Long-Term Oxidation Behavior of SOFC Interconnects Alloy ZMG232G10 for 40,000 Hours

Abstract

Solid Oxide Fuel Cell (SOFC) is known as clean and high efficiency co-generation systems. SOFC operates at high temperatures (600 – 900°C), therefore, heat resistant materials are used for SOFC components. For SOFC interconnects that connect between ceramic cells electrically, the required properties are as follows; good oxidation resistance at operating temperature, high electrical conductivity at operating temperature and thermal expansion close to ceramic cells.There are many research works about long term degradation behavior of anode, cathode and electrolyte materials, because the lifetime of SOFC is expected from 40,000 hours (about 5 years) to 90,000 hours (about 10 years). On the other hand, degradation behavior of metal interconnects, that means oxidation behavior of metal interconnects, has not been evaluated sufficiently despite relevance to ohmic loss of SOFC stack.Fe-Cr ferritic alloy for SOFC interconnects “ZMGTM232G10” was developed and its long term oxidation resistance has been evaluated. At this time, we will report 40,000 hours’ oxidation test result about ZMG232G10, for example the changes of oxidation weight gain and microstructure.The alloy was melted with 10kg lab-scale vacuum melting furnace, and then hot forged to bar shape. After that, forged material was cut and cold rolled to sheets with some target thicknesses. Sample sizes cut from sheets are 10mm square with 3mm thickness and 15mm square with 0.3 and 0.5mm thicknesses. The surfaces of samples were ground with #1,000 dry paper.Oxidation test was performed at 850°C in air, which is an accelerated condition compared to 750°C. The test pieces were put in the ceramic containers, and put into electric furnaces. The oxidation weight gain of each test piece was measured after each 500 hours’ oxidation test period. After the measurement, some specimens were cut, plated with Ni, mounted in the resins, ground and polished, and then the cross-sectional microstructures were observed with optical microscope and EPMA and analyzed with EPMA.Figure1 shows the oxidation weight gain of ZMG232G10 vs. exposure time at 850°C in air. In general, oxidation behavior of the alloy with dense oxidation layer is explained with parabolic law. ZMG232G10 with 3mm thickness (bulk sample) showed stable oxidation behavior for 40,000 hours according to parabolic law. However, oxidation behavior of thinner samples deviated from parabolic law, and anomalous oxidation was observed at the edge of the samples.Microstructure of oxide layer of ZMG232G10 consisted of (Mn, Cr, Cu) spinel layer and Cr2O3 layer located beneath spinel layer. Ferrite phase and oxide layer were observed in almost all samples, however martensitic transformation under the oxide layer was observed in thinner sample after long term oxidation test. It indicated that ferrite phase transformed to austenite phase during long-term oxidation test and then austenite phase transformed to martensite during cooling to room temperature. This is the reason why martensite was observed at room temperature.Because the protective oxide layer of ZMG232G10 is mainly Cr2O3, Cr are consumed gradually from the alloy matrix during high temperature oxidation. Then, Cr contents in the alloy matrix were analyzed with EPMA.ZMG232G10 with 3mm thickness contained Cr amount enough to keep protective oxide layer after 40,000 hours oxidation test. However Cr amount in thin samples decreased faster with exposure time than thick one. It was difficult to keep protective oxide layer dense because of less total Cr amount and decreasing Cr in the alloy matrix. As a result, faster oxidation and earlier anomalous oxidation occurred in thin samples than thick one. Furthermore, low Cr content was not able to keep ferrite phase stable and then caused transformation from ferrite phase to austenite phase at high temperature.Through long term oxidation test on ZMG232G10, we obtained the knowledge as follows ; (1) although bulk sample of ZMG232G10 shows stable oxidation behavior according to parabolic law, thin ones show faster oxidation and earlier anomalous oxidation than thick one. (2) In thin samples, ferrite phase may transform to austenite phase in long term oxidation. Both phenomena were caused by less total Cr amount in thin samples and rapid decrease of Cr at high temperature for long term oxidation than thick one. Figure 1