Tensile stress and corrosive liquid can cause corrosion cracking of copper pipe
Copper is susceptible to a failure mechanism known as stress corrosion cracking, where a combination of tensile stress and corrosive liquid (e.g. ammonia) results in intergranular cracking of the copper microstructure. Flexible copper tubing is particularly susceptible to this mechanism. When flexible tubing coiled for packaging is uncoiled for installation, permanent stresses develop. If the tubing is exposed to a corrosive environment, premature failure due to stress corrosion cracking can occur.
Flexible copper tubing conforming to ASTM B280—Standard Specification for Seamless Copper Tube for Air Conditioning and Refrigeration Field Service—is used in HVAC systems for the transfer of a refrigerant, such as glycol, between a compressor and a heat exchanger. Although chilled copper lines typically develop an external corrosion patina due to condensation in a humid environment, no significant section loss occurs. However, copper tubing in a stressed state can fail if exposed to certain environments.
The authors’ company was engaged in a project where HVAC lines in a newly built apartment complex had developed leaks. During the construction of this wood-framed building, an extreme weather event had resulted in the wetting of the building’s interior frame. In the subsequent months, mold grew on these wood surfaces, leading to the implementation of a mold-abatement program. The manufacturer’s literature indicated the mold abatement chemicals were compatible with copper, so there should have been no corrosion issues. However, when the HVAC units were activated upon completion of the building, multiple leaks were discovered.
The HVAC lines in this building consisted of an uninsulated liquid line, bundled with a foam-insulated vapor line, held together at regular intervals by adhesive tape (Photo 3). A laboratory analysis determined discrete cracks had formed in the uninsulated liquid line at locations where the tape was present (Photo 4). Metallographic and fractographic analysis confirmed the failure mechanism as stress corrosion cracking (Photo 5). The authors’ company conducted experiments to demonstrate how the exposure of the tape and foam insulation to the mold-remediation chemicals resulted in the production of ammonium ions, which generated ammonia.
The HVAC copper lines in the building were replaced. This failure demonstrated caution should be used in spraying chemicals in a building containing plumbing piping, even if the data sheets suggest the materials are compatible. Temporarily wrapping exposed piping prior to spraying chemicals would be a prudent approach.
Internal corrosion of steel sprinkler pipe
Wet fire suppression systems, consisting of steel piping conforming to ASTM A135—Standard Specification for Electric-Resistance-Welded Steel Pipe—are normally resistant to internal corrosion. This is because electrochemical corrosion reactions typically require oxygen, and dissolved oxygen in the water standing in sprinkler pipes is quickly consumed, and corrosion is suppressed. However, fire suppression piping exposed to potential freeze conditions requires the use of dry sprinkler systems, where the pipes are charged with compressed air rather than water.
Whereas wet sprinkler systems are filled with water and contain a finite amount of dissolved oxygen, dry sprinkler systems, which often contain inadvertent small quantities of water, can rapidly corrode, as there is an infinite amount of oxygen available for the electrochemical reaction. Standing water can collect in dry systems from the condensation of humid compressed air or from the accumulation of residual water from the testing of the alarm system. To combat this, fire codes such as NFPA 13—Standard for the Installation of Sprinkler Systems—typically require dry sprinkler pipes to be sloped at a pitch of at least 6.3 mm (0.25 in.) per 3 m (10 ft) of pipe length towards a drain. An experienced mechanical contractor will know to install a properly pitched dry sprinkler system.
The authors’ company investigated a project where significant internal corrosion of a dry sprinkler system developed after only three years of service (Photo 6, above). The sprinkler pipes were located in an unheated attic space, which also included ductwork for an HVAC system that was installed after the sprinkler pipes. When checking the sprinkler pipe pitch, they encountered local runs of pipe which were back-pitched, preventing water drainage and resulting in advanced corrosion. Upon further investigation, it was determined the HVAC contractor had altered the position and caused back-pitch of the sprinkler pipe at several locations, by adjusting pipe hangers or altering pipe position to accommodate duct installation (Photo 7).
The owner had the incorrectly pitched pipe adjusted and installed a nitrogen charging unit into the system. Nitrogen purges oxygen from pipes, thus preventing further corrosion. This is a good example of a contractor unwittingly affecting the performance of a system installed by another contractor with what appeared to be minor adjustments. Best practice is to make a final check of the sprinkler system after all components in the space are installed.