When aluminum is exposed to air, what is formed on its surface?

When aluminum is exposed to air, an oxide layer forms on its surface due to a chemical reaction between the aluminum and oxygen in the atmosphere. This oxide layer is typically composed of aluminum oxide (Al2O3), which develops rapidly when aluminum is exposed to air. This layer is significant because it is very thin but effectively protects the underlying aluminum from further corrosion and gives aluminum its characteristic resistance to rust.

The formation of aluminum oxide occurs almost instantaneously and serves as a protective barrier, making aluminum highly durable in various environments. The properties of aluminum oxide contribute to its use in many applications where corrosion resistance is crucial, such as in the construction and aerospace industries.