Engineering · Comparison

Anodize Type II vs Type III.

Both are aluminum oxide grown electrolytically on aluminum surfaces. Type II is the decorative-and-corrosion finish on most aluminum brackets; Type III is the structural wear coating that turns aluminum into a viable bearing surface. The choice comes down to whether you need cosmetic options or a hardness north of 400 HV.

Spec	Type II (decorative)	Type III (hard)
Common name	Sulfuric / decorative anodize	Hard anodize
MIL-spec	MIL-PRF-8625 Type II (formerly MIL-A-8625)	MIL-PRF-8625 Type III (formerly MIL-A-8625)
Typical thickness	0.0001–0.001″ (2.5–25 µm)	0.0005–0.004″ (12–100 µm)
Hardness	200–400 HV	400–600 HV (harder than the base alloy)
Color options	Wide range — black, red, blue, gold, clear	Limited — natural gray to dark gray/black
Wear resistance	Moderate	High — used as wear surface
Corrosion resistance	Good	Excellent
Dimensional growth	~100% of coating thickness adds to a diameter (≈50% per surface × 2)	~100% of coating thickness adds to a diameter (≈50% per surface × 2); significant for tight tolerance
Electrical	Insulating	Insulating, higher dielectric strength
Cost / sq ft	$0.50–2.00 (process)	$2.00–8.00 (process; thicker = higher)
Best alloys	6061, 7075, 5052	6061, 7075, 2024 (with masking)

How the coating builds

It grows into the part — and out of it.

Anodizing converts the aluminum surface itself into aluminum oxide, so roughly half the coating grows down into the base metal and half builds outward. Only the outward build-up changes your dimensions — about 50% of the total coating thickness on every surface.

Dimensional growth — worked example

1.0000″ OD + Type III 0.002″ coat
→ +0.001″ per surface × 2 → 1.0020″ final

Machine the OD to 0.998″ and let the coating bring it to size — or mask off where coating isn’t allowed.

Cross-section illustration of anodized aluminum: a thin porous oxide layer (Type II, left) and a much thicker oxide layer (Type III, right) on an aluminum substrate. A red line marks the original machined surface — oxide grows above it (build-up) and into the metal below it (penetration). — The red line is the original machined surface. Oxide builds up from it (this is what grows your dimension) and also penetrates **down** into the metal — and Type III runs several times thicker than Type II.

By the numbers

Hardness, performance, and color at a glance.

Surface hardness — Vickers (HV)

Type III is harder than the aluminum it grows on — which is why it works as a wear and bearing surface where Type II would polish away.

Performance scorecard

Type IIType III

Wear resistance
Corrosion resistance
Color range
Dielectric strength
Cosmetic / gloss

Color availability

Type II

+ more

Type III

gray → black only

The appearance difference

You can see it on the bench.

Type II takes a wide range of vivid dyed colors — the red, blue, gold, and black anodize you see on brackets and hardware. Type III hard anodize is limited to natural gray through black, but it’s far harder and built to wear.

Left: Type II decorative · Right: Type III hard anodize

CNC-machined aluminum parts side by side: brightly colored Type II anodized brackets and knobs in red, blue, gold and black on the left; matte gray and black Type III hard-anodized parts on the right.

Decision rule

Three questions in order.

1. Will the surface see sliding wear or repeated contact?
If yes, Type III. Bushing bores, bearing surfaces, sliding interfaces. Type II will polish away under wear.
2. Is color the priority?
If you need a specific color (red, blue, gold, etc.) or a high-gloss decorative finish — Type II. Type III is restricted to gray/black tones because the coating is denser.
3. Is dimensional tolerance critical?
Both grow the part. Type III at 0.002″ thickness adds ~0.001″ per surface — 0.002″ across a diameter (so a 1.0000″ OD becomes 1.002″). For tight tolerance, machine smaller and let the coating bring it to size — or specify maskoff areas where coating isn’t allowed.
For the highest-performance combination, dye Type III.
Type III with black dye + sealer gives near-Type-II appearance with hard-anodize wear properties. Standard for aerospace and defense hardware that needs both.
2024 needs masking.
Copper-bearing alloys (2024) anodize poorly — the copper inclusions disrupt the oxide layer. Either mask copper-rich features or specify Type I (chromic acid anodize) instead.

Common applications

Where each fits.

Type II — decorative

Color and corrosion protection where wear is light. Dye it any color.

Aerospace brackets
Drone frames
Electronics enclosures
Color-coded fasteners
Cable carriers
Hardware kits

Type III — hard

Wear and bearing surfaces. Dye black for low-reflectance defense hardware.

Hydraulic cylinder bores
Valve trim
Slide-rail bearings
Weapons-mount surfaces
PTFE self-lubricating bearings

Keep exploring

Related tools & references

Coatings & finishes reference→

All the post-machining finishes we coordinate, with specs.

6061 datasheet→

Anodizes the lightest and most consistently of the common alloys.

7075 datasheet→

Higher copper content anodizes slightly darker than 6061.

How to machine 6061→

Machine to size accounting for anodize build-up.

6061 vs 7075 vs 2024→

Pick the base alloy before you pick the finish.

Machining aluminum that needs anodizing?

Tell us about your part and we'll get back to you promptly. Aerospace, medical, defense, and semiconductor production work welcome.Email CAD models and drawings to sales@digitalmachine.com, including ITAR, EAR, CUI and AS9100 work. Keep controlled technical data out of web forms.

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