Corrosion Behavior of model zirconium alloys in deaerated supercritical water at 500°C

Several zirconium alloys with differing weight percentages of Cr, Fe, Cu, and Mo were exposed to flowing, pure supercritical water at 500 degrees C for up to 150 days in an effort to determine their corrosion behavior for consideration in the supercritical water reactor. The weight gains of the alloys were measured, and oxides were characterized after various times. The test results showed a wide range of corrosion behavior depending on the alloy composition and process temperature. The alloys most resistant to corrosion were those containing Cr and Fe, three of which showed protective stable oxides, low corrosion rates, and no breakaway behavior. The ZrCr, ZrCu, ZrMo, and ZrCuMo alloys all exhibited high corrosion rates and non-protective oxides. Analysis of the oxide layer showed that the oxide consisted mostly of monoclinic zirconia (ZrO2). The structure of the oxide-metal interface in the five protective alloys exhibited characteristics that were also seen in protective oxides formed at low temperature, especially the presence of a suboxide layer and an intense (002)T peak at the interface, indicating the presence of a highly oriented tetragonal phase associated with the protective oxide. The change in corrosion kinetics from cubic to linear was directly linked to the size and density of cracks in the oxides.