Microstructural aspects of Zircaloy nodular corrosion in steam

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