have a steam superheater bundle made of 347 SS tubes and tubesheet. Steam on the shell side and process gas on the tube side. The tube side process gas is directly from the hydrogen plant after passing thru a steam generator so the temperature of the gas is around 900 deg F; it contains lots of hydrogen, CO, steam, etc. The bundles recently failed due to chloride SCC, that is carryover in the steam from the steam drum. This is the only superheater we have in the whole plant that is stainless steel. Question: - Why is this bundle SS and not 1 1/4 Cr. Is it the fear of caustic cracking, carbonate cracking or am I missing something else. The steam generator upsteam of this unit takes the process gas directly from the hydrogen heater and is 1 1/4 Cr but the tubesheet is refractory lined because the temps are quite a bit higher.

We too had similar problem in the Boier feed water heater in hydrogen plant where reactor effluent is in tube side and boiler feed water is at the shell side.The tube metallurgy is SS304L.The failure is mainly due to chloride concentration in Steam.Bring chloride level as below 2-3 ppm so that CLSCC can be avoided.Other wise go to duplex grade like sanicro-28. regards, krish