Corrosion evaluation of austenitic stainless steels in Li2CO3-K2CO3 eutectic salt for thermal energy storage

Abstract

Molten carbonate salt is one of the promising candidates for high-temperature thermal energy storage tailored towards advanced pumped-thermal energy storage and next generation concentrated solar power technologies. However, severe corrosion of structural materials exposed to molten carbonate salts poses one of the most pivotal threats in terms of limiting their large-scale application at ever-escalating temperatures. Here, we elaborate the compatibility of three commercial austenitic stainless steels specifically SS310, SS316L and Incoloy 625 with Li2CO3-K2CO3 (28–72 wt%) eutectic salt. The corrosion behavior and corrosion mechanism of Fe-Cr (SS316L), Fe-Cr-Ni (SS310) and nickel-based (In625) alloys at 600 °C in air were also investigated. It revealed that SS310 exhibited better corrosion resistance compared to SS316 and In625, with weight loss of 23.8 mg/cm2 and corrosion rate of 522 μm/year after 500 h of exposure at 600 °C. This implies that higher Ni content of the alloy may not necessarily improve the corrosion resistance, it is associated with the basicity of the molten and the solubility of Ni salt in the melt. ICP analysis indicated that the salts exposed to SS310 and In625 at the same exposure time both had nearly 11 times more Ni than that exposed to SS316. Precisely, it is related to the behavior of the Cr-enriched zones formed in the corrosion layers of different alloys. The corrosion mechanism of SS310 specimens in molten carbonate can be generally divided into the following steps: (i) selective oxidation of the metal elements and lithiation reactions of the metal oxides to generate a passivation scale, (ii) an ever-growing middle transition scale consisted of mixed metal oxides and lithium-containing compounds, (iii) cracking and peeling off of the corrosion scale.