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

Abstract

Stainless steel is being investigated for use in cathode air pre-heater for solid oxide fuel cell systems because of its high thermal conductivity, formability, manufacturability, and superior mechanical properties. However, high temperature steels 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 stainless steels including Inconel 625, SS309, and AluChrom 318 have been investigated by varying temperatures and water content of the exposure atmosphere. For the influence of water content, samples of Inconel 625, SS309 and AluChrom 318 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 hours. For the influence of temperature, samples of Inconel 625, SS309, and AluChrom 318 were isothermally exposed at 650°C, 750°C and 850°C in a 6.0 L/min air stream containing 3% H2O for 168 hours. The results of this study show that Cr2O3 evaporation and oxidation rates were dramatically reduced with decreasing temperature for Inconel 625 and SS309. AluChrom 318 exhibited a decreased oxidation rate with decreasing temperature, but it demonstrated a reverse trend to the temperature dependent Cr2O3 evaporation compared to Inconel 625 and SS309. The major effect of water vapour on the three tested materials appeared to be the further enhancement of Cr2O3 evaporation.