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Thread: Carbon Fiber Panels - Touching Aluminum Frame

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    Carbon Fiber Panels - Touching Aluminum Frame

    Hello Group, 10-year EAA member - but brand new to the Forum Site here. (My return key doesn't appear to work, so apology for the run-on format) We are looking for Builders' wisdom on how to properly protect & separate carbon fiber panels from aluminum airframe structures. We understand that if carbon fiber panels come in direct contact with aluminum that more active "galvanic" material (the aluminum airframe) would suffer significant aggravated corrosion. We heard of possibly using tape or even layers of primer coat, etc. (1) Is this true? (2) Has anyone dealt with this? (3) What is the best method of separating the two materials, like for floorboards which will inevitably move and shift somewhat in flight and use? (4) Does this concern also apply to the stainless steel fasteners (bolts) that would pass through the carbon fiber into the aluminum? Do the fasteners need to be separated from the carbon fiber in some way also? - Dale, Milwaukee, Wisconsin

  2. #2
    Matt Gonitzke's Avatar
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    Put a fiberglass isolation ply on the side of the carbon fiber laminate that is contacting the aluminum. That's what's done with large aircraft.

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    AcroGimp's Avatar
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    OEM's usually use a single/double prime on the AL part and a fiberglass layer over the CFRP, which is also sometimes primed.

    AL to CFRP has the worst galvanic potential evaluated from a corrosion protection standpoint, it is important to deal with it.

    Personally, I'd go single-prime on the AL part and a fiberglass layer over the carbon part - and make sure you check for corrosion as part of your conditional inspection.

    For fasteners, we usually use passivated (e.g., cad plated, etc.) or corrosion resistant steels for removable fasteners (bolts/screws), and either passivated or wet install for rivets and the like.

    'Gimp (Consultant Reliability, Maintainability & Safety Engineer)
    Last edited by AcroGimp; 11-15-2013 at 08:46 PM.
    Whether you think you can, or think you can't, you're right.

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  4. #4

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    Also remember that most carbon fiber panels are encapsulated in epoxy, with additional finish coats, such as paint or just a simple clear coat.
    --
    Bob Leffler
    RV-10 Flying
    www.mykitlog.com/rleffler

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    Thanks much or the input and detail. These panels could also be made out of simple aircraft-grade plywood; do you feel that would be wiser? (We are not concerned with the higher cost or effort with carbon fiber. Our primary goal with carbon fiber would be weight savings.) However, the potential for any protective material to 'rub away' causing the two materials to come in contact and corrode aggressively over time concerns us. What are your thoughts compared to plywood?

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    Quote Originally Posted by D Egan View Post
    Thanks much or the input and detail. These panels could also be made out of simple aircraft-grade plywood; do you feel that would be wiser? (We are not concerned with the higher cost or effort with carbon fiber. Our primary goal with carbon fiber would be weight savings.) However, the potential for any protective material to 'rub away' causing the two materials to come in contact and corrode aggressively over time concerns us. What are your thoughts compared to plywood?
    Your wings could fall off in flight too, but the odds of that happening are almost nil. I would think the odds would be the same for any carbon fiber product installed properly.

    Carbon fiber is not only used for weight savings, but strength too. I have a carbon fiber panel in my RV-10 and I don't believe that plywood would be strong enough to support my panel layout unless it was 1/2" thick or more. (too many holes) I would install an aluminum panel if carbon fiber wasn't an option.

    In my particular case, the aluminum fuselage is primed AKZO epoxy primer and top coated. The carbon fiber is incapsulated in epoxy resin, then primed, and top coat applied. It would take quite a bit of rubbing to wear through all those layers with a panel that is rigidly installed to the fuselage (movement is severely restricted)

    If this really keeps you up late at night, put it on your conditional or annual inspection checklist.
    --
    Bob Leffler
    RV-10 Flying
    www.mykitlog.com/rleffler

  7. #7

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    Good points. Thanks for putting it in context. I guess as I read - the epoxy resin / fiberglass / primer is a thick barrier. Ok - thank you. We are going to build up one full section, add the resin and primer coats, and see what weight comparisons we find.

  8. #8

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    The normal aerospace method for galvanic isolation is a layer of glass on the carbon to separate from alum. If faying/contact surfaces will be rubbing, any painted coating will eventually wear thru and allow electrical contact and eventual corrosion of the alum. Preferred fastener materials are clearance fit titanium or A-286 SS which won't react. These are expensive for homebuilders but give the best results.
    You don't ever want to use any bucked rivets, no matter the material, as the radial swelling of rivet will preload fibers at hole surface and cause lower bearing/shear allowables.
    Bob H

  9. #9

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    Quote Originally Posted by rleffler View Post
    Your wings could fall off in flight too, but the odds of that happening are almost nil. I would think the odds would be the same for any carbon fiber product installed properly.

    Carbon fiber is not only used for weight savings, but strength too. I have a carbon fiber panel in my RV-10 and I don't believe that plywood would be strong enough to support my panel layout unless it was 1/2" thick or more. (too many holes) I would install an aluminum panel if carbon fiber wasn't an option.

    In my particular case, the aluminum fuselage is primed AKZO epoxy primer and top coated. The carbon fiber is incapsulated in epoxy resin, then primed, and top coat applied. It would take quite a bit of rubbing to wear through all those layers with a panel that is rigidly installed to the fuselage (movement is severely restricted)

    If this really keeps you up late at night, put it on your conditional or annual inspection checklist.

    Did you think about the fasteners going thru the carbon to secure the panel and instruments. I am sure it is touching somewhere.

  10. #10

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    Quote Originally Posted by D Egan View Post
    We understand that if carbon fiber panels come in direct contact with aluminum that more active "galvanic" material (the aluminum airframe) would suffer significant aggravated corrosion.
    Check out a galvanic corrosion chart where the potential voltage indicates how likely corrosion is to occur:

    http://www.thelen.us/1galv.php

    If aluminum is -1.670 and carbon is +.810, the potential is 2.48. The potential between aluminum and cadmium is about 1/2 that, at 1.268.

    Titanium pins and bolts are often used with aluminum in large aerospace structures. The potential there is 1.725, but 7000 series aluminum alloys are somewhat worse than 2000 series alloys. In most large aerospace structures fasteners are installet "wet" using sealant to keep electrolytes (moisture) out. BTW, monel "pop" rivets as used in some homebuilt aircraft have more galvanic potential with aluminum than titanium. Also, an "upgrade" for spam can owners is to install stainless steel screws to hold on fairings in aluminum aircraft. This is a "Bozo no no" in the aerospace industry because of the galvanic potential, yet it is commonly done in GA. A cadmium plated carbon steel screw, like came from the factory, would be much better.

    The bottom line here is that aluminum corrodes with aplomb in seaplanes exposed to even fresh water without being in contact with dissimilar materials. It depends upon your environment.

    The standard aerospace method is to add a fiberglass corrosion barrier ply to the carbon fiber panel and use a titanium screw. The aluminum would be coated with primer and topcoat.

    In real life, a carbon fiber panel in a dry interior space in a homebuilt isn't at much risk, in my opinion. A external carbon fiber fairing on an aluminum stabilizer used in a seaplane would be much more worrisome. Airliners and cargo aircraft that see lots of cycles in inhospitable weather are where most of these measures are most needed.

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