"Stress Corrosion Cracking in Stainless Steel"
"We have experienced repeated failures on seal flush piping on the naphtha and distillate reflux pumps in our Crude Fractionation Unit. The piping is currently constructed of 316L tubing. The process stream in low in organic chlorides, but high in hydrogen sulphide. Inspection of the failures(cracks) suggests stress corrosion cracking; likely sulphide induced. I am considering replacing the stainless steel piping with either carbon steel, or 5% chromium 1/2 molybdenum. Do you have any thoughts or suggestions?I was not aware that h3S increases the susceptibility of austenitic stainless steels to chloride induced stress corrosion cracking. This relates to another persistent problem that we have experienced; cracking of 347 valves in hydrotreating service. The valves that fail are typically small diameter, A182 TP347 forged steel valves. The service conditions are about 800°F and 2500psig. The fluid in the piping circuits is heavy oil; high in sulfur, hydrogen and hydrogen sulphide. We currently neutralize the piping circuits during turnarounds using a soda ash/sodium nitrite wash as per NACE recomendations. This procedure was developed to prevent polythionic acid attack on the stainless steels when the piping is exposed to oxygen. Although this does not specifically address chloride contamination problems, it does help to flush contaminants high in chlorides from the system. It also leaves a thin protective layer of crystalline soda ash/sodium nitrite on the piping which helps to limit oxygen exposure to the piping. The reactor circuits(feed and effluent) in our plant have been constructed with A297 HF Modified piping(cast and machined 347SS). The smaller diameter piping is typically A312 TP347 with A182 F347 fittings. We have seen chloride induced stress corrosion cracking in valves, forged fittings and butt-welded connections. The cracking of small diameter forged valve bodies(drains and vents) has been the most common failure. Do you have any suggestions that may help to eliminate the problems that we are experiencing?"
You are probably correct with respect to chloride stress corrosion cracking being responsible for failure of the 316 piping. Hydrogen sulfide significantly decreases the threshold quantities of chlorides need to promote chloride SCC. (A laboratory analysis would easily verify your theory of chloride SCC). The question of replacement metallurgy depends on the nature of your process stream. The proper selection of carbon steel or a chromium-molybdenum low alloy steel depends several factors, including amounts of hydrogen sulfide, sulfur and temperature. The McConomy curves are a widely used reference for materials selection in h3S environments. If the cracking has been correctly diagnosed as chloride stress corrosion cracking, and the problem is confined to small drain, flush fittings, etc. you might consider upgrading those specific components to an alloy not susceptible to chloride SCC while still maintaining resistance to polythionic acid SCC, i.e., alloy 825 or something similar..
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