While some people want to remove aluminum oxides for aesthetics, there are other good reasons to remove them. Oxide removal is essential to guarantee high quality for the following reasons:
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Aluminum oxidation is a form of corrosion that appears as a white powder or a gray layer on the surface of aluminum. When the oxide layer is extremely thin (a few nanometers thick), it is near transparent. If oxides are visible to the naked eye, it indicates a thicker layer and an advanced form of oxidation.
The chemical formula for aluminum oxide is Al2O3. It is formed by the chemical reaction of aluminum molecules (Al) with oxygen molecules (O2). Because oxygen is present everywhere from ambient air to water droplets, the oxidation of aluminum is difficult to prevent.
Aluminum is said to be corrosion resistant because once the oxide layer is formed, it acts as a protective barrier that slows down further oxidation. This natural barrier is one of the reasons why aluminum is so popular.
Still, aluminum oxide is a porous material. For this reason, further corrosion can form beneath the oxide layer over time—especially in moist environments.
Several methods can be used to clean oxidized aluminum. Laser cleaning, mechanical abrasives, pickling chemicals, and other chemicals have all proven to be effective. Let’s see how they work and their benefits and drawbacks.
Laser cleaning is a process that removes aluminum oxides by ablating them, turning them into dust and fumes. As the energy of the laser beam is absorbed by the surface, oxides and surface contaminants are heated up to the point where they vaporize.
Aluminum oxides have a lower ablation threshold than the aluminum substrate underneath. This means that the laser can send just enough energy to ablate the oxides and not the aluminum. If you’re curious about the details of the process, you can learn more about how laser cleaning works here.
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Pickling is the process of using acids or other chemicals to remove oxides and impurities from the surface of aluminum. During pickling, the aluminum is exposed to an acidic solution that reacts with the oxide layer, causing it to break its bond with the surface and dissolve in the acidic solution.
Pickling is typically done to prepare aluminum for a welding or coating process. The most common pickling method is immersion, where the aluminum is immersed in an acidic solution. For welding, other methods can be used, such as manually applying a heated carbon brush to the welding joints (as shown in the previous video) or using robots to apply acids with a brush or a spray.
After the pickling process, the acidic cleaning agent needs to be removed from the aluminum surface and neutralized. This can be done manually by wiping the surface with a dampened clean cloth, then by applying a neutralizing agent, and finally by rinsing off with water. This can also be done by immersing the aluminum part in a series of baths. Thorough drying is essential to ensure that no residual acidic or neutralizing agents remain on the surface.
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Mechanical abrasives such as wools, sandpapers, wire brushes, and grit blasters rely on the principle of abrasion to remove oxides from aluminum. Whether it's the grit of sandpaper, the bristles of a rotating brush, or the abrasive particles of a grit blaster, physical force and friction will gradually wear away the oxide.
Aluminum oxide is a hard and durable material that has a strong bond with the aluminum underneath. This makes it difficult to remove using abrasive methods (unlike the flaky orange rust on steel). To do this manually, you must be prepared for some elbow grease.
Mechanical abrasion also generates lots of dust that needs to be managed properly to prevent the following issues:
In 2009, the OSHA provided precautions when grinding aluminum to avoid explosions:
Such a reaction occurs when an aluminum particle and a metal oxide, such as rust, are ignited by a heat source and chemically burn […]. The reaction is similar to a fireworks explosion, can create 4,500ºF sparks, and can occur when a grinder is used on ferric material (e.g., steel) prior to being used on an aluminum material or vice versa.
Hazards associated with aluminum grinding, October 8, 2009
For example, steel wools are not ideal to scrub oxidized aluminum, as they can introduce iron particles onto the aluminum surface. This can lead to galvanic corrosion, a form of corrosion between different types of metals that can significantly reduce the structural integrity of the aluminum.
For manual scrubbing, a stainless steel wire brush is a good tool, as stainless steel is harder than aluminum oxide and will not introduce steel particles onto the surface.
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Using alkaline and acid solutions, DIY enthusiasts can use common household items for aluminum cleaning.
Alkaline solutions can be used to neutralize and break down greases, oils, dirt, and grime, but they are not suited to remove oxides. They are composed of chemicals with a pH greater than 7 and include items such as baking soda and soap mixed with water. After alkaline cleaning, contaminants are easy to remove using water and a clean cloth.
Alkaline cleaning is ideal to prepare surfaces for oxide removal, such as for mechanical abrasion with a sandpaper. You might find this guide on alkaline cleaning interesting.
Acidic solutions can break down and dissolve light oxidation and tarnish. They are composed of chemicals with a pH lower than 7 and include items such as lemon juice, white vinegar, and cream of tartar mixed with water.
While acid cleaning can remove light oxidation, it is not a good method to remove firmly embedded oxides or thick oxide layers. For heavy-duty applications, specialized aluminum cleaners with stronger compounds are needed.
After using acid or alkaline solutions, it’s essential to remove all chemicals from the aluminum surface (especially before scrubbing it with abrasives). You can also apply protective coating, such as a clear coat or an anodizing treatment, to further improve corrosion resistance and extend the lifespan of your part.
In the following videos, you can see examples of how laser oxide removal is done to prepare various aluminum products for adhesive bonding, welding, and coating processes.
Now that you understand how to remove oxides, you should have a good idea of which method is right for you. For laser oxide removal, you can reach out to our experts to discuss your application and find a solution that meets your needs.
With a PhD in Laser Processing, Alex is one of the two laser experts who founded Laserax. He is now Vice President and Chief Technology Officer, overseeing the team that develops laser processes for laser marking, cleaning, texturing, and welding applications.
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