Effect of a pre-oxide on the high temperature steam oxidation of Zircaloy-4 and M5Framatome alloys

Breakaway oxidation of zirconium alloys exposed to steam around 1000 °C

“…Optical microscopy observations of cross-sections of samples oxidized 5000 and 15000 s at this temperature confirmed that the oxide thickness was uniform on average (however, there were local variations in oxide thickness since the metal-oxide interface appeared scalloped), similar at the inner and outer surfaces and consistent with the measured weigh gains. These results showed longer oxidation times to breakaway oxidation than those previously reported [5] [23], which were about 5000 s at 1000°C (weight gain of 8-9 mg/cm 2 ). While the oxidation tests were done using the same facility, there are some differences, such as one-side oxidization as done in the tests presented in [5][23] versus two-side oxidation in the present study.…”

“…Oxidation at HT was carried out by using the DEZIROX 1 facility [5][6][27][22] [23]. For oneside oxidation, the sample hanged from an alumina holder via a hole in the sample upper endcap.…”

“…When the thickness of the oxide layer was expected to be low, a nickel coating was applied before imbedding and polishing the sample on its inner and outer surfaces in order to stabilize the oxide layer and minimize the end effects during polishing. As already done in [6], [23] and [27], Scanning Electron Microscopy (SEM) (mainly in back scattered electron mode) and a specific image analysis procedure were used to quantify the fractions of αZr(O) and prior-βZr across the cladding wall thickness. In the following, the so-called αZr(O) phase layer corresponds to the "continuous" αZr(O) phase layer containing more than 90% of αZr(O).…”

“…Within the temperature range investigated, the pre-breakaway oxidation is slower for M5Framatome than for Zircaloy-4. It must be recalled that the oxidation kinetics of the two alloys are very close to one another above 1100°C [5][6] [23].…”

“…Furthermore, during in-service operation in PWR, an oxide layer grows on the cladding outer surface due to corrosion by the water environment (155 bar, ~300-340°C), and a fraction of the hydrogen released is absorbed by the cladding. It was obseved for low tin Zircaloy-4 and M5Framatome that the occurrence of breakaway oxidation was not significantly modified by pre-hydriding up to 600 weight parts per million (wppm) [5] or by 10-13 µm-thick pre-oxides formed under water at 340-360°C [23].…”