Failure Analysis of Stainless Steel Repair Clamps Used in the City of Boise Geothermal System
The city of Boise, Idaho is home to the nation’s largest and oldest direct-use geothermal system. The system provides heat to Boise State University, local businesses, government and residential spaces from a natural, renewable source. Stainless steel repair clamps are utilized throughout the underground network of pipes for sealing off leaks and joining new connections. On occasion these clamps fracture and fail prematurely requiring expensive excavation and repairs. Sometimes failure has occurred in as little as 2 years in service. The goal of this research was to determine the cause of failure of the repair clamps and provide a recommendation to mitigate future failures. Various testing and characterization of the failed repair clamps was conducted including mechanical tensile tests to determine any degradation of the material properties and imaging with optical and scanning electron microscopy to observe the microstructure and failure mode. Based on the results of the testing conducted and the known environmental conditions, it was found that the cause of failure was stress corrosion cracking. Stress corrosion cracking is a synergistic degradation mode in which both the applied stress and corrosion damage on the clamp in combination cause a much greater detrimental effect on the repair clamp material than either one acting alone. To avoid similar failures in the future it is recommended that a different repair clamp material be utilized that is more resistant to stress corrosion cracking, such as duplex stainless steel.