Is corrosion inside tubing a concern?

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TFF

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Powder coating can be hard enough to hide a crack where as paint will show the crack. The argument goes both ways and no one side is totally better.

I personally prefer paint as long as it’s Polyfiber for 4130 or milspec equivalent. It called a primer. It is really a hard shell epoxy paint.

If you have it powder coated, you have to vet the process. Lawn furniture powder coating misses some of the higher quality prep needed for airplane parts. Rust can have fun under that perfect powder paint job. A job done right, either kind, will be fine. A bad job is still a bad job.
 

cvairwerks

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Unless you are using a power coater that does aerospace work already. you are going to run a big chance that the work will not be done correctly. Parts need to be quite clean before powder application and most shops are not going to do the necessary steps due to costs. In the case of an airframe, it actually needs to be baked first to burn off any contamination on the tubing internals. Anything that cooks off during the coating process is now contaminating the coating itself. IN the case of my fuselage, it wasn't baked first and during the baking process, quite a bit of tube oil burned off, contaminated the powder and within a year, there was rust starting to show. Additionally, the guys doing the work did not understand the time needed to actually wipe down every corner and nook to be sure it was clean for adhesion. We found places that the powder was flaking off within a few weeks of getting the fuselage back.

As mentioned before, powder can hide cracks easily. It's impossible to patch the coatings if repairs are needed too. Paint in almost any form is better in the long run, quicker to apply, doesn't require a huge curing oven and somewhat less labor intensive overall.

Everything we do since my fuselage was done, has been paint. We've even stopped powder coating at my normal work, as far as I know.
 

Lendo

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Powder coating can be very stiff and will crack when the Chrome Moly flexes and you will incur unseen rust under the coating.
George
 

akwrencher

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You also run the risk of warpage. I dont have a ton of experience with power coating, but I think I would opt for good quality paint. Also much easier to repair in the future if it gets scratched up or needs welding, etc. Just my .02
 

Rhino

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Yeah, I think I'm convinced on the paint vs. powder coating debate. I was more interested on what to do about the corrosion issue. Everyone has given me some great input on that as well, so I think I won't have any issue making the right decision there either. Thanks to all.
 

Paul Saccani

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Good info. Thanks. The linseed oil is preventative, and it's advisable to remove existing corrosion before painting. I assume that applies to corrosion inhibiting paint as well. Given the circumstances, it's probably safe to assume that surface corrosion in my case has occurred on the inside as well. So I think I may just err on the side of caution and order a new cabin frame from Zenith so I can restart the anti-corrosion process from the start.
I guess the other thing is, edition 1, 2 or 3? Changing to a 3rd edition cabin frame is a bit of a value add, compared to 1st or 2nd edition. So you can get more out of the exercise than better corrosion control.
 

Heliano

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Inner surfaces: treat with zinc phosphate (as cars are treated when manufactured), then linseed oil, and seal it 100%. Sealing is important because, as TFF said, the air inside is O2-starved, and also because it is impossible to properly inspect the inside of a tube frame, unless one has very sophisticated borescope equipment, and even so it depends on the geometry. Undetected corrosion is a major cause of unexpected cracks and fractures, especially next to welded joints.
Now a quick reminder for those who have antique aircraft:
Older Piper Colt/J3 were built with 1025 steel tubing. That kind of steel is way more subject to corrosion than 4130.
 

Rhino

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I guess the other thing is, edition 1, 2 or 3? Changing to a 3rd edition cabin frame is a bit of a value add, compared to 1st or 2nd edition. So you can get more out of the exercise than better corrosion control.
I've heard that, but I don't know what benefits it would give me or what else it might change.
 

PTAirco

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Older Piper Colt/J3 were built with 1025 steel tubing. That kind of steel is way more subject to corrosion than 4130.
I've seen more claims to the contrary; that 025 has better rust resistance then 4130. From personal experience I would say this is true; I have a lot of 1940s 1025 steel parts around my shop that show very little corrosion.

Could somebody with some knowledge on the subject chime in here?
 

Turd Ferguson

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I've seen more claims to the contrary; that 025 has better rust resistance then 4130. From personal experience I would say this is true; I have a lot of 1940s 1025 steel parts around my shop that show very little corrosion.
Could somebody with some knowledge on the subject chime in here?
Definitely have no knowledge (which is why I fly planes for a living) but I can noodle google. It looks like the 2 materials have 98% commonality in alloy composition. For all practical purposes, I would say neither has an advantage over the other in corrosion resistance.

 

Dana

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The added chromium in 4130 gives it slightly better corrosion resistance, but obviously not as much as stainless which has a lot more chromium.
 
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