While most grades of aluminum have adequate appearance and corrosion resistance in many instances, it is sometimes required to increase these properties further. This can be achieved through a process known as anodizing. Anodizing is a metal coating process that increases the amount of aluminum oxide on the surface of an aluminum substrate, thus potentially increasing its ability to withstand corrosion and alter its appearance. While many different aluminum alloys and grades can be anodized, some are better suited to the process than others.
What Aluminum Alloys Can Be Anodized?
The following aluminum alloys are best suited to the anodizing process:
- 5XXX Series
- 6XXX Series
- 7XXX Series
The anodizing process can increase the size of the aluminum oxide layer on most aluminum alloys. However, the coating of aluminum oxide may lack the desired amount of protection on some alloys. Furthermore, some alloys may have a layer of aluminum oxide after the anodization process that leaves an undesirable color, such as an unattractive yellow, brown, or dark grey. While there are some variations from each alloy to alloy, here is a summary of what one will likely encounter when anodizing an aluminum alloy by their series type:
This series covers pure aluminum. The aluminum in this series can be anodized. The resulting layer of aluminum oxide that forms is clear and somewhat shiny. Since the underlying pure aluminum is relatively soft, these anodized aluminums can be easily damaged and be lacking in mechanical properties when compared with other series of aluminum alloys.
This series is used to designate aluminum alloyed with copper. The copper in these alloys create a very strong and hard aluminum alloy. While the copper is useful for improving the mechanical properties of aluminum, it unfortunately renders these alloys poor candidates for anodization. When anodized, the 2XXX aluminum series alloys have an oxide layer that is a shade of yellow that is generally not considered appealing. Furthermore, the layer created by anodization offers poor protection for the underlying aluminum alloy.
Aluminum alloyed with manganese is categorized in this series. While the anodized layer offers decent protection for the manganese-alloyed aluminum substrate, it creates an undesirable brown color. Also, this brown color can differ from substrate to substrate and especially from grade to grade. This makes it difficult to keep a similar color across a 3XXX series aluminum assembly.
The 4XXX series consists of aluminum alloyed with silicon. Anodized 4XXX material is well protected by the aluminum oxide layer created from the anodizing process. However, it is important to note that the 4XXX series has a dark gray color that lacks aesthetic appeal. 4XXX aluminum alloys are often used to weld other alloys such as 6XXX, but if these welded assemblies are anodized, the weld metal will not match the color of the base metal.
This series designates an aluminum that is alloyed with manganese. When anodized, the alloys in the 5XXX series have a resulting oxide layer that is strong and clear. They are excellent candidates for anodizing; however, there are some important considerations that go along with carrying out the anodizing process on the 5XXX series alloys. For instance, certain alloying elements such as manganese and silicon need to be kept within a range; also, the anodizing process used is important. These alloys can often be substituted with a 4XXX series alloy for welding filler metal such that the resulting weld is not a different color than the rest of the anodized aluminum assembly.
The 6XXX series was created for aluminum alloyed with magnesium and silicon. These alloys are excellent candidates for anodizing. The oxide layer that follows the anodizing process is transparent and offers excellent protection. Since the 6XXX series alloys offer great mechanical properties and are readily anodized, they are frequently used for structural applications.
This series of alloyed aluminum uses zinc as its primary alloying element. It takes to the anodizing process very well. The subsequent oxide layer is clear and offers great protection. If the zinc level becomes excessive, the oxide layer created by anodization can turn brown.
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