Coloring a Robot

[Pat Fairbank]

Originally Posted by Ben.V.293

Is this something you do yourselves? If so I'd really like to learn how to do it. Could you post an overview of the process?

Sure, no problem.
Please keep in mind that this is a very general overview, since we only started doing this ourselves this year, and haven't quite perfected the process yet.

First off, it may be obvious, but you can only anodize aluminum. Any other kind of metal simply won't work with this method. The part should be really well cleaned before, and keep in mind that the anodized layer is so thin that if you leave a bad finish before anodizing, it will show through afterwards.

The first step is to immerse the part in a basic solution (2 molar NaOH will do the trick) for a short amount of time. This is known as "etching", and the idea behind this, I believe, is that it increases the surface area of the part on an atomic level, making the aluminum more receptive to the oxygen later on. If your part came already anodized, like extrusion, then leaving it in the basic solution for half an hour or so will remove the anodized layer, if you help it along by wiping every few minutes.

The next stage is the actual anodization, in which the part is immersed in a sulfuric acid solution (1.5 molar H₂SO₄ is what we use). This is done usually by suspending the part with aluminum wire (having any other type of metal in the solution will cause the electricity to flow through it instead of the aluminum). The part gets attached to the positive terminal of a power supply, which is why it's called "anodization". You also need a cathode, which usually is an aluminum plate at the bottom or side of the anodizing tank, hooked up to the negative terminal. We use a 35A manual 12V battery charger as our power supply. The power supply gets turned on, and should be set so that it supplies 4-12 amps per square foot of the surface area of your part. At this current, it usually takes about an hour and a half to anodize a part. What happens during this process is that the water in the solution is electrolyzed, and the hydrogen is released while the oxygen bonds with the aluminum at the anode to form a porous aluminum oxide layer.

The following step is to dye the oxide layer to whatever colour you want. Ordinary fabric dye works fine for this, mixed at twice the normal concentration. This takes 15 minutes to half an hour, and during this stage the dye is absorbed into the pores of the aluminum oxide layer.

The last stage is the sealing, during which the pores are closed, trapping the colour inside. Some people will use special chemicals to seal their parts, but immersing them in boiling water for a few minutes works just fine.
The result of all this is a colourful, slightly matte layer a couple thousandths of an inch thick.

If you're considering trying your hand at anodizing yourself, there are some excellent websites which you should consult, such as this one and this one.

Good luck with your robot colouring!