Microstructural aspects of zircaloy nodular corrosion in steam

Zircaloy-2 becomes susceptible to nodular corrosion in high-temperature, high-pressure steam when the total solute concentration of the {beta}-stabilizing alloying elements Fe, Ni and Cr in the {alpha}-zirconium matrix falls below a critical value C{sub c} that is characteristic of the test conditions. C{sub c} for typical commercial Zircaloy-2 in a 24hr/510 C/10.4MPa steam-test is the precipitate-free a-matrix concentration in equilibrium with solute-saturated {beta} phase at about 840 C, the corresponding critical temperature T{sub c}.Thus, immunity to nodular corrosion is a metastable condition for {alpha}-Zircaloy that requires fast cooling from above T{sub c} to achieve adequate solute concentration throughout the matrix. Annealing Zircaloy at any temperature below T{sub c} for a sufficiently long time makes it susceptible to nodular corrosion. In the ({alpha}+{chi}) phase field, where {chi} collectively designates the Fe-, Cr-, and Ni-containing precipitate phases, lowering the solute concentration to less than C{sub c} by Ostwald ripening can require many hundreds of hours. Above about 825 C, the temperature of the ({alpha}+{chi})/({alpha}+{beta}+{chi}) transus, solute-saturated {beta} phase surrounds each precipitate and a strong inverse activity gradient promotes equilibration with the much lower solute concentration in the {alpha} matrix. Sensitization to nodular corrosion occurs most rapidly at about 835 C between the ({alpha}+{chi})/({alpha}+{beta}+{chi}) transus and T{sub c}. Annealing Zircaloy at temperatures above T{sub c} for a sufficiently long time will raise the solute concentration above C{sub c} and, with rapid cooling, heal any degree of susceptibility. Annealing within the protective coarsening window between T{sub c} and about 850 C, the temperature of the ({alpha}+{beta}+{chi})/({alpha}+{beta}) transus, achieves rapid precipitate growth in a matrix immune to nodular corrosion