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Two vertical American Iron and Steel Institute (AISI) 316L (Unified Numbering System [UNS] S31603) stainless steel (SST) nozzles on the head of a process vessel operating at 120°C leaked after six years in service. It has been plant practice to apply mastic reinforced with glass cloth as a sealer over insulation around nozzles on vessel heads. The cracking occurred where the fire retardant brominated mastic met the nozzles. Chloride stress corrosion cracking developed when the mastic seal with the nozzles broke, allowing water to enter and chloride to concentrate. The water source is the fire protection sprinkler system (50-ppm chloride), which deluged the vessel eight to ten times in six years. Up to 60 000-ppm chloride was detected on the fracture face and, significantly, no bromide. Tests showed that the mastic contained 363-ppm leachable chloride and 36 300-ppm leachable bromide. The remedy was to terminate insulation with a metal cover and seal the gap between cover and nozzles with caulking compound.
stainless steels, thermal insulation, fire retardants, calcium silicates, UNS S31600, chloride analysis, bromide analysis
E. I. du Pont de Nemours and Company, Inc., Beaumont, TX
E. I. du Pont de Nemours and Company, Inc., Victoria, TX