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51318-11016-The Passive Film Evolution of Carbon Steel in Hanford Waste Simulants

Nuclear wastes are stored in large carbon steel storage tanks. Carbon steels can become susceptible to localized corrosion and stress corrosion cracking. Results of ongoing electrochemical tests to understand effects of a maturing passive film on the corrosion.

Product Number: 51318-11016-SG
Author: K. J. Evans / S. Chawla / K.M. Sherer / K. D. Boomer
Publication Date: 2018
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Nuclear wastes are stored in large, underground carbon steel storage tanks at the Hanford site. Most of the liquid wastes are highly alkaline, typically with pH values between 12 and 14. Under alkaline conditions, carbon steels tend to be passive and undergo relatively slow, uniform corrosion. However, carbon steels can become susceptible to localized corrosion (e.g., pitting) and stress corrosion cracking (SCC) in the presence of certain aggressive constituents under passive conditions. Furthermore, the passive films that form on carbon steel evolve over time, resulting in changes in the open circuit potential (OCP). The pitting and SCC susceptibility of the steel, as the passive film evolves, is not well understood in these conditions. In this work, carbon steel coupons were immersed in alkaline waste simulants and then retrieved at various time intervals to examine the changes in passive film properties as a function of time. Electrochemical impedance spectroscopy (EIS), Mott-Schottky analysis, and potentiodynamic polarization measurements were employed to characterize the changes that were occurring to the passive film. This paper presents the results of ongoing electrochemical tests to understand the implications of a maturing passive film on the corrosion behavior of carbon steel.

Key words: Carbon steel, passive film, Mott-Schottky, EIS, Hanford, radioactive waste

Nuclear wastes are stored in large, underground carbon steel storage tanks at the Hanford site. Most of the liquid wastes are highly alkaline, typically with pH values between 12 and 14. Under alkaline conditions, carbon steels tend to be passive and undergo relatively slow, uniform corrosion. However, carbon steels can become susceptible to localized corrosion (e.g., pitting) and stress corrosion cracking (SCC) in the presence of certain aggressive constituents under passive conditions. Furthermore, the passive films that form on carbon steel evolve over time, resulting in changes in the open circuit potential (OCP). The pitting and SCC susceptibility of the steel, as the passive film evolves, is not well understood in these conditions. In this work, carbon steel coupons were immersed in alkaline waste simulants and then retrieved at various time intervals to examine the changes in passive film properties as a function of time. Electrochemical impedance spectroscopy (EIS), Mott-Schottky analysis, and potentiodynamic polarization measurements were employed to characterize the changes that were occurring to the passive film. This paper presents the results of ongoing electrochemical tests to understand the implications of a maturing passive film on the corrosion behavior of carbon steel.

Key words: Carbon steel, passive film, Mott-Schottky, EIS, Hanford, radioactive waste

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