The effect of oxide microstructure on kinetic transition in out-of-pile steam corrosion test for Zircaloy-2 and Nb-added Zircaloy-2

in order to study the mechanism of kinetic transition of corrosion rate for zirconium alloys, oxide films formed on Zircaloy-2 (Zry-2) and Nb-added Zi rcaloy-2 (0.5Nb/Zry-2) in steam at 673 K and 10.3 MPa were examined with TE M and SIMS. Kinetic transition occurred at almost the same oxide thicknesses for both Z ry-2 and 0.5Nb/Zry-2, but the corrosion rate after the transitions were qui te different for the two alloys. Zircaloy-2 showed cyclical oxidation, whil e the weight gain of 0.5Nb/Zry-2 increased linearly. The morphology and crystal structure were similar for the oxides of the two alloys and both the oxide films still mainly consisted of columnar grains even after the transition. Interface layers which mainly consisted of alpha -Zr crystallites were observed for both alloys and the oxygen content in t he interface layers increased after the transition. The solute concentrations of Fe, Cr and Ni became higher, accompanying the increase of oxygen concentrations at columnar grain boundaries in the oxide films after the transition for 0.5Nb/Zry-2. It was thought that the proper -ties of grain boundaries of the 0.5Nb/Zry-2 oxide films changed after the transition, and the increase in oxygen diffusivity at grain boundaries caus ed the linear increase in weight gain.