Nodular corrosion resistance of zircaloy-2 in relation to second-phase particle distribution

A large number of Zircaloy-2 standard tube lots and tube lots beta-quenched in intermediate size were corrosion tested in steam at 490 to 5150C. In addition, specimens heat treated in the temperature range 675 to 8500C were corrosion tested in steam at 5000C. To study the correlation between nodular corrosion susceptibility and type and size distribution of second phases, specimens from one beta-quenched and two standard tube lots, one with high resistance and one with low resistance to nodular corrosion, were examined by optical and electron microscopy. In addition, specimens from the heat treatment program were studied in order to investigate heat treatment effects. For quantitative analysis, a JEOL 200 CX scanning transmission electron microscope (STEM) equipped with a Link Systems energy dispersive X-ray spectrometer (EDX) was used. The STEM studies showed that two main types of second phases exist irrespective of heat treatment. These are hexagonal Zr(Cr,Fe)2 and tetragonal Zr2(Ni,Fe). Of the different microstructural features studied, the size distribution of particles was found to be the main factor distinguishing the different materials. In the low susceptibility standard tube lot, the particles were smaller than in the high susceptibility lot. The smallest particles were found in the beta-quenched lot, where no particles coarser than about 0.2 μm could be seen