Development of electrochemical sensing techniques for the determination of activity-composition relations in liquid alloys and slags at 1873 K

Abstract

A plug-type oxygen probe containing a magnesia stabilized solid zirconia electrolyte rod, was developed. This oxygen sensor was evaluated in the Fe-Cr-0 system at 1873 K by determining the Raoultian chromium and Henrian oxygen activities of an Fe-Cr alloy which was in equilibrium with pure Cr20 3 and a Cr20rsaturated liquid slag respectively. However, the slag reacted with the oxygen probe and therefore, a crucible assembly was developed to prevent direct contact between the slag and the oxygen sensor. The results obtained with this crucible assembly proved that no interaction between the oxygen probe and slag occurred and that the smaller slag/metal interface did not effect equilibrium between the slag and the metal. The Raoultian chromium activities of the Fe-Cr alloy containing 5 to 25% chromium determined in this study, deviate negatively from ideal Raoultian behaviour. The oxygen and chromium activities were used to determine the standard free energy change for the formation of solid Cr20 3 from the elements at 1873 K, and the value obtained is in good agreement with existing data. Evidently, the plug-type oxygen probes and the redesigned crucible assembly can be used to determine the oxygen activity of an Fe-Cr alloy, in equilibrium with a liquid slag at 1873 K, accurately and reliably. Therefore, these measurements were extended to the determination of the activity of MnO in (MgO-Si~-MnO)-slags. The chemical composition of the equilibrium phases pertaining in (Mg0-Si0Si02-MnO)slags saturated with (MnO-MgO) solid solutions and in contact with manganese metal, were determined by microprobe analyses. These results together with the known activity-composition relations of (MnO-MgO) solid solutions at 1873 K, were used to determine the activity of MnO in the slag. This method proved to be reliable and the MnO activities so obtained, could serve as a check for activities determined with the electrochemical technique. Furthermore, the same method could be used for the determination of the high temperature phase constitution of the slag when the oxygen potential of the slag/metal system is determined by electrochemical means. In the course of the determination of the oxygen potential of (MgO-SiOz-MnO)-slags at 1873 K by means of electrochemical oxygen probes, the magnesia stabilized zirconia solid electrolytes reacted with manganese metal and vapour which had a detrimental influence on the accuracy and reliability of the EMF (electromotive force) measurements. Accordingly, it was illustrated that zirconia solid electrolytes are not suitable for application in slag/metal systems containing manganese metal at 1873 K. Previous workers have shown that thoria solid electrolytes are not attacked by manganese metal at these high temperatures and should consequently be used with the crucible assembly developed in this study, for the determination of activities in (Mg0-Si02-Mn0)-slags at 1873 K.