What Is Anodized Aluminum?

Anodized aluminum is aluminum whose surface has been converted into aluminum oxide by passing current through the part in an acid bath. The oxide isn't applied on top of the metal the way paint or plating is. It grows out of the metal itself, which is why an anodized surface can be scratched but can't chip, flake, or peel.

Use anodizing when an aluminum part needs corrosion and wear resistance without added thickness and should still read as metal. A standard Type II film runs 1.8–25.4 µm according to the Aluminum Anodizers Council, thin enough that it moves a hole diameter by about 0.015 mm. Use powder coating instead when color has to match a specification, when you need more than a few hundred hours of salt spray life, or when the part isn't aluminum.

Anodizing gives an aluminum part four things: corrosion resistance, a harder surface, a porous film that will hold dye, and an electrically insulating skin. The last one is easy to overlook, and it's the one that bites on grounded enclosures.

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How Does Anodizing Work?

Five steps, in order:

1. Clean. Alkaline soak to strip oils, cutting fluid, and fingerprints.
2. Etch or brighten. Caustic etch gives a matte surface; a phosphoric-nitric bright dip gives a specular one. This is where the final appearance is decided, not in the anodizing tank.
3. Anodize. The part goes into sulfuric acid as the anode. Standard decorative conditions per the AAC are a 15% sulfuric solution at 20 °C and 12 amps per square foot, run 10 to 60 minutes depending on how thick a film you want.
4. Dye. The fresh oxide is porous. Organic dye or metal salts get drawn into the pores.
5. Seal. Hot water or a nickel acetate bath hydrates the oxide and closes the pores, locking in the dye and finishing the corrosion protection.

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Type I, Type II, Type III: What's the Difference?

MIL-A-8625 is the specification almost every anodizing callout traces back to. It covers six types and two classes. The three that matter in practice:

MIL-A-8625 Anodizing Types at a Glance

Thickness ranges are the AAC's. Types IC and IIB are non-chromate substitutes for the chromic acid coatings, brought in as environmental restrictions tightened around chromium. The classes are simpler than the types: Class 1 is undyed, Class 2 is dyed.

There's a second numbering system that trips people up. The Aluminum Association's architectural designations also use "Class 1" and "Class 2," but they mean thickness, not color: Architectural Class 2 is 10–18 µm, Architectural Class 1 is 18 µm and above, and exterior architectural service is where that thicker film is expected. If a drawing says "Class 1 anodize" with no spec number next to it, ask which system the designer meant before you quote it.

For sheet metal parts, Type II is the only one worth discussing. Type III is a machined-component finish, run at bath temperatures near 0 °C at high current density. It takes hours and costs several times more. On a 2 mm bracket it buys you nothing. On a hydraulic valve block it's the difference between a part that survives and one that galls.

And if you're specifying hardcoat expecting a hardness number, Products Finishing's technical column is blunt about it: there is no typical hardness for Type III coatings. It depends on alloy and process. Under favorable conditions you can reach 520–700 HV; on 7075 you might see 350–360 HV. ISO 10074 sets a floor of 400 HV for Class 1 wrought alloys, which is a more useful thing to put on a drawing than a vendor's marketing figure.

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Which Aluminum Alloys Anodize Well?

This is the part most anodizing articles skip, and it's the part that determines whether your parts come back looking the way you expected.

The alloying element controls the color of the film. The AAC's alloy reference maps it series by series:

Aluminum Alloy Series and Anodized Appearance

The AAC also flags which specific alloys are sold as "anodizing quality." In 5xxx that's 5005 and 5657; in 6xxx it's 6063 and 6463. Note what isn't on that list: 5052 and 6061, both stocked as standard sheet metal grades. They anodize perfectly well and they're used for anodized parts constantly. They just aren't controlled for cosmetic consistency the way an A.Q. grade is, so batch-to-batch color can drift.

Practical translation for the aluminum grades stocked for sheet metal work:

• 5052-H32: the default for anodized sheet metal. Good formability, clear film, reliable protection. Pick this unless you have a reason not to.
• 6061-T6: anodizes cleanly, stronger, less formable. Fine for flat or lightly formed parts.
• 6063-T5: the best cosmetic result of the group, since it's an anodizing-quality grade.
• 7075-T6: anodizes, but expect a bronze cast on clear finishes.
• 1060: bright, but soft enough that racking marks and handling damage show.

One rule with no exceptions: don't mix alloys in a single anodized assembly and expect them to match. Two brackets, one in 5052 and one in 6061, will come out of the same tank on the same day looking different. If they bolt together on a visible surface, use one alloy for both.

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How Much Does Anodizing Change Part Dimensions?

Anodizing consumes metal as it grows. The AAC's rule is 50% penetration and 50% build-up per surface: half the film thickness is below where the original surface was, half is above it. Metal Finishings Ltd puts the same rule in the terms you actually need at the drawing stage. External dimensions grow by half the film thickness per surface, and internal diameters shrink by roughly the full film thickness, because both walls of the hole grow inward at once.

Now run the numbers for a real sheet metal part. Take a 15 µm Type II film, a normal thickness for a dyed black finish:

• Build-up per surface: 7.5 µm
• Growth on an outside dimension: 0.015 mm
• Reduction in a hole diameter: 0.015 mm

Compare that to standard laser cutting tolerances of ±0.45 mm linear and ±0.12 mm on hole diameter. The anodize eats about 12% of the hole tolerance band. On the high-precision option at ±0.08 mm, it's still under a fifth.

So here's the position, which runs against most of what's written about anodizing tolerances: on Type II sheet metal parts, anodic growth is not your problem. It's a rounding error next to the tolerance you're already living with. Design the clearance holes normally and move on.

