Let's say a drop mercury is placed on clean aluminium sheet; mercury amalgam is formed. The solubility of aluminium in mercury and vise versa at room temperature is fairly low. If the entire sheet is now placed under water, hydrogen evolution is observed. Where is this hydrogen evolution from - the aluminium rich surface on the edges or the mercury rich surface on the drop?
(For reference, please look up videos of aluminium amalgam online, say on YouTube.)