| The effect of β-heat treatment on the nodular corrosion behavior of Zircaloy-4 alloy was investigated from autoclave tests at 500℃ under 1500psi for the specimens ,β-treated at 1050℃ and cooled in various cooling processes such as liced brine, water, oil, air and furnace cooling. It was observed that the corrosion resistance of β-treated Zircaloy-4 specimens increased with increase in cooling rate, showing the worst corrosion resistance in the furnace-cooled specimen resulting in nodular corrosion. It was also found that the size of nodule on the surface of specimens increased with increase in exposure time in the autoclave, but the number of nodule remained unchanged. From the SEM observation in microstructure of oxide, precipitate and alloy elements, it was also observed that the corrosion resistance decreased with increase in the size of precipitate, and the corrosion of furnace-cooled specimens progressed mostly in the interior of grains where Fe and Cr alloying elements were largely depleted during the cooling process. However, the grain boundary showed relatively high corrosion resistance due to higher concentration of alloying elements. From combining the present results with other works, it is suggested that the nodules nucleate in the local region where some of alloying elements are depleted. |
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