Stability of Metal in Molten Chloride Salt at 800˚C

Abstract

The stability of Haynes 230 and Hastelloy C-276 nickel alloys exposed to high temperature molten salt with trace contaminants (i.e., water and oxygen) is found to be acceptable for using these metals to house anaerobic MgCl2-KCl and NaCl-KCl-ZnCl2 molten salts at 800oC. The corrosion rate determined by gravimetric tests range from -98 µm/year to 20. 13 µm/year at 800˚C. The corrosion rate is estimated to be 16.14 µm/year for Haynes 230 and 10.03 µm/year for Hastelloy C-276 based on the weight loss and surface area of the coupons when the coupons of Haynes 230 and Hastelloy C-276 alloys are immersed in molten MgCl2-KCl salt in sealed quartz containers and left in an oven at a temperature of 800˚C up to 16 days. The corrosion rate is estimated to be -20.46 µm/year for Haynes 230 and -7.36 µm/year for Hastelloy C-276 based on the weight loss and surface area of the coupons when the alloys are immersed in molten NaCl-KCl-ZnCl2 salt in sealed quartz containers and left in an oven at 800˚C up to 56 days. The corrosion rate of the alloys are well below the DOE requirement of 50 µm/year for the alloys in molten chloride salts to be considered acceptably stable. Ultimate tensile strength (UTS) after immersion of Haynes 230 and Hastelloy C-276 in molten salt ranged from 634 MPa to 860 MPa. The UTS of Haynes 230 is estimated to be 642 MPa after exposure to NaCl-KCl-ZnCl2 for 4 weeks at 800˚c and 841 MPa after exposure to MgCl2-KCl for 4 weeks at 800˚c compared to an untreated sample which achieved a UTS of 851 MPa. Likewise, the UTS of Hastelloy C-276 is estimated to be 692 MPa after exposure to NaCl-KCl-ZnCl2 for 4 weeks at 800˚c and 842 MPa after exposure to MgCl2-KCl for 4 weeks at 800˚c compared to an untreated sample which achieved a UTS of 830 MPa. Molten chloride salts, such as NaCl-KCl-ZnCl2 and KCl-MgCl2, are pretreated by heating and bubbling dry Argon gas in the salt in order to remove oxygen and water and thereby reduce the corrosion of metal containers of molten salt. Monitoring the relative humidity and percent oxygen of the exhaust gas during the sparging of dry Argon at 240 sccm into 150 g of molten chloride salt at 500˚C for NaCl-KCl-ZnCl2 and 700˚C for KCl-MgCl2 allows an estimation time to reach a low level of oxygen and water in the salt and to estimate the amount of oxygen and water removed. Results show water is more difficult to remove than oxygen from the salt. Ten minutes of sparging with dry argon brings oxygen content of exhuast gas to<0.1% O2. Approximately fifty minutes of sparging leaves the exhaust gas only containing<0.7% RH. The total moles of oxygen removed from the salts are estimated to be 0.0043 moles for molten NaCl-KCl-ZnCl2 and 0.0076 moles for KCl-MgCl2. The total moles of water removed from the NaCl-KCl-ZnCl2 salt is estimated to be 0.016108379 moles and 0.002321214 moles from molten KCl-MgCl2.