Custom anodized aluminum parts

How anodizing works

Anodizing is an electrochemical process. The aluminum part is the anode in an acid bath (sulfuric acid for Type II and Type III), and current converts the surface into aluminum oxide. Unlike paint or plating, the layer is grown from the base metal itself, so it does not peel. The fresh oxide is porous; it can absorb dye for color and is then sealed to close the pores and improve corrosion resistance.

We machine the part, anodize it and inspect it as one order, so you get custom anodized aluminum parts without coordinating a separate finisher. Prototypes from 1 piece and low-volume batches use the same process.

Type II vs Type III anodizing

Both types are commonly specified to MIL-A-8625. The difference is process temperature, current density and resulting thickness and hardness.

MIL-A-8625 Class 1 means undyed (clear) and Class 2 means dyed. If you only need corrosion protection and a cosmetic finish, choose Type II. Choose hard anodizing when a surface slides, wears or sees grit.

Anodizing thickness and tolerances

About half of the anodic layer grows outward from the original surface and half penetrates into it. That means each surface moves outward by about half the coating thickness:

• A 20 µm Type II coat adds about 10 µm per surface: an outside diameter grows ≈20 µm and a bore shrinks ≈20 µm.
• A 50 µm hard anodize adds about 25 µm per surface: a diameter changes ≈50 µm, which is larger than a ±0.03 mm tolerance band.
• On 60° threads, pitch diameter changes by about four times the per-surface growth, so even Type II can make a class 2B tapped hole tight.

For Type II on general features, the change is usually inside standard tolerance. For hard anodizing and for precision fits, tell us whether dimensions apply before or after anodize. If after, the machinist leaves the allowance; if not stated, we ask in the DFM notes. Read our tolerance guide for how to call this out.

Masking threads, bores and contact points

Masking keeps the coating off selected areas using plugs, caps, tape or lacquer. Typical masked features:

• Tapped holes — mask, or chase threads after anodizing. Threaded inserts installed after anodizing avoid the issue.
• Bearing and dowel bores with press or slip fits.
• Electrical grounding points — anodize is an electrical insulator, so any face that must conduct needs masking.
• Sealing faces where a specific roughness is required.

Mark masked areas clearly on the drawing. Each masked feature adds manual work, so mask only what matters. Also note that the part is held on a rack during anodizing and the contact point stays uncoated; if you care where that small mark goes, show an acceptable location.

Sharp edges and part design

The oxide grows perpendicular to the surface, so it cannot form a continuous layer on a knife-sharp outside corner. Edges end up thin, chip easily and may look lighter. A small edge break or radius (about 0.5 mm is a common recommendation for hard anodize) gives a more durable coat. Deep, narrow holes and blind pockets anodize unevenly because acid flow and current are restricted; expect thinner coating deep inside.

Black anodized aluminum parts and color matching

Black anodized aluminum parts are the most common request. Black is forgiving, but exact color matching across batches and alloys is a limitation of the process, not something any finisher can fully control.

• 6061 anodizes well and gives the most consistent, even color. It is the default for cosmetic parts.
• 7075 anodizes well for protection, but zinc in the alloy shifts the color. Clear coats can look slightly yellow and black can look different from 6061 parts in the same batch.
• 2024 contains a lot of copper, which makes anodizing harder and results less uniform. Hard anodizing 2024 needs care, and cosmetic results are limited.
• Cast alloys with high silicon anodize gray and patchy. Machined plate is a better choice for cosmetic anodizing.
• 5052 sheet anodizes reasonably well for formed parts, though color may differ from machined 6061 parts.

To keep parts in an assembly matching, make them from the same alloy, anodize them in the same batch, and send a color reference if you have one. Bead blasting before anodizing also hides small variations by giving every part the same matte texture. Dyed colors other than black can fade under long UV exposure; black and clear are the most stable outdoors.

See aluminum grades for strength and machinability, and the surface finish chart for alternatives like powder coating. For cost effects of finishing and masking, read CNC machining cost.