Role of the oxide film formed on the surface of molten aluminum as a membrane for chemical pumping of hydrogen

The properties of the oxide film formed on the surface of the aluminum as a chemical potential barrier separating the melt and the oxidizing atmosphere were investigated from the electrochemical point of view. The emf generated across the film was measured under various potential conditions and analyzed theoretically considering that the oxide film works as the solid electrolyte having a certain amount of proton and oxide ion conductivity. The emf was found to coincide with the value expected for pure proton conductor but considerably lower than that estimated for pure ox ide ion conductor. This phenomenon is explained by the fact that the high hydrogen potential in the molten metal is sustained by the "chemical polarization" mechanism induced by the extremely large potential difference of oxygen between the inside and the outside of the film.