MS21042-3 oxidization... replace or treat?

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pfarber

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My plane is basically bolted together with AN nuts and bolts. Specifically MS21042-3 nut... 1200 of them. They mostly appear to be covered in oxidization (aka rust) and no longer seem to have any cad-plating.

At $0.22 its $250 to replace them. Other options are treat them with a chemical solution (phosphoric acid) or sand blast (both are within my abilities) and re-install.

The AC43.13 would classify the formation of visible rust as 'light' and both mechanical and chemical removal processes are approved.

What is the general consensus?
 

pfarber

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Can’t really offer a suggestion without knowing more, but you may be able to apply a primer or corrosion inhibitor directly to the nuts, assuming that aesthetics is not a big factor.

In any event, if you haven’t already done so, refer to https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_43-4B.pdf#page25
Cad plated alloy steel is addressed on page 6-39.


BJC

6.81.2 Under no condition should such a coating be cleaned with a wire brush. If protection is needed, apply a touch-up with primer or a temporary preservative coating. Restoration of the plate coating is impracticable in the field.

By spec the nuts were cad plated. Since that is gone, the base metal has been compromised (although only on the surface).
Re-plating is not gonna happen. Seems like I'll order new nuts.
 

TFF

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If they are so rusty you want to blast them, get rid of them. If it is just light rust, I would be wiping them with some type of penetrating oil and maybe something thicker after. You just need to grade the rust. Arresting the future rust would be my goal if structurally good. Soda blasting usually does not do much for rust but might be enough. I guess you might paint them if you don’t oil them?
 

pfarber

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If they are so rusty you want to blast them, get rid of them. If it is just light rust, I would be wiping them with some type of penetrating oil and maybe something thicker after. You just need to grade the rust. Arresting the future rust would be my goal if structurally good. Soda blasting usually does not do much for rust but might be enough. I guess you might paint them if you don’t oil them?
All of the nuts are internal. So its baffeling how/why they would lose the cad plating to begin with. But it could also be that a previous version of the plans spec'd a different part than what it called for now (original part was MS21040-3 but that's been superseded by MS21042-3) The airframe is at least 10 years old using plans that are from the 80s/90s (it has fiberglass wings vs current metal wings).

For $250 it might be worth it to bite the bullet and replace all 1200 vs remove, clean, reinstall.

I just was not sure what the AC had to say... it seems the answer is 'it depends' lol
 

TFF

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Is this something like a BD4? Maybe not cad but zinc plated nuts? Is it from a place with harsh conditions?
 

cvairwerks

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They could be MS21040L3 nuts, which have a dry lube coating. Could be the coating is simply oxidizing and changing color. I'd pull a couple of the worst looking ones and get them into some good light and under a loupe if there's any question before dropping the cash and time to replace them universally. They can be found from known lots on evilbay for less money.
 
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