Oxygen fluxes and the measurement of the surface tension of liquid metals

The surface tension of liquid metals is very sensitive to impurities and, in particular, is often strongly dependent on the oxygen concentration in the liquid phase. In this paper the problem is examined with reference to unsteady conditions: a physicochemical description in terms of rate phenomena is presented, which takes into account diffusion and reaction steps controlling the gas-metal exchanges. The evaluation of the Thiele modulus, and of the parameter ?°, allows the definition of the regimes in which fast or slow reactions in the gas phase are possible in the presence of different oxygen concentrations. The definition of such regimes is in turn necessary in order to evaluate the oxygen concentration near the surface of the liquid metal, and the effective oxygen supply to the liquid phase. Results of these calculations are presented for different metals under well-defined flow conditions, and discussed on the basis of experimental results. In particular, it is shown that surface tension values in high-temperature metallic systems often evolve very slowly with time.