Behaviour of various glass–ceramic sealants with ferritic steels under simulated SOFC stack conditions

The suitability of various combinations of glass–ceramic sealants with high-chromium ferritic steels under conditions simulating SOFC stacks has been evaluated, i.e. three glass–ceramic sealants and seven types of ferritic steels. The test method used is based on test samples consisting of two metallic sheets, joined together with a glass–ceramic sealant. The outer side of the specimens was exposed to air for 400 h at 800 °C, whereas the inner side was exposed to hydrogen saturated with 3 vol.% water vapour. In particular, attention is paid to the influence of small amounts of additives to both the glass–ceramic sealant and the steel on the electrical and chemical behaviour of the specimens. Under experimental conditions simulating SOFC stacks, it appeared that excessive internal Cr oxidation of the ferritic steels, sometimes accompanied by external Fe-oxide formation, only occurred in the case of glass–ceramics containing minor amounts of PbO. This internal oxidation finally resulted in a volume change of the ferritic steel, which was manifested in bulging of the steel. As a consequence, the glass–ceramic was pushed away from the steel surface and crack formation at the glass–ceramic–steel interface occurred. The rate of corrosion attack strongly depended on the detailed steel composition. Increasing Si content apparently increased the rate of the corrosion attack, and thus possibly decreasing the time for the occurrence of short-circuiting.