1. Abstract Questions Results Methods & Materials A Learning Process
Proposal: Corrosion Resistance of Alumina Forming Stainless Steels for Use in Concentrated Solar Power Systems
Author: Miguel A. Jaimes, Undergraduate
Faculty Adviser: Benjamin C. Church, PhD
Material Science & Engineering
Abstract
Questions
Results
The EIS data will be interpreted by software to produce an equivalent circuit that best matches the oxide layer’s response to voltage applied by the potentiostat to the sample. It will also visualize that response with a Nyquist plot, like the one shown below, which will show how impedance of the oxide layer changes. The data will also be used to calculate the thickness of the oxide layer.
The surface of the oxide layer will be imaged and analyzed.
One of the biggest challenges of concentrated solar power plants is the corrosion of the storage containers and pipes used to transport molten salts which serve to store the sun’s heat energy. Stainless steels that are currently used  for this application form a chromium oxide layer that is not very resistant to hot corrosion, but new alumina forming steel alloys show very high resistance due to a protective Al-Cr rich oxide layer. Properties of this oxide layer such as thickness, surface structure/composition, and resistivity are of interest in this study. 
This oxide layer can be analyzed with electrical impedance spectroscopy (EIS), a technique that models the oxide layer’s macro electric properties with an equivalent circuit made with a combination of resistors and capacitors. Composition and surface structure of the oxide layer will be investigated through microscopy techniques such as confocal microscopy and scanning electron microscopy. A furnace will be used to melt a 60 wt% NaNO3 40 wt% KNO3 salt solution and when kept at 390 and 590 °C, samples of alumina forming steel will be tested against a graphite reference electrode inside the molten solution while connected to a potentiostat that records EIS data. All data will be analyzed using software that will interpret the raw EIS data into Nyquist plots and equivalent circuits which will be used to characterize the sample's oxide layer structure.  
Do alumina forming stainless steels show good corrosion resistance at working temperatures(500+ °C)?
How does electrical impedance spectroscopy provide insight into the nature of the resulting oxide layer?
Fig. 3 A Nyquist plot modeling impedance of a simple Randles cell(circuit).
(Gold Standard Corrosion Science Group)  
Fig. 2 SEM image of coated 310SS stainless steel oxide scale. (Gomez-Vidal)
Methods & Materials
To test the samples, the materials needed are a furnace, a thermocouple & controller, alumina crucibles and containers, nickel chromium wire, protective refractory coating for metal contacts, electrodes, steel samples, nitrate salts, and a potentiostat.
A controller will be built that will read data from a thermocouple to control the temperature of the furnace during testing.
A 60 wt% NaNO3 40 wt% KNO3 salt solution will be prepared and melted inside an alumina crucible. Samples and reference electrodes will sit inside this crucible.
Metal samples will be weighed before and after the testing on an analytical balance.
Wire will be welded onto the metal samples, and the wires will be coated with a sodium silicate and kaolin slurry. The wire will be encased in an alumina cylinder sealed with the refractory slurry for ease of handling.
The electrodes will be hooked up to the potentiostat for EIS readings.
After 10 hours in the melt, the metal samples will be removed, allowed to cool, and washed with DI water to remove salt, and the test will be complete.
EIS data will be used to create Nyquist plots for later analysis.
SEM and confocal microscopy will be used to examine the surface of the oxide layer.
Fig. 1 Schematic of a concentrated solar power plant. (Lydersen Kari, Jay Smith)
A Learning Process .
Preparing for this project has been a dynamic learning process. All of the readings have had me referencing course material from past semesters, and learning about EIS has inspired me to dive deeper into the fields of electrochemistry and corrosion. Working out practical issues for setting up experiments has been challenging and rewarding.
References
1.Fernández, A. G., et al. “Corrosion of Alumina-Forming Austenitic Steel in Molten Nitrate Salts by   Gravimetric Analysis and Impedance Spectroscopy: Corrosion in Molten Nitrate   Salts.” Materials and Corrosion, vol. 65, no. 3, Mar. 2014, pp. 267–75. CrossRef,   doi: /maco
2.Gomez-Vidal, Judith C. “Corrosion Resistance of MCrAlX Coatings in a Molten Chloride for   Thermal Storage in Concentrating Solar Power Applications.” Npj Materials   Degradation, vol. 1, no. 1, Dec CrossRef, doi: /s
3. Lydersen, Kari. “April 2018.” Discover Magazine, 28 May 2015, discovermagazine.com/2015/july-aug/26-power-stash.
4. “Electrochemical Impedance Spectroscopy (EIS).” Electrochemical Impedance Spectroscopy -   Gold Standard Corrosion Science Group - Boston MA,