Absorption Mechanism of Hydrogen into Molten Aluminum

众所周知，铝熔体在空气中的吸氢量比在常压下的吸氢量要大得多。为了证实熔体上形成的氧化膜中的质子传导本质上与吸收机制有关，进行了以下研究。首先，利用新研制的氧化物质子导体氢气渗透仪测量了氢气在多晶烧结氧化铝中的渗透率。渗透量与高压侧氢气压力的10次方成正比。这一结果表明，氧化铝中可移动的物种不是二氧化硅玻璃中的双原子分子，而是质子等带电物种。当铝液与氧化铝试件的高压侧接触时，渗透量减小。这一现象表明，氧化铝的渗透是由于质子和铝离子向相反方向的单极扩散所致。接下来，用新设计的装置测量了铝熔体表面形成的氧化膜上产生的电动势。测得的电动势与假设薄膜作为具有质子和铝离子导电的固体电解质的电动势的极性是一致的。在假设质子输运数和铝离子输运数分别为1的情况下，计算值在理论值的范围内。这些观察结果有力地支持了我们提出的模型，即铝在空气中熔融时的异常吸氢是由于铝在熔体表面形成的薄氧化膜上的铝势差极大而产生的化学泵浦机制。

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.