Session: 04-02: Particles and Materials for Energy Storage

Paper Number: 160863

160863 - Evolution of Elemental Distribution in Structural Alloys Exposed to High Temperature and Mgcl2 Pcm as Revealed by High Resolution X-Ray Fluorescence 

Abstract: 

Efficient energy storage technology is a primary requirement for the global transition towards clean and renewable energy sources like solar and wind that are inherently intermittent in nature. Phase change materials (PCMs), which store/release heat during endothermic/exothermic phase-change reactions, are promising candidates for next generation electrothermal energy storage applications because of their higher energy density. Chloride-based salt PCMs such as MgCl₂, are among the most promising candidates owing to their higher temperature and enthalpy of melting, and low material cost. However, due to the highly corrosive nature of chloride salts, corrosion of structural materials used to contain the PCM remains a challenge for their practical applications. During corrosion, unwanted reactions and mass transfer of alloying elements are observed near the alloy-PCM interface which changes the elemental composition near the interface and within the PCM. Such selective diffusion of elements and the formation of corrosion products near the alloy-PCM interface not only alters the mechanical and chemical strength of the container alloys but also degrades the thermophysical properties of the PCM system. Although the interaction of MgCl₂ with metallic containers has been studied to some extent, a comprehensive understanding of the interfacial interactions and corrosion mechanisms at alloy-PCM interface remains unknown, particularly for the phase-change case where the PCM interactions with the alloy can be affected by the transition between liquid and solid states. Such understanding of corrosion mechanisms is critical for both the development of predictive multiscale corrosion models, and the engineering of high performing PCM materials for TESM applications.

In our approach, we utilize synchrotron-based X-Ray Fluorescence (XRF) and X-Ray Absorption Near Edge Structure (XANES) spectroscopic imaging with submicron resolution available at 5-ID beamline of National Synchrotron Light Source – II (NSLS-II) to elucidate the chemical state, distribution, and possible segregation of alloying elements in the corroded region and near the alloy-PCM interface.  Synchrotron XRF studies are complemented with scanning electron microscopy (SEM) imaging providing us with the morphological and distribution of alloying elements near the alloy-PCM interface. Such detailed information of the interfacial region provides mechanistic insights about mass transfer near the interface and the consequential degradation of containment materials in MgCl₂ PCM system.

In this study, we investigate the corrosion of commercial structural alloys (Stainless Steel 304 and Inconel 617) in MgCl₂ PCM as a function of number of thermal cycles (exposure time). Owing to their varied corrosion-resistance properties (e.g. reduction potentials), the alloying elements are non-uniformly corroded and we observe migration, segregation and preferential dissolution of the alloying elements near the alloy-PCM interface using submicron resolution XRF imaging. Simultaneous XANES measurements inform us of the chemical changes (oxidation state) of the alloying elements and the possible formation of corrosion products. We map the chemical evolution of these alloys near the alloy-PCM interface, showing preferential dissolution of alloying elements and MgCl₂ penetration into the alloy. We observe sensitization of steels after 500 cycles and increased alloy-PCM interaction with longer exposure times, with onset of corrosion as early as 100 thermal cycles and becoming severe around 500 thermal cycles. These results inform us about the initial stages of corrosion, mechanistic understanding of structural degradation at molecular level as well as corrosion as function of alloy type and number of thermal cycles (exposure time). Such insights will aid in developing efficient and robust electrothermal energy storage systems employing energy-dense chloride-based PCMs.

Presenting Author: Diwash Dhakal Brookhaven National Laboratory

Presenting Author Biography: Diwash Dhakal is a postdoctoral researcher at Brookhaven National Laboratory working on synchrotron X-ray characterization of advanced materials for energy applications. He uses XAS, XRF and XRD to probe the structural and chemical properties of materials especially under in-situ or operando conditions.