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External PTA SCC of stainless steel tubes in low sulfur fuel gas

External PTA SCC of stainless steel tubes in low sulfur fuel gas

External PTA SCC of stainless steel tubes in low sulfur fuel gas

What is everyone's opinion of the probability of external PTA SCC of sensitized furnace tubes due to low sulfur (<10ppmw) fuel gas?  Do you take precautions like keep the fire box temperature up or soda ash the outside?

I read in refincor 93F5.7-16: HYDRODESULFURIZATION UNIT
Norm Mack (Amoco) was certain that this was polythionic acid stress corrosion cracking.  He reported on a case at CORROSION/93 of a unit with 347 SS tubes in service for 25 years.  Tubes that had been replaced eight or nine years previously first showed cracking, but they also found crack initiation on the inside of the older tubes that had not propagated through the wall.  In one case cracking occurred from the outside.  The furnace primarily used gas firing.  One section of the furnace ran fuel-rich, the other oxygen-rich.  The fuel-rich box had sulfur containing deposits.  They experienced cracking in both 347 and 321 SS.  In the 321, the entire tube was sensitized and cracking was not confined only to welds; it also occurred in the tube material.  In the 347, cracking was only found in the weld heat-affected zones.  They have been using thermally stabilized 321 and 347 since 1986 or 1987 and, when the welds are also stabilized, they have performed well without failures.  A paper was presented by Amoco at CORROSION/93 which addressed the effects of stabilizing heat treatments.  The paper is "Effects of Welding and Thermal Stabilization on the Sensitization and Polythionic Acid Stress Corrosion Cracking of Heat and Corrosion Resistant Alloys," by D. V. Beggs and R. W. Howe, CORROSION/93, paper 541.

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