THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Friday, June 12, 2020

9:00 AM

via

BlueJeans Video Conferencing

https://bluejeans.com/378093395

will be held the

DISSERTATION PROPOSAL DEFENSE

for

Kasey Hanson

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

Committee Members:

Prof. Preet M. Singh, Advisor, MSE

Prof. Chaitanya Deo, ME

Prof. Arun Gokhale, MSE

Prof. Hamid Garmestani, MSE

Prof. Joshua Kacher, MSE

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 degradation mechanism 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 at the alloy surface or not. Thermodynamic calculations were used to predict the equilibrium phase that can form in two model systems representing the two categories of molten salts. The first being an oxygen containing molten salt environment, in the presence of oxidizing gases, of superheater tubes in recovery boilers used in the biomass industry. Here, fluxing of protective oxides due to salt deposits expose alloys to gaseous environments as well as molten salts causing accelerated corrosion. As a model system for the second category, eutectic mixture of metal chloride salts was studied in the absence of any oxidizing gaseous environment. These molten salts are used due to their excellent heat-transfer properties as a coolant in molten salt reactors and energy storage systems. Materials for coolant loops experience attack from molten salt by selective dissolution of alloying elements that can be predicted by thermodynamic calculations.

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.