Anodizing Aluminum Bicycle Components

Steve Maas, Long Beach, California

I have a special-edition 1978 Colnago Super, which was created to commemorate Colnago's 25th anniversary. It is red, white and green (betcha can't guess why!) with brake calipers and levers anodized to match the frame color: red in front and green in the rear. Unfortunately, the calipers were badly faded, especially the rear one.

Aluminum anodizing, I was told, is not difficult to do. So, I studied the subject, experimented a little, and decided I'd do the job myself. Having learned how to do it, of course, I now regularly anodize other bicycle parts; the only problem is preventing myself from going totally nuts with it, and anodizing things that just aren't appropriate.

Aluminum Anodizing

The anodizing process is conceptually pretty simple. Dunk a piece of aluminum into a sulfuric acid bath and pull a current through it, with the aluminum part connected to the positive terminal, or anode. This grows a porous oxide layer on the surface of the part. Under an electron microscope, the oxide layer looks a lot like a honeycomb. The layer easily takes up a dye, and the dye is finally set by heating. The result is a hard, brightly colored finish.

Of course, there are a lot of variables in this process: the current density, anodizing time, acid concentration, and so on. Fortunately, books on anodizing are plentiful, and there is a lot of information on the web. Unfortunately, there is a lot of variability in the parameters suggested by the various sources. This probably means that nothing is terribly critical.

Below is some information on my anodizing setup. Click on a small pictures to see a larger one; use the back button of your browser to return to this page.

My Process and Setup

Some pictures of my setup. It is not a permanent installation; I keep the chemicals stored in their tanks and set them up when needed. I have one vat of sodium hydroxide solution, one of sulfuric acid, a bucket of clean water, and an appropriate dye. The sodium hydroxide is simply household drain cleaner, mixed about 25% by volume with water. It is used to remove the aluminum's oxide layer before anodizing. The sulfuric acid is automotive battery acid mixed 50-50 with distilled water. I mix the acid carefully, adding it to the water, not the water to the acid. This is in accordance with the old college chem lab admonition: "Remember this order: add acid to warter!" Adding water to concentrated sulfuric acid creates an acid explosion, not exactly a good thing to do.

I use a laboratory power supply to provide the DC current. This is a bit grand, but certainly does the job nicely. A useful  feature of such an instrument is its ability to regulate current; the current, not the voltage, is important in this process.

The part to be anodized first must be cleaned and degreased. The hydroxide solution easily removes old anodizing, although some cleaning and polishing is necessary after the removal. (Note that I didn't bother to remove all the anodizing from the inside of the brake-pad holders, where it's difficult to remove, and in any case, won't show.) I polish the part with sandpaper and buff it on my buffer. Then, I use naphtha to remove the buffing compound, and finally wash the part in strong detergent. Once it's cleaned, I don't touch it with bare fingers again until it's finished. Before anodizing, I dip the cleaned part in the hydroxide solution about a minute, when it starts bubbling violently. The bubbles indicate that the oxide layer, which always builds up on aluminum on exposure to air, is gone. I then rinse the part in clean water and transfer it to the anodizing tank.

The growth of the oxide layer is a function of current density and time: more current, and it grows faster. Within reason, the current x time product is what's important; you get the same oxide layer with, say, 1 amp for 30 min as 1/2 amp for an hour. Unfortunately, there is a lot of variability in suggested times and current densities. I have seen current-density recommendations of 100 to 500 mA per square inch, and anodizing times from 30 min (even at low current!) to 70 min. The best combination probably depends a lot on the acid concentration. I use about 200-250 mA per square inch and anodize for 30-45 min. If the current density is much higher, or the time longer, the surface becomes microscopically pitted. The part then has a matte finish, which usually is not what I want.

It should be obvious that nothing but aluminum can be used in the anodizing tank. Copper wire or steel fittings are etched rapidly by the acid, contaminating it. I use aluminum welding wire connected to the part by aluminum screws or other aluminum fittings. It is necessary to make a tight connection, because the oxide builds up around the connection. Then, the slightest movement can shift the wire to an oxide-covered area, which is nonconductive.

I monitor the current carefully with an inexpensive multimeter. This is important. Many amateur metal finishers simply use a car battery as a power source and don't monitor the current, but this could be a real disaster. With a voltage source (that is, instead of a current-regulated power supply), the current varies substantially with the quality of the connection. Without monitoring it, the current could be very different from what is desired.

The anodizing process liberates hydrogen, oxygen, and sulfur dioxide. The first two are explosive and the last causes respiratory irritation. I work in my garage with the door open, so the area is well ventilated.

After anodizing the part, I rinse it thoroughly to remove all acid and then place it into the dye solution. I use dyes specifically designed for anodizing; these are available from hobby metal-finishing stores (see below). Many people use fabric dyes like Rit; I have never tried them. Black is a difficult color to obtain, so for black anodizing, special dyes, unfortunately much more expensive, are necessary.  The part spends a half hour suspended in the dye solution.

It is important, throughout this process, to keep all the solutions cool; if the part heats up, the oxide film "seals" and will not accept the dye. For small parts, this is no problem, but the high currents needed for larger parts can heat the acid solution. Some people keep their tanks in ice water; I have not found this to be necessary, but I rarely use more than 3 amps.

Finally, to set the dye, the part must be boiled or steamed. I have found that boiling leaches more of the dye than I would like, but steaming for a half hour followed by a half hour of boiling works fine. I do this in an unused paint can, with holes drilled in the sides near the top, and an old spoke poked through the holes. I then hang the parts from the spoke, place the lid on loosely, pour an inch of water into the bottom of the can, and steam away. After steaming, I just drop the part into the water at the bottom of the can and boil it.

The Colnago Super

The original brake calipers, as I received them, are shown below. the front caliper is actually much worse than it appears. There is also a lot of peeling chrome on the screws, something I can also fix. The color is almost gone from the rear brake. I'm not sure why the anodizing faded; probably just from the use of poor dyes, which couldn't withstand the California sunshine. (Again, click on a small pictures to see a larger one; use the back button of your browser to return to this page.)


The idea of painting the bike three colors, with red paint and caliper in the front and green in the rear, is indeed a neat design touch. Consistent with the red front, the bike still has its original red handlebar tape, and the brake levers are red anodized. I added tires with white and red sidewalls, and even a red and white water bottle. I also had to replace the wheel rims; the ones on the bike when I bought it were not original and had started to crack around the spoke holes.  I rebuilt the wheels with an oldish set of Mavic MA-2 rims and DT spokes. 

The bike's drive train is Campagnolo Nuovo Record, and the brakes (if you haven't figured it out by now) are Modolo. I'm a little surprised that Colnago didn't use top-end components for their anniversary bike, but, after all, the Super Record parts were pretty expensive in the 1970s. Lots of builders just stopped using them, substituting high-quality parts from Modolo and Gipiemme.

A few more pictures of the Colnago Super are shown below. The pictures below were taken before the rims were replaced.

The Finished Product

Here are a couple pictures of the finished brakes and the bike, as it exists at this writing (late 2003). You can see the new rims, and I have replaced the tires since the other pictures were taken. I also put on new, smaller chainrings. I found a set of new Modolo brake pads on eBay, which the bike badly needed, and I replated the hardware that had peeling chrome. So, now, the brakes are like new.

Anodizing for Others

I am frequently asked if I could anodize bicycle parts for other people. Unfortunately, I'm just not set up to do this. If you need something anodized, and are in the Los Angeles area, try

Aerodynamic Plating
13629 South Saint Andrews Street
Gardena, CA 90249

310-515-1670
310-515-1585
910-644-4521

I have never used these guys but they have been recommended to me by people who have. If you're not in LA, look in your local yellow pages under anodizing or metal finishing. Plenty of places do this. 

Some Useful Links

Caswell Plating My source of anodizing dyes.

Ron Newman's anodizing page

Anodizing at home by Jim Bowes

Wikipedia entry on anodizing

Do-it-yourself anodizing

Anodizing aluminum by Bryan Pryor

Disclaimer

This page is a description of what I do. It is not intended to be a lesson in anodizing. If you want to create your own anodizing setup, I suggest that you do what I did: research the topic and experiment. Be sure to learn about, and observe, safety precautions for handling the chemicals and dealing with other hazards.