Since its discovery in 1827, aluminum has become a vital material in several industries. Its physical and chemical properties, along with its ability to go through multiple manufacturing processes, make it an ideal construction material with a life span that exceeds most other known metals.
What Makes Aluminum a Desirable Material?
Aluminum stands as one of the most abundantly available metals on earth. About 8% of the earth’s crust contains aluminum. This metal is also 100% recyclable, with the ability to be recycled indefinitely without any significant reduction in quality.
The overall weight of aluminum is one-third that of steel, which makes it a highly prized in the automotive and aerospace industry. Aluminum’s tenacity (the measure of a material’s resistance to deformation) is also very similar to steel, making it ideal for the construction of durable, lightweight components and structures.
Moreover, aluminum does not need much maintenance and generally lasts for years in appropriate environmental conditions. These desirable properties are complemented by the fact that this material mixes well with other metals to create stronger and more durable metal alloys.
Interesting Facts About Corrosion of Aluminum
Despite being considered a highly durable material, aluminum has its limitations. In the following sections, we will go through some important facts about aluminum corrosion that industry professionals must know.
1. Aluminum Is Naturally Corrosion Resistant
Aluminum’s corrosion resistance is mainly responsible for its popularity in many industrial and domestic applications. It belongs to the Boron family; as such, it oxidizes quickly to become highly inert under normal conditions.
In its pure, natural form, aluminum is very reactive. However, as soon as it is exposed to air, the surface reacts with oxygen to form an oxide layer of aluminum oxide (Al2O3). This oxide layer protects the underlying aluminum from further reactions and helps it maintain its durability. The oxide layer is very thin (about 2.5 nm) and can be removed by scratching. However, the exposed metal quickly reacts with the atmosphere again, and the protection resumes.
2. Aluminum Corrodes Rapidly Under Some Conditions
Aluminum is the perfect choice for many conditions. The oxide layer on its surface remains unaffected in a wide pH range of 4.0 to 8.5. Additionally, the layer can repair itself, which allows aluminum structures to continue to function even if they are exposed to harsh environments for short periods of time. (Further reading: A Look at Self-Healing Metal Oxides as a Corrosion Prevention Method.)
There are, however, some instances where aluminum is susceptible to corrosion if left untreated; these include:
- Extreme acidic/alkaline conditions (pH below 4 and above 9)
- Constant exposure to humidity
- Unventilated and enclosed indoor spaces
- Marine environments
- Conditions supporting galvanic corrosion
3. Aluminum Has the Best Durability in Dynamic Environments
The durability of aluminum in dynamic environments is unmatched. Many materials are considered to be corrosion-resistant; however, they can only remain stable under strict conditions. Aluminum, on the other hand, has few environmental restrictions as long as the oxide layer is given enough time to repair itself.
For example, an outdoor aluminum shed will work perfectly all year round even if it is exposed to rain, snow and high temperatures. As long as it gets time to dry and regenerate itself, the aluminum shed components will remain mostly unaffected. However, if the same structure is kept in an enclosed and unventilated environment, or an area with consistent exposure to moisture, corrosion may occur.
4. Concrete Can Be Damaging for Aluminum
While aluminum is the go-to material for window frames and doors, it doesn’t mix well with concrete. The alkaline compounds in wet concrete, in particular, along with the presence of steel nails and rivets, react with aluminum to initiate corrosion processes.
In building frames, aluminum surfaces are often treated with an inert coating that prevents concrete from reacting with the metal. It is widely recommended to use bituminous paint to coat aluminum members. (Learn how by reading our download Bituminous Coatings: When and How to Use Them.) Using a concrete mix without soluble chlorides can also help reduce the chances of aluminum corrosion.
5. Galvanic and Pitting Corrosion Are the Most Common Corrosion Types for Aluminum Surfaces
Galvanic corrosion and pitting corrosion are the biggest culprits when it comes to corrosion-related deterioration of aluminum.
Galvanic corrosion occurs when two dissimilar metals react with each other in the presence of an electrolyte. In such cases, the more inert metal (i.e., less reactive) gets protected at the expense of the other. In the case of aluminum, common metals that contribute to galvanic corrosion are copper, zinc and galvanized steel.
Steel fasteners and rivets are commonly used in most aluminum frames. These are harmless in dry conditions, but in the presence of an electrolyte, the aluminum frame will start to break down. To prevent this, architectural aluminum is painted and mixed with either magnesium or silicon. These elements increase its corrosion-resistant properties and make the frame suitable for use in humid conditions. Magnesium, especially, is frequently used in marine-grade aluminum sheets because it significantly reduces the chances of galvanic corrosion.
The other likely type of corrosion, pitting corrosion, occurs on the surface of aluminum. The primary cause of pitting in this material is humidity in the presence of different salts. Pitting corrosion can even occur when aluminum is not in contact with any other material. In general, while mild pitting corrosion may only make the surface unappealing, the metal’s structural integrity can be severely compromised if left untreated.
When it comes to corrosion-resistant materials, aluminum is not the only choice. However, very few materials can match the versatility it offers. While aluminum corrosion is usually not a worry under normal conditions, it can corrode in certain environments.
For most applications, aluminum is a cost-effective and durable material choice. Depending on the application, aluminum members may have to be treated before they are put into use. It is, therefore, crucial for engineers and designers to understand the limits of aluminum and the specific conditions under which it may deteriorate.