Aluminum is one of the most widespread elements, making up about 8.1% of the Earth’s crust. In addition to its abundance, it’s one of the most commonly used industrial metals. This is due to its unique combination of desirable properties, such as a high strength-to-weight ratio, good electrical and thermal conductivity, cost-effectiveness, and formability.
Aluminum is also known for its natural corrosion resistance and durability. However, just like any other metal, it can undergo corrosion in certain conditions. This can affect its mechanical properties and lead to structural weakness or even premature material failure in extreme cases.
In this article, we’ll delve deeper into aluminum corrosion, its causes, types, and how we can prevent it. Let’s get started!
What is Aluminium corrosion?
Aluminum corrosion is a gradual process where the metal deteriorates, forming oxides that degrade its physical and chemical properties.
Aluminum is fundamentally a reactive metal, but it typically behaves like a passive metal. This is because it instantly reacts with oxygen or moisture in the environment to form a tough aluminum oxide layer on its surface. This non-reactive oxide layer protects the underlying metal and shields it from further corrosion.
If the oxide layer keeps getting removed, aluminum corrodes at a fast pace due to its high affinity to oxygen. This situation may arise when the metal is exposed to certain conditions, such as acidic or alkaline solutions.
Corrosion vs Rust: What is the Difference?
Rust forms when iron or its alloys react with oxygen in the presence of moisture to form iron oxide. It is a reddish-brown color and typically disintegrates over time.
Aluminium is a non-ferrous metal, meaning it doesn’t contain iron. As such, it does not rust but corrodes in some environments. Aluminum corrosion typically appears as a thin, whitish layer that does not flake.
What Causes Aluminum Corrosion?
Aluminum is highly resistant to corrosion thanks to its innate protective oxide layer. However, there are several factors that can affect this oxide film, leading to aluminum degradation. They include:
Changes in pH
The optimal pH range for the oxide film is between 4 and 9. In highly acidic or alkaline environments, the protective layer degrades, leading to corrosion of the exposed aluminum surface. Common factors that can cause pH changes include industrial emissions, acidic rain, and exposure to chloride anions.
Environmental factors
Humidity, pollutants, and temperature variations can have a huge impact on untreated aluminium. Exposure to moisture and high temperatures can break down the oxide layer. Also, aluminum corrodes in the presence of pollutants such as sulfur dioxide.
Electrochemical reactions with other metals
Metal fabrication projects often require aluminum to be paired with other metals. Unfortunately, aluminum can corrode if it is in contact with a noble metal in the presence of an electrolyte. This corrosion phenomenon is common in structures where aluminum is linked to copper, zinc, or brass in marine environments.
Mechanical stresses
When exposed to sufficient stress levels, aluminum and its alloys will develop microcracks and openings. This allows corrosive elements to reach the underlying, vulnerable metal surface. 7079-T6 is an example of a susceptible alloy.
Types of Aluminium Corrosion
There are different forms of corrosion that can affect aluminum and its alloys. They include:
Atmospheric corrosion
When aluminium is exposed to the environment, it can react with rainwater or condensing water, oxygen present in the air, and atmospheric pollutants, resulting in corrosion. This type of damage is known as atmospheric corrosion, and it is the most common type of aluminium corrosion by far. Atmospheric corrosion is often categorized as dry, damp, and wet, depending on the humidity levels of the environment.
Since environmental conditions can vary greatly in different geographical locations, some regions will experience more corrosion than others. Factors such as precipitation, wind direction, temperature changes, and closeness to large bodies of water can have a significant impact on atmospheric corrosion.
Also, you may observe a greater corrosion rate in designs that do not support efficient drainage of moisture.
Galvanic corrosion
Galvanic corrosion, or dissimilar metal corrosion, is one of the biggest concerns for aluminum alloys. This type of corrosion occurs when aluminum is connected through an electrolyte (such as salt water) to a more noble metal. Any metal that is less reactive than aluminum (e.g., zinc, copper, brass, and some types of steel can be considered to be a noble metal.
For galvanic corrosion to take place, three conditions must be met:
Dissimilar metals
Metal-to-metal contact
The presence of an electrolyte solution to complete the electrical circuit
There is a higher risk of galvanic corrosion in marine environments due to the presence of saltwater. This is a problem in boats and yachts, where aluminium parts are often in close contact with dissimilar metals.
A simple way of preventing galvanic corrosion is through electrical insulation. By insulating the different metal parts, you eliminate the electrical contact generated by the electrolyte solution. Also, paint and other surface treatments can be applied to the most noble metal to prevent corrosion.
Pitting corrosion
Pitting corrosion is a localized type of degradation that results in small holes, or “pits” on the aluminium surface. It occurs quickly and, if left unchecked, can affect the metal structure. Unaddressed pit corrosion can lead to other damaging mechanisms, like stress corrosion cracking or corrosion fatigue.
Common causes of pit corrosion include:
Surface defects such as scratches, scuffs, and excessive roughness
Defects in the protective coating, including uneven application and cracks
Chemical attack by corrosive agents such as chloride solutions, organic acids, and fluoride ions
Non-uniform stress
Environmental factors such as humidity, temperature, pH levels, and condensation
You can prevent this type of corrosion using protective coatings such as paints, epoxies, and anodizing. Another effective way of controlling pitting corrosion is cathodic protection. With this technique, aluminium is coated with a more reactive metal, which serves as the sacrificial anode. It will oxidize and corrode first, protecting the underlying aluminum surface.
Crevice corrosion
Crevice corrosion occurs in narrow, confined spaces. These tight gaps allow for moisture or electrolytes to get trapped and stagnant, resulting in corrosion.
