Microstructural study of oxide layers formed on Zircaloy-4 in autoclave and in reactor part 11: Impact of the chemical evolution of intermetallic precipitates on their zirconia environment

Oxidation behaviour of zirconium alloys and their precipitates – A mechanistic study

“…For all conditions for which suitable SEM observations of the iron oxide on the surface and of the precipitate in TEM samples are available, this estimation of the diffusion coefficient was performed ( Table 9). The obtained diffusion coefficients (Table 9) are in a similar range as the one reported by Iltis et al [70]. The zirconia formed during oxidation exhibits much higher diffusion coefficients for iron than sintered yttria stabilized zirconia (single and polycrystalline material [72][73][74]), which are in the range of 10 À21 -10 À27 cm 2 /s when extrapolated to 415°C.…”

“…Therefore it was of interest to estimate the effective transport coefficient of the iron to the surface. Studying the literature, only Iltis et al [70] have provided an estimate of iron diffusion coefficient in the zirconia layer formed during oxidation, the value was reported to be 10 À15 cm/s. Other literature sources provide only a relative ranking of the different alloying elements, but do not give diffusion coefficients [17,18,22].…”

“…To be able to compare the diffusion coefficient provided by Iltis et al [70] and others [72][73][74], an estimate for the diffusion of the iron to the surface was made using the following procedure:…”

Influence of light ion irradiation of the oxide layer on the oxidation rate of Zircaloy-4

“…This assumption may explain only a part of the high burnup acceleration of the alloy. A second assumption assigned the accelerated corrosion to the amorphisation and the dissolution of the Zr(Fe,Cr) 2 precipitates under irradiation [2,[8][9][10][11][12]. Others authors suggest that the kinetic acceleration is due to the tin distribution in the alloy in service [13][14][15].…”

Cathodoluminescence imaging of oxidised zirconium alloys

“…Bright CL emissions originate only from the areas around the Zr 3 Fe precipitates (which remain dark) and these emissions do not seem to change as the oxide thickens over time at 723 K. The absence of high CL emission areas in the pure Zr, ZrCr and ZrNi alloys indicates the low diffusion rate of Fe in the 723 K temperature range since the base Zr material contains 0.05% Fe. A published value of 190 nm for the diffusion distance of Fe was reported for an oxidation period of 41 days at 673 K [18]. Samples oxidised at 873 K all show high CL signals at the grain boundaries, indicating the diffusion of Fe to these locations (whether deliberately added or present as an impurity, Fig.…”