OXYGEN MASS-TRANSFER AT LIQUID-METAL-VAPOR INTERFACES UNDER A LOW TOTAL PRESSURE

The problem of oxygen exchange at the interface between a gas and a li quid metal is treated for systems under a ''vacuum'' (Knudsen regime, pressures lower than 1 Pa), where, due to the large mean free path of gas molecules in a vacuum, transport processes in the gas phase have n o influence on the total interphase mass exchange, which is controlled by interface phenomena and by oxygen partition equilibrium inside the liquid. Owing to the double contribution of molecular O-2 and volatil e oxides to the oxygen flux from the surface, non-equilibrium steady-s tate conditions can be established, in which no variations in the comp osition of the two phases occur with time, as the result of opposite o xygen exchanges. The total oxygen and metal evaporation rates are eval uated as a function of the overall thermodynamic driving forces, and a n account of the transport kinetics is given by using appropriate coef ficients. A steady-state saturation degree s(r), is defined which rela tes the oxygen activity in the liquid metal to the O-2 pressure impose d and to the vapour pressures of volatile oxides. When metals able to form volatile oxides are considered, pressures of molecular O-2 higher than those defined under equilibrium conditions have to be imposed in the experimental set-up in order to obtain a certain saturation degre e, as a consequence of the condensation of the oxide vapours on the re actor walls. Effective oxidation parameters are determined, which defi ne the conditions leading the liquid to a definite steady-state compos ition under a ''vacuum'' when it is out of equilibrium. The effective value of the oxygen pressure which corresponds to the complete oxygen saturation of the metal, P-O2.s(E), is evaluated at different temperat ures for the systems Si-O and Al-O. The results are represented as cur ves of IgP(O2.s)(E), against T, which separate different oxidation reg imes; these results agree well with the experimental data found in the literature.