Vapor pressure and corrosivity of ternary metal-chloride molten-salt based heat transfer fluids for use in concentrating solar power systems

Abstract

Higher operating temperatures increase efficiency of the concentrating solar power plants but promote faster corrosion of the pipes and vessels made of Hastelloy or stainless steel materials for the molten-salt mixtures. Hastelloys C-276 and C-22 and stainless steel 304 coupons evaluated in the present study in a eutectic molten salt consisting of 13.4mol% NaCl, 33.7mol% KCl and 52.9mol% ZnCl2 showed substantially lower corrosion rates in the absence versus presence of air from 200 to 800°C as determined by electrochemical and gravimetric methods. In the presence of air, the corrosion rate for the Hastelloy C-276 in the molten salt was found to diminish with immersion time and converges around ∼50μm per year after 4weeks of immersion at 500°C, which is close to the value ∼40μm per year obtained using the electrochemical method at 500°C. For anaerobic corrosion rate estimation, the corrosivity of an alloy sample was examined by immersing in molten salt inside a sealed quartz container without any contact with air, which is possible because the vapor pressure of the eutectic molten salt is only about 0.7atm at 800°C. The corrosion rate of the Hastelloy C-276 was only 10μm per year in the molten salt in the absence of air at 800°C, which is extremely low compared to 500μm per year in conducting corrosion studies in the presence of air at 800°C. The Hastelloy coupons after immersion testing in the absence of air have then been examined also by SEM, and the images did not show any significant changes in the surface. This behavior indicates that, from a corrosion standpoint, the eutectic molten salt in the absence of air is suitable as a heat transfer fluid in Hastelloy C-276 pipes and containers up to 800°C.