Dimensionless Analysis for Predicting High Temperature Alloys Corrosion in Molten Salt Systems for Concentrated Solar Power Systems
Authors: Cho Hyun-Seok , Van Zee John William , Shimpalee Sirivatch , Tavakoli Bahareh , Weidner John W. , Garcia-Diaz Brenda L. , Martinez-Rodriguez Michael J. , Olson Luke Christopher , Gray Joshua R. 
Published: February 2020

In the development of concentrated solar power systems (CSPs), the high-temperature corrosion of alloys in receivers and heat exchangers can affect the performance on CSPs and influence the apparent durability. [1-4] The extent to which high-temperature corrosion is reversible may improve the commercialization. To understand corrosion mechanisms and their impact on alloys, experimental coupon (1.2mm×12mm×30mm) studies were conducted to the 700-1000°C within halide and fluoride salts in a thermosiphon. The thermosiphon reactor was designed to allow exposure the coupons to the both isothermal and non-isothermal condition. That is, the thermal gradients between top and bottom of the reactor could induce natural convection of the salts. The corrosion experiments such as coupon test and post mortem analysis highlighted main corrosion mechanisms of Cr depletion and diffusion along the grain boundaries. [5-6]
A multi-physics corrosion model that include the main corrosion mechanism of Cr oxidation along with diffusion-limited corrosion as the limiting step allow to predict corrosion rates (i.e., current density) and corrosion potentials. Also, a CFD model has shown that temperatures and fluid flows can be accurately predicted in molten salt heat transfer systems including coupons. The CFD predictions allow to characterize natural convection flows in the reactor by analyzing engineering parameters such as Rayleigh number, Nutssel number, and Grashof number, etc. As the temperature and mass transports are coupled in the natural convection system, the analysis of these dimensionless numbers helps to correlate different physics to understand the system better and to reduce complication to solve the problem. These dimensionless numbers around coupons incorporated into the corrosion model allow to predict mass transfer coefficients and boundary layer thickness that determine corrosion rates (i.e., current density).
The model predictions and experiment results suggest that the use of thicker materials to improve the lifetime may not be a good strategy in molten salt systems due to the localized nature of the corrosion.

Acknowledgements

The authors gratefully acknowledge the support for this work by the DOE EERE SunShot Initiative under a subcontract from SRNL to the University of Alabama and to the University of South Carolina.

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