"Cracking of 304L SS"

Question:

"We have product storage tanks that in-breathe air. The vapors generated in the tanks contain small amounts of nitrogen dioxide, carbon dioxide, water, and succinic anhydride. The tanks are under a slight vacuum and operate at 130 C. The material of construction is 304L SS. The tank nozzles exposed to the vapor space crack at the welds in a month or so after repair. The bottom nozzles, exposed to only liquid are fine. The cracks propogate from pits on the inside to the outside. Succinic anhydride crystals grow out through the cracks despite the tank being under a slight vacuum. We can detect no chlorides in the liquid feed to the tanks. We are located about 30 miles from the Gulf of Mexico in south central Texas. What components besides chlorides can cause the pitting and cracking? Is Teflon(R) the only suitable material of construction where nitrogen dixoide and whatever is causing the pitting and cracking are present?"

Answer:

Your problem with the pitting and cracking of vapor space 304SS storage tank nozzles calls for a formal failure analysis. However, pitting of 304SS is typically caused by chlorides. They may be entering the tank from chlorides in the air when the tank breathes. You may want to have a nozzle analyzed for the cracking morphology and to check for chlorides. Stress corrosion cracking (SCC) of 304SS from chlorides is readily apparent. I would determine the source of the problem before considering alternate materials.

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