Stainless Steel Exposed to Free Chlorine Gas

[Rabbit12]

Have a project where we are having issues with corrosion of stainless steel. The issue is occurring in the head space of a mostly buried clearwell. The top slab is exposed and maybe a foot or so of the walls. Overall depth is 30' or so.

We have generally used 316 stainless steel in these scenarios with good success. This site is different in that we see fairly severe corrosion of any stainless steel that is above the water line. I've attached a picture. This particular element went from brand new to this in 6-8 months or so. We are currently working on installing a new chlorine analyzer to confirm the concentration of free chlorine. A previous analyzer measured 2-3 ppm, but we aren't sure of the accuracy of these measurements. We believe the temperature to be in the mid 70's F in the summer months.

I'm working to identify alternate materials to test in the environment to replace the existing 316 SST. I'm thinking a 2205 duplex could be a good option, but I'd like other options as well. Can anyone provide recommendations alloys to test or lead me to a document where I may be able to get some information on how different alloys perform in the presence of free chlorine gas?

 

[EdStainless]

The concentration isn't that important. The metal will be damp with condensation.
Any (and nearly all) of the free Cl will dissolve in the condensation making the concentration anything up to saturation.
Are there any organics in the water?
If so then what you are really dealing with is attack from the volatile chloramines.
I would guess that this is the case since 316 does fairly well plain in chlorinated water.
I don't believe that 2205 will give you much better corrosion resistance.
Though it would give you a lot better cracking resistance.
Have you ever looked into FRP?

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P.E. Metallurgy, consulting work welcomed

 

[Rabbit12]

Ed, thanks for the response. I'm guessing we have some level of TOC in the water where we are having this issue.

If indeed we are dealing with volatile chloramines is there a stainless steel or some other metal that can withstand that? Titanium has been thrown out too.

FRP is an option we have discussed but we have certain elements like anchors and the rail in the picture I attached that really can't be FRP.

 

[EdStainless]

Ti will work great but remember that it has a very low modulus.
Alloys like 800 and 600 would work as well, at a price.
Ni alloy mounts for FRP hardware might be good option.

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P.E. Metallurgy, consulting work welcomed

 

[Rabbit12]

Ed, when you say Ni alloys would you be referring to like a Hastelloy C type alloy?

 

[EdStainless]

Actually 800 and 600 work well in Cl environments.
If you had lots of other contamination (acids and salts) then a C alloy would make sense.
No harm in using C if that is what hardware is available in.

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P.E. Metallurgy, consulting work welcomed

 

[ShakeU]

Do not use Ti unless the gas stream is fully saturated. You will have TiCl4 formation and roast everything to the ground.

 

[0707]

this maybe useful for your choice

https://nickelinstitute.org/media/8...loricacid-hydrogenchloride-chlorine_10020.pdf