Lead effect on the corrosion and passivation behavior of Alloy 600

Dissolved Pb is considered as the most aggressive chemical species involved in the initiation and growth of stress corrosion cracking (SCC) in a pressurized water reactor (PWR) power generating system. The results from laboratory studies indicate that Pb-induced SCC (PbSCC) covers a range of potential and pH which is the largest of all the submodes of SCC occurring in steam generators (SG) and it occurs at threshold concentrations as low as 0.1 ppm. It is hypothesized that PbSCC is caused by the incorporation of Pb into the passive film, which reduces the passivity of the film and enhances the selective dissolution of Ni from the base metal. This investigation is focused on studying the effect of Pb on the dissolution and passivation of Alloy 600 MA in order to provide information for understanding the PbSCC mechanism.; The effect of Pb on Alloy 600 MA was investigated in the solutions containing 110ppm Cl- and different concentrations of Pb 2+ at 90°C with pH4.5. Potentiodynamic polarization scans, electrochemical impedance spectroscopy, Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were used to study the electrochemical behavior of Alloy 600 MA in the active and passive potential region in the solutions with/without Pb2+ in order to compare the effect of Pb on the dissolution and passivation of Alloy 600.; The results indicated that the corrosion of Alloy 600 induced by Pb 2+ was ascribed to the enhanced dissolution of Ni by the reduction of Pb2+ at potentials slightly more cathodic than EPb2+/Pb0G Pb0=1 . The lower boundary of Pb2+ concentration for the occurrence of the displacement reaction is 2.5ppb according to thermodynamic calculations. The passivation of Alloy 600 was retarded by the Pb2+ and it was ascribed to the inhibited growth of NiO at passive potential and the increased conductivity of Cr2O3 by doping Pb 2+ in Cr2O3 lattice.