Effect of Alloy Composition on the Oxidation Behaviour and Cr Vaporisation of High-Cr Steels for SOFC Cathode Air Preheater
Authors: Zhang Kun , Hong Jong-Eun , Steinberger-Wilckens Robert 
Published: February 2020

Solid oxide fuel cell (SOFC) stacks require the inlet air to be pre-heated prior to entry into the fuel cell and produce exhaust gas containing a high level of thermal energy which can be recovered and transferred elsewhere within the system. The Cathode Air Preheater (CAPH) functions as a heat exchanger, recovering heat from the exhaust gas to heat air before it enters the fuel cell. The latest generation of the CAPH design uses an advanced alloy containing Cr to provide improved oxidation resistance under isothermal and cyclic conditions and provide good high temperature strength. However, reactive evaporation of Cr-species from high-Cr steels is a technical challenge for SOFC systems and other devices operating in high-temperature (>600 °C) oxidising environments. The vaporisation of Cr-rich volatile species from stainless steel materials is a major source of degradation that limits the lifetime of SOFC systems with metallic CAPH. For a more systematic approach, it is crucial to measure the oxidation rate and quantify the amount of evaporated Cr-species of coated and uncoated stainless steel materials.
This study compares the oxidation performance of the aluminium containing Aluchrom 418 alloy and the nickel based Inconel 625 alloy at high temperature. Furthermore, the oxidation behaviour of uncoated stainless steel 309 is also compared with PVD aluminised version of same material. The materials have been researched with respect to oxidation rate and Cr evaporation. High temperature oxidation measurements have been implemented both isothermal and discontinuously in tubular furnaces at 850 °C, with 6 L/min airflow and 3% H2O to simulate the cathode atmosphere in SOFC. In addition to the oxidation test, the Cr evaporation tests have been performed by applying a developed denuder technique, which allows time-resolved and accurate quantification of Cr evaporation from alloys at high temperature. Chemical analysis of the formed oxide scales are subsequently studied by XRD and SEM/EDX. SEM results indicate that all the materials produce an oxide scale with an inner layer of Cr2O3 and a spinel layer on top. EDX analysis reveals that transitional alumina layer is formed for the Aluchrom 418 and the aluminised stainless steel 309. The Cr evaporation tests show that the aluminium containing alloys effectively decrease Cr evaporation by forming a stable and protective Al2O3 scale. Furthermore, the quantity of Cr evaporated from alloys increases with operating temperature and humidity. High temperature mechanical properties of these materials for application of CAPH in SOFC have been investigated with the tensile and creep tests. Tensile properties are evaluated at temperature of 850 °C. Creep properties are investigated by constant-load measurements at 850 °C. The nickle based 625 alloy shows the highest mechanical strength among all these materials, which could be ascribed to precipitation strengthening effect of the Laves phase with the addition of Nb and Ti elements.