Condensed Answer: Unlike metal, carbon doesn’t rust. Consequently, carbon frames are not prone to rust like steel ones.
That said, it’s possible for galvanic corrosion to take place when carbon is in contact with metal. Also,
UV light and heat negatively affect carbon frames that do not have a protective layer of paint.
What Is Rust?
Rust is a form of corrosion that happens when certain metals are exposed to oxygen. (The process of exposing a material to oxygen is known as oxidation).
During the exposure, the oxygen “steals” electrons from the metal and creates a new compound known as iron oxide a.k.a. rust. Rust is weaker than the original material and spreads over time.
Whether a metal will rust depends on its ability to protect its electrons from the oxygen in the air.
Aluminum, for example, oxidizes too but forms a protective layer that essentially seals the exposed area and protects it from further corrosion. Precious metals such as gold and silver do not rust.
The simplest way to avoid rust is to paint the material and isolate it from the oxygen in the environment.
Unlike metal, carbon fiber does not oxidize/rust and is therefore very stable from that perspective.
Is There An Equivalent to Rust In The World of Carbon Fiber?
Carbon may not rust, but it’s also subject to degradation caused by the environment. Hot weather and exposure to U.V. can cause the disintegration of carbon fiber layers.
By themselves, carbon fiber strands are not susceptible to UV damage. The problem arises when epoxy resin is added to the mix for extra shine and structural integrity.
Epoxy resin is used for three main reasons:
- It effectively gets into carbon and adheres to it.
- It increases the strength of the carbon fiber composition by permanently securing the positions of each strand.
- Adds shine to the finished product.
UV light damages the molecules of the epoxy resin and causes discoloration and cracking. Since the epoxy and the carbon fiber strands are essentially one, the whole unit disintegrates.
The standard solution to this problem is to use UV-resistant epoxy resin and a quality clear coat to mitigate the effect of UV light.
That said, with enough exposure and time, all carbon products will showcase some sort of degradation even when relying on special epoxy resins.
In the best-case scenario, the damage will be superficial and primarily aesthetic (discoloration).
How Are Carbon Fiber Frames Protected Against UV Light?
In most cases, the frames are painted and then covered with a clear coat (transparent paint) that’s UV-resistant. If the integrity of the frame’s paint is compromised then the exposed area is susceptible to UV light and should be repainted.
That said, ceramic coating is considered the most advanced form of protection since it’s much harder than standard paint and keeps the shine of the material longer. However, it’s more expensive and difficult to apply and is therefore not common in the bike industry.
Another advantage of ceramic coating is that it makes the frame a lot easier to clean since the contaminations do not penetrate nearly as deep.
Galvanic Corrosion and Carbon Fiber
Galvanic corrosion is a deterioration process occurring when two dissimilar metals are in contact and in the presence of a conductor like water.
Carbon composite isn’t a metal, but it’s highly conductive and leads to galvanic corrosion too.
When the necessary conditions are met, a transfer of electrons begins from one of the materials to the other.
For example, an aluminum seat post in a carbon fiber frame or a carbon seat post in an aluminum frame are two scenarios when galvanic corrosion can take place.
In those instances, the aluminum is acting as an anode giving up electrons whereas the carbon element is a cathode receiving electrons.
Galvanic corrosion may result in the seat post getting permanently trapped in a frame.
The only way to prevent the occurrence of galvanic corrosion is to isolate the two materials electrically and stop the transfer of electrons.
The most common choice is to apply lubricating paste. (read more)
What About Heat?
Carbon fiber is highly resistant to heat and can withstand temperatures above 2000℃.
However, the epoxy holding the carbon units together has a much lower tolerance to heat (about 100℃).
Since bike frames are unlikely to be exposed to the aforementioned temperatures during expected use, the tolerance to heat is more than adequate.
How Should A Carbon Fiber Frame Be Stored For Maximum Longevity?
Ideally, the carbon frame will be stored indoors away from the sunlight. It’s best to avoid extreme temperatures too. In general, a simple basement with no windows or with covered ones is a fine place for a carbon fiber bike to “sleep”.
Avoid storing carbon frames in a location with lots of windows and sunlight exposure because of the excessive UV light and heat.
It’s also recommended to avoid placing a frame in a car with all the windows closed during the summer for an excessively long period of time for the same reasons.
Note: Before all, the storage location should be fairly protected against theft. UV light may damage carbon, but thieves make it disappear.
How Long Do Carbon Frames Last?
This is a hard question to answer with extreme accuracy. Carbon does not corrode nor does it fatigue like aluminum, for example. Therefore, it can theoretically last an incredibly long time.
Nonetheless, one has to admit that carbon isn’t as tough as metal and can’t handle external abuse very well. If you hit a steel frame, it may dent, but it will not break.
In different, a small crack of a carbon frame can result in an unusable bike. This is one of the reasons why carbon frames do not age like fine wine on the second-hand market.
Another issue is the epoxy resin which deteriorates much faster than the carbon itself. Once the resin is compromised so are the carbon strands.
Ultimately, a carbon frame that isn’t subject to external stress can last decades.
If maximum frame longevity is the goal, however, nothing will ever beat steel for the following reasons:
- Steel does not fatigue (unlike aluminum) and can flex forever without degradation as long as the stress is low enough not to cause permanent deformation.
- Steel is the toughest frame material when it comes to external stress.
- When painted and internally treated, steel frames do not rust or do so at a level that doesn’t cause structural damage.
- Steel can withstand substantial clamping force without cracking.