Hard Anodizing Aluminum: Type III vs Type II for Wear Parts

A sliding block in 6061 with a standard black anodize wore through the coating in three months, polished a groove into the bore, and took the seal with it. The drawing said "anodize black" and the shop delivered exactly that. What the application needed was Type III, four times the thickness and twice the hardness — and an allowance in the bore so the shaft still fit afterwards.

Hard anodizing is the finish for aluminum surfaces that rub, slide, or take abrasion. It is also the finish most likely to cause a dimensional argument, because the coating grows into the part as well as out of it, and that growth has to be planned before the part is machined.

Here is what changes between Type II and Type III, and how to specify the harder one without losing your tolerances.

What changes between Type II and Type III

Both processes are sulfuric acid anodizing. The part is the anode in an acid electrolyte, and the surface converts to aluminum oxide.

Type II runs at about 18 to 22 °C with a moderate current density and produces 5 to 25 µm of coating, usually 10 to 15 µm for decorative work. It is the finish on most consumer aluminum: phone housings, camera bodies, architectural trim.

Type III, hard anodizing, runs at 0 to 5 °C with a higher current density and produces 25 to 75 µm as standard, with 100 to 125 µm for severe wear. The cold bath slows dissolution of the oxide as it forms, which is what allows a thick, dense layer instead of a powdery one.

The color is a useful tell. Type II dyes easily across a wide range. Type III is usually black or dark bronze, because the denser coating accepts dye differently and the alloys that hard anodize well produce darker natural tones.

Thickness, build and penetration

This is the part that breaks drawings.

Anodic coating grows roughly half outward and half inward. A 50 µm Type III coating adds about 25 µm to the outside of the part and penetrates about 25 µm into the substrate.

A shaft measured before anodizing will be 50 µm larger in diameter after. A bore will be 50 µm smaller. A flat surface will sit 25 µm higher.

For a Ø20 H7 bore with a 21 µm tolerance band, a 50 µm coating closes the bore by more than twice the tolerance. The bore must be masked, machined oversize by the calculated growth, or honed after anodizing — which removes the coating you just paid for.

Mask every bore that carries a fit, and state the pre-anodize dimension so the shop and the anodizer work from the same number.

Alloy suitability

Not every aluminum alloy hard anodizes equally well.

The 6000 series is the sweet spot, which is convenient because it is also the most common machining alloy. If your part is 6061 and needs wear resistance, hard anodizing is straightforward.

The 2000 and 7000 series hard anodize, but copper and zinc produce a darker, more brittle coating with lower hardness and more variability. Aerospace parts in 7075 are routinely hard anodized, and the specification accepts the color variation as normal.

High-silicon die castings are a different problem: silicon does not anodize, so the coating forms around the particles and the result is patchy and porous. For castings, powder coating is usually the better route.

Masking and precision features

Masking is the largest hidden cost in hard anodizing, and the largest source of scrap.

Threads are the classic case. Anodize an M6 thread and the coating adds roughly 50 µm to the flanks and crest, leaving it tight, brittle and prone to chipping. Mask the thread, cut it oversize before anodizing, or chase it afterwards with a tap and accept bare metal in the flanks.

Bores with fits, O-ring grooves, sealing faces and contacts need the same decision. Plugs, tape and caps each cost labor.

A drawing note that says "mask all threads and Ø10 H7 bore" is worth more than a paragraph of general instruction. Without it, the anodizer will coat everything and the assembly will not go together.

Wear, fatigue and sealing

Hard anodizing reaches 450 to 500 HV, harder than most steel at 30 HRC. It resists sliding wear, abrasion and galling, and it does not gall aluminum against aluminum the way bare metal does.

It is also brittle. Impact can crack the coating, and a crack becomes a corrosion site. Radius sharp corners before anodizing, because the coating is thin at edges and cracks there first.

Fatigue is the trade nobody mentions. A thick anodic layer can reduce fatigue strength by 30 to 60 percent, because cracks initiate in the coating and propagate into the substrate. For a cyclically loaded part, hard anodizing may be the wrong choice, or the coating may need to be restricted to non-critical surfaces.

Sealing is optional. Sealed coatings resist corrosion better; unsealed coatings absorb oil and perform better in sliding wear. Hydraulic bores are often unsealed and lubricated, outdoor parts are sealed.

What to put on the drawing

A complete callout reads: "Hard anodize per MIL-A-8625 Type III, Class 1 (dyed black), 50 µm nominal, sealed. Mask all threads and Ø10 H7 bore. Dimensions apply after coating except masked features."

Four elements: the specification, the thickness, the masking, and the dimensional reference. The last one is the one most often missing, and it is the one that decides whether the part is accepted or rejected at incoming inspection.

At JSHS we machine to the pre-anodize dimensions and pass the masking instructions to the finisher as part of the routing, so a hard-anodized bore does not become an assembly problem. If a drawing is ambiguous about where the tolerance applies, we raise it in the DFM notes that accompany the quote.

Frequently asked questions

How much does hard anodizing change a bore diameter?

A 50 µm coating closes the bore by about 50 µm on diameter, since half grows inward from each wall. Mask the bore, or machine it oversize by that amount.

Is Type III anodizing harder than Type II?

Yes, roughly twice as hard: 400 to 600 HV against 200 to 300 HV. The thicker, denser coating also lasts far longer in sliding wear, which is why it is specified for cylinders, guides and wear plates.

Can hard anodized parts be machined afterwards?

Only by grinding, and only if the coating survives it. Milling through a hard anodic layer chips it and leaves bare metal. Mask critical features rather than machining after coating.

Does hard anodizing work on 7075?

It works, but the coating is darker, more brittle and less uniform than on 6061, and the natural color is bronze-black rather than clear. It is common in aerospace applications where the alloy is chosen for strength and the finish is accepted as-is.