Analysis of Oxide on Steam Generator Tubing Material in High Temperature Alkaline Leaded Solution

Abstract

Nuclear power plants (NPP) using Alloy 600 as a heat exchanger tube of the steam generator (SG) have experienced various corrosion problems such as pitting, intergranular attack (IGA) and stress corrosion cracking (SCC). In spite of much effort to reduce the material degradations, SCC is still one of important problems to overcome. Secondary water pH which affects SCC behavior substantially spans widely from acid to alkaline in crevice depending on water chemistry control, water chemistry in crevice, plant specific condition, etc. Especially, specific chemical species are accumulated in the crevice as the sludge leading to a specific condition of crevice chemistry. Among these chemical species, lead is known to be one of the most deleterious species in the reactor coolants that cause SCC of the alloy (Sarver, 1987; Castano-Marin et al., 1993; Wright and Mirzai, 1999; Staehle, 2003). Even Alloy 690, as an alternative of Alloy 600 because of outstanding superiority to SCC, is also susceptible to lead in alkaline solution (Vaillant et al., 1996; Kim et al., 2005; Kim and Kim, 2009). 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 10,000 ppm have been detected (Fruzzetti, 2005). The best method to prevent lead induced SCC (PbSCC) is to eliminate the harmful lead from the NPP chemistry, which is not possible and most NPPs are already contaminated by lead. Moreover only a very low level of sub ppm affects PbSCC. During a long exposure time of more than 30 years under a high temperature and high pressure water chemical environment, an Alloy 600 surface experiences an oxide formation, breakdown and modification depending on the nature of the grown oxide, combined with a residual stress induced by a tube expansion which is introduced to fix a tube to a tube sheet. Therefore it is strongly anticipated that a SCC is inevitably related to an oxide properties, formed on an Alloy 600 surface, because a crack initiates and propagates through a breakdown and alteration of a surface oxide, fundamentally speaking. An oxide properties should be investigated for the elucidation of a lead induced mechanism and its countermeasure.