Determination of Aluminum Concentration in Molten Zinc by E.M.F. Method Using Calcium Fluoride Solid Electrolyte

摘要

Calcium fluoride solid electrolyte has been extensively used in galvanic cells for the measurement of fluorine potential to obtain the activity of alloys and compounds, because the ionic transference number of the electrolyte is about unity even at P_(F2)/P~°= 10~(-62) above 550 deg C. So far there were no literature data available for the measurement below 500 deg C by use of the calcium fluoride solid electrolyte. The purpose of this work is to develop the sensors for the control of aluminum concentration in a hot dip galvanizing bath, which is of industrial importance for producing high quality galvannealed steel sheets. In this report, a trial has been made to measure the fluorine potential in a molten zinc bath artificially equilibrated with ZnF_2 at the temperatures between 450 approx 500 deg C using CaF_2 cells with some kinds of reference electrode. By use of Bi, BiF_3 or Zn, ZnF_2 as the reference electrode, the cells respond quickly and stable emf values which agree fairly well with the literature data are obtained. Subsequently, a trial has been made to measure the fluorine potential in molten Zn-Al baths artificially equilibrated with A1F_3 at the temperatures between 450 approx 500 deg C using CaF_2 cells with the above reference electrodes. The initial response of these cells after immersion of the cells into these baths is relatively slow. However, once emf of the cell becomes stable, each cell responds quickly to the aluminum concentration change. The response time is not longer than 5 min. From the analysis of such emf measurements, the emf values are found to increase linearly with the logarithm of the aluminum concentration. The possibility of the present method has been ascertained in Zn-Al baths.