Aluminum does not rust, but it will corrode, especially when exposed to saltwater. Rust specifically refers to the corrosion of iron-based metals like steel, resulting in flaky, reddish-brown iron oxide. Corrosion is the general degradation of a material due to chemical reactions with its environment, which is the process aluminum undergoes. Aluminum’s degradation is a distinct chemical process that leads to a different type of damage than rust.
Why Aluminum Does Not Rust
Aluminum does not rust due to its unique chemical reaction with oxygen, a process called passivation. When bare aluminum is exposed to air, it instantly reacts to form a microscopically thin layer of aluminum oxide, or alumina. This layer is durable and adheres tightly to the metal’s surface.
The aluminum oxide layer acts as a permanent barrier, sealing the underlying metal from further atmospheric exposure. Unlike the crumbly, porous rust that flakes off iron, the aluminum oxide film is non-porous and self-healing. If the layer is scratched, a new protective film forms almost instantly, preventing the deep degradation characteristic of iron rust.
The Specific Threat of Saltwater Corrosion
While the natural oxide layer protects aluminum in air, saltwater poses a distinct and aggressive threat to this passive film, primarily through localized attack. The high concentration of chloride ions (Cl-) in seawater is the main culprit. These ions actively penetrate and break down the protective alumina layer, often at weak points or defects in the film.
Once the passive layer is breached, corrosion concentrates in small areas, leading to pitting corrosion. Chloride ions trap themselves in these microscopic holes, accelerating the localized breakdown of the aluminum. This creates deep, structurally damaging pits rather than a uniform surface loss, severely compromising the metal’s integrity.
Saltwater also increases the risk of galvanic corrosion if aluminum is in contact with a dissimilar metal, such as stainless steel or copper. Because saltwater is an excellent electrolyte, the electrical current between the two metals is amplified. This causes the less noble aluminum to corrode rapidly, as the aluminum essentially sacrifices itself to protect the other metal.
Practical Methods to Prevent Aluminum Degradation
Protecting aluminum in a marine environment requires isolating the metal from chloride ions and enhancing its natural defenses. One effective solution is the application of specialized surface coatings, such as marine-grade paints, powder coatings, or epoxy. These barriers physically prevent saltwater from contacting the aluminum, which drives corrosion.
Anodizing is an electrochemical process that significantly thickens the naturally occurring aluminum oxide layer. This makes the layer more robust and resistant to chloride penetration, creating a harder, more durable surface. For applications where galvanic corrosion is a risk, sacrificial protection uses a more reactive metal, typically zinc or magnesium, called a sacrificial anode. These anodes corrode first, diverting the electrical current away from the aluminum structure.
Selecting the correct material provides a layer of defense against saltwater attack. Marine-grade aluminum alloys, specifically the 5000 and 6000 series, are formulated with elements like magnesium to offer superior resistance to chloride-induced pitting. These are the preferred choice for boat hulls and marine structures. Isolating dissimilar metals with non-conductive materials like plastic washers or insulating tape also prevents electrical contact and stops galvanic corrosion.