Formation and Evaluation of Protective Layer over Magnesium Melt Under CO₂/Air Mixtures

Abstract

Molten magnesium oxidizes rapidly when exposed to air causing melt loss and handling difficulties. The use of certain additive gases to form a protective MgO layer over a magnesium melt has been proposed. The oxidation behavior of molten magnesium in air that contains various concentrations of CO₂ was investigated, including the kinetics of the oxide layer growth. 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 MgO-C layer was formed under the test conditions. The thicknesses of this layer formed under CO₂/air ranged from 500 nm to 12 μm. Rate parameters were calculated and a model for the process was developed. The morphology and composition of the surface films were studied using SEM and EDS.

Appendix A

Evaluation of CO₂ Diffusion Through the MgO Layer

Let us assume that the diffusion of CO₂ through the non-protective MgO layer was the rate-controlling step. In this case, as soon as CO₂ reached the Mg surface, Reaction [2] would take place rapidly. The rate of CO₂ diffusion is given by the following Fick’s law:

$$ N_{{{\text{CO}}_{2} }} = - D\frac{{C_{{{\text{CO}}_{2} ,b}} }}{{L_{\text{MgO}} }}. $$

(A1)

The diffusion rate is then related to the growth rate of carbon layer by the following relationship:

$$ \rho_{{{\text{MgO}}/{\text{C}}}} \cdot\frac{{{\text{d}}L_{{{\text{MgO}}/{\text{C}}}} }}{{{\text{d}}t}} = D\frac{{C_{{{\text{CO}}_{2} ,b}} }}{{L_{\text{MgO}} }}, $$

(A2)

where L _MgO/C = thickness of the protective layer, t = reaction time, D = diffusion coefficient, $C_{{{\text{CO}}_{2} ,b}}$ = equilibrium concentration of CO₂ at the gas/oxide interface, $\rho_{{{\text{MgO}}/{\text{C}}}}$ = moles of C per unit volume of MgO-C (mol/cm³). (Note that $1/\rho_{{{\text{MgO}}/{\text{C}}}}$ represents the volume of the MgO-C layer per 1 mol of C contained therein.) The effective diffusivity of CO₂ can be estimated by:[19]

$$ D_{{{\text{CO}}_{2} }} = 1.19\left( {\varepsilon^{2} } \right){\text{cm}}^{2} /{\text{s}}, $$

(A3)

where ε is the porosity of the non-protective 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 MgO-C layer thickness increased from 4.48 to 2.50 μm in 4 minutes of exposure to 90 vol pct CO₂ in air at (993 K) 720 °C, the experimental growth rate was ~8 × 10⁻⁷ cm/s. However, a growth rate of ~1.95 × 10⁻³ cm/s was calculated using Eq. [A2] with 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 MgO-C layer was rate-controlling and CO₂ diffusion through the porous MgO layer did not affect the overall rate.

Comparison was made for the fastest rate of MgO-C 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.