Influence of second-phase particles on zircaloy corrosion in BWR environment

The influence of morphology, chemical composition, and crystal structure of second-phase particles in Zircaloy-2 material upon its corrosion behavior is investigated. Five different thermochemically treated Zircaloy-2 cladding tubes with known boiling water reactor (BWR) corrosion properties are examined. The characterizations of second-phase particles are carried out by scanning transmission electron microscopy, where particles in the range of 50 to 5000 A are analyzed. The morphology of larger particles is characterized by scanning electron microscopy. Chemical composition is evaluated by an energy dispersive X-ray system. Crystal structure of some of the particles is determined by the electron microdiffraction technique where the chromium-bearing particles are identified as a face center cubic (fcc) phase. It is found thats the oxidation rate (oxide thickness) is an increasing function of particle size. It is also agreed that the oxidation rate is an increasing function of the ratio of the number of particles with the hexagonal close packed (hcp) structure to the total number of chromium-bearing particles (with both hcp and fcc structures). The beta-quenched claddings with a high degree of supersaturation of low-soluble alloying elements indicates small oxide weight gains, while the nonquenched claddings with a low degree of supersaturation show large oxide weight gains