10 Things to Know About Carbon Fiber Reinforced Polymer and Corrosion
While carbon fiber reinforced polymer (CFRP) materials possess the advantage of a very high specific strength coupled with very low weight, galvanic activity with aluminum and other popular high strength metals remains a major concern.
Since their introduction, carbon fiber reinforced polymers (CFRP) are finding increased usage in several industries. Despite their inherent advantages, there are several important things to keep in mind when selecting them for a particular application.
1. The Basics of Carbon Fiber Reinforced Polymer (CFRP)
Carbon fiber reinforced polymers (CFRPs) are an advanced composite polymeric material where carbon fibers are mainly used to enhance mechanical strength (load capacity) and provide other essential features. (Mechanical strength is discussed in the article 6 Tests to Measure a Material's Strength.) CFRPs are preferred in marine, sports, automotive and aerospace applications due to their superior strength-to-density ratio. Additionally, CFRPs have a high fatigue strength, thus contributing to safety, fuel economy and durability. They are also eco-friendly.
The carbon fiber in carbon fiber reinforced polymers provides the advantage of very high specific strength; therefore it is used to design fuel-efficient lightweight vehicle parts and engineering structures. (Note: Specific strength is calculated by dividing the yield strength of the material by its mass density.)
Studies have shown that carbon fiber reinforced polymers have a specific strength that is almost 2.6 times greater than the strongest steel (maraging steel) and almost 3.6 times the specific strength of aluminum 7075-T6. In other words, for a given tensile strength, if we replace aluminum 7075-T6 with CFRPs, the weight of the component will decrease by almost 70%. By substituting CFRPs for maraging steel, the weight of components will decrease by more than 55%. For aerospace, automotive and marine applications, this substitution translates to lighter vehicles that consume less fuel and indirectly have a smaller environmental footprint.
2. The Automotive Industry's Take on CFRPs
As the US government is pursuing a target fuel consumption rate of 54.4 miles per gallon to be achieved by 2025, vehicle designers are continuing their quest for lightweight materials with a higher specific strength to achieve this average fuel economy goal even sooner.
Initially, CFRPs were intended for race cars and transformed the car racing industry by enabling robust, lightweight cars. But today, CFRPs are used in almost every new car design. They are increasingly used for mainstream vehicle structures and component parts. Automobile manufacturers such as GM, Ford and BMW are slashing the weight of their latest models by using CFRP structures, thus improving fuel economy and safety. Toyota is using CFRP fuel tanks in their next generation fuel cell electric automobiles. The US Department of Energy (DOE) aims to reduce the manufacturing cost of carbon fiber by a whopping 50% so that it is affordable and also reduce the vehicle's weight by 50% without impairing safety or vehicle performance in any way.
3. The Ever-Widening Scope of CFRP Applications
Apart from their use in the transportation sector, other emerging applications of CFRPs include:
- Medical devices such as orthopedic implants
- Infrastructure maintenance repairs, retrofitting and new bridge construction
- Submarine and marine applications
- Reconstruction of damaged offshore oil and gas assets such as pipelines due to corrosion or other failures
- Production of electric power transmission lines with a CFRP core (carrying the mechanical load), an aluminum conductor and a fiberglass barrier to prevent galvanic corrosion
4. Corrosion Risk of Carbon Fiber Reinforced Polymers
An area of research concerns the extent to which metals in electrical contact with CFRPs are susceptible to corrosion.
Most metals electrically connected to CFRPs in multi-material constructions can corrode due to galvanic corrosion because carbon is more noble than common metals. Additionally, the polymers used in CFRPs are prone to degradation by corrosion. (To learn more, see The Corrosion of Polymeric Materials.)
A research review indicates that CFRP deterioration can occur due to either anodic polarization or cathodic polarization. Anodic polarization may occur due to adsorbed oxygen, which causes deterioration of carbon fiber.
