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 Mn 1?5 Co 1?5 O 4 coated Crofer 22 APU were studied using cross-sectional scanning and transmission electron microscopies. Alloy samples were coated with a Mn 1?5 Co 1?5 O 4 slurry and then reduced in moist H 2 based forming gas at 850uC 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 MnCr 2 O 4 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.

Effect of coating density on oxidation resistance and Cr vaporization from solid oxide fuel cell interconnects

“…The microstructural development of the coatings from deposition to long term aging at 800 °C in air is summarized schematically in Figure 6. Gambino et al [30] showed that during heat treatment of Mn Based on results in the current study, it is argued that the formation of (Mn,Cr) 3 O 4 is a prerequisite for the formation of the Cr-rich reaction layer. Gambino et al [30] proposed that Cr 2 O 3 is formed first on…”

“…The first step is heat treatment in N 2 +H 2 (or equivalent reducing atmosphere) where the spinel oxide decomposes into MnO and Co [29]. The second step is heat treatment in air where the spinel phase is re-formed and densification is promoted by a reactionsintering type mechanism [30,31]. The two-step sintering procedure has in some assessments been estimated to make up approximately 20 % of the total interconnect coating costs [32,33].…”

“…Simultaneously, a ca. 3 µm thick Cr 2 O 3 + (Mn,Cr) 3 O 4 scale forms at the coating/alloy interface[30]. After a re-oxidation heat treatment, a reaction layer comprising Mn, Cr, Co and Fe is detected between the oxide scale and the re-oxidized coating[51].…”

Comparison of iron and copper doped manganese cobalt spinel oxides as protective coatings for solid oxide fuel cell interconnects

“…The partial densification of the MCCu coating can be explained by Fe diffusion from the alloy into the coating leading to a volume expansion. Lower quantities of Fe (up to 5 wt.%) have previously been observed in (Mn,Co) 3 O 4 coatings after sintering and were attributed to rapid diffusion of Fe in the Co metal that forms when the coating is reduced during the first heat treatment step in N 2 -H 2 [32,33]. Here, Cu in the MCCu coating will also be reduced to its metallic state during heat treatment in N 2 -H 2 .…”

“…The average reaction layer thickness was determined to 5.1±1.5 µm. The uneven thickness may be a result of local differences in composition after the reduction heat treatment procedure, during which the coating first is reduced to MnO and Co, and then re-oxidized to MnCo 2 O 4 [32,33].…”