Anodising of cast aluminium parts is accepted practice and often forms part of the production process; however it is important to note that the anodising process for cast parts differs from wrought or extruded material and a number of important considerations need to be taken into account prior to specifying the surface finish.
Why Castings Behave Differently in the Bath
Wrought aluminium is made up of consistent grain structure. This allows it to anodize consistently and provide an even finish. The cast aluminium part, on the other hand, is made up of a less uniform microstructure. As a result of this the oxide layer will grow at different rates. This can lead to areas on the part being thicker than others and resulting in inconsistent appearance.
How Silicon and Porosity Affect the Coating
High silicon alloys (found in many die cast and sand cast products) can form a thick oxide barrier which can prevent the formation of aluminium ions in solution. Hence, silicon particles in the casting surface do not anodise and appear as dark spots against the coated surface. Subsurface porosity can cause pitting or smut in the coating and also cause ‘burning’ when trying to anodize at high current densities.
Surface Preparation for Cast Parts
Surface preparation on cast parts differs also to that required on machined billets and requires more care. An aggressive alkaline etch can open up surface porosity and result in smutting. This can be made worse by longer etch times and therefore a milder surface preparation such as etching and grit polishing is required prior to anodising. Shot blasting the surface prior to anodising can also close up surface pores thus improving the coating’s adhesion but will result in component dimensional tolerance reduction.
Choosing the Right Alloy
When specifying a casting that will be anodized it is worth bearing in mind the level of silicon in the alloy and choosing one of the lower silicon casting alloys (i.e. below 7%) such as 6061 or LM25 as opposed to a high silicon die casting alloy. The lower the silicon the cleaner and more even the anodized finish will be.
Setting Realistic Expectations on Appearance
On cast parts the colour will tend to vary from part to part. This is particularly evident with dyed anodized parts where the overall appearance will be non-uniform.
When to Specify It Anyway
Sulphuric acid anodising is still a good option for cast components where corrosion protection and a slight increase in surface hardness are the major considerations and appearance is not so important.
There is more on Sulphuric acid anodising at //www.poeton.co.uk/surface-treatments/anodising/sulphuric-acid-anodising/.
Appearance however remains a critical factor and high-silicon alloys are often best completed with a conversion coating or paint finish.