Iron Oxide and Aluminum oxide are no different in their "protective" qualities. The "barrier" you are talking about simply refers to the fact that the aluminum (or iron) has reacted with atmospheric oxygen to form aluminum oxide. Once the oxide is formed, no more reaction can take place because the oxide compound is stable. Just as stable as the iron oxide compound. Atmospheric oxygen has a hard time making its way through the layer of stable oxide to the underlying metal to form more oxide, that is why it "protects" the underlying metal. But, for these reasons, aluminum oxide is no more protective than iron oxide. They will both eventually corrode through. However, iron generaly accepts oxygen much more readily than alumum, therefore producing the oxide compound more quickly.