Electrochemistry!
DIY Anodizing of Aluminum
Here I have my experiences with anodizing aluminum. This is only my test case, which seemed pretty successful for how much I invested in it. Soon I hope to have up some results for larger pieces of aluminum. I would like to start out by saying that this can be dangerous if you don't treat the set up with respect. Sodium hydroxide is corrosive, sulphuric acid is dangerous, electricity is dangerous, their powers combined are even more dangerous. Lead has done in a few civilizations, too. Don't try this unless you feel comfortable working with the materials,and willing to take full responsibility for any accidents!
My parts list here is very generic; I had access to laboratory equipment, but I'll give some tips for at-home set ups as well.
Parts List:
- A bucket
- Aluminum
- Lead (or more aluminum)
- A power supply
- Sulfuric acid
- Sodium bicarbonate
- Dye (small organic dyes)
- ALUMINUM WIRE (all caps = important)
- Sodium hydroxide (NaOH)
- An open, well ventilated workspace
- Several jars for water, NaOH, and dyes
I picked up a sturdy plastic construction bucket at Wal*Mart to perform the anodizing process at home. It's deep enough to anodize some sizeable pieces of aluminum and durable enough to clean up and reuse. It also makes great transportation for your other supplies.
As you can see in the bucket I have some RIT clothing dye (my cheap organic dye),
some lead fishing sinkers (my cathode),
and my test-piece of aluminum.
Note that commercials dyes can be purchased on a number of web sites, as well as proper sealants. We won't be using either of those here, I'm mainly interested in coloring the aluminum cheaply.
You'll want to have set-up a bath of sulphuric acid (diluted to about 10-20%), a glass of hot water (nearing a boil), a glass for the dye, and a glass for the sodium hydroxide. The sodium bicarbonate (arm+hammer baking soda) is to be kept around to neutralize any sulphuric acid spills/ end waste. It is also a good idea to wear protective gloves as well. I also used a larger bath of water to help keep the sulphuric acid solution cooler during the anodizing.
You'll want to suspend your aluminum part by aluminum wire, so any side chemical process don't occur.
You will then want to place your piece of aluminum into a sodium hydroxide bath. This etches the outside oxidized layer off, and prepares the surface for a proper anodizing.I dissolved some NaOH pellets is distilled water to make a rather concentrated solution. Leave it in until the piece turns a nice dark dull grey. Once ready, take it out and wash it off with water to remove any excess NaOH If you want to be thorough about it, you can take the etched aluminum and soak it in some Nitric acid; this should remove any impurities in the metal. I did not do this here, as my piece of aluminum was rather pure and I had no desire to fool around with nitric acid.
At this point, it's time to set your lead cathode and aluminum anode into the sulphuric acid solution (make sure they're not touching!), and turn on the power. Our power supply was set at 12V and let it take as much current as it wanted, which was not much for a piece of aluminum this size. I hear that old computer power supplies can do the trick quite nicely, and they're pretty easy to come by. I will mention that the read-out on the power supply didn't indicate the proper voltage it should have been anodizing at.
After a short while, I noticed a bluish color in the acid solution.
Anybody who has taken general chemistry is likely to know what color copper sulfate is in solution ...
As it turns out, the "aluminum wire" my room mate provided me was actually tinned copper wire. This explains the blue tinge, and of course, all that copper on the cathode! At this point we restarted the whole thing; this time I anodized a longer piece of aluminum (which would stick out of solution) and got a proper hunk of lead.
So we did everything again, this time without any contamination. This time the power supply read the proper voltages and everything seemed to be going smoothly.
Since this was a test run, we let it anodize for only 5-10 minutes. We switched the power off, washed off the aluminum with distilled water, and set it in the (warm) red dye solution. After a 5-10 minute soak there, we took it out and put it in our (nearly) boiling water. We raised the heat a bit and made it an actually-boiling beaker of water and, again, let it go for 5-10 minutes.
Our finished product doesn't look too shabby for such a short anodizing time (and using cheap clothe dyes with no sealant). I'll mention that the darker red spots up near where I am holding the aluminum could be washed off; it was more of an effect of being right above the liquid line in the acid solution. The leftover acid solution can be neutralized with the sodium bicarbonate, and then everything properly disposed of.
So all in all, this seems simple enough to do, provided you are careful and safe enough. I've seen that one can use another piece of aluminum for the cathode as well, which would eliminate the need to work with lead. I would like to mention two sites that also have some wonderful instructions and information on anodizing aluminum. The first one is what inspired me to give this a shot in the first place, and the second one has some other excellent information.
http://astro.neutral.org/anodise.shtml
http://www.engineeringhobbyist.com/projects/completed/anodizing/
mark[at]markslaboratory.com