Journal · Shop Notes

What is anodized aluminum? Color that won't chip

Shop Notes, from the Key Bored workshop · October 18, 2026 · 3 min read

Anodizing is an electrochemical process that grows a hard layer of aluminum oxide out of the metal's own surface. That layer is full of tiny pores, so it soaks up dye before it's sealed shut. The color ends up inside the metal's skin instead of sitting on top of it, which is why anodized parts don't chip or peel the way painted ones do.

How it works

A machined aluminum part goes into a bath of acid, usually sulfuric acid, and becomes the positive side of an electrical circuit. That's where the name comes from: the part is the anode. As current flows, oxygen bonds with the surface and builds a layer of aluminum oxide, which is very hard.

The new layer grows partly into the metal and partly out of it, and it's thin. For decorative anodizing, often called Type II, it's typically a few thousandths of a millimeter. Hard anodizing, or Type III, builds a much thicker layer for parts that need to survive real wear.

Where the color comes from

Freshly anodized aluminum is covered in microscopic pores, a little like a honeycomb seen end-on. Dip the part in dye and those pores fill with color. Then the part is sealed, often in hot water, which closes the pores and locks the dye in.

Getting the color exactly right is harder than it sounds. The alloy, the surface finish, the layer thickness, and the dye batch all shift the result. Matching a color between two production runs is part science and part patience, which is why makers with a big color range tend to obsess over it.

Anodized vs painted

Paint sits on top of the metal. If you scratch through it, it can chip and lift around the scratch. An anodized finish is part of the surface, so even a deep scratch doesn't flake.

Anodizing also keeps fine detail. Because the layer is so thin, crisp chamfers and machined edges stay crisp. A coat of paint softens them. That's a big part of why anodized aluminum reads as premium: you're still looking at the machining.

What can't be anodized

Anodizing works on aluminum and titanium. Titanium is a strange case, because it can produce color without any dye. The oxide layer forms at a thickness that interferes with light, the same effect that makes soap bubbles colorful, and changing the voltage changes the color.

Steel, brass, and copper can't be anodized. They get their color from coatings, plating, or chemical treatments instead. Stainless steel, for example, is often colored with a thin vapor-deposited film called PVD.

Caring for an anodized finish

Anodized aluminum is easy to live with. A soft cloth handles fingerprints, and warm water with a drop of dish soap handles anything worse. Dry it afterward so water spots don't sit on the surface.

The thing to avoid is harsh chemistry. Strong alkaline cleaners, like oven cleaner or some bathroom sprays, can dissolve the oxide layer and leave a dull, blotchy patch. Abrasive pads will also scuff the surface. If something is safe on a nice phone case, it's almost always safe on anodized aluminum.

How we use it

The case of the KB OneKey Pro is machined from 7075 aluminum and anodized in the full OneKey color range. The brass weight inside stays raw brass, so you get two finishes on one small object: dyed aluminum where you look, and warm metal underneath.