What is anodising?

In anodising the aluminium part is the anode in an electrolytic bath, usually based on sulphuric acid. The surface is converted into aluminium oxide: a conversion layer, not a layer of another material laid on top. The porous layer can be dyed, black for example, and is then usually sealed. Hard anodising is a variant with a thicker, harder layer for wear resistance. The oxide layer is an electrical insulator.

What do layer thickness and growth in anodising depend on?

The layer grows partly into the material and partly outwards. As a rule of thumb the split is roughly half and half: each face then moves out by about half the layer thickness, so a shaft grows by about one layer thickness on diameter and a bore shrinks by about the same. The actual split, the achievable thickness and the colour depend on the process, bath conditions and the alloy. Alloys high in copper or silicon, such as some free-machining and casting alloys, usually give a less even layer. Deep bores get a different layer thickness, and on a sharp outside edge the layer does not grow together well; a small edge radius helps.

What does the standard say about anodising?

Drawings often refer to ISO 7599 for decorative and protective anodic coatings and to ISO 10074 for hard anodising. American drawings often cite MIL-A-8625, with Type II for sulphuric acid anodising and Type III for hard anodising. An unambiguous specification states the alloy, the type of treatment, the layer thickness, the colour, whether the layer is sealed, which faces must stay bare and whether dimensions apply before or after anodising. On Dutch drawings the note ‘maatvast’ usually means the dimensions apply after anodising. If any of these is missing, the requirement is ambiguous and worth querying.

How does anodising work in practice?

With a thin protective layer the growth often stays within the tolerance of ordinary dimensions, but not necessarily within that of a fit. For a fit such as ⌀12 H7, or with hard anodising, you allow for the growth beforehand or mask the bore. Threads are masked or made beforehand to a size that allows for the layer. Faces for earthing or electrical contact stay bare. Masking is often hand work that the anodiser charges separately. The price is usually a minimum charge per batch plus an amount per piece, area or rack; the extra lead time extends delivery.

What is anodising often confused with?

Anodising is not a coating or paint: those add a layer of another material, anodising converts the aluminium itself. Nor is it the same as a chemical conversion coating such as chromating, which is much thinner and largely keeps electrical contact possible. And anodising does not make a surface smoother: the layer follows the surface beneath and can change the roughness.

How do you see anodising in Blake?

In Blake anodising falls under finishing: it sits with the part as a proposal from the drawing, and once it is confirmed or corrected, who did so and when stays visible. Subcontracted anodising gets its own line in the estimate, priced from your subcontract library. Without a price, that line stays open. More on estimating.