I see the same thing on maybe one in three aluminum part drawings that come through here: the finish callout just says "anodize" or "clear anodize." No type. No thickness. No color code. No spec reference. Then I email back and ask five questions before I can give an accurate quote.
Anodizing is the most common surface finish for CNC machined aluminum parts. It's also the one with the most variables that nobody puts on the drawing. This covers the three types of aluminum anodizing that matter for machined parts, what they cost, and what you should actually write in the finish block.
What anodizing actually does
Anodizing is an electrochemical process that grows a controlled aluminum oxide layer on the part surface. Unlike paint or plating, it's not a coating applied on top — the oxide forms from the aluminum itself, converted and grown outward.
Two things follow from this. First, anodizing has near-zero dimensional build-up in the traditional sense. Half the oxide grows inward, half outward. A 10μm anodize layer adds roughly 5μm to each surface. Second, you can't peel it. The bond between aluminum and its oxide is as strong as it gets — aluminum oxide is what sapphire and ruby are.
The resulting oxide is harder than raw aluminum, electrically insulating, and porous enough to absorb dyes before sealing. That combination — hardness, color options, durability — makes it the default finish for aluminum parts that need to look decent, resist wear, or both.
Type II (sulfuric acid anodize): the standard
Type II, per MIL-A-8625, is what most people mean when they say "anodize." Sulfuric acid bath, room temperature to slightly chilled, typical thickness 5-25μm (0.0002-0.001 inch).
For machined parts, Type II handles three jobs:
- Corrosion resistance for indoor and mild outdoor environments
- Base layer for dye — the pore structure absorbs dyes well
- General appearance — uniform matte to semi-matte finish
Type II works on all common machining alloys: 6061, 7075, 5083, 2024. 6061 anodizes beautifully — uniform color, consistent thickness. 7075 comes out darker and slightly uneven because of its zinc and copper content. 2024 (high copper) is darker still, more bronze than gray. If you expect 7075 or 2024 to match a 6061 cosmetic part, you'll be unhappy with the result.
Thickness control with Type II is decent, not precision. A callout for 10μm Type II really means 8-15μm across the part. Sharp corners build thicker oxide on the corner itself and thinner in the zones next to it.
Type III (hardcoat): when wear matters
Type III uses the same sulfuric acid chemistry but at near-freezing temperatures (0-5°C) and higher current density. The result is denser, harder, thicker oxide — 25-100μm (0.001-0.004 inch).
Hardcoat is specified for wear surfaces, sliding components, and parts that see mechanical abuse. The oxide hardness runs 350-500 HV, versus 200-300 HV for Type II. Raw 6061-T6 sits around 100 HV for reference.
Color options are limited with Type III. Black is common. Dark green and dark gray work. Light colors and bright colors generally don't — the oxide is too dense and its natural gray-to-charcoal base color overwhelms the dye.
Two practical things about hardcoat on machined parts:
Threaded features grow. A 50μm (0.002 inch) hardcoat layer per surface shrinks the pitch diameter of a threaded hole by roughly 0.004 inch total. Threads below M4 or 8-32 usually need masking during anodizing or chasing with a tap afterward. Say which on the drawing, or the shop will either mask everything (adds cost) or anodize everything (threads won't fit).
Fatigue life takes a hit. Hardcoat reduces aluminum fatigue strength by 15-30%, depending on alloy and thickness. For structural aerospace parts, this is a design input, not an afterthought. Type II has a smaller effect (5-10%). Type I chromic acid anodize has almost none — which is why Type I still exists in fatigue-critical aerospace.
Type I (chromic acid): the niche you probably don't need
Type I uses chromic acid instead of sulfuric. The oxide is thin (2-5μm), softer than Type II, gray-green, and has essentially no effect on fatigue life. It accepts paint well.
Unless you're in aerospace or defense and the print calls for it by spec, you won't specify Type I. It costs more than Type II because chromic acid handling is heavily regulated. Most general CNC shops don't run it in-house.
Type II vs Type III at a glance
| Property | Type II | Type III |
|---|---|---|
| Thickness | 5-25μm | 25-100μm |
| Hardness | 200-300 HV | 350-500 HV |
| Color range | Full (clear, black, blue, red, gold, green) | Black, dark green, dark gray |
| Corrosion resistance | Indoor, mild outdoor | Outdoor, marine, chemical |
| Wear resistance | Moderate | High |
| Cost multiplier | 1x baseline | 2-3x |
| Common spec | MIL-A-8625 Type II | MIL-A-8625 Type III |
| Thread interference | Minor (compensate in machining) | Significant (mask or chase taps) |
| Fatigue reduction | ~5-10% | ~15-30% |
| Dye absorption | Good | Limited |
What to put on your drawing
The minimum the shop needs:
- Anodize type: Type II or Type III (or Type I if you're in aerospace)
- Color: Clear (natural), black, blue, red, gold. Reference a standard if you have one — though exact color matching in anodize is closer to cooking than chemistry
- Thickness: Only if it matters. "Type II, Class 2" implies 10-18μm per MIL-A-8625
- Masking: List every feature that must stay bare aluminum — threaded holes, press-fit bores, electrical contact pads, grounding surfaces
- Seal: Sealed is standard for Type II (closes pores, locks dye). Unsealed has better lubricity for oil-impregnated wear surfaces but worse corrosion resistance. Nickel acetate seal at temperature is standard for Type III
- Spec reference: "MIL-A-8625 Type II, Class 2, Clear" covers most commercial work
Example for a typical cosmetic part:
ANODIZE PER MIL-A-8625 TYPE II, CLASS 2
COLOR: BLACK
MASK ALL THREADED HOLES
Example for a hardcoat wear surface:
ANODIZE PER MIL-A-8625 TYPE III, CLASS 1
THICKNESS: 0.002" (50μm) MIN
MASK: PRESS-FIT BORES Ø12.00 +0.01/-0.00
Masking: the step that gets forgotten
Any surface that gets anodized stops conducting electricity. Any surface that gets Type III changes dimension by 25-100μm per side. Threaded holes, press-fit bores, bearing seats, grounding pads — all need masking.
Masking is done with plugs, caps, tape, or paint-on resist. It's manual labor. Someone installs the mask before anodizing and removes it after. Complex masking — lots of small threaded holes, tiny features — can add more to the finishing cost than the anodizing itself.
Call out every feature that needs masking on the drawing. If you skip this, the shop will either anodize everything (your threads won't fit) or call to ask, which adds a day to your quote.
What anodizing costs
Anodizing cost splits into the batch minimum plus per-part processing.
For Type II, a finishing house charges $50-150 batch minimum plus per-part cost that drops with quantity. A single prototype runs $80-120 (you're paying the batch minimum). 50 pieces: $3-5 each. 500 pieces: $1-2 each.
Type III costs 2-3x Type II. Longer process time, more electricity, tighter bath control.
Color adds 20-50% over clear for Type II. Black is cheapest — every shop runs black tanks daily. Custom colors cost more because you might be paying for a dedicated batch.
| Finish | 1 part | 10 parts | 50 parts | 100 parts |
|---|---|---|---|---|
| Type II, clear | $80-120 | $8-15/ea | $4-8/ea | $2-5/ea |
| Type II, black | $90-150 | $10-18/ea | $5-10/ea | $3-6/ea |
| Type II, custom color | $120-200 | $15-25/ea | $8-15/ea | $5-10/ea |
| Type III, black | $150-250 | $20-35/ea | $12-20/ea | $8-15/ea |
Most CNC shops don't anodize in-house. Parts go to a finishing house, which adds 2-4 days to your lead time. Rush anodizing (24 hours) runs about 50% extra.
Mistakes I see regularly
"Anodize" with no type. Type II and Type III are completely different. The shop has to ask, and your quote loses a day.
Wrong thickness for the type. "Type II, 50μm" — that's Type III territory. Type II above 25μm gets uneven and tends to burn. If you need 50μm, write Type III.
No masking callout. Your threaded holes, press fits, and ground pads matter. Call them out.
Expecting 7075 or 2024 to look like 6061. High-copper and high-zinc alloys anodize darker and less evenly. If cosmetic match to a 6061 part matters, say so upfront.
Assuming dye lot consistency. Black from batch A won't exactly match black from batch B. If parts sit next to each other in an assembly and color matters, order them in one batch with that note on the PO.
The short version
For most CNC aluminum parts, "MIL-A-8625 Type II, Class 2, Black" is the right callout. Specific enough for the shop, standard enough for any finishing house, cheap enough that nobody debates it.
If your part is a wear surface, sliding component, or lives outdoors, step up to Type III and call out your masking.
Put the spec on the drawing before you send the RFQ. It saves everyone a day of back-and-forth and gets you a real quote the first time.
Need aluminum parts machined and anodized? Send us your drawing. We quote both machining and finishing in one number — no separate finishing house markup, no back-and-forth between shops.