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Electrochemical studies were performed in lithiated water (2.5 ppm Li and 1 ppm dissolved hydrogen) at 315°C in autoclave tests, on Zircaloy-2. Only the outside surface of the cladding was exposed to the environment. Specimens were corroded at open circuit potentials and also with externally imposed constant currents and potentials. The effects of weld regions and of a thin sputtered palladium coating on corrosion potential, the polarization behavior, and hydrogen pickup were investigated. Post-transition oxides were grown in steam at 12.5 MPa and 400°C, and electrochemical measurements were performed at 315°C in the lithiated water. The intrinsic electrochemical coupling effect, inherent in welding Zircaloy components, was also investigated in a loop test. Infrared interferometry and differential scanning calorimetry were used to measure the oxide thickness and hydrogen pickup along the specimen length.
Hydrogen pickup at the weld regions, relative to the body of the cladding, was high for specimens from both the autoclave and loop tests. Palladium considerably reduced both the corrosion potential and the hydrogen pickup. A comparison of pickups, from experiments with externally imposed cathodic polarization, identified two effects: (i) a galvanic coupling that resulted in the transfer of hydrogen evolution and, hence, pickup from the body of the cladding to the weld regions, and (ii) an ohmic contact between palladium and the corrosion film that transferred the evolution reaction to palladium. SIMS depth profiles, following incorporation and exchange of isotopes (H, Li, and B), revealed the presence of microporosity in the oxides.
Referring to the energy levels on the SHE scale, the hydrogen electrode potential in the lithiated water environment is calculated to be -0.88 V in the autoclave tests and -0.97 V in the loop test. For the reaction H+ + e = H, the favorable energy level would have a maximum value of ∼+ 0.15 V, after allowing a splitting of the energy levels on the order of ∼2.3 eV for the red-ox couple. For semiconducting (n-type) ZrO2, the conduction band edge is at +1.4 V, too much higher compared to SnO2 with the band edge at +0.1 V. A band overlap for proton reduction is unlikely on zirconium oxide, but would be favored on oxidized tin. It is concluded that hydrogen evolution during the corrosion of Zircaloys is localized at semicon ducting tin sites on the oxide surface and at the exposed alloy inside the pores. The palladium coating differentiated the semiconducting and porous properties of the oxide. Between the two processes of hydrogen evolution, the one occurring inside the pores is associated with the pickup. For bulk intermetallics to be a contributing factor to pickup, they have to be in contact with the alloy and also intersect the pores.
Zircaloy, corrosion, hydrogen pickup, reduction reaction, electron energy levels, tin in the oxide, palladium coating
ECCATEC Inc., St. Paul-lez-Durance,
CEA, Saclay, Gif-sur-Yvette,
EPRI, Palo Alto, CA