NODULAR CORROSION OF ALPHA-ANNEALED ZIRCALOY-4 IN STEAM AT 500-DEGREES-C

Abstract

The early stage of nodular corrosion was investigated on alpha-annealed Zircaloy-4 in 500-degrees-C steam under various pressures. The preoxide film thicker than 1 mum, which was formed in atmospheric pressure, markedly restrained the nodule nucleation. The restraining effect was reduced when the preoxidation pressure increased. The careful observations of the initial stage of nodule nucleation revealed that accelerated corrosion appeared selectively on certain grain groups, and small oxide crystallites were formed at the metal/oxide interface underneath the selective grains and they induced the cracking of surface oxide film. The crystallites in the selective grains were enlarged rapidly after the oxide film was cracked. When the crystallites grew a certain size, the nodules were appeared on those grain groups with large crystallitic oxides at metal/oxide interfaces. Cold working increased the resistivity against the nodular corrosion and showed the maximum resistivity at 30% cold working. Conclusively, the nodular corrosion may be explainable as a locally accelerated corrosion due to the combined effect of the preoxide, the oxidation rate difference among grains, and the oxide shape.