Influences of Mo Addition on the Microstructure, Mechanical Properties and Corrosion Resistance of Alloy 690

Steam generator is the interface between primary side and secondary side of nuclear power plant(NPP). If the primary water with radioactivity may leak to secondary water, it threatens the safety of NPP and shortens the lifetime of steam generator. This work was carried out to improve the corrosion resistance of steam generator tubing materials of NPP. New designed alloy 690 was manufactured, and this molybdenum-bearing alloy 690 was evaluated for microstructure, mechanical properties and corrosion resistance by annealing. With the addition of molybdenum, grain size was decreased. It seemed that grain growth was suppressed due to the precipitation by molybdenum containing carbides. Also, molybdenum addition increased the hardness and, improved tensile strength and yield strength without the loss of elongation. The corrosion resistance of alloy 690 was greatly improved by molybdenum addition from the results of pitting test and anodic polarization test. Molybdenum did not also decreased the corrosion resistance in the various ratios of Cu2+, Cl- and SO42- ions in corrosion environments. It can be concluded that molybdenum addition in alloy 690 has a beneficial effect on the mechanical properties and corrosion resistance