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.
The Hanford site contains approximately 55 million gallons (2.08 × 108 liters) of radioactive and chemically hazardous wastes arising from weapons production beginning with World War II and continuing through the Cold War. The wastes are stored in 177 carbon steel underground storage tanks, of which 149 are single-shell tanks (SSTs) and the remaining are double-shell tanks (DSTs). Substantial additions of sodium hydroxide to the waste have raised the pH to 12 or higher, allowing for passivity to occur on the carbon steel tanks and reducing the general corrosion rate down to acceptable levels. However, the wastes may contain low concentrations of Cl- and F-, both of which are known to destabilize passive films and increase susceptibility to localized corrosion and SCC. High concentrations of NO3- are also common to these wastes. NO3- is another aggressive anion that is known to promote localized corrosion and SCC of carbon steels.1 In addition to these aggressive species, there are also beneficial species present in the waste that help inhibit localized corrosion and SCC of carbon steel, most notably NO2-. An essential part of managing the integrity of these storage tanks is to define certain waste chemistry control limits with respect to pH, aggressive species, and inhibitive species.