Impact of Solar Salt ageing on the corrosion of SS316

Abstract

The increasing demand for renewable energy sources subsequently led to the development of new energy storage solutions. Thermal Energy Storage (TES) systems that use molten Solar Salt (60 wt.% NaNO3/40 wt.% KNO3) as storage medium and heat transfer fluid are one of the most promising solutions regarding scalability to the GWh-scale, economics and intrinsic compatibility with a multitude of thermal processes. Despite their commercial utilization in Concentrated Solar Power (CSP) plants, long term corrosion studies under controlled salt decomposition, are not available. Hence, the impact of molten salt ageing on the lifetime of the TES system is mostly unknown. Due to thermal decomposition of the nitrate ion in the Solar Salt, multiple types of soluble oxide ion species (O2-, O22-, O2-) may develop at elevated temperatures [1]. These impurities are acknowledged to possess significant corrosive properties towards metallic components. In earlier work, Slusser et al. showed the increase in corrosion rate with increasing sodium peroxide concentration in molten nitrate-nitrite salt [2], also Federsel et al. published results on the increase in corrosion rate depending on the concentration of sodium oxide in a ternary nitrate/nitrite salt [3]. However, the interaction of these oxide species during corrosion and the underlying corrosion mechanisms are unexplored, mostly due to the complexity of reproducing and controlling the salt chemistry in the lab scale. In this work a systematic approach is used to identify the critical concentration of oxide ion species and compare the corrosive properties of different oxide ion concentrations in Solar Salt.