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October 01, 2017 4 Comments
As we northern climes head into damp weather and cyclocross, issues of corrosion deserve attention. Corrosion cannot be ignored but, for cycling, most of its challenges can be addressed.
Beware viewing extreme cases of corrosion on the Web. An incident only matters in terms of frequency. Being one of 1,000 witnesses to a wild case may not have statistical validity. Shape your practice around incidents that are representative.
Corrosion ~ oxidation ~ redox ~ rust
Beware, I’ll be interchanging these terms, most unscientific! To begin, corrosion is universal. Everything oxidizes under some conditions, life depends on this chemistry. It simply must be controlled and limited to an acceptable rate so we get good use from our wheels.
In general, corrosion is not a main driver of component failure, but is always present at the scene and often a player. Where a failure has occurred, note all the detail you can but be cautious assigning relative importance to each factor. A badly rusted hub bearing may show extreme wear but as it takes time and/or negligence to blow bearings, the coincidence does not define causality.
Corrosion is related to both material and environment. Warm, humid, coastal areas with onshore breeze and mist are hugely more corrosive than elsewhere. Components designed for average conditions need more care (rinsing) in FL or HI. In those areas, brass nipples can turn black with corrosion and crumble to dust. If you respond by cleaning and lubricating you might not feel aluminum is such a bad nipple material. Its corrosion is similar to brass and, maintained, can be reliable.
Corroded hub shells are ugly. Chalky stains and pits can be impossible to remove. Anodizing provides a first line of defense. For hubs that are uncoated, a high polish is more corrosion resistant. Campagnolo’s early hubs were polished and builders cleaned and shined them before a build so subsequent maintenance was easier. Surface treatments like original WD40 go beneath water, clinging to metal surfaces. A light spray or wipe can minimize corrosion.
Today’s better spokes are made of stainless steels, corrosion resistant enough to not require protective coatings. These steels can corrode so don’t be surprised to see a light haze of rust. Wipe it off with an oiled rag. The black color applied to many stainless spokes is decorative only and not as corrosion resistant as the underlying metal. A light haze of rust on a black oxide coating is easily wiped off with an oiled rag and will not rust again until the oil is removed.
Remember that multiple stainless alloys are used for spokes, each with different metallurgy and corrosion resistance. Likewise, there is no single black or color coating. Integrity of a coated spoke is difficult to predict without access to manufacturing details.
For non-stainless spokes, galvanizing is a common corrosion prevention. It begins as a shiny, attractive layer, quickly becoming dull, industrial gray. This is normal. On show motorcycles and antique bicycles we see chrome or nickel plating. Neither has much corrosion resistance without regular wiping and application of a protective polish. I’ve seen bad rust on non stainless spokes but rarely breakage for which the rust could be blamed. Of course, it happens but is rare on in-use wheels.
Wiping spokes with a rag lightly soaked in an oil of your choice is good practice. Pressure washing removes dirt but dirt doesn't cause corrosion. Washing removes the protective coatings all components need. We're in a modern world of materials and coatings but superficial oils and waxes still play a key role for weather resistance.
The debate over brass vs aluminum nipples is eternal. The issue should be decided on mechanical attributes, not corrosion resistance (IMO). Both are quite similar and a well anodized aluminum nipple can equal brass with a nickel plated finish. Brass is more ductile and self lubricating. High grade (2024 and 7075) aluminum is stronger, can be brightly colored, and a bit lighter.
Check this chart for comparable corrosion. Notice columns for aluminum and brass. For exposure to sea water, aluminum is better. For distilled water, both are excellent. For vinegar, they are equivalent. For whiskey, brass is better. Two metals with different outcomes depending on environment. I'm for the whiskey.
In navigating the “controversy” remember that much unwelcome behavior by any single batch of nipples may depend on specific conditions. What, if any, lubricant is used? What process shaped the nipple (forged, cast, machined)? What plating and specification (depth, penetration) was used? Silver nipples are not necessarily anodized. Are yours? Notice also that many millions of aluminum nipples have been used on production wheels over the past 20 years; most outlasting their spokes, hubs, and rims.
Brass contains lead for formability. Recent law restricts the amount but the historic presence of lead in brass taints its otherwise classical charm. Regardless of nipple material, regular lubrication is key. If you soak nipples in heavyweight oil prior to building, the coating can last years. If your wheels see pressure washing, relubrication is required.
To lube a built wheel, touch a drop of light oil to the spoke as it enters the nipple. Watch it wick down the bore. A second drop belongs at the nipple-rim contact, applied from the same side. Spinning the wheel creates centrifugal force to drive lube into the assembly. Many of today's chain lubes are ideal for this as they are thin for application and penetration but as their carriers evaporate the remaining formula becomes dense and long lasting.
Cracking of aluminum rims under tension at nipple holes can be accelerated by corrosion. After anodizing, rims are purged of liquids used in the electrochemical process. Inadequate flushing can leave seeds of corrosion deposited inside the rim. Anodizing can be too deep and hard, leading to surface cracking where the rim may be deforming from spoke tension.
Corrosion and cracking at once.
Tire sealants can also be corrosive agents and leak into rim interiors, especially those with ammonia.
Carbon rims are neutral for corrosion but can act as anodes when in contact with metals, stimulating corrosion. It is relatively easy to isolate and protect such galvanic combinations with anodizing, washers, and lubricants. For better or worse, the vast majority of carbon wheels are built with aluminum nipples, showing the combination is not automatically bad news. Enve’s recent switch from aluminum to brass for internal nipples does not indict all use of aluminum. Many of the problems they observed are owed to ammonia from tire sealants and poor anodizing on nipples. Substantial numbers of carbon wheels show no aluminum nipple corrosion. There is often more to the story than simply nipple material.
Storage batteries in use or awaiting disposal can create electrical fields that produce very corrosive conditions. When you see an “impossible” corrosion example, wonder about the context. With growth in e-bikes, e-cars, and home energy storage, we can expect more such cases.
In following corrosion, make careful observations and notes, utilize magnification and photo images, and use components with highest material and coating properties.
This upcoming La Niña winter should be fun, not corrosive!
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