Development of electrical insulator coatings for fusion power applications

In the design of liquid-metal cooling systems for fusion blanket applications, the corrosion resistance of structural materials and the magnetohydrodynamic (MHD) force and its subsequent influence on thermal hydraulics and corrosion are major concerns. The objective of this study was to develop stable corrosion-resistant electrical insulator coatings at the liquid-metal-structural-material interface, with emphasis on electrically insulating coatings that prevent adverse MHD-generated currents from passing through the structural walls. Vanadium and V-base alloys (VTi or VTiCr) are leading candidate materials for structural applications in fusion reactors. When the system is cooled by liquid metals, insulator coatings are required on piping surfaces in contact with the coolant. Various intermetallic films were produced on V, V5Ti, and V20Ti, V5Cr5Ti, and V15Cr5Ti, and Ti, and on types 304 and 316 stainless steel. The intermetallic layers were developed by exposure of the materials to liquid Li containing 3–5 at.% dissolved metallic solute (e.g. Al, Be, Mg, Si, Ca, Pt, and Cr) at temperatures of 416–880°C. Subsequently, electrical insulator coatings were produced by reaction of the reactive layers with dissolved N in liquid Li or by air oxidation under controlled conditions at 600–1000°C. These reactions converted the intermetallic layers to electrically insulating oxide-nitride or oxynitride layers. This coating method is applicable to reactor components. The liquid metal can be used over and over because only the solutes are consumed within the liquid metal. The technique can be applied to various shapes (e.g. inside or outside of tubes, complex geometrical shapes) because the coating is formed by liquid-phase reaction. This paper discusses initial results on the nature of the coatings (composition, thickness, adhesion, surface coverage) and their in situ electrical resistivity characteristics in liquid Li at high temperatures.