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Radioactive and chemically hazardous wastes are stored at the Hanford site in underground double-shell tanks (DSTs) constructed of carbon steel. The corrosion management of these tanks has mainly focused on the complex waste chemistries that are contained in the primary tank. More recently, attention has been given to the corrosion that has been found on the exterior of the secondary liners of the DSTs. The cause of the external corrosion could be related to the intrusion and accumulation of water in the tertiary leak detection systems, which exist under the concrete foundation of the DSTs. However, site investigations have discovered significant reductions in wall-thickness (up to 70%) for DSTs that do not have a history of water intrusion. Furthermore, site measurements have indicated that wall-thinning may not be as severe at locations where the tank exterior is in contact with the slotted concrete foundation. Laboratory tests involving periodic wetting and condensing moisture on steel specimens were conducted to better understand this external corrosion threat. A simulated groundwater solution of near-neutral pH served as the wetting or moisture source in the tests. The laboratory tests consisted of weight-loss evaluations along with more involved studies using a multi-electrode array (MEA). The effect of concrete on the corrosion of steel was also examined by incorporating areas of metal-to-concrete contact on the weight-loss specimens and MEA. The results of this testing revealed that corrosion rates, calculated from weight-loss, can underestimate wall-thinning due to the severe localized attack that was found. Also, the inhibition of corrosion due to concrete-contact may relate more to the obstruction of condensation on the steel rather than an influence on the local pH.
Keywords: Hanford, carbon steel, vapor phase corrosion, pitting, multi-electrode array, concrete
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Aboveground storage tanks (AST) soil-side corrosion protection is commonly implemented using cathodic protection (CP) technology. U.S. Department of Transportation mandate use of CP system for the tanks falling under federal regulations purview. In case of a double bottom tank i.e. a tank where original (old) bottom has been rendered useless and a second (new) bottom has been installed couple of inches above the original bottom the anodes are laid in the sand bed just above the old bottom. An impressed current cathodic protection (ICCP) system with an MMO anode is a preferred for this application however there are instances where a galvanic anode (GCP) system is also used for tank bottom protection. For effective operation of the CP system the current generated by the anode is a determining factor which mainly depends on the interface resistance between the anode and sand bed. In an ICCP system the current output can be driven to counter the circuit resistance of CP system whereas for a GCP system the potential difference between the anode and cathodeis the key driving factor. In other words the GCP system cannot function in a high resistivity sand bed system. Vapor Corrosion Inhibitors (VCI) are being used in combination with CP systems for the tank bottom soil side applications. VCIs tend to change the interface resistance by contact with the metal (anode and cathode) surfaces.In this paper investigations are carried out to understand the effect of VCIs on impressed current (MMO) and galvanic anodes and their ability to function in a VCI saturated electrolyte system. The passivation behavior of the anode is analyzed using electrochemical tests and the performance is evaluated. The paper also reports the simulated long-term performance of the ICCP and GCP system in the presence of VCIs.