Surface reactivity of liquid metal with oxygen and its relationship with surface tension measurements: a kinetic-fluodynamic model

Abstract

A kinetic and fluodynamic description of processes that may take place near the liquid-vapour interface in the presence of an inert carrier flow containing definite oxygen amounts is presented, in relation to surface tension measurements. The evaluation of the Thiele modulus, φ, and of the parameter ε⁰ (which compares the magnitude of the fluxes of interest to and from the surface), allows definition of the regimes in which fast or slow reactions in the gas phase surrounding a drop of liquid metal are possible in the presence of variable oxygen concentrations. The kinetic features of the process are described in terms of these different reaction regimes, and on the basis of the thermodynamic stability of the possible oxidation products. Diagrams on a plane log φ²/log ε⁰ are drawn, immediately allowing one to know the range of $P_{O_2 }^0$ and T in which the reaction considered is thermodynamically possible, and to foresee the evolution in this field from a kinetic point of view, as a function of $P_{O_2 }^0$ and T. Therefore, the treatment presented here (and applied to selected cases of technological interest) allows a “stability field” for a chosen oxide on the log φ²/log ε⁰ plane to be defined, and the behaviour of the system from a kinetic-fluodynamic point of view to be predicted when the flow parameters and the geometry of the experimental set-up are known. In particular, the reactive gas fluxes at the liquid-vapour interface can be evaluated as a function of the different experimental conditions. Kinetic-fluodynamic diagrams for liquid metal-oxygen systems containing Pb, Sn, Zn, Cu and Al are presented, as well as an application of the model to surface tension measurements on liquid tin, which gives semi-quantitative confirmation of the theory developed here.