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. Here we'll explain the properties of carbon composites and look at the galvanic corrosion of metals when they are electrically connected to CFRCs.
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 mentioned application, it is not the mass density that's important throughout the material selection. In fact, the important issue is a concept called specific strength, which is defined as a ratio of a material's yield strength to its mass density, as follows:
Engineers are always looking for a material with a high specific strength instead of low mass density. The following table compares the specific strength of different industrial materials.
|Material|| Mass density
| Tensile Strength
|Specific Strength (kN.m/kg)|
|Magnesium alloy AZ91D||1.7||230||135|
|Carbon Steel (0.45 % C)||7.8||850||108|
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 automobile applications, this means a lighter vehicle that consumes less fuel. Low crack growth due to impact or fatigue, the ability to produce in 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.
Figure 1: Carbon composite materials are lighter and stronger than traditional materials such as aluminum and steel.
Figure 2: Wotton Bridge in Quebec, Canada, is one of the prototypes of using carbon composites in the infrastructure industry. They can be used to wrap concrete columns or use as bars inside of the concrete.
Source: Transports Quebec.
Carbon Composites' Drawbacks
Despite all of the excellent properties of CFRCs, there are issues with using CFRC and metals together. 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. 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. In the following section, the behavior of different 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. Figure 3 shows the anodic and cathodic polarization curves of aluminum alloys and carbon composites, respectively. 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.
Figure 3: Cathodic polarization curves for (A) virgin carbon composite, (C) a surface-treated carbon composite, and (E) a pure graphite electrode in neutral air-stirred 3.5 % NaCl.
Source: A. Bellucci, Journal of Applied Electrochemistry,16, 1986
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 and crevice) 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 pittting 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.) Figure 4 shows blistering in a carbon composite that was connected to an aluminum component in seawater.
Figure 4: Blisters formed on vinyl ester composite coupled with steel in seawater.
Source: W.C. Tucker, Journal of Composite Materials, 23, 1989
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 party 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.