Unfortunately cast aluminium parts don’t perform as well in anodising as machined or wrought material. Knowing in advance what to expect and how best to specify them will no doubt save much time and potential expensive rework.
Casting Alloys Behave Differently
Castings are typically made from higher silicon content alloys than wrought products. Silicon does not anodize well and appears as dark patchy areas in the coated surface. The higher the silicon content of the alloy the more pronounced the effect is likely to be. Information on how aluminium alloy composition affects surface treatment can be found here.
Porosity and Inclusions Affect Coating Quality
Many casting defects such as pores, shrinkage voids and surface defects can be centres for poor anodic growth. This can result in local areas of thinner anodic film or even no film. In addition any process liquids can penetrate into these pores and cause local staining in service.
Pre-Treatment Matters More Than Usual
Releasing agents, oxides, sand marks and machining swarf can adhere to the surface of castings. A good degreasing and etching sequence is required to remove these to ensure a strong bond between the coating and part. However, some etching solutions can open up sub-surface porosity thereby increasing the visibility of such features. It is essential to discuss the pre-treatment to be used with the processor before processing.
Set Realistic Expectations for Appearance
Mottling and overall appearance of a Hard anodised part made from castings can vary greatly from that of the same part machined from bar as billet. While for some applications the appearance will not be important, others will, and so you should clearly outline which faces are of most importance for visual inspection and be sure to clearly advise the anodiser as to what parts need to be cleaned prior to processing the surfaces to be inspected.
Account for Coating Build When Specifying Thickness
The anodic coating grows to approximately half the thickness of the coating within the substrate and half as a surface coating.
More on Hard Anodising can be found at https://www.poeton.co.uk/surface-treatments/anodising/hard-anodising.
Hard anodising on aluminium castings can be as good as expected once you have specified around the known limitations of the process and communicated these adequately to be ensured in practice.
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