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Anodizing aluminum is another one of the projects I wanted to try. After reading up on it, it seemed fairly straight forward to do. Being in the grips of a Midwest winter, it seemed like a good time to get started. That, and the fact that I'm using aluminum more and more with each new rocket project.

Why anodize aluminum? Three reasons:

1) It increases the surface hardness of aluminum.

2) Anodizing protects the aluminum from corrosion.

3) It's the best way color an aluminum part.

For my first attempt I'm going to use the absolute basic approach. I'll use sulfuric acid as the electrolyte, Rit brand dye and the boiling water seal method. I picked up a five gallon container of battery acid, which is about 50/50 sulfuric acid and water mix from an auto parts store. The Rit brad dye is available from pretty much any grocery store. A 10 gallon Rubbermaid plastic tub was purchased for the anodizing tank as well. Total cost, about $30.

Always wear safety glasses, gloves and an apron or really old clothes. Both the lye and acid used here could be lethal, and are capable of doing serious damage to people and property. Keep a good supply of baking soda on hand to neutralize acid spills, and have a large supply of fresh water available if you spill on yourself.

For my first test, I used an extra piece of 3.5" 6061 tube. The aluminum was washed with soap and water, rinsed, then immersed in a bath of 3) tablespoons lye in one gallon of water. All of the water used in this first batch was distilled water. The caustic bath will etch, or eat into the surface of the aluminum to clean the part. You'll know the surface is perfectly clean when the water sheets over the surface, as opposed to forming drops, or areas of wet and dry spots on the surface. Two or three minutes and this part was etched nicely.

I picked up 5 gallons of battery acid (sulfuric acid) at an auto parts store for the electrolyte. You can see my small battery charger and the acid tank in the back. The tank is a 10 gallon Rubbermaid tub. Initially, I added three gallons of distilled water to the tub, then added one gallon of battery acid. Remember the 3 A's, Always Add Acid to water, not the other way around.

Here is the acid bath tub. Two aluminum (or lead) plates are used as cathodes and hook up the negative lead of the battery charger. The part to be anodized (anode) is hooked up with aluminum wire to the battery charger positive lead. The part is suspended off the bottom by the wire. My cathodes are pretty small, and really should be larger I think. Notice I'm using a disposable piece of particle board over my work bench, any acid spills can eat the old wood rather than my work bench.

Make sure you vent the gas from the process, a good respirator wouldn't hurt either. I doubt breathing the sulfuric acid gas would be very good for the lungs!

Power was turned on, I had the charger set to 2 amps at first. Then increased the setting to 10 amps after a few minutes. Notice the white "fog" or bubbles coming off the cathodes, that indicates current is flowing. The part will emit a few small bubbles too, but most of it is coming from the cathode plates. At the charger 2 amp setting the charger was drawing 2.5 amps, when I increased the power to the 10 amp setting, the meter indicated a draw of about 3.5 amps.

After 105 minutes the part was showing a slight yellow tint and surface wasn't quite as shinny. The bubbles coming off the cathodes had also changed the direction they were moving. I took all those as signs the part had been in the acid bath long enough. Power was turned off, and the part moved to a rinse bath of distilled water.

While the part was in the acid bath I had mixed up a batch of dye, 4) tablespoons in 1/2 gallon of distilled water. The part was immersed in the dye at room temperature, then the temperature slowly increased to 100 degrees F. A pot of distilled water was started boiling for the final sealing process. After 15 minutes the part was removed from the dye, rinsed in fresh water, then placed in the boiling water for 30 minutes to seal the surface.

After 30 minutes in the boiling water, the now anodized part was removed. It seems to have worked very well. I used a navy blue dye, and it came out very dark. Darker than I really wanted, but very nice regardless. For batch number 2, I have my newest PIRM2 in the lye bath to etch.

The second batch was done exactly the same as the first, only I left the charger on the 2 amp setting for 90 minutes, current draw was 1.75 amps. I mixed up a batch of crimson red dye, 3 tablespoons to 1/2 gallon, and started the dying process. It was quickly apparent the release pin hadn't anodized properly, no doubt a loose wire on the part in the acid bath. The other two parts took the dye, but they weren't as red as I wanted. So I ended up etching the all three parts again in the lye bath, and reanodizing the whole batch.

I added another 1/2 gallon of acid to the acid bath, and this time increased the power to the 10 amp setting on the charger. The draw on the charger was still 1.75 amps, but the voltage on the higher setting increased from 9.45 volts to 11.6 volts. After 104 minutes the parts were showing the slight yellow tinge I had seen in the first test, so I removed the parts from the acid bath, rinsed them, and started the dying.

I left the parts in the dye for about 10 minutes, until they looked about the way I wanted them, knowing they would fade slightly when rinsed and sealed. The parts were then sealed in boiling water for 30 minutes.

Here's the end result. Again, not bad for my second try at it.

I received my dye and sealer from Caswell, Inc., so I decided to give the new dye a try on a motor casing. I cleaned up one of my old LW-1 motors, and gave it a real quick polishing with a mild rubbing compound. After polishing, I cleaned the motor thoroughly with Dawn dish soap, hot water and a scrub brush. I finished up the cleaning with a mild abrasive pad. I did not etch this time in lye, as I wanted to keep a little shinier surface on the casing.

