spar corrosion?

[Jerrytex]

So working on my new found project.... I cleaned off tons of dirt from the quick built wings. They were hanging in a barn and have had years of accumulated dust on them. I used maroon scotch brite pads and a stiff brush so as not scratch the spars. So after taking a close look at the spars, I noticed, where the dirt was, there is staining of the aluminum. I can pretty much scrub these areas with the scotch brite and get the stain off but I am not sure how much of the aluminum I am removing. I wanted to get opinions from those who know a lot more than me about aluminum and the integrity of these spars. After blowing or brushing the dust off, the spars felt rough. I ran the maroon scotch brite pad over these areas and it smooths it out. I can't catch any of these stained areas with a finger nail and I don't see any pitting. So...thoughts? I was thinking about expoxy priming them anyway.

[jiott]

In my opinion only, I think you are OK Jerry. If all stains and rough areas are gone and can't be felt with a fingernail after scotchbrite then I think you meet the criteria in the builder's manual that says no pits, dings or scratches deeper than 0.005" (definitely check this number out yourself, my memory may be wrong). It also says to rub the scotchbrite longitudinally, not circumferentially, to avoid stress risers in bending. I also doubt you could remove too much aluminum just using scotchbrite by hand; using it on a power tool would be a different story.

[jiott]

If you have scratches, pits, corrosion on the inside of the spars this is a different situation altogether because of the difficulty in cleaning them up. You will have to use your own good judgement here, and I would recommend talking it over with the factory also.

[Delta Whisky]

Jerry - It is really tough to get a good evaluation of corrosion from pictures but I'd go cautiously here because the patient is one or more spars. I'm guessing that we are looking at either pitting or intergranular corrosion and neither are good if the corrosion goes very deep into the aluminum. It would be interesting to see if you can remove any of the darker areas completely and how deep you have to go? (If bay 9 is affected, you can feel pretty safe do remove to your heart's content there - especially if on the front or back surface.) Do I remember correctly that you are in Aggie Land? I recommend that you get in touch with someone in the engineering or aero departments at one of the best engineering schools in the hill country and see if they won't take a look at them and offer a little advice.


BTW - you said you were using a brush on them but not what kind of brush. I wouldn't use anything but non-metallic, brass or aluminum.


Good luck and keep us informed as to what you find out.

[jrevens]

...
BTW - you said you were using a brush on them but not what kind of brush. I wouldn't use anything but non-metallic, brass or aluminum.


It would not be advisable to use a brass brush on the aluminum. The 2 metals are far enough apart on a galvanic series chart to be a potential problem from microscopic particles embedding in the aluminum, causing galvanic corrosion. To my knowledge, it would not be good practice.

[Jerrytex]

Thanks for the replies. I am using only a nylon brush, compressed air and the maroon scotch brite pads. I can pretty much rub all the stains out but it’s a lot of scrubbing and I am thinking it might not be necessary especially if I epoxy the spars. I am planning on running a camera inside and looking around. I can see some Dirt dobber nests but it’s not too dirty inside. I think the fact that the spars had so much dust on them, it held any moisture from condensation/temp differences. I was mainly concerned about pitting which I haven’t found any yet. Not much staining on the underside of the spars as they were hanging so the tops of the spars as they hung, caught the most dust. This kit was is a drier part of Texas so I think that helped.

[jrevens]

Jerrytex,
If they are truly just surface stains, have you tried to remove them with solvent? Beginning corrosion does not always leave discernible "pitting". Covering actual corrosion with primer would not be considered proper, especially on something like a wing spar, and it would hide and not necessarily keep the corrosion from advancing. As has already been said, I don't think you could take enough material off of the spar to be a problem by hand with a Scotchbrite pad. If the "stains" are deeper than that, they are probably corrosion. Just my opinion for whatever it's worth.

[rv9ralph]

I would recommend an inspection of the discolorations with a 10x magnification looking for pitting.

Ralph

[Jerrytex]

Hmm. I wonder what kind of solvent? I didn't want to try any kind of acid. I might just be Scrubbing....

A magnified inspection is a good idea too. I'll explore that too.

[Delta Whisky]

John Evans - you are quite correct and my comment about the use of a brass brush was bad advice. I'll use them on cleaning jobs in my automotive world but they shouldn't be used here for the very reason you stated. I shouldn't mix the two worlds.

[rv9ralph]

I recommend that you download AC 43-13.1B and review Chapter 6. It addresses recommended procedures for remediation of corrosion.
Link:
https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_43.13-1B_w-chg1.pdf

Ralph