Soft X-ray study on electronic structure of passive oxide layer of carbon steel and low alloy steel in flow-accelerated corrosion environments of pressurized water reactors

Due to susceptibility of carbon steels under flow-accelerated corrosion environments, 2.25Cr-1Mo has been adopted as replacement. However, the mechanism of enhancement in the passivity has not been completely elucidated. We employed synchrotron scanning transmission X-ray microscopy to test the microstructure and chemistry of the alloys after flow-accelerated corrosion tests at 150 degrees C. Carbon steel exhibited a mixture of Fe2+ and Fe3+ states as a passive layer while 2.25Cr-1Mo presented two oxide layers composed of Fe2O3, Fe3O4, and FeO. A small quantity of Cr2O3 was observed which was not enriched. Furthermore, Mo was not observed in the oxide layer of 2.25Cr-1Mo.