Carbon and Al

[LAMPSguy]

Seeing a lot more Carbon parts. Isn't there a galvanic corrosion issue there?

[David Paule]

Originally Posted by LAMPSguy

Seeing a lot more Carbon parts. Isn't there a galvanic corrosion issue there?

Sure there is. But any non-conductive barrier layer between them will prevent that. The fasteners could be an issue, so the same care is needed for them.

Another issue is that carbon parts have a coefficient of thermal expansion quite close to zero. This is about as big a mismatch with aluminum as Plexiglas is, but in the other direction. This can lead to distortion or cracking.

Dave

[LAMPSguy]

So, for fasteners do you just ensure they are primed?

[BJohnson]

If the structure never gets wet, corrosion won't occur.

If you are planning of bolted joints for carbon fiber composites, there are other considerations.

Edge margin is wider (from the fastener center). Don't quote me on this but I think 4D is typical.

Composites are very weak in bolt bearing so the laminate must be thicker than what is needed for the rest of the structure.

The fiber orientation plays a large role in stresses around the hole. For metals, the stress concentration of a hole is 3 times the field stress. In composites, with the wrong layup, the stresses concentration can be as high as 6 times the field stresses. A symmetric, quasi-isotropic layup [0/45/90/-45]S is typical to keep stress concentrations down around 3.

Hole tolerance is much more critical than with metals because the material will not yield around the fastener. But the material is more difficult to drill properly, making the right drill selection and process critical.

If riveting, the swelling of the shank will load the hole in compression (good in metal, bad in composite) and can initiate a delamination.

[DanH]

Brice, please post more often.

[fixnflyguy]

I am in the heavy aircraft overhaul bussiness with a backround in corrosion prevention and dissimilar materials compatability..These two items are very galvanic if allowed to touch "uncoated". Virtually all principal structural elements of aircraft where this is accepeted, such as wing panels,cowlings ect. require the aluminum to be well prepared with primer and topcoat, and the carbon panel is primed as well. the fastener issue is critical, as aluminum rivets will eventually disintegrate, so the standard practice is most often a titanium rivet,bolt,screw or hilok, with a special coating applied (usually aluminum to protect the aluminum structure from the titanium...so as you can see, it is an issue to consider, however in our world of RV's, I believe we are not going to see these conditions, as long as the carbon part is painted, the aircraft aluminum is at least primed, and I would recomend using stainless hardware. I think the carbon fairings will retain shape better than some of the glass stuff,and I would not be afraid to use them...may even be cool clearcoated. Just take a look ata Carbon Cub cowling some time.

[Mike S]

Whatever you do with attachment, and protection of same, I would think that inspection in these areas for corrosion should be a mandatory part of your "annual".

[BGS]

How about using carbon fiber tape? Was thinking of using this on the joint between the fiberglass tips and aluminum on empennage. Maybe not now!!

[NickAir]

Corrosion w/dissimilar metals on the RV's. It is reported that Boeing uses this option.

A post from another thread has addressed this for one option seen below:


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I used light grey 2 part Teflon paint in lieu of the tape, and really like the results. Its the same paint we use on the flaps of all Boeing products where the seals rub the flap leading edges.It is available in white,grey and several other colors..I masked so it does not show when flaps retracted, and applied with a roller to the flap LE as well as the bottom of the wing skin,the same as we do on the big birds.We use Akzo Nobel paint, but others sell it also.This teflon paint is also applied to all of my fairing mating surfaces,gear leg upper and lower ends and cowl flanges. Its super easy to brush on a fresh coat at annual time and keep everything fresh, plus it doesnt peel, yellow or degrade from oil. Try it, you'll like it !

__________________
Bill E.
RV-4/N76WE
Advance NC/8A7