Anodize Type II vs Type III for Machined Aluminum

Anodize Type II vs Type III is the finish call that shows up after the mill work is done. Both grow an oxide on aluminum. They are not the same thickness, look or job.

Should a Machined Aluminum Part Get Type II or Type III Anodize?

Pick Type II for corrosion protection, color and most cosmetic parts. Pick Type III (hardcoat) when wear or abrasion matters more than appearance. Plan for coating growth either way.

Protolabs compares both in The Different Surface Finishes for CNC Machining. The description lists "anodizing type 2 and 3 and powder coating," and the chapters walk Type II at 05:35 and Type III at 06:16 after a short anodizing overview.

Anodize Type II vs Type III: What Changes on the Part

Protolabs' written guide on comparing Type II and Type III anodizing gives useful ranges: Type II thickness about 0.00007-0.001 in (1.8-25 μm), Type III about 0.0005-0.006 in (13-150 μm). Type II is listed with good corrosion resistance and dyeing for color. Type III is listed with very good corrosion and wear resistance, and is "generally best for non-visible parts," at somewhat higher cost.

That thickness eats into fits. Threads, bearing bores and press fits need stock left (or masking) before the bath. We cover that in how much stock to leave for anodize.

Material choice still matters first. Aluminum 6061 and Aluminum 7075 are both on our Materials page for CNC. 7075 anodizes differently than 6061 in practice; if the alloy is still open, see 7075 vs 6061.

Specifying It on the Drawing

Call out Type II or Type III, color if Type II is dyed, and which faces must stay uncoated. Our CNC standard is "ISO 2768-m as standard, down to ±0.01 mm where the drawing calls for it," so fit features that get coated should be called out before finishing, not after.

Finishing options on a given job depend on the order. Ask us when you quote whether Type II, Type III or another finish is available for that part, and how to note masked areas.