Which coating is best for aluminium?

The excellent properties of aluminum make it a regular choice for extrusion designers in a wide range of industries. It has a fantastic strength-to-weight ratio, good corrosion resistance, good thermal and electrical conductivity, and more. While this may be enough for some applications, designers will frequently include additional finishing steps during production.

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Why is this? Simply put, the right surface finish can elevate products from &#;good&#; to &#;outstanding!&#; Surface finishes can improve corrosion resistance, wear resistance, and aesthetic appearance to make extrusions last longer and look better.

Keep reading to learn what finishing options are available for your custom aluminum extrusions and what makes each unique. Many of these finishes can also be applied to die cast, rolled, or machined aluminum products.

At first, all aluminum extrusions have a mill finish. This reflects the state of the aluminum after it leaves the extrusion die. Die and tooling marks, a dull appearance, oxidation, and white &#;rust&#; can all be present on the aluminum after extrusion.

Generally speaking, a mill finish is only suitable for products that need to be made as cheaply as possible, where appearance doesn&#;t matter and they are only expected to last a short amount of time. Otherwise, you should at least consider one of the other aluminum finish options.

Anodizing is commonly used to improve the corrosion resistance and wear resistance of aluminum parts. It can also provide other desirable benefits, like improved heat dissipation and the ability to accept dyes.

Unlike paints and coatings, anodizing does not apply an external product to the exterior of the aluminum. Instead, an electrochemical process is used to stimulate the naturally-occurring layer of aluminum oxide into growing thicker. This protects the underlying aluminum from its environment, and can even accept dyes. However, the color possibilities for anodizing are not as abundant as in powder coating.

For bright, long-lasting colors with high UV resistance, many product designers will choose powder coating. For this process, aluminum parts are cleaned and prepped, then sprayed using an electrostatic gun to apply dry paint powder to the aluminum surface. After this, the powder-coated aluminum parts are baked to melt and cure the coating into an even finish.

Powder coatings are not as hard and durable as anodized aluminum, but they provide an exceptional appearance and great corrosion resistance as long as the coating remains intact. The powder coating process is also considered more environmentally friendly than liquid painting, which uses solvents containing volatile organic compounds (VOCs).

Polyvinylidene fluoride (PVDF) coatings are resin-based coatings that often have embedded color pigment particles. They must be applied to the aluminum product in a factory setting and can be used to create a wide variety of matte-finish colors.

PVDF coatings are highly corrosion resistant, highly resistant to fading, and chemically resistant, making them excellent for exterior applications like aluminum wall cladding. They also don&#;t require an oven for curing and can be applied in a thinner layer than powder coatings. Finally, while it can be considered slightly more wasteful than powder coating, this aluminum coating does not contain any harmful VOCs.

While PVDF is technically a liquid paint, it is a somewhat distinct process from other liquid paints. Liquid paints are an enticing option when you need a very particular color, especially for small batches. That&#;s because liquid paints offer the widest selection of color possibilities, and paint can be mixed up in the quantities needed for the products in question.

Unfortunately, the solvents used in these types of paints contain harmful VOCs that are bad for both people and the environment. Painters need to use VOC abatement systems to mitigate the risks associated with these types of aluminum surface finishes.

Rather than one type of process, mechanical finishes encompass an assortment of methods used to affect the surface of aluminum parts. Common examples include abrasive blasting, grinding, and polishing.

Instead of applying chemicals, mechanical force is applied using abrasive media to smooth the aluminum&#;s surface. This can improve its appearance or prepare it for another finishing process.

Commonly seen in more artistic applications, sublimation is a process for creating colored patterns on aluminum. The patterns are transferred from a film wrapping onto the aluminum surface using heat.

A popular choice for sublimation in extruded aluminum is to create a wood pattern. This allows designers to have more flexibility in the products they create. Like creating outdoor furniture or window frames with a wooden appearance that has improved durability and lighter weight from the aluminum.

A cheaper alternative to anodizing, Alodine coatings can be used to enhance the passivating oxide layer on the surface of aluminum products, improving corrosion resistance. While the Alodine chem film is not as strong or wear resistant as anodized aluminum, it has a high electrical conductivity that may be necessary for some applications.

It should be noted however that the Alodine solution is not as environmentally sustainable as other options on this list.

Finally, bright dipping is a process for creating a brilliantly shiny surface finish on your aluminum parts. By bathing them in acid, the microscopic fluctuations in the surface of the extruded aluminum are smoothed to create a &#;mirror-like&#; finish.

While it may be required to create certain products, bright dipping is a potentially dangerous and environmentally unsafe process. It also only works well with certain grades of aluminum, namely .

Summary

Overall, aluminum surface finishes are a great idea to get more out of your product. While mill finish aluminum will cost less upfront, any applications where corrosion and wear are a concern will often need more frequent replacement of unprotected aluminum products. This means money savings over the long run.

Surface finishing extruded aluminum is also important if visual appeal is important to your design. Depending on your needs, you can even combine different processes to get the benefits of both.

To learn more about aluminum finishes, joining, and other important considerations for aluminum extrusions, download our Aluminum Extrusion Design Guide.

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When you&#;re working with aluminum components, it&#;s vital to consider whether the pieces need engineered coatings to give the performance specifics you need and want. Those specialized layers could bring desirable results by protecting the surface underneath, plus providing additional characteristics that promote or prevent certain scenarios.

Here are five examples of engineered coatings that work well for aluminum or aluminum alloys.

They&#;ll give you some valuable inspiration for thinking about possibilities associated with your aluminum parts and highlight how necessary it is to think about how the aluminum components you want to coat will get used.

