Effect of Temperature and Water Content on the Oxidation Behaviour and Cr Evaporation of High-Cr Alloys for SOFC Cathode Air Preheaters

Abstract

High temperature alloys are being investigated for use in cathode air pre-heater for solid oxide fuel cell systems because of their high thermal conductivity, formability, manufacturability, and superior mechanical properties. However, it is well-known that high temperature alloys often contain a high concentration of Cr which presents a risk of evaporation and contaminating the cathode of the solid oxide fuel cell (SOFC). The oxidation and Cr2O3 evaporation mechanisms of the alloys including alloy 625, SS309, and alloy 318 have been investigated by varying temperatures and water content of the exposure atmosphere. For the influence of water content, the alloys were isothermally exposed at 850 °C in dry air and air containing 1%, 3% and 9% of H2O at a high flow rate for 168 h. For the influence of temperature, the alloys were isothermally exposed at 650 °C, 750 °C and 850 °C in a 6.0 L/min air stream containing 3% H2O for 168 h. The results of this study show that Cr2O3 evaporation and oxidation rates were dramatically reduced with decreasing temperatures for alloy 625 and SS309. Alloy 318 exhibited a decreased oxidation rate with decreasing temperature, but it demonstrated a reverse trend to the temperature-dependent Cr2O3 evaporation compared to alloy 625 and SS309. The major effect of water vapour on the three tested materials appeared to be the further enhancement of Cr2O3 evaporation.