Guides / Finishing

Choosing Between Decorative Anodize and Hardcoat

September 21, 2026 by The Alu CNC Parts editorial desk / how this is checked

Almost every machined aluminum part gets anodized. The choice is usually between two specifications from MIL-A-8625: Type II, the decorative and corrosion resistant coating, and Type III, the hard wearing one.

They are the same chemistry run under different conditions, and they give you very different parts.

The comparison that matters

	Type II	Type III hardcoat
Typical thickness	5 to 25 um	25 to 50 um, sometimes more
Hardness	Moderate	Very high, comparable to hardened steel at the surface
Colour	Clear or dyed almost any colour	Dark grey to black, dyes look muted
Dimensional growth	Small but real	Significant, must be designed for
Electrical insulation	Yes	Yes, better
Corrosion resistance	Good	Good
Cost	Lower	Higher, longer process time
Typical use	Housings, panels, visible parts	Bores, sliding faces, wear surfaces

Both of them change the size of your part

This is the single most common surprise, and it applies to both types. The oxide layer grows both into the metal and out from it, roughly half and half. A 20 um Type II coating therefore adds about 10 um to each surface, which closes a bore by about 20 um across the diameter.

Type III at 50 um adds around 25 um per surface. That is enough to turn a slip fit into an interference fit, and it is enough to jam a thread.

The full mechanics, and what to write on the drawing, are in the dimensional growth guide. The short version: either dimension the feature after coating, or mask it.

Design rules the anodizer will thank you for

• Break every external edge. A minimum 0.25 mm radius for Type II. Coating does not build on a knife edge, so a sharp corner ends up thin, chipped or burnt.
• Radius internal corners. Type III wants around 0.75 mm minimum. A sharp internal corner concentrates current and the coating grows unevenly.
• Mask threads, or tap after coating. A 50 um hardcoat in an M4 thread will stop the screw.
• Leave a contact point. Parts hang on racks and the rack marks the part where it touches. Tell the anodizer where the mark is allowed to be, or they will decide for you.

The alloy decides how it looks

Type II colour consistency depends heavily on the alloy. 6061 and 6082 take dye evenly. 7075 comes out darker and more variable because of its zinc and copper content. High silicon casting alloys and MIC-6 cast plate look patchy.

If a part is going to be a specific colour and it has to match something else, the alloy choice is part of the finish specification, not separate from it.

What to put on the drawing

A complete callout names the type, the class, the thickness and the colour:

ANODIZE PER MIL-A-8625 TYPE II CLASS 2, BLACK, 15 um NOMINAL. MASK BORE O20 H7 AND ALL TAPPED HOLES.

Class 1 means undyed, Class 2 means dyed. Naming the thickness matters because “anodize” without a number can mean anything from 5 to 25 um, and that range is wider than most fits can absorb.

If you do not know what you need, describe what the part does. A shop that knows the surface slides against something will suggest Type III without being asked, and one that knows the part sits on a desk will steer you to Type II and save you money.

Sources

Figures in this guide are checked against the following. Published values are typical ranges for test material, not a guarantee for your part.

• MIL-A-8625 and anodizing types
• Aluminium oxide

Frequently asked questions

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