Corrosion mechanism of borosilicate sealing glasses in molten carbonates studied by impedance spectroscopy

The commercial development of molten carbonate fuel cells (MCFC) implies a demand for new sealing materials to ensure the tightness of the cell. The optimisation of the sealing conditions and the sealing glass used requires the knowledge of the corrosion mechanisms and their influence on the stability of the metal–seal–ceramic junction. The impedance of a glass-coated (60SiO2·24B2O3·16Na2O mol%) Pt rod was measured after immersion into a 62.5 Li2CO3+37.5 K2CO3 (mol%) eutectic mixture at 650 °C, the working temperature of the MCFC. The frequencies applied ranged from 1 Hz to 500 kHz, the ac amplitude was 10 mV and the dc potential was 0 V. Since the eutectic melt of the carbonates has a very high conductivity, the resistance of the glass is expected to be orders of magnitude higher than that of the melt. Both resistances are in series, therefore, the measured resistance is caused only by the resistance of the glass coating, and so changes in the resistance will be caused by changes in the thickness of the coating. On the other hand, a ln(Δl (the thickness of the corroded material)) versus ln(t (time)) plot allows conclusions to be drawn on the corrosion mechanism taking place. The slope of this plot tends progressively to 0 with time because of the formation of a corroded layer mainly composed by crystalline Li2SiO3, which prevents further corrosion. This investigation will prove and extend a corrosion model obtained by mass loss measurements and chemical analysis within a short time and in-situ.