Understanding the interaction of thiosulfate with Alloy 800 in aqueous chloride solutions using SECM Da-Hai Xia a , Ren-Kang Zhu a , Yashar Behnamian a , Jing-Li Luo a,⇑ , Chang-Jian Lin b , Stan Klimas c a Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada b State Key Laboratory of Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China c Atomic Energy of Canada Ltd, Chalk River Laboratories, Stn. 80, Chalk River, Ontario K0J 1J0, Canada article info Article history: Received 5 November 2014 Received in revised form 27 February 2015 Accepted 3 March 2015 Available online 5 March 2015 Keywords: Alloy 800 Chloride Thiosulfate SECM STM EIS abstract In situ scanning electrochemical microscopy (SECM) and scanning tunneling microscopy (STM) assisted scanning reference electrode technique (SRET) were applied for the first time to study the interaction of thiosulfate with Alloy 800 surfaces in aqueous chloride solutions. Electrochemical impedance spec- troscopy (EIS) and scanning electron microscope (SEM) were also performed to understand the interac- tion mechanism. The results showed that the effect of 0.075 mol/L thiosulfate in 0.6 mol/L chloride solutions strongly depended on the potential. There was no aggressive effect at the corrosion potential where the passive layer was intact, but a combined effect was observed with the presence of chloride ions at high potential where the passive layer was broken down. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Alloy 800 is one of the materials used for steam generator (SG) tubing in CANDU Ò ™ (CANada Deuterium Uranium) nuclear power stations and is also used in other pressurized water reactor (PWR) SGs due to its proven excellent corrosion resistance [1–3]. However, impurities on the SG boiling side, such as sulfate, chlo- ride and lead, may collectively cause passivity degradation of Alloy 800 [4]. Sulfate concentrations measured in the SG blow- down could be up to 10 lg/kg, the concentrations of non-volatile impurities at the hideout in crevices may increase by several orders of magnitude. Moreover, sulfate may reduce to lower valence species such as H 2 S, elemental S and S 2 O 3 2 under SG operating conditions; these species can be the powerful ther- modynamic activators for copper, nickel, and iron through the for- mation of solid sulfide phases at more active (negative) electrochemical potentials than those at which the oxides form [5]. Thiosulfate (S 2 O 3 2 ) is known to cause accelerated corrosion of SG tubing materials [6], and more severe effects if other ions (such as Cl  ) are present [7–9]; however, the interaction of thiosul- fate with the passive film in the presence of chloride ions is unknown. In recent years, scanning electrochemical microscopy (SECM) has been developed for a wide range of applications in corrosion science [10–12]. An advantage of SECM is its capability to probe charge transfer occurring nonuniformly at interfaces [13,14]. Scanning reference electrode technique (SRET) has been used for determination of the potential distribution on corroding surfaces [15–20] and scanning tunneling microscope (STM) is a technique to study corrosion at micro- and nano-scale [21–24]. Zhu et al. [10] investigated the impact of tensile and compressive stress on reactivity of the Alloy 800 C-ring sample using SECM and found that both tension and compression increased the localized surface reactivity, indicating corrosion susceptibility of Alloy 800 under stress in thiosulfate-contained chemistries. Lu et al. [25] investi- gated the pitting and stress corrosion cracking of welded austenitic stainless steel using in situ scanning reference electrode technique (SRET). Mott–Schottky measurements were also used to investigate the electric properties as well as the susceptibility to the presence of chloride ions and tensile stress of passive films. Marcus et al. inves- tigated the passive film growth mechanism at nano scale using STM combined with molecular dynamics simulation [22,23,26–29]. Limited studies of thiosulfate–chloride induced passivation degra- dation were performed using a combination of techniques [10]. In this work, the passive film on Alloy 800 was studied by SECM, STM assisted SRET, and EIS to observe the corrosion of Alloy 800 in thiosulfate–chloride solutions. http://dx.doi.org/10.1016/j.jelechem.2015.03.006 1572-6657/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: jingli.luo@ualberta.ca (J.-L. Luo). Journal of Electroanalytical Chemistry 744 (2015) 77–84 Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem