On the Effect of Preoxidation of Nickel Alloy X-750
Nickel Alloy X-750 is a Ni-Cr–Fe alloy with good corrosion properties and high strength at elevated temperature. It is commonly used for spacer grids in Boiling Water Reactors (BWR). In this environment, the material can suffer from significant corrosion, leading to weight loss by metal dissolution. To further improve the characteristics of this material, a process called preoxidation is often performed. This results in the formation of strengthening γ’-Ni3(Ti, Al) precipitates and a thin oxide on the surface. In this paper, preoxidized and non-preoxidized specimens are compared with respect to their oxidation properties. We report about microstructural studies made on specimens exposed in simulated BWR environment for 24 h and 840 h. Electron microscopy techniques have been used to investigate the oxide microstructures. A comparison between these specimens shows the complexity of the corrosion process and the impact of preoxidation. Preoxidized specimens show thinner and more homogenous oxides than non-preoxidized ones. They lose less mass and build thinner oxides. The preoxidation layer consists of a bilayer oxide of NiFe2O4 and Cr2O3 that is preserved during the long exposure. NiFe2O4 spinel crystals are present on the surface of all exposed specimens, a result of re-precipitation of dissolved metal ions.
KeywordsHigh temperature water X-750 Corrosion NiFe2O4 Oxide formation BWR Nickel based alloys
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