Anodized Aluminum Parts

Type II versus Type III hardcoat

Type II is the sulfuric anodize most aluminum parts get: a thin layer (roughly 0.0002-0.001 in) that adds corrosion and scratch resistance and gives a clean matte look. It’s also the layer that accepts dye, so clear (natural) and black are the standard callouts, with colors like red, blue, and gold available on top of that.

Type III, hard anodize or hardcoat, is the same idea taken further: a thicker layer (roughly 0.001-0.004 in) that comes out much harder and far more wear resistant. It’s the right call for sliding faces, bearing surfaces, and anything that takes load or abrasion. Hardcoat naturally reads dark gray to bronze or near black, so think of it as a wear finish first and a cosmetic one second. Both types are written to MIL-A-8625, the spec that defines the class and thickness.

Dimensional growth and what gets masked

Because anodize grows oxide into and out of the surface, it changes the part’s dimensions. Roughly half the layer builds outward past the original surface and the rest penetrates, and hardcoat moves more than Type II does. Tight-tolerance bores, ground points, and press fits have to account for that growth, which is why threaded holes are often chased after anodize and critical bores get masked off so they stay on size.

Anodize is also electrically non-conductive, so any face that needs grounding or electrical contact has to be masked to leave bare metal. Call those surfaces out on the print, the same way you’d call out a tolerance, and the masking is planned in with the rest of the finish.

Color, blasting, and alloy

A bead blast before anodize evens out tool marks for a uniform matte; an as-machined part can anodize with the tooling pattern still visible, which is fine for function but shows on cosmetic faces. If the look matters, say so up front. Clear and black are the most consistent options across a mixed-alloy assembly, since dyed color can shift batch to batch and a 6061 and a 7075 part in the same dye bath can read slightly different.

On alloy: 6061 and 7075 both anodize well and are the common picks for anodized aluminum parts. High-copper 2000-series alloys and many castings anodize blotchy, so they’re worth flagging early. This is a coating story for aluminum only: titanium ‘anodizing’ is a different process where color comes from oxide thickness set by voltage rather than dye, and that’s covered on the titanium material page.

Anodize or powder coat

Anodizing keeps the part dimensionally tight and integral to the metal, which is why it’s the default for precise aluminum work like brackets and housings. If you need a thicker, more impact-resistant color layer and the tolerances can give a little, powder is the other route; see powder coating, and the full menu of options on CNC surface finishes. Either way the anodizing service is specified with the part and quoted as one line, not bolted on afterward, so the finish, the masking, and the tolerances are all priced together.