The Mechanism Behind the Oxidation Protection of High Mg Al Alloys with Beryllium
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BERYLLIUM additions in ppm levels have long been used by the aluminum industry as a means of reducing the excessive oxidation that can occur when producing Al alloys with an elevated Mg content. Through TGA experiments, the effects of Be were well documented by Balicki already in 1958, Thiele in 1960, and Cochran in 1976.[1–3] The benefits of Be additions come in reducing the total amount of oxidation over short timeframes, as well as increasing the time until the onset of breakaway oxidation. However, owing to the limited ability of many quantification techniques to detect trace amounts of Be, the mechanism by which Be influences the oxidation has not been fully understood. Due to the significant health risks posed by Be and BeO, especially to the workers in the aluminum casthouses, the use of Be is restricted, and alternative methods are preferred.[4] The addition of Mg to an Al alloy causes an MgO oxide layer to form, which is further transformed to MgAl2O4; once all the Mg is oxidized from the melt, these oxides are considered nonprotective in terms of preventing further oxidation. Mg-containing alloys will initially oxidize at a low rate; however, after a period of time ranging from minutes to hours, there will be a sudden and sharp increase in the oxidation rate, known as breakaway oxidation that will continue until all the Mg in the melt is
NICHOLAS SMITH and GABRIELLA TRANELL are with the NTNU, Alfred Getz Vei 2, 7034 Trondheim, Norway. Contact e-mail: [email protected] ANNE KVITHYLD is with the SINTEF, Sluppen, P.O. Box 4760, 7465 Trondheim, Norway. Manuscript submitted February 20, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
oxidized. The oxidation will result in a notable loss of Mg from the melt. Therefore, extra steps are generally required when producing alloys with a high Mg content to minimize the oxidation. Ppm level additions of Be have been shown to reduce both the amount of oxidation and delay the onset of breakaway oxidation. This results in a reduction of the amount of Mg lost to oxidation and a reduction in the number of oxide inclusions that enter the melt during casting.[1–3,5] The focus of this study will be on describing the formation of the oxide layer in the presence of Be during the incubation period of oxide growth, rather than breakaway oxidation, as the former is the most important time period with respect to industrial Al production—as the primary aim of oxidation control in the casthouse is to remain in the incubation stage and avoid catastrophic breakaway oxidation. To better understand the mechanism behind how trace Be additions can result in a marked drop in the oxidation of Al-Mg alloys, a series of experiments were undertaken to characterize the oxide layer and the effects of Be on the layer.
II.
EXPERIMENTAL
The primary experimental study focused on oxidation experiments conducted in a horizontal tube furnace; however, supplementary metal–oxide reaction and oxide-stability experiments were carried out to support the findings. A. Oxidation Experiments Figure 1 s
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