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. © 2013 The Electrochemical Society.