Controlled Growth of α-Al 2 O 3 Nanofilm on FeCrAl Alloy as an Effective Cr Barrier for Solid Oxide Fuel Cell (SOFC) Cathode Air Pre-Heaters

Solid oxide fuel cell (SOFC) systems often employ metallic cathode air pre-heaters (CAPHs), frequently made from alloys with high chromium (Cr) content, to recover thermal energy from exhaust gases and pre-heat incoming air and fuel. Cr evaporation from metallic CAPHs can poison SOFC cathodes, reducing their durability. To mitigate this, we investigated controlled pre-oxidation of a FeCrAl alloy (alloy 318) to form a protective alumina scale by self-growing, assessing its impact on and oxidation resistance and Cr retention capability for CAPH applications. The effects of pre-oxidation were investigated across a temperature range of 800 to 1100 °C and dwelling times of 0.5 to 4 h. The formed oxide scales were characterised using gravimetry in combination with advanced analytic techniques, such as SEM/EDX, STEM/EDX, TEM, and XRD. Subsequently, the pre-oxidised FeCrAl alloys were characterised with respect to the oxidation rate and Cr 2 O 3 evaporation in a tubular furnace at 850 °C, with 6.0 L/min air flow and 3 vol% H 2 O to simulate the SOFC cathode environment. TEM analysis confirmed that the FeCrAl alloys formed alumina scales with 10 nm and 34 nm thickness after 1 h of pre-oxidation at 900 and 1100 °C, respectively. The corrosion and Cr 2 O 3 evaporation rates of the FeCrAl alloy at 850 °C in humidified air were shown to be dramatically decreased by pre-oxidation. It was found that the mechanisms of oxidation and Cr 2 O 3 evaporation were found to be controlled by the formation of different alumina phases during the pre-oxidation. Measurements of Cr 2 O 3 evaporation and weight gain revealed that the alloy 318 pre-treated at 1100 °C for 1 h will form an α-Al 2 O 3 scale, leading to a 98% reduction of the oxidation rate and 90% reduction of Cr 2 O 3 evaporation compared to the non-oxidised alloy 318 under simulated SOFC cathode conditions.