Vapor space corrosion data collected by electrochemical noise (EN) based corrosion probes installed in double shell tanks (DSTs) at the Department of Energy's Hanford Site in Richland, Washington have historically been characterized by surprisingly high levels in current. In late 2003, a program was established to assess the significance of archived Hanford DST vapor space EN data. This program showed that the high vapor space current levels are likely the result of crevice corrosion on the vapor space electrodes. The design of DST vapor space electrodes provides tight metal-to-metal and gasket-to-metal interfaces necessary for this type of localized corrosion to occur. In-tank activities (splashing, etc.), or more likely condensation of water vapor in the vapor space, provide the necessary moisture. Because crevice corrosion appears to be active on the vapor space EN electronics, data collected from these electrodes are not likely to be applicable to the large flat metal surfaces that make up the bulk of the DST domes and upper walls. The data do, however, indicate that conditions in the DST vapor spaces are conducive to accelerated crevice corrosion at creviced areas in the tank vapor space (overlapping joints, riser interfaces, equipment penetrations, etc.) under high humidity conditions.
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