Anodizing Aluminium at Home

The faceplate for the Gauges project is 1/8” Aluminium sheet metal. I bought a 12”-square sheet through McMaster-Carr, and after cutting it to size decided it would be best to anodize the finished product to keep it from scratching or corroding. After a few successful tests, I ran the final process, though I switched the anode and cathode polarities by accident, resulting in a strange thin-film oxidation on the work piece, which gives rise to varied rainbow interference patterns.

Step 1: Etch

After sanding and thoroughly cleaning the Aluminium with detergent (dish soap), I etched it in a solution of lye (Sodium Hydroxide) to remove surface defects and give a matte finish, like that of the Apple MacBook. I bought the lye in a solid form, as crystals of drain cleaner, and assumed that the crystals were pure lye (a stretch, though it was the best conservative estimate I could make). I read a guide about the etching process that suggested a 15% lye solution, so I mixed 1 part lye to 6 parts water in a 5-gallon plastic bucket. Wearing gloves and goggles, I submerged the metal in the solution, and it heated up and bubbled furiously as the lye ate away the surface. I only left the piece in solution about 2 minutes, to give a consistent matte look without making pock marks.

Step 2: Desmut

I don’t have access to proper desmut solution, the solvent that takes off the impure “smut” residue that accumulates on the surface of treated Aluminium. I rinsed the part in water and cleaned it with dish soap to fair success, and proceeded to anodizing.

Step 3: Anodize

Well, this step is supposed to involve using the work piece as an anode (positive terminal) in a direct-current circuit. The theory is that the metal will form a thin oxide layer, a clear ceramic that adds scratch- and corrosion-resistance to the finish. However, I accidentally wired the system in reverse, so that the work piece was the cathode (negative terminal). The result was a strange thin film of oxide that creates rainbow interference patterns. I like the look, and I assume the coating is indeed Aluminium oxide as it should be, but I cannot know for sure. It seems to have moderate scratch resistance, but I may try again with the correct circuit some time in the future to get it right. In the mean time, I am enjoying the vintage effect the “cathodizing” created, and I may just decide to keep it this way.

Following the guide I found, I bought battery acid from the local auto parts store and mixed it into water in the ratio of 1 parts acid to 4 parts water. The acid comes as 50% solution, and the goal is to have around 10%, so a solution of about 12% acid is just about right. The solution stays at room temperature.

I fashioned an electrode (supposedly a cathode, though due to my error ultimately an anode) out of Aluminium stock, which I placed in the bottom of the bucket of acid solution. I used an old computer ATX power supply to run a current of around 16 Amps through the workpiece, dangling from a bar of Aluminium, into the acid solution and back out through the “cathode”. Because I had wired the whole thing in reverse, the intended “cathode” ended up with the MacBook-like finish while my part looked just about the same as it did going in.

I sealed the part with boiling water for about an hour, and the rainbow effects showed up. Not exactly what I was after, but cool!

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