Examination of Mechanisms for Liquid-Air-Interface Corrosion of Steel in High Level Radioactive Waste Simulants

Abstract

Several hypothesized mechanisms of liquid-air-interface (LAI) corrosion in simplified simulants of high level liquid nuclear waste were investigated. The salt concentration cell theory is based on the possible development of an enhanced nitrate concentration or a depleted nitrite concentration in the meniscus solution compared to the bulk solution. In situ Raman spectroscopy was employed to monitor the time-dependent changes in nitrate concentration, nitrite concentration, and the nitrate-to-nitrite concentration ratio in the meniscus region during LAI corrosion. However, there was no evidence to support the proposed nitrate-or nitrite-salt concentration cell theory. The IR drop theory was ruled out as being the cause of LAI corrosion based on local potential measurements using customized micro-reference electrodes. A theory of CO 2 -induced local hydroxide depletion was tested with a small pH electrode. However, the meniscus and bulk solutions exhibited no difference in pH before the initiation of LAI corrosion under anodic potentiostatic polarization. A local acidification mechanism, which results from borderline passive dissolution of steel in the meniscus and subsequent hydrolysis similar to the phenomenon of crevice corrosion, remains possible. Mild steels are susceptible to corrosion at the Liquid-Air Interface (LAI) in high-level liquid nuclear waste and simulants, 1-9 which contain inhibiting sodium nitrite and sodium hydroxide, and highly concentrated aggressive sodium nitrate. Although LAI corrosion has not yet been reported to cause any leaks in service for the nuclear waste storage tanks in Hanford, WA or Savannah River, SC, a long-term LAI corrosion event could potentially penetrate through the tank wall. The absence of a precise understanding of LAI corrosion has affected the tank chemistry control program for corrosion mitigation.10 The mechanisms of LAI corrosion must be understood to provide useful insights for appropriate formulation of waste chemistry specifications that will not accelerate LAI corrosion.Several hypotheses have been proposed in the literature for the mechanism of LAI corrosion. [3][4][5][6][7]11 They are based on a corrosion cell established by a potential or chemical gradient formed in the meniscus region. Schematic illustrations for several possible gradients of variables that could cause LAI corrosion are shown in Figure 1.The oxygen concentration cell theory has been used to successfully explain the common 'waterline corrosion ', 12-14 where the localized corrosion initiates just below the waterline on a partially immersed metallic structure. However, this mechanism is not consistent with the experimental observation of the initiation of LAI corrosion at the very top of the meniscus and also does not explain the presence of LAI corrosion under deaerated conditions. 6-9 Furthermore, LAI corrosion can readily form under potentiostatic polarization in an aggressive solution, but an oxygen concentration cell should not form under potentiostatic polarization because the main ...

Liquid-Air Interface Corrosion of A537 Steel in High Level Liquid Radioactive Waste Simulant in a Sealed Container

“…It has been reported that mild steels may be subjected to corrosion at the liquid-air interface (LAI) in high-level liquid radioactive waste, [1][2][3][4][5][6] although severe leaks due to this issue have not been reported yet. 6 The liquid radioactive waste is mainly alkaline, consisting of NaNO 2 , concentrated NaNO 3 and sometimes a relatively small amount of chloride.…”

“…It has been reported that mild steels may be subjected to corrosion at the liquid-air interface (LAI) in high-level liquid radioactive waste, [1][2][3][4][5][6] although severe leaks due to this issue have not been reported yet. 6 The liquid radioactive waste is mainly alkaline, consisting of NaNO 2 , concentrated NaNO 3 and sometimes a relatively small amount of chloride. 1,[6][7][8][9][10][11] Nitrite has been reported to inhibit corrosion of alloys, [12][13][14][15][16] while nitrate was regarded as either a corrosion inhibitor [17][18][19][20][21][22] or an aggressive species.…”

“…6 The liquid radioactive waste is mainly alkaline, consisting of NaNO 2 , concentrated NaNO 3 and sometimes a relatively small amount of chloride. 1,[6][7][8][9][10][11] Nitrite has been reported to inhibit corrosion of alloys, [12][13][14][15][16] while nitrate was regarded as either a corrosion inhibitor [17][18][19][20][21][22] or an aggressive species. 6,[23][24][25] Several hypotheses of LAI corrosion in this environment have been proposed, which include increase of aggressive chloride concentration due to evaporation, 2,26 ohmic potential drop due to crevice-like meniscus geometry, 6,27 and depletion of OH − due to CO 2 from air.…”

Deciphering the structural dynamics in molten salt–promoted MgO-based CO 2 sorbents and their role in the CO 2 uptake

“…4 is confirmed through the occurrence of a Raman band at 1325 cm −1 , due to the internal mode of NO 2 − , after the first regeneration step (2; Fig. 4B) (51). As in situ XRD (vide supra) did not evidence peaks due to crystalline NaNO 2 or Na 2 O, we postulate that ionic pairs of [Na + …NO 2 − ] and [2 Na + …O 2− ] (products of NaNO 3 decomposition) remained dissolved in the melt…”

Compatibility of materials for macroencapsulation of inorganic phase change materials: Experimental corrosion study

“…Literature reports that ALI corrosion can appear in pairs that do not show corrosion in fullysubmerged metal surfaces [28,29]. Although the mechanism of ALI corrosion is still not well understood [28,30,31], some authors relate it to the dissolved oxygen at the airliquid interface [32,33].…”