A novel in-situ corrosion sensor for structural alloys exposed to molten salts has been initially demonstrated. The measurement is based upon observing the change in magnetic susceptibility of salt wetted structural alloys as corrosion occurs. In halide salts corrosion of structural alloys proceeds primarily through dissolution of the least noble component of the alloy into the melt. All currently available structural alloys intended for use with molten salt reactors (MSRs) include nickel, chromium, and iron. Chromium is preferentially oxidized from the alloy surface by exposure to halide salts at high temperature. Diffusion within the alloy results in progressively deeper depletion of chromium from the alloy surface. Relevant chromium bearing structural alloys are paramagnetic. However, once the chromium has been depleted, they become ferromagnetic. Thus, structural alloy corrosion in an MSR results in development of a ferromagnetic surface layer whose depth increases with increasing corrosion. The corrosion sensor functions by employing the progressive increase in ferromagnetism as a transduction mechanism through including the corroding alloy in a magnetic circuit. To date we have characterized the sensor response of structural alloy samples with varying degrees of corrosion at room temperature. Over the next year, we plan to demonstrate sensor performance at MSR operating temperatures (up to 750 °C) in a piping geometry. Development of the sensor remains a work in progress as the aim is to install a corrosion monitor to operate over extended periods with only the corroding component exposed to salt (which could be the pipe itself). This configuration can be accomplished so that measurement magnetics and electronics are external to the pipe. Presumably, the instrument would continuously relay corrosion progress via electronic communications.
