What forms immediately after aluminum is exposed to weather, making it resistant to most chemicals?

• A. Oxide
• B. Hydroxide
• C. Patina
• D. Alloy

Answer: (A)

Aluminum quickly forms a thin, tightly adherent layer of aluminum oxide on its surface when exposed to air. This oxide film acts as a protective barrier, blocking further chemical attack and giving the metal resistance to most chemicals. The layer forms almost instantly and is self-healing if scratched, so the protection remains even after minor abrasion. This protective behavior is called passivation and is why aluminum is so durable in many environments.

The other ideas don’t fit the observed protection. A hydroxide film isn’t the standard protective layer created by weathering, and a patina is a general aging film seen on some metals rather than the specific oxide barrier aluminum relies on. An alloy would involve mixing aluminum with other elements, which isn’t what forms just from weather exposure.