Carbon fiber reinforced composites (CFRC) are a sort of advanced material used in the aerospace, automobile, marine and sport industries. Their use is growing drastically because of their fantastic properties, particularly their high strength and low weight.
Properties of Carbon Composites: Mass Density, Specific Strength and Yield Strength
We’ve heard about aluminum and its alloys as light metals that are very suitable for use in the aerospace and automotive industries. But for a materials engineer who wants to select an adequate material for a given application, it is not the mass density that's important throughout the material selection.
Figure 1. Video about the desirable physical characteristics of carbon fiber composite materials.
Engineers are always looking for a material with a high specific strength along with a low mass density. The following table compares the specific strength of different industrial materials.
| Material || Mass density |
| Tensile Strength |
| Specific Strength (kN.m/kg) |
|Aluminum 7075-T6 ||2.8 ||600 ||214 |
|Magnesium alloy AZ91D ||1.7 ||230 ||135 |
|Titanium ||4.4 ||950 ||216 |
|Carbon Steel (0.45 % C) ||7.8 ||850 ||108 |
|Maraging Steel ||8.1 ||2500 ||300 |
|Carbon composite ||1.6 ||1240 ||785 |
Table 1: A comparison of mechanical strength, mass density and specific strength of different industrial materials.
The materials with a high strength and low mass density provide a high specific strength and are ideal for engineers. It can be seen that carbon fiber reinforced composites possess the highest specific strength. The specific strength of carbon composites are at least two times more than maraging steel, which is characterized with the highest strength among all types of steels.
This means that for a certain required strength, the weight of a component is reduced to half if CFRC is used instead of maraging steel. For automotive applications, this means a lighter vehicle that consumes less fuel. Low crack growth due to impact or fatigue, the ability to produce in desirable directional mechanical properties, and being cost-effective in mass production are the other highlighted properties of carbon-reinforced-polymer composites.
As a result, this material is considered to be an advanced structural material.
All of the above-mentioned properties of carbon fiber reinforced polymers make this material a potential candidate for automobile, aerospace, infrastructure and marine applications. (To learn more about the properties of carbon fiber, read 10 Things to Know About Carbon Fiber Reinforced Polymer and Corrosion.)
Figure 2. Carbon composite materials are lighter and stronger than traditional materials such as aluminum and steel.
Carbon Composites' Drawbacks
Despite all of the excellent properties of CFRCs, there are issues with using carbon fiber reinforced composites and metals together. The carbon fibers in CFRPs cause this material to become electrically conductive. The carbon fibers are electrically conductive and electrochemically very noble. Therefore, when a metal is electrically connected to a CFRP, it is more susceptible to galvanic corrosion. (To learn more about galvanic corrosion, read An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion.)
There are five conditions which need to be met for galvanic corrrosion to occur between a metal and the composite material:
- An actively corroding metal
- An electrically conductive composite (usually carbon fiber)
- An electrical connection between the composite and the metal
- An electrolyte (usually salt water)
- The presence of oxygen
This situation becomes worse when a large surface area of carbon composite components is coupled to small metallic parts (such as fasteners, bolts and nuts). In these circumstances, the rate of galvanic corrosion is extremely high due to the high cathode to anode surface area ratio (Ac/Aa).
The galvanic corrosion of metals coupled to carbon composites is not a new issue. It has been reported since the 1960s. But this issue has not been resolved yet. The morphology and intensity of the galvanic corrosion strongly depends on the type of metal connected to the carbon composite, cathode-to-anode surface area ratio, and environmental conditions. (For further discussion, see Why Do Two Dissimilar Metals Cause Corrosion?)
In the following section, the behavior of dissimilar metals in this situation will be discussed in more detail.
Aluminum Coupled to Carbon Composite
Aluminum alloys are extremely vulnerable when they are coupled to a carbon composite. According to the anodic and cathodic polarization curves of aluminum alloys and carbon composites, it is clear that the rate of galvanic corrosion in seawater is controlled by the oxygen reduction reaction.
What this means is that any condition that leads to an increase in the rate of oxygen reduction will cause an increase in the rate of galvanic corrosion. During the galvanic corrosion, a white, jelly corrosion product will be formed on the surface of the aluminum.
