In one paragraph: Type II (sulfuric) anodizing builds a 2–25 µm decorative-and-protective layer that takes color well — the default for corrosion protection and appearance. Type III (hardcoat) builds 25–75 µm of dense oxide at low temperature for wear surfaces — harder, thicker, and naturally dark gray. Choose by function: sliding wear or abrasion demands Type III; everything else on aluminum usually wants Type II.
The spec behind the names
Both processes are defined by MIL-A-8625: Type II is conventional sulfuric-acid anodizing, Type III is hard anodic coating. Class 1 means non-dyed, Class 2 dyed. When a drawing says “anodize per MIL-A-8625 Type III Class 1”, it is asking for a hard, undyed coating of specified thickness.
Side-by-side
| Type II (sulfuric) | Type III (hardcoat) | |
|---|---|---|
| Typical thickness | 2–25 µm | 25–75 µm (spec min ~50 µm for true hardcoat) |
| Process temperature | 18–24 °C | 0–5 °C, high current density |
| Hardness | Moderate | 350–500+ HV |
| Color options | Wide (black, clear, gold, blue…) | Naturally dark gray; limited dyeing |
| Best for | Corrosion protection, appearance | Wear surfaces, sliding fits, abrasion |
The dimensional trap: 50/50 growth
Anodic coatings grow roughly half into the base metal and half outward — a 50 µm Type III coating adds only about 25 µm per surface, but it consumes 25 µm of your machined dimension. On precision parts, specify whether dimensions apply before or after anodizing, and flag tight-fit holes and threads (which often need masking or allowance). This single note prevents most anodizing rework.
Alloy matters more than you’d expect
- 6061 anodizes evenly and takes color best — the default choice for dyed Type II and uniform dark-gray Type III.
- 7075 yields higher substrate strength but anodizes darker and less predictably, especially when dyed.
- 2024 (high copper) is the hardest to anodize consistently — avoid it for cosmetic parts. See our 6061 vs 7075 comparison for substrate-level trade-offs.
Choosing in 30 seconds
- Part lives outdoors or needs brand color → Type II, 8–15 µm.
- Surface slides, pivots, or takes impact → Type III, 25–50 µm, and mask bearing bores.
- Electrical contact required → mask the zone or specify chromate conversion instead.
- Working with us? Anodizing runs in-house next to machining — see our anodizing capability page, or send the drawing via the RFQ form and the coating callout gets reviewed with the part.
