Microstructural aspects of zircaloy nodular corrosion in steam

Technical Report · Thu Jul 01 04:00:00 EDT 1999

DOI:https://doi.org/10.2172/754941· OSTI ID:754941

Taylor, D F ^[1]

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1. Lockheed Martin

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.

• Research Organization: Knolls Atomic Power Lab., Niskayuna, NY (US)
• Sponsoring Organization: US Department of Energy (US)
• DOE Contract Number: AC12-76SN00052
• OSTI ID: 754941
• Report Number(s): KAPL-P-000317; K99108
• Country of Publication: United States
• Language: English

Related Subjects

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS
36 MATERIALS SCIENCE
BWR
BWR TYPE REACTORS
CORROSION RESISTANCE
CORROSIVE EFFECTS
FUEL CANS
MICROSTRUCTURE
NODULAR CORROSION
NUCLEAR FUEL CLADDING
STEAM
ZIRCALOY
ZIRCALOY 2