Stainless Storage Tanks - Chloride SCC Potential 1

[gearteeth]

I have some atmospheric storage tanks constructed of solid 316L stainless steel, that contain spent caustic produced as a by-product of refinery processes. They operate at ambient temperatures, 100F is the hottest temperature seen by temperature indicators. pH is in the range of 13-13.5. Sampling shows chloride concentrations to be around ~1000 ppm as measured by chlora analyzer following a recent process change. O2 ingress into the tank is a given, no nitrogen blanket. H2S is present as well.

I’m concerned about the potential for chloride stress corrosion cracking. Industry documentation reports that temperature and pH are in our favor, but both come with caveats. There have been instances of cracking below 140F, and even though the bulk environment is alkaline, local environments may be different particularly within equipment as large as a tank.

I’ve been reading MTI publication on SCC, Nickel institute document on material selection for caustic service, and API publications. Is there anything out there that can help me better understand our risk? Any inspection recommendations?

 

[SJones]

Try working through an API RP 581 analysis to put the risk in context. NACE Corrosion 2007, Paper 07480 also has some useful information to assist.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[EdStainless]

The lower temp CSCC cases that I know of involved chemicals other than simple chlorides, such as chloramines.
What concerns me about your application is what other chemicals may be present.
If this was pure caustic with 1000ppm Cl I wouldn't worry at all.

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P.E. Metallurgy, Plymouth Tube

 

[gearteeth]

Sjones, I had worked through the 581 assessment but some of the literature I was reading gave me pause. It may be that simple. I will take a look at the NACE paper.

EdStainless, The caustic is used in petroleum refinery applications to remove sulfur species like H2S, mercaptans, disulfides, etc. I'm chasing sample data on the fresh and spent caustic, may be more to come there. Let me know if you have any specific recommendations on testing or information on low temperature cases.