Aluminum does not rust, but it does undergo corrosion, a process of degradation. While “rust” is often used broadly to describe metal deterioration, it refers specifically to the oxidation of iron, which aluminum does not experience. Aluminum is a highly reactive metal, yet it maintains remarkable resistance to its environment due to a unique chemical phenomenon on its surface. This process creates a stable barrier protecting the underlying metal, though this protection is not absolute.
Rust vs. Corrosion
Corrosion is the general term for the deterioration of any material, typically a metal, due to a chemical or electrochemical reaction with its surroundings. This process converts the refined metal back into a more chemically stable form, such as an oxide, hydroxide, or sulfide. Rust is a specific type of corrosion reserved for the oxidation of iron or iron-containing alloys like steel.
Rust forms when iron reacts with oxygen and moisture, creating hydrated iron(III) oxide, a flaky, reddish-brown substance. The corrosive process in aluminum results in a thin, whitish or grayish build-up on the surface. This substance is aluminum oxide, which looks very different from the characteristic red-orange flakes of rust.
The Protective Aluminum Oxide Layer
Aluminum’s natural resistance to corrosion stems from its strong affinity for oxygen, causing it to react instantly with the air. This rapid reaction forms an extremely thin layer of aluminum oxide on the metal’s surface. This self-forming layer is dense, tightly adhering, and non-porous, effectively sealing the underlying aluminum from further environmental contact.
The formation of this oxide layer is an example of passivation, where the metal becomes non-reactive in a specific environment. The layer is incredibly thin, but acts as an electrical insulator and a physical barrier against corrosive agents. Unlike porous iron oxide, which flakes away to expose fresh metal, the aluminum oxide layer remains intact. It is also self-healing under neutral conditions, allowing aluminum to endure for decades when exposed to the atmosphere.
Types of Aluminum Corrosion
While the passive oxide layer offers significant protection, specific environmental conditions can compromise it, leading to aluminum degradation. This deterioration typically occurs in localized forms, unlike the uniform decay seen with rusting iron. The corrosive output is usually a white powder or crystalline deposit.
Pitting Corrosion
Pitting corrosion is characterized by the formation of small holes or micro-dents on the surface. This localized attack is often initiated by aggressive ions, particularly chlorides found in salt water or de-icing chemicals. These chloride ions penetrate the protective oxide film, leading to concentrated corrosion beneath the surface.
Galvanic Corrosion
Galvanic corrosion occurs when aluminum is in electrical contact with a more noble metal in the presence of an electrolyte like moisture or salt water. Aluminum is more electrochemically active than metals like copper or steel. This causes aluminum to act as the anode and corrode preferentially to protect the other metal, which can quickly deplete the aluminum, especially in marine environments.
Crevice Corrosion
Crevice corrosion occurs in tight gaps or shielded areas where oxygen access is restricted, such as under washers or between overlapping parts. The stagnant moisture and depleted oxygen within the crevice create an acidic environment. This environment destabilizes and dissolves the protective oxide layer, leading to aggressive, localized attacks that can be difficult to detect until structural damage has occurred.
Protecting Aluminum Surfaces
Several methods can reinforce aluminum’s natural defense mechanism and prevent various forms of corrosion. Anodizing is an electrochemical process that artificially thickens the natural oxide layer. This creates a much more durable and controlled surface, enhancing the passivation layer and making it an integral part of the metal.
Applying a protective coating is another common strategy, acting as a physical shield between the metal and corrosive elements. Powder coating, which uses a dry powder cured into a tough barrier, is highly effective and resistant to chipping. Liquid paint can also be used, but it requires a suitable primer designed for aluminum.
Regular maintenance, such as cleaning, is a simple yet effective protective measure. Washing aluminum with mild detergents and water helps remove corrosive agents like dirt, salt, and pollutants before they compromise the oxide layer. When joining aluminum to dissimilar metals, using insulating materials like nylon washers or non-conductive sealants is necessary to prevent damaging galvanic corrosion.