Corrosion Protection of Metallic Waste Packages Using Thermal Sprayed Ceramic Coatings
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ABSTRACT Ceramic coated carbon steel coupons were corrosion tested in water with dissolved salts to simulate exposure to evaporation concentrated groundwater in an underground nuclear repository. Metallography revealed no corrosion at the ceramic metal interface of dense coatings, even though electrical measurements demonstrated that the coatings were slightly porous. Experimental results and a model to predict corrosion rates influenced by a porous ceramic coating and coating lifetimes are presented. INTRODUCTION Certain refractory ceramic oxides have desirable properties for the construction of containers for long-term use in nuclear waste disposal applications such as at the potential repository site under study at Yucca Mountain. Ceramics are thermodynamically stable against further oxidation and far less prone to environmental corrosion than metals under realistic repository conditions. In studies by the Swedish Corrosion Institute [1], the aqueous dissolution rate of alumina (A120 3) in distilled water was measured to be in the range of 10 nm/year and was virtually unmeasurable in groundwater. Spinel (MgAl20 4) and titania (TiO 2) are expected to behave similarly. Oxide ceramics are unlikely to be subject to microbiologically influenced corrosion (MIC), which may attack most of the available engineering metals over time. Ceramics have a reputation for poor mechanical performance, and sufficiently large, impermeable vessels are not easily fabricated in large numbers. Current waste package designs are based on multiple metallic layers to provide handling capability and "defense in depth" because of varying corrosion mechanisms for the various layers. The most promising approach for incorporating ceramics in large waste packages is as a low porosity protective coating to a supporting metallic structure, such as the steel "corrosion allowance material" (CAM) that has been part of the primary design focus of the Yucca Mountain Project (YMP). Coatings applied by thermal-spray can be effectively seamless and offer a method for coating over the final closure weld while not unduly raising the average temperatures within the waste package. Without liquid or vapor phase water, electrochemical corrosion and MIC processes on metals are considered impossible, so if an impervious ceramic coating could be fabricated, it could protect the metal vessels for a long time related to the ceramic corrosion rate. Even an imperfect coating should extend the life of the package, delaying the onset and reducing the severity of corrosion by limiting the transport of water and oxygen to the ceramic-metal interface. If oxygen transport is impeded, the corrosion rate will decrease. There is a presumption that all thermal sprayed coatings will be porous at some level, so the model which follows is an attempt to account for increased impedance to oxygen transport due to a porous coating and predict a resulting corrosion rate. EXPERIMENT Porous (19% porous) alumina coatings were produced on cylindrical carbon steel substrates using convention
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