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 515°C. In addition, specimens heat treated in the temperature range 675 to 850°C were corrosion tested in steam at 500°C. 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. The coarsest particles, about 1 µm, were observed after annealing at 750°C, which was the temperature showing a dramatic weight gain peak in 500°C steam testing.

It is evident that material with coarse second-phase particles is particularly susceptible to nodular corrosion. The STEM observations support the idea that there is a critical particle size for nucleation of a nodule, a size which is directly related to the test temperature. This model can explain the lot-to-lot variation observed for standard material and the improved resistance for tubing beta-quenched in intermediate size.