Electrochimica Acta 56 (2011) 7871–7879 Contents lists available at ScienceDirect Electrochimica Acta jou rn al hom epa ge: www.elsevier.com/locate/electacta Electrochemical investigation of oxide films formed on nickel alloys 182, 600 and 52 in high temperature water Célia de Araújo Figueiredo a,∗ , Rik-Wouter Bosch b , Marc Vankeerberghen b a Nuclear Technology Development Centre (CDTN/CNEN) Av. Antônio Carlos, 6627, 31270-901 Belo Horizonte/MG, Brazil b SCK CEN, Boeretang 2400, Mol, Belgium a r t i c l e i n f o Article history: Received 16 July 2010 Received in revised form 18 May 2011 Accepted 19 May 2011 Available online 27 May 2011 Keywords: Electrochemical impedance Mott–Schottky Oxide film Nickel-based alloys PWR a b s t r a c t Nickel-based alloys 182, 600 and 52 were exposed to simulated Pressurized Water Reactor (PWR) primary water (1000 ppm B, 2 ppm Li, O 2 <10 ppb, 325 ◦ C) under different dissolved hydrogen (DH) conditions [0, 2, 25 and 50 cm 3 H 2 (STP) kg -1 ] for times up to 1 month in a recirculating autoclave. The influence of exposure time and DH on oxide films formed on the alloys was evaluated by means of electrochemical tests; electrochemical impedance spectroscopy (EIS) and Mott–Schottky (M–S). The in situ EIS was performed every day, allowing the monitoring of the oxide layer formation and change. M–S was performed at room temperature after the full exposure time. The results showed that the maximum in the defect concentrations obtained via M–S analysis and the maximum crack growth rate are at the same DH content, thus relating electrochemical testing to stress corrosion cracking observations. A conceptual separation between the electrochemical behavior of (1) the oxide layer (visible in the higher frequencies of EIS) and (2) the oxide layer – solution interface (visible in the lower frequencies of EIS) was able to explain the effect of hydrogen on the low-frequency EIS impedance results. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Stress corrosion cracking (SCC) is one of the concerns, and one of the major corrosion concerns, in plant life management of age- ing nuclear power plants (NPP). The mechanical response of NPP materials to an applied load and their chemical response to the environment, together with any synergy between the mechanical and chemical response, are major factors in determining the mate- rial’s SCC behavior. This SCC behavior is not constant and evolves in time for it is modified by ageing phenomena related to the long- term exposure of the material to temperature, stress, environment and irradiation. The main classes of NPP structural materials that are poten- tially subject to SCC are austenitic stainless steels and nickel-based alloys. Here, our main purpose is to study the oxidation behavior of selected nickel-based alloys in simulated PWR environments and correlate it with their known susceptibility to SCC. Three nickel- based alloys (600, its weld metal 182 and alloy 52) were selected for the study. Alloy 600 and its weld metals 182 and 82, were origi- nally selected for use in NPPs due to their high corrosion resistance but showed, after many years of plant operation, susceptibility ∗ Corresponding author. Tel.: +55 31 3069 3319; fax: +55 31 3069 3285. E-mail addresses: caf@cdtn.br, celia.fig2009@gmail.com (C. de Araújo Figueiredo), rbosch@sckcen.be (R.-W. Bosch), mvankeer@sckcen.be (M. Vankeerberghen). to SCC. The alloys are used in steam generators and as dissimilar metal weld materials for nozzles of components such as the reac- tor pressure vessel and the pressurizer. Alloy 52 was included in the study because it has been considered as a proper material for weld repairs, such as the weld overlay technique. Alloy 690, although of relevance, was omitted from the study due to a lack of electri- cal connections through the autoclave lid. Besides three alloys, the effect of the dissolved hydrogen (DH) concentration was included as a parameter in the study for it is known to affect the SCC behav- ior of nickel-based alloys [1–3], e.g. showing a maximum in crack growth rate around 15–20 cm 3 H 2 (STP) kg -1 . It is generally recognized that nucleation and propagation of localized corrosion, and stress corrosion in particular, are strongly related to the properties of the oxide film formed on the metal sur- face [4]. These oxide films make the material resistant to uniform corrosion but simultaneously susceptible to SCC. This phenomenon is affected by the electrochemical processes that take place at the metal–oxide–solution interface. The oxide film formed on metallic materials in high tempera- ture and high pressure water can be characterized, after exposition, by surface and microscopic analysis techniques. Since corrosion of metals and alloys in high temperature aqueous environments is electrochemical in nature, it is also possible to investigate the prop- erties of the oxide film in high temperature aqueous environments by in situ EIS and ex situ M–S, as done here. Although there have been many studies on SCC of Alloy 600, lit- tle is known about the influence of hydrogen on the electrochemical 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.05.077