Georgia Institute of Technology Materials Science and Engineering

Committee Members: 

Prof. Preet M. Singh, Advisor, MSE

Prof. Chaitanya Deo, ME

Prof. Arun Gokhale, MSE

Prof. Hamid Garmestani, MSE

Prof. Joshua Kacher, MSE

"Understanding Degradation Mechanisms of Metallic Alloys in High-Temperature Molten Salt Environments"

Abstract:

Selection of structural alloys for harsh environments require understanding of material degradation mechanisms to enhance material lifespan, process efficiency, and overall safety.  One category of harsh environments for metallic materials is molten salts where individual salts or mixtures of carbonates, sulfates, nitrates, halides etc. can induce significant material degradation at elevated temperatures and is commonly referred to as molten salt corrosion.

This study separates molten salt corrosion into two different categories: oxygen bearing salts and non-oxygen bearing salts. Investigation of the material degradation mechanisms for each category is performed through the use of two different model systems. Metallic alloy selection and performance in the two categories of molten salt environments depends on whether a stable protective oxide can form on the alloy surface or not. The first category is an oxygen containing molten salt environment. The model system selected for this category was superheater tubes in biomass recovery boilers. Here, protective oxides can be fluxed due to the molten salt mixture: Na2SO4-Na2CO3-KCl-K2SO4. Moreover, the desired operating temperature range approaches this mixture’s first melting temperature, thereby accelerating corrosion not only from the molten salt mixture, but by oxidizing gaseous environments simultaneously. As a model system for the second category, eutectic salt mixture of KCl-MgCl2 was studied in the absence of any significant oxidizing gaseous environment. Material candidates for coolant loops in Molten Salt Reactors (MSRs) experience attack from molten salt by selective dissolution of active alloying elements. Thermodynamic calculations can be used to predict the extent to which alloys will suffer corrosive attack. The severity of attack can be controlled through the use of impurities in the molten chloride salt.

Similarities and differences in the corrosion mechanisms for the two very different categories of molten salt environments is systematically studied in this project. Understanding of the various mechanisms of material degradation induced by molten salts can be used to better inform materials selection, engineering and design in the application of structural alloys.