High-resolution and in-situ characterization of the material behavior of Fe-based CrNiMo alloys exposed to geothermal-type fluids

• Applicant: Dr. Niklas Mundhenk
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2015 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 290015776

In applications where corrosion is a critical factor, typically corrosion-resistant and relatively inexpensive Fe-based CrNiMo alloys are employed. Among these materials, austeno-ferritic duplex alloys are particularly promising for a wide range of applications and are regarded as showcases for the importance of microstructure on the corrosion behavior. The proposed research aims at studying the behavior of duplex alloys, simpler model systems (austenite, ferrite), and elemental reference materials (Cr, Mo) under conditions relevant to geotechnical applications (e.g. utilization of geothermal energy, CO2 storage, nuclear waste disposal).Reflecting the current state of geothermal research, corrosion environments with temperatures up to 300°C are mimicked in the lab. Multi-component fluids (Na-K-Ca-Mg-Cl-SO4-H2O system) with varying TDS (total dissolved solids) that closely resemble real world fluids from geothermal sites in Germany are synthesized. Such corrosion environments have not yet been adequately researched, but are crucial for future subsurface activities.Central aspects are:- Testing the stability limits of the test alloys in geothermal corrosion environments- Studying the fundamental processes with regard to passivity/breakdown of passivityInstead of just prioritizing the macroscopic corrosion resistance, a special emphasis is laid upon the interfacial reactivity, which requires a combination of electrochemical (cyclic voltammetry CV, impedance spectroscopy EIS), spectroscopic (in-situ surface-enhanced Raman spectroscopy SERS), and microscopic methods (atomic force microscopy AFM) to be employed. CV and EIS are used to determine the thermodynamic domains of the test alloys and to probe the passive film with respect to its electronic properties and protectiveness. SERS enables to quasi-continuously monitor the dynamic of growth and modification of the passive film, while the sample is kept under potentiostatic or -dynamic control. Compositional and structural changes will reveal breakdown mechanisms (e.g. chloride-induced pitting) resulting in high local dissolution rates. AFM and in-situ hydrothermal AFM (HAFM) are employed for sub-nanometer scale surface characterization, HAFM being capable of real-time imaging at up to 150°C.Following an interdisciplinary approach, the project encompasses a range from fundamental to applied research and provides valuable insights into the dynamic electrochemical and physical behavior of CrNiMo alloys exposed to fluids with varying TDS in an application-relevant temperature range. On the basis of an deeper understanding of the interfacial dynamic, it assesses the suitability of important alloy representatives and helps to improve the corrosion resistance. Due to the superior nature of in-situ techniques and the synergistic combination of methods, the results have a high relevance for material assessment, not solely limited to the alloys and environments considered in this study.

• DFG Programme: Research Fellowships
• International Connection: USA
• Hosts: Professor Thomas Devine, Ph.D.; Kevin Knauss, Ph.D.