Abstract
Millions of liters of liquid radioactive waste are stored in double shell tanks (DSTs) at Hanford Site. The DSTs were constructed of carbon steel from 1968 to 1986 and have a comprehensive chemical control program which mostly relies on hydroxide additions, thus preventing localized corrosion specially stress corrosion cracking (SCC). Most of the radioactive waste is now below 50°C and the pitting factor was determined to provide limits in the control program using significant chemical species in the waste. However, several tanks have reached temperatures up to 75°C, so the pitting factor equation may not be adequate to address localized corrosion conditions at higher temperatures than 50°C. Using statistical analysis, a series of electrochemical tests were designed and developed to account for the increase in temperature with different chemistries and at pH lower than 13. Additionally, testing was performed to differentiate between the contribution from the halides (i.e., chloride and fluoride) in the pitting factor (PF) equation since we have only been focusing on chloride. Cyclic potentiodynamic polarization (CPP) was used to identify regions of localized corrosion susceptibility at different concentrations of statistically significant chemicals in the waste up to 75°C. Using the results of this study, the pitting factor was analyzed and validated for elevated temperatures.
