Chromium ion implantation of in625 for in-situ tem molten chloride salt corrosion

Abstract

Concentrated Solar Power plants (CSP) capture the Sun's thermal energy by concentrating sunlight to a central tower which transfers the heat to a thermal energy storage material (TES) such as molten salts. The TES containers can raise the temperature of water and create steam which powers a turbine generator to output electrical power. CSP is a renewable energy technology with flexible energy delivery for moments of high-energy demand, variable regional solar flux, and non-daylight hours. But for CSP to be utilized in a greater capacity, there remains the challenge to increase the overall efficiency and keep costs competitive. Use of chloride based salts could achieve this goal with their lower material cost, lower melting point (compared to other salt compositions), and stability at higher operating temperatures which increases efficiency being operated over a wider temperature range. However, chloride based salts have been found to be very corrosive when contaminated with water or oxygen, jeopardizing the lifetime of the container materials which use nickel (Ni) alloys. This study investigated corrosion in a NaCl-MgCl2-KCl salt environment to observe the influence of chromium (Cr) on Ni-Cr based alloy Inconel 625 (In625) which was patterned with local Cr concentrations using ion implantation. Two different masking methods for ion implantation- one with PMMA resist and the other a FIB milled physical mask- were used to create discrete Cr-enhanced regions in the alloy. The physical mask approach was successful in achieving a patterned alloy surface which could be suitably prepared for Transmission Electron Microscope (TEM) imaging. The implanted samples were then used in a recently developed in-situ technique using the TEM to study the corrosion at high-magnification and inert ambient of N2. The diffraction rