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What's inside my steel tubing?

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Anthony Goetz
Homebuilder or Craftsman
32
Posts
5
#1 Posted: 4/21/2010 00:04:09

Hello all,

I'm currently working on a project which includes a welded steel tube fuselage. I obtained the project partially built. Among the work that had been done was the construction of this fuselage. The project had obviously been sitting for some time, and the tubing had a healthy coating of rust on the surface which was pretty easily removed with some sanding.
 
However, this makes me wonder what the INSIDE of the tubing looks like. From the looks of it, the original builder had not gotten around to drilling holes and adding Tubeseal (or similar), and not all the tubing is closed off. There are a few places I could probably cut samples from to have a look, or borescope into one of the open tubes, but I suspect it will look like the outside did.

Can I do anything to protect the tubing at this point, or is the damage done and the clock ticking on the metal's integrity? Is it even worth using this fuselage? I hate to throw away a (basically) perfect fuselage, but I would also hate to scrap it a few years down the line when something breaks - assuming it doesn't do so catastrophically in flight...

 

-Tony

 



Joanne Palmer
Young Eagles Pilot or VolunteerHomebuilder or Craftsman
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#2 Posted: 4/21/2010 15:19:50

Short of drilling some holes and then borescoping, you really can't tell.  

 

However... With the outside rusty, what other attack has taken place?  You said that you sanded the rust off.  But how much metal was sanded off?   Another issue is were the welds done correctly or were they done cold and with what filler metal??  It might be more trouble, but these questions will be answered if you rebuild the fuselage structure yourself.



Anthony Goetz
Homebuilder or Craftsman
32
Posts
5
#3 Posted: 4/21/2010 21:11:51

I have yet to check the ODs of the tubing to estimate how much material is left, but that's something I plan to check. I suppose it's possible to inspect the inside of the tubes (even if they require cutting/drilling/borescoping), but the weld condition I obviously know nothing about. I'm becoming less hopeful about the prospects for this fuselage which is a shame because it's quite nicely done and it is complete. Plus, what do I do with the thing if I replace it? Minor issue, I realize...

Really, I guess it wouldn't cost much - cash wise - to build a new one, and I would get the experience of doing so myself. It's just lousy to take a step backward on the project's progress when I thought I was so far ahead.

-Tony

 



Jeff Point
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#4 Posted: 4/21/2010 22:36:51

Anthony,

The standard advice applies here- find a local EAA chapter and join it (if you haven't already.)  There should be at least one knowledgable person in the chapter who can assist you.

I doubt that removing a light coat of rust would reduce the tube diameter by a significant amount.  This is a very common thing with homebuilt projects that are built on the stop and start plan.



Anthony Goetz
Homebuilder or Craftsman
32
Posts
5
#5 Posted: 4/21/2010 22:44:25

Jeff, thanks - I couldn't imagine that either a.) I would sand off too much material or b.) the rust itself consumed that much material, but like they say, you don't know what you don't know. It would be an interesting thing to try to quantify, in any case. I do worry about that which I can't see on the inside though.

As for a local chapter, the plane is actually based at Flabob (Chapter 1) and is a Ray Stits design, so I would say it has a good home! It's a long drive from MY home, however, so I thought I would do a little research here between visits. I'm eager to get this project going.

 

-Tony



Joanne Palmer
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#6 Posted: 4/21/2010 22:57:01

Anthony:

 

It is actually a little bit more involved than just the diameter reduction.  There's also the fact that rusting is not a uniform process in a granular material such as steel.  Oxidation does not travel linearly across the face.  Some corrosive elements attack grain boundaries and sometimes this is difficult to see and stop.  And corrosion attacks heat affected zones (near the welds) at a differing rate. 

 

You also need to apply a chemical conversion coating to the finished product to prevent corrosion.  For steels this is either cadmium plating (the goldish color on aircraft bolts) or a phosphate coating which would be more appropriate in the case of a fuselage.  Then it should be painted.  So if it were me, and you seem to have some doubts already, I'd use this as a pattern to build the jigs and make a new one. 



Anthony Goetz
Homebuilder or Craftsman
32
Posts
5
#7 Posted: 4/21/2010 23:46:22 Modified: 4/21/2010 23:47:15

Hi Joanne,

So is there then a chance that corrosion could have already started working "in," whether via pitting or some other mechanism? I imagine the safe answer is "maybe." Is it possible to determine the extent of this damage by inspection and hopefully remedy it? My doubts at this point are centered around the fact that I'm a first time builder and just don't know how careful I need to be about things like this. I want to build it right, and build it well. But where to set the bar for anal retentiveness? I don't mind building a new fuselage if needed, I just hate to throw away a perfectly good one if I don't have to.

I've not yet looked too deeply into the preparations to be made for protecting the metal, but I was under the impression that e.g. an epoxy primer (in the Poly-Fiber system) alone was sufficient. A quick browse through the Poly-Fiber manual didn't find a recommendation for a conversion coating for steel while the same chapter dealing with metal prep suggests their E-2300 conversion coating for aluminum. I'm not about to attempt this without the recommendations of people much more knowledgable than I (including Poly-Fiber employees at the local chapter), but I hadn't heard of a chemical conversion coating for steel thus far.

