Basant L. Tiwari

Thermodynamic properties of liquid Al-Mg alloys measured by the Emf method

The thermodynamic properties of liquid Al-Mg alloys containing 0.027 to 95.50 at. Pct Mg were determined by measuring the emf, between 973 and 1073 K, of a magnesium concentration cell of the type Mg(l)|MgCl2-CaCl2 (eutectic melt, l)|Mg (in Al, l). Special attention has been given to low-magnesium aluminum alloys which are most commonly used in industry and for which definitive thermodynamic data are not reported in the literature. Alloys containing up to 12 at. Pct Mg follow Henry’s law, and the magnesium activity is given by the relation aMG = 0.88 XMG at 1073 K. Above 12 at. Pct Mg, the magnesium activity shows a small negative deviation from the ideal solution behavior. The activity of aluminum, however, closely follows the ideal solution behavior up to 75 at. Pct Mg and thereafter it shows a small negative deviation. The emf data have also been used to determine the free energy, entropy, enthalpy, and excess free energy for liquid Al-Mg alloys.

Demagging Processes for Aluminum Alloy Scrap

Demagging processes can be divided into three general categories: 1) chlorination processes, in which chlorine gas is bubbled into the molten aluminum to react preferentially with magnesium which is finally removed as magnesium chloride dross; 2) solid flux processes, in which a solid-flux (AlCl3, AlF3), is mixed into the molten aluminum to react with magnesium which is again removed as chloride or fluoride dross; and 3) electrochemical processes, in which magnesium is removed as metal on the cathode by passing a current between the molten aluminum anode covered with an electrolyte and an inert cathode dipped into the electrolyte. Of the processes reviewed, chlorination methods are the most widely practiced in the industry and seem reasonably efficient. However, most of the chlorination processes appear to have emission and waste generation (MgCl2 dross) problems. The electrochemical processes have been tested only on laboratory scale so far, but they look promising for commercial use. If successfully developed, the electrochemical processes would be pollution-free, and magnesium metal would be recovered as a by-product.

Electrolytic Removal of Magnesium from Scrap Aluminum

This paper examines an experimental process designed to electrolytically remove magnesium from scrap charges in secondary smelting. The principle involves selective oxidation of magnesium from the molten alloy into mixed chloride salt electrolyte and simultaneous deposition at the cathode-electrolyte interface. Magnesium is recovered in the form of salt-coated globules, which might be useful in ferrous desulfurization practice. This process may provide a feasible alternative to traditional chlorination, without attendant environmental problems.

A Novel Technique to Evaluate the Corrosion Behavior of Magnesium Alloys

A novel technique for measuring the intrinsic corrosion rate of magnesium alloys has been developed. As a magnesium alloy sample dissolves in a 5% NaCl solution, the dissolution rate is determined by measuring the amount of HCl added to the NaCl solution to control the pH between 5 and 7. The corrosion rate is determined from the slope of the curve where the dissolution rate reaches a steady state. The technique was used to compare the corrosion behavior of newly developed creep resistant Mg-Al-Ca (AC) alloys with several known magnesium alloys. It was shown that the corrosion rate of AC52 (Mg-5%Al-2%Ca) is comparable to AZ91, meeting one of the several design criteria for new, creep-resistant alloys. This technique provides a fast way to screen the effect of small composition changes on corrosion behavior of Mg alloys. Because of the short test time, this technique provides a powerful tool to accelerate alloy and product development with Mg alloys.