Concentrated salt water solution in 316 SS reactor

[brianhamel]

We are considering running a process which generates sodium chloride in an organic reaction. To remove the salt, water is added to dissolve the salt and the salt water is decanted off - this salt water will be about 20-25% sodium chloride by mass at 70C.

There is a concern about corrosion with salt water but I only find data at low concentrations - what impact do high concentrations have on pitting and SCC in 316SS? Also, if the pH is high (around 12) how does this impact pitting and SCC?

Assuming we go ahead and run the process, how could I determine the life expectancy of my equipment and is there anything I can do to it to help prolong the life?

 

[EdStainless]

I have some limited data, and it doesn't look good.
I wouldn't use 316 at these Cl levels and temps, even at high pH.
I would suggest that you consider 2205 for the equipment. You will gain pitting resistance, CSCC resistance, and significant strength. You may be able to cut thicknesses in half, or more.

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[brianhamel]

Thanks for the information. At this point replacing the vessel is not an option.

Are there any NDT's that can be used to check the vessel for SCC periodically?

 

[metengr]

You have several options, what are the dimensions of the vessel?

 

[EdStainless]

Penetrant inspection is good option. You can use fluorecent penetrant and get various sensitivites.
You might also want to consider giving this a good cleaing and passivation.
You need to monitor temp and pH. Any spike in temp or drop in pH and you had better inspect soon.

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[brianhamel]

The reactor is 4000 gallons so about 8 feet in diameter and 9 feet on the straight side. I spoke with someone at TesTex and they suggested some sort of electronic scan of the interior surface for pitting and cracks every 6 months to see how it trends.

If a failure did occur because of SCC, would it be a simple pinhole leak that would grow over time or would a big crack develop? The process is run at atmospheric pressure.

 

[EdStainless]

What would probably happen is areas that under higher stress (welds, connections) will have hundreds of tiny cracks. If the stresses are low they won't grow very deep very fast. Once they are there crevice corrosion will take over and you will get through wall pitting.
My gut feel is that failure would take weeks.

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Rust never sleeps
Neither should your protection

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[StoneCold]

You could do the reaction in the 4000 gallon reactor and then send a percentage of the liquid over to a small glass lined reactor where you could do the water wash and phase separation. That might save your 4000 gallon reactor.

 

[brianhamel]

If failure occurs in a matter of weeks - that's gonna' be a problem.

I read where treating the stainless to minimize tensite stresses can help minimize SCC. Do you think we could shot peen the welds and connections on the inside of the vessel to reduce the residual stresses?

 

[metengr]

brianhamel;
I would agree with edstainless that you will have significant problems switching process conditions with the existing tank material.

If this vessel is exposed to only 70 deg C at atmospheric, pressure why don't you consider installing a rubber liner/mastic coating to avoid direct exposure of the stainless steel vessel to the concentrated chloride solution?

 

[EdStainless]

Heck, 70C you could use an epoxy coating.
I don't think tha tyou will have through wall failures in a couple of weeks, but hte inside surface may be crazed with fine cracks that quickly. Then crevice corrosion becomes the worry.

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Rust never sleeps
Neither should your protection

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[brianhamel]

I'm obtaining recommendations from at least 3 coating manufacturers and applicators: Belzona, Thortex, Carboline and still going. An internal coating would be our best bet provided it sticks and we don't lose too much heat transfer from the reactor jacket.

Thanks for all your help guys.

 

[gr2vessels]

Brian,

The process you have mentioned is agressive and the coating is probably not going to help, except for a short while. The pinholes in the coating will generate concentrated, extremely aggressive corrosion points, which will chew your stainless steel, literally, in weeks. By far metengr's advice is the safest option. Good luck.

Cheers,

gr2vessels