Superior Corrosion Resistance of Ti-Al-Zr Alloy in Aggressive Nitric Acid Environments
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JMEPEG (2020) 29:8441–8450 https://doi.org/10.1007/s11665-020-05300-z
Superior Corrosion Resistance of Ti-Al-Zr Alloy in Aggressive Nitric Acid Environments Prafful Kumar Sinha and Vivekanand Kain Submitted: 6 June 2020 / Revised: 14 October 2020 / Accepted: 18 October 2020 / Published online: 13 November 2020 This paper describes the corrosion behavior of a Ti-based alloy, Ti-Al-Zr for aggressive nitric acid applications. Corrosion behavior of the materials was established using electrochemical polarization technique, for three process variables encountered in reprocessing plants: (a) nitric acid (1, 3 and 6 M), (b) temperature (25 and 75 °C) and (c) oxidizing ions (Ce+4 and V+5 ions). Electrochemical polarization tests established that the Ti-Al-Zr alloy was in passive condition up to a potential of 4000 mVAg/AgCl even at 75 °C, and it showed uniform corrosion. SS304L, however, showed severe intergranular corrosion at a much lower anodic potential, thus establishing better corrosion resistance for Ti-Al-Zr alloy in the aggressive nitric acid environment. Keywords
electrochemical polarization, pseudo-trans-passivity, titanium alloy, uniform dissolution
1. Introduction For nuclear power production, two types of fuel cycles are followed: (1) once-through cycle in which spent fuel is disposed off, e.g., in a long-term disposal site and (2) closed cycle in which spent fuel is reprocessed to obtain uranium (U) and plutonium (Pu) and the recovered U/Pu is again used for making fuel for reactors (Ref 1). India follows a closed cycle for nuclear power production. The reprocessing of spent fuel is typically done by plutonium uranium reduction extraction (PUREX) process (Ref 1, 2). Nitric acid is the main process medium employed in the PUREX process for dissolving the spent fuel and recovering the U and Pu (Ref 1, 2). For reprocessing the spent fuel from water-cooled reactors and fast breeder reactors, nitric acid is used in varying concentrations ranging from dilute (1-4 N) to concentrated (10-14 N) and from room temperature to boiling temperature (Ref 2). Austenitic stainless steels (ASS) are commonly used as structural and storage materials for handling nitric acid media (Ref 2-7). These ASS (predominantly SS 304L) are used in reprocessing/waste management plants owing to their excellent corrosion resistance due to the formation of a passive film of Cr2O3 on its surface (Ref 2, 5). Whenever a passive film is present on the surface, ASS would undergo either uniform corrosion at a very low rate (in a solution annealed condition) or intergranular corrosion (IGC) in a sensitized condition (Ref 3). In fact, IGC is the dominant cause of degradation of stainless steels in reprocessing/waste management plants where nitric acid is used (Ref 2, 3, 5-8). Most cases of IGC degradation are due to the use of sensitized stainless steels (e.g., Prafful Kumar Sinha and Vivekanand Kain, Material Processing and Corrosion Engineering Division, BARC and Homi Bhabha National Institute, Mumbai 400085, India. Contact e-mail: [email protected]
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