Dimensionless Analysis for Predicting Fe-Ni-Cr Alloy Corrosion in Molten Salt Systems for Concentrated Solar Power Systems

In high operating temperature concentrated solar power (CSP) systems, the use of molten salt heat transfer fluids causes corrosion of alloys in receivers and heat exchangers that decreases both heat transfer efficiency and system lifetime. Mitigation of high-temperature corrosion would improve the system lifetime, increase heat transfer efficiency, and thus improve the economics of high-temperature CSP systems. A corrosion model has been developed that includes fluid flow and the selective oxidation of Cr, which has been observed as the main corrosion mechanism for high-temperature alloys in contact with molten salts. This model is able to predict corrosion rates and corrosion potentials by coupling the reaction mechanism with computational fluid dynamics (CFD) to accurately predict temperatures and velocity profiles in molten salts. Experimental studies were also conducted in the temperature range of 700°C to 1,000°C with MgCl₂-KCl salt in a thermosiphon designed to allow exposure of the coupons to controlled non-isothermal conditions for model validation. The CFD simulations predict the flow resulting from natural convection in the thermosiphon and allow analysis of dimensionless engineering parameters such as the Rayleigh number, Nusselt number, and Grashof number. The values of these dimensionless parameters allow estimation of mass transfer coefficients and boundary layer thicknesses for determination of corrosion rates. The model predictions and experiment results suggest that the selective oxidation of Cr is mass transfer driven and could be further aggravated in CSP systems with forced convection.