Corrosion within the laminate

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Charles
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Corrosion within the laminate

Post by Charles »

As I mentioned in the fuel probe thread, there's a small corrosion problem with my DA40 (if a corrosion problem can be small). As it turns out, it seems to be a common thing with older DA40s as 2 other DA40s around here are also affected. My plane is a 2003 with 600TT.

The problem was discovered while doing the pre-purchase inspection. I noticed a small bump on the skin of the wing around the 3 screws on the top near the fuselage. The bump is so slight that it's hard to see on a picture, so we called Diamond to discuss the issue and they sent us a picture of a more severely affected plane to compare it to. We confirmed that it was most likely the same thing. Here's the e-mail they sent:

I’ve attached an example of extreme wing corrosion. Yours does not appear to be as bad but would require scarfing back skin layers, removing the strap corrosion and treating the metal and putting carbon and fiber glass skin back in. However we cannot make a full determination of your aircraft without wing removal, close inspection from your end and submission of a damage report for engineering repair information.

Image

And here's what the repair involves (from that same plane):

Image

I got an estimate of $5k to fix this on one wing. And I have to admit that even though we found this out before I bought the plane (and the price was adjusted accordingly), it's a little scary to think that it might pop up elsewhere on the plane.

Anyone else have the same issue? Any experience in fixing this problem?

Charles
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Gary
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Re: Corrosion within the laminate

Post by Gary »

Thanks for the photos, very helpful. Was your airplane previously kept in a hangar or outside? Was is based near the coast with exposure to salt?
If you can't hangar the airplane, it might make sense to get a complete set of covers to include wing covers. I would also like to know how many other DA40s have had this problem.
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Charles
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Re: Corrosion within the laminate

Post by Charles »

Seller claims it was kept in a hangar all the time. In any case, one of the other affected planes up here has just 120TT and we know for a fact that it was always kept inside.

Again, the problem is barely noticeable at this stage, but we know we'll need to fix it eventually, and probably sooner rather than later. I might wait until the 1000hr mark when we have to take the wings apart anyway.
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Gary
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Re: Corrosion within the laminate

Post by Gary »

This information is disturbing but very valuable. The cost of repair might be lower if the corrosion can be recognized before the damage progresses. I wonder if Diamond engineers have identified the reason corrosion occurs here. Are dissimilar metals involved?
Thank you for taking the time to report this problem and post photos.
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Charles
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Re: Corrosion within the laminate

Post by Charles »

I showed the plane to a Boeing-trained composite expert at a nearby airport and he agrees that the sooner you fix it, the cheaper it's going to be. But there's no way around grinding the laminate all the way down to the bonding strip, and there's about 20 layers needed to rebuild the laminate afterward, plus painting, which may require painting the entire wing to do a perfect job. It will take time regardless.

Just to be clear, those pictures are not from my plane. The problem is very subtle (yet unmistakable) on my plane and the 2 others around here.
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Gary
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Re: Corrosion within the laminate

Post by Gary »

Charles wrote:I showed the plane to a Boeing-trained composite expert at a nearby airport and he agrees that the sooner you fix it, the cheaper it's going to be. But there's no way around grinding the laminate all the way down to the bonding strip, and there's about 20 layers needed to rebuild the laminate afterward, plus painting, which may require painting the entire wing to do a perfect job. It will take time regardless.

Just to be clear, those pictures are not from my plane. The problem is very subtle (yet unmistakable) on my plane and the 2 others around here.
I believe that this is a design flaw due to inadequate corrosion protection. Does your composite expert have any idea of the cause and what (if anything) can be done to prevent it or slow the progress?
Now I can't wait to run out to the hangar to check my airplane.
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Charles
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Re: Corrosion within the laminate

Post by Charles »

There's nothing you can do without exposing the corroded copper strip and the only way to get to it is to remove the laminate.
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Re: Corrosion within the laminate

Post by Steve »

Wow - that is pretty scary! I always wondered what those screws were for, now I know - to provide a fluid path to start corrosion. My Star is 9 years old - and luckily nothing like that is apparent. It spends most of its life in the hangar, but has been outside in the rain (and snow) on occasion.

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Re: Corrosion within the laminate

Post by Gary »

This might be caused by dissimilar metals, the steel screws and copper. I suspect that moisture has been leaking around the screws acting as an electrolyte.

http://www.corrosion-doctors.org/Forms- ... rosion.htm
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Re: Corrosion within the laminate

Post by Charles »

I wouldn't be surprised if it was galvanic corrosion, at least in part, with the conductivity exacerbated by any humidity that seeps through the screws (which we've since caulked). In addition to the copper of the bonding strip and the steel of the screws, there's an aluminium mesh throughout the laminate. And carbon is pretty conductive too. So all the ingredients are there.
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