Anodizado Tipo II o Tipo III: ¿cuál necesita su pieza?
Espesor, dureza, crecimiento dimensional, colores y coste comparados, además de cómo compensar el espesor en cotas toleradas y roscas.
Escrito por Engineering Team
La documentación técnica se mantiene en inglés para preservar la exactitud de los términos de ingeniería.
Anodising converts the surface of aluminium into aluminium oxide — a hard, non-conductive, corrosion-resistant layer that is grown from the substrate rather than deposited on it. Because the layer grows into and out of the part, it changes dimensions, and that is where most anodising problems on precision parts come from.
The two types that matter
Type II (sulphuric acid anodising) produces a 5 – 25 µm layer. It accepts dye readily, giving the full colour range, and provides good corrosion resistance. This is the standard decorative and general-purpose finish.
Type III (hard anodising) produces a 25 – 80 µm layer at lower bath temperature and higher current density. The resulting coating reaches roughly 60 HRC equivalent surface hardness, with far better wear and abrasion resistance. Colour options are limited to dark grey and black because the thicker layer masks dye.
| Type II | Type III | |
|---|---|---|
| Thickness | 5 – 25 µm | 25 – 80 µm |
| Surface hardness | ~200 HV | ~500 HV (≈60 HRC) |
| Dimensional growth | ~50 % of thickness | ~50 % of thickness |
| Colours | Full range | Dark grey, black |
| Dielectric strength | ~500 V | ~1500 V |
| Relative cost | 1.0 | 1.8 – 2.5 |
| Standard | MIL-A-8625 Type II, ISO 7599 | MIL-A-8625 Type III |
The dimensional problem
About half the coating thickness grows outward from the original surface, and half grows inward. For a 25 µm Type III coating, that means each surface gains roughly 12.5 µm.
The consequences compound on internal features:
- An external diameter grows by ~25 µm (12.5 µm on each side)
- A bore shrinks by ~25 µm
- A slot narrows by ~25 µm
- A threaded hole loses ~25 µm of clearance
On a part with an H7 bore, a 50 µm Type III coating will close the bore right out of tolerance. There are three ways to handle this:
- Machine undersize / oversize to compensate. Tell us the finished dimension you need and which features are toleranced after coating, and we will adjust the machining dimensions. This is the normal approach.
- Mask the feature. Bores, threads and mating faces can be masked so they are not coated at all. Masking costs money and leaves a visible transition line, so use it where compensation is impractical.
- Post-machine after anodising. Effective but expensive, and it removes the coating from the machined surface.
Threads deserve special mention. A threaded hole coated with Type III can refuse the mating screw entirely. Either mask internal threads, or specify them one class larger before anodising. We flag this on every quotation where a drawing shows both hard anodising and threads.
Colour and cosmetic consistency
Dyed Type II colours vary with alloy, temper and surface preparation. The same black on 6061 and 7075 will not match exactly — 7075's higher zinc content shifts the tone. If colour match across different alloys matters, tell us; we can process them together and cross-check against a master sample.
Bead blasting before anodising evens out machining marks and produces a uniform matte appearance. Without it, tool paths remain visible through the coating, which can look either purposeful or careless depending on the part.
Clear (undyed) Type II gives a natural silver appearance and is the most consistent option across alloys.
Which alloys anodise well
- 6061, 6063 — excellent. The default choice for anodised parts.
- 5052 — very good.
- 7075 — good, but expect a slightly darker, warmer tone than 6061.
- 2024 — acceptable, with reduced corrosion protection due to copper content.
- Cast alloys (A380, ADC12) — poor. High silicon content produces a patchy, dark grey finish. If you need a cosmetic anodised finish, machine from wrought stock.
When to specify which
Type II when you need colour, corrosion resistance, electrical insulation or a decorative finish. Consumer products, enclosures, brackets, optical mounts, general-purpose parts.
Type III when the surface slides, seals or takes abrasion: pistons, valve bores, wear plates, guide rails, gun components, hydraulic components. Also where the dielectric requirement is high.
Chromate conversion (Alodine / Iridite, MIL-DTL-5541) when you need corrosion protection and electrical conductivity — grounding paths, RF enclosures, EMC-critical assemblies. It is not anodising, it is a thin conversion coating, and it does not change dimensions meaningfully.
How to call it out on a drawing
State the standard, the type, the class, the colour and the thickness:
"Anodise per MIL-A-8625 Type II, Class 2, black, 15 – 20 µm. Mask threads and bore Ø12 H7. Dimensions apply after coating."
That single line answers every question the finisher would otherwise have to ask — and every question left unanswered becomes an assumption that may not match yours.