Effect of fluoride contamination on the growth of ZrO 2 films

Abstract

The mechanism by which fluoride ion degrades the oxide film on Zircaloy-2 has been investigated by deliberately contaminating specimens. Delaying the washing of specimens for 0, 60 and 1800 s after pickling gave sets of, respectively, well-pickled, poorly-pickled and pickle-stained specimens. These were oxidised initially in dry steam (300°C, 3.5 MPa) and were then transferred to water (300°C) for short periods (1, 2 or 7 days). The oxides produced were examined by weight gain, interferometry, impedance spectroscopy and optical, SEM and TEM microscopy. The initial oxidation rates in steam were little different for the three groups of specimens (1 or 2 days), although the interference coloured oxides showed a very different distribution of oxide thicknesses between the well-picked specimens and the other groups. Transfer to water rapidly resulted in thick, friable, porous oxides on the pickle-stained, but not the other specimens, that could not be examined by many techniques because of ready loss of oxide. The techniques that could be applied to these specimens showed that they consisted of apparently large oxide crystallites in multiple layers nearly normal to the oxide metal interface. The original surface topography was still visible in areas where this surface had not spalled, showing that the degradation occurred within the oxide. The severity of this attack was determined by the extent to which the original preparation technique had left oxyfluoride layers on the initial surfaces. It was deduced that these oxyfluoride layers developed porosity in which concentrated fluoride solutions could form during high temperature exposures in water. These solutions attacked the ZrO 2 film by hydrothermal dissolution and recrystallisation to give the large layered platelets in the degraded films. The oxyfluorides appear to be sufficiently hygroscopic that the same degradation process occurred generally in 300°C, 3.5 MPa steam, only locally in 0.1 MPa steam and not in moist air.