Type I vs Type II vs Type III: Choosing the Right Anodizing Class

MIL-A-8625 defines three families of anodic coating on aluminium, and the difference between them is not cosmetic — it's coating thickness, and thickness is what drives both performance and fatigue impact. Specifying the wrong class is one of the more common, and more expensive, drawing mistakes on aluminium parts.

Type I — Chromic Acid Anodizing

The thinnest of the three, typically 0.00002–0.0001 in (0.5–2.5 microns). Because the coating is so thin, it has the lowest impact on fatigue life of the three types — which is exactly why it's the default choice for fatigue-critical structural aluminium and, alongside BSAA, for surfaces that will be adhesively bonded. Chromic anodizing uses hexavalent chromium, which is why boric sulfuric acid anodizing (BSAA) has become the preferred chromium-free alternative on new aerospace programmes.

Type II — Sulfuric Acid Anodizing

The general-purpose workhorse, typically 0.0002–0.001 in (5–25 microns), governed by AMS 2471 (undyed) and AMS 2472 (dyed). It's what most people picture when they hear "anodized" — good corrosion resistance, takes dye well for colour anodizing, and a reasonable fatigue debit for non-critical or moderately loaded parts. This is the default for cosmetic and general corrosion-protection applications.

Type III — Hard Anodizing

Thicker again — 0.002 in (50 microns) and up, per AMS 2469 — grown at lower bath temperature and higher current density to produce a denser, harder oxide layer. Hard anodizing is specified where wear resistance matters as much as, or more than, corrosion resistance: hydraulic cylinder bores, sliding mechanical interfaces, gauges and tooling. The trade-off is a larger fatigue debit and more dimensional build-up, both of which need to be accounted for on tight-tolerance features.

Choosing between them

• Fatigue-critical or bonded structural aluminium — Type I or BSAA, not Type II or III.
• General cosmetic or moderate corrosion protection — Type II, dyed or undyed.
• Wear surfaces, hydraulic bores, tooling — Type III hard anodizing, with dimensional build-up designed into the tolerance from the start.
• When in doubt, the governing specification (customer drawing, AMS, or MIL standard) should call the class explicitly — "anodize per spec" without a class is an invitation for the wrong coating to get run.

We run all three classes, plus BSAA, and size the process to the class the drawing actually calls for — not the class that's easiest to run that week.