Microstructural aspects of Zircaloy nodular corrosion in steam

Zircaloy-2 becomes susceptible to nodular corrosion in high-temperature, hi gh-pressure steam when the total solute concentration of the beta-stabilizi ng alloying elements Fe, Ni and Cr in the alpha-zirconium matrix falls belo w a critical value C-c that is characteristic of the test conditions. C-c f or typical commercial Zircaloy-2 in a 24 h/510 degrees C/10.4 MPa steam-tes t is the precipitate-free alpha-matrix concentration in equilibrium with so lute-saturated beta phase at about 840 degrees C, the corresponding critica l temperature T-c. Thus, immunity to nodular corrosion is a metastable cond ition for alpha-Zircaloy that requires fast cooling from above T-c to achie ve adequate solute concentration throughout the matrix. Annealing Zircaloy at any temperature below T-c for a sufficiently long time makes it suscepti ble to nodular corrosion, in the (alpha + chi) phase field, where chi colle ctively designates the Fe-, Cr- and Ni-containing precipitate phases, lower ing the solute concentration to less than C-c by Ostwald ripening can requi re many hundreds of hours. Above about 825 degrees C, the temperature of th e (alpha + chi)/(alpha + beta + chi) transus, solute-saturated beta phase s urrounds 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 degre es C between the (alpha + chi)/(alpha + beta +chi) transus and T-c. Anneali ng Zircaloy at temperatures above T-c for a sufficiently long time will rai se the solute concentration above C-c, and with rapid cooling, heal any deg ree of susceptibility. Annealing within the 'protective coarsening window' between T-c and about 850 degrees C, the temperature of the (alpha + beta chi)/(alpha + beta) transus, achieves rapid precipitate growth in a matrix immune to nodular corrosion. (C) 2000 Elsevier Science B.V. All rights res erved.