Which Aluminium Alloys Can Be Anodised?Shea Bethell
While it is widely known that aluminium has a great general appearance and good corrosion resistance, there are many instances where further increasing these properties is required. This is where the anodising process comes into play. Aluminium anodising is an electrolyticpassivation process used to increase the thickness of the natural oxide layer on the surface of the metal, thus greatly increasing its ability to withstand corrosion and alter its aesthetics. While many different aluminium alloys and grades are able to be anodised, there are some that are better suited than others.
Which Aluminium Alloys Can Be Anodised?
The following aluminium alloys are best suited to the anodising process:
5XXX Series – Aluminium alloy containing a small amount of Magnesium.
6XXX Series – Aluminium alloy containing a small amount of Magnesium and Silicon.
7XXX Series – Aluminium alloy containing a small amount of Zinc.
The anodising process should increase the thickness of the aluminium oxide layer on most aluminium alloys. However, the aluminium oxide layer may lack adequate protection on some alloys used for more rigourous applications. It has been observed that some alloys may develop a layer of aluminium oxide after the anodisation process with an undesirable color, such as a dirty-yellow, brown, or dark grey. While there are some variations between alloys, below is a summary of what one will likely encounter when anodising an aluminium alloys by series type:
The 1xxx series consists of pure aluminium with no other elements added in the manufacturing process. The aluminium in this series can be anodised. The resulting layer of aluminium oxide that forms is clear and noticeably shiny. Since the underlying aluminium is relatively pure and soft, these alloys can be lacking in mechanical properties and easily damaged when compared with other series of aluminium alloys.
The 2xxx series is alloyed with copper. This makes for a very strong and hard aluminium alloy, however, while the copper is useful for improving the mechanical properties of the aluminium, it makes these alloys rather poor candidates for anodisation. When anodised, the 2XXX aluminium series alloys have an oxide layer that provides poor protection of the underlying aluminium and is a shade of yellow that is generally not considered appealing.
The 3xxx series is alloyed with manganese. Although the anodised layer offers sufficient protection for the underlying aluminium, it also adds an unattractive brown color to the finish. This brown color differs from grade to grade and makes it rather difficult to keep a similar color across a 3xxx series aluminium assembly.
The 4xxx series is alloyed with silicon. Although anodised 4xxx series material is rather well protected by the anodising process, it is important to note that the 4xxx series has a dark gray color that lacks aesthetic appeal. The 4xxx series of aluminium is often used as a filler material to weld other alloys such as the 6xxx series, but if these welded products are anodised, the weld metal will unfortunately not match the colour of the base metal.
The 5xxx series is alloyed with manganese. Once anodised, the 5xxx series alloys have a resulting oxide layer that is clear and strong. This makes them excellent candidates for anodising. However, there are some important factors to consider when carrying out the anodizing process on 5xxx series alloys. As an example, certain alloying elements such as manganese and silicon need to be kept within a range, additionally, the anodising method used is also very important. These alloys can often be substituted with a 4xxx series alloy for welding filler metal, resulting in the weld metal taking a different colour to the rest of the anodised aluminium product.
The 6xxx series is alloyed with magnesium and silicon. These alloys are excellent candidates for anodising. The oxide layer that follows the anodizing process is both transparent and provides excellent protection. Because the 6xxx series alloys provide great mechanical properties and are easily anodised, they are frequently used in structural applications.
The 7xxx series is alloyed with zinc as its primary alloying element. It takes to the anodising process very well. The resulting oxide layer offers great protection and is particularly clear. However, if the zinc levels become too excessive, the resulting oxide layer can turn a brownish colour.