Two spherical storage tanks for ammonia (volume 1000 m3) have been tested in 1998 and1999 by Non-Destructive Techniques (NDT) in accordance with the Periodic InspectionRegulations. These storage tanks have been constructed in 1979 using microalloyed steelSt.E460 according to DIN, with thickness of 30 mm. The large number of transverse cracksin weld metal and longitudinal cracks in heat-affected-zone and along fusion line of innerwelded joints has been detected. In order to estimate the residual strength, from both storagetanks, plates 500x500 mm were cut for the chemical, metallurgical and mechanicalinvestigations. These investigations encompassed detailed toughness testing, includingstandard Charpy specimens, tested on instrumented pendulum, and fracture mechanics testingon standard SENB specimens for fracture toughness evaluation. Using data obtained byCharpy and fracture mechanics testing, the structral integrity of cracked storage tanks hasbeen assessed.