5. Galvanic Corrosion of Aluminum Alloys in Contact with CFRPs
A research article concludes that there exists the risk of galvanically induced corrosion of aluminum alloys connected to CFRPs. Factors affecting the corrosion include the carbon fiber content (concentration) in the matrix as percent by volume and the surface features of CFRPs. The percent of carbon fiber content by volume determines the electrical conductivity of the carbon fiber reinforced polymer system, which in turn determines the galvanic corrosion risk to metals.
Further research on galvanic corrosion of aluminum in contact with CFRPs has concluded that high carbon fiber content by volume in the polymer matrix makes the polymer structure adequately conductive to enable the galvanic corrosion of connecting metals. Oftentimes the fibers of carbon in the polymer matrix are not fully covered by the insulating polymer covering.
Additionally, aging factors, wear and tear and environmental and climatic conditions often cause the aluminum surface to come in contact with exposed carbon fibers. This situation will result in aluminum corrosion because aluminum is the less noble material. The researcher found that the corrosion rate is dependent on the area of the exposed carbon fiber's surface, whereas the exposed aluminum surface area has no impact on the corrosion rate. The researcher also found that the overall corrosion rate is limited by oxygen availability on the carbon fiber's surface area, the diffusion rate of oxygen and the overall geometry of the CFRP component or structure.
In the case of CFRP cored aluminum conductors, the fiberglass protective shield of the composite core can become damaged due to factors such as excessive bending, aging, impact loads and fatigue. Due to a high percentage of carbon fiber content by volume in the polymer structure (designed to carry heavier loads), a metal to carbon fiber contact can often occur and the entire composite assembly can be considered conductive for the galvanic corrosion process.
6. Corrosion of Galvanized Steel in Contact with CFRPs
Galvanic activity of galvanized steel in electrical contact with CFRPs was studied by researchers that found that DP590 steel with a zinc coating in contact with CFRPs was initially corrosion resistant, however the corrosion rate increased over time. Thus, corrosion resistance of galvanized steel was not sustained for the long term.
7. The Corrosion Rate is Also Linked to the CFRP's Surface Condition
In the above mentioned study, a carbon fiber reinforced polymer sample with a thick epoxy layer was found to be more corrosion resistant than another sample with a thin nylon layer. (Nylon coatings are discussed in Advances in Liquid Nylon Multipolymer Coatings for the Transportation and Renewable Energy Industries.) Therefore, it was established that the provision of a thick corrosion resistant polymer layer around the carbon fiber is essential for corrosion prevention of galvanized steel and other metals in contact with CFRPs.
8. Corrosion of Stainless Steel or Titanium in Electrical Contact with Carbon Fiber
Titanium was found to be corrosion resistant and the connected CFRP was also free from corrosion. The initial corrosion product found in this case actually protected the metal from further deterioration.
In the case of stainless steel, however, the prevalence of crevice corrosion and pitting corrosion was confirmed on a stainless steel screw sample fitted onto a CFRP system in a saltwater environment. In addition, the CFRP system in electrical contact with the stainless steel screw suffered deterioration due to delamination.
9. Corrosion of Steel in Contact with Carbon Fiber in Cement Mortar
Research conducted on steel in cement mortar in contact with CFRPs concluded there was only negligible corrosion in cement mortar that was free from chlorides. In contrast, significant galvanic activity was observed in steel in contact with CFRPs in mortar containing chlorides. This investigation is significant for predicting the corrosion behavior of steel in contact with CFRPs in an environment containing seawater or deicing chemicals (e.g., roads and bridges).
10. Corrosion Proofing the Carbon Fiber Connected to Metals
Isolating the conductive carbon fibers in CFRP composite structures by using electrical insulators (e.g., fiberglass reinforced polymers) as an outer shield over the carbon fiber is one way to mitigate the corrosion risk. Another solution is to replace corrosion-prone metals with corrosion-resistant metals such as titanium alloys. (See Your Guide to Corrosion-Resistant Metals for more information on these metals.)
Most metals, including aluminum and stainless steel, are prone to galvanic corrosion when in contact with carbon fiber reinforced polymers. In spite of this disadvantage, CFRPs are being increasingly used in aerospace, automotive and marine applications in combination with metals, due to their tremendous advantage of light weight and very high specific strength.