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marcobevilacqua (Mechanical) (OP)
18 Feb 09 6:00
Hello!
I'd like to know an aeronautical process/treatment to protect from corrosion AISI 4130 tubes.
Is there anyone can help me?
Thank you
RPstress (Aerospace)
18 Feb 09 7:16
Back in the eighties standard protection for non-corrosion resisting steel was cad plate, prime + paint (typically epoxy prime polyurethane paint). Since then we've tended not to use non-corrosion resisting steel, and cad is now frowned upon environmentally. There's been some limited use of zinc plate, prime and paint.

Thermal environment is important. If it's part of an engine mount how hot will it get? Cad/zinc are a bit limited.

Have to make sure the inside is drained (a hole at least 6 mm diameter is advisable).

Make sure any fastener holes, etc., are protected, and any fastenings should be wet assembled.

If it's a part for a major like A or B (or Embraer, Bombardier, etc.) they'll have their own standard protection requirements.

4130 sees only limited use due to its corrodibility, limited ruling section (about 1/2"), relatively poor strength/weight and relatively low toughness.
 
marcobevilacqua (Mechanical) (OP)
18 Feb 09 8:31
The tubes are used for a 2 seats rotorcraft (New design CS-27) for the engine throttle control. There aren't thermal problems since the controls are protected by a firewall.
Do you suggest an alternative material?
Thanks a lot
smile
 
RPstress (Aerospace)
18 Feb 09 9:47
If fact, this sounds like a reasonable application for a corrodible steel. Steel concentrates the material in a thinner tube than Al, and this gives an increase in structural efficiency (local (cylinder) instability is very unlikely to be a worry). Theoretically a higher modulus carbon part could probably be a bit lighter but is unlikely to be cost-effective. For a non-underfloor cockpit application corrosion is most unlikely to be a problem, so there's no real point in making it stainless (you lose out a bit on cost and toughness with (e.g.) 17-4 or similar).

Depends on what's driving the sizing. If it's beam-column static stability/strength then low alloy steel is a pretty good choice, as is a round tube. If there's a bigg-ish influence of damage tolerance (tension with the presence of cracks) then a tougher alloy (like 4340) would save some weight compared with 4130 at the same strength.

Also depends a bit if it's desirable for the part to be welded. This would make quite a nice little strut and would count against higher carbon steels like 4x35 and 4x40.

An awful lot of the cost and weight will likely be in the end-fittings/connections (depends on overall length). It's there that Al alloy might show a benefit versus steel.
 
bf109g (Aerospace)
18 Feb 09 10:18
I've seen sealed control rods made from 4130, with good epoxy/polyurethane external corrosion protection.  These are generally tubes with tapped barrels or threaded fittings welded to each end which accept rod end bearings.  This creates a sealed tube assembly.

In a few cases, I've seen these types of rods have a small hole drilled into them and then a small amount of preservative injected into the tube.  The hole is then closed of with either a rosette weld or some other type of plug.  
RPstress (Aerospace)
19 Feb 09 9:48
bf109g: so these tubes were not internally protected?

We used some goop based on lanolin for some 15-5 struts' insides once.
 
thruthefence (Aerospace)
19 Feb 09 10:39
They used to use linseed oil. Surely there is something better these days?

Cosmolene?
johnSchwaner (Aerospace)
19 Feb 09 11:10
Use boiled linseed oil as it has passed the time test.

They tried something better on the Fokker F27 which resulted in the following statement from the Airworthiness Directive:

"The interior of the tubes and end-fittings of the engine mount frames have been preserved with a film of preservation oil. Premature degradation of this synthetic preservation oil is considered to be the cause of the corrosion. "

see the following link for a discussion on using boiled linseed oil:

http://www.mechanicsupport.com/aircraft_tube.html
bf109g (Aerospace)
19 Feb 09 14:56
RP - it depended on the application of the tube as to whether or not it had preservative applied to the interior.

Linseed oil would work.  Have also used Val Oil cut with  Stoddard solvent.  Normal handling during production would completely coat the inside of the tube.  Verified by sawing several units in half.
berkshire (Aeronautics)
20 Feb 09 2:38
Here is a proprietary tube preservative used for the inside of 4130 fuselage tubes : http://www.aircraftspruce.com/catalog/cspages/tubeseal.php

B.E.

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