Sunday, April 15, 2007

Adventures in Anodizing Aluminum

My wife told me to get a hobby or die. So my friend and I are building a tube amp - a copy of the venerable Fender 5F1 circuit. First, it is amazing how involved it's been to convert that schematic to a real thing. There are the parts called out for by the schematic, but then the nuts, bolts, bushings, washers, wire, mounting hardware, you name it. It's been eye opening and educational which is the whole point of the project. I could buy a vintage amp for less than I spent making this one, lol.

Now, we're making everything that a couple of hacks can make. The control panel plate is the thing the guitar plug and volume knob are mounted to. We made ours from aluminum. It's what we had. But aluminum is soft and scratches easy. I could could spray it with a Krylon enamel rattle-can, but that's boring and I have spent a LOT of time on this amp. I don't want to cheese out now. We decided to anodize it.

Aluminum rusts, just as steel does. That's why el-cheapo lawn furniture can turn your hands black. Anodization is a process where the aluminum gets a different type of rust than normal. The anodized surface is a thin coating that is almost as hard as diamond. Remarkably, the basic process is simple and as safe as anything that involves sulphuric acid and a battery charger.

It's a three-step process.

First first step is where we run current from the battery charger through a lead cathode (- side of the charger) through the acid, through the aluminum parts to be anodized, and to the anode (+ side) of the charger. This causes a layer of aluminum hydroxide to form on the aluminum parts.

The second step is where you drop the now-anodized aluminum part into a bath common clothing dye. The aluminum hydroxide surface has microscopic pores that are open. The dye can sneak right into those pores.

The third stage is where you boil the parts in water. The heat makes the pores close, trapping the dye and hardening the surface which, magically, becomes a 'hydroxide monohydrate' which I do not pretend to understand.

All this, and you can't really over-do it. As you apply the current to the acid bath, the oxide layer starts forming on the parts. This layer does not conduct electricity well! As the layer gets thicker and thicker, it conducts electricity less and less. So the process is self-limiting. After a while, electricity can't flow, and it all just... stops. Crazy. The Lord provides.

Armed with our knowledge, we tried an experiment yesterday. We produced lovely un-anodized aluminum and burned our anode hangers in half. Funny, but really, I didn't expect total failure. I thought we'd get a mediocre result and have to make an adjustment. We did deviate from The Plan a little, so next time we'll follow the instructions better.

Here is an example of some parts anodized by a professional. Look around, you're probably surrounded by anodized aluminum. Stay tuned.

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