If the carbon frame is fitted with aluminum parts, corrosion can occur at the contact points. If e.g. an unpainted aluminum seat post in the carbon frame or a carbon post in the aluminum frame has not moved for a long time, it can clamp tightly. Tip: Once a year, remove the seat post and clean it.
Also a cable stop, unpainted areas allow the emergence of corrosion. Therefore, it is always recommended to seal the frame with spray wax, to expel moisture.
Corrosion of carbon and metal is rare, but not impossible. Basically, the very good conducting carbon fiber is embedded in epoxy resin, a dielectric. Nevertheless, it is possible by mechanical working that metal at the cutting edge comes in contact with exposed carbon fibers, e.g. the manufacturer-side milling of the inner diameter of a seat tube.
The electrochemical potential of unalloyed steel is -0.35 V, of aluminum -1.66 V, that of the carbon fiber is +0.75 V. Direct contact of two materials with a large difference in electrochemical potential starts a corrosion process in which electrons from the less noble, the material with low potential, to the nobler material with higher potential, drain - it comes to a leakage current, the beginning of the corrosion process.
The anodic partial reaction is the actual corrosion. This is an oxidation and takes place at the less noble reactant. The material (e.g., aluminum seatpost) releases electrons and thus dissolves. The cathodic partial reaction, however, is a reduction. Here, the nobler reaction partner absorbs electrons and thus promotes anodic dissolution.
The recognition of the damage is often not possible without disassembly of the entire compound on the example mentioned, it is advisable to dismantle the seat post once a year and to free from any moisture through rain rides. This is called crevice corrosion.
The corrosive attack of a carbon-metal compound can cause a large number of damages. For example, the development of corrosion products and their deposits in the contact area between aluminum / steel and carbon can make it impossible to loosen the connection. Dismantling then leads in most cases to uncontrolled release of the connection, whereby the further reliability of the carbon frame is seriously compromised.
The metal dissolution is due to an increasing oxygen depletion in the cleavage. By additional mechanical stresses of the clamp connection, the corrosive attack can be favored, this is called tension corrosion and vibration corrosion. The interaction of corrosion and mechanical stresses leads to cracks that continue throughout the entire component cross-section and can lead to component failure.
A rubber seal as protection against moisture and the installation of a intermediate dielectric (plastic or fiberglass sleeve), the electrolyte exchange at the contact point can be reduced and the corrosion current (compensation in the potential) can be minimized. Often this is not very practical, as this makes the clamping of the seat post difficult or even impossible and in the worst case, an increase in torque above the predetermined maximum value, the clamp connection is destroyed. It is best to ride without a metal-carbon compound.
Current state of the art is the use of a material combination whose potential difference does not exceed a maximum amount of 0.4V.
But what is difficult with a material combination of carbon with the electrochemical potential of +0.75V.
Carbon +0.75 V to aluminum -1.66 V = difference 2.41 V
Carbon +0.75 V to steel -0.35 V = difference 1.10 V
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