Corrosion..

Discussion in 'Maintenance Bay' started by Tom-D, Apr 22, 2012.

  1. DouglasBader

    DouglasBader Line Up and Wait

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    Tom,

    I understand corrosion very well, being a long time career mechanic and inspector, as well as former director of maintenance (several times).

    Intergranular is internal to the metal, but is often found having surfaced, and may appear to be minor surface corrosion. Sometimes it's like chasing a gopher hole; a small amount on the surface becomes veins or areas beneath the surface, and an attempt to remove corrosion quickly leads to discovery of a very unairworthy panel or part.
     
  2. Tom-D

    Tom-D Touchdown! Greaser!

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    What you say it true, but the difference between intergrainular and any other is where it starts. Only intergrainular starts internally and breaks out to the surface. all other types of corrosion starts on the surface and eats away the metal and may cause the alclad to exfoliate. That doesn't mean it is intergrainular.
     
    Last edited: May 4, 2012
  3. RotorAndWing

    RotorAndWing Final Approach

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    I thought N2801C was a 1954 Model? Also, are you sure you actually replaced any formers or fuselage skins? :dunno:
     
  4. Tom-D

    Tom-D Touchdown! Greaser!

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    how many 170s have I owned?

    Proving once again you make conclusions based upon what you think you read. not what was actually said.

    If you really were a FAA employee you could look that info up.
     
    Last edited: May 4, 2012
  5. kontiki

    kontiki Line Up and Wait

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    Photo 3 looks like it's it's got a hole corroded all the way through!
     
  6. RotorAndWing

    RotorAndWing Final Approach

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    You owned 2623V (1948) and 2801C (1954).
     
  7. Tom-D

    Tom-D Touchdown! Greaser!

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    And..

    I have mentioned both of them here, but not the others.

    Feeble at best.
     
    Last edited: May 4, 2012
  8. Tom-D

    Tom-D Touchdown! Greaser!

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    We will know in the next couple days, he says he has it nearly cleaned up.
     
  9. DouglasBader

    DouglasBader Line Up and Wait

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    No, it means it's intergranualar when you begin treating the surface, remove material and discover that the entire piece is infected with intergranular corrosion, which is often how it's found.
     
  10. Tom-D

    Tom-D Touchdown! Greaser!

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    That's not what it says in the AC 43,13 :


    6-17. INTERGRANULAR CORROSION.
    Inter-granular corrosion is an attack on the grain boundaries of a metal. A highly magni- fied cross section of any commercial alloy shows the granular structure of the metal. It consists of quantities of individual grains, and each of these tiny grains has a clearly defined boundary which chemically differs from the metal within the grain. The grain boundary and the grain center can react with each other as anode and cathode when in contact with an electrolyte.
    (See figure 6-9.)
    Rapid selective corrosion of the grain boundaries can occur. High-strength aluminum alloys such as 2014 and 7075 are more susceptible to inter-granular corrosion if they have been improperly heat-treated and then exposed to a corrosive environment.

    6-18. EXFOLIATION CORROSION. Ex- foliation corrosion is an advanced form of in- ter-granular corrosion and shows itself by lift- ing up the surface grains of a metal by the force of expanding corrosion products occur- ring at the grain boundaries just below the sur- face. (See figure 6-10.) It is visible evidence of inter-granular corrosion and is most often seen on extruded sections where grain thick- ness are usually less than in rolled forms.It is visible evidence of inter-granular corrosion and is most often seen on extruded sections where grain thickness are usually less than in rolled forms.
     
    Last edited: May 4, 2012
  11. DouglasBader

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    No one is arguing that, Tom.

    Often it goes undiscovered until it surfaces, unless the part, panel, or component is subject to an inspection procedure that might otherwise identify that corrosion.

    Not uncommonly, intergranular goes undiscovered until it surfaces somewhere. By then, it's far too late.
     
  12. Tom-D

    Tom-D Touchdown! Greaser!

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    The argument was "what is Intergrainular corrosion."

    I hope every one now knows it starts from with in the metal, it is the only type that does.
     
  13. DouglasBader

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    There was no argument, Tom. I never disagreed with that. I think we're all familiar with the concept of intergranular corrosion.

    What some may not be familiar with is what it does, how it is and isn't detected, and where it can be found.
     
  14. Tom-D

    Tom-D Touchdown! Greaser!

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    the AC 43,13 pretty much tells it all.

    Intergranular is seldom found in the Cessna and pipers, Direct chemical attack and filliform yeah they all have it.

    I don't believe we fully understand what each other writes.
     
  15. kontiki

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    Assuming is something common and easy to work with (like 2024-T3), you could drill it off. Make a forming block out of wood, get some dead soft stock and form a replacement. Heat treating it might be an issue, I don't know where I'd look for a heat treat oven on a GA field. Given it's a Cessna it's probably not real thick sheet Metal. It would come out of the oven a little twisted, but usually you can pull it into place when you shoot it back in.
     
  16. Henning

    Henning Ejection Handle Pulled

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    Thanks Tom, excellent reference; been a long time since I looked at that. So any alloy family is susceptible but it is the heat of the grade that makes for Intergranular activity. The effect itself is electrolytic because aluminum is a poorly behaved metal; it is easy to work with though. If you heat incorrectly you set up a potential charge for a battery within the grain structure of the alloy just waiting for some electrolyte to turn it loose. This ends up with a fizzing of the area on the edge of each grain.

    It's the exfoliation which is an advanced feature of the reaction brought about by the pressure of the forging process. 50,000 tons of weight bearing down.:eek: That's how they achieved it, just stacks of mass, Iron for the most part. From the looks of the one I attached it appears as a steam actuated, cam driven, mechanically raised structure and stack of weights.
     
  17. N801BH

    N801BH Touchdown! Greaser!

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    I can just picture a OSHA sign attached to it with the warning that says "Pinching Hazzard".. ;)
     
  18. Cap'n Jack

    Cap'n Jack En-Route

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    Company that owns it now claims it's hydraulic.

    http://www.wymangordon-grafton.com/forging-capabilities.html
     
  19. Tom-D

    Tom-D Touchdown! Greaser!

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    I finally got to see the cleaned up cabin former today. Basically no serious damage, primed it and that's all it required.

    We installed new BAS shoulder harness and sent him home to install his new head liner.

    I didn't get pictures, forgot the camera.
     
  20. Henning

    Henning Ejection Handle Pulled

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    Question in my mind is are the hydraulics openning the press or closing it. It would seem to me using them to open the press an let the weight forge would do a better job. Neat stuff though, big old machines still knocking out parts.
     
  21. Piloto

    Piloto Line Up and Wait

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    Glad to know that it turn out a minor issue, like I suspected. What baffle me is that with your vast expertise on the subject you needed to consult the forum. But sometimes getting a second opinion is a good idea.

    José
     
  22. Tom-D

    Tom-D Touchdown! Greaser!

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    Where wasn't a "NEED"