What is responsible for aluminum's excellent corrosion resistance?

What is responsible for aluminum's excellent corrosion resistance?

• A. The aluminum oxide layer; it changes to MgO or MgAl2O4 when Mg % exceeds 0.05%.
• B. The hydrogen passivation layer forms on aluminum.
• C. The aluminum oxide layer is not protective.
• D. A chromium-rich layer provides protection.

Answer: (C)

The main concept is that aluminum’s corrosion resistance comes from a self-passivating oxide film. When aluminum meets air, a thin, dense layer of aluminum oxide (Al2O3) forms almost instantly and sticks tightly to the surface. This film is a highly effective barrier to moisture and ions, so it dramatically slows further oxidation of the metal beneath. If the surface is scratched, new oxide forms quickly, reestablishing protection. That’s why aluminum doesn’t keep corroding once the film is in place. So the idea that the oxide layer isn’t protective isn’t correct. The oxide film is exactly what provides aluminum’s resistance to corrosion. Hydrogen passivation layers aren’t how aluminum protects itself, and a chromium-rich layer is the mechanism for stainless steels, not aluminum. Also, magnesium content doesn’t convert the oxide into MgO or MgAl2O4 as the primary protective film; the protective layer remains aluminum oxide.

The main concept is that aluminum’s corrosion resistance comes from a self-passivating oxide film. When aluminum meets air, a thin, dense layer of aluminum oxide (Al2O3) forms almost instantly and sticks tightly to the surface. This film is a highly effective barrier to moisture and ions, so it dramatically slows further oxidation of the metal beneath. If the surface is scratched, new oxide forms quickly, reestablishing protection. That’s why aluminum doesn’t keep corroding once the film is in place.

So the idea that the oxide layer isn’t protective isn’t correct. The oxide film is exactly what provides aluminum’s resistance to corrosion. Hydrogen passivation layers aren’t how aluminum protects itself, and a chromium-rich layer is the mechanism for stainless steels, not aluminum. Also, magnesium content doesn’t convert the oxide into MgO or MgAl2O4 as the primary protective film; the protective layer remains aluminum oxide.