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Black Anodized Aluminum: What to Know Before You Spec It

Then there's UV. Organic black dyes break down under sunlight and, more surprisingly, under fluorescent lighting. Faded black anodize typically shifts toward purple or bronze rather than simply lightening. Dye lightfastness is rated 1–8 under ISO 2135, and only dyes rated 8 belong outdoors. Even then, Products Finishing notes that an 8-rated dye can still fade under real operating conditions, and that the anodizing and sealing quality matters as much as the dye rating.

The alternative is inorganic coloring: metal salts driven into the pores electrolytically instead of organic dye. Electrolytic blacks and bronzes are far more UV-stable. They're also a limited palette, which is why bronze and black are the architectural colors you see and bright blue isn't.

If your part sits in direct sun for years and the color has to hold, don't dye it. Powder coat it.

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Anodizing vs Powder Coating for Sheet Metal

These get compared as if they're interchangeable. They're not, and the surface finishing properties table makes the difference obvious:

Anodizing vs. Powder Coating for Sheet Metal

One caveat on that salt spray row: 336 hours is a pass/fail threshold in the specification, not a measured service life, so read it as a floor rather than a rating.

Powder coating wins on corrosion, color choice, and part size. It's also the only option if the part isn't aluminum.

Anodizing wins on three things: it adds essentially no thickness, so fits and threaded holes stay where you put them; it can't chip, so there's no edge for corrosion to start under; and it keeps the aluminum looking like aluminum, which for consumer-facing hardware is often the whole point.

The decision usually comes down to one question. If color is a specification, matched to a housing or a brand palette or a mating part, powder coat it. If the finish exists to protect the part and look like machined metal, anodize it.

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When Should You Use Anodized Aluminum?

Anodize an aluminum part when it needs corrosion or wear resistance, has to hold its dimensions, and should still look like metal. It's the wrong choice when color is a specification or the part isn't aluminum. Four situations where it's the right call:

The part gets wet or salty. The oxide is chemically stable in conditions that pit bare aluminum, which is why anodizing is a standard callout for marine and saltwater service. Komacut's finishing reference lists it as a common choice for exactly that.

Something rubs against it. Hinges, slides, panels that get handled every day. The oxide is harder than the aluminum beneath it, so it absorbs handling wear that would scuff bare or painted metal. For genuine metal-on-metal sliding, though, that's Type III territory, not Type II.

Fit matters more than color. A 15 µm film moves a hole by 0.015 mm against a ±0.12 mm tolerance. Nothing needs re-tapping, no clearance holes need opening up, and threaded features come back usable. A 70–150 µm powder coat has to be designed around.

The aluminum should still read as aluminum. At a few microns thick, the finish leaves brushed grain and machining texture visible. Paint and powder bury both. If the part lives outdoors permanently, push the thickness up: the Aluminum Association's architectural Class 1 starts at 18 µm.

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When You Shouldn't Anodize

Four situations where anodizing is the wrong choice, whatever the finish table says:

Aluminum oxide is a dielectric - anodizing insulates every mating face, so mask contact zones or design in a bare pad on any part that carries a ground.

The assembly is welded. Filler alloy chemistry shows through the anodize. ESAB's guidance on filler selection is explicit: 4043 turns dark gray after anodizing and shouldn't be used where color match matters; 5356 gives a much closer match. Grinding the weld flush before anodizing doesn't fix it and weakens the joint. If you're building welded aluminum assemblies that need a uniform anodized appearance, that's a job to scope with an engineering team up front. Komaspec handles welded and turnkey assembly work.

The alloy is wrong for it. 2xxx copper alloys give yellow, poorly protective films. High-silicon casting alloys go dark gray. If the material was chosen for strength and the finish was chosen for looks, one of those two decisions needs to change.

The color has to match across batches. Anodized color depends on alloy, temper, mill lot, surface prep, and bath chemistry. Two production runs six months apart can come back different. Parts that must match visually should be anodized in the same batch, or finished with something that comes out of a can.

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FAQ

Does Anodized Aluminum Rust?

No. Aluminum doesn't contain iron, so it can't rust. It does corrode, forming white oxide pitting in salt or alkaline conditions. Anodizing thickens the natural oxide barrier and slows that process. MIL-A-8625 requires sealed coatings to survive 336 hours of neutral salt spray without meaningful pitting.

Can You Paint Over Anodized Aluminum?

Yes, and the porous oxide is a good paint base. Type I anodizing exists largely as a paint pre-treatment. For best adhesion the film should be unsealed or only lightly sealed, since a fully sealed surface gives the primer less to key into. Tell your finisher the part will be painted before it goes in the tank.

Does Anodized Aluminum Wear Off?

The coating can't peel or flake because it's part of the metal, but it can be worn or scratched through. A typical Type II film of 5–25 µm resists handling marks, not metal-on-metal contact. Sharp edges are the weak point: the oxide is brittle and the aluminum under it is soft, so corners wear first.

Can Any Aluminum Alloy Be Anodized?

Technically yes, cosmetically no. 5xxx and 6xxx alloys give clear films with good protection. Copper-bearing 2xxx alloys go yellow with poor weather resistance, and high-silicon 4xxx alloys and castings go dark gray. Check the alloy before promising a customer a clear or dyed finish.

Does Anodizing Add Thickness to a Part?

It adds about half the film thickness per surface, because the other half grows into the metal. A 15 µm Type II coating adds roughly 0.0075 mm per face and closes a hole by about 0.015 mm. At Type III hardcoat thicknesses of 50 µm or more, the change is large enough to require pre-machining allowances.