Sulfuric Acid Leak onto Galvanized Structural Steel

[albanyYEM]

I'm a structural engineer who designed a galvanized structural steel tower (exterior) to support a storage tank a year ago for an industrial process client. They contacted me recently and said there had been a limited leak of 6% sulfuric acid from the tank equipment onto some of the galvanized steel structure. The galvanized steel has apparently darkened in some areas and they are wondering if the structure has been compromised. Have not seen it yet with my own eyes. My understanding is that the leak has been corrected and this is not an ongoing exposure. Not being an expert in chemistry or chemical corrosion, does this sound like a particularly bad corrosion issue? Mild? Minor? I'd appreciate any comments on this. Thanks.

 

[EdStainless]

It depends.
First the Zn is gone, that is for sure.
If the acid just ran down some steel then there may not be an issue.
But it is more likely that there were placed that it could puddle, and crevices or joints that it ran into.
Those locations could be in serious trouble.
They probably won't fail tomorrow, but they sure could see greatly accelerated corrosion going forward.
And if acid managed to run down to where the steel in contact with concrete then both the steel and concrete may be compromised there.

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P.E. Metallurgy, Plymouth Tube

 

[bimr]

As EdStainless posted, the degree of corrosion will depend on the duration of time that the metal was exposed.

 

[albanyYEM]

I'm heading to the site next week and will try to post a photo of the condition.

Would any potential corrosion damage generally be visually apparent or are there underlying concerns that wouldn't be visually apparent?
Other than atmostpheric exposure to the elements, would there be concern of ongoing corrosion from the acid after it has been removed/cleaned?
If the steel seems in good shape, would it be reasonable/possible to simply recoat with a zinc-rich or high performance coating?

 

[EdStainless]

You want to take a small bottle of high purity (deionized) water and some Litmus paper with you.
That way if there are joints you can wet them and check the pH.
My hunch is that if there are joints that have seen acid they will need to disassembled, neutralized, flushed, dried, re-coated, and then put back together.
Typically people would power wash with soda water to neutralize.
Coating with a zinc rich primer and sealing with the appropriate epoxy or urethane top coat should work fine.

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P.E. Metallurgy, Plymouth Tube

 

[albanyYEM]

Here are some photos from my site visit today. You can see in the first photo what appear to be streaks where the zinc galvanizing is no longer present. The streaks look consistent with the dripping leak that was fixed as well as the direction of the prevailing wind. All things considered, the steel superstructure looked pretty good to my eyes and I did not see any immediate structural concerns. I plan to advise further cleaning, disassembling affected joints, and re-coating either in place or prior to re-assembly. My client contact stated that they had neutralized the acid with baking soda.

The concrete base is another matter. It looks quite discolored as you can see from the second photo. Is this a case of just waiting for the concrete to physically deteriorate, removing these areas, and then patching over and over?

Thanks again for your comments as they are helpful.

 

[cap1a79]

I think (from my personal aspect though) the photos do not show any serious corrosion damages.
If there is corrosion from the sulfuric acid, at first, there will be actual thickness reduction and the reduction(or pits corroded) will result in visible mechanical symptoms such as bending or cracking.
If the clients are really concerned about it, then advise them to remove all the scales (by the reaction between the acid and the steel). Paint can be applied after confirming the remaining thickness(which is the one scale removed) whether it can withstand all the loads for the steel. (This would be based on kind of quantative approach.)





Lee SiHyoung,
Engineer - Material & Corrosion Specialist
WorleyParsons Oman Engineering,
cap1a79@gmail.com

 

[BlackJackJacques]

The exposed steel is an easy fix as suggested here by others. As you have seemed to guess, the concrete is indeed another matter. There is steel rebar in the concrete and the acid will work on steel and concrete. Spalling, flaking of the concrete will be the first signs of corrosion colocating with telegraphing of the iron oxide (rust) of the rebar to the concrete surface. It is possible that the acid will compromise the footers supporting the beams. The strength of the acid will be a factor as well as how much acid has permeated the concrete. A baking soda solution wash to saturate the concrete and ground can't hurt to neutralize the acid and check further acid action, but you will need to keep an eye on the entire base.