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

Abstract

A plate of Zircaloy-2 received in the β-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°C and at 500°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°C tests whereas those with smaller SPPs performed better at 500°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°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.

EELS and atom probe tomography study of the evolution of the metal/oxide interface during zirconium alloy oxidation

“…Whereas the usual approximation made is that all the oxygen ingress results in the formation of ZrO 2 , this is not strictly true: the region ahead of the advancing oxide front has been reported to show a complex microstructure with a variety of oxide phases exhibiting lower oxygen content than 66 at.% [13][14][15][16]. Particularly studies of the oxide-metal interface demonstrated by both electron energy loss spectroscopy (EELS) and atom probe tomography (APT) that one of the lower stoichiometry oxide phases is equiatomic suboxide ZrO [17][18][19][20], which is only present in pre-transition alloys and disappears after transition [21,22].…”

Oxidation of Zircaloy-2 studied using a wedge-shaped specimen

“…However, it is known that the initial oxide does not generally consist of stoichiometric ZrO 2 but is in fact a sub-oxide having the approximate composition of ZrO [6][7][8]. For Zircaloy materials oxidised in autoclaves, as in the present case, the thickness of the sub-oxide layer is in the range 200-300 nm.…”

“…3. This was the same material as described in a previous publication where more details were provided [8].…”

“…Several researchers using different experimental methods [6][7][8] have reported the existence of a layer of oxide with a composition close to ZrO beneath the main ZrO 2 oxide. In other cases [9] this was not observed.…”

“…The explanation for these differences [3] appears to be that the sub-oxide layer disappears immediately after each transition, when it converts to ZrO 2 , but it then re-establishes itself. The actual structure therefore varies with time but there is reason to believe that the sub-oxide layer is present in Zircaloy for the majority of the time [8] during autoclave exposure. Fig.…”