On the susceptibility to stress corrosion cracking of Zircaloy in an iodine containing environment

The role of iodine and zirconium iodide in the process of stress corrosion cracking (SCC) of Zircaloy is studied by an internal pressurization SCC test and independent corrosion and creep tests, using iodine and zirconium tetraiodide crystals as the attacking species. Measurement of the critical values of iodine potential reveals that irradiated fuel rods should possess enough iodine potential to induce SCC failure of Zircaloy cladding. The difference between tests with I 2 and ZrI 4, and the morphology of microcraks at the site of corrosion pits suggest that the quantity of free iodine may play an important role in the crack initiation stage. Although the thick surface oxide can protect Zircaloy from iodine attack, it will rupture under sustaining a tensile stress, and then the iodine-bearing carrier will penetrate through the cracks to supply the necessary iodine potential and to proceed the propagation.