Anodizing
Anodizing grows a hard porous oxide layer on aluminum electrochemically, which can then be dyed and sealed for wear and corrosion resistance.
- Part
- Finishing
- Revised
- 2026-08-11
At a glance
- Family
- Additive
- Typical tolerances
- Type II adds 0.0002–0.001 in (5–25 µm) total, roughly half of it outward; Type III adds 0.0005–0.004 in (13–100 µm). Budget the full coating thickness on a diameter and 4× the radial buildup on thread pitch diameter
- Surface finish
- Reproduces the substrate. Bead-blast plus Type II typically lands at 32–125 µin (0.8–3.2 µm) Ra; Type III raises Ra above the as-machined value
- Typical volumes
- 1 to millions of parts; racked in batches and priced by area and rack space
- Lead time
- 3–7 business days at a job shop; 1–2 days expedited; add a week for a first-article custom color match
- Materials
- Metal
What it is
Anodizing is an electrochemical process that converts the surface of an aluminum part into a hard, porous aluminum oxide grown out of the base metal rather than deposited onto it. Because the oxide is part of the substrate, it cannot chip or peel the way paint can. Three coating types dominate, all defined by MIL-A-8625: Type I (chromic acid, 0.00002–0.0001 in / 0.5–2.5 µm), Type II (sulfuric acid, 0.0002–0.001 in / 5–25 µm), and Type III hardcoat (sulfuric acid run cold, 0.0005–0.004 in / 13–100 µm). Type II is the cosmetic workhorse — it accepts dye and is what most consumer aluminum enclosures wear. Type III is the wear coating, commonly quoted at 60–70 HRC equivalent surface hardness.
The number that governs every design decision: roughly half the coating grows into the part and half builds outward, so a 0.002 in (50 µm) hardcoat adds about 0.001 in (25 µm) per surface. Anodizing applies to aluminum, titanium and magnesium — never to steel.
How it works
- Clean and degrease. An alkaline soak cleaner at 140–160 °F (60–70 °C) removes oil, coolant and fingerprints. Anything left behind prints through as a visible defect, because the finish is transparent.
- Etch and desmut. A sodium hydroxide bath at 120–150 °F (50–65 °C) for 1–3 minutes removes roughly 0.0001–0.0005 in (2.5–13 µm) and creates the uniform satin base that "matte anodized" parts show; a bright dip instead of an etch preserves gloss. A nitric acid dip then strips the dark smut of copper, silicon and iron that the etch leaves on alloyed and cast material.
- Anodize. The racked part becomes the anode in a sulfuric acid electrolyte of roughly 15–20% concentration. Type II runs at 65–75 °F (18–24 °C) and about 12–18 A/ft²; Type III runs the same chemistry chilled to roughly 28–45 °F (−2 to 7 °C) at 24–36 A/ft², with the voltage climbing as the insulating film thickens. A colder bath and higher current density produce a denser, harder, thicker oxide. Time on the rectifier sets thickness — Type II clear typically takes 20–40 minutes, Type III 30–90 minutes.
- Color (optional). The as-formed oxide is porous. Organic dye at 120–140 °F (50–60 °C) penetrates the pores and gives the full color range but fades fastest outdoors. Two-step electrolytic coloring deposits tin or nickel salts at the base of the pore for lightfast bronzes and blacks. Type III dyes black reliably; other colors come out muted because the natural hardcoat is already gray-bronze.
- Seal. Hydrating the oxide swells it and closes the pores — boiling deionized water at 200–212 °F (93–100 °C) for 20–30 minutes, or a nickel-acetate seal at 180–190 °F (82–88 °C), or a cold seal. Sealing is what delivers corrosion resistance: MIL-A-8625 requires sealed coatings to withstand 336 hours of ASTM B117 neutral salt spray. Hardcoat destined for PTFE or oil impregnation is deliberately left unsealed.
Design guidelines
Subtract the buildup before you machine
Type II clear at 0.0003 in (7.6 µm) puts about 0.00015 in (4 µm) on each surface, which disappears inside a ±0.005 in tolerance. Type III does not: a 0.002 in (50 µm) hardcoat closes a bore by about 0.002 in on diameter and grows a shaft by the same amount. Machine toleranced features undersize or oversize by the full coating thickness on diameter, and state on the drawing whether the dimension applies before or after anodize.
Allow four times the radial buildup on threads
A 60° thread multiplies radial coating thickness by roughly 4 on pitch diameter. Type II at 0.0002 in per surface consumes about 0.0008 in of pitch-diameter clearance — usually acceptable on a Class 2B thread. Type III at 0.001 in per surface consumes about 0.004 in and will bind small threads outright. Mask the thread, tap after anodizing, or specify an oversize tap.
Mask everything that must stay conductive or dimensioned
Anodize is a dielectric. Ground pads, connector mating faces, press-fit bores, bearing journals and dowel holes need plugs, tape or lacquer masking, or a secondary machining pass after the tank. Where a whole face must stay conductive, a chromate conversion coating to MIL-DTL-5541 Class 3 is the usual substitute on that surface.
