Failure Analysis of a Corroded Threaded Fitting in an Ammonia Refrigeration System

A corroded threaded fitting on a direct-expansion ammonia fan-coil unit was found to be leaking in a refrigerated food-handling warehouse. Efforts to stop the leak by tightening the threaded connection resulted in the failure of the male fitting, causing a catastrophic release of ammonia refrigerant. Subsequent analysis of the subject threaded connection found that the threads of the male carbon steel fitting were severely deteriorated by atmospheric corrosion. The installation location of the subject fan-coil unit was such that it repeatedly experienced significant condensation and frost accumulation and, therefore, needed to be regularly defrosted. It was this combination of repeated condensation/frost accumulation and a cyclic freeze/thaw process which resulted in the accelerated atmospheric corrosion of the subject threaded connection.