The casing is 2.25" OD and 15.5" long. While 15.5" doesn't seem that long, you'd be surprised to find how few containers are large enough. My anodizing tank is plenty big, I used my second Rubbermaid tub for the dye, but a container for the sealer was the problem. I ended up using an old enamel roasting pan, it really should be plastic I think. I'll have to come up with something new in the future, but the roasting pan had to suffice this time.

Once the casing was in the anodizing tank, I noticed the leads from the battery charger getting very hot. So I switched to a larger 10/30/200 amp battery charger, on the 10 amp setting the charger was putting out some 17 amps. Enough that in time, the acid bath started to get warm. So I turned off the heat in my shop to keep the temp under control, it seemed to at least stabilize the temperature. For large parts, it looks like a cooling system of some sort my be needed.

After about 90 minutes I removed the motor casing, gave it a rinse and moved it to the dye tank. The aluminum wire turned a nice deep blue, but the casing stayed a nice bright aluminum color. It seems the wires going to the casing didn't maintain good contact with the part. So I rinsed off the casing, crimped some new wire to the casing and started anodizing again.

This time I left the part in for over 2.5 hours, it never did change to the light yellow I had seen in my first tests, but I decided it had to be long enough. So I rinsed off the casing, and moved it to the dye tank. This time, the motor took the color very quickly. In fact, I didn't want it too dark, so after about 60 seconds I removed the casing from the dye, rinsed it off, then put it in the sealing tank (ok, old roaster pan) for 25 minutes.

Here is the end product.

I think it turned out pretty nice. I'm not sure how much better the anodizing dye is than off the shelf Rit dye. But it really took the color quickly, and didn't wash out at all in the rinse or sealing process. I'm sure the nickel acetate sealing solution is better than a simple boiling water seal too, but I really can't see much difference in the finished product.

I've always wondered if an aluminum nozzle would hold up to long burn sugar motors. The anodizing layer is supposed to make the surface more heat resistant too. Perhaps an anodized 6061 would work, sounds like a good next project.


Jonathan Carter started doing some anodizing and it gave me the itch to do some more too. So I pulled out my equipment, cleaned up an old 1.75" aluminum casing and threw it in the tank for 90 minutes. I had some new dye I hadn't tried yet, Caswell's Deep Red. I mixed it up, and after anodizing and rinsing the casing I dyed it for about 10 minutes, rinsed and then tried sealing it in boiling water. The dye pretty much all rinsed off. I pondered why the dye didn't take. If the part had lost contact with the power wires while in the tank, the wires would anodize, but not the part. In this case, even the wires didn't take the dye, so it had to be some other reason. I suspected the acid bath temperature was too low.

For the next batch, I increased the temperature of the bath to 70 F. and after etching the casing in lye, tried it again, this time using Casewell Blue dye. I had pretty much the same results. I wondered if the boiling water bath used to seal the part wasn't working with the dye, so for the next batch I decided to use the nickel acetate solution recommended by Caswell. Same results, the dye just washed off the part leaving a very, very pale color on the casing.

So I tried a different part this time, the nozzle retaining ring for my big 6" motor. Same results, the dye just washed off. I had another casing already cleaned up and ready to anodize, an old LW-1 motor. So I was going to try it in the anodizing tank, perhaps less current would be better... So I hooked it up in my tank and applied current. Then I thought for a while, what has changed from my first anodizing to now. Then it hit me, I was using four cathodes in the tank rather than two, but that shouldn't be a problem, in fact, more cathode area should help. But, one of the cathodes I was using was not 6061 aluminum, it was a piece of old aluminum C-channel, of unknown alloy. So I removed the C-channel cathode, and anodized with the 3 cathodes of 6061.

After 90 minutes I removed the casing, rinsed, then double dipped the casing 50/50, one end in the Caswell red and the other end in the Caswell blue dye. This time the dye held, even in the nickel acetate bath.

To the right is the casing, to the left is the 6" nozzle retainer.

Just to be sure I had solved the problem, I anodized the 6" nozzle retainer again. I dyed it in Rit clothing dye and sealed it in the nickel acetate bath. The Rit dye worked OK, but wasn't going to get much darker purple than what you see here. It seems the alloy of aluminum is important for the cathodes, stick with 6061 and you shouldn't have any problems.

Notes :

Acid bath temperature is best between 65 and 75 degrees F.

Rinse water after the acid bath should be room temperature.

Dye works best at about 140 degree's F.

Power density of about 4 to 10 amps per square foot of work piece is best, with a duration of about 10 amp hours per square foot. Example: If you have a work piece with a total surface area of .5 square feet, and a current draw of 4 amps. You would have a power density of 8 amps per square foot ( 4 / .5 = 8 ). So your duration should be 10 amp hours per square foot / 8 amp current density = 1.25 hours, or 1 hour and 15 minutes. This should give you just under (20% or so) the maximum anodization layer.

Cathode area works best if it is at least as big as the part being anodized. But I have gone both larger and smaller with good results.