Anodizing @ Home

The other night I finally finished buying all the supplies I needed to try to anodize some aluminum parts. Anodizing is a process that causes aluminum hydroxide to grow into and out of the surface of aluminum metal. Aluminum hydroxide is very hard, which provides scratch protection and is also porous which allows dyes to color it. Dying the metal is reason for a lot of commercial anodizing and is the main reason I am doing it.

The basic process of anodizing is to form an electrical circuit between the part to be anodized (the anode), dilute sulphuric acid and (usually) another piece of aluminum which is the cathode. Voltage is passed through the circuit which causes the anodizing to take place. The part is then dyed and finally the pores are sealed to keep the dye in.

This post describes how I did it.

First, some safety. Most of the stuff that I am doing to anodize parts can blind or kill you. Always wear gloves. Always wear goggles. Never mix chemicals unless you know what will happen. A few of these chemicals mixed will produce horrible gases that will kill you and your family and probably your dogs too.

Next, the supplies. I bought the following:

  • Battery acid, 5 gallons, from NAPA Auto Parts
  • Lots of distilled water
  • Several plastic buckets from Home Depot
  • 1 pound tube of 1/16″ aluminum tig welding rod from Central Welding Supply
  • 40 amp battery charger from Harbor Freight
  • Black RIT Dye from the supermarket
  • 2 single burner hot plates from Target
  • 2 6 quart pots for the hot plates

The first step is to get the parts nice and clean. I first washed them in warm water and hand soap and then in a dilute mixture of Simple Green and distilled water.

Next I used the welding rod as simple aluminum wire and attached pieces about 6 inches long to each part I wanted to anodize. You have to make sure the wire is attached very well, and also keep in mind that wherever the wire is attached the metal will not anodize. Lots of people use threaded holes and jam the wire in. I just made some little clips by bending the wire.

Next I took 1.5 gallons of my battery acid and mixed it with 1.5 gallons of distilled water. I kept in mind the AAA rule. Always Add Acid. If you add water to acid the water can boil and “explode”. I put 1.5 gallons of distilled water in a plastic bucket and slowly poured the 1.5 gallons of acid in. Very slowly. Probably took me about 5 minutes.

Next I used some aluminum scrap I had laying around to make a cathode and used some as a rack for the anodes to hang from. It’s pretty clear from the below picture:

I drilled a bunch of holes into the anode rack to use as hook hangers and bent the ends of the anode wires into hooks. Hang the parts.

Next step is to apply the juice! Hook up the negative from the battery charger to the cathode and the positive to the anode rack. I’ve read that it’s best to start at low current and ramp it up. My charger has settings for 2 amps, 10 amps and 40 amps so I set it to 12 volts, 2 amps and let it run for 5 minutes. Almost immediately there were bubbles forming around the cathode. The bubbles are hydrogen gas which comes to the surface. Make sure you have some ventilation so you don’t blow up your house. Remember the Hindenburg.

5 minutes later I turned it up to 10 amps and then 5 minutes later I turned it up to 40 amps.

(I should probably note that I am running this batch as I type this post. Most of the experiences for this post come from a batch I did the other night with slightly different gear. )

At this point I let the anodizing bath run for about 80 more minutes. I have read that anywhere from 45 minutes to 90 minutes is good.

There’s a few ways to tell the anodizing has finished. The current draw should drop quite a bit. An anodized surface does not conduct electricity well, so as the surface becomes more anodized the part conducts less electricity. You may also notice a slight lemon yellow tint to the part. I just let it run for 90 minutes and called it good.

Once the parts are finished, turn off your power supply and rinse the parts very well in cold, clean water. The water must be cold or the parts will start to seal.

Next up is the dye. I used RIT Dye which seemed to work pretty well. There’s lots of places to get proper anodizing dye but RIT seems to work for me. I dumped a bottle the black, liquid RIT into 1 gallon of distilled water and put it on the hot plate. The dye works best heated and most people recommend about 140 degrees so that’s what I used. Once the dye was up to temperature I put the parts in and let them soak for about 15 minutes. I have read that you might get the color you need in as few as 15 seconds depending how dark you want it. I wanted dark, dark black so I left them in for 15 minutes.

At this point you can pull the parts out and you may find that some didn’t take any dye or the dye drips right off. This happened on my first batch with one part. What this means is that you didn’t have a good electrical connection to the part. You can remove the dye and anodizing by etching it in a bath of water and lye. I used a few tablespoons of lye in about a half gallon of distilled water.

Once the parts are dyed the only thing left to do is seal them. This is done with boiling water. I read that steaming the parts before putting them in boiling water can help you lose less dye, so I put a pot of water on my other hot plate, put a rack over the pot and placed the parts on the rack. Then I boiled the water letting the steam wash over the parts for about 10 minutes. Then into the bath they go. 30 minutes in boiling water and the parts should be sealed and ready for use! Hooray!

I’m still a total newbie at this, and I’m actually going to a professional anodizing shop next week to see how they do it. That said, I did get really nice results and it seemed pretty easy. Takes some time and a few dollars but the result is amazing!

My pictures of the finished results are here and Courtney’s pictures of the process are here.

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