"316L S.S. and Stress Corrosion Cracking"

Question:

"I have a 3-phase separator on an offshore platform. The separated gas contains 8 mol% CO2; the liquid phase has 3.2 mol% CO2. The produced water will contain 10000 ppm chlorides. Operating pressure=1200 psig. Operating temperature is between 120 to 150 F. Vessel ID=84"; Wall thickness= 4". To protect against wet CO2, vessel material is specified as 316L S.S. A fabricator suggested that we use carbon steel cladded (explosion bonded) with 316L S.S. instead of an all stainless steel vessel. He says a cladded vessel will protect against wet CO2 and if chloride SCC occurs in the 316L SS cladding, the fracture will not propagate through the carbon steel layer. A failure, therefore, will not be catastrophic. I am not convinced by this argument. Is a 316L cladded C.S. vessel a better choice than an all 316L vessel? Is chloride SCC a serious problem in this application? Is there some other cladding material I can use without substantial cost increase? What other options do I have?"

Answer:

At the chloride levels and temperatures that you mentioned, chloride SCC could potentially be a problem, depending on other specifics, including design internals, welding, etc. The suggestion to use a carbon steel vessel cladded with 316LSS is an appropriate one, based on chloride SCC concerns. Any SCC cracks that develop should terminate at the carbon steel interface. Another suggestion would be to investigate use of a duplex stainless steel, such as 2205SS. It should provide resistance to CO2 and also provide resistance to chloride SCC.

 

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