Carbon fiber reinforced polymer (CFRP) is a composite material prized across aerospace, automotive, and sporting industries for its high strength-to-weight ratio. This material consists of strong carbon fibers embedded within a protective polymer resin, typically epoxy. The short answer to whether carbon fiber corrodes is no; it does not undergo the traditional electrochemical process of corrosion seen in metals. Understanding the durability of a carbon fiber part, however, requires looking beyond just the fibers themselves, as the composite structure has different vulnerabilities than pure metal.
Why Carbon Fiber Does Not Corrode
Traditional corrosion, such as the rusting of steel, is an electrochemical process where metal atoms oxidize. Carbon fiber is composed almost entirely of carbon atoms, which are in a highly stable, non-metallic state. These atoms form strong covalent bonds, making the material chemically inert under normal operating conditions. Carbon fibers lack the metallic ions necessary for electrochemical corrosion to occur. This inherent stability means the fibers are highly resistant to degradation from most chemicals, acids, and bases, and will not rust or oxidize like iron-based alloys.
Degradation of the Polymer Matrix
While the carbon fibers remain stable, the polymer resin that binds them together is the most susceptible component of the composite to environmental degradation. This resin matrix, often an epoxy, is responsible for transferring load between the fibers and protecting them from the environment. Degradation of the matrix is a non-corrosive form of material failure that reduces the composite’s overall mechanical performance.
One common degradation mechanism is photo-oxidation caused by ultraviolet (UV) exposure from sunlight. UV radiation breaks down the polymer chains on the surface of the composite, which can lead to a chalky appearance and erosion of the resin. This surface damage is significant because it exposes underlying material, potentially limiting the effective load transfer to the reinforcing fibers over time.
Moisture absorption is another significant factor in matrix degradation, particularly in humid or hot-wet environments. Water molecules can diffuse into the polymer, leading to plasticization, where the matrix softens and its glass transition temperature is lowered. This process reduces the resin’s stiffness and strength, decreasing matrix-dominated properties like flexural and interlaminar shear strength.
Prolonged moisture exposure can also cause the weakening of the bond between the fiber and the resin, known as the fiber-matrix interface. This interface damage can lead to micro-cracking within the composite and, in severe cases, to delamination, which is the separation of the composite layers. The presence of strong chemicals, such as certain acids or solvents, can also directly attack and dissolve the resin, further accelerating the composite’s overall deterioration.
The Risk of Galvanic Corrosion
The most significant exception to carbon fiber’s corrosion resistance occurs when it is placed in direct contact with certain metals. Although the carbon fiber itself does not corrode, it is highly electrically conductive, which makes it a powerful driver of galvanic corrosion in adjacent metals. This type of corrosion requires three conditions: two dissimilar conductive materials, electrical contact between them, and an electrolyte like moisture or saltwater.
In this electrochemical reaction, carbon fiber acts as the noble material, or the cathode, due to its high electrochemical potential. A less noble metal, such as aluminum, mild steel, or magnesium, acts as the anode and is preferentially attacked. The carbon fiber accelerates the dissolution of the metal component, often leading to rapid and severe deterioration of metallic fasteners or structural fittings.
This risk is particularly high when a large carbon fiber surface area is coupled with a small metal part, creating a severe cathode-to-anode surface area ratio. To mitigate this destructive process, engineers must introduce an electrically insulating layer, such as a fiberglass ply or a non-conductive polymer coating, between the carbon fiber and the vulnerable metal. Alternatively, using highly corrosion-resistant metals like titanium for adjacent parts can reduce the potential for galvanic reaction.