Investigation of a Lead Induced SCC and Its Mitigation

In spite of much effort to reduce the material degradations, SCC is still one of important problems to overcome. Especially lead is known to be one of the most deleterious species in the reactor coolants that cause SCC of the alloy. Even Alloy 690 as an alternative of Alloy 600 because of outstanding superiority to SCC is also susceptible to lead in alkaline solution. Lead has been effectively detected in all tubesheet samples, crevice deposits and surface scales removed from SGs. Typical concentrations are 100 to 500 ppm but in some plants, concentrations as high as 2,000 to 10,000ppm has been detected. According to previous reports, lead induced SCC (PbSCC) is more dependent on the crack initiation time than the crack propagation rate. An oxide on Alloy 600 is formed and modified expanding to complex sludge throughout hideout return (HOR) of various impurities including Pb, combined with a residual stress induced by a tube expansion which is introduced to fix a tube to a tube sheet during a long time operation under a high temperature and high pressure water chemical environment. Therefore it is expected that a passivity of an oxide formed on Alloy 600 is deeply related to PbSCC and an inhibitor to hinder oxide modification by lead efficiently can be found. In the present work, an SCC susceptibility was investigated in the presence of lead and NiB as an inhibitor as well as in the absence of both impurities by using a slow strain rate tensile (SSRT) test. Moreover the oxides formed on Alloy 600 in aqueous solutions with and without lead were examined by using a scanning electron microscopy (SEM), a transmission electron microscopy (TEM), an energy dispersive x-ray spectroscopy (EDXS), an Auger electron spectroscopy (AES), an x-ray photoelectron spectroscopy (XPS) and an electrochemical impedance spectroscopy (EIS). The results were compared with the results obtained in aqueous solutions with NiB