Understanding Aluminum Corrosion: Causes, Types, and Prevention

Aluminum is a highly versatile metal. 

Its applications span virtually every aspect of daily life, often serving as a key component in products ranging from automobiles and airplanes to marine vessels, roofing and consumer goods. To that point, the Aluminum Association reports that about 75% of all aluminum ever produced in America is still in use today, often repurposed into different forms.

While aluminum may be resistant to corrosion, it’s not immune. Read on as we cover the causes of aluminum corrosion, its various types and how you can best prevent the corrosive process. 

How does aluminum corrosion occur? 

Aluminum corrosion occurs when aluminum comes into contact with oxygen and water, leading to the formation of aluminum oxide. 

This reaction forms a protective oxide layer that adheres to the surface, shielding the underlying metal from further corrosion. However, if this layer is damaged or exposed to corrosive substances like chlorides or if galvanic corrosion occurs due to contact with more noble metals in the presence of an electrolyte, the aluminum can continue to corrode. 

With that said, the nature of the corrosion depends on environmental conditions and the physical and chemical properties of the aluminum alloy.

Rust vs corrosion

While “rust” specifically refers to the oxidation and deterioration of iron and steel, aluminum does not rust because it lacks iron content. However, aluminum does undergo corrosion, particularly forming a protective oxide layer that prevents further degradation under normal circumstances. 

Unlike rust, which typically leads to flaking and structural weakness in iron, aluminum corrosion results in the creation of a hard, protective surface layer. This layer, unless physically damaged or chemically attacked, helps preserve the integrity of the aluminum beneath it.

12 Types of aluminum corrosion 

Aluminum corrosion comes in multiple forms, each with different properties and causes. 

Let’s look into 12 common types of aluminum corrosion. 

1. Atmospheric corrosion

Atmospheric corrosion occurs when metals, such as aluminum and its alloys, are exposed to the environment and react with moisture, pollutants and elements such as oxygen. This type of corrosion is facilitated by the presence of an oxide layer on metals like aluminum, which leads to the formation of aluminum oxide. 

This form of corrosion is prevalent in environments with high levels of moisture and airborne contaminants, and it affects not only pure aluminum but also aluminum products and aluminum alloys used in various applications. The rate and severity of atmospheric corrosion can be influenced by factors such as the presence of protective coatings, environmental conditions and the physical properties of the metal.

2. Galvanic corrosion

Galvanic corrosion occurs when two dissimilar metals, such as aluminum and a noble metal, are in electrical contact in the presence of an electrolyte — leading to the accelerated corrosion of the less noble metal due to a galvanic reaction. 

In these interactions, aluminum, used in aluminum alloys and products, acts as the anode and corrodes faster than it would alone, while the more noble metal acts as the cathode. This type of corrosion is commonly prevented through the use of corrosion inhibitors or by isolating the metals from direct contact, for instance, by employing non-conductive barrier layers or suitable protective coatings to minimize the electrochemical potential differences.

3. Pitting corrosion

Pitting corrosion is a localized form of corrosion that affects metals such as aluminum, leading to the formation of small, deep pits on the metal surface. This type of corrosion is particularly insidious due to its ability to penetrate deeply into the metal while leaving the surrounding areas relatively unscathed. 

Pitting in aluminum and aluminum alloys often begins at microscopic surface defects where the protective oxide film is damaged or disrupted, allowing corrosive agents to attack the underlying metal. 

4. Crevice corrosion

Crevice corrosion is a type of localized corrosion that occurs in confined spaces where the oxygen concentration is significantly lower than in the surrounding environment. This form of corrosion is commonly seen in metal assemblies — such as those involving aluminum or stainless steel — where gaps or crevices exist due to imperfect joint design, the presence of gaskets or overlapping surfaces. 

These crevices create an environment conducive to corrosive processes, as they trap electrolytes and foster a localized chemistry different from that of the open surface. This type of corrosion can be particularly damaging to aluminum products and components, leading to accelerated deterioration within the crevices, often undetectable until severe damage has occurred.

5. Intergranular corrosion

Intergranular corrosion specifically attacks the grain boundaries of metals such as aluminum and its alloys rather than the whole grain itself. This type of corrosion arises when precipitates accumulate at the grain boundaries, typically as a result of improper heat treatments or when exposed to corrosive environments. 

In aluminum alloys, this can lead to a weakening of the microstructural integrity, as the oxide layer at the grain boundaries fails to protect the metal effectively. Intergranular corrosion can be particularly damaging because it preserves the bulk appearance of the metal while potentially leading to fundamental structural failures.

6. Exfoliation corrosion

Exfoliation corrosion is an advanced form of intergranular corrosion found primarily in aluminum alloys, characterized by the lifting or peeling away of thin layers of the metal along the grain boundaries. 

This type of corrosion typically occurs in rolled or extruded aluminum products, where the elongated grain structure is susceptible to moisture and corrosive agents. These agents infiltrate the grain boundaries, causing the layers of the metal to separate and flake off. The visible effect is often a layered deterioration, somewhat resembling the pages of a book. 

