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Thread: Carbon Fiber Panels - Touching Aluminum Frame

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  1. #1

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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.

  3. #3
    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 07:46 PM.
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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?

  6. #6

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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.
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    Bob Leffler
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  7. #7

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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.

  8. #8

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    Quote Originally Posted by c322348 View Post
    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.
    Good post! I don't know why anyone would vary from the plans to add carbon to a aluminum aircraft as the benefits would be negligible and it would just introduce another pontenal problem down the road. I just had to retake a corrosion control class at work and aluminum and carbon are about as far apart on the chart as you can get.

  9. #9

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    Part of the answer to your question is that there are a lot of compound curved components on aluminum airplanes that get done in composite these days. The list includes wing tips, cowlings, spinner assemblies, canopy structure, landing gear doors, wheel pants, etc. All are much easier to construct using composites vs traditional materiels. And since weight control is a primary focus of the design build process, many of these parts and others can in fact be built lighter using carbon.

    An example is the Extra series of airplanes that are designed so that the entire top of the fuselage is composite and may be removed in two pieces for inspection and maintenance of the inside of the airplane. This would really not be practical in aluminum and even if done would be heavier than the composite used. And being able lean over the side of the airplane into a large open space to inspect and lube rather than crawl into the tail cone after removing the pilot seat obviously makes service easier which means better inspection and service. So looking at what you might call a holistic view of the aircraft design, working around the potential for corrosion that comes with carbon is an overall win.

    Best of luck,

    Wes
    N78PS

  10. #10

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    Quote Originally Posted by WLIU View Post
    Part of the answer to your question is that there are a lot of compound curved components on aluminum airplanes that get done in composite these days. The list includes wing tips, cowlings, spinner assemblies, canopy structure, landing gear doors, wheel pants, etc. All are much easier to construct using composites vs traditional materiels. And since weight control is a primary focus of the design build process, many of these parts and others can in fact be built lighter using carbon.

    An example is the Extra series of airplanes that are designed so that the entire top of the fuselage is composite and may be removed in two pieces for inspection and maintenance of the inside of the airplane. This would really not be practical in aluminum and even if done would be heavier than the composite used. And being able lean over the side of the airplane into a large open space to inspect and lube rather than crawl into the tail cone after removing the pilot seat obviously makes service easier which means better inspection and service. So looking at what you might call a holistic view of the aircraft design, working around the potential for corrosion that comes with carbon is an overall win.

    Best of luck,

    Wes
    N78PS

    I understand all of that but some builders want to use carbon for the cool factor for a straight sheet panel. I just don't uderstand why thy would want to deviate from the plans and add a future potential problem.. It is hard enough treating corrosion as it is. I worked for a major airline in the composite shop and now I treat and repair corrosion on navy aircraft and it is bad enough treating aluminum on aluminum. Once stared it is the gift that keeps on giving. There some very informed people posting on here with very good advice.
    Last edited by RV8505; 11-24-2013 at 06:01 PM.

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