How molten nitrate salt altering affects steel corrosion

Abstract

Thermal Energy Storage (TES) coupled with concentrating solar power (CSP) has proven competitive with conventional solar harvesting techniques such as PV but offers substantial advantages due to the sheer presence of the TES unit itself. Modern CSP-TES plants accommodate 2-tank based storage units which operate between 290°C and 565°C using molten nitrate salts, such as the classical Solar Salt, a 60-40 wt.-% NaNO3-KNO3 mixture. Despite their expected life-time of 20+ years and the availability of numerous corrosion studies being published to date, a critical and comprehensive view on both the molten salt (MS) chemistry and the corrosion effects is often not presented and it remains unclear how these phenomena are associated. The results of a series of experiments conducted recently by DLR and MPA outline that the MS chemistry has drastic effects on corrosion rates, and additionally that the type of impurity drives the corrosion mechanism. This impact will be presented using a martensitic high temperature steel (T91) and an austenitic stainless steel (AISI 316Ti) exposed to isothermal storage for 1.200 h at 560 °C in Solar Salt with precise control of the atmosphere, either synthetic air or nitrogen, to gain chemically equilibrated or continuously decomposing MS. Moreover, the same set of experiments has been carried out with the addition of impurities using 0.5 wt. % chlorides. As a short outlook it can be stated that AISI 316Ti and T91 show different susceptibility to impurities, as well as differential dependence on atmospheres. While the corrosion behavior of AISI 316Ti was influenced by the nitrite- and oxide-ion ratio in the MS, T91 was more sensitive to the presence of chloride impurities forming thicker oxide layers with the tendency to spall. The work demonstrates that the gas environment and thus the MS chemistry, investigated by chromatography, titration and atomic absorption spectroscopy, directly affect the corrosion behavior. It can be concluded that a simultaneous examination of MS chemistry and metallic corrosion is indispensable to understand metallic corrosion phenomena in molten nitrate salts.