TECH: UNDERSTANDING ANODIZING

The term “anodizing” may sound technical and electrical, and perhaps it is. After reading this article, you will have a much better understanding of the anodizing process, what it is, what it isn’t, and why it is so difficult to achieve color consistency.

THE ANODIZING PROCESS

Anodizing is not a coating or paint. It isn’t sprayed on and it doesn’t have any drying time. Instead, anodizing is a process that uses electricity and the object itself as part of an electrical circuit. In the process, the surface of the component being anodized is electrochemically converted. In the case of aluminum, the surface of the component being anodized becomes aluminum oxide.

Most simply stated, anodizing is the process of electrically etching the surface of an aluminum component to make it porous so it will readily accept and retain a colored dye, and then dying the component the desired color.

When anodizing bicycle parts, the anode could be a the parts of a headset, a hub shell, a pedal, a skewer lever, a seat post clamp, or any other metal part that the manufacturer would like to feature in a color. Normally, only components made of aluminum can be successfully anodized, although there are processes for Titanium and other metals that are also said to be successful.

To begin the process, all metals used in this process must be surgically clean or the process will not succeed. Both the anode (the component to be colored) and the cathode, the other part of the circuit, are submerged in a liquid, which is usually a mild acid. The cathode, the negative side of the electrical circuit, is often a plain but very clean piece of metal such as lead.

Once the negative connection is made to the cathode, and the positive connection to the anode, a very important part of anodizing is controlling all of the many variables during the preparation and setup process. When everything is ready and in its proper position, the circuit is switched on and the electrical current applied. Note that the most popular belief is that electrons flow from positive to negative, but the opposite is true. Electrons flow from negative to positive, and that is why the bicycle component is connected to the positive terminal of the power source, so electrons will gather upon it.

BENEFITS OF ANODIZED SURFACES

A short time after the electricity has been turned on, the piece to be anodized begins to form a layer of oxidation on its surface. This layer of oxidation “etches” the surface of the aluminum, making it rougher than it was. This etched, oxidized surface is only microscopically deep, usually a little over 2/100ths of a single millimeter, or 1/1000 of an inch. Thin as it is, the completed anodized surface has (at least) three important properties that make the anodizing process successful and desireable:

The surface oxidation is very hard, making the surface of the soft aluminum much more durable
The purposely created surface oxidation prevents any further oxidation and corrosion from forming under normal conditions
The layer of oxidation is porous, making it able to absorb and hold a colored dye

After anodizing is complete, the seat post, stem, or other component is now ready to be dyed. As in anodizing, the component must remain meticulously clean to before and during the dying process. The dying process involves immersing the freshly anodized component into a bath of dye, similar to dying an Easter egg. The bath of dye can be any color. The porous anodized surface of the component accepts the dye readily. When the desired color of the component is achieved, the components are removed from the dye solution. The component is then sealed in boiling de-ionized water or a hot-nickel acetate solution. Like an Easter egg, the longer the component stays in the dye, the darker its color will be.

Note: A component can be anodized without being dyed (leaving it silver) but it cannot be dyed without being anodized. The etching of the surface of the metal that the anodizing process provides is similar to the etching that takes place when dying Easter eggs. Without the acid (vinegar) to make the surface of an egg porous, the egg shell won’t accept the color.

WHY ARE THERE COLOR VARIATIONS?

Like all processes, a manufacturer can’t anodize just one piece at a time. Usually, many pieces are anodized at one time and then dyed at one time in a batch. This makes anodizing much more affordable to the manufacturer and ultimately, to you and me. For both economic and environmental purposes, it is also important to reuse the acids and dyes as many times as possible. Each reuse can result in a slightly different shade of color.

One of the biggest challenges in anodizing is all the possible variations of the factors that affect the anodizing process. These variations are an unavoidable part of the anodizing process, and they affect every manufacturer and every anodized part. In short, there are so many variables in the anodizing and dying process that color consistency is very difficult and perhaps nearly impossible to achieve. Like natural gemstones formed in the molten magma deep in the earth, the chemistry and all of the surrounding environmental variables affect what the final color will be.