1. Polyvinylidene Fluoride (PVDF) Resin

Polyvinylidene fluoride (PVDF) resin is an engineered coating often used for architectural applications due to its excellent weather resistance. Aluminum roof sheets and wall claddings are some of the most common components that get coated with PVDF.

It&#;s also possible to get PVDF coatings infused with pigments, such as to match other parts of a building&#;s exterior or complement the look of the surrounding structures. Since PVDF coatings also resist fading, corrosion, and chalking, they&#;re good choices for aluminum components in highly noticeable areas, such as on building exteriors.

One example involved the Federal Building in Hawthorne, California. The structure, completed in , features anodized aluminum panels and glass wrapped around concrete. Those design characteristics give the building a mirror-like finish. But, since time is typically unkind to many outdoor surfaces that encounter the weather, a team had to take action to preserve the building&#;s appearance.

An examination of the building before a recent improvement project showed that people had coated the aluminum with silver paint several times over the years. However, it flaked off, and the effect was so severe in some cases that it exposed the aluminum to the elements.

The team overseeing the building&#;s renovations applied an engineered water-based PVDF coating to the aluminum. The additive was a custom color to match the aluminum&#;s original hue. The work also included using a silicone sealant to give the coating a lifespan of at least two to three decades.

2. Henna Extract-Infused Paint

The best coatings for aluminum are not always the ones people most expect. In one example, a researcher tested the effectiveness of coating aluminum alloys with henna leaf extract mixed into a paint.

The investigation involved testing aluminum alloy in a simulated environment that exposed the pieces to saltwater. Aluminum is a popular choice in the maritime industry. However, there are ongoing efforts to protect the material from corrosion due to the near-constant saltwater exposure.

The researchers created three henna leaf-infused paints to determine the optimal mixture. The results showed that making a coating with 10% henna leaf extract showed the best performance for curbing corrosion.

Other options exist for managing corrosion, too. Titanium alloys resist corrosion, making them popular choices for marine use. Aluminum-titanium alloys are often chosen when the goal is to make lightweight components. Since many industries need corrosion-resistant aluminum, making a coating or alloy to meet specific requirements is often the best bet.

3. Specialty Anodic Coatings

People commonly apply anodic coatings to aluminum medical devices to improve their lifespan and reduce the abrasive effect that prolonged and regular use can cause.

However, stipulations for medical-grade anodic coatings state that they must tolerate 50-100 cycles of sterilization with vaporized hydrogen peroxide or peracetic acid or the use of a high-alkaline cleaner without showing color degradation or a breakdown of the coating.

Thus, many companies catering to the medical device industry offer specialized anodic coatings that withstand such sterilization regimens and provide other desirable characteristics. For example, some are biocompatible.

One company has an engineered polytetrafluoroethylene (PTFE) coating suitable for permanently marking the aluminum device during the application. That characteristic makes a coated product easy to trace or establish ownership of when needed.

4. Low Interfacial Toughness (LIT) Coatings

Aluminum is the Earth&#;s third most abundant element. It&#;s also widely used in a wide variety of products. Perhaps that&#;s why researchers commonly experiment with aluminum when aiming to come up with new engineered coatings. They know that if the layer of material works as expected on aluminum, that&#;s already an indicator of excellent progress.

Researchers at the University of Michigan sought to develop a coating for aluminum and other surfaces that would cause a de-icing effect. They ended up with a spray-on material that offered low interfacial toughness (LIT). LIT encourages cracks to form between the icy surface and the ice itself. Then, instead of ice adhering to whatever&#;s underneath, it slides off, similar to food cooked in a non-stick pan.

The team developed an assortment of possible LIT coatings to use. They then tested them on various surfaces, including a large aluminum sheet and a piece of thin aluminum mimicking a power line. Tests showed that the ice fell off immediately due to its weight.

Conversely, the ice stayed stuck to the control group surfaces. That was even the case with material coated with an ice-phobic substance. The researchers need to do more research to enhance the performance and see what happens with other surfaces. In any case, this example shows the promise of engineered coatings when scientists have a clear goal in mind.

5. Germicidal and Antimicrobial Coatings

Aluminum components may also need germ-killing coatings or those that curb bacterial growth. For example, some aluminum parts of climate control equipment feature antimicrobial coatings to prevent mold formation.

It&#;s also increasingly common for aluminum food packaging to include coatings that stop bacteria from flourishing. Aluminum does not offer such properties on its own, but when specialty coatings are easy to apply, they become appealing for a wide variety of reasons.

A Texas A&M University team created a hydrophobic antimicrobial surface coating that can reportedly help the underlying surfaces retain their germicidal properties for longer. The researchers were particularly interested in using their invention to safeguard against the cross-contamination of surfaces regularly used to handle food.

Many of them, such as produce buckets and conveyor belts, often contain aluminum. That&#;s why the researchers used aluminum as their testing material for an engineered coating made from silica and a naturally occurring germicidal protein called lysozyme.

They started by placing a layer of silica on the aluminum using high heat, then adding a silica-lysozyme coating to that surface. The silica-aluminum substrate bound with the new layer, creating a coating with microscopic roughness that gave the hydrophobic quality. When testing it against salmonella and listeria, the coated surfaces had a 99.99% lower prevalence of the bacteria than the non-coated ones.

Careful Consideration Brings Favorable Results

There&#;s no single best engineered coating for all aluminum components or alloys. However, people are likely to get the best results when they think about their goals. For example, must the treated aluminum offer corrosion resistance, or are you trying to prevent bacteria buildup? Having a clear idea of what you want to accomplish with an engineered coating will steer your efforts while creating a new one or choosing an existing option for your metal surface.

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