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What Aluminum Alloys Can Be Anodized?

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Aluminum is one of the most popular metals in use today; it has had a profound impact on nearly every industry.  Though it was only first commercialized a little over a century ago, constant developments since then have led to an increasing diversification of what aluminum can do and the introduction of even more advanced specialty alloys.  One of the major benefits of aluminum is its ability to be anodized, which offers greater protection against corrosion and provides a major boost to the material’s long-term durability.

Aluminum offers a wide range of benefits that makes it preferable to other metals.  It is a modern, efficient material that has tremendous adaptability and versatility. Perhaps best known for its light weight and high strength-to-weight ratio, aluminum has many other advantages as well.  These include being extremely hygienic, aesthetically pleasing, easy to recycle and corrosion resistant.

The metal’s ability to be anodized means that manufacturers and designers have even more options when it comes to aluminum as they create quality applications that will stand the test of time.

What is anodizing?

Anodizing involves electrolytic passivation, which forms or thickens the oxide layer on a metal surface.  The name comes from the fact that treatment creates an anode electrode during the process.  This bolsters the corrosion and wear resistance of the metal while also providing better adhesion for paint primers and bonding agents.  Although many metals can be anodized, including titanium and zinc, it is most commonly used with aluminum.

Aluminum was first commercialized in the early twentieth century, and anodizing followed soon after, with the process reaching an industrial scale in 1923.  The procedure actually changes the microscopic texture of the metal, both at and below its surface.  It’s important to note that while anodized aluminum might be harder than raw aluminum, it can exhibit low wear resistance unless a sealing substance is added.  There’s also a higher susceptibility to thermal stress in anodized surfaces. On the plus side, in addition to increased corrosion resistance, other benefits include improved lubrication and better dyeing.  The anodic surface is also non-conductive.

Some aluminum grades are enhanced with alloying agents such as copper and silicon in order to increase the strength of the metal and improve other properties.  These additives can also decrease the corrosion resistance.  This is especially true of alloys in the 2xxx, 4xxx and 6xxx series.  Pure aluminum passivates when it comes in contact with oxygen and forms a protective aluminum oxide layer that is 2 to 3 nm thick.  While the layer may actually be thicker in typical alloys, it will also be more porous, the reason for its increased susceptibility.  This is why so many aluminum parts are anodized.

The anodizing process involves passing a direct current through an electrolytic solution. The aluminum forms an anode.  As the current releases hydrogen at the cathode, oxygen is simultaneously released at the surface of the anode, leading to a build-up of aluminum oxide.  The voltage typically ranges from 15 to 21 V, with higher voltages necessary for thicker coatings.  This normally takes place in an acid solution.  It’s crucial to monitor the electrolyte concentration, acidity, solution temperature and current in order to ensure the anodized layer is consistent and of the desired thickness.

Which alloys can best be anodized?

Let’s consider the different aluminum alloy families and see which ones are best suited for anodizing, beginning with the 1xxx series, which can be anodized and includes pure aluminum.  The anodized layer that results is soft compared to many other alloys and can be more easily damaged.

The main alloying agent is copper in the chemistry of the 2xxx series.  This makes the aluminum extremely hard, which may be beneficial, but also makes it an inferior choice for anodizing.  Furthermore, the anodized layer of a 2xxx series alloy has a yellowish hue that is cosmetically undesirable.

As for the 3xxx series, while the manganese found in these grades leads to good protection when anodized, the process leaves a brown color that will differ from grade to grade.  This means anodizing is generally avoided with this series. Likewise, 4xxx series aluminum, alloyed with silicon, is well suited for anodizing.  Unfortunately, the process leaves a dark gray color.

The 5xxx series, alloyed with manganese, has a strong, clear oxide layer after anodizing.  This is also the case with the 6xxx series of aluminums, which are alloyed with magnesium and silicon.  These alloys are often found in structural applications.  Finally, in the 7xxx series, alloyed with zinc, the anodizing results in a clear layer of protection.  It should be noted that if the zinc level is particularly high, it can lead to a brown anodized layer.

What are some examples of applications that use anodizing?

When done properly with the right alloy, anodized aluminum is both extremely durable and cosmetically pleasant, making it an extremely popular choice for manufacturers. Examples of products that might be anodized include aluminum aircraft parts, architectural structures and a wide array of electronics and consumer goods.  MP3 players, mobile phones, flashlights, televisions, kitchen appliances, cookware, refrigerators, guns, roofing, sporting equipment, window frames, patio covers, boats, golf carts, solar panels and many other products are all made with anodized aluminum.

Work with a technical services professional

Aluminum products continue to evolve, and developers are constantly coming up with new types of alloys and applications based of these advancements.  The anodizing of aluminum alloys is no exception; thanks to continued development, today’s aluminum is more durable, adaptable and environmentally sustainable than ever before.  It doesn’t matter what industry you work in, there’s probably an anodizing solution that’s right for you. Best of all, aluminum alloys are cost effective.

At Clinton Aluminum, our philosophy is to always help find the right material for the job. Our sales professionals pride themselves on working closely with our customers and suppliers to assist in selecting the correct alloy for every application. Contact us today to speak with one of our friendly customer service representatives and learn more about which aluminum alloy is right for you.

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