Elsevier

Solid State Ionics

Volume 143, Issues 3–4, 2 July 2001, Pages 355-365
Solid State Ionics

Kinetic study of the oxidation by oxygen of liquid Al–Mg 5% alloys

https://doi.org/10.1016/S0167-2738(01)00861-XGet rights and content

Abstract

The oxidation into MgO of an industrial Al–Mg 5% alloy in the liquid state has been studied at 700°C by thermogravimetry, under controlled oxygen partial pressure. Since the kinetic curves were not reproducible, it was not possible to obtain directly from them the variations of the areic conversion rate (mol of MgO s−1 m−2) versus the oxygen partial pressure. Therefore, it has been necessary to use a method based on the isolation method, which allowed us to overcome the problems of non-reproducibility of the curves. It was found that the areic rate of growth of MgO decreases when the oxygen pressure increases. A reaction mechanism in elementary steps has been proposed to account for these results, involving two competitive oxygen adsorptions on the MgO surface.

Introduction

Aluminium–magnesium alloys are widely used in industry (packaging, aerospace, transportation, building…).

During the manufacture of these alloys, the surface of the liquid metal may be oxidised, mainly leading to the formation of MgO, especially in the case of alloys containing more than 3% in magnesium [1], [2]. To avoid this drawback, beryllium was added to the alloys in the past. Present knowledge of the potential hazards of this element leads most aluminium manufacturers to discard its use in spite of the very low level involved. In any case, it is planned to be forbidden soon. Thus, it is necessary to find alternative ways to prevent oxidation.

It is, therefore, important to understand the mechanisms involved in the formation of magnesium oxide from the alloy. The gaseous atmosphere involved in the industrial process is quite complex, since several gases 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. This article deals with an experimental study of the influence of the oxygen pressure on the oxidation, and an interpretation of the results in order to have a better knowledge of the reaction mechanism.

The transformation of a solid involves the processes of nucleation and growth of the new phase (MgO in our case). The experimental rate corresponds to the growth of MgO, thus, the modelling of the transformation is reduced to the growth model. It is simplified using the following assumptions:

  • (i) the system is in a quasi-steady state

  • (ii) the derivative of the fractional conversion α versus time, which will be also called the rate, can be written:R=dαdt=ΦE

where Φ is the areic growth rate of MgO (in mol s−1 m−2), which depends on the physicochemical variables (pressure P, temperature T, magnesium activity…), and E is the ‘space function’ (m2 mol−1), characteristic of the extent of the area where the rate-limiting step of the growth occurs. E depends on the time and on the history of the solid from the beginning of the transformation up to the considered instant [3]. These two assumptions can be verified experimentally [3], [4], [5].

Then, an experimental method, based on the isolation method [6], can be used to obtain directly the variations of Φ with the physico-chemical variables (particularly the oxygen pressure).

In this article, we present first the results of the kinetic study of the oxidation of an Al–Mg alloy into MgO; then a mechanism is proposed in order to account for the experimental results on the variations of the areic growth rate with oxygen pressure.

Section snippets

Experimental

The alloy is an industrial Al–Mg 5% alloy, supplied by Pechiney (purity about 99%). The samples are cylinders of 1 mm height and 9 mm diameter. Before each experiment, they were manually polished with grade 500 SiC paper and rinsed with acetone (the polishing method of the samples has no influence on the kinetic curves).

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,

Kinetic curves

As shown in previous studies [7], [8], [9], [10], [11], for alloys containing more than 3% in magnesium, MgO is the first phase which appears during the oxidation. We present in Fig. 1 the results of thermodynamic calculations giving, at equilibrium, the oxygen pressure versus magnesium activity, for the alloy at 700°C. Fig. 1 indicates that MgO is the thermodynamically stable phase as long as the residual magnesium activity in the alloy is higher than 0.023, which corresponds to a weight

Interpretation of the variations of Φ with PO2: growth mechanism

We have verified the assumption of quasi-steady state, and that the oxidation rate could be written as a product ‘ΦE’. These results are important to find the elementary steps of the growth mechanism, because they mean that it will be possible to use the assumption of the rate-limiting step to calculate the corresponding possible rate laws. Comparing these laws to the experimental variations of Φ with PO2 (Fig. 8) will normally lead us to determine the rate-limiting step of the reaction and,

Conclusions

The oxidation kinetics of a liquid industrial Al–Mg 5% alloy has been studied in an attempt to validate a reaction mechanism. The use of the isolation method turned out to be necessary to overcome the problems due to the non-reproducibility of the kinetic curves and to obtain the variations of the areic growth rate of magnesium oxide Φ with the oxygen pressure.

It has been shown that Φ decreases when oxygen pressure increases, which was accounted for by a mechanism involving two parallel oxygen

Acknowledgements

This work has been carried out in the frame of a CPR (Contrat de Programme de Recherches): ‘REACTIVITE des ALLIAGES LIQUIDES à HAUTE TEMPERATURE et RECYCLAGE’: Contrat de Programme de Recherches between CNRS, INP Grenoble, INP Lorraine, INP Toulouse, the Ecole nationale supérieure des mines de Saint-Etienne and PECHINEY RECHERCHE.

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