2007 Volume 47 Issue 5 Pages 689-698
The deoxidation of molten steel has been studied by employing an electrochemical cell which utilizes a solid-oxide electrolyte. This technique enables deoxidation of liquid steels without leaving deoxidation products in the steel. Y2O3-stabilized ZrO2 (YSZ) is used as an oxygen-permeable membrane due to its reasonably high mixed-ionic and electronic conductivity and thermal stability at elevated temperature in low oxygen partial pressure (PO2). In order to investigate the rate-determining steps involved in this electrochemical deoxidation process, the oxygen-permeation experiment using a YSZ disk is performed to measure the oxygen flux as a function of PO2 and temperature. The measured oxygen flux is about one order of magnitude smaller than the estimated value from Wagner equation that describes permeation under bulk-diffusion limit. Thus, oxygen permeation is mostly controlled by surface-exchange kinetics even at high temperature (~1600°C) in reducing atmosphere. In the experiment of deoxidation of molten steel using a YSZ tube, the deoxidation rate is largely dependent on the surface-exchange kinetics as well. The deoxidation rate is much smaller than that calculated under the assumption of bulk-diffusion limit. Based on these results, the surface modification of YSZ is investigated as a way to enhance the deoxidation kinetics. Both surfaces of the membrane are coated with porous YSZ (Zr0.84Y0.16O2-δ), GDC (Ce0.8Gd0.2O2-δ) and LSC (La0.7Sr0.3CrO3-δ) during oxygen permeation experiment in the gas phase. During the deoxidation experiment in the steel melt, only the permeate side is coated with Pt, Mo-YSZ cermet and LSC. The oxygen permeation rate of YSZ drastically increased with coating. The increase also depended on the coating materials. Thus, it is shown that the oxygen permeation through the membrane is mostly controlled by the surface-exchange kinetics. Similar trend can be seen in the deoxidation experiment, confirming the surface-exchange kinetics limit.
