Why do Fe-Al Alloys Show Good Passivation Behavior in Acids Compared to Elemental Al?

We present some general concepts and pose questions connected to the difference between the ambient temperature passive film formed on elemental aluminum in acid vs that which forms on iron-aluminum alloys containing less than about 35-40 at% Al. Data is presented which demonstrates that the non-protective oxide that forms on aluminum is not related to impurity effects, either in the matrix or in grain boundaries.We argue that the ability of aluminum to form a protective passive film in a single-phase solid solution alloy is connected to atomic-scale size effects that vanish once the aluminum concentration increases to about 60 at%.