The effect of Mn_(1.5)Co_(1.5)O_4 coatings on the development of near surface microstructure for Haynes 230 oxidized at 800 °C in air

摘要

The effect of reactively sintered Mn_(1.5)Co_(1.5)O_4 (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~(?9) mg~2cm~(?4) s~(?1), and forms a two-layer oxide scale with a continuous chromia layer and a thinner discontinuous MnCr_2O_4 overlayer. For the MCO-coated H230, the reduction step of the reactive sintering process converts the MCO coating to a mixture of Co and MnOwith a thin Cr-rich oxide layer at the interface with the alloy substrate. Following the re-oxidation step, there is a 200 nm chromia layer and a 400 nm cubic spinel reaction layer (RL) between the alloy and the MCO. These layers thicken to 800 nm and 1.2 μm, respectively after 1000 h 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.