The Influence of Cr Evaporation on Long Term Cr Depletion Rates in Ferritic Stainless Steels

It is shown that well defined flow rates are an essential parameter in oxidation experiments influenced by scale volatilisation. The 22 % Cr ferritic steel Sanergy HT, intended for use as an SOFC interconnect material, was exposed discontinuously over 3100 h at 850 °C in air + 3 % H2O @ 6000 sml min−1 (27 cm s−1) and stagnant conditions in a tubular reactor. Time resolved isothermal Cr evaporation measurements over 1000 h were also performed in the 6000 sml min−1 case while the stagnant exposure environment was saturated in Cr(VI) species to suppress any evaporation reactions. Mass balances based on oxidation and evaporation data were in good agreement with SEM/EDX bulk Cr concentration measurements. The time to a bulk concentration of 15 wt% Cr increased by ≈2× from ≈3000 to ≈5500 h when evaporation was suppressed. Further, it was established that a suppressed evaporation reaction affected not just the magnitude of Cr depletion in the steel but even its long term depletion rate.