Two allotropic forms of zirconium oxide, monoclinic and tetragonal, have been identified in the scales formed on zirconium alloys. The transition from tetragonal to monoclinic has been followed by X-ray measurements and Raman laser spectroscopy. Information on the average content of the tetragonal phase was obtained by X-ray diffraction, whereas Raman laser analyses on tapered sections revealed its distribution through the scale thickness. Oxidation exposures were made in an autoclave, using H2O18 and D2O18 to determine the overall diffusion coefficients. In particular, oxide scales have been studied on Zircaloy-4 with three different precipitate sizes, and on a Zr-1Nb alloy, after exposure in an autoclave for between 3 and 100 days. The specimens were analyzed in detail in the vicinity of the kinetics transition point, where the acceleration of corrosion occurs. Raman spectroscopy analyses enabled the crystallographic nature of the ZrO2 to be determined. Close to the interface, the tetragonal phase content is about 40%, when after the transition the tetragonal phase is transformed into monoclinic. The O18 diffusion treatment was carried out in an autoclave at 400°C under pressure on specimens previously oxidized for between 3 and 100 days in natural water vapor pressure. The diffusion profiles were determined by nuclear microanalysis using the O18(p,α)→N15 reaction. Based on these profiles, the volume and grain boundary diffusion coefficients were calculated for each material and for each oxidation time. The results show clearly that oxygen transport occurs principally by grain boundary diffusion. The boundary diffusion coefficient is 108 times higher than those for bulk diffusion (Db is equal to 10-13 to 10-15 cm2/s). The results of these analyses are in agreement with the changes in kinetics observed at the transition point. The stabilization of the tetragonal phase by stress is discussed.
In order to get a better understanding of the mechanisms governing corrosion of Zr-based alloys, several examinations have been performed on a variety of samples with uniform and nodular corrosion and different oxide layer thicknesses. The results point to a barrier layer concept. The oxide layer becomes porous at a critical thickness. Open porosity increases from 0.01% at 10 μm to 3% at 100 μm. Between the outer porous oxide and the metal, a dense interlayer exists. This is only ≤30 nm in nodular oxide but has been found to be several hundred nm in uniform post-transition oxide. The barrier layer is obviously influenced by the crystallization of the oxide at the interface. This crystallization leads either to a columnar monoclinic, an equiaxed tetragonal, or to a fine equiaxed monoclinic oxide. The latter, which probably forms only under the mineralizing effect of hydrogen, was found in nodular oxide. It easily cracks at the grain boundaries. Well developed columnar oxide is seen in uniform oxide, when corrosion resistance is high. Recrystallization seems to be responsible for the pore or microcrack formation at the transition. The intermetallic precipitates influence the corrosion behavior significantly. They probably oxidize slowly. This oxidation starts with the zirconium and is accompanied by iron diffusion into the surrounding oxide.
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