Water intrusion in a sealed beam 2

[toothless48]

Hello,

I am dealing with a sealed, welded box beam (~8" x 8" x 36", 8mm wall thickness, low carbon structural steel) which acts as the backbone of a chassis frame. At some point, some small #6 tapped holes were drilled through the beam to mount a bracket. We have discovered, during a repair, that water has intruded into the cavity. We poured out about 1 liter of water. The tube has been inspected via boroscope, to reveal mild to moderate corrosion, but no significant material thickness loss. The interior surface of the beam is bare steel.

While the bulk of the water has poured out, the interior of the beam is still wet. In addition, the boroscope revealed small puddles of water due to internal welded features, which we will not be able to easily remove. The chassis frame undergoes a lot of motion, so the water will get splashed everywhere. Since the beam is effectively sealed, the water will not evaporate.

Two questions:
1. Will a wet surface, in a sealed environment, corrode significantly over time? Will the limited amount of water limit the corrosion that can take place?
2. Is there any kind of expanding foam, silica gel, or desiccant that would work well to dry the interior beam surfaces?

Thank you for your help.

Mike

EDIT: The source of the water intrusion is being corrected with Loctite, so we are just dealing with existing moisture.

 

[ornerynorsk]

A drain hole or two is always good practice. I've sprayed WD-40 into sealed cavities before to help displace moisture, using the thin straw that usually comes with the aerosol can version. WD-40 is good for very little, but that is one of its better uses.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[BridgeSmith]

I would unseal it, orient it so the opening is on the top and heat the bottom until the whole thing hits at least 250 degrees, let it cool in a dry environment before sealing it up. If you can leave a vent hole at a location where water won't get in through it, it would be best not to seal it up at all.

 

[BridgeSmith]

The "WD" in WD-40 supposedly stands for "water displacement". Don't know if that's really true, but that what I heard.

 

[SAIL3]

if it is already in place , it may be difficult to manuever the bm....is it possible to apply heat carefully using a torch that could evaporate the moisture and draw a vacuum on it to remove the saturated air...

 

[BridgeSmith]

...maybe get one of the tire shops to fill it with Nitrogen for you...

 

[dhengr]

FEAintern:
It is damn near impossible to keep something like that stl. box section dry, as much as you think you’ve seal welded it. The best solution is usually a couple drain/breather holes at low spots so it will drain and dry, on its own. Right now, the heat and a couple holes would dry it out. You’re really likely to have some mechanic mount a bracket, or some such on it, at a later date, and you are right back where you started. You’ve got a tire or something splashing water up on it, and then changes in temps. and atmospheric pressure will actually make a sealed (?) box section suck water in, under the right conditions.

 

[Tmoose]

High vacuum, like from a $100 Harbor freight Air conditioning service vacuum pump, can dry things out pretty well.

 

[toothless48]

Thanks all for your replies. This forum is a lifesaver, 10 heads are better than 1.

For our application, a vacuum pump is going to be the best solution, being non-invasive and relatively quick. This is not a road vehicle, and we need very high reliability, so removing all of the moisture will be important. We can guarantee that the beam will truly remain sealed, as our designs are tightly controlled, the added bracket was an oversight. I think we will be filling the tapped holes, and mounting the bracket in another way.

If I can ask for one more piece of input, would you expect a small pump like this to be adequate to boil moisture over an hour or so? My feeling is that it would be - 10 Pa max vacuum seems more than adequate, based on what I have read on boiling pressures.

 

[3DDave]

I suggest looking at weld studs for future mounting provisions. Take a few practice shots on some scrap to dial in the procedure; otherwise they go on faster, don't create a hole, and they can be ground off if anyone changes their mind.

 

[oldestguy]

Ther is a room here for "corrosion engineering". I'd see what they might recommend. Ask about using the treatments used by automobile undercoating specialists getting inside the body of a car.

 

[Ron]

Yes...corrosion can occur in a sealed cavity. Common is MIC (microbiologically influenced corrosion) in its anaerobic form.

 

[msquared48]

Ron's comment would also apply to the Stacking Roof deck thread too...

Mike McCann, PE, SE (WA)

 

[BUGGAR]

We treat old car gas tanks all the time. I think its Eastwood that sells kits. It cleans and coats the inside of gas tanks. You have to swirl your part around, though.

 

[BUGGAR]

I forgot to mention, if you can get some POR 15 in there, it saved the rocker panels on my old car for years. I applied it right over the rust.

 

[BUGGAR]

Also curious about your chassis. Any drawings? The length sounds short, are you using front and rear assemblies to carry the suspension?
Thanks.

 

[IFRs]

You asked if it will corrode. I'm not an expert but I was taught that steel corrodes (rusts) by combining iron Fe with oxygen O to form ferric oxide. To the extent there is oxygen inside, it will attempt to combine with free iron and rust. When it is used up it will stop. Water contains quite a bit of oxygen.

 

[toothless48]

Thanks for the additional input! A corrosion inhibitor sounds like a good idea, but we have very little access to the beam interior (without being destructive!), which is also divided by a baffle. I will probably seek the advice of a corrosion engineer moving forward.

I wish I could share more details, Buggar! Thanks for the POR 15 suggestion, it may come in handy another day! The chassis to to big to swish anything around, unfortunately.

 

[SAIL3]

maybe one could drive the vehicle aggressively to help swish the anti-corrosive liquid around....

 

[Ron]

Install a pipe plug. Insert a paint nozzle through the pipe plug hole and paint or coat the entire interior surface. Use a moist-cured urethane coating....it will deplete the moisture during the curing. Insert plug in hole and seal with teflon tape...done.