Abstract
Molten magnesium oxidizes rapidly when exposed to air causing melt loss and handling difficulties. The use of certain additive gases such as SF6, SO2, and CO2 to form a protective MgO layer over a magnesium melt has been proposed. The oxidation behavior of molten magnesium in air containing various concentrations of SF6 was investigated. Measurements of the kinetics of the oxide layer growth at various SF6 concentrations in air and temperatures were made. Experiments were performed using a thermogravimetric analysis unit in the temperature range of 943 K to 1043 K (670 °C to 770 °C). Results showed that a thin, coherent, and protective MgF2 layer was formed under SF6/Air mixtures, with a thickness ranging from 300 nm to 3 μm depending on SF6 concentration, temperature, and exposure time. Rate parameters were calculated and a model for the process was developed. The morphology and composition of the surface films were studied using scanning electron microscope and energy-dispersive spectroscope.
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Acknowledgments
This work was supported in part by Research Institute of industrial Science and Technology (RIST), Pohang, Korea 790–600.
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Manuscript submitted November 11, 2013.
Appendix: Evaluation of SF6 Diffusion through the MgO Layer
Appendix: Evaluation of SF6 Diffusion through the MgO Layer
Let us assume that diffusion of SF6 through the MgO layer was the rate-controlling step. In this case, as soon as SF6 reach the Mg surface, Reaction [2] would take place rapidly. The flux of SF6 diffusion is given by the following Fick’s law:
The diffusion flux is then related to the growth rate of magnesium fluoride layer by the following relationship:
where \( L_{{{\text{MgF}}_{2} }} \) is the thickness of the protective layer, \( t \) is the reaction time, \( D \) is the diffusion coefficient, \( C_{{{\text{SF}}_{6,b} }} \) is the bulk molar concentration of SF6 at the gas/oxide interface, and \( \rho_{{{\text{MgF}}_{2} }} \) is the molar density of MgF2. The effective diffusivity of SF6 in large excess air can be estimated by Reference 15:
where ε is the porosity of the MgO layer. In this work it was calculated to be ~0.65 to 0.70. A much more accurate value of diffusivity is not warranted because we are verifying the fact that the diffusion rate is much greater than the observed reaction rate. When the MgF2 layer thickness increased from 1.31 to 1.83 μm in 4 minutes of exposure to 2 pct SF6 in air at 993 K (720 °C), the growth rate was calculated to be ~2 × 10−7 cm/s. However, a growth rate of ~1.5 × 10−3 cm/s was calculated using Eq. [A2] using the effective diffusivity value computed from Eq. [A3]. The growth rate under diffusion control would be orders of magnitude faster than the experimental growth rate, indicating that the chemical reaction of forming the MgF2 layer was rate controlling and SF6 diffusion through the porous MgO layer did not affect the overall rate.
Comparison was made for the fastest rate of MgF2 layer growth determined in this work to ensure the validity of this conclusion. The loose and porous structure of magnesium oxide observed under a microscope also supports this conclusion.
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Emami, S., Sohn, H.Y. & Kim, H.G. Formation and Evaluation of Protective Layer Over Magnesium Melt Under SF6/Air Atmospheres. Metall Mater Trans B 45, 1370–1379 (2014). https://doi.org/10.1007/s11663-014-0059-2
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DOI: https://doi.org/10.1007/s11663-014-0059-2