produced by consolidation of combustion-synthesized oxide powders. The cation site occupancies in these samples have been studied by X-ray spectrometry-based Atom Location by CHanneling Enhanced MIcroanalysis (ALCHEMI) experiments in the transmission electron microscope, with the data being analyzed using the ordering tie-line approach. In Cr-substituted samples, the Cr ions lie on the octahedral B sites and the Co ions reside on the tetrahedral A sites with Mn occupying the remaining sites. In Ni-substituted samples, all of the Ni ions occupy the B sites and the Co and Mn ions tend to lie on A and B sites, respectively. In contrast to the Cr-substituted samples, there is some mixing of the Co and Mn ions on the two types of sites at lower Ni contents. In Fe-substituted samples with lower Fe contents, all of the Mn ions occupy the B sites, roughly equal proportions of Fe ions occupy the A and B sites, and Co ions fill the remaining sites. With increasing Fe content, the degree of order decreases which ultimately results in the High Fe sample exhibiting no channeling evidence for preferred site occupation. These ALCHEMI data could provide a useful insight into the role of cation sub-lattice site preference in the formation of reaction layers in MCO-coated stainless steels and superalloys.

ALCHEMI Studies of Spinel Oxides for SOFC Interconnect Alloy Coatings

Manganese cobalt oxide (MCO) spinels are used as coatings on metallic interconnects for solid oxide fuel cells (SOFCs) because they inhibit outward diffusion of Cr while exhibiting polaronic conductivity that gives acceptable contact resistances [1]. We have recently shown that MCO-coated Crofer 22 APU develops a complex reaction layer (RL) at the alloy/coating interface, and we have proposed that the mechanism of formation is related to the site occupancies of Cr, Fe and/or Ni ions diffusing into the MCO from the alloy [2,3]. Here we report a study of these effects in which we firstly consider a series of model ceramic MCO spinels with Cr-, Feand Nisubstituted compositions. Powders of the appropriate compositions were produced by glycine nitrate combustion synthesis, and these powders were consolidated into ceramic pellets by pressing and pressure-less sintering. Conventional ceramic TEM samples produced by dimpling and Ar ion milling were analyzed using planar ALCHEMI on grains oriented close to the 400 systematic row. The data were plotted using the ordering tie line (OTL) approach introduced by Hou et al. [4]. In each case, the OTLs extrapolated to identify the sub-lattice compositions for the most highly ordered state consistent with the sense of order measured experimentally. These data were then compared with measurements from the RL in MCO-coated Crofer 22 APU coupons that had been oxidized in air at 800 ̊C for 1000 h. FIB-cut cross sections through the coating/alloy interfaces were analyzed and ALCHEMI data were obtained at various points through the RL thickness corresponding to different levels of cation substitution into the MCO structure. Examples of the OTLs for ALCHEMI data obtained from Cr-substituted ceramic MCO samples are shown in Figure 1. For the samples with low and intermediate Cr contents the site occupancies show the expected trends with Co on the tetrahedral A sites and Mn and Cr on the octahedral B sites. For the high Cr sample, however, the Cr displaces some of the Mn onto the A sites so that at this composition the proportions of Mn are the same on both sites. The microstructure of the interface in the MCO-coated Crofer 22 APU after 1000 h oxidation as shown in Fig 2(a) contains: the alloy, a 1-2 μm thick fine-grained chromia layer, a RL ≈5μm in thickness, and the unaffected MCO coating. There is a compositional gradient within the RL and Fig. 2(b) shows the regions of low (10%), medium (28%) and high (44%) Cr content within the RL. The corresponding OTLs are shown in Fig. 3. For the regions of medium and high Cr content within the RL, the sense of the ordering is broadly consistent with that expected on the basis of the data from the Crsubstituted ceramic samples. For the low Cr sample, however, both the sense and extent of ordering are very different. This can be explained on the basis of the effects of other substitutional cations (mainly Fe) in the RL, and the fact that the MCO coatings are undergoing a structural transformation under the influence of a diffusive flux whereas the ceramic samples adopt an equilibrated cation distribution.

Analysis of Titanium Microalloying in As-Received and Oxidized Crofer ® 22 APU

The addition of titanium to ferritic stainless steel alloys has been shown to benefit the high temperature characteristics of these alloys for use in solid oxide fuel cell (SOFC) interconnects. TiN precipitates can reduce the fractional softening of interconnects at high temperatures by preventing recrystallization and recovery of the cold-worked alloy [1]. TiN has a lower solubility in austenite than ferrite [2]. During solidification of molten steel, TiN precipitates in the austenitic region and is then annealed in the ferritic region [2]. During long term SOFC operation, titanium oxide forms and provides a keying effect at the interface of the alloy and chromia scale [3]. Titanium oxide also forms at the surface of the chromia scale to reduce chromium volatilization [4]. Analysis of an interconnect alloy, Crofer ® 22 APU, before and after oxidation was performed to show the initial development of titanium precipitate morphology, structure, and diffusion.

3D EDS Applications Using Destructive and Non-Destructive FIB-Based Techniques

The introduction of improved, large area silicon drift detector (SDD) technology for energy dispersive spectrometry (EDS) along with advances in focused ion beam (FIB) techniques provides for a unique opportunity to perform chemical analysis of fine structures in 3-D. The improvements in data collection efficiency, increased count rates and better computing power enables routine collection of 3D EDS datasets for materials science applications.