 

-Tony

 



Joanne Palmer
Young Eagles Pilot or VolunteerHomebuilder or Craftsman
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#8 Posted: 4/22/2010 12:20:33

Tony
You're right, the answer really is "Maybe".  But the remedies aren't too intrusive.  It would have been nice if you had left the rust on, actually rather than removing it as rust can be a "conversion' coating of its own under the right circumstances.  Since it has been removed, i do have a question about the condition of the rust.  "Was it smooth(ish) like a paint coating or were there rough and "scaly" spots?  If the latter then it is good that you removed it. 

 

I would suggest that you find a person with a large aircompressor and get a small portable blast hopper gun from Harbor Friegth.  Blast it with Baking Soda only.  That will remove the rust and the blasting will get into some of the small crevises where sanding won't.  Then if you want to use it support the fuselage at the engine mount and the tail plane maounting points and proof load the structure at the wing attach points with 3 times the maximum gross weight of the aircraft.  This will take some construction af a suitable test fixture.   Whter you want to do that or rebuild but you will have at least a firm data point  of the strength of the fuselage and it will put your mind to rest.



Joanne Palmer
Young Eagles Pilot or VolunteerHomebuilder or Craftsman
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#9 Posted: 4/22/2010 12:36:09

Chemical conversion coatings for steel are usually a Manganese Phosphate which usually is applied by a hot tank dip.  But Aircraft Spruce sells this stuff which may be good enough  http://www.aircraftspruce.com/catalog/cspages/convcoat.php  but I have never used it.



Anthony Goetz
Homebuilder or Craftsman
32
Posts
5
#10 Posted: 4/22/2010 13:01:47

Joanne,

Well, for better or for worse, there is still a nice coating of rust on it since I had to set the project aside about 3 years ago. Most of the sanding work I had put into it is now covered back up. I remember the original rust being a fairly even coating, feeling like a fine grit sandpaper to the touch and sanding off easily as a powder rather than flaking. You mention that rust can - sometimes - be its own conversion coating. I would imagine that if you could put up a barrier between the steel and oxygen, the existing rust damage would already be done but you would prevent more. Does existing rust pose a threat to the underlying metal (other than what it's already attacked)? My knowledge of metallurgy is limited to a 10 week material science course I took several years ago, so I'm not sure - can rust eat its way further into the surface even without access to oxygen?

I'm not overly concerned that there has been so much corrosion that the fuselage is structurally unsound. My concern is that in 5 years, 10 years, 20 years, it will get to that point, possibly with me or someone else suspended a mile above the Earth. I don't necessarily expect this airplane to be flying in 200 years, but I would sure like to put a lot of time on it and hopefully pass it on in one piece. The engineer in me likes the idea of the proof testing, and I think that's something I'll do with whatever fuselage I use.

 

I had planned to blast the fuselage just prior to coating, but the baking soda is a new idea to me. I will definitely try that. I wasn't going to use anything too abrasive - walnut shells or similar are what I had in mind. I'll add the baking soda to the list.

 

-Tony



Joanne Palmer
Young Eagles Pilot or VolunteerHomebuilder or Craftsman
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68
#11 Posted: 4/22/2010 15:26:52

Tony

The stuff that AIrcraft Spruce sells is supposed to convert the rust to another form of Iron Oxide that is black in color.  Most likely this is FeO or Ferrous Oxide.  It will prevent rusting, but it is not absolutely a rust preventative.  It is similar to some gun bluing and the gunsmiths used to boil the parts in pure water to convert rust to blue/black oxide.  that is probably too cumbersome to do for a fuselage so the chemical process is used.  After you get the conversion coating on the steel, painting with a primer is still mandatory to preclude rusting.  I recommend one of the PTI primers that Spruce also sells. 

 

 

 

 



Anthony Goetz
Homebuilder or Craftsman
32
Posts
5
#12 Posted: 4/23/2010 00:10:32

Joanne, you've definitely given me some food for thought and some new subjects to read up on. I think I'll spend some time doing just that. Thank you for all the input!

 

-Tony

 



Bill Berson
Homebuilder or Craftsman
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#13 Posted: 4/26/2010 19:34:03

 Some Ospho (phosphoric acid steel treatment) on a rag will remove light surface rust nicely. And the Ospho treatment is a good prep for steel as well. My hardware store sells Ospho.



A Ormsby
Homebuilder or Craftsman
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#14 Posted: 5/31/2010 00:26:28

There is nothing lost in doing a proof test. You would proof test if you built a new one. You should do a die-penetrant inspection of the welds. The kit is cheap. If at all possible contact the former owner and try to determine the weld composition. If it was TIG welded there is less choice in filler rod and it might be easy to qualify the welds. If acetylene welded then there is a wide range of filler materials and a good chance of a wrong application. I’m more worried about the integrity of the welds than the rust. They need to be inspected by an expert. Welding 4130 steel has to be annealed. If the integrity of the welds can’t be established throw the thing away. Or cut out sections and use it for various stands or garage racks. Maybe even a tow bar :-)

To further alleviate suspicion for internal rust you should do ultrasonic testing. They are accurate to 0.001”. You could rent or borrow a unit from a local automotive machine shop. They are fairly expensive but with the amount of use you want to put it to, you might consider buying a unit. All automotive paint stores sell a product called “metal-prep”. A generic name for phosphoric acid plus other constituents that removes rust and converts the steel for primer. All the major brands have their own version. I would cut open closed sections and wash the insides of all tubes as well. The clean wash after the metal prep will neutralize any residuals. Obviously, your major focus must be on complete drying after the wash. Then follow up immediately with coating the insides. Good Luck 
goggles