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