The effect of reactively sintered Mn1.5Co1.5O4 (MCO) coatings on the development of surface microstructure for Haynes 230 (H230) oxidized in air at 800°C has been studied using a combination of thermo-gravimetric analysis and electron microscopy techniques. The bare alloy exhibits a parabolic rate constant of 8.8×10−9mg2cm−4s−1, and forms a two-layer oxide scale with a continuous chromia layer and a thinner discontinuous MnCr2O4 overlayer. For the MCO-coated H230, the reduction step of the reactive sintering process converts the MCO coating to a mixture of Co and MnO with a thin Cr-rich oxide layer at the interface with the alloy substrate. Following the re-oxidation step, there is a 200nm chromia layer and a 400nm cubic spinel reaction layer (RL) between the alloy and the MCO. These layers thicken to 800nm and 1.2μm, respectively after 1000h oxidation. These observations are compared to our previous studies of MCO-coated Crofer 22 APU, and the implications for long-term SOFC performance are discussed.

The effect of alloy heat treatment on the oxidation kinetics and oxide scale microstructure of Crofer 22 APU has been studied. Parabolic oxidation rate constants were measured for the as-received alloy and after pre-oxidation heat treatment in argon at 1050 degrees C for 1 and 4 h. The oxide scale microstructure was investigated using scanning electron microscopy, focused ion beam milling and transmission electron microscopy. It was found that the alloy forms a two-layer scale with a continuous chromia layer and a discontinuous MnCr2O4 overlayer. Two forms of internal oxides were also formed: subscale pockets of spinet and isolated TiOx precipitates in the underlying alloy. The pre-oxidation heat treatment had a profound effect on the grain size and morphology of the Cr2O3 and MnCr2O4 layers in the scale. The heat-treated samples exhibit a 3.5x lower parabolic oxidation rate constant than the as-received Crofer 22 APU. This improvement in oxidation resistance is attributed to the dramatic differences in the morphology of the oxide scale that forms during the earliest stages of oxidation (<5 h). The implications of these findings for oxidation mechanisms and long-term SOFC performance are discussed. (C) 2013 United Technologies Corporation and Elsevier B.V. All rights reserved.

The microstructural development of Mn1.5Co1.5O4-coated Crofer22 APU has been studied using cross-sectional transmission electron microscopy. Alloy samples were coated via a slurry process involving consolidation by reduction and re-oxidation, and these samples were then oxidized at 800 degrees C for times of up to 1000 h. All samples exhibited a thin chromia scale at the alloy/coating interface plus spinel phases as a reaction layer between the chromia and the manganese cobaltite coating. The oxidized samples also exhibited pockets of stoichiometric MnCr2O4 spinel at the chromia/alloy interface and internal Ti-rich oxides in the alloy below the chromia. The reaction layer spinels exhibit remarkable changes in thickness, morphology and composition, and these effects are explained on the basis of changes in the diffusive fluxes during the different stages of coating application and subsequent exposure. The possible consequences of these observations for the degradation mechanisms that could affect SOFC interconnects produced from MCO-coated Crofer22 APU are discussed. (C) 2012 United Technologies Corporation. Published by Elsevier B.V. All rights reserved.

Interaction of current collectors with coated interconnect alloys was examined using dual-coating specimens. Oxidation kinetics of (La,Sr)(Co,Fe)O3 (LSCF)/Mn1.5Co1.5O4 (MCO) and (La,Sr)(Mn)O3 (LSM)/MCO coated Haynes 230 (H230) was determined at 750°C in air via a gravimetric method. The LSCF/MCO dual coating samples exhibited a rate constant 2 times larger than that of the LSM/MCO coated H230. SrCrO4 that formed at the LSCF/MCO coated H230 interface contributed to the increased rate constant. No SrCrO4 was detected at the interface of the LSM/MCO coated H230. Chemical stability of LSCF in air at 750°C was confirmed by TGA. Area specific resistance (ASR) of MCO coated Haynes 230 (H230) and Crofer 22 APU with LSCF and LSM (coated H230 only) current collectors was measured at 800°C for 1100h. No gross difference in ASR evolution was observed between the LSCF/MCO-H230 and LSM/MCO-H230 assemblies. Activation energy of conduction of the LSCF/MCO-H230 and LSCF/MCO-Crofer 22 samples evolved from 0.63eV to 0.80eV and from 0.53eV to 0.63eV respectively over a period of ca. 1100h, suggesting somewhat different interface chemistry evolution. Sr transport was observed in the LSCF/MCO-H230 samples subjected to both exposure test and ASR measurement, whereas it was not detected on the LSCF/MCO-Crofer 22 sample. Mechanism of the Sr transport is discussed on a qualitative basis to account for the observations.

The oxidation behavior of Mn1.5Co1.5O4 (MCO)-coated Haynes 230 (H230) and Crofer 22 APU was investigated between 700 and 900 degrees C. The oxidation kinetics of the coated alloys was compared with that of base alloys at 800 degrees 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. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3391820] All rights reserved.