Abstract
The oxidation into MgO of an industrial Al-Mg alloy in the liquid state has been studied at 700°C by thermogravimetry. Since the kinetic curves were not reproducible, it was not possible to use them directly. Thus, it has been necessary to use a method based on the isolation method to obtain the influence of oxygen partial pressure on the areic reactivity (mole of MgO.m-2.s-1) of growth of MgO. Introduction During the elaboration of aluminium-magnesium alloys, the surface of the liquid metal may be oxidised, mainly leading to the formation of MgO. The industrial system is quite complex, since several gases of the atmosphere can react with the alloys (oxygen, water vapour, nitrogen, carbon dioxide...). So, we have studied a « simplified » system : the oxidation of an Al-Mg 5% alloy by oxygen. The aim of this work is to obtain the influence of the oxygen pressure on the areic reactivity (mole of MgO.m-2.s-1) of growth of MgO, and to propose a reaction mechanism able to account for the experimental results, in order to predict the behaviour of the alloy in industrial conditions of elaboration and to limit its oxidation. After giving the oxidation kinetic curves, the methodology used for the kinetic study of the MgO growth will be detailed. Finally, the influence of the oxygen pressure on the areic reactivity of growth of MgO will be obtained, and thanks to the results of an IR study, some indications for a reaction mechanism will be given. Experimental The alloy is an industrial Al-Mg 5% alloy, supplied by Pechiney. The samples are cylinders of 1mm height and 9mm diameter (this height was chosen in order to ensure an constant oxygen pressure around the sample). For higher samples (15mm to 3.5mm), more oxidised parts were observed (a ring of white powder at the surface of the samples), the localisation of this ring along the cylinders varying with the oxygen pressure fixed for the experiment. Thus, we have assumed that a gradient of oxygen pressure exist along the high samples. This was not observed for the 1mm-high cylinders. The oxidation of the liquid alloy was followed by isothermal thermogravimetry at 700°C (thermobalance SETARAM TAG 24). The experiments were carried out at atmospheric pressure, under a flowing mixture of helium and oxygen (1l.h-1), the partial pressures being controlled by mass-flowmeters (Brooks 5850E). The heating and the melting of the samples were carried out under oxygen at atmospheric pressure since it has been observed that a high oxygen pressure allows to limit the oxidation of the liquid alloy; the oxygen pressure chosen for the experiment is established after 30 minutes of isotherm at 700°C. The experiments of simultaneous calorimetry and
Submitted Version (
Free)
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call