Aluminum Anodizing Thickness: How to Specify Type II and Type III

Anodizing is a conversion coating, not a layer sitting on top of the metal. The oxide grows out of the aluminum itself, which means thickness is not a cosmetic number: it decides how long the surface resists corrosion, how well it stands up to abrasion, and how much the finished part has grown.

What anodizing thickness actually controls

Three properties move together as the coating gets thicker. Corrosion resistance improves because the oxide is a barrier the environment has to defeat. Wear resistance improves because the oxide is far harder than the aluminum beneath it. And dimensional growth increases, which is the one that causes trouble in assembly.

A 5 µm coating and a 25 µm coating do not behave like the same material with a different look. On a sliding face the difference is the service life of the part. On a press fit it is whether the parts go together at all.

Type II and Type III: the practical difference

Type II sulfuric anodizing typically runs 5–25 µm. It is the default for appearance parts, takes dye well, and covers the full range of colours.

Type III, usually called hard anodizing, runs roughly 25–50 µm and produces a denser, markedly harder oxide. It belongs where the anodized surface is a working surface — bearing bores, sliding rails, wear pads — rather than only a finish. Type III darkens the aluminum on its own, so colour choice narrows considerably and very light colours are generally not achievable.

Growth per surface: the dimension nobody plans for

The oxide grows roughly half outward and half into the metal. A 20 µm coating therefore adds about 10 µm to each surface it covers. That figure is small enough to ignore on a cosmetic panel and large enough to ruin a controlled fit.

Threads, sealing faces, dowel holes and press fits are the usual casualties. If a feature must remain at nominal, say so on the drawing and specify masking or post-machining. Deciding this after anodizing means rework, and rework on a coated part usually means stripping and starting again.

Sealing, and what it adds

An unsealed anodic layer is porous and will absorb oils, hold fingerprints and continue to shift in colour. Sealing closes the pores and stabilises the appearance. For architectural and outdoor work it is not optional. State the sealing requirement rather than leaving the shop to assume it.

Writing the callout on a drawing

A useful callout names the standard, the type, the thickness range, the colour and the sealing requirement:

Anodize per MIL-A-8625 Type II Class 2, 10–15 µm, black, sealed.

That is a specification. “Black anodized” is a wish: it leaves the thickness, the sealing and the colour tolerance to whoever reads it first.

Where colour matters, approve a physical sample produced on the production alloy and the production line. Photographs do not survive the anodizing tank, and a sample made on a different alloy will not match the parts it is supposed to govern.

Mistakes worth avoiding

• Calling for tight colour control on Type III, where the darkening is inherent to the process.
• Giving a single thickness instead of a range, which forces the shop to guess at the tolerance.
• Treating anodizing as a filler. It reproduces machining marks and die lines faithfully, often more visibly than bare aluminum does.
• Leaving masking off the drawing, which converts a cosmetic requirement into a rework.

From Design Review to Quotation

Chengdacai supports custom aluminum extrusion, precision cutting, CNC machining, drilling, tapping, and both anodizing and powder coating. A complete drawing package lets the engineering team identify tooling, secondary operations and quality controls before production starts.

See our anodizing finish reference for the process range, and the guide to anodizing versus powder coating if the finish type is still open. Colour control on anodized parts has its own guide: anodizing colour consistency.

Explore our custom aluminum profiles, aluminum heatsinks, enclosure and frame profiles, or send your drawing for an engineering review.