Anodized Aluminum: Types, Colors & Tolerances
The standard cosmetic and protective finish for aluminium — hard, corrosion resistant, and it grows the part slightly.
Request a quoteWhat actually happens to the part
An aluminium part is used as the anode in an electrolytic cell. Current converts the surface metal into a hard aluminium oxide layer that is bonded to the part, rather than sitting on it like paint. The layer is porous, so it is dyed before being sealed.
| Parameter | Value |
|---|---|
| Type I (chromic) | Decorative, rare in new work |
| Type II (sulfuric) | Standard decorative and protective |
| Type III (hardcoat) | Wear and abrasion resistant, 25–80 µm |
| Clear anodising | Natural metallic, 5–25 µm |
| Black anodising | Most requested colour, 5–25 µm |
| Colour anodising | Architectural colours, 15–25 µm |
| Surface roughness | Ra 0.8–1.6 µm before sealing, slight increase after |
| Typical lead time added | Add 2–4 working days to the machining time |
What this finish does to your tolerances
This is where most prototype surprises come from. A finish is not decoration — it moves dimensions, and the drawing has to account for it.
Anodising adds material, not a coating. Expect the surface to grow 50% of the nominal oxide thickness, so a 25 µm layer adds roughly 12.5 µm per surface. A hole becomes smaller and an external feature becomes larger — critical fit dimensions should be masked or machined undersize.
Suitable for
- Al 6061
- Al 7075
- Aluminium die-castings (with care)
Not suitable for
- Steel, brass, copper and titanium — anodic oxidation is an aluminium process
- Threads that need to mate — the coating fills and stiffens the thread
- Precision bores and bearing seats without masking
- Parts that will be welded afterwards — the coating must be stripped
What engineers ask about Anodized Aluminum: Types, Colors & Tolerances
Why did my anodised part not match the colour sample?
Anodising dyes an oxide layer, so the colour depends on the alloy, the electrolyte, the bath age and the immersion time. On a cosmetic part we run a three-piece trial and match against a physical approved sample before releasing the batch.
Does anodising make the part harder?
It hardens the surface layer, not the whole part. For wear resistance, hardcoat Type III is the one that delivers, and the dimensional cost is roughly triple.
Can I machine anodised parts afterwards?
You can, but you will cut through the coating and expose raw metal. Machine first, finish second — we flag this if the drawing shows a finish note and a machined surface together.