The high temperature steam (500°C, 10.3MPa) test and observation of microstructure were conducted in order to investigate the effect of the solid solutional alloying elements on the nodular corrosion resistance of Zircaloy-4.
Needle-like structure, which resulted from rapid cooling from β- and α+β-temperature range, contained martensite produced by diffusionless transformation. The content of solid solutional Fe and Cr in the matrix increases with the increasing of quenching temperature and cooling rate. A good correlation between the weight gain after the steam test and the volume fraction of precipitates (fvol) was obtained. Nodular corrosion did not occur on the heat-treated specimens, in which fvol was less than 3×10
-5.
In the β-furnace cooled specimen, nodular oxides were observed in the α-grain near the grain boundary, where the precipitation of intermetallic compound Zr(Cr, Fe)
2 was remarkable.
From these results, it is considered that nodular corrosion initiates at the solid solutional Fe and Cr depleted zone.
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