Oxidation of Liquid Aluminum — Magnesium Alloys

Silva, M. P.; Talbot, David E.J.

doi:10.1007/978-3-319-48228-6_17

Cited by 7 publications

(5 citation statements)

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“…This is in contrast to other literature which indicates that the amorphous oxide is more protective than the crystalline form, and indeed it is the breakdown of the amorphous film into crystalline forms which causes the so-called breakaway oxidation [69,[86][87][88][89][90].…”

Section: Experimental Observations Of Oxidation Kineticscontrasting

confidence: 73%

“…This result is of significant practical importance since, in practice, industrial reverberatory furnaces are often gas fired, with the combustion products (which contain significant amounts of CO 2 ) coming into direct contact with the melt surface. Silva and Talbot [89] in a paper based on a PhD thesis by Silva [79], investigated the oxidation kinetics of molten Al-Mg alloys, with Mg concentrations ranging from 1.3-9.3%, at temperatures up to 725°C in an oxygen/helium mixture, and characterised the resulting oxide films. The authors questioned the validity of previous work in the literature because of uncertainties inherent in the experimental techniques used and identified three sources of uncertainty.…”

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confidence: 99%

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A microstructural and kinetic study of molten aluminium oxidation in relation to dross formation

Bonner¹

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Oxidation and dross formation on molten aluminium is estimated to directly cause losses of tens of millions of dollars per annum to the Australian smelting industry. Although there has been significant study of molten aluminium oxidation over the past few decades, the formation mechanisms are still not fully understood. There is also lack of reliable and useful kinetic data that can be applied to computational simulations of industrial melt handling processes involving the oxidation of either quiescent pools or turbulent transfers of molten aluminium. There is additional uncertainty in the literature relating to the physical processes underlying molten aluminium oxidation, and the microstructural evolution of the growing oxide film. Past oxidation studies have been confounded by the detrimental effect of the pre-existing oxide ubiquitously present on aluminium specimens. Recent studies conducted at The University of Queensland have indicated a way forward in this respect, with the development of an experimental technique that enables the melt surface to be skimmed free of pre-existing oxide prior to each oxidation experiment. This project is based on previous outcomes and expands on them to create new insights into the oxidative behaviour of molten aluminium in response to various key parameters, such as melt temperature and isothermal holding time. It also aims to resolve some ambiguities in the literature.Melts of commercial purity aluminium were isothermally held at various temperatures ranging from 750°C to 900°C, and exposed to ambient air for times ranging up to ~20 h. The oxide grown on each melt was removed and treated to separate the oxide itself from the underlying aluminium metal, using a molten salt flux. The mass of oxide was then determined as a function of melt temperature and exposure time. These gravimetric experiments were complemented by morphological and microstructural characterization of grown oxide films, using scanning electron microscopy, transmission electron microscopy, and electron diffraction. These two approaches were combined to develop a simple kinetic model based on the parabolic rate law.The amount of oxide formed was found to be significantly lower than what has previously been reported, and was attributed to the removal of the pre-existing oxide. The amount of oxide that formed increased with increasing temperature, and the oxidation rate continuously decreased with increasing exposure time. These observations are consistent with diffusion-limited growth.Microstructural characterisation showed two stages of oxidation. In the early stage, nano-sized, planar, polyhedral crystallites of γ-alumina nucleate at the metal/oxide interface. In the later stage, flake-like γ-alumina crystallites were observed to grow outwards from the oxide/air interface. The time at which the second stage of growth became active decreased with increasing temperature, and the flake-like crystallites continued to increase in size with increasing exposure time.iii Crystallographic texture was obser...

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Section: Experimental Observations Of Oxidation Kineticscontrasting

confidence: 73%

mentioning

confidence: 99%

A microstructural and kinetic study of molten aluminium oxidation in relation to dross formation

Bonner¹

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“…Regarding pure aluminum and some of its alloys, it is commonly accepted that an increase in temperature, 12,13 humidity 14 or oxygen partial pressure 15 leads to a faster and more intense oxidation. Magnesium addition is also well known to deliver adverse effects on the oxidation of aluminum alloys leading to a significant increase in the reaction kinetics 16‐20 . In this case, the composition of the oxide film mainly depends on the magnesium content insofar as the latter is the most reactive chemical compound among all additives commonly used in aluminum alloys, as also confirmed by the Ellingham diagram.…”

Section: Introductionmentioning

confidence: 77%

Deal–Grove model revisited for a new insight into the oxidation of molten aluminum alloy: The A356 alloy, as a model system

2021

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Oxidation of A356 aluminum alloy at molten state is investigated from thermogravimetric analyses. Linear kinetics followed by unexpected logarithmic kinetics is observed during the oxidation process. Scanning electron microscopy imaging coupled with wavelength‐dispersive X‐ray spectroscopy analyses reveal a heterogeneous oxide layer composed of spinel (MgAl2O4) embedded in a nonoxidized alloy matrix. Nonlinear diffusion of magnesium through the heterogeneous oxide layer is the key mechanism proposed to explain the logarithmic kinetics. The growth of spinel phase causes a growing complexification of the aluminum network through which magnesium is diffusing. To model the magnesium transport through the oxide layer, an effective diffusion coefficient, build from the pioneering approach developed by Davies et al. (Davies, D., Evans, U. & Agar, J. (1954) Proc. Roy. Soc. London. A., 225, 443), is introduced in the Deal–Grove model.

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“…In contrast to this is the rather remarkable behavior of Mg (a major alloying element) among the salt, oxide, gas and metal phases: the results show that Mg can be removed as chloride than oxide. Typically, when Mg is added as an alloying element, it forms MgO by oxidation and then forms spinel oxide (MgAl 2 O 4 ), by the following reactions [ 21 , 22 , 23 ]: Mg(l) + 0.5O 2 (g) = MgO(s) 2Al(l) + 1.5O 2 (g) = Al 2 O 3 (s) MgO(s) + Al 2 O 3 (s) = MgAl 2 O 4 (s) …”

Section: Resultsmentioning

confidence: 99%

Thermodynamic Analysis for the Refining Ability of Salt Flux for Aluminum Recycling

et al. 2014

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The removability of impurities during the aluminum remelting process by oxidation was previously investigated by our research group. In the present work, alternative impurity removal with chlorination has been evaluated by thermodynamic analysis. For 43 different elements, equilibrium distribution ratios among metal, chloride flux and oxide slag phases in the aluminum remelting process were calculated by assuming the binary systems of aluminum and an impurity element. It was found that the removability of impurities isn’t significantly affected by process parameters such as chloride partial pressure, temperature and flux composition. It was shown that Ho, Dy, Li, La, Mg, Gd, Ce, Yb, Ca and Sr can be potentially eliminated into flux by chlorination from the remelted aluminum. Chlorination and oxidation are not effective to remove other impurities from the melting aluminum, due to the limited parameters which can be controlled during the remelting process. It follows that a proper management of aluminum scrap such as sorting based on the composition of the products is important for sustainable aluminum recycling.

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Oxidation of Liquid Aluminum — Magnesium Alloys

Cited by 7 publications

References 5 publications

A microstructural and kinetic study of molten aluminium oxidation in relation to dross formation

A microstructural and kinetic study of molten aluminium oxidation in relation to dross formation

Deal–Grove model revisited for a new insight into the oxidation of molten aluminum alloy: The A356 alloy, as a model system

Thermodynamic Analysis for the Refining Ability of Salt Flux for Aluminum Recycling

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