STP754

    Chemical Aspects of Iodine-Induced Stress Corrosion Cracking of Zircaloys

    Published: Jan 1982


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    Abstract

    The thermodynamics of the zirconium-iodine system are summarized. In the gas phase are four stable species: ZrI4, ZrI3, ZrI2, and ZrI. Their enthalpies of formation and free-energy functions are presented. The only one of those species relevant to stress corrosion cracking (SCC) of zirconium alloys by iodine is the tetraiodide. There are also four solid compounds corresponding approximately in composition to the tetra, tri, di, and monoiodides. Ranges of solid solution composition, enthalpies of formation, and free-energy functions for those phases are given. Thermodynamic information for iodine chemisorbed on zirconium surfaces is also presented. These thermochemical results are used to analyze chemical behavior in situations related to SCC. In particular, reaction of cesium iodide with uranium dioxide in a fuel rod may produce an iodine pressure large enough to result in chemisorbtion of iodine on clean zirconium.

    Crack initiation sites in commercial Zircaloy tubing were found to be associated with impurities (iron, aluminum, silicon, and chromium) in the Zircaloy surface in microsurface examination of failures produced in iodine-induced SCC tests. It is suggested that the impurity site may react with iodine to form a locally embrittled region that fails under stress and acts as a crack initiator.

    Keywords:

    zirconium, nuclear industry, zirconium halides, iodides, thermodynamic properties, crack initiation, stress corrosion, chemisorption


    Author Information:

    Cubicciotti, D
    Project manager, program manager, and project manager, Electric Power Research Institute, Palo Alto, Calif.

    Jones, RL
    Project manager, program manager, and project manager, Electric Power Research Institute, Palo Alto, Calif.

    Syrett, BC
    Project manager, program manager, and project manager, Electric Power Research Institute, Palo Alto, Calif.


    Paper ID: STP37052S

    Committee/Subcommittee: B10.02

    DOI: 10.1520/STP37052S


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