What Aluminum Alloys Can Be Anodized?

Although many aluminum grades possess sufficient corrosion resistance and appearance for certain applications, sometimes it is desirable to enhance these properties. This is accomplished through the process of anodizing. The metal coating process of anodizing increases the quantity of aluminum oxide on the aluminum substrate surface. This has the effect of potentially increasing the metal’s ability to prevent corrosion and change its appearance. At Howard Precision Metals, we offer an extensive array of products made from aluminum alloys to meet the needs of your industrial applications.

Applications That Utilize Anodized Aluminum
Anodized aluminum is perfect for many applications in the architectural and construction industries. The colors of anodized aluminum do not peel, chip, or fade after exposure to sunlight making them highly suitable for many consumer, industrial, and commercial applications. Some common applications include construction items for the exteriors of buildings, doors, window frames, vents, and more.

Some aluminum alloys are better suited to anodizing than others. Different aluminum alloys contain varying types and amounts of alloying metals which result in coatings with different properties – some more desirable and some less desirable.

What Aluminum Alloys May Be Anodized?
The series 5XXX, 6XXX, and 7XXX series aluminum alloys are the most ideal for undergoing the anodizing process.

The Effect of Anodizing on Various Aluminum Alloys
On most aluminum alloys, the anodizing process can increase the size of the aluminum oxide layer. However, the amount of protection afforded by the aluminum oxide coating may be lacking the desired amount on some aluminum alloys. Also, after the anodization process, the layer of aluminum oxide may leave behind an unattractive dark grey, brown, or yellow color.

Below is what is likely to happen with the different aluminum series types when they are anodized:

1XXX Series
This is the pure aluminum series. When anodized, the aluminum oxide layer formed is somewhat shiny and clear. Due to the fact that the pure aluminum is relatively soft, the anodized aluminum may lack certain mechanical properties that are present in other aluminum alloys and be damaged easily.

2XXX Series
Aluminum alloyed with copper is what defines this alloy series. The addition of copper produces a hard and very strong aluminum alloy, enhancing the mechanical properties of the aluminum. However, the copper makes the alloys less than adequate for anodization. When 2XXX series aluminum alloys are anodized, a generally unappealing yellow colored oxide layer result. Also, the layer produced by anodizing delivers inadequate protection to the underlying aluminum alloy.

3XXX Series
This series of aluminum alloy consists of aluminum alloyed with manganese. The anodized layer produced provides reasonable protection for the substrate, but also produces an unappealing brown color. The brown color produced may also vary between the various grades and substrates. Thus, it can be challenging to maintain a similar color throughout various 3XXX series aluminum alloy metals.

4XXX Series
With the 4XXX series, silicone is alloyed with aluminum. The aluminum oxide layer produced from anodization protects the material very well. At the same time, the 4XXX aluminum alloy series possesses a gray color that is aesthetically unappealing. Oftentimes, 4XXX alloys are used to weld 6XXX alloys and other alloys. However, after anodizing these welded assemblies, the color of the base material will not match the weld metal.

5XXX Series
In this series, the aluminum is alloyed with manganese. Upon anodization, the alloys obtain a clear and strong oxide layer. Therefore, the 5XXX series of alloys is great for anodizing. However, it is important to take note of the following considerations when anodizing this aluminum alloy series.

For example, some of the alloying elements (silicone and manganese) must be kept inside a certain range. The types of anodization processes used with the 5XXX series is also important. A 4XXX series alloy may be used as welding filler metal instead of 5XXX to ensure the weld does not have a different color than other parts of the anodized assembly.

6XXX Series
The 6XXX alloy series has magnesium and silicon alloyed with the aluminum. This is an excellent series for the anodization process. Upon anodization, the oxide layer offers superb protection and it is also transparent in appearance. Along with being a great candidate for anodizing, the 6XXX series is also commonly used in structural applications due to its great mechanical properties.

7XXX Series
Zinc is the primary alloying element used in the 7XXX series of aluminum alloys. It responds the anodization process very well. The oxide layer is clear and provides superb protection against corrosion. If the level of zinc becomes excessive, the layer of oxide produced by anodization can change to a brown color.