Oxidation behavior of Ni-Cr-Fe alloys with different Cr content in simulated PWR primary water

Ni is released from the surface of various Ni-base structural materials such as steam generator tubes and CRDM nozzles because these not only consist of 58-72% nickel but also occupy over 70% of the total surface area exposed to the coolant. Therefore, it is apparent that mitigation of Ni release from the Ni-base alloys should be a crucial strategy for source term reduction in PWRs. Since the steam generator tubing material has been replaced from Alloy 600 to Alloy 690, the Co-58 radiation filed has been reported to be significantly reduced. The diffusion of metal elements into the coolant through the passive layer is depressed due to the increase of the Cr content because increasing Cr content in Ni-base alloys enhances the protectiveness of passive film. Actually, the passive film, Cr₂O₃, was reported to be stably formed when the Cr content in Ni-base alloys is larger than about 17wt%, although the critical concentration of Cr for passive film formation depends on many environmental factors such as hydrogen concentration, water chemistry and coolant flow rate. Based on such a background, Alloy 600 (Ni-16Cr-9Fe) and Alloy 690 (Ni-30Cr-9Fe) have been employed as a significant structural material in PWRs. In addition, the corrosion characteristics of these materials have been widely studied in various environmental conditions. The release rate of metals has been known to be proportional to their general corrosion rate and depends on a kinds of materials. In this work, the corrosion rate and oxidation behaviors of the Ni-Cr-Fe alloys with increasing a Cr content were investigated in simulated primary water at 330oC. The structural and electrochemical properties of the alloys were analyzed by using XRD, SEM and potentiostat analyzer.