Rust on an aluminum baking sheet is a bit of a misnomer because aluminum itself does not rust. Rust is the result of the oxidation of iron and steel when exposed to moisture and oxygen. However, aluminum can corrode when exposed to certain conditions.
The process of aluminum corrosion is known as "oxidation," similar to rusting in iron and steel. When aluminum comes into contact with oxygen in the air, it forms a thin layer of aluminum oxide on its surface. This aluminum oxide layer acts as a protective barrier against further corrosion. It prevents the underlying aluminum from reacting with oxygen and moisture, thereby preserving the metal.
However, if the aluminum surface gets damaged or scratched, this protective oxide layer may be compromised. When this happens, the exposed aluminum can react with moisture and oxygen, leading to localized corrosion. The result is the formation of a white or grayish powdery substance, known as aluminum oxide.
While this aluminum oxide is not the same as rust, it can be mistaken for rust due to its appearance. It can sometimes create stains on baking sheets and might make people think that the sheet is rusting.
To prevent aluminum baking sheets from developing corrosion or stains:
Avoid using harsh or abrasive cleaners that can scratch the surface of the baking sheet.
After using the baking sheet, wash it with mild soap and water, and dry it thoroughly to prevent moisture from lingering on the surface.
If the baking sheet develops stains, you can try removing them using a mixture of baking soda and water or by applying a small amount of vinegar. Rinse and dry the sheet afterward.
If you prefer to use a baking sheet without any stains, consider lining it with parchment paper or a silicone baking mat before placing food on it.
Remember that aluminum baking sheets are generally safe to use, even if they develop some discoloration over time. As long as the sheet is clean and free from excessive corrosion, it should still be effective for baking.