Surface Finish
Anodizing Types Explained: Type II vs Type III
Both grow an oxide layer on aluminium. Type II gives you color and corrosion protection. Type III gives you wear resistance.
Key takeaways
- Anodizing converts the surface of the aluminium into aluminium oxide.
- Type II is 5 to 25 µm thick, takes dye well and is the common choice.
- Type III, or hard anodize, is 25 to 50 µm and much harder.
- The coating grows outward and inward, so holes and threads change size.
What anodizing does
Anodizing is an electrochemical process. The part goes into an acid bath and current is applied. Oxygen joins the aluminium at the surface and forms a layer of aluminium oxide.
That layer is not paint. It is part of the metal. It does not chip or peel the way a coating can, because there is no coating to peel.
The oxide layer is porous. That is what lets dye soak in and what makes sealing possible afterward.
Type II: the standard finish
Type II uses a sulfuric acid bath and produces a coating of roughly 5 to 25 µm. It is what most people mean when they say anodized.
- Good corrosion resistance for indoor and moderate outdoor use.
- Takes dye well, so black, blue, red, gold and custom colors are all possible.
- Clear anodize keeps the metal look with a slight sheen.
- Lower cost and shorter lead time than Type III.
Type III: hard anodize
Type III, often called hard coat or hard anodize, runs colder and at higher current density. The result is a thicker, denser layer, typically 25 to 50 µm.
- Surface hardness around 400 to 500 HV, close to case-hardened steel.
- Much better wear resistance for sliding and rubbing parts.
- More abrasion resistance in dirty or high-contact environments.
- Usually darker in color and less colorful, because dye does not take as evenly.
Side by side
| Type II | Type III | |
|---|---|---|
| Common name | Standard or decorative anodize | Hard anodize, hard coat |
| Thickness | 5 to 25 µm | 25 to 50 µm |
| Hardness | ~200 to 300 HV | ~400 to 500 HV |
| Color range | Wide | Limited, mostly dark |
| Wear resistance | Moderate | High |
| Corrosion resistance | Good | Good, but more brittle |
| Relative cost | Baseline | 1.5x to 2.5x |
| Best for | Housings, panels, visible parts | Sliding parts, wear surfaces |
Design rules that matter
Holes and threads grow
The coating builds about half outward and half inward. A 50 µm coating adds roughly 25 µm to each side, so a hole closes by about 50 µm on diameter.
For precision fits, mask the hole or specify the finished size. For threads, mask them or plan a slightly oversized tap.
Sharp corners thin out
Coating builds thinner on sharp edges. Break edges with a 0.3 to 0.5 mm chamfer so the finish holds.
Masking costs money
Every masked area is hand work. If a surface must stay conductive, design a small flat pad instead of asking for a complex mask.
Common questions
Which anodizing should I choose for a visible enclosure?
Type II, usually black or clear. It gives a clean, even color at a reasonable cost.
Does anodizing change the part color?
Clear anodize darkens the metal slightly. Dyed colors are applied after anodizing, so the shade depends on the alloy and the bath.
Can anodized parts be welded later?
No. Anodizing must be stripped first. Weld, then finish.
The bottom line
Type II for looks and general protection. Type III when metal rubs on metal. Decide before you finalize hole sizes, because the coating changes them.