Oxidation of Aluminium-Magnesium alloys at elevated temperature in the solid, semi-liquid and liquid states
Sensitive thermogravimetric equipment was used to monitor the oxidation rates of A1-1 to 9% Mg alloys in 0.21 oxygen/0.79 helium mixtures with and without 0.03 atm water vapour pressure, in the temperature range 500-725°C which include their respective solid, semi-liquid and liquid states. These measurements were supported by structural and topographical studies of oxide films using SEM, X-ray diffraction, and TEM with electron diffraction and EDX techniques. The objective was to provide information in the context of dross formation during melting operations. The only oxidation product observed was MgO. The oxidation rates do not follow simple rate laws. The rate normally increases as the temperature is raised except for a curious inverse relationship for liquid alloys just above the liquidus temperatures due to the formation of undulating surfaces. An amorphous MgO film formed on liquid alloys restricted the initial oxidation but, following an incubation period, crystallisation of the film induced breakaway oxidation. The crystallisation was promoted by high magnesium contents, high temperatures and moist atmospheres. The oxidation rates for alloys in the semi-liquid state were as follows: (i) for < 50% liquid, the rates were faster than those for wholly solid alloys due to preferential oxidation of magnesium-enriched liquid fractions along grain boundaries; (ii) for > 50% liquid, the rates were initially faster than those for wholly liquid alloys due to the presence of solid phase particles which acted both as disruptive stress-raisers and as sources of nuclei for crystallisation of the amorphous oxide film over the liquid phase. For the moist atmosphere: (i) wholly solid alloys and semi-liquid alloys with < 50% liquid oxidised initially faster but eventually slower than in the dry atmosphere. These effects are explained by enhanced initial oxidation but suppressed subsequent nucleation of tertiary MgO particles and by the injection of OH - ions into the oxide; (ii) wholy liquid alloys and semi-liquid alloys with > 50% liquid oxidised faster than in the dry atmosphere because of the injection of OH- ions into the oxide with consequent disruptive effects caused by hydrogen absorption by the alloy. The addition of 0.003% Be to Al-8% Mg alloy decreased the oxidation rate markedly by toughening the MgO films preventing cracking and hence the nucleation of tertiary MgO. Small additions of Mn or Zr offset the effect of Be and probably enhanced the Mg 2+ ion conductivity in the oxide by the 4+ injection of Mn 2+ , Mn 4+ or Zr ions. For alloys with Be, although breakaway eventually occurred it was not initiated by crystallisation of the amorphous film and moisture in the atmosphere increased the oxidation throughout the temperature range, 500-725°C.