Anodizing

Anodize when you need hard, durable corrosion and wear protection or a dyed cosmetic finish; chem film (MIL-DTL-5541) when you need corrosion protection plus electrical conductivity or a paint base with minimal dimensional change; plate (nickel, silver, tin) when the aluminum needs a solderable, conductive, or wear surface that an oxide cannot provide. The three are complementary, not competing, and many assemblies use more than one. The deciding questions are conductivity, wear, cosmetics, and whatever the governing spec already mandates.

Type II (sulfuric, per MIL-A-8625) is the general-purpose choice for corrosion protection and dyed cosmetic finishes at roughly 0.0001 to 0.0005 inch; Type III hardcoat is thicker (typically 0.001 to 0.002 inch) and much harder, chosen for wear surfaces, abrasion, and dielectric duty. Hardcoat grows more into and out of the surface, so dimensional callouts matter more. Each type has its own page here with thickness, sealing, and spec details.

Yes – a common construction is anodize over most of the part with masked areas receiving chem film, which keeps those pads electrically conductive for grounding and bonding while the rest of the part gets the harder anodic coating. Masking for threads, bores, and interfaces is routine rack work. Define masked zones explicitly on the drawing with dimensions, since ‘mask threads’ alone leaves too much to interpretation.

Significantly – high-copper 2xxx and high-zinc 7xxx alloys anodize with more color variation and somewhat lower corrosion performance than 5xxx and 6xxx alloys, and cast alloys with high silicon anodize darker and less uniformly than wrought material. 6061 is the benchmark alloy for predictable, cosmetically consistent results. If color matching across a multi-alloy assembly matters, say so up front, because alloy-to-alloy shade differences cannot be fully processed out.

MIL-A-8625 is the umbrella specification for both, with AMS 2471 commonly cited for Type II (undyed sulfuric) and AMS 2468/2469 for hardcoat, per the revision on your drawing. Callouts should include type, class (Class 1 undyed or Class 2 dyed), color, and sealing method, since dichromate, nickel acetate, hot DI water, and Teflon-impregnated seals serve different requirements. Gleco anodizes to these specs across 1xxx through 7xxx wrought alloys plus sand and die castings.