Influence of different atmospheres on molten salt chemistry and its effect on steel corrosion

Abstract

Dispatchability of renewable solar energy can be realized by integrating thermal energy storage units. In concentrating solar power plants the use of sensible heat storage based on molten nitrate salts, typically Solar Salt (Na,K//NO3), has proven most beneficial in the last decade. However, a crucial parameter that affects the long-term performance of the TES unit however, is the sustainability and reliability which is directly linked to the material performance of the molten salt and the construction material. Metallic corrosion is one of the biggest concerns due to the harsh conditions provided by the redox active molten nitrate salts. The mechanisms of corrosion are thought to be well understood which is reflected by a significant number of publications in the last decades. Despite, an essential parameter is often ignored or underestimated in many studies: the molten salt chemistry. This work is one of the first addressing directed degradation of Solar Salt by controlling the gas atmosphere in the storage system, thus driving the formation of corrosive impurities, especially oxide species. Austenitic, stainless Cr,Ni-steel and ferritic Cr-steel samples are subjected to the different operating conditions to demonstrate the variations in corrosivity as a function of gas atmosphere and additionally of artificially added chloride impurities. The experimental matrix allows for a comprehensive analysis of the influence of different corrosive species on the stability of different steel types over the course of 1.200h experiments performed at 560°C.