Microstructural aspects of Zircaloy nodular corrosion in steam

Abstract

Zircaloy-2 becomes susceptible to nodular corrosion in high-temperature, high-pressuresteam when the total solute concentration of the~-stabilizing alloying elements Fe, Ni and Cr in the a-zirconium matrix falls below a critical value CCthat is characteristic of the test conditions. CCfor typical commercial Zircaloy-2 in a 24hr / 5 10"C / 10.4MPa steam-test is the precipitate-free a-matrix concentration in equilibrium with solutesaturated f) phase at about 840"C, the corresponding critical temperature TC.Thus, immunity to nodular corrosion is a metastable condition for a-Zircaloy that requires fast cooling from above TCto achieve adequate solute concentration throughout the matrix. Annealing Zircaloy at any temperature below T, for a sufficiently long time makes it susceptible to nodular corrosion. In the (a+%) phase field, where x collectively designates the Fe-, Cr-, and Ni-containing precipitate phases, lowering the solute concentration to less than CCby Ostwald ripening can require many hundreds of hours. Above about 825"C, the temperature of the (a+~)/(a+~+~) transus, solute-saturated~phase surrounds each precipitate and a strong "inverse" activity gradient promotes equilibration with the much lower solute concentration in the a matrix. Sensitization to nodular corrosion occurs most rapidly at about 835°C between the (a+~)/(a+~+~) transus and TC. Annealing Zircaloy at temperatures above T, for a sufficiently long time will raise the solute concentration above CCand, with rapid cooling, heal any degree of susceptibility. Annealing within the "protective coarsening window" between T, and about 850°C, the temperature of the (a+~+~)/(a+~) tra.nsus, achieves rapid precipitate growth in a matrix immune to nodular co~osion.