Aluminium is one of the few metals that can be surface treated. Not all surface treatments are suitable for all parts and here we outline when anodising of aluminium is suitable.
Check Your Alloy First
Not all aluminium alloys anodize equally well. The 1000, 5000 and 6000 series aluminium alloys anodize well and produce a consistent and even coloured finish. The high copper 2000 series and some of the 7000 series of aluminium will anodize but can produce an uneven, patchy finish or produce areas that are significantly darker than others. It is wise to check with your surface finish supplier to see if they have any experience in processing your specific alloy before specifying anodizing as the surface finish for your parts.
Match the Type to the Job
There are two main forms of anodising. Type II or ‘standard’ anodising is generally used for decorative finishes, light and engineering components that require reasonable corrosion resistance. The coating thickness is generally in the order of 2 – 10 microns, depending on process parameters. Type III or ‘hard’ anodising produces a much thicker, more dense coating typically between 25 – 50 microns. This form of anodising is often used for wear resistance on sliding parts and also for other engineering components that require good mechanical wear resistance in addition to reasonable corrosion resistance.
Account for the Dimensional Change
The coating does grow outward from the surface, but it also penetrates into the surface, so roughly half the thickness of the coating ends up on the surface of the part and the other half penetrates into the part. This growth can be a problem for parts that require tight tolerances, such as bores and surfaces that are to be in contact with other parts. This growth can not be ‘dialled back’ after anodising so must be taken into account at the design stage.
Corrosion Environment Fit
Anodised surfaces are suitable for mild to moderate environments and are particularly common in architectural, marine and general engineering applications. However, as with all surface finishes, anodised coatings are not suitable for strong alkaline environments as these will attack the oxide coating. Therefore parts to be used in caustic washes or in contact with cement for long periods are better finished with an alternative treatment.
When to Choose Something Else
Anodising does not provide any lubrication and so other surface treatments are more suitable for parts requiring low friction. More on Anodising can be found at poeton.co.uk/surface-treatments/anodising/.
So if you are looking for a finish for your aluminium components, which are subject to wear, but are not exposed to a highly caustic environment, which will allow for a few microns of dimensional change, then anodising could be a good option.