Chloride Stress Corrosion Cracking
Chloride Stress Corrosion Cracking
(OP)
Experts,
I am being asked by a customer to fabricate two replacement tanks out of either 304 or 316 Gr. SS for a water demineralization skid for a boiler feed water application. The original vessels are constructed of SA-516 Gr. 70 and coated inside and out with a Plasite 7122 coating. These vessels have been in service for 20+ years without failure - it appears that the coating has now begun to fail and they are experiencing internal corrosion. Design pressure will be 125psi. @ 450 deg. F.
I'm not a corrosion engineer, but it seems to me that if we replace these vessels with an austenitic stainless, it is an almost sure bet that they will soon start to experience chloride stress corrosion cracking. I want to suggest to my customer to instead allow us to fabricate two replacement vessels out of the original carbon steel material and apply the coating per the original manufacturer - 20+ years of continuous service isn't bad.
Any input would be greatly appreciated. Agree??? Disagree???
numberfive
I am being asked by a customer to fabricate two replacement tanks out of either 304 or 316 Gr. SS for a water demineralization skid for a boiler feed water application. The original vessels are constructed of SA-516 Gr. 70 and coated inside and out with a Plasite 7122 coating. These vessels have been in service for 20+ years without failure - it appears that the coating has now begun to fail and they are experiencing internal corrosion. Design pressure will be 125psi. @ 450 deg. F.
I'm not a corrosion engineer, but it seems to me that if we replace these vessels with an austenitic stainless, it is an almost sure bet that they will soon start to experience chloride stress corrosion cracking. I want to suggest to my customer to instead allow us to fabricate two replacement vessels out of the original carbon steel material and apply the coating per the original manufacturer - 20+ years of continuous service isn't bad.
Any input would be greatly appreciated. Agree??? Disagree???
numberfive





RE: Chloride Stress Corrosion Cracking
RE: Chloride Stress Corrosion Cracking
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RE: Chloride Stress Corrosion Cracking
Thanks for the quick response. I will definitely look at a cost comparison.
RE: Chloride Stress Corrosion Cracking
Perhaps all you need to do is update the coating (they have come a long way in 20 yr) and do periodic out-of-service inspections.
"If you don't have time to do the job right the first time, when are you going to find time to repair it?"
RE: Chloride Stress Corrosion Cracking
If the demin then the chloride levels should be very low.
As much as I like SS I would echo some of what has been said.
Look either at S32205 duplex stainless, or carbons steel with a modern coating inside of it.
There is no guessing which will cost less, or be easier to build. It could be a tossup.
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Plymouth Tube
RE: Chloride Stress Corrosion Cracking
RE: Chloride Stress Corrosion Cracking
If it is for raw water purification, irrespective of the design temperature, the service temperature is unlikely to be above 60 C and the pressure is likely to be modest. Accordingly your risk of chloride SCC in these tanks is likely quite low, assuming you use good fabrication practice for the 316SS tanks.
That said, it's also an easy service for lined carbon steel, which has been proven in service. Which will be cheaper? Probably the stainless, but that depends on lots of stuff you haven't mentioned.
RE: Chloride Stress Corrosion Cracking
RE: Chloride Stress Corrosion Cracking
If they are for chlorine removal then they will operate near room temp.
If you go the SS route then making them from 2205 will probably cost less than 316L because they will be ~50% of the wall thickness. Much less metal and much easier to fabricate.
You would also get better corrosion resistance and some reasonable SCC resistance.
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Plymouth Tube
RE: Chloride Stress Corrosion Cracking
Carbon is used for all purposes at a temperature as low as possible, because low temperatures increase the sorption capacity of everything that you'd want to adsorb on carbon.
Just in case some are confused, CHLORINE and CHLORIDE are NOT the same thing. Carbon removes CHLORINE, and does NOT remove CHLORIDE whatsoever. Chloride ion will elute with the water. In fact, it is likely that at least some of the chlorine in the feedwater will react with the carbon to produce chloride ion rather than organochlorine species. However, feedwater free chlorine levels are really low- less than 1 ppm- much smaller than the amount of chloride ion naturally found in most feedwaters. The chlorine levels are low enough to not represent much of a worry from a stress corrosion cracking perspective in the raw water itself (eg the OP's feedwater carbon tanks). That chlorine CAN generate a worrisome amount of chloride though if the downstream process used can concentrate that tiny amount of chloride many-fold over time - e.g. boiling with inadequate blowdown- hence the need for removal.
RE: Chloride Stress Corrosion Cracking
Seems like that would save some oney - at the expense of field work working inside a tank.
RE: Chloride Stress Corrosion Cracking
I discussed the stated concerns with my customer - he much appreciated the insight. We are going to quote two new coated carbon steel vessels and two duplex stainless vessels as another alternative - as always the final decision will come down to $$$.
Thanks again!