ALTERNATIVE AND ENHANCED CHEMICAL CLEANING: CORROSION STUDIES RESULTS: FY2010

Wiersma, B.

Savannah River Site (S.C.)

关键词: Waste Storage;    36 Materials Science;    Electrochemical Corrosion;    Nitric Acid;    Oxides;   

DOI  :  10.2172/990341
RP-ID  :  SRNL-STI-2010-00555
RP-ID  :  DE-AC09-08SR22470
RP-ID  :  990341

美国|英语

来源: UNT Digital Library

【 摘 要 】

Due to the need to close High Level Waste storage tanks, chemical cleaning methods are needed for the removal of sludge heel materials remaining at the completion of mechanical tank cleaning efforts. Oxalic acid is considered the preferred cleaning reagent for heel dissolution of iron-based sludge. However, the large quantity of chemical reagents added to the tank farm from oxalic acid based cleaning has significant downstream impacts. Optimization of the oxalic acid cleaning process can potentially reduce the downstream impacts from chemical cleaning. To optimize oxalic acid usage, a detailed understanding of the chemistry of oxalic acid based sludge dissolution is required. Additionally, other acidic systems may be required for specific waste components that have low solubility in oxalic acid, and as a means to reduce oxalic acid usage in general. Electrochemical corrosion studies were conducted with 1 wt. % oxalic acid at mineral acid concentrations above and below the optimal conditions for this oxalic acid concentration. Testing environments included pure reagents, pure iron and aluminum phases, and sludge simulants. Mineral acid concentrations greater than 0.2 M and temperatures greater than 50 C result in unacceptably high corrosion rates. Results showed that manageable corrosion rates of carbon steel can be achieved at dilute mineral acid concentrations (i.e. less than 0.2 M) and low temperatures based on the contact times involved. Therefore, it is recommended that future dissolution and corrosion testing be performed with a dilute mineral acid and a less concentrated oxalic acid (e.g., 0.5 wt.%) that still promotes optimal dissolution. This recommendation requires the processing of greater water volumes than those for the baseline process during heel dissolution, but allows for minimization of oxalic acid additions. The following conclusions can be drawn from the test results: (1) In both nitric and sulfuric acid based reagents, the low temperature and dilute concentration environment resulted in carbon steel corrosion rates that were less than 150 mpy. These rates are manageable in that chemical cleaning processes could proceed for limited time without significant wall loss. Further optimization of the Alternative Enhance Chemical Cleaning (AECC) process should focus on testing in solutions of this dilute concentration and low temperature regime. (2) In general, for the nitric acid based reagent, the aluminum oxide phase environments resulted in higher corrosion rates than the iron oxide phase environments. (3) In general, for the sulfuric acid based reagent, the iron oxide phase environments resulted in higher corrosion rates than the aluminum oxide phase environments. (4) In general, for the nitric acid based reagent, the HM sludge simulant environments resulted in higher corrosion rates than the PUREX sludge simulant environments. This result agrees with the previous observation that the aluminum oxide phases are more aggressive than the iron oxide phase environments in the nitric acid reagent. (5) Pitting was more likely to occur in the sulfuric acid based reagents than in the nitric acid based reagents. (6) Pitting occurred only in the iron based pure oxide phases and the sludge simulants. No pitting was observed in the aluminum based pure oxide phases. (7) Pitting tended to occur more frequently in tests that involved the dilute mineral acid reagent. (8) Pitting was more severe at the higher temperature for a given mineral acid concentration. (9) Pitting was more severe at a higher mineral acid concentration for a given temperature. (10) Based on the combined results of the open circuit potential and cathodic polarization testing, there was a low propensity for hydrogen evolution in solutions where sludge has been dissolved.