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The oxidation behavior of Mn1.5Co1.5normalO4 (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.

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.

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