Relation between Surface Oxide and SCC of Alloy 600

Abstract

Stress corrosion cracking (SCC) is one of major threats against the integrity of the structural materials composing a nuclear power plant (NPP). Lead is deleterious element accelerating the SCC of Alloy 600 used as a steam generator tubing material in an NPP. In the present work, the oxide property grown on an Alloy 600 surface was evaluated as a function of PbO content using electrochemical impedance spectroscopy (EIS) and a field-emission transmission electron microscopy, equipped with an energy dispersive X-ray spectroscopy (TEM –EDS) for the specimen prepared using a focused ion beam (FIB). Alloy 600 was immersed in 0.1M NaOH containing PbO in the range of 0-5,000 ppm at 315oC for 2 weeks. The oxide property was compared with SCC susceptibility obtained from a slow strain rate tension (SSRT) test for Alloy 600 in 0.1M NaOH containing PbO. The impedance value was greatly decreased by adding PbO into the solution indicating a decrease in passivity. The composition of the oxide was also changed by Pb in an aqueous solution. The duplex oxide layer consisting of outer porous nickel-rich oxide and inner dense chromium-rich oxide is modified to a Pb incorporated nickel-rich oxide layer. Modification of the oxide property induced by lead incorporation caused an obvious increase in SCC susceptibility. It is expected that lead observed at the crack tip of an early cracked pulled Alloy 600TT tube was considerably responsible for SCC acceleration of this tube among numerous sound tubes in an NPP. Keywords: Alloy 600, impedance, passive oxide, PbO, steam generator, stress corrosion cracking.