For example, when seawater collects in a small crevice, oxygen levels will deplete over time. This leads to a surplus of positive metal ions in the crevice, turning its interior into an anode. In an attempt to restore balance, the aluminum combines with negative ions in the crevice (usually chloride ions), further corroding the metal.
Crevice corrosion is prevalent in small spaces where the flow of fresh, oxygen-rich fluid is restricted. Common locations include:
Bolted and riveted connections
Under gaskets and seals
At lap joints and washers
Weld seams
Under insulation
In fastener threads
You can control crevice corrosion by eliminating small gaps and promoting electrolyte flow to prevent stagnation.
Intergranular corrosion
Like other common materials, aluminium metal is composed of many tiny crystals known as grains. Intergranular corrosion, or intercrystalline corrosion, involves a localized corrosive attack at and adjacent to grain boundaries with minimal attack on the grains themselves.
Intergranular corrosion in aluminum is caused by impurities and alloying elements that precipitate at grain boundaries. Heat treatment, alloy composition, and electrochemical potential are some of the factors that can exacerbate this type of corrosion.
Some aluminum grades are more susceptible to intergranular corrosion than others. This includes aluminum grades 2024, 7075, 5083, and 7030.
Exfoliation corrosion
This is a severe type of intergranular corrosion that occurs when grains are flattened without recrystallization during processes such as hot or cold rolling. It is characteristic of aluminum alloys that have marked directional structures.
Exfoliation corrosion happens along grain boundaries in the microstructure. As corrosion products accumulate beneath the aluminum metal surface, a wedging force lifts the overlying layers, resulting in a bulging or swelling appearance. If disturbed, this bulge reveals layers of material flaking off. The term “exfoliation” comes from this peeling appearance.
Aluminium alloy systems containing copper (Al-Cu), Magnesium (Al-Mg), and a combination of Zinc and Magnesium (Al-Zn-Mg) are especially susceptible to this corrosion form due to their highly directional grain boundaries. You can avoid exfoliation in these alloys by using heat treatment methods to prevent the formation of susceptible microstructures.
General corrosion
General corrosion occurs when a chemical or electrochemical reaction causes uniform degradation across the surface of the aluminum part. For this reason, this type of attack is also known as uniform corrosion.
General corrosion can be caused by exposure to corrosive agents, such as alkaline or acidic solutions. It may also take place when the aluminum surface is in an electrolyte. These conditions result in a uniform attack on the surface, causing a reduction in the material thickness and overall structural strength. The slow and gradual nature of uniform corrosion makes progression predictable and easy to monitor.
Deposition corrosion
As the name suggests, deposition corrosion occurs when a corrosive coating or contaminants are deposited on the aluminium surface, causing a localized attack. With this corrosion process, the aluminum metal will react with the deposited material, resulting in its degradation.
For example, when water flows through copper piping, it may pick up copper ions. When the resulting solution comes into contact with an aluminum part, the copper ions are deposited on its surface. This generates a galvanic cell where the aluminum acts as the anode and corrodes rapidly.
Other metals that can affect aluminum in this way include lead, tin, and nickel.
Stress corrosion cracking
Stress corrosion cracking (SCC) is the fracture of aluminum or its alloys due to the combined effect of tensile stress and a corrosive environment. In addition to these two conditions, the specific alloy has to be susceptible to SCC for this form of corrosion to occur.
High-strength aluminum alloys, with elements such as copper, magnesium, zinc, and silicon, are more prone to stress corrosion cracking. There are two types of SCC processes: intergranular stress corrosion cracking (IGSCC) and transgranular stress corrosion cracking (TGSCC).
SCC can cause sudden structural failures at stress levels far below the yield strength of aluminum. This makes it a major concern in the aerospace and automotive industries since they often utilize high-strength alloys.
You can improve the stress corrosion resistance of aluminum alloys through surface treatment, controlling the environmental conditions, and heat treatment.
Erosion corrosion
Erosion corrosion is caused by the rapid flow of a turbulent fluid on the aluminum body. Fluids moving at high velocities can accelerate damage by eroding the protective oxide layer. This exposes the underlying metal surface and leaves it susceptible to further chemical attack.
Some factors that can aggravate erosion corrosion are:
pH levels
Existence of abrasive solid particles in the fluid
The chemicals present in the fluid
Erosion corrosion is prevalent in piping systems and commonly affects fittings, particularly at bends. You can minimize this type of corrosion by implementing good design practices, filtration to remove solid particles, and the use of de-aeration and corrosion inhibitors.
Filiform corrosion
Filiform or wormtrack corrosion is a localised corrosion process that manifests as fine, thread-like filaments under painted aluminum.
Filiform corrosion occurs at coating defects such as scratches and bruises, with moisture and electrolytes penetrating through the damaged regions. This creates an electrochemical cell under the coating. The “head” of the wormtrack is the anode, and its “tail” is the cathode. As a result, corrosion products that occupy more space than the original metal will precipitate, causing the coating to blister or lift.
Humid environments and the presence of chlorides favor wormtrack corrosion. You can protect aluminium alloys from this corrosion form through surface pretreatment procedures, proper coating selection, and the careful inspection of coatings to minimize imperfections.
Aluminium Corrosion Vs Laser
Corrosion on aluminum can interfere with processes such as welding, painting, and other surface treatments. One of the most effective ways to remove oxide layers and contaminants is through laser cleaning. This method utilizes a high-energy, focused laser beam.
Once the oxide layer absorbs the laser energy, it heats up and vaporizes, leaving the underlying aluminum metal surface untouched. One of the key benefits of laser cleaning is its precision. This technique allows you to remove the unwanted layer without causing damage to the metal surface. Unlike traditional cleaning methods, there is no risk of scratching or deformation.
Aluminum laser cleaning is mainly used for welding preparation, surface restoration, and pre-coating treatment.