아노다이징 II형 vs III형(하드코트): 경도·성장·비용 엔지니어 가이드 (2026)

Type II vs III at a glance

What anodizing actually is

Anodizing is an electrochemical process that thickens aluminium’s natural oxide layer into a hard, integral aluminium-oxide skin. Unlike paint or plating, the oxide is converted metal surface — it can’t chip or peel because it isn’t a separate layer sitting on top. That’s why anodized aluminium dominates aerospace, consumer electronics, medical and architectural parts. Both Type II and Type III are covered by the US defence spec MIL-A-8625 (opens in new tab), which also defines Type I (chromic acid).

The process, step by step

1. 1

   1. Clean & degrease

   Remove oils and machining residue so the oxide forms uniformly.

2. 2

   2. Etch / desmut

   A light etch evens the surface; desmutting removes alloying-element residue.

3. 3

   3. Anodize (the electrochemical step)

   The part is the anode in a sulfuric-acid bath. Current grows the oxide. Type III runs colder and at higher current for a thicker, denser layer.

4. 4

   4. Colour / dye (optional)

   The fresh oxide is porous and accepts dye. Type II dyes to almost any colour; Type III mostly goes dark.

5. 5

   5. Seal

   Hot water, nickel-acetate or dichromate seal closes the pores, locking in colour and maximising corrosion resistance.

The chemistry is nearly identical for both types — the difference is the bath temperature and current density. Type III (hardcoat) runs near 0 °C at higher voltage, which grows a thicker, harder, denser oxide than Type II’s room-temperature bath.

Type II — decorative / conventional

Type II is the everyday anodize: a thin (5–25 µm) sulfuric-acid oxide that resists corrosion, takes dye in a full spectrum of colours, and adds a modest hardness bump. It’s what’s on most consumer-electronics housings, brackets, trim and general aluminium hardware.

• Thickness: 5–25 µm (0.0002–0.001").
• Best for: colour, corrosion resistance, cosmetic finish, general-purpose parts.
• Colour: excellent — clear, black, red, blue, gold and custom colours.
• Hardness: harder than bare aluminium, but still scratchable by hard metals.
• Dimensional impact: small (typically ≤5 µm on a dimension) — often ignorable on loose tolerances.

Type III — hardcoat

Type III (hardcoat / hard anodize) is the engineering finish: a thick (25–75 µm), dense oxide with surface hardness comparable to hardened steel. It’s specified where parts slide, wear, or take abuse — pistons, valves, gears, aerospace and defence hardware.

• Thickness: 25–75 µm (0.001–0.003"), sometimes thicker.
• Best for: wear resistance, abrasion, load-bearing sliding surfaces, harsh environments.
• Hardness: very high — often quoted around 60–70 on the equivalent Rockwell C scale.
• Colour: limited — naturally grey to bronze/black; bright colours aren’t reliable on hardcoat.
• Dimensional impact: significant — must be compensated in machining (see below).

Head-to-head comparison

Dimensional growth & tolerance compensation

This is the section most guides skip and most first-time buyers get burned by. Anodizing changes the part’s dimensions, and on Type III the change is big enough to blow a tight tolerance if you don’t plan for it. Here’s the rule that matters:

The practical consequence: machine the part to compensate before anodizing. If a bore must be Ø10.00 mm after a 50 µm hardcoat, machine it to about Ø10.05 mm so the coating brings it into spec. A press-fit pin diameter is machined slightly under. This is exactly why anodizing should be on the drawing before the part is cut — our in-house anodizing service works from the finished-part tolerance backwards so the machined dimension already accounts for growth. For how to specify these tolerances, see our GD&T guide.

Colour & appearance

Colour is where Type II shines and Type III is limited. Type II’s thinner, more transparent oxide dyes cleanly to almost any colour and seals bright. Type III’s thick oxide is naturally dark and only reliably holds black and dark bronze; asking for a bright colour on hardcoat usually disappoints.

• Type II colours: clear (natural silver), black, red, blue, gold, and most custom colours with dye matching.
• Type III colours: natural grey/bronze to black. Black hardcoat is standard and repeatable; other colours are unreliable.
• Finish before anodize matters: anodizing is translucent, so it reveals the underlying surface. Bead-blast for a uniform matte; polish for gloss. See our surface finishing guide.
• Batch colour consistency depends on alloy and temper — critical cosmetic parts should be anodized in one lot from one material batch.

Which aluminium alloys anodize well

Not all aluminium anodizes equally. Alloying elements — especially copper and silicon — affect colour uniformity and coating quality. This surprises engineers who assume "aluminium is aluminium".

For cosmetic anodized parts, design in 6061 or 6063 wherever you can. If the part must be a high-copper or high-silicon alloy, set colour expectations accordingly. More on grade selection in our aluminium CNC machining guide.

Masking & design for anodizing

Which one should you choose?

Still unsure? Send the drawing and the service environment to our engineers via the anodizing service page and we’ll recommend the type, colour and tolerance compensation before the part is cut.

Frequently asked questions

The questions engineers ask most about anodizing aluminium parts.