Sulphuric acid anodising can be applied in various layer thicknesses and affects dimensions, appearance and the properties of the surface in service.
Thickness Ranges and When to Use Them
There are three general thicknesses that are specified for sulphuric acid anodising, decorative-thin, general-purpose and protective-heavy. These are typically specified as follows: decorative-thin (5-8µm) for parts that are to be lightly used, for cosmetic purposes etc.; general-purpose (15-20µm) for average engineering, architectural etc. applications; and, protective-heavy (20-25µm) for applications where maximum corrosion resistance is required. It should be noted that thicker coatings take longer to build and cost a little more per square meter than the average coating thickness of 18µm.
How Coating Growth Affects Dimensions
The anodic coating grows roughly 50% outward from the surface of the part and 50% inward into the substrate. As an example, a coating thickness of 20µm will add approximately 10µm to each surface. This must be taken into account when designing parts with bores, mating surfaces and other features with tight tolerances. Critical dimensions should be highlighted on the processing instructions to ensure that adequate allowance is made.
How Alloy and Pre-Treatment Affect Finish
The surface finish and appearance of an anodized layer of a given thickness can vary. The alloy used will have a large effect on the finish, with high-silicon casting alloys having a greyer, mottled appearance. The surface finish of the part prior to anodizing will also have a large effect, with parts that have been bright-dipped or mechanically polished prior to anodizing having a much brighter appearance than parts that have not. It is therefore important to confirm the pre-treatment process with the processor at the quoting stage to ensure that the required finish is achieved.
Sealing Options and What They Change
There are a number of sealing options available to protect the porous anodic surface. Hot water sealing provides a good clean finish and does not add any colour to the coating. Dichromate sealing on the other hand provides a pale gold colour and also has good corrosion resistance. Dry-film PTFE sealing leaves a low friction surface and is particularly suitable for sliding and threaded components. However, this coating does affect the finish and may appear to be dull. Sulphuric acid anodising is covered in more detail at https://www.poeton.co.uk/surface-treatments/anodising/sulphuric-acid-anodising/.
Writing a Clear Specification
For sulphuric acid anodising of precision parts it is useful to outline the specified characteristics of the coating and of the subsequent sealing in the form of a short note on the drawing (e.g. 15 to 20 µm coating, finish class ‘smooth’, hot-water sealing). This avoids confusion between the specified dimensions on the drawing and the processor’s default values for similar parts. For applications where there are formal quality requirements, e.g. for aerospace, it is also useful to refer to the relevant ISO standard for anodic coatings on aluminium.
So there, by getting the 3 key elements right from the start you can avoid the usual surprises that surface after your parts have been anodized.
