Sensing and Arresting Corrosion of Haynes 230 Alloy in Molten Chloride Salts at 800°C

Abstract

The corrosion of metal in molten chloride salt is studied and lowered using a power supply. A particular emphasis is on the ternary eutectic sodium chloride, potassium chloride and magnesium chloride (MgCl2-KCl-NaCl) salt with a melting point of 387°C, because it is the high temperature heat transfer fluid of choice in electrical power generators and Haynes 230 alloy (H230), because H230 is a ductile metal which retains its strength at high temperatures (800oC). A potential negative of the open circuit potential of H230 metal alloy in ternary eutectic MgCl2-KCl-NaCl is applied, the cathodic potential generates a negative (cathodic) current for the reduction any oxidants, such as metal ions, oxygen and water, in the molten salt. The magnitude of the cathodic current is a signal of the level of oxidants present in the salt. Applying the cathodic potential also arrests ionization of metal, that is, corrosion of the metal. The increasing level of oxidant impurities, (particularly water) in the molten chloride salts causes the open circuit potential (OCP) of H230 versus a silver and silver chloride reference electrode (SSE) to shift positive, which gives a warning that the molten chloride salt heat transfer fluid is corrosive to metal. The OCP is crudely measured by direct readings of H230 and SSE with a voltmeter and refined by potentiodynamic scans of current versus potential of H230 versus SSE, where the H230 potential is scanned 30 millivolts starting negative of to positive of the OCP found with the voltmeter. A linear rate equation, called the Stern-Geary method is used to find corrosion potentials and estimate corrosion rates (CR) of H230 alloy in molten salt at various relative humidity (RH) of Argon atmospheres equilibrated with the molten salt. In oxidant free ternary MgCl2-KCl-NaCl eutectic molten salt, the OCP of H230 vs SSE is -866±24 [mV] and CR of 85±9 [micron/year]. In ternary eutectic molten salt equilibrated with a 40% RH Argon flow the OCP of H230 vs SSE is -363±1 [mV] and CR of 4670±780 [micron/year]. When a negative potential, of -200mV from OCP in anaerobic salt, is applied to a H230 working electrode in eutectic molten salt at 800oC and 100% RH Argon flow is flowed over salt for 20h, it was found that this H230 working electrode (WE) was cathodically protected, because the WE (cathode) lost only 0.0552g while the counter H230 electrode (CE), a “sacrificial” anode, lost 0.4513g. This gives a preliminary assessment a cathodic potential is effective for arresting corrosion of H230 metal in oxidant-contaminated salt at temperatures up to 800oC.