Formicary Corrosion and EAC of Copper Tubes in Contact with Building Sealant

Abstract Copper tubes for carrying medical-grade gasses were installed during construction in multiple medical facilities. Where the tubes penetrated drywall separating individual rooms, the perimeter surfaces of the tubes were sealed with building sealant to suppress noise, smoke, and fire transmission. During construction, discoloration of the copper tube where sealant had been applied was observed. A laboratory investigation determined the observed discoloration was copper corrosion product. Further laboratory evaluation of the system was performed to characterize the discoloration, the underlying copper tubes, and sealant using a variety of techniques. Evaluation of the copper tubes indicated nitrogen-containing deposits on the outside diameter (OD) surface as well as features consistent with formicary corrosion and environmentally assisted cracking (EAC). While through wall-cracking or corrosion had not occurred, the investigation indicated that failures were possible if the discolored pipes were placed into service as constructed. Evaluation of the sealant indicated it contained nitrogen-containing compounds and formic acid. Certain nitrogen-containing compounds are associated with EAC of copper, and formic acid is associated with formicary corrosion of copper. The results of the investigation determined that the sealant was not compatible with the copper tubes.