Effect of Composition, Microstructure and Component Thickness on the Oxidation Behaviour of Laves Phase Strengthened Interconnect Steel for Solid Oxide Fuel Cells (SOFC)

The high-Cr, Laves phase strengthened ferritic steel, Crofer 22 H, has recently been proposed as construction material for interconnects in solid oxide fuel cells (SOFC). Aim of the present investigation was to get more insight in the details of SOFC relevant properties, mostly oxidation behaviour and oxide electronic conductivity, in simulated SOFC service environments as a function of temperature, with main emphasis on themost commonly used operating temperature 800 °C. The present studies of Crofer 22 H, showed that the consumption of chromium from the alloy as a result of chromia surface scale formation ends up in formation of a subsurface layer in which the Laves phase is depleted. In parallel, formation of a thin zone near the scale/steel interface occurs in which the Laves phase is enriched. Modeling revealed that this depletion/enrichment process can be explained by the influence of chromium concentration on the niobium activity in the alloy matrix. Moreover, experiments with solution annealed and pre-aged materials showed that the subsurface phasetransformation processes are at least partially affected by preferential Laves-phase nucleation at the scale/steel interface. Experimental results and theoretical considerations show that the specimen/componentthickness dependence of the oxidation rate is related to relaxation of oxide growth stress by creep of the metallic substrate. Due to poor intrinsic creep resistance, thin Crofer 22 APU specimens exhibit enhanced oxidation rates in air at 800 °C. For the more creep resistant steel Crofer 22 H this only occurs after Laves phase coarsening which results in a substantial decrease in intrinsic creep strength of the alloy. The resistance of thethinnest samples against substantial creep deformation then becomes gradually too small to prevent substantial relaxation of oxide growth stress, leading to enhanced oxidation rates. However, in anode gas hardly any specimen/component thickness dependence of the oxidation rate was found, which is believed to be related to a different oxide scale composition and microstructure