Mechanisms of corrosion of stellite-6 in lithiated high temperature water

Abstract The corrosion of the cobalt-based alloy Stellite-6 in lithiated high temperature water, simulating one chemistry regime of an operating pressurized water reactor (PWR) coolant circuit, has been investigated using a combination of surface analytical, microscopic and radiotracer techniques. X-ray photo-electron spectroscopy (XPS) has been employed to determine the chemical composition at the outer surface of the corrosion layer. The morphology of the corroded surface has been characterized by transmission and scanning electron microscopy. Scanning Auger microscopy (SAM) has been used to determine the spatial distribution of elements within the oxide layer and underlying alloy. Composition depth profiles have been constructed by sequential ion-sputter etching and either XPS or Auger electron surface analysis. Further details of the corrosion processes have been elucidated using ion-implanted radiotracers. For lithiated coolant under reducing conditions, the oxide film on Stellite-6 appears to grow predominantly via solid-state diffusion processes. Preferential dissolution of cobalt at the oxide-solution interface leads to a strongly enhanced chromium concentration within the oxide layer. A modest increase in the oxygen activity of the coolant can drive the corrosion mechanism into a fast dissolution-precipitation regime.

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