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 atmospher e were investigated from the electrochemical point of view. The emf generat ed across the film was measured under various potential conditions and anal yzed theoretically considering that the oxide film works as the solid elect rolyte having a certain amount of proton and oxide ion conductivity. The em f was found to coincide with the value expected for pure proton conductor b ut considerably lower than that estimated for pure ox ide ion conductor. Th is phenomenon is explained by the fact that the high hydrogen potential in the molten metal is sustained by the "chemical polarization" mechanism indu ced by the extremely large potential difference of oxygen between the insid e and the outside of the film.