The dynamic oxidation of aluminum and its alloys
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I.
INTRODUCTION
A L U M I N U M alloys have received attention from a number of authors with the overall aim of reducing the oxidation of the metal during teeming and casting. Drouzy and Mascr6 ~ cite the work of Thiele who found that traces of sodium produced scatter in the results and that the oxidation did not follow a simple parabolic law. It was found that initially a thin film of'y-Al203 formed, inhibiting the oxidation. After an incubation period this oxide transformed to the c~ form which is less effective in inhibiting reaction, and a faster rate of oxidation occurred which appeared to obey a parabolic law. Thiele also studied the effect of alloy additions on the oxidation of liquid aluminum and reported that iron, copper, zinc, and manganese had little effect, although zinc increased the oxidation above 900 ~ due to zinc vapor disturbing the oxide film. A similar effect was observed with additions of selenium. It was found that sodium increased the rate of oxidation while beryllium hindered reaction through the formation of BeO. Sleppy 2 employed a monometric technique to follow the oxidation of aluminum by dry oxygen at a partial pressure of 120 torr over the temperature range 660 ~ to 850 ~ A logarithmic law was found below 700 ~ and was accounted for by a disarray theory for the amorphous oxide. Above 750 ~ eta-alumina formed and a parabolic law was obtained which was interpreted in terms of diffusion of cations through the oxide and subsequent reaction at the gas interface. The oxidation of aluminum-magnesium alloys is complicated as magnesium increases the rate by forming a spinel, but oxidation returns to a low level once all the magnesium has been oxidized. Belitskus and co-workers 3"4"5have made a number of measurements of quiescent oxidation of aluminum-magnesium alloys and have studied the inhibiting effect of compounds of boron, sulfur, and carbon. Initially, a layer of magnesium oxide formed at the molten metal surface and, after an incubation period, spinel crystals (MgO'AI203) formed and breakaway oxidation began, which ceased only when all the magnesium had been oxidized. G. WIGHTMAN, formerly Graduate Student in the Department of Metallurgy and Materials Science, University of Cambridge, England. is now with Davy McKee Research and Development, Stockton-on-Tees, England. D.J. FRAY is a University Lectt, rer. Department of Metallurgy and Materials Science, University of Cambridge, England. Manuscript submitted July 12, 1982.
METALLURGICAL TRANSACTIONS B
As none of the above experiments pertains to oxidation under dynamic conditions, it was decided to investigate the oxidation of aluminum and its alloys when carbon dioxide was bubbled through the metal and the oxide film was continually deformed.
II.
EXPERIMENTAL PROCEDURE
The apparatus and technique were the same as had been reported in previous papers 6,v although two different sizes of reaction vessel were employed (55 mm i.d. and 38 mm i.d.) and the alumina dip tube was approximately 5 mm o.d. and 8 mm o.d. Experiments on pure alu
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