DETERMINATION OF CORROSION INHIBITOR CRITERIA FOR TYPE III/IIIA TANKS DURING SALT DISSOLUTION OPERATIONS INTERIM REPORT
Preparation of high level waste for vitrification involves in part the dissolution of salt cake from the carbon steel storage tanks. During dissolution, a point is reached in which the corrosion inhibitors, hydroxide and nitrite, are diluted below established guidelines, and nitrate stress corrosion cracking (SCC) is possible. Because the addition of inhibitors may be counterproductive to process efficiency and waste minimization, corrosion testing was initiated to revisit and possibly revise the guidelines for inhibitor limits. The bases for the work summarized in this status report are results from previously-completed phases of study. In the first two phases of study, several reduced-inhibitor levels were tested in HLW simulants with nitrate concentrations ranging from 4.5 M to 8.5 M. The first two phases of work determined, among other things, the reduced-inhibitor levels and solution chemistries in which heat-treated and non-heat-treated A537 carbon steel is susceptible to SCC, crevice corrosion, and pitting. The work covered in this current task both builds on and verifies the conclusions of the previous work. The current work involves testing of low levels of inhibitors in HLW simulants with 5.5 M to 8.5 M nitrate concentrations. Stressed U-bend specimens, both polarized and non-polarized, were tested. Non-polarized U-bend testing is ongoing, with the U-bends currently in test for 100 days. The purpose of the testing is to determine SCC susceptibility in the vapor space (VS) and liquid air interface (LAI) regions of the HLW tanks under conditions expected during salt dissolution, and also to verify previous accelerated testing. The simulated wastes being tested have nitrate concentrations of 5.5 M and 8.5 M and inhibitor levels of 0.01 M/0.01 M hydroxide/nitrite and 0.1 M/ 0.1 M hydroxide/nitrite. The open circuit potential measurements being monitored and the corrosion morphology of the U-bends are in agreement with results and observations of previous phases of work. No SCC has occurred in the first 100 days of testing. The LAI specimens experienced minor corrosion at the liquid line with corrosion products visible on the weld material and in the heat-affected zones on either side of the welds. The VS specimens are more evenly and slightly more corroded. Polarized U-bend testing is complete after approximately 80 days of testing. No SCC occurred, but the results are inconclusive due to a competing, unexpected galvanic corrosion mechanism that interfered in the last 50 days of testing. No cracking was indicated during the first month. The tests will be repeated in order to satisfy the original objective which was to determine the effect of grinding HLW tank welds and heat treating the tanks had on corrosion. Both the non-polarized and polarized U-bend tests will continue. Additionally, cyclic polarization (CP) testing will be performed to examine the effects of surface oxides on corrosion and the differences in corrosion susceptibility between welded and un-welded areas.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC09-96SR18500
- OSTI ID:
- 925849
- Report Number(s):
- WSRC-STI-2007-00552; TRN: US200807%%541
- Country of Publication:
- United States
- Language:
- English
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