Abstract The implementation of inexpensive scalable thermal energy storage will play a crucial role in the successful establishment of dispatchable renewable energy technologies. Storage based on molten nitrate salts is one of the most relevant technologies implemented in the GWh-scale. Yet, there is significant cost reduction potential in replacing the conventional two-tank system by a single-tank thermocline system. The latter involves the use of a single-tank design where a large fraction of the costly salt is replaced by an inexpensive filler material. Industrial waste materials such as AlferRock (red-mud-derived Fe2O3-rich ceramics), have significant potential since they are readily available in the Mt-range and have adequate thermal and mechanical resistance at the intended temperature of use. This study explicitly investigates the corrosion resistance of AlferRock in Solar Salt, 60% NaNO3-40% KNO3 mixture, at 560 °C. By variation of particle sizes classical exposure tests as well as accelerated test methods can be applied to understand the long-term stability of this waste material under relevant conditions. Salt chemistry and compositional changes in the filler are analyzed in terms of ion chromatography, titration, diffraction techniques (XRD) as well as electron microscopy (SEM) coupled with element mapping (EDX).