Effect of the Composition of Nickel Alloys on the Anodic Behavior in Aqueous Solutions of Chloride and Bicarbonate
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Effect of the Composition of Nickel Alloys on the Anodic Behavior in Aqueous Solutions of Chloride and Bicarbonate Natalia S. Zadorozne1, Ricardo M. Carranza2,3, Mabel C. Giordano2,3, Alicia E. Ares4, and Raul B. Rebak5 1 CONICET, Argentina. 2 Departamento Materiales, Comisión Nacional de Energía Atómica, Argentina. 3 Instituto Sabato, UNSAM / CNEA, Argentina. 4 Universidad Nacional de Misiones, Posadas, Misiones, Argentina 5 GE Global Research, Schenectady, NY, USA. ABSTRACT The stress corrosion cracking (SCC) of Alloy 22 in concentrated ground waters was associated to the presence of bicarbonate and chloride ions in the water. SCC occurred when an anodic peak appeared in the polarization curve of Alloy 22. The aim of this work was to investigate further which element in Alloy 22 was responsible for the anodic peak and therefore SCC. Four alloys (22, 800H, 600 and 201) were mainly used for this study. It has been found that Alloy 22, 600 and 800H show anodic peaks; which are affected by chloride and temperature. It is likely that the anodic peak is associated to the presence of either chromium, molybdenum or both. INTRODUCTION The ability of nickel (Ni) to dissolve a large amount of different alloying elements has led to the development of several Ni alloys families. The most versatile family is the NickelChromium-Molybdenum (Ni-Cr-Mo) or C-type, which provides corrosion resistance in reducing and oxidizing hot acids. Alloy 22 (UNS N06022) is one of the most multipurpose members of this family since it can be used under reducing and oxidizing conditions. Due to its excellent corrosion resistance in a wide variety of environments, Alloy 22 was selected for the fabrication of the corrosion-resistant outer shell of the high-level nuclear waste container in the USA [1-3]. Since over their life-time the containers may be exposed to multi-ionic concentrated ground water environment, it is estimated that this material could suffer three different types of deterioration: general corrosion, localized corrosion (especially crevice corrosion) and stress corrosion cracking (SCC). Generalized and localized corrosion has been extensively studied in the last ten years in conditions that would simulate the underground emplacement of the containers [4-6]. On the contrary, fewer experimental studies were conducted to understand the processes of stress corrosion cracking of Alloy 22. U-bend specimens of C-22 (N06022) and other nickel alloys such as C-4 (N06455), G-3 (N06985), 825 (N08825) and 625 (N06625) were used to characterize their stress corrosion cracking susceptibility in a variety of environments. Gas Tungsten Arc Welded (GTAW) and non-welded U-bend specimens were exposed for more than 5 years at the corrosion potential to the vapor and liquid phases of three different solutions (pH 2.8 to 10) simulating up to 1000 times the concentration of ground water both at 60°C and 90°C. None of these Ni alloys suffered any indication of environmentally induced cracking [3, 7]. Alloy C-22 was found susceptible to SCC when slow strain r
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