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 nitrogencontaining 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.

Introduction

Blue discoloration of off-white sealant in contact with copper tube at medical facilities under construction was observed. The copper tube was being installed to transport medical-grade gasses and the sealant was used as an acoustical and smoke sealant at through-wall penetrations. In some areas of one facility, galvanized steel pipes inserts were used as sleeves for the copper pipes through the drywall, while in other areas, the copper pipe penetrated directly through the drywall. Observations of the discoloration prompted an evaluation of the copper tube, sealant, and potential adverse interactions.

Laboratory evaluation of samples removed from the buildings was performed to evaluate potential interactions of sealant with copper, evaluate the condition of the copper tubes, and the composition of the sealant. Testing indicated that the discoloration occurred as a result of chemical interaction of the copper with uncured sealant applied from the tube. Furthermore, the sealant led to discoloration of the copper and buildup of corrosion products which was greater if the copper tube was sealed inside a galvanized steel pipe insert. Microscopic evaluation of the copper tubes in contact with the sealant indicated features associated with formicary corrosion and environmentally assisted cracking (EAC). Chemical analysis of the sealant indicated the presence of water-soluble formic and acetic acid salts, and their conjugate acids, which are known to cause formicary corrosion of copper. Additionally, water-soluble ammonium (in some samples) and possible amines, which, along with acetic and formic acids, are associated with EAC. As a result of the evaluation, the copper tubes were recommended to be removed and replaced prior to building completion.

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