Oxidation Behaviour and Phase Transformations of an Interconnect Material in Simulated Anode Environment of Intermediate Temperature Solid Oxide Fuel Cells

The oxidation behaviour and the phase transformations associated with high temperature exposure of a commercial ferritic interconnect steel, Crofer 22 H, was studied in a simulated solid oxide fuel cell (SOFC) anode atmosphere at 700 °C. Special emphasis was placed on the formation of the intermetallic sigma phase. No sigma phase was detected in the bulk alloy after 500 h of exposure of bare specimens. However, specimens which were pre-coated with a layer of nickel showed formation of an interdiffusion zone after as little as 2 h of exposure and sigma phase formation occurred after 10 h. The presence of the nickel layer, which simulates the contact between ferritic steel interconnects and a nickel mesh in a SOFC results in the formation of an austenitic zone and accelerated formation of a σ-phase rich layer at the ferrite/austenite interface. The ferritic steel is transformed into austenite due to the inward diffusion of nickel, σ-phase started to nucleate at the transformed austenite grain boundaries. The nucleation is enhanced by an increased Cr/Fe-ratio at that interface due to more pronounced diffusion of Fe, compared to Cr, in the direction of the Ni-layer. Different possible mechanisms for the nucleation and growth of σ-phase were identified. The experimental results led to the conclusion that sigma nucleates in the austenite and grows following an isothermal eutectoid-like decomposition