There is an assertion that the galvanic corrosion rate of aluminum could be mitigated by the anodization of aluminum and the formation of a thick, protective aluminum oxide layer on the surface. However, it has to be mentioned that in the case of a breach of the oxide layer by mechanical damage, the situation becomes much worse due to a really high cathode-to-anode surface area ratio (Ac/Aa).
The galvanic corrosion rate of mild steel coupled to a carbon composite has been investigated in different environments: concrete, deicing solution and seawater. The results show that much like aluminum, the corrosion rate of plain steel is controlled by an O2 cathodic reaction. Sometimes the corrosion rate of plain steel increases by a factor of 25 and 60 when it is coupled with a carbon composite in deicing solution and seawater, respectively.
There is no evidence of the formation of corrosion for stainless steels coupled to carbon composite. However, it is reported that some types of stainless steels (such as types 410 or 301) are susceptible to localized corrosion (pitting corrosion and crevice corrosion) when they are connected to a carbon composite in aerated 3.5% NaCl (simulated seawater). It is believed that the pitting corrosion will be enhanced by increasing Ac/Aa. (For more information about pitting corrosion, see A Look at High Nitrogen Stainless Steels .)
By looking at the standard electrochemical potential of titanium, it seems that this metal is an active metal. However because of the formation of a dense stable and protective oxide layer, titanium is placed among the noble materials and just below graphite or carbon in the galvanic series table. (For a primer, see the article An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion .)
Therefore, there is no significant gap between titanium and carbon-fiber-reinforced composite to create galvanic corrosion. This means that commercially pure titanium and its alloys are completely resistant to galvanic corrosion when they are coupled with carbon composites.
Are Carbon Composites Safe When Coupled with Metals?
The galvanic coupling of metals to carbon composites will not only cause problems for the metal, but also for the composite itself. Due to the hydrogen gas evolution in defect sites of the composite (such as voids and cracks), hydrogen-filled blisters can form on the composite surface. (Discover more in The Corrosion of Polymeric Materials.)
The other issue that might be a problem for carbon composites as a cathode in a galvanic couple is the formation of calcareous deposits on the surface of the carbon composite. In stagnant seawater, a huge number of cathodic reactions happen on the surface of carbon fibers, including hydrogen evolution and oxygen reduction, which can lead to the creation of a localized alkaline solution on the surface.
In this condition, the carbonate salts in seawater are not soluble and will deposit in the form of the aragonite phase (calcium and magnesium carbonate). Because a high hydrogen reduction rate is needed to create such calcareous deposits, this phenomenon happens when an active metal, such as aluminum or magnesium, is connected to a composite material.
How to Mitigate the Corrosion of Metals Connected to Carbon Composites
Here are some remedies to control the galvanic corrosion of metals connected to carbon composites:
- Substitute the metallic part with a high-corrosion resistance alloy. In this case, the best option is titanium and its alloys.
- Disconnect the electrical connection of two parts by placing an electrically insulating material, such as fiber-glass-reinforced composite, between those parts.
- Use epoxy resins without any hydrolysable linkage, such as ester bonds, to mitigate water penetration into the composite and then to decrease the real cathodic surface area.
- Use sizing agents as a sealant on the surface of the carbon fibers before fabricating composites.
- A combination of sulfuric acid anodizing and a hybrid sol-gel coating is quite effective against galvanic corrosion in carbon fiber metal laminates.
Testing Method for Galvanic Corrosion of Composites and Metal Assemblies
The potential benefits to CFRP-metal users of implementing a galvanic corrosion test based on this standard are:
- Addressing the corrosion risks related to CFRPs of drastically nobler galvanic potential than metals to form a global cell between the CFRP and the metal
- Expanding CFRP applications to include corrosive environments that still require the inclusion of metallic components
- The detection or the prevention of galvanic current insulation loss, such as ion migration and time-related degradation in sealant film, injected caulking layer and glass fiber reinforced plastics (GFRPs)
- Demonstrating conformity to specified conditions for type certification requirements for aircraft
- Evaluating the corrosion related procedures for maintenance, repair and overhaul (MRO) of CFRP aircraft