Electrochemical Study of Corrosion in High Temperature Molten Salts

Concentrating solar power systems are projected to need to produce energy with a cost of $0.06 / kWh when implemented on a very large scale.  It is generally believed that temperatures of the heat transfer fluids will need to be higher than 800^oC to drive an advanced Brayton cycle or Rankine cycle with superheated steam to reach this target.  However, current state-of knowledge of corrosion science indicates a lack of published data on the impact of molten salts at temperatures above 800°C on corrosion of molten salt contacting materials such as steels or high temperature alloys and ceramics. This work investigates the corrosion mechanism of selected alloys and ceramics in chloride and fluoride molten salts used for high operating temperature heat transfer fluids using a combination of electrochemical techniques, theoretical calculations, and thermodynamic predictions. The electrochemical characterizations included linear polarization resistance scans, electrochemical impedance spectroscopy, and cyclic potentiodynamic polarization.  Results show that the Fe-Ni-Cr alloys tested have high general corrosion and require corrosion inhibition. Advanced materials, such as SiC-SiC and TZM, have been shown to have much lower corrosion and good passivation behavior.  Experimental tests using coupon immersion in the molten salts support the mechanism determined from electrochemical testing that the main element being oxidized is Cr. Analysis of samples also indicates selective Cr oxidation creates porous layers in the sample. The electrochemical impedance tests suggest that the corrosion is diffusion controlled.