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Thread: Corrosion treatment , Intergranular exfoliation

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    Corrosion treatment , Intergranular exfoliation

    Hi Guys, have some serious corrosion on the spar capstrips of our wildcat. Some of this will have to be replaced with new parts, but some sections are small, and would be best treated in situe, bearing in mind that the aircraft is for ststic display when finished. We thought of cleaning all the loose stuff and painting on a coat of epoxy resin??
    Any thoughts?

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    lower spar capstrip.jpgPicture as described above.

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    If corrosion products are not completely removed, a coat of epoxy will just make it worse. Because the moisture remains trapped under the coating. The corrosion powders absorb moisture and it just festers and never dries.

    A corrosion control penetrating fluid might work for this ( static display).

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    Hi Bill, do you mean an oil based penetrating fluid, or something like alocrome 1200

    Jim.

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    EAA Staff / Moderator Zack Baughman's Avatar
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    Jim,
    I passed your question along to two of my friends who are in the warbird restoration business. Without actually seeing it in person, here is what they advise:

    "Nothing wrong with that as long as it will not fly. Treating the area with Alodine prior to painting will help as well. Noting original material thickness, and amount of material remaining will let you know if you left enough thickness to support the remaining structure of the airplane. This only works as long as it's the powdery form of corrosion and hasn't gone too deep. If it's the delaminating, exfoliation type of corrosion, then I don't think it would help too much. Also, there should be some permanent record made noting unairworthy repairs were made for static display purposes."

    Another person you might want to try contacting for advice is Chris Knapp, who is the head conservator at the Imperial War Museum. Unfortunately I do not have his email address, but you can probably connect with him here: http://www.iwm.org.uk/connect/contact-us

    Zack
    Zack

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    Quote Originally Posted by Jim.UAS View Post
    Hi Bill, do you mean an oil based penetrating fluid, or something like alocrome 1200

    Jim.
    Something like ACF50 or similar. I don't think alodine (chromic acid) will help unless all corrosion is first removed to bare metal. I suppose its even possible that acid in the remaining powder might eat the metal over time. Alodine must be washed off after conversion (a few minutes) not left to dry. Alodine over powder is not something I would do. The corrosion should be scrubbed off with phosphoric etch first, then alodine may help. If it is intragranular, then the part is destroyed and nothing will help.
    Is this indoor storage or outdoor?

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    Thanks Zack, and for the contact, will give that a try for future reference.

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    Bill, I all indoor storage, thanks for your advice.
    Jim.

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    Jim-
    section 7 of AC 43.13 has info about corrosion treatment if you are interested.

  10. #10

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    Quote Originally Posted by Jim.UAS View Post
    Hi Guys, have some serious corrosion on the spar capstrips of our wildcat. Some of this will have to be replaced with new parts, but some sections are small, and would be best treated in situe, bearing in mind that the aircraft is for ststic display when finished. We thought of cleaning all the loose stuff and painting on a coat of epoxy resin??
    Any thoughts?

    Your plane has the cancer and it is what it is. I would just clean off what you can with a stainless steel brush and paint it with primer. There is little to nothing you can do about it. I have found corrosion like this on commercial aircraft in overhaul and sometimes you would see a little bitty spot and then I would start digging with a scribe and the damaged area would become very large. Paint can hide alot and that type of corrosion can work itself between sheets of material and follows along the grain of the aluminum. From the surface it would apear to be undamaged. Commonly, I would have to go in there with a die grinder and chop off sections ( stringers, skin, ribs, Spars ) untill you could get a visual on a cross section that is absent of corrosion. I usally check with a 3 power magnifing glass and then I take another 1/2 inch just to be sure. I have found repairs where material was replaced the the repairer didn't remove enough material and the corrosion continued and grew. It is just like cancer in people , if you don't get it all it will come back. Anything you put on top of corrosion like that is just going to mask the symtoms however it will keep on working. Also, Etch and Alodine is a bad choice as well because you won't be able to wash it completely out and you will only add moisture the problem.

    There is nothing you can do to stop it other than removal. The best you can hope for with the primer is to slow it down. Anyway, rule of thumb for airworthiness is if you grind away more than 3% of the original thinkness of the part in question the part is reject and is in need of replacement.
    Last edited by RV8505; 02-14-2013 at 01:56 PM.

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