Water intrusion in a sealed beam 1

[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.

 

[Compositepro]

It is not actually the water that causes corrosion, it is the oxygen dissolved in the water. So if the cavity is tightly sealed the oxygen will get consumed and limit corrosion. I would not bet on that, though.

Pulling a very good vacuum will cause the water to boil off. Doing this on a warm day helps a lot.

Any closed cavity that is not perfectly sealed will "breathe". That is, temperature changes cause the air to expand and contract, moving in and out of the cavity. Often this results in condensation in the cavity during cold times, which, as you noted, will not evaporate, so water accumulates. Having weep holes at all low points will let this water drain out freely.

 

[TugboatEng]

You're right about slower corrosion with a lack of oxygen. The nozzles on tugboats get pinholes and cracks all of the time. We drain the water, repair the leak, and return to service. Once sealed the corrosion just about stops. I should test the air for O2 content next time we open one up.

 

[Compositepro]

Linseed oil is an excellent anti-rust preservative for steel. It is what alkyd (oil-based) paints are made from. It reacts with oxygen to crosslink, and in thin films will create a tough plastic coating. This reaction is catalyzed by iron, so steel surfaces are perfect. This reaction will consume the oxygen in sealed cavities and the bulk oil will become a gummy mess, but with no oxygen there will be little corrosion.
Cast iron cookware is seasoned in this way, using vegetable oil to make a non-stick, non-rusting coating, which can be easily replenished, as needed.

 

[TugboatEng]

I would hesitate to put oil inside of a structural member in any industry that may not have a well established hot work approval procedure.

 

[EdStainless]

There are commercial vapor phase corrosion inhibitors that are commonly used in closed (or nearly closed) spaces.
Contact someone like Cortec for some options.

http://www.cortecvci.com/index2.php

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P.E. Metallurgy, Plymouth Tube

 

[Dhurjati Sen]

Dear FEAintern,

The idea of putting silica gel is quite revolutionary, but in your case would serve the purpose quite well.

FYI, the corrosion reaction could be 2H[sub]2[/sub]O + O[sub]2[/sub] + 4e[sup]-[/sup] = 4(OH)[sup]-[/sup]

So, removing moisture is as effective as removing oxygen (in your case).

However, the catch is something else. How tight is loctite?

Regards.

DHURJATI SEN