New methods for the determination of hydrogen content of aluminum and its alloys: Part III. Quantitative Vacuum Gas Test (Straubbe-Pfeiffer Test) by use of a mass spectrometer

Abstract

A new method is proposed for determining quantitatively the hydrogen content of molten aluminum directly in the cast house. It consists in measuring, with a small mass spectrometer (an industrial helium leak detector), the partial pressure of hydrogen evolved during solidification of a liquid metal sample at a reduced pressure of 1 Torr in a vacuum chamber,e. g. under operating conditions similar to the usual Vacuum Gas Test. A detailed investigation permits a definition of the following characteristics for the method. 1) The detection limit is on the order of 0.03 ml/100 g for a 200 g metal sample. 2) Dehydrogenation occurs essentially at the end of the solidification of pure metal or eutectics, and can be explained by the difference in solubility of hydrogen in liquid and solid aluminum. 3) The correlation between the phase diagram of the alloy to be studied and dehydrogenation permits the consideration of the dehydrogenation curve as a characteristic spectrum of the alloy. 4) No simple correlation could be established between hydrogen content and bubble nucleation. 5) Hydrogen contents derived from this method before casting an alloy are not significantly different from those obtained by a laboratory method on the solidified final product. 6) The time required for a determination (10 to 20 min) is short enough to allow a plot of the evolution of hydrogen content during a whole metal treatment cycle. 7) The new Vacuum Gas Test combines the information from the usual test for metal clean-liness (visible bubbles rucleated by the presence of inclusions) and the possibility of obtaining a quantitative value of the hydrogen content.