Lesson 11.6CoatChapter 11 · Coatings and surface finishing

Anodizing

Current grows a hard oxide out of aluminum itself

Anodizing grows a hard, protective oxide skin on aluminum by running electric current through it in an acid bath. The colored metal of a laptop case, a carabiner or a flashlight is anodized aluminum. Unlike paint or plating, nothing is added on top: the metal's own surface turns into a ceramic-like layer.

Photo: Polyparadigm, Public domain, via Wikimedia Commons

Anodizing · the tank, then the oxide layer grown, dyed and sealedCircled numbers match the steps below. The three lower panels zoom into the oxide layer, which is far thinner than drawn.

How it works

Clean and etch. The part is degreased, then etched in a caustic (strongly alkaline) bath and desmutted (dipped in acid to remove loose residue). Etching gives the matte look; chemical polishing gives a bright one.
Rack it as the anode. The part is clipped tightly to an aluminum or titanium rack and lowered into the acid, typically sulfuric acid kept at about 20 °C. Here the part is the anode (positive electrode), the opposite of electroplating.
Switch on the current. A DC power supply, typically at around 15–20 V for standard sulfuric anodizing, pushes current from the part to cathode plates in the tank.
The oxide grows. Oxygen from the water reacts with the aluminum surface to form aluminum oxide. The layer grows partly into the metal and partly outward, full of tiny straight pores, typically for 20–60 minutes.
Dye (optional). The open pores soak up organic dyes like a sponge, giving black, red, blue, gold and other colors. Clear anodizing skips this step.
Seal. Hot water near boiling, or a nickel or other sealing bath, swells the oxide and closes the pores. This locks in the dye and greatly improves corrosion resistance.

Hard anodizing runs colder, near 0 °C, and at higher voltage to grow a much thicker, harder layer for wear parts. Titanium and magnesium can be anodized too; on titanium the thin oxide gives bright colors by light interference, with no dye.

Key facts

Materials: Mainly aluminum alloys; also titanium and magnesium. Alloys rich in copper or silicon anodize less well
Typical parts: Laptop and phone cases, extruded window frames, cookware, climbing gear, bike parts, aircraft parts
Part size: From small screws to extrusions several meters long; the tank sets the limit
Precision: Thin and even: standard layers typically 5–25 µm, hard anodizing about 25–100 µm; about half the thickness adds to each surface
Surface finish: Copies the surface below (matte if etched or blasted, bright if polished), with an even satin or glossy look
Batch size it suits: From single parts to continuous production of profiles and consumer products
Tooling and machine cost: Racks are cheap; a full anodizing line with tanks, rectifiers, chillers and waste treatment costs hundreds of thousands or more
Speed: Roughly 20–60 minutes in the anodizing tank, plus cleaning, dyeing and sealing; a whole cycle takes about 1–2 hours

Strengths and limits

Strengths

Part of the metal, so it cannot peel or flake like paint
Hard and wear-resistant, especially hard anodizing
Thin and predictable, so tight fits are kept
Many durable colors, with the metallic sheen showing through

Limits

Works only on aluminum, titanium, magnesium and a few other metals
The oxide is brittle and can crack if the part bends
Electrically insulating, so contact points need masking or machining
Some dyed colors, especially reds and blues, fade in strong sunlight

Where it's used

Consumer electronicsLaptop, phone and tablet cases, headphone parts, PC cases
ArchitectureWindow frames, curtain walls, facade panels, handrails
Aerospace and defenseAirframe parts, hard-anodized actuator parts and housings
Sports and outdoorCarabiners, bike components, flashlights, fishing reels
KitchenHard-anodized cookware, utensils, espresso machine parts
MachineryPneumatic cylinders, pistons, pulleys, valve bodies

How to recognize it

A colored or silvery metal with a satin, slightly deep sheen, where the metal texture shows through the color.
No layer that can be scratched through to a different color; a deep scratch shows bright bare aluminum.
Small uncolored marks where the part was clipped to its rack, often on an edge or inside a hole.
A surface that a simple tester shows does not conduct electricity, though the part is metal.

Compare with

Electroplating11.5the part is the cathode and receives new metal from the bath; in anodizing it is the anode and its own surface becomes oxide.Powder coating11.2adds a thicker plastic layer and is common on aluminum too, but it sits on top and can chip.PVD coating11.8deposits very thin, hard metallic layers in a vacuum, for example on tools and watches.

Watch it

Yijin Solution · A beginner explanation of the oxide grown in an acid bath

Captain Corrosion · Clean, etch, anodize, dye and seal, step by step

RAPID DIRECT · A tour of an anodizing line, tank by tank

Gamers Nexus · Racks of PC case parts anodized and dyed in a real factory

See real photos

Wikipedia: Anodizing — photos of colored anodized parts and a diagram of the porous oxide structure
Wikimedia Commons photo search

Remember

Anodizing turns the aluminum surface into oxide; nothing is added on top.
The part is the positive electrode, and the oxide grows full of pores that can take dye.
Sealing closes the pores, locking in color and corrosion resistance.