Failure of a Tank Coupling

Leakage was identified around a coupling welded into a stainless steel holding tank that stored condensate water with low impurity content. The tank and fitting were manufactured from type 304 stainless steel. The coupling joint consisted of an internal groove weld and an external fillet weld. Cracking was found to be apparent on the tank surface, adjacent to the coupling weld. Chlorine, carbon, and oxygen in addition to the base metal elements were revealed by energy-dispersive x-ray spectrometric analysis. A great number of secondary, branching cracks were evident in the weld, heat-affected zone, and base metal. The branching and transgranular cracking was found to emanate primarily from the exterior of the tank. It was concluded that the tank failed as a result of stress-corrosion cracking that initiated at the exterior surface as aqueous chlorides, especially within an acidic environment, have been shown to cause SCC in austenitic stainless steels under tensile stress.