7. General corrosion

General corrosion, also known as uniform corrosion, occurs evenly across the entire aluminum surface and is characterized by a consistent and predictable material loss. This form of corrosion is generally considered less harmful than localized corrosion types because it’s easily detectable and its effects are uniformly distributed. This allows for better control and mitigation through techniques such as the application of protective coatings or anodization.

General corrosion typically results from exposure to a corrosive environment that reacts uniformly with the metal, leading to oxidation and the formation of an oxide layer that might offer some level of subsequent protection.

8. Deposition corrosion

Deposition corrosion occurs when metal ions from a more noble metal are deposited onto a less noble metal surface, such as aluminum, in the presence of an electrolyte. This type of corrosion can lead to the formation of a galvanic cell where the deposited metal acts as a cathode and the underlying metal as an anode, accelerating the corrosion of the anodic metal. 

Deposition corrosion is particularly problematic in environments where metals like copper or other heavy metals are present in the fluid flow that contacts aluminum surfaces, leading to increased corrosion rates and potential structural failure of the affected parts.

9. Stress corrosion cracking (SCC)

Stress Corrosion Cracking (SCC) is a form of corrosion that results from the combined effects of tensile stress and a corrosive environment on a metal, such as aluminum or stainless steel. SCC causes the formation of fine cracks on the surface of metals such as aluminum or stainless steel. These cracks can extend deep into the material while the surrounding areas remain largely intact. 

10. Erosion corrosion

Erosion corrosion occurs when a corrosive fluid flows against a metal surface with sufficient velocity, causing mechanical wear and chemical attack simultaneously. This dual action accelerates the deterioration of the metal’s surface, leading to the formation of grooves, gullies or wave-like patterns on the aluminum. 

Erosion corrosion is especially common in systems like piping, pumps and heat exchangers, where the combination of high-speed water flow and corrosive agents intensifies the damage to metal surfaces. Preventative measures typically include material selection with better resistance and modifications to system design to reduce fluid velocities or turbulent flow.

11. Corrosion fatigue

Corrosion fatigue results from the cyclic stress and strain on aluminum in a corrosive environment, leading to the initiation and propagation of cracks over time. Unlike pure mechanical fatigue, corrosion fatigue is exacerbated by chemical or electrochemical reactions at the crack tips.

This type of corrosion is a major concern in dynamic structures where repetitive load and exposure to aggressive environments are common. 

12. Filiform corrosion

Filiform corrosion is a type of localized corrosion that occurs under protective coatings on aluminum, manifesting as thread-like or filamentous patterns. This corrosion form typically starts at small defects in the coating, where moisture and oxygen penetrate and initiate an electrochemical reaction. The corrosion progresses beneath the coating, creating long, worm-like tunnels of corroded metal that can significantly undermine the aesthetic and structural integrity of the underlying metal.

Filiform corrosion is especially problematic in humid environments or where the coated metals are exposed to salt-laden air, making it a frequent concern for aluminum products used in coastal areas. 

Preventing aluminum corrosion 

To prevent aluminum corrosion, consider the following three strategies: Apply a protective coating, opt for the right aluminum grade and test. 

• Apply a protective coating: Applying a protective coating effectively shields aluminum surfaces by creating a durable, corrosion-resistant barrier that can also enhance the product’s aesthetic appeal.

• Opt for the right aluminum grade: Choosing an aluminum grade with high corrosion resistance — like 5052 or 6063 — ensures that the material can withstand harsh environments without degrading.

• Test: Test chambers function by simulating environmental conditions (like humidity, salt spray and temperature changes) to assess how aluminum reacts and to verify the effectiveness of corrosion prevention measures. These tests can inform engineers and product managers on the optimal materials and coatings needed for specific applications and environments, and the potential weaknesses of existing materials. 

Leading aluminum corrosion testing chambers with Auto Technology 

At Auto Technology, we offer a comprehensive range of environmental testing chambers specifically designed to simulate various corrosive environments — allowing for thorough and precise corrosion testing of aluminum and its alloys.

As the premier manufacturer of high-quality innovative environmental corrosion testing equipment, you can trust that our test chambers will provide you with accurate, reproducible results that help ensure the durability and longevity of your aluminum products. 

Click here to learn more about our range of test chambers, or get in touch with an Auto Technology team member today.

FAQs

What does aluminum corrosion look like?

One distinctive characteristic of metals is their reaction to environmental conditions. 

Unlike iron, which rusts into a classic orange-brown flake, aluminum corrodes into a lighter, often white shade. Instead of flaking, aluminum oxide creates a hard, protective layer that encapsulates the affected area — akin to container rust. 

How long does it take for aluminum to oxidize?

Determining the precise duration for aluminum to oxidize is challenging due to the many variables involved. However, it generally oxidizes more quickly than steel because of its anodized nature and its reaction to oxygen. The way aluminum oxidizes — forming a hard outer layer — contributes to its resilience. This protective layer prevents peeling or softening, maintaining aluminum’s durability over time.