Tube Steel Inside Corrosion 9

[jcali]

Has anyone dealt with exterior painted structural steel tubes that have begun to corrode on the inside? I've been asked about a building in coastal NJ with exposed tube steel stair framing. It's painted outside, but it is corroding from the inside out. Is there any way to repair this?

-JCali

 

[SlideRuleEra]

In coastal SC, I have seen this happen often - things like broken boat trailers to collapsed tubular towers. The only action to "deal" with the corrosion directly, that I have been aware of, was to spray an oily mist into the tubing - if it worked at all, it was not for long.

 

[JTMcC]

Tubes should be closed (welded caps) to control corrosion. Leaving the ends(and any other openings) open is poor practice.

JTMcC.

 

[jcali]

But, even if caps are welded to the ends, isn't it possible that water vapor could still get inside? The weld caps don't make it airtight, do they? If condensation occurs inside, won't it be trapped if the ends are capped?

-JC

 

[JTMcC]

No.
My goodness yes.
Yes, and the amount will be small enough to be immaterial.

This is the standard method of solving your problem. If you work with tube, I'm suprised you're not familiar with it.

JTMcC.

 

[jcali]

Yes, we usually do use the end caps in our designs because that's the way it has "always been done". I'm just rethinking it and raising the question. Isn't that what this forum is for?

The tube structure in question isn't my design. I'm trying to help a colleague resolve a problem at his property. I don't see how adding an end cap is the standard way of solving a corrosion problem that already exists.

-JC

 

[JTMcC]

This REALLY is a common thing when using structural tube, if I took the time to look, I could refer you to many printed references to the practice.
Everybody I know that works with tube is aware of this, from structural engineers to welders helpers in the shop.

JTMcC.

 

[BFPartners]

Tube is a poor choice in situations where moisture is present and corrosion can be an issue. Usually end caps or galanization from the start are the best alternatives. One could try finding a rust sealer and may be injection spraying the inside of the tube to seal the rust (remember no air/moisture = no rust). A question would be is there a potential for injury or economical lose if collapse occurs? How much steel is required to hold the loads? Unfortunatly, unless you use a metal sonar thickness measurer these types of situations can become ticking timebombs... (ie no way of inspecting the inside of the member until the rust has created a viewport)

 

[Ron]

jcali...back to your original question...what to do. Not much. You can do as SlideRuleEra correctly suggested, but he also gave you the long term result.

At this point, you have to investigate the amount of section loss and compare it to the expected loads, and recommend replacement if you have significant capacity reduction.

You can seal up the ends at this point, but if you have a lot of corrosion product in there, it has already imbibed enough water to last the corrosion process for quite a while, even without addition oxygen or moisture. If the tubes are short (10 feet or less) you can blast clean the inside and coat them internally. Not cheap to do this, but it will prolong the life. You can also clean them, cap them, then evacuate the air from them. This helps a lot.

 

[aerodog]

This thread reminds me of an ongoing 70 year old debate within the aircraft industry of how to corrosion proof the insides of 4130 structural tube used for engine mounts and fuselage frames in tube and fabric airplanes.

One school believed in the sealed tube theory and made no effort to treat the insides of the tubes. Others would coat the insides with linseed oil before welding. Some after welding the assembly would drill small holes in the tubes, inject linseed oil, drain them and plug the holes with small rivets.

After 70 years the jury is still out because you can find examples of success and failure with each of the methods.

 

[epr]

Thanks aerodog for being the voice of reason.

This discussion was beginning to sound less than professional. Once in awhile, a question may seem outright basic, or border on stupidity based on the knowledge and experience of the person reading it. But don't forget, no one engineer is all-knowing, and everyone is subject to different experiences. This forum should be a place that an engineer, or engineer-in-training, can ask a basic question, if need be. It should also be a place where a very involved technical question can be asked, as the more opinions gathered, the better the possible solution. I have found many helpful hints, or at least some information that has pointed me in the right direction in these fora. Many engineers are working as the only engineer in a construction firm, or a manufacturing facility, and may have no peer with which to consult. I don't believe a professional should ever put down a fellow professional for asking a question. It never hurts to get a second opinion. My only warning to all (and I believe most of you already are aware of this) is that because of the variety of experience levels of the many persons in these fora, the information provided should always be verified with further document research, etc. You alone, as the professional-of-record, are liable for your final work product.

Now, back to the subject at hand . .

I believe, as aerodog does, that the jury is still out. However, there are several items to consider . . .

Large pipes and tubes, such as single-pole sign posts can be "seal" welded using E60 electrodes after the structural welding is complete. Even with this, the large interior volume is susceptible to moisture and subsequent condensation. Usually, the deterioration of these poles begins at the weld between the pole and the base plate, where the condensation usually collects.

