Corrosion under carbon fiber composite

[awol]

We are reinforcing a steel plate with a wet-applied carbon fiber composite. The bare steel is primed with epoxy primer. The carbon fabric (PAN) is wetted with an epoxy and, while the primer is still wet and uncured, the wet carbon is layed over the steel. Several layer are used, all wet when applied and pressed into place.

Now the composite is fully cured and we have cut the part in half to reveal a layer of steel and a layer of carbon composite. When we check the outside surface of the composite to have continuity with the bare steel, we have none. The epoxy is thick enough on the surface to isolate the carbon surface from the steel. When the carbon composite edge (that was exposed by cutting, the part in half) is checked for continuity, it is hit and miss. Some of the carbon is isolated from the steel and some is not. Also, some of the carbon is isolated from itself and some is not. We are trying to establish that the carbon may or may not contact the steel directly and , if it does, has no corrosive impact on the steel. Is this a true statement, "The area under a carbon composite repair will not corrode due the the absence of an electrolyte." Corrosion must hav a cathode, anode, and electrolyte. Has anyone researched this area?
PAN fibers would be more noble and cathodic while the steel will be the anode. If no moisture is present, what if anything, will act as the electrolyte after the epoxy has cured?
Thanks, awol

 

[MechanicalChef]

I'd suspect the electrolyte is humidity. All I can say at this point is galvanic corrosion and isolation is a fundamental issue that I we continuously have to worry about.

 

[Oceania]

I have reinforced a reasonable amount of steel with carbon fibre over the years and have undertaken a number of simple corrosion tests. Due to the variation in consolidation you will get touching points, and in time depending on the steel and the humidty some element of corrosion. Early work with carbon on aluminium the corrosion was quite spectacular. As you are using a wet-layup I assume you are applying the carbon to a complex surface. If you use pre-cured laminates then the use of a thick glue line epoxy adhesive will reduce the probability of corrosion. Alternatively you can do as I have over the years and use an insulating ply between the steel and carbon.

 

[MechanicalChef]

Has anyone tried epoxy powder-coat as a dielectric between carbon and steel/aluminum?

As a small piece or trivia, a large portion of the Ti being used on the 787 isn't for its superior structural abilities. Instead, it's simply because graphite likes to consume aluminum.

 

[Blackhorse007]

We used to have the same problem too when we applied carbon plates to bare Steel structures. One of the reason i could come up with is that carbon is conductive for electrostatic, thus when it comes into contact with the steel, there some electron transfer be it if there is epoxy or not. One of the way we proposed and worked is to add a layer of glass veil to in between the carbon and the steel. Glass is not conductive to electricty or electrolyte thus, there after there is no more rusting occurring. prolly u might want to work on this area?

 

[phoenix221]

I will be applying a layer of glass fiber between the steel and the carbon fiber. I'll see how that works out.

Thanks.

 

[TapasLaha]

If there is no moisture, which is almost impossible, there should not be any kind of corrosion problem, as you have rightly pointed out the absence of any electrolyte. But how is the interfacial bonding between c-fiber and steel? With epoxy at the interface, I do not assume good adhesion. I doubt glass coating will improve the situation. What about electroless coating of Ni on the fiber?

Tapas Laha
Miami