Investigation of Oxide Property on Alloy 600 with the Immersion Time in a High-temperature Leaded Alkaline Solution

摘要

Oxide on a stress corrosion cracked region of IT Alloy 600 exposed to a nuclear power plant for over 20 years was analyzed using TEM-EDS for a sample prepared by an FIB. Pb above 10 wt% was observed in the oxide on the pulled TT Alloy 600 tubes presuming that Pb affects SCC to some extent. The oxide structure in the crack was fairly consistent with the laboratory grown oxide, indicating that the oxide grown in the real operational condition is formed in a similar way. The oxide property on TT Alloy 600 was investigated as a function of the immersion time in 0.1 M NaOH with 1000 ppm PbO at 315 degrees C using SEM, TEM, TEM-EDS, and EIS in view of the passivity and chemical composition. Electrochemical impedance was increased to an almost saturation value with the immersion time in a leaded solution while the oxide was increased to a steady state thickness. However, electrochemical impedance was decreased to an almost constant value with the immersion time in un-leaded solution suggesting that the Pb mainly affects the oxide passivity on early days. Moreover the electrochemical behavior of IT Alloy 600 was understood by analyzing the potentiodynamic polarization curve in 0.1 M NaOH with/without PbO.