Carbon fiber corrosion

[Webb]

As I get ready to add stiffening strips to the cowl, I have seen several references to putting a layer of glass on top of the carbon fiber layer. Especially if steel touches the carbon fiber layer. In this case it’s aluminum.

Would someone please explain why carbon fiber layups are so corrosive.

 

[RVDan]

Carbon Graphite) is a metal and the issue is dissimilar metal corrosion. Carbon is at one end of the galvanic chart and aluminum is pretty near the other end. The contact between the 2 creates an electro chemical couple and a current will flow, causing a breakdown in the aluminum. You can research galvanic corrosion.

 

[BJohnson]

Carbon Graphite) is a metal and the issue is dissimilar metal corrosion. Carbon is at one end of the galvanic chart and aluminum is pretty near the other end. The contact between the 2 creates an electro chemical couple and a current will flow, causing a breakdown in the aluminum. You can research galvanic corrosion.

An electrolyte needs to be present also, so if it never gets wet, it won’t corrode. But for a cowling that is not likely. The fiberglass electrically isolates the aluminum from the carbon. Primer and adhesive would do the same thing. Another thing to consider is that the surface of the laminate might have a resin rich layer, unless it has been sanded, preventing contact between carbon fiber and aluminum.

When I was building my -9a almost twenty years ago, I riveted together two pieces of carbon fiber/epoxy with countersunk rivets and left it in the garage. Still no signs of corrosion, but I’m not looking closely either. And yes, don’t rivet composites with expanding rivets. It will start delaminating the material.

An Introduction to the Galvanic Series: Galvanic Compatibility and Corrosion

When metals are incorrectly combined and exposed to water and other types of electrolytes, the effects can be very harmful. Find out more about the galvanic se

www.corrosionpedia.com

 

[scsmith]

It is also much more to worry about if you have an adhesive bond between the carbon and aluminum vs. two parts that are fastened together. In an adhesive bond, the bond-line corrosion can/will degrade the bond until it fails. A cowl attached to an aluminum flange might maybe show a little bit of topical corrosion on the aluminum if the primer/paint is chaffed through. But really, so what? If you are at all concerned, put a thin fiberglass vale cloth layer over the carbon, which, combined with being fully wetted with epoxy will form an effective insulator.

 

[Webb]

Let me see if I get this straight. If I have bare carbon fiber cloth on bare aluminum and water is present, a galvanizing reaction will occur time. If I have carbon fiber encased in resin and no moisture is present, no reaction will occur.

 

[RVDan]

Let me see if I get this straight. If I have bare carbon fiber cloth on bare aluminum and water is present, a galvanizing reaction will occur time. If I have carbon fiber encased in resin and no moisture is present, no reaction will occur.

Not exactly true.An electrolyte will greatly accelerate the process, but galvanic corrosion will proceed without an electrolyte just at a greatly reduced rate. It also takes very little contamination of the joint to have an electrolyte. Just use a scrim layer of glass. scrim fabrics are very light. Something like .75 oz per sq yard Will do.

 

[Desert Rat]

Let me see if I get this straight. If I have bare carbon fiber cloth on bare aluminum and water is present, a galvanizing reaction will occur time. If I have carbon fiber encased in resin and no moisture is present, no reaction will occur.

Pretty much. Epoxy, paint, fiberglass; pretty much anything that you would normally expect to act as an electrical insulator. On something like a cowling that sees a lot of wear at the mating surface, sooner or later primer is going to wear off, A layer of fiberglass is a much more robust insulator.

 

[fixnflyguy]

As others have stated, Carbon fiber and Aluminum is not good together. I'm a structures specialist on Boeing/Airbus for my day job. Mating Carbon and Aluminum is a difficult process to prevent dissimilar metal corrosion, and aluminum rivets through CG is no bueno, let alone bonding aluminum to it. Most CG panels are well coated and sealed with fiberglass layer and or paint between mating surfaces then attached with titanium fasteners. We could go deep into this subject, but I would stay away from aluminum in contact with CG..it wont happen immediately, but corrosion will likely develop. Stiffeners on composite panels are usually layed up bonded areas with either honeycomb or foam strips to achieve the rigidity desired, and will become one with the panel. Search up Dan Horton on VAF here and see some of his well documented glass/CG composite work.

 

[Webb]

Amazing what can be found on the internet. Here is what the FAA has to say about it.

https://www.tc.faa.gov/its/worldpac/techrpt/tctn17-42.pdf

There are five conditions that must be satisfied for composite-to-metal galvanic corrosion to develop:
1. An actively corroding metal
2. An electrically conductive composite (usually carbon fiber)
3. An electrical connection between the composite and the metal
4. An electrolyte (usually salt water)
5. Oxygen
If any of these conditions are absent, galvanic corrosion will not occur.

 

[RVDan]

Amazing what can be found on the internet. Here is what the FAA has to say about it.

https://www.tc.faa.gov/its/worldpac/techrpt/tctn17-42.pdf

There are five conditions that must be satisfied for composite-to-metal galvanic corrosion to develop:
1. An actively corroding metal
2. An electrically conductive composite (usually carbon fiber)
3. An electrical connection between the composite and the metal
4. An electrolyte (usually salt water)
5. Oxygen
If any of these conditions are absent, galvanic corrosion will not occur.

Nice find. The one thing I would say about the electrolyte aspect, is that condensing moisture from the air and normal air pollution contamination becomes an electrolyte.