Smaller pipes and tubes are best protected by galvanizing inside and out, followed by capping, etc. Many assemblies have small penetrations (bolt holes, etc.) where water can enter, particularly when rain is followed by a drop in temperatures, and the reduced pressure inside will suck the moisture inside. Other assemblies, that may be fully sealed, still seem to allow moisture penetration, as the welding and sealing may not be perfect. It doesn't take very much moisture to begin corrosion. If moisture can penetrate, so can the air that is needed for oxidation. Low points in the assembly are collection points for the condensation, and this is usually where the corrosion begins. Ambient temperature changes may cause the water to evaporate and condense within the assembly, exposing the steel to further oxidation potential. I have seen many welded steel tube amusement rides, railings, etc. that have corroded from the inside with no apparent means of moisture penetration.

I believe it is virtually impossible to completely seal steel pipes and tubes against corrosion from within. There is no easy answer, and in some cases, it may be more suitable to provide small weep holes at low collection points, such that any condensate can weep out.

In short, you should study each pipe and tube assembly as an individual case, and ascertain which is the most effective means of protecting against corrosion from the inside.

 

[71corvette]

I do very little work with hollow sections but...

I've heard of some contractors placing lit candles inside a hollow member prior to installing the end caps. As it was explained to me, the logic is that the caps create an airtight seal and the candle burns until all oxygen within the member has been used up. No oxygen means no corrosion, regardless of the presence of moisture.

I've never seen this done, or specified it, but thought I would throw it out there for discussion. I'm not sure what effect the additional CO2 & CO would have on the steel though.

How about adding sacrifical anodes to the structure? Although they aren't a permanent solution, they can certainly extend the life of a structure.

 

[apsix]

Document below discusses the issue at length. Conclusion is that it is not a problem, based on their research.

I certainly thought it was a reasonable question, obviously resulting in some discussion.

On a related topic, it is considered good (essential?) practice by some to ensure that hollow sections subject to freezing temperatures are drained rather than sealed to avoid bursting due to freezing of the water contained within the section. The reason given is that sealing to prevent water ingress due to condensation is extremely difficult and can not be guaranteed.

http://www.corusgroup.com/file_sour...ce_to_Corrosion_in_SHS_-_to_go_on_website.pdf

JdG

 

[pba]

Going back to the original post - The requirement is to repair an already defective hollow section (or sections).

First - verify that the existing section is strong enough in its current corroded state.
Second - attempt a fix.
Third - Check your work to ensure that it is doing the job on-site and that it will continue to do the job into the future.
Now, the second part is actually the least important! I don't have any definitive answers but suggest the following.
IF the moisture and air are getting into the section through known holes, these should be sealed either as the whole solution or following some internal treatment.
IF the moisture and air are not getting in through known holes then the welding is suspect and should be ground back and replaced...
Internal treatment of a sealed section is difficult and can involve drilling holes and pumping fluid or air through to remove debris. This can be followed by protective coatings but there is no good method of checking how effective this might be. The new coating is less likely to adhere to the corroded portions. Once this is done the section can be re-sealed - I like the candle idea!
I'd guess that the cost of the above is high. It's always worth considering replacement, either right now or at some future date when the corrosion will become more of an issue. Most metal work in coastal environments needs replacing at some stage.

 

[epr]

Treating the interior of a hollow section "after-the-fact" is indeed a matter that is very difficult to handle.

In the past, the now-outlawed oil-based "red lead" primer was a very good penetrant of existing corrosion, and provided the most suitable protection in my experience. Unfortunately, it is now unavailable.

The closest product that I have been able to find to the red lead primer that IS available is PENETROL (by The Flood Company 800-321-3444). This product is currently manufactured, and can be sprayed, poured, etc., onto and into corroded surfaces of steel, and basically encapsulates each rust partical (as long as the application rate is adequate) to prevent further corrosion, or at least place further corrosion on hold. If the surface is not subject to abrasion from wind and weather, the benefits should remain for a long time.

 

[HgTX]

Termarust (formerly Bridge-Cote) has penetrants that they claim work wonders on crevice corrosion. Deduct for the inevitable "wonderpaint" factor and it still might be useful for your application--if you can get inside the tubes.

I have no direct personal experience with the products.

Hg

 

[btrueblood]

In a similar vein, I have heard of people forming tube structures placing a "lump" of an asphalt-type compound inside. After welding was complete, the entire structure was placed in a warm oven (for paint bake out), and after warming, tumbled around as the part cooled. The idea was that the asphalt would melt, and tumbling would allow the stuff to coat internal surfaces.

 

[HgTX]

If you're gonna be able to prep the tube as a separate component, then hot-dip galvanizing would have been the way to go. I was assuming in this case that the tube can't be removed from the structure.

Hg

 

[CESSNA1]

JCALI: Another method is to seal all the openings and then presurize the inside with dry nitrogen, at the same time removing the air. Install a pressure gauge in a conveneint place to record the pressure. If it drops to near zero, find the leak, fix it and recharge the column.

Regards
Dave

 

[L775]

I usually specify that the tubes be filled solid with concrete thru a hole in the cap plate. this practice will provide higher fire rating and helps keep the moisture from building on the inside.