DETERMINATION OF CORROSION INHIBITOR CRITERIA FOR TYPE III IIIA TANKS DURING SALT DISSOLUTION OPERATIONS

Wiersma, B

Savannah River Site (S.C.)

关键词: Breakdown;    36 Materials Science;    Wastes;    Efficiency;    Oxides;   

DOI  :  10.2172/927594
RP-ID  :  WSRC-STI-2006-00029
RP-ID  :  DE-AC09-96SR18500
RP-ID  :  927594

美国|英语

来源: UNT Digital Library

【 摘 要 】

Preparation of high level waste for vitrification involves in part the dissolution of salt cake from the carbon steel storage tanks. The salt crystals composing this cake are high in nitrate concentration with the interstitial liquid being high in hydroxide and nitrite concentration. During the salt dissolution process, a stage is reached in which the inhibitors, hydroxide and nitrite, are insufficient to prevent nitrate stress corrosion cracking (SCC) and fall outside the requirements of the corrosion control program. Additional inhibitors, which are necessary to meet the requirements, may be counterproductive to the efficiency of the process and waste minimization. Corrosion testing was initiated to better characterize the necessary inhibitor concentration for high nitrate waste during salt dissolution processing. A four-phase test program is being conducted: (1) electrochemical characterization, (2) accelerated or polarized U-bend testing, (3) long-term (non-polarized) U-bend testing and (4) vapor space U-bend tests. Electrochemical testing, which included cyclic potentiodynamic polarization (CPP), linear polarization resistance (LPR) and open-circuit potential (OCP) measurements, was performed to identify stress corrosion cracking susceptibility, to characterize pitting resistance and to determine the general corrosion rate. Polarized U-bend tests were utilized to assess the effect of minimum inhibitor concentrations and heat treatment on SCC and to determine test parameters for future long-term U-bend testing. Results from CPP, LPR and OCP tests demonstrated that carbon steel formed a protective oxide film and the potential became electropositive during exposure to the waste at all inhibitor concentrations. The tenacity of this film improved as the inhibitor concentration level was increased and the temperature was decreased. This passive film increased the resistance to localized corrosion significantly. Therefore if any of these inhibitor levels are selected for storage of dissolved salt solutions, no changes to the service life estimates that were based on general corrosion are necessary. The breakdown potential for SCC as well as the other electrochemical parameters were independent of nitrate concentration (4.5-8.5 M). The breakdown potential, however, was strongly affected by temperature (i.e., 25 and 50 C) and inhibitor concentration. These results indicate that for this nitrate concentration range a critical inhibitor level is necessary for minimizing the occurrence of SCC. The polarized U-bend tests were in good agreement with the electrochemical tests. The U-bend testing clearly demonstrated that the heat treating of the samples clearly improved the SCC resistance of A537 carbon steel even at the low inhibitor concentration (0.01 M hydroxide and 0.01 M nitrite). This concentration was insufficient to prevent cracking for any tested nitrate concentration (4.5-8.5 M). At a 7 M nitrate concentration, SCC was prevented for inhibitor concentrations as low as 0.3 M hydroxide and 0.1 M nitrite. The current inhibitor requirements for a waste containing 7 M nitrate are 0.6 M hydroxide and 1.1 M hydroxide and nitrite. Thus, a considerable reduction in the amount of inhibitor necessary may be attained. It will also be recommended that the temperature of the dissolved salt solution be maintained below 50 C.