Electrochemical Control for Corrosion in Molten Chloride Salts

Abstract

Concentrated solar power (CSP) with thermal energy storage (TES) has the ability to help solve the renewable energy grid integration challenge with its electricity dispatch capability. However, utility-scale adaption of CSP-TES has been slow due to the high levelized cost of electricity (LCOE). One avenue for cost reduction is the increase in operating temperature of the CSP plant to increase the thermal efficiency of the Carnot cycle. This requires finding a new salt blend with higher thermal stability up to 800 ℃. A ternary MgNaK chloride salt is a prime candidate with its low cost, high thermal stability, and good heat transfer properties. However, its commercial deployment is challenged by the high corrosion rates, leading to the use of expensive alloys; thus, increasing the capital cost and maintenance cost of the CSP plant.A corrosion control system in MgNaK chloride salts using active metals such as Mg has been demonstrated under laboratory conditions. This mitigation strategy involves controlling the corrosive impurities, such as MgOHCl, by using liquid Mg. The elemental Mg scavenges oxygen and water derived impurities and provides redox reaction control by shifting the electrochemical potential of the alloys towards cathodic protection while Mg acts as the sacrificial oxidant. However, employment of Mg is challenged in the cold temperature section of the CSP plant (400 – 550 ℃) due to its slow dissolution kinetics in the solid state (melting point of 650 ℃).Here, we propose an electrochemical approach using bulk Mg anodes and W cathodes to provide corrosion control. We used electrolysis to enhance dissolution kinetics of solid Mg to the molten salt at temperatures relevant to the cold section of the CSP plant (500 ℃). Our data suggests electrolysis of Mg effectively enhances the rates of removal of corrosive impurities from molten chloride salts. Preliminary results indicate significant opportunity for the electrochemical process to be scaled up commercially. The novel approach eliminates impurities in-situ from oxygen and moisture ingress in the CSP plant, in turn reducing corrosion rates and therefore reducing capital and maintenance cost of the CSP plant.