Metal-Ionic Phase Reactions in Molten Salt Ionic Liquids: Experimental, Thermodynamic and Kinetic Analysis of the Alteration of Preformed-Oxides on Fe-Cr Alloys

Abstract

The object of this research is to develop a better experimental, thermodynamic and kinetic understanding of effects of various “harsh” environments on the passive preformed-oxide developed on model structural materials. Ultimately the passive oxide layer governs corrosion processes in molten salts. To this end, systematic studies were conducted on a range of model alloys from Fe to Fe-Cr and Cr as well as Ni and Ni-Cr alloys after passivation in aqueous solutions or by thermal “pre-oxidation in air”. The tested materials were Fe-8Cr, Fe-18Cr binary alloys and nominally pure Fe, Cr as reference metals. These model alloys were subsequently exposed to halide containing aqueous solutions, room temperature and high temperature ionic liquids with various salt species and impurities added in isolation. These may alter or dissolve the preformed-oxide film and ultimately govern the corrosion behavior. The final goal of this project will be to improve the scientific understanding using selected multi-component and model materials in classes of ionic liquids (molten salts) with and without radiation. Experiments have been focused on the non-irradiated materials to date. However, the irradiated materials will also be investigated in various molten salts and room temperature ionic liquids. The specimens were tested in a borate buffer solution to form a passive oxide layer (2~5 nm thickness) without forming compounds with elements of structural materials. This oxide layer was firstly analyzed in an aqueous chloride containing solution in order to understand its electrochemical properties under more common aqueous condition. Thermally grown pre-oxides at various temperatures, T = 200, 400, 600 and 800°C, and oxides designed by controlled sputter deposition were also investigated. Diagnostics was used to elucidate the type of films on metals, the growth rate and the controlling factors as a function of driving force such as formation potential or temperature. Electrochemical impedance spectroscopy (EIS) was used to first characterize the preformed-oxide. Capacitance and Mott-Schottky plots were utilized in order to quantify film thickness, growth rates versus time, semiconductor dopant type and defect density. Altered oxides and intact oxides were also characterized by XPS and FIB TEM analysis. Changes in these attributes as a function of time of exposure of preformed-oxides to ionic liquids and molten salts will be discussed. Acknowledgments This work was supported as part of Fundamental Understanding of Transport Under Reactor Extremes (FUTURE), an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Figure 1