Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials

Abstract

Molten chloride salts are promising advanced high-temperature (400-800°C) thermal energy storage (TES) and heat transfer fluid (HTF) materials in next generation concentrated solar power (CSP) plants for higher energy conversion efficiencies. However, severe corrosion of structural materials in contact with molten chloride salts is one of the most critical challenges limiting their applications at elevated temperatures. In this work, two corrosion mitigation strategies are investigated to alleviate the hot corrosion of structural materials in molten chloride salts: (1) adding corrosion inhibitor and (2) using a Fe-Cr-Al alloy with a protective alumina layer on the surface after pre-oxidation. Three commercial high temperature Fe-Cr-Ni alloys (SS 310, Incoloy ® 800 H and Hastelloy ® C-276) were exposed to molten MgCl 2 -NaCl-KCl (60-20-20 mole.-%) mixed salts with 1 wt.-% Mg as corrosion inhibitor, for 500h at 700°C under inert atmosphere. By addition of the Mg inhibitor, the corrosion rates of the studied alloys were found to be significantly reduced, more precisely by ~83 % for SS 310, ~70 % for In 800 H and ~94 % for Ha C-276 compared with the exposure tests without Mg addition. To assess the second mitigation strategy two pre-oxidized alumina forming Fe-Cr-Al alloys were exposed to the same molten chloride salts without Mg corrosion inhibitor under the same conditions. It is observed that the adherent alumina scales can effectively inhibit the dissolution of Cr and Fe and the bulk penetration of corrosive impurities. Overall, both strategies offer enormous potential for enhancing the expected lifetime of commercial alloys in molten chloride salts. Highlights  By Mg-addition the corrosion rates of alloy SS 310, In 800 H and Ha C-276 are significantly reduced. 2  Adding Mg inhibitor not only mitigates the corrosion caused by impurities, but also promotes the formation of protective MgO layer on metal surfaces.  The corrosion mitigation mechanism of Fe-Cr-Ni based alloys in molten chloride salts by adding Mg is discussed. Pre-oxidized alumina forming Fe-Cr-Al alloys show promising corrosion resistance and stability in molten chloride salts at 700°C for 500h exposure.

Purification strategy and effect of impurities on corrosivity of dehydrated carnallite for thermal solar applications

“…ICP-MS/OES results on non-treated, thermally purified, and thermally/chemically purified DC salts This Unported Licence.been known from literature16,[18][19][20] that intergranular attack of Cr by MgOH + is a common corrosion mechanism in heavily corroded Ni-Fe-Cr alloys such as in the case of the non-treated salt (estimated at >3200 mm per year) and thermally puried salt ($950 mm per year). When an active metal such as Mg is added into the molten chloride, intergranular corrosion can be signicantly reduced, as shown by Ding et al,21 who investigated corrosion of Hastelloy C-276 (C276) at 700 C in a ternary MgCl 2 -KCl-NaCl (60-20-20 mol%) made from commercial NaCl, KCl, and MgCl 2 with 1 wt% of elemental Mg addition. The corrosion rate of the C276 alloy in Ding et al (29.8 AE 8.7 mm per year) is very close to that of the Haynes 230 corroded in the thermally and chemically puried salt (at 800 C) in this work.…”

Facility to Alleviate Salt Technology Risks (FASTR): Preliminary Design Report with Failure Modes and Effects Analysis

“…Based on the previous purification experiences using Mg and the basic salt chemistry of the system of interest, it follows that adding Mg to the FASTR system will assist in maintaining the salt purity and ensuring a sufficiently low redox potential to inhibit corrosion of the system hardware. Indeed, adding Mg to chloride salt has been shown to reduce corrosion during static corrosion tests [5,11,12]. In addition, SETO recently sponsored development of a thermal convection corrosion loop constructed of Alloy 600 and operated with a peak temperature of 700°C for 1,000 hours.…”

Molten Chloride Thermophysical Properties, Chemical Optimization, and Purification

“…Molten chloride salts have been proposed as an alternative HTF chemistry for the next generation (Gen3) CSP technology operating at higher temperatures in order to maximize the benefit of using the sCO2 power cycle [1,2]. A significant number of research efforts [3,4,[13][14][15][16][17][18][19][20][21][22]5,[23][24][25][26][27][28][6][7][8][9][10][11][12] have been devoted to investigating molten chlorides' chemistry, thermodynamics, corrosion, and properties, especially the ternary system of NaCl-KCl-MgCl2 due to its availability and low cost.…”

“…1. The assumption in #2 above is not accurate because (1) physically absorbed moisture can be removed at the pre-drying step, 20 and (2) not all H2O released from dehydration of MgCl2 hydrates can react with MgCl2 to form MgOHCl at 100% efficiency.…”