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.

 

[drbrainsol]

This is an interesting problem and one I had to research before. I would recommend you have a read through Michel Tournay's report titled Internal Resistance to Corrosion in Steel Hollow Sections (CIDECT 10 B 78/3). You can find the report on google.

In short, internal corrosion of hollow sections is negligible even if not sealed properly. In addition to moisture, corrosion also needs quite a lot of oxigen. As long as the sections look good on the outside, the interior shouldn't be a problem. I wouldn't bother with a corrosion inhibitor but if drying the inside of the hollow sections is possible you should do it for peace of mind.

 

[BUGGAR]

Another vote for oxygen deprivation: Many corrosion engineers recommend that sprinkler systems not be flushed because the "stale" water has used up the oxygen and rust stops. POR 15 is supposed to be air tight to stop rust.

 

[toothless48]

drbrainsol, thanks for the source.

Does anyone have experience with vacuum drying? I am wondering if ice formation will slow us down. I am also somewhat concerned about the ice expanding in any crevices in the beam.

 

[BridgeSmith]

It would seem that how much effort you need to put into this depends on how much additional corrosion is tolerable. If no additional corrosion is tolerable, you will need to ensure it gets dry and probably coat the inside. If some additional very minor section loss would not put it at risk of failure, then adding some small drain holes and/or vent holes in shielded locations would likely be sufficient.

Perhaps something as simple as This could work for you.

 

[Compositepro]

Vacuum drying rate will first be limited by vacuum pump capacity, which is in liters per minute. So if the vacuum drops from 10 Torr to 1` Torr, then 1 liter of vapor will only have 10% of the mass.

Then heat transfer will become a limit. Water can only vaporize if heat is absorbed. So if the pressure is reduced to where the boiling point of water is reduced to, say 10C, while ambient is 20C, you have a temperature difference of only 10C to drive boiling.

Then there is liquid surface area. Boiling will generally only occur at the liquid surface because pressure increases with depth in the liquid. In crevices and low spots water can pool with little exposed surface.

Vacuum drying can be very effective but it can take along time. Ice expansion is not a real concern.

 

[btrueblood]

You don't have to have boiling to evaporate water, but yeah, heat helps. Heat the outside of the beam with a hot air gun, torch, electric blanket.

 

[Compositepro]

Evaporation and boiling are two different and distinct processes. Evaporation requires the flow of another gas, such as dry air or N2 to sweep away the water vapor above the liquid surface. This is very ineffective in crevices or dead ends. If the water vapor is moving away from the liquid surface due to a vacuum pump, then the water is boiling.

 

[berkshire]

If you can rotate the chassis, you might look at the method that has been used to preserve the insides of steel tube aircraft fuselages, a drying oil that can be injected into the cavity.

https://www.aircraftspruce.com/catalog/cspages/tubeseal.php

B.E.

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

 

[charliealphabravo]

OT but Aircraft Spruce is a great site. I find some unique stuff there. Here is a 3" carbon fiber digital caliper I carry to every job. Only like $15 on that site.