---
title: "Anodizing Spec Chart: Type I, II, III (MIL-A-8625) | OpenSpindle"
description: "Anodizing spec chart: Type I, II, and III thickness, color, and alloy guidance per MIL-A-8625, and how to call it out on a drawing."
canonical: https://openspindle.com/reference/anodizing-spec
---

# 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.

## On OpenSpindle

- [Aluminum 6061 machining](https://openspindle.com/materials/aluminum.md)
- [Powder coating spec](https://openspindle.com/reference/powder-coating-spec.md)
- [Surface finishes compared](https://openspindle.com/reference/surface-finishes-compared.md)

## 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.