Corrosion Protection Method for Welded Steel Tube Structures 1

[ModmanPME]

We have some welded steel tube structures that have been in use for several years. Currently we call out hot linseed oil treatment for these structures (weld, drill small hole, inject oil, drain, plug holes). In the past we have not had any issues with this method. However, we have had several vendors come back and no-bid new versions of these items due to the linseed oil treatment. Is there a process spec that replaces this method?

 

[berkshire]

There was an earlier discussion on this subject thread2-237859


From that link
Boiled linseed oil has been the gold standard for preserving the inside of tubes for decades see link :

http://www.mechanicsupport.com/aircraft_tube.html

There is a mil spec for a preservative oil MIL- L- 21260 but as far as I know this is just a mineral oil. There is a company that adds a vapor phase preservative to MIL-L-21260 they are somewhere in Texas, whether this is experimental or a production item I do not know.
However Consolidated aircraft coatings formerly Randolph and Stits do make a tube oil pointed to in the above Eng tips link.
I have done repairs to 70 year old welded tubular airframes preserved with linseed oil and found them clean as a whistle inside.
I would not give up what you are doing.
B.E.

You are judged not by what you know, but by what you can do.

 

[Sparweb]

It's been a long time since I discussed this with anyone, but IIRC the point that person wanted to impress upon me at that time was that BOILED Linseed oil is not the same as linseed oil that hasn't been boiled. The details escape me, but presumably the boiling removed any water or broke down some other product in suspension in the oil, leaving just the oil after boiling. Which then makes the ideal coating for tubes as you use it.

You could find a definitive answer by contacting members of a local EAA (Experimental Aircraft Association) and talking to the old-timers that welded tubular airframes back in the day (riveted sheet metal and composites are popular today but some guys still around will know).


STF

 

[dgapilot]

From an old school tube welder, there are two theories. First is weld everthing up and don't do anything. If it is sealed it won't let moisture in. This is what Piper did. I can tell you I've repaired a lot of Piper fuselages that had corrosion from the inside work it's way to the surface. Of course this is on fuselages that are 50+ years old that spent most of it's life outside in the north-east.

The other thought is as stated previously, drill a hole and pour in boiled linseed oil, then let it drain and plug it. Here you also have 2 methods, drill every tube at both ends, pour it in one end until it comes out the other, or drill a hole in the cross tube of every cluster so pouring the linseed oil in the longeron will flow to all the tubes welded to it. The only way to really be sure with this method is to pump the oil in at the bottom and have another hole at the top and keep pumping until it comes out, then let it drain. You would be supprised how much oil a fuselage will hold, and how much will still be there after you let it drain for a couple days.

When using boiled linseed oil, you have a couple options for closing the holes. I've seen PK screws, and drive plugs, and I've also seen roset welds to plug the holes.

One of the advantages of using linseed oil is that if you do get corrosion that works it's way through a tube, the residual oil will start to leak out and you have a way to pinpoint where the corrosion is.

David

 

[berkshire]

Dave,
On the hot linseed oil issue, I should mention that it is a drying/ oxidizing oil, and if sufficient air is present it will dry like a paint film.
Having worked for a company that preserved tubular metal fuselage sections in this way, our method was to drill 1/8" holes at every crossing so oil could flow to all members, fill the entire assembly with oil, plug the fill holes, allow it to sit in one position for a day, then turn it over, allow it to sit another day. Then pull the plugs, rotate the fuselage assembly several times over a period of several hours, until we were sure all the oil was out that we could get out, then plug the holes.
Thinking about it, that may be why Modman's vendors did not want to mess with this item, because of the time involved.

I think on the Piper tube corrosion issue there were a couple of issues working there , on some models PK screws were used to attach belly skins, and wear strips, this allowed air into the structure, on the Tri pacers in particular the lower drain grommets allowed a small pocket of moisture to remain against the tube promoting corrosion, once the tube corroded through from the outside , it was Katy Bar the door. Looking with a bore scope up fuselage tubes that, have, been preserved inside, it is not uncommon to see the tube shiny bright except where a screw has penetrated the tube, then find a rust pocket 2 to 3" long, where the tube has corroded all the way through, with the rust lifting the tube oil off like paint.
B.E.

You are judged not by what you know, but by what you can do.

 

[btrueblood]

Isn't part of the idea of linseed oil that it will (via its polymerization reaction) consume residual oxygen from the air inside the tube, thus helping prevent oxidation-driven corrosion? Boiled linseed oil is just partly polymerized oil (by heating or addition of catalysts in more modern formulations) that hardens more quickly, to speed finishing time. For the purpose of corrosion inhibiting, I'd think you would not want to lose any of the polymerization ability (oxygen sucking) of the product.

 

[dgapilot]

Part of the problem with steel tube structures (and wood) is that in the early days of aviation we covered them with cotton fabric that needed to be repalace every 10 to 15 years. That gave the oportunity to inspect the underlying structure at that 10 to 15 year interval. In the mid 60s, the use of dacron (and fiberglass) was certified, and advertised as a "lifetime" fabric. This resulted in these structures being covered and not properly inspected for 30+ years. In that amount of time, the tubing can deterierate much more and in the case of wood it can be even worse. What was acceptable corrosion protection for 10 years may not be acceptable for 30 years.

Now in the case of the fuselages that Brian is talking about, the C-70 has a steel tube cabin structure coverd with fiberglass and resin. It is not a fabric airplane, but a fiberglass glider, one of the early glass high performance gliders. Knowing that the steel tubing is inaccessable inside the fiberglass sheel, extra corrosion protection was needed. If all the airplanes were done like the C-70 was, we most likely wouldn't have issues with internal corrosion in steel tube structures. Just look at the Piper and Tcraft wing strut ADs and fixes to get an idea of what is being done today.

 

[tbuelna]

If you are TIG welding steel tube structures, the current practice is to make sure the tube inner surfaces are clean and free from corrosion prior to welding. During welding, the tube inner volumes should be back-purged with an inert gas to displace any moisture or oxygen at the back side of the welds. After welding, the vent holes should be welded closed.

As long as the weld joints are sound, there should be no moisture and very little oxygen present inside the tubes. So corrosion should not be a concern. The drawback with filling the tubes with oil is that the oil will make it impossible to make weld repairs because it will contaminate the weld joints.

 

[berkshire]

tbuelna (Aerospace)
Been there done that, You almost always have to cut out a section of tube so you can get inside and clean the crud out with a rotary wire brush and some heat before you can effect the repair. I much prefer the eastern European system of pressurizing the fuselage frame with inert gas and putting a pressure guage on the aircraft that the pilots and AMTs can monitor.
B.E.

You are judged not by what you know, but by what you can do.