Absorption Mechanism of Hydrogen into Molten Aluminum

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It has been well known that the extremely larger amount of hydrogen is absorbed in molten aluminum kept in air than that expected from the hydrogen pressure in syrrounding atmosphere. The following investigations were performed to confirm that the proton conduction in oxide film formed on the melts is concerned essentially with the mechanism of the absorption. At first, the hydrogen permeation through the polycrystalline sintered alumina were measured by the apparatus which is newly developed using oxide proton conductor. The amount of permeation was found to be proportional to about the tenth power of the hydrogen pressure at the higher pressure side. This result suggests that the movable species in alumina is not diatomic molecule as in silica glass but charged species such as proton. The amount of permeation was observed to decrease when liquid aluminum was contact with the higher pressure side of the alumina specimen. This phenomena imply that the permeation through alumina results from the unbipolar diffusion of proton and aluminum ion to the opposite directions with each other. Next, the electromotive force generated across the oxide film formed on the surface of the molten aluminum was measured by the newly designed apparatus. The polarity of measured emf was coincide with that estimated from the assumption that the film worked as the solid electrolyte having protonic and aluminum ionic conduction. The value was in the range of the theoretical ones calculated assuming that the transport number of proton and that of aluminum ion was unity respectively.These observations support strongly the model proposed by us that the extraordinary hydrogen absorption of aluminum melted in air is originated from the chemical pumping mechanism due to extremely large difference of aluminum potentials across the thin oxide film formed on the surface of the melt.