Effects of Manganese and Chromium on the Oxygen Potential in Liquid Zinc Bath

Control of the free 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 small. Aluminum sensor based on the emf method using the zirconia solid electrolyte has been devised by utilizing the above equilibrium relation.In this paper, pure manganese or chromium has been employed as a deoxidizer for molten zinc and the effect of these additives on the oxygen potential in molten zinc has been investigated by emf measurements using the cells below.(-) W|O in (Zn-Me) |ZrO2(+8mol%Y2O3) In, In2O3|W(+) Me; Mn or CrThe above cells responded quickly and showed stable emfs with fairly good reproducibility. From the analysis of such emf values at temperatures between 450650°C, the following relationships between additive concentration and cell emf for two cells at 450°C were obtained;E/mV=34.5 ln[Mn]/mass%+380.1, 0.03≤[Mn]/mass%≤0.99E/mV=11.0 ln[Cr]/mass%+408.7, 0.0007≤[Cr]/mass%≤0.276Also, the analysis of above results suggests that the emf method using zirconia solid electrolyte is applicable down to Po2/Pa=10-40 at 450°C. It has been confirmed by calculating the value of P_??_ that the portion of electronic conduction in the above zirconia solid electrolyte was negligibly small.