Determination of the sodium activity in aluminum and aluminum silicon alloys using sodium beta alumina
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INTRODUCTION
S O D I U M is an important element in aluminum alloys and, due to the chemistry of the Hall Heroult process, a small amount of sodium is codeposited with the aluminum at the cathode. Generally, some of the sodium is lost during holding and, if the metal is to be used for aluminum magnesium alloys, the sodium content is reduced by chlorine injection to less than 2 ppm to prevent hot shortness occurring during rolling. However, if aluminum silicon alloys of high strength are required, sodium is added to modify the eutectic structure. As sodium is a particularly volatile element, this leads to problems in sampling, and the analytical results may only be of post mortem use. Because of its considerable significance in aluminum alloys, it is important to have a thorough knowledge of the thermodynamics of the aluminum sodium system and to develop methods for the instantaneous analysis of sodium. The activity of sodium in aluminum has been determined independently by Dewing ~ and Mitchell and Samis.-" The latter authors equilibrated lead, aluminum, and sodium under argon, quenched the two phases, analyzed the sodium content of both phases, and calculated the activity coefficient of sodium using data for the lead sodium system. The AI-Na system was found not to obey Henry's law, with the activity coefficient increasing from 107 at 21 ppm to 287 at 45 ppm at 999 K. The method used by Dewing ~ was to measure the vapor pressure of sodium over a mixture of aluminum, sodium fluoride, and cryolite by a gas transference technique. The aluminum was then analyzed for sodium and the activity coefficient found to be 427 at 998 K and independent of composition between 99 and 670 ppm and in agreement with the value obtained from the phase diagram of Ransley and Neufeld. 3 Mitchell and Samis attempted to reconcile their data with Dewing's by suggesting that the activity coefficient increases rapidly with increasing sodium content below 50 ppm and varies only slightly with composition above 100 ppm. ttowever, this is at variance with later work by Dewing. 4 Fray5 used a/3-alumina pellet hot pressed into an alpha alumina tube with sodium reference consisting of a mixture of ct and/3-alumina under a controlled oxygen partial pressure to detect changes in sodium content of an aluminum-
R. J. BRISI.EY, formerly Graduate Student in the Department of Metallurgy and Materials Science, University of Cambridge, is now with the British Gas Corporation, Solihull, England. D.J. FRAY is a University Lecturer, Department of Metallurgy and Materials Science, University of Cambridge, England. Manuscript submitted September 22, 1982. METALLURGICAL TRANSACTIONS B
silicon alloy. The potential given by the cell stainless steel, Na(a-~3A1203) I/3-Al20_~1AI-Na(/), stainless steel was related to the sodium concentration, but, as the activity of sodium in the reference was not accurately known, it was not possible to determine the sodium activities in the alloy, but the results indicated that there were considerable deviations from Henry's l
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