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Why does aluminum foil develop an oxide layer when it is exposed to air?
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A scanning-electron-microscope photograph of pyroxene and plagioclase crystals (the long and the short crystals, respectively) that grew in a cavity in a fragment of Moon rock gathered during the Apollo 14 mission.
Aluminum foil develops an oxide layer when exposed to air because aluminum is a chemically active metal with a strong natural affinity for oxygen. When the metal comes into contact with air, it reacts to form a hard, tough, and stable film of aluminum oxide () on its surface.12
This oxide film acts as a protective barrier that isolates the underlying metal from further environmental exposure, which is why aluminum is highly corrosion-resistant despite its chemical reactivity. While this layer forms naturally, its thickness can be intentionally increased through a process known as anodizing, which further enhances the metal's corrosion resistance.12