On-line monitoring of oxygen potential and structure of oxide layer in liquid metals by electrochemical methods

Abstract

Lead lithium alloy (Pb-17Li) is one of the candidate tritium breeders of fusion reactors. Lead bismuth eutectic alloy (LBE) is one of the candidate materials for the coolant of fast reactors and the target of accelerator driven systems. The functional layers such as an anti-corrosion layer and a tritium permeation barrier have been developed to be applied in these liquid metal systems. Zirconium oxide (ZrO₂) are considered to be the candidate material of the functional layers due to its excellent chemical stability. The online monitoring of oxygen potential in the liquid metals is essential technology. The development of in-situ analysis technique for the layer properties in the liquid metals at high temperature is important issue. The purpose of the present study is to develop the on-line monitoring system for the oxygen potential and the layer properties both by the solid electrolyte (SE) oxygen sensor and the electrochemical impedance spectroscopy (EIS). The YSZ type SE sensor and the ZrO₂/Zr specimen for the EIS were simultaneously immersed to liquid Pb at 698-898K. The responses of the oxygen sensor and the EIS were successfully obtained, and these signals indicated that the ZrO₂ oxide layer is thermodynamically stable in liquid Pb. The EIS response was evaluated based on some equivalent circuit models as the oxide layer had a lot of horizontal cracks in its structure. The influence of the cracks on the capacitance is larger than that on the electrical resistance, when the covering ratio of the cracks in the layer is low. The influence on the electrical resistant becomes larger when the ratio is close to 100%.