Stress-Corrosion Cracking of a Brass Tube in a Generator Air Cooler Unit

An arsenical admiralty brass (UNS C44300) finned tube in a generator air cooler unit at a hydroelectric power station failed. The unit had been in operation for approximately 49,000 h. The cooling medium for the tubes was water from a river. Air flowed over the finned exterior of the tubes, while water circulated through the tubes. Investigation (visual inspection, leak testing, history review, 100X micrographs etched in potassium dichromate, chemical analysis, and EDS and XRD analysis of internal tube deposits) supported the conclusion that the cause of the tube leaks was ammonia-induced SCC. Because the cracks initiated on the inside surfaces of the tubes and because the river water was not treated before it entered the coolers, the ammonia was likely present in the river water and probably concentrated under the internal deposits. Recommendations included either eliminating the ammonia (prohibitively expensive in cost and time) or using an alternate material (such as a 70Cu-30Ni alloy or a more expensive titanium alloy) that is resistant to ammonia corrosion as well as to chlorides and sulfur species.