Aluminum cans are common, but they do not rust when exposed to moisture. Rust is a term exclusively reserved for the corrosion of iron. Aluminum instead undergoes corrosion, which is the broader deterioration of a material due to chemical reactions with its environment. Although aluminum is highly reactive, its unique interaction with oxygen prevents the common form of decay known as rust.
Understanding Rust: The Role of Iron
Rust is a term exclusively reserved for the corrosion of iron and its alloys, such as steel. This process is an electrochemical reaction that requires the presence of three components: iron, oxygen, and water or moisture. The resulting chemical compound is a reddish-brown, flaky substance known as hydrated iron(III) oxide (\(\text{Fe}_2\text{O}_3\cdot \text{nH}_2\text{O}\)).
The formation of rust is a destructive, self-sustaining process because the iron oxide it creates is porous and does not adhere tightly to the metal’s surface. This porous quality allows oxygen and water to continue their attack on the underlying iron, causing the material to flake away. This continuous exposure is why iron objects can completely convert to rust over time if left unprotected. Rusting is accelerated by electrolytes, such as salt, which help facilitate the movement of electrons during the electrochemical reaction.
Aluminum’s Protective Shield: Passive Oxidation
Aluminum’s chemistry is fundamentally different from iron, leading to its superior resistance to widespread corrosion. When aluminum metal is exposed to air, it spontaneously reacts with oxygen to form a layer of aluminum oxide (\(\text{Al}_2\text{O}_3\)).
This naturally occurring chemical process is known as passive oxidation. The aluminum oxide layer forms an extremely thin, dense, and non-porous film that is chemically inert and tightly bonded to the underlying aluminum surface. This protective barrier effectively seals the metal from further reaction with oxygen and moisture. This phenomenon, called passivation, is why aluminum does not visibly degrade under normal atmospheric conditions.
The oxide layer is also self-healing. If the surface is scratched, the exposed aluminum immediately reacts with surrounding oxygen to reform the protective film.
What Makes Aluminum Cans Corrode
While the passive oxide layer provides excellent general protection, certain environmental factors can compromise this defense, leading to localized corrosion. The most common form of aluminum degradation is pitting corrosion, which appears as small holes or cavities on the surface. Pitting is initiated by chloride ions, found in high concentrations in salt spray, seawater, and de-icing salts. These chloride ions can break down the oxide film, creating a localized electrochemical cell where the corrosion accelerates rapidly.
Highly acidic or alkaline substances can also destabilize the aluminum oxide layer, which is only stable within a pH range of roughly 4.5 to 8.5. For this reason, aluminum beverage cans are manufactured with a thin polymer lining, often made of epoxy resin, on the inside surface. This coating acts as an additional chemical barrier to prevent acidic contents, such as soda and fruit juice, from reacting with the metal from within. Mechanical damage, like a deep scratch or dent, can also physically breach the oxide layer.