Corrosion of Aluminum and Aluminum Alloys

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Introduction: Aluminum's Natural Corrosion Resistance

Aluminum is widely valued in industry for its excellent corrosion resistance, owed to a robust oxide film that forms naturally and adheres strongly to its surface. This barrier oxide layer, just 1 nanometer thick on freshly abraded surfaces, is highly effective at preventing corrosion and reforms quickly if damaged. The formation and stability of this natural film result from a dynamic equilibrium between forces that build and break down the barrier. In dry air, the film reaches its maximum thickness rapidly, while in highly destructive environments, it may hydrate faster than it can form, reducing its effectiveness. In most service conditions, a balance between these forces results in relatively thick (20 to 200 nm) protective films.

Pitting Corrosion of Aluminum Alloys

Pitting corrosion is the most common localized corrosion mechanism affecting aluminum in passive environments. It typically manifests as random pits on the metal's surface. The pitting potential principle determines when metals in the passive state become susceptible to pitting corrosion.

For aluminum, pitting is most often caused by halide ions—especially chloride (Cl⁻)—which are frequently encountered in service. In aerated halide solutions, aluminum is easily polarized to its pitting potential due to the presence of oxygen. Conversely, in aerated solutions containing non-halide salts, aluminum generally does not pit because the pitting potential is much more cathodic, and typical service conditions do not reach these levels.

The Role of Solution Potentials in Corrosion

Corrosion processes are largely electrochemical, making the solution potentials of various aluminum alloys—and the relationships between constituents—crucial to understanding corrosion behavior. The solution potential of an aluminum alloy is primarily determined by the composition of the aluminum-rich solid solution, which forms the majority of the alloy's microstructure.

Microscopic second-phase particles often have solution potentials different from the matrix, creating localized galvanic cells that can accelerate corrosion. Since most commercial aluminum alloys contain several alloying elements, their effects on solution potential are approximately additive. The proportion of elements retained in solid solution, especially in highly alloyed materials, depends greatly on fabrication and thermal processing, so heat treatment and processing have a significant influence on the final electrode potential.

Solution potential measurements are valuable for evaluating heat treatment, quenching, and aging in alloys containing copper, magnesium, or zinc. For example, in 2xxx aluminum-copper and aluminum-copper-magnesium alloys, potential measurements can indicate the effectiveness of solution heat treatment and the susceptibility to intergranular corrosion and stress corrosion cracking (SCC). In 7xxx zinc-containing alloys, these measurements help track aging and distinguish among tempers, while in 5xxx magnesium alloys, they can detect low-temperature precipitation and assess stress-corrosion behavior.

Corrosion in Different Environments

Atmospheric Corrosion

Most aluminum alloys exhibit outstanding resistance to atmospheric corrosion, often requiring no protective coatings or maintenance in outdoor environments.

Corrosion in Water

Aluminum alloys from the 1xxx, 3xxx, 5xxx, and 6xxx series are generally resistant to corrosion in many natural waters. Critical factors affecting the corrosivity of water include temperature, pH, conductivity, presence of cathodic reactants, heavy metals, and the specific corrosion and pitting potentials of the alloys.

Effects of Alloy Composition and Microstructure

1xxx Wrought Alloys

These high-purity aluminums have excellent corrosion resistance, though resistance may decrease slightly with higher impurity content. The main impurities are iron, silicon, and copper, with copper and some silicon remaining in solid solution.

2xxx Wrought and Casting Alloys

Containing copper as the primary alloying element, 2xxx series alloys are less resistant to corrosion. Originally developed as the first heat-treatable high-strength aluminum materials, they see widespread use in aerospace. Electrochemical effects, such as changes in electrode potential with varying copper content and inhomogeneities in solid solution, can exacerbate corrosion. However, the primary reason for reduced resistance is the formation of galvanic cells by minute copper particles or surface films during corrosion.

Note: Many thin sheets of 2xxx alloys are produced as alclad composites for additional protection, while thicker sections may not require cladding.

2xxx Alloys with Lithium

Lithium additions reduce density and increase modulus, making aluminum-lithium alloys attractive for aerospace. However, the impact on corrosion must be considered in design.

3xxx Wrought Alloys

Aluminum-manganese and aluminum-manganese-magnesium alloys (3xxx series) offer very high resistance to corrosion. Manganese is present as a solid solution as well as in precipitate particles, which typically have solution potentials similar to the matrix.

4xxx Wrought and 3xx.x and 4xx.x Casting Alloys

Silicon appears as second-phase particles in 4xxx wrought alloys and in brazing, welding, and casting alloys of the 3xx.x and 4xx.x series. The corrosion resistance of 3xx.x casting alloys is strongly influenced by copper content and impurity levels. Alloys with tighter impurity limits generally exhibit superior corrosion resistance and mechanical properties.

5xxx Wrought and 5xx.x Casting Alloys

Aluminum-magnesium and related alloys in the 5xxx and 5xx.x series are highly resistant to corrosion, making them suitable for construction, chemical processing, food handling, and marine applications.

6xxx Wrought Alloys

Heat-treatable aluminum-magnesium-silicon alloys (6xxx series) offer a balance of moderate strength and excellent corrosion resistance, making them popular in structural, marine, and processing equipment applications.

7xxx Wrought and 7xx.x Casting Alloys

Alloys in the 7xxx and 7xx.x series contain zinc, often with magnesium or magnesium plus copper, resulting in high strength. Copper-containing 7xxx alloys are primarily used in aerospace structures. Copper-free 7xxx alloys provide moderate-to-high strength and excellent toughness, workability, and weldability, leading to their increased use in automotive, structural, and military applications. However, due to their anodic nature, 7xxx alloys are among the most susceptible aluminum alloys to stress corrosion cracking.

The general corrosion resistance of copper-free 7xxx alloys is good, comparable to 3xxx, 5xxx, and 6xxx alloys. Alloys with copper, such as 7049, 7050, 7075, and 7178, are more susceptible to corrosion but still outperform 2xxx alloys. Notably, the presence of copper, while reducing general corrosion resistance, improves resistance to stress corrosion cracking.