Determination of hydrogen absorption and desorption processes in aluminum melts by continuous hydrogen activity measurem
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I.
INTRODUCTION
H Y D R O G E N is an important impurity component in aluminum and its alloys, which has to be controlled carefully in casting and working processes. It is usually absorbed as a result of the reaction of water vapor with aluminum during melting or annealing. The reaction 2A1 + 3H20 = A1203 + 3H2 is strongly exothermic with an equilibrium constant PH2/PH~o = 10t4 at 700 ~ However, fast hydrogen absorption of aluminum, as well as effective H2 desorption, at atmospheric pressure is prevented by the oxide skin on the melt surface. Hydrogen is removed from liquid aluminum under vacuum conditions, where Hz bubbles are formed and break through the oxide skin, or by inserting bubbles of inert gases with more or less oxide-free internal surfaces. Stirring of the melt destroys the oxide skin, too, and enhances hydrogen absorption and desorption. The crucial role of oxide skins on reaction kinetics has been discussed in many papers. It had to be considered in experiments on the solubility t4-1~ and diffusivity [11-14] of hydrogen in liquid aluminum as well as in experimental [15-181 and t h e o r e t i c a l [15,19,2~ work on degassing of melts by purge gases or vacuum processes. Quantitative data on reaction kinetics published so far are usually measured under conditions where the oxide skin has been destroyed or partly removed and H diffusion in the melt and/or bubble transport is considered to be the rate-controlling step. Almost no quantitative information is available on hydrogen absorption and desorption rates of A1 melts covered with undisturbed oxide skins and on parameters affecting their protective properties. This lack of knowledge is not surprising, since such experiments require reliable determination of small concentration changes in a molten sample without affecting the oxide skin by the analytical technique applied. Determination of the hydrogen content in melts or in J. WEIGEL, formerly Ph.D. Student, Max Planck Institut for Metallforschung, Institut fiir Werkstoffwissenschaft, is with Alusingen GmbH, Alusingen Platz 1, D-7700 Singen, Federal Republic of Germany. E . FROMM, Senior Scientist, is with the Max Planck Institut for Metallforschung, Institut f'tir Werkstoffwissenschaft, SeestraBe 92, D-7000 Stuttgart 1, Federal Republic of Germany. Manuscript received November 7, 1989. METALLURGICAL TRANSACTIONS B
the final products is usually performed by taking samples which are subsequently analyzed by various techniques, such as vacuum fusion, effusion techniques in solid state, or assessment of bubble formation in cast samples. [21-27] A more direct access to the hydrogen content of aluminum melts is provided by the Alu-melt tester, which determines the pressure when bubbles are first visible on a melt sample in a receiver during evacuation, t2s,291 and the Telegas test, where nitrogen or argon is bubbled through the melt and the H2 concentration in the gas is determined according to the carrier gas method. [3~ However, even these methods do not provide a direct or continuous signal, since a
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