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

Control of the dissolved aluminum concentration in a hot dip galvanizing bath is of great importance for producing galvannealed steel sheets. Since aluminum has much greater affinity for oxygen than zinc, the oxygen potential of Zn-Al-O bath may be determined only by the following equilibrium reaction; Al2O3(s)=2Al(in Zn)+3O(in Zn) when the aluminum concentration in the bath is relatively low. Aluminum sensor based on the emf method using the zirconia solid electrolyte has been devised by utilizing the above equilibrium relation.In this paper, emf measurements have been carried out in Zn-Al-O baths having aluminum concentration of 0.02-0.50 mass% under an air atmosphere by using some of the most appropriate combinations of zirconia solid electrolyte and liquid reference electrode for Zn-O baths.From the analysis of such emf values at temperatures between 450-500°C, the following relationships between aluminum concentration and cell emf for two reference electrodes were obtained; E/mV=57.19 ln[Al]/mass%+506.4, 0.02≤[Al]/mass%≤0.30 (In-In2O3 as ref. electrode)E/mV=68.27 ln[Al]/mass%+270.4, 0.05≤[Al]/mass%≤>0.20 (Zn-ZnO as ref. electrode)The external AC voltage applied on these sensors immediately after immersion into the bath showed to be effective to their responsibility and stability. The emf measurements were also carried out in Zn-Mn-O and Zn-Cr-O baths, respectively, and it has been ascertained that the sensors work much better in these baths, even though the partial oxygen pressure PO2 of the bath is as low as 10-40 Pa at 450°C. The posssibility of the present aluminum sensor has been ascertained experimentally.