Ferric oxide is brittle. Aluminium oxide forms a protective film at the surface.

I agree with Igor's answer.

•Three types of oxides may form, depending on the volume ratio between the metal and the oxide: (a) magnesium produces a porous oxide film, )b) aluminum forms a protective, adherent, nonporous oxide film, and (c) iron forms an oxide film that spills off the surface and provides poor protection.