Break outside corners before hardcoating
Hardcoat grows perpendicular to every surface it touches, so at a sharp external corner the two advancing films collide and the coating cracks or forms a brittle ridge that chips in service. Break outside corners to at least 0.020 in (0.5 mm) radius before Type III.
Alloy choice decides the color
6061, 6063 and 5052 anodize clear and consistent. 7075 runs slightly darker. High-copper alloys such as 2024 produce thinner, less corrosion-resistant coatings and are difficult to hardcoat. High-silicon casting alloys (A380, A383) come out dark gray and mottled and cannot be color-matched to wrought parts. Never mix alloys inside a color-matched assembly.
Designate the rack point and control the prep
Every part carries at least one contact mark where the rack gripped it; call out an acceptable location on the drawing. The coating is thin and transparent, so machining marks, extrusion die lines and weld heat tint read through — a bead blast beforehand hides tool marks and yields a uniform satin. See the surface finish chart for the Ra band each prep leaves.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Type II thickness | 0.0002–0.0005 in (5–13 µm) | 0.001 in (25 µm) | Thicker Type II coatings soften and craze |
| Type III thickness | 0.002 in (50 µm) | 0.004 in (100 µm) | Very thick hardcoat becomes brittle and micro-cracks |
| Bore/shaft allowance | Full coating thickness on diameter | — | Half the coating builds outward from each wall |
| Thread pitch-diameter allowance | 4 × radial buildup | — | 60° thread geometry multiplies radial coating by 4 |
| Outside corner radius before Type III | 0.020 in (0.5 mm) | 0.010 in (0.25 mm) | Films from adjacent faces collide and crack |
Cost drivers
Anodizing is a tank process billed by surface area and rack space over a lot minimum. The variables that move the price:
- Rack density. Cost tracks how many parts fit per tank cycle. Small flat parts nest tightly; a large weldment occupies a full rack for the same cycle and carries the whole cost.
- Coating type. Type III needs a chilled bath, more current and a longer cycle than Type II, and is priced accordingly.
- Color. Clear and black are stock. A custom match adds bath setup, a first-article approval loop and usually a dedicated lot.
- Masking. Every plug, cap and tape mask is manual touch time applied and removed once per part. On a precision machined part it frequently costs more than the anodizing.
- Cosmetic grade. Forbidding rack marks in a visible zone, water staining or lot-to-lot color drift raises reject and inspection cost far more than it raises processing cost.
Cost-reduction tactics:
- Standardize on one alloy and one color across an assembly so everything runs in a single lot.
- Design an intentional rack point instead of accepting whatever the shop picks.
- Use Type II unless a wear, dielectric or salt-exposure requirement genuinely demands Type III.
- Replace a masked feature with a post-anodize machining operation — reaming a bushing bore is often cheaper than plugging it.
- Release full racks; cost per part drops steeply with lot size because the tank cycle is fixed.
Questions
7 questionsHow much thickness does anodizing add to a part?
Roughly half the oxide grows into the base metal and half builds outward. Type II at 0.0002–0.001 in (5–25 µm) total adds about 0.0001–0.0005 in (2.5–13 µm) per surface. Type III hardcoat at 0.002 in (50 µm) adds about 0.001 in (25 µm) per surface, which closes a bore by 0.002 in on diameter.
What is the difference between Type II and Type III anodizing?
Both use a sulfuric acid electrolyte. Type II runs at 65–75 °F (18–24 °C) and produces a 0.0002–0.001 in (5–25 µm) decorative coating that dyes in any color. Type III runs the bath chilled to roughly 28–45 °F (−2 to 7 °C) at higher current density and produces a 0.0005–0.004 in (13–100 µm) wear coating, commonly quoted at 60–70 HRC equivalent hardness but with a limited color range.
Can you anodize steel or stainless steel?
No. Anodizing requires a metal that forms a coherent, adherent oxide under anodic current — aluminum, titanium and magnesium. Steel and stainless steel are finished by black oxide, passivation, electroplating, phosphating or paint instead.
Is anodized aluminum electrically conductive?
No — the oxide is a dielectric. Ground paths, connector faces and bonding pads must be masked or reworked after anodizing. Where a surface must be both protected and conductive, a chromate conversion coating to MIL-DTL-5541 Class 3 is specified instead.
Do threads need to be masked before anodizing?
For Type II, usually not: a 0.0002 in per-surface buildup consumes about 0.0008 in of pitch-diameter clearance, which normally fits within a Class 2B thread. For Type III, yes — 0.001 in per surface consumes roughly 0.004 in of pitch diameter and will bind small threads. Mask the thread or tap after anodizing.
Which aluminum alloys anodize best?
6061, 6063 and 5052 give clear, consistent results and are the default choices for anodized parts. 7075 anodizes slightly darker. High-copper alloys like 2024 form thinner, less protective coatings, and high-silicon die-casting alloys such as A380 come out dark gray and mottled with no reliable color match.
How corrosion resistant is anodizing?
It depends almost entirely on sealing. MIL-A-8625 requires sealed coatings to withstand 336 hours of ASTM B117 neutral salt spray. Unsealed coatings, used when the pores will be impregnated with PTFE or oil, offer far less corrosion protection.