Abstract
Radioactive liquid waste has been stored in underground carbon steel tanks for nearly 60 years at the Savannah River Site (SRS). The site is currently in the process of removing the waste from these tanks in order to place it into vitrified, stable state for longer term storage. The last stage in the removal sequence is a chemical cleaning step that breaks up and dissolves metal oxide solids that cannot be easily pumped out of the tank. Oxalic acid (OA) will be used to chemically clean the tanks after waste retrieval is completed. The waste tanks at SRS were constructed from carbon steel materials and thus are vulnerable to corrosion in acidic media. In addition to structural impacts, the impact of corrosion on the hydrogen generated during the process must be assessed. Electrochemical and coupon immersion tests were used to investigate the corrosion mechanism at anticipated process conditions. The tests were conducted at OA concentrations between 2.5 and 8 wt.% and at temperatures between 60 and 75 °C. In general, the corrosion rates increased with temperature and oxalic acid concentration. The testing also showed that the corrosion rates were dependent upon the reduction of the iron species that had dissolved in solution. Initial corrosion rates in 8 wt.% OA at 75 °C were approximately 140 mpy (3.6 mm/yr) due to the reduction of the ferric species to ferrous species. After 4 weeks, as the ferric species depleted, the corrosion rate at this condition decreased to approximately 30 mpy (0.75 mm/yr). On the other hand, as the electrochemical tests demonstrated, the hydrogen evolution reaction became more dominant at these later times.
