Thermodynamic properties of zinc-rich zinc-aluminum melts
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I.
INTRODUCTION
ZINC-rich aluminum alloys are being used increasingly in industry. Additions of up to 0.005 pct aluminum are used as a brightener, and additions up to 0.06 pct aluminum are employed for controlling alloy layer growth in bath galvanizing, lu Large amounts of aluminum are added to zinc for die castings, galvanizing, and sand casting, t21 the alloys of industrial interest have aluminum contents of 4.5, 8, 11.5, and 27 pct. [3] In order to improve the understanding of the nature of Zn-A1 melts and model the Zn-A1 alloying process, it is essential to have accurate thermodynamic data for the liquid Zn-A1 system in the composition and temperature range relevant to alloying. The investigation of liquid Zn-A1 alloys started during the early 1960s. Eremenko,t< Batalin and Belaborodova, 15J and Predel and Schallner IrJ studied the thermodynamics of liquid Zn-A1 alloys by the electromotive force (emf) method using fused-salt electrolytes at temperatures from the melting point of aluminum (660.45 ~ to 900 ~ over the composition range 0.1 to 0.9 atom fraction of aluminum. Witting and Schoffl iv1 measured the heat of mixing in liquid Zn-A1 alloys up to 1000 ~ by high-temperature calorimetry. Lutz and Voigt t81 measured the vapor pressure of zinc over liquid Zn-A1 alloys by means of the radioactive-trace dew point method. Bolsaitis and Sullivan tgl determined the activities of zinc in liquid Zn-A1 alloys by the isopiestic technique. Yazawa and Lee I1~ reinvestigated liquid Zn-A1 alloys using an improved dew point method. Most of the data on Zn-A1 alloys were assessed by Hultgren e t al. till and Murray tl21 and, most recently, by Mey and Effenberg) 131 It is apparent that considerable thermodynamic data are available for liquid alloys at temperatures above 660 ~ However, experimental X.Y. YAN, Graduate Student, D.E. LANGBERG, Research Fellow, and W.J. RANKIN, Director, are with the Department of Chemical Engineering, G.K. Williams Cooperative Research Centre for Extractive Metallurgy, University of Melbourne, Parkville 3052, Australia. Manuscript submitted January 22, 1993. METALLURGICAL TRANSACTIONS B
thermodynamic data in the liquid zinc-rich region below 660 ~ which are significant for Zn-A1 alloying processes, have not been reported. The purpose of this investigation was to determine the thermodynamic properties of liquid zinc-rich aluminum alloys in the range 400 ~ to 650 ~ and to investigate the behavior of dilute solutions. The emf of a reversible concentration cell was measured over the composition range 0.0016 to 0.43 atom fraction of aluminum and the results were used to calculate the activities of aluminum and related thermodynamic properties of Zn-A1 melts.
II.
EXPERIMENTAL PROCEDURE
The electrochemical cell used is shown in Figure 1 and has the form W,A1 IMgC12-NaC1-KC1-A1C131 Zn-AI (1),W A eutectic mixture of MgC12-NaCI-KC1 of composition 50:30:20 on a molar basis (mp = 396 ~ with 2 to 5 tool pct anhydrous A1CI3 was chosen as the electrolyte. Alloys of the required composition were prepared from
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