A Study of the Structure and Chemistry in Zircaloy-2 and the Resulting Oxide After High Temperature Corrosion

A plate of Zircaloy-2 received in the P-quenched condition was heat treated to vary systematically the size of the second phase particles (SPPs) and the content of alloying elements retained in solid solution. Scanning electron microscopy and thermoelectric power measurements were used to quantify these microstructural parameters. Specimens were exposed for different periods of time in autoclaves at 415 degrees C and at 500 degrees C in steam, and corrosion was assessed according to the gain in weight. Effects of the microstructure were not great but it was observed that materials having larger SPPs were somewhat more resistant to oxidation in the 415 degrees C tests whereas those with smaller SPPs performed better at 500 degrees C. Influences on corrosion rates that could be attributed to alloy elements in solid solution were small or perhaps nonexistent. The chemistry of oxide layers formed at 500 degrees C has been examined by etching away the zirconium metal and then depth-profiling using sputtered neutral mass spectroscopy. There was some difference in surface topography between the materials with smaller and larger SPPs. The compositional profiles showed that the oxide was substantially under-stoichiometric below the surface in all cases, with O/Zr ratios reaching unity or even less, and extending to depths between 150 nm and 400 nm from the metal oxide interface. These results are discussed in terms of their implication for existence of a diffusional barrier layer and for the stress states that can be expected within the oxide.