Characterization of oxide film on CANDU reactor feeder pipe steels in high temperature water

Abstract

Corrosion is a well-known concern for all nuclear plants. The wall thinning of feeder pipes observed in CANDU reactors has been attributed to "Flow-Accelerated Corrosion (FAC)". In previous work, it was shown that the stability of the oxide film is important for maintaining the integrity of the feeder pipes. This study investigated the effects of coolant velocity and exposure time on the oxide film properties. The FAC model developed from previous studies was applied to the experimental conditions of this work. There were two sets of experiments, the static experiment (0 m/s coolant velocity) and the flow experiment (5 m/s coolant velocity). Each set of experiments was conducted with different exposure times. All prepared samples were subjected to visual linspection and surface characterization techniques, Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Analysis (EDXA), and Raman Spectroscopy. Some results from previous studies at CNER were used for comparison with the present study. The results showed that there were two types of oxide particles formed on carbon steel surface - fine grain particles, mainly magnetite, and crystalline particles, magnetite (Fe3O4) or ilmenite (FeTiO3). The presence of titanium or nickel in the system can affect the formation of the oxide film. Longer exposure times resulted in a thicker oxide film. High velocity coolant can erode the oxide film formed on the surface and limit its thickness. The FAC Model developed can be applied only to the high velocity coolant system of the plant at this time.