In-situ Monitoring of SCC of Alloy 600 SG Tubing in PWR using EN Analysis

Abstract

Nickel based alloys such as Alloy 600 and 690 have been used as the steam generator (SG) tubing materials in a pressurized water reactor (PWR) due to their high corrosion resistance. However, many types of corrosion have occurred in highly caustic environments containing some oxidizing impurities, especially in SG sludge piles, because the highly caustic conditions can be developed in the heated crevices of PWR SG’s (Jacko, 1990). Among those impurities, Pb is well known to assist in stress corrosion cracking (SCC) of the SG tubing in the caustic environments (Sakai et al., 1990). Many authors reported on the cracking modes of SCC in Pb-contaminated solutions and the role of Pb on the passive films formed on nickel-based alloys to explain the mechanism of SCC (Hwang et al., 1997; Hwang et al., 1999; Kim et al., 2007; Kim et al., 2008). Recently, there was an approach to investigate the mechanism by distinguishing between the initiation and propagation stages of Pb-assisted SCC using an electrochemical noise (EN) technique (Kim & Kim, 2009). The EN is defined as a fluctuation of the electrochemical potential or current which is observed experimentally to be associated with localized corrosion processes (Cottis et al., 2001; Stewart et al., 1992). From the analyses of the EN parameters such as the frequency of events, the average charge of events, the noise resistance (Al-Mazeedi & Cottis, 2004; Cottis, 2001; Sanchez-Amyay et al., 2005), and the mean free time-to-failure (Kim & Kim, 2009; Na & Pyun, 2007, Na et al., 20007a, Na et al., 2007b), various types of localized corrosion are distinguishable from each other. Therefore, the EN monitoring technique has become a useful tool for characterizing such localized corrosions as pitting corrosion, crevice corrosion and SCC. This work is aimed to analyze the EN generated during Pb-assisted SCC of Alloy 600 at high temperature. The EN was measured from C-ring specimens in the highly caustic solution containing oxidizing impurities in two different ways: in a potentiostatic controlled current noise (PCCN) mode, the electrochemical current noise (ECN) was measured from the stressed C-ring specimen by applying an anodic potential. In an uncorrelated three electrode current and potential noise (UCPN) mode, the electrochemical potential noise (EPN) and the ECN were measured simultaneously from the stressed C-ring specimen. Changes in an amplitude and time interval of the ECN and EPN, and variations in power spectral density of the ECN and EPN were analyzed in terms of the initiation and propagation of Pb-assisted SCC of nickel-based alloys in the highly caustic solutions at high temperature.