Reference

Anodizing Spec Chart

Type I, II, and III anodize compared by thickness, color, and alloy, per MIL-A-8625, so you can spec the right finish and call it out cleanly.

Anodizing converts the surface of an aluminum part into a hard, corrosion-resistant oxide that can be dyed. The type controls how thick and hard it is. Type II (sulfuric) is the common decorative and corrosion finish; Type III (hardcoat) is the thick, wear-resistant one; Type I (chromic) is the thin, fatigue-friendly aerospace finish. All three are defined by MIL-A-8625.

Anodize types compared (MIL-A-8625)

Class 1 is undyed; Class 2 is dyed. Thicker coatings add dimensional growth: roughly half the coating thickness grows on the surface per side.

Type	Process	Typical thickness	Color	Best for
Type I	Chromic acid	0.5-2.5 um (0.02-0.1 mil)	Thin, gray-ish; Class 2 dyeable	Fatigue-critical aerospace parts, paint base
Type II	Sulfuric acid	5-25 um (0.2-1.0 mil)	Clear or dyed (black, red, blue, gold...)	Decorative and general corrosion protection
Type III	Sulfuric hardcoat	25-100 um (1.0-4.0 mil)	Gray to black (dye optional)	Wear and abrasion resistance, tooling, wear surfaces

Typical ranges; the achievable thickness and color depend on alloy and shop. Confirm on your part.

How different alloys anodize

Anodizing quality is alloy-dependent. Wrought 6000-series parts anodize clean and uniform; high-copper and high-silicon alloys can come out patchy or dark.

Alloy	Anodizes well?	Notes
6061 / 6063	Yes	Best all-around; clean, uniform, dyes evenly
7075	Yes	Good, slightly darker/grayer than 6061
2024 (high copper)	Limited	Duller, less uniform; hardcoat can look mottled
High-silicon cast (e.g. A380)	Poor	Grays/darkens unevenly; often not spec'd for cosmetic anodize

How to call it out on a drawing

A complete callout names the standard, type, and class, plus color and thickness where they matter. Example: "Anodize per MIL-A-8625, Type II, Class 2, black." For hardcoat, add the thickness, e.g. "Type III, 0.002 in." Note masked areas and threaded holes, which are usually excluded.

Values reflect MIL-A-8625F and common practice. Verify the exact type, class, thickness, and color a shop can hold on your alloy before release.

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Frequently Asked Questions

What is the difference between Type II and Type III anodizing?

Type II is sulfuric anodize, roughly 5-25 um thick, used for decorative color and general corrosion protection. Type III is hardcoat, roughly 25-100 um thick and much harder, used for wear and abrasion resistance. Type III is thicker and tougher but adds more dimensional growth.

How much thickness does anodizing add?

About half of the coating thickness grows outward from the original surface per side. For a 25 um (0.001 in) Type III coat, expect roughly 12 um (0.0005 in) of growth per surface, which matters on tight-tolerance features and threads.

Which aluminum anodizes best?

Wrought 6061 and 6063 anodize the cleanest and dye most evenly. 7075 anodizes well but a little darker. High-copper 2024 and high-silicon cast alloys anodize poorly and can look mottled, especially with hardcoat.

How do I specify anodizing on a drawing?

Call out the standard, type, and class, e.g. "Anodize per MIL-A-8625, Type II, Class 2, black." Add a thickness for hardcoat (Type III) and note any masked or excluded areas such as threaded holes.

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