Development and validation of electrical-insulating Al2O3 coatings for high-temperature liquid PbLi applications

Electrical-insulating coatings are of great importance for liquid-metal breeder/coolant based systems relevant to fusion power plants. In specific to Pb-16Li eutectic, a candidate breeder material, such coatings are being actively investigated for their criticality in addressing various functionalities. For such applications, a candidate coating must be demonstrated for its compatibility with corrosive media, high operational temperatures and integrity of electrical-insulation over long durations without substantial degradation. At present, no relevant in-situ insulation resistance (IR) data is available for performance assessment of coated substrates within PbLi environment. To address this shortfall, an experimental study was performed at IPR towards application of high-purity alumina coatings on SS-316L substrates and further rigorous validation in static PbLi environment. The adopted coating process required a low-temperature heat treatment (< 430C) and could yield average coating thicknesses in the range of 100-500 micron. Coated samples were validated for their electrical insulation integrity in static PbLi over two test campaigns for continuous durations of over 700 h and 1360 h, including thermal cycling, at operational temperature between 300C-400C. Estimated volumetric electrical-resistivity remained of the order of 109-1011 ohm-cm without significant degradation. In-situ estimations of thermal derating factors establish excellent electrical-insulation characteristics after long term exposure to liquid PbLi. This paper presents details of utilized coating application methods, coating thickness estimations, liquid-metal test set-up, insulation performance and critical observations from SEM/EDX and XRD analysis on the tested samples.