Influence of Glycolate Anion on Aqueous Corrosion in DWPF and Downstream Facilities – SCC Testing

Abstract

Glycolic acid is being evaluated as an alternate reductant in the preparation of High Level Waste for the Defense Waste Processing Facility (DWPF) at the Savannah River Site (SRS). During processing, the glycolic acid may not be completely consumed with small quantities of the glycolate anion being carried forward to waste facilities. The SRS liquid waste contractor requested an assessment of the influence of the glycolate anion on the corrosion of the materials of construction (MoC) throughout the waste processing system since this concern had not been previously evaluated. The influence of glycolic acid on the occurrence of stress corrosion cracking (SCC) had not been evaluated specifically in the previous testing. The present testing used electrochemical and coupon immersion testing to assess this influence. Additionally, the primary guidance on corrosion for operating DWPF with a formate-based flowsheet is based on testing that utilized test solutions with a small number of constituents. Recent testing used more complex solutions based on flowsheet development for DWPF. Additional electrochemical tests were also conducted to assess the influence of this difference on test results. The results from both the electrochemical and coupon immersion tests in formate-based and glycolatebased solutions with chlorides concentrations between 800 and 1200 parts per million showed no active forms of localized corrosion. The Cyclic Potentiodynamic Polarization (CPP) scans in these solutions had passive regions with some degree of positive hysteresis which indicate a possibility for the occurrence of localized corrosion. The positive hysteresis occurred at potentials greater than 200 mV more electropositive than the corrosion potential indicating that, at least, crevice corrosion should not occur in service. The threemonth coupon immersion tests, however, showed no form of localized corrosion (SCC, crevice corrosion and pitting) occurred under simulated operating conditions. The coupon immersion test also showed that deposits are likely to form on C276 during service especially near the vapor/liquid interface. The CPP testing also showed that some differences occur with using five-constituent DWPF simulants (i.e. simple) as opposed to the more complex multi-constituent solution chemistry used in this testing. The testing was performed to establish a comparison point between older corrosion data (1980-90's) obtained with simple solution chemistry and the newer data. Tests with glycolate-based solutions, which were not tested prior to this study, the CPP data did not show a significant difference in corrosion behavior. For formate-based solutions, the difference was more significant with a change in operable corrosion mechanisms (i.e., anodic corrosion for a simple chemistry versus passivity for a complex chemistry). However, the lack of susceptibility to localized corrosion was found to be the same.