Anodizing
Also: Type II anodizing, sulfuric acid anodizing, MIL-A-8625 Type II, anodising, clear anodize, color anodize, dyed anodize
Anodizing grows a hard, dyeable aluminum oxide layer on aluminum parts, 8-25 µm thick, for corrosion resistance, colour and a durable satin finish.
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Anodizing is an electrolytic conversion of the aluminum surface into aluminum oxide. The part is the anode in a sulfuric acid bath; the oxide grows into and out of the metal as a porous, columnar film, which is then dyed if a colour is wanted and sealed to close the pores. Nothing is deposited: the coating is the part's own metal, so it cannot peel or flake the way paint can.
MIL-A-8625 is the spec everyone quotes. Type II is conventional sulfuric acid anodize, 8-25 µm (0.0003-0.001") thick; Type I is chromic acid, thinner and used on fatigue-critical aerospace parts and assemblies that trap electrolyte; Type III is hardcoat, covered in its own record. Within a type, Class 1 is undyed and Class 2 is dyed to a specified colour. The sealing step matters as much as the anodize: a hot-water or nickel-acetate seal gives the corrosion resistance, and an unsealed coat is only used as a paint base or before PTFE impregnation.
The coating is an electrical insulator, it is harder than the base alloy but not a wear coating, and dyes fade in sunlight unless the colour is black or an inorganic (two-step) process. Colour and clarity depend on the alloy: 6061, 6063 and 5052 anodize clear and even; 2024 and 7075 come out darker and less uniform because of their copper; castings with high silicon turn grey. Keep one alloy per visible assembly if the parts must match.
At a glance
| Typical tolerances | Type II thickness 8-25 µm (0.0003-0.001"), typically held to ±5 µm (±0.0002") on a stated nominal. About half the thickness grows outward, so a 20 µm coat adds roughly 10 µm (0.0004") per surface and 20 µm on a diameter. Type I chromic is 2-8 µm (0.0001-0.0003") with negligible growth. Threads, press fits and bearing bores are masked or dimensioned after anodize. |
|---|---|
| Size limits | Job-shop tanks commonly take parts up to 2.5-3 m (8-10 ft) long and 1 m (40") deep; architectural anodizers run 6-8 m (20-26 ft) extrusions. Small parts are racked in batches of hundreds; every part needs an electrical contact point, which leaves a small bare mark. |
| Surface finish | A satin to semi-gloss surface that follows the underlying texture: bead-blast before anodize for an even matte, or polish for a bright finish. Clear anodize on 6061 is silver-grey; dyes give black, red, blue, gold and most other colours, black being the most UV-stable. Type II is about 250-350 HV, twice the hardness of the alloy, with a small increase in roughness (about 0.2-0.5 µm Ra added). |
| Lead time | Clear or black Type II in 3-5 business days; custom colours, colour-matched lots or Type I in 1-2 weeks. Same-week service is common for small racks. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Aluminum only: wrought 6061, 6063, 5052, 5083 and 7075 anodize well, 2024 and 7075 with darker, less uniform colour, and sand or die castings poorly because their silicon shows as grey smut. Titanium and magnesium have their own anodizing chemistries (different spec, different shop). Steel, stainless, brass and plastics cannot be anodized.
What drives the cost
- Racking labour: how many parts fit on a rack and how many contact points they need
- Masking of threads, bores, contact faces and cosmetic areas
- Colour: clear and black are stock; custom or matched colours carry a setup charge and a minimum lot
- Coating thickness and Type I versus Type II (chromic lines are rarer)
- Pre-treatment: bead blasting, chemical brightening or polishing before anodize
- Certifications and test coupons: coating weight, seal test, salt spray, MIL-A-8625 conformance
When to use it
- Corrosion protection on aluminum parts, especially 7075 and 2024, which pit if left bare
- A colour or a uniform cosmetic finish on machined or extruded aluminum enclosures, panels and fittings
- A harder, scratch-resistant surface on knobs, handles and covers that see handling
- Electrical insulation of an aluminum surface, or a paint and adhesive base
- Any aluminum part that goes outdoors or in a marine or coastal environment
When not to
- Surfaces that must conduct electricity or ground an enclosure: use chem film there, or mask the contact points
- Wear surfaces and sliding fits: specify Type III hardcoat instead
- Exact colour matching across different alloys, castings or lots; the chemistry makes small shifts unavoidable
- Parts that cannot tolerate a small dimensional growth on tightly toleranced fits and threads
- Steel, stainless, brass or plastic parts, which need a different finish entirely
Design tips
- Call out MIL-A-8625, the type, the class and the colour, and the thickness if it matters: "Anodize per MIL-A-8625 Type II Class 2, black, 15-20 µm".
- State whether dimensions on the drawing apply before or after anodize, and mask or post-machine anything with a fit tighter than ±0.025 mm (±0.001").
- Give the anodizer a rack point: name a surface that can carry a small contact mark, or add a tab or a tapped hole for it.
- Anodize cannot hide machining marks, scratches or die lines; it makes them more visible. Bead blast first for a cosmetic part.
- Keep one alloy and one temper across a matched cosmetic assembly, and ask for parts from the same lot to be run together.
- Blind holes and deep pockets trap acid; add drain holes or ask the shop to orient the part so it drains.
- Do not weld after anodizing, and expect the coating to be burned off in the heat-affected zone if you weld before.
- For outdoor colour, specify black or an inorganic two-step colour and a nickel-acetate seal; organic reds and blues fade.
Anodizing by material
Frequently asked questions
- How thick is Type II anodizing?
- Typically 8-25 µm (0.0003-0.001"), with 12-18 µm the usual production window. MIL-A-8625 sets a minimum coating weight rather than a thickness, so state a thickness on the drawing if a fit or a wear requirement depends on it.
- Does anodizing change part dimensions?
- Yes, by about half the coating thickness per surface. A 20 µm Type II coat adds roughly 10 µm (0.0004") to each surface, which closes a bore by about 20 µm on diameter. Mask or post-machine tight fits, or dimension the part after anodize.
- Can 7075 aluminum be anodized?
- Yes. Type II and Type III both work on 7075, but its copper makes the coating darker and less even than on 6061, so clear cosmetic parts are usually 6061 and 7075 is anodized for protection, often dyed black.
- Is anodizing conductive?
- No. Aluminum oxide is an insulator, and a sealed Type II coat will not carry a ground. Mask the grounding points, or specify chem film (MIL-DTL-5541 Class 3) on parts that must conduct.
- What is the difference between Type II and Type III anodizing?
- Type II is the decorative and corrosion coating: thin, dyeable, cheap. Type III hardcoat is grown colder and thicker, 25-75 µm, and is much harder, for wear surfaces. Type III is darker, rougher, harder to colour, and costs more.
