Interfacial electrical resistivity, oxidation and thermomechanical properties of X24 ferritic stainless steel for SOFC interconnects

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Microhardness, tensile and yield strength, thermal expansion, surface oxidation and interfacial electrical resistivity of a novel ferritic chromium steel containing 24.6% Cr (X24) were appraised with respect to commercial Crofer 22 APU, one of most common materials used for the solid oxide fuel cell (SOFC) interconnects. Testing of the mechanical properties at room temperature and 850 °C revealed no significant differences between these steels, as for their thermal expansion measured by dilatometry. At the same time, X24 possesses a better formability advantageous for machinability and resistance to thermal cycling. Higher Cr and La concentrations in X24 (24.6% and 0.39%, respectively, vs. 22% and 0.08% in Crofer 22 APU) promote the uniform formation of strongly adherent oxide scales. In contact with lanthanum-strontium manganite (LSM) cathodes under a constant current load of 0.5 A/cm²at 850°C and atmospheric oxygen pressure, the formed scales consist of two layers, namely inner Cr₂O₃ and outer (Mn,Cr)₃O₄. The resultant increase in the area-specific resistance (ASR) can be suppressed applying protective nickel coatings. After 7,900 hours of testing under the SOFC cathode chamber conditions, the achieved ASR values were as low as 5–10 mOhm cm². The protective effects are associated with the complete transformation of metallic Ni coating into a mixture of high-conductivity spinel phases (NiFe₂O₄, MnFe₂O₄, Fe₃O₄), whereas chromium remains beneath the conductive interfacial layer in the form of (Cr,Mn)₃O₄.

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