Experience with Stainless Steel in Chlorine Environments

[damorim]

Hi all,

Question for you (also posted in the structural subforum) - We installed an alternating tread stair system into a potable water reservoir two years ago. The railing on the platforms was spec'd as Type 316L SS while the alternating tread stairs were fabricated from Type 304 SS. As you can see in the photo below, both of these components are experience significant signs of rust/corrosion - especially considering they're only two years old!

Just wondering if any of you have experience in the fabrication of stainless steel components of similar size/nature. Our theory is that the materials were improperly handled and cross-contaminated with carbon steel (eg. cutting/grinding with equipment used on carbon steel, lifting with steel chains, skid steer forks, etc.). The worst areas are definitely where the SS pipes have been manipulated (at the railing corners/welds for example) which supports our theory.

I'm curious if any of you have any experience with passivation of stainless steel after fabrication? We spoke briefly with a local fabricator who suggested that for members such as these, they would have re-passivated the members by spraying them with nitric acid after fabrication... Is this standard industry practice? Ever hear of anyone doing this? Is it called up in any ASTMs or other codes?

If you can share your experience with stainless steel fabrication (specifically worked members such as these) and what the industry norms are, that would be appreciated!

The other theory is simply that SS does not fare well against high-chlorine environments - being a potable water reservoir, there is a fair amount of chlorine in the air. That said, we are well below the chlorine levels that are documented to cause a concern for SS.

Cheers,

 

[MintJulep]

http://nem.org.uk/rust2.htm

http://www.ssina.com/corrosion/igc.html

 

[EdStainless]

Nitric acid is the best method for passivation, but it can only be used on clean material.
There are ASTM specs, A380 and A967 cover methods and testing.
If you wanted to you could take these and sand blast them and then passivate.
There are also issues with the poor quality welds corroding in these.
A better solution long term would be to have these fabricated from 2205 and have them shop passivated.
You also need to be careful when installing them to prevent iron contamination.
You shouldn't have much airborne Cl in a water treatment plant unless you have problems.
I have seen issues where water with high organics is being chlorinated, and volatile chloramines are being generated, these are very aggressive.

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P.E. Metallurgy, Plymouth Tube

 

The pipe within about 1" of the welds does seem more affected.
As EdS says, these can be remediated, but I expect they would eventually end up with a similar appearance.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[damorim]

Thanks for the comments guys.

EdS - if I have to remove these now, I would just go with aluminum... We've done aluminum stairways into the sister reservoir and 20 years later they are still immaculate. As you can imagine, whatever the action is moving forward, the question is at who's expense? Design error or poor fabrication (not carried out in accordance with a standard which should have been followed)?? The literature out there is pretty split on the use of stainless in these type of environments. There was a great paper published in NACE International's Materials Performance Vol. 37, No. 11 (1998) entitled "Effect of Chlorine on Common Materials in Fresh Water". It states that "Types 304SS and 316SS [are] resistant to localized corrosion in unchlorinated and chlorinated fresh waters up to 2 ppm chlorine". As you mention EdS, we are about half of that and seeing significant corrosion... It goes on to discuss a number of other potential considerations, most of which are admittedly above my head. It describes the formation of under-deposit corrosion (UDC) and/or microbiologically influenced corrosion (MIC)in the HAZ of welds that are covered by heat-tint scale in low chloride waters in which iron manganese deposits cover the weld area. Another important consideration is that chlorine in the gaseous state appears to be quite aggressive towards SS and therefore, vapours shall be well vented. There is no specific information on what level of vapour is harmful to SS but these steps are definitely exposed to gaseous chlorine vapours from the reservoir water. They are located in the louvre house so the entire reservoir vents up the stairs.

In any case, it is not just the cost of dealing with the stairs. The entire reservoir needs to be drained to complete the work. Then disinfected and of course re-filled. Another huge in-direct cost due to this issue.

 

[EdStainless]

Better fabrication would depend on what you put on the purchase requirements, if you left it wide open then the fabricator would presume that it didn't matter to you.
This is real ugly, but is it actually causing any issues?
If you really have gaseous Cl then you have other problems.
And Al is not immune in this service either.

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P.E. Metallurgy, Plymouth Tube

 

[RolMec]

Marine environment experiences:

https://www.assda.asn.au/technical-...0-corrosion-resistance-in-marine-environments

http://www.worldstainless.org/Files/issf/non-image-files/PDF/Marineapplications.pdf

Roland Heilmann