MECHANISTIC UNDERSTANDING OF CAUSTIC CRACKING OF CARBON STEELS
Liquid waste generated by the PUREX process for separation of nuclear materials is concentrated and stored in Type IV single-shell carbon steel tanks at the Savannah River Site (SRS). The Type IV tanks for this waste do not have cooling coils and have not undergone heat treatment to stress-relieve the tanks. After the waste is concentrated by evaporation, it becomes very alkaline and can cause stress corrosion cracking (SCC) and pitting corrosion of the tank materials. SRS has experienced leakage from non-stress-relieved waste tanks constructed of A285 carbon steel and pitting of A212 carbon steel tanks in the vapor space. An investigation of tank materials has been undertaken at SRS to develop a basic understanding of caustic SCC of A285 and A212 grade carbon steels exposed to aqueous solutions, primarily containing sodium hydroxide (NaOH), sodium nitrate (NaNO{sub 3}), and sodium nitrite (NaNO{sub 2}) at temperatures relevant to the operating conditions of both the F and H area plants. This report presents the results of this corrosion testing program. Electrochemical tests were designed using unstressed coupons in a simulated tank environment. The purpose of this testing was to determine the corrosion susceptibility of the tank materials as a function of chemical concentration, pH, and temperature. A285 and A516 (simulates A212 carbon steel) coupons were used to investigate differences in the corrosion of these carbon steels. Electrochemical testing included measurement of the corrosion potential and polarization resistance as well as cyclic potentiodynamic polarization (CPP) testing of coupons. From the CPP experiments, corrosion characteristics were determined including: corrosion potential (E{sub corr}), pitting or breakdown potential (E{sub pit}), and repassivation potential (E{sub prot}). CPP results showed no indications of localized corrosion, such as pitting, and all samples showed the formation of a stable passive layer as evidenced by the positive hysteresis during the scan. Analysis of the CPP data was performed to compare the corrosion susceptibility of the samples under different environmental conditions. Test results indicated that the most important factors affecting corrosion of the steel are the solution temperature, hydroxide concentration, and the material used in constructing the tanks. Variables that did not significantly affect the corrosion susceptibility of the steel were the nitrate or nitrite concentration and the atmosphere in the tank. The passivation current of the coupons increased exponentially with temperature. Longer-term studies of the passivation current are suggested based on results from the literature for iron in highly caustic environments. Polarization resistance studies showed a significant increase in corrosion rate at 125 C and 12 M hydroxide concentration when compared with tests at lower temperatures and lower hydroxide concentrations. Within the temperature and pH range of these tests, iron oxide, Fe{sub 3}O{sub 4}, becomes unstable and could account for the increased corrosion susceptibility. The applicability of these conditions should be confirmed and detailed surface studies should be conducted to determine the corrosion resistance of A285 and A516 carbon steels under these conditions. Surface science studies should also be conducted to determine the role of the carbon steel composition in preventing corrosion under these conditions.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC09-08SR22470
- OSTI ID:
- 966688
- Report Number(s):
- SRNL-STI-2009-00596; TRN: US0904080
- Resource Relation:
- Conference: NACE CORROSION 2010
- Country of Publication:
- United States
- Language:
- English
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