Effect mechanism of heat treatments on nodular corrosion resistance of Zircaloy-4

The effects of heat treatments on the nodular corrosion resistance of Zircaloy-4 have been investigated in the super-heat steam of 500 degrees C and 10.3 MPa by autoclave tests. It is found that the nodular corrosion resistances are obviously different for the specimens heat treated at 600 degrees C, 820 degrees C and 1000 degrees C, respectively. The key factors improving the nodular corrosion resistance are the supersaturated solid solution contents of Fe and Cr alloying elements in alpha-Zr matrix, but the size of the second phase particles. In order to study the relationship between the nodular corrosion resistance and the-microstructure of oxide films, the morphologies of inner surface and fracture surface of oxide films were examined by high resolution scanning electron microscopy. According to the nucleation and growth of nodule corrosive spots, the effect mechanism of heat treatments on nodular corrosion resistance in Zircaloy-4 was -discussed. It is shown that the supersaturated solid solution contents of alloying elements in a-Zr matrix could be changed by different heat treatments, and the growth of oxide films is anisotropic on the crystallographic plane with different orientations.