Think of coloring a batch of Easter eggs one day and you’ll find that the eggs aren’t all the exact same color, even though they were dyed in the same batch. Dye another batch of eggs on the next day and attempt to reproduce the previous day’s exact temperatures, measurements of water, vinegar, dye, and time. You’ll find that exactly matching the previous day’s color is not likely to happen. You could also prepare a batch of Easter egg dye and dye a batch of eggs. Using the same dye, color another batch of eggs. And another, and another, and then 20 more batches. You will see that all else being equal, the last batch of eggs will be noticeably lighter than the first batch because the dye solution has been gradually depleted with each batch.

As you see, dying Easter eggs involves a number of variables that affect their color. During anodizing of aluminum components, there are many more variables, and each of them affect how thick the anodized surface becomes and how porous it is:

Some of the variables in the anodizing process are:

Total surface area of all the components (anodes) being anodized in one batch
Temperature of the anodizing acid solution
Strength of the acid solution
Ph of the acid solution
Type of acid solution used (chromic, oxalic, organic, & others)
Agitation applied to the acid bath and components
Time in which the anodes are exposed to the process
Original smoothness of the component being anodized
Composition of the aluminum alloy being anodized
Directness of “line of sight” between anode and cathode
Position of each anode component in relation to each other
Size of the cathode
Composition metal of the cathode
Conductivity of the anode component
Voltage applied to the cathode / anode circuit
Level of contaminants in the acid bath
All of the various combinations of all of the above

That’s just the anodizing process. The dying process also has many variables:

Porosity of the anodized surface to be dyed
Concentration of the dye in the solution
How many times the dye has been used (affects concentration)
Type of dye used (organic, etc.)
Length of time components are exposed to dye
Temperature of the dye
PH level of the dye
How many components there are in the dye at one time
Variations in any agitation that is used
Presence of contaminants in the dye
Sealing process used (water or nickel-acetate)
Sealing time
All of the various combinations of all of the above

DID YOU SAY SPECIAL "LIMITED EDITION" COMPONENTS?

Considering all those possible variations and their combinations, its amazing that manufacturers can get colors to come out even reasonably close. Recent advancements in anodizing and anodizing automation have improved the process and consistency over the years, but so far, absolute color consistency does not result.

Sometimes, a batch of components comes out of the anodizing and dying process with an unexpected and unintended color. From the manufacturer’s standpoint, they have to make a decision. Many thousands or tens of thousands of dollars, euros, pounds, or yen have been invested in producing the components before anodizing. Manufacturers sometimes decide to market their uniquely colored anodized components rather than throw away and waste perfectly good components.

Color variations do have their bonuses as well as their drawbacks. Sometimes, one particular batch of a color doesn’t come out quite as expected, but it is unique and still a beautiful color. One example of this were some components we had several years ago that were intended to be gold. Instead, they were much more of a “lemon-lime.” They were very unique and a great color. Just one batch was turned out that way, and what a great color it was.

The downside comes when you’re trying to match components. Say you’ve purchased one component and a while later, want another component in the same color from the same manufacturer. While it might be the same color, it may not be the exact same shade of the same color. This is a normal variation of the anodizing process. Also, components purchased at the same time from the same manufacturer still does not guarantee that the colors will match. Thankfully on bikes, most components are far enough away from each other that reasonable variations in the shade of color aren’t a problem. Or, depending upon how you think about it, those variations show the uniqueness and individuality of each component and add character to a bike. There’s also nothing wrong with a preference towards color matched components as long as a purchaser is willing to wait or if a color-matched set happens to be available.

This article isn’t intended to be anywhere near a complete technical description of anodizing, but I hope it has given you a better understanding of what anodizing is and how color variations are a natural part of the process.

Happy Riding from Mike & the Team @ AVT.BIke!

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Special thanks to Mike and the Team @ AVT.Bike (friendly online store for some of the best component brands) for the content share. Stay tuned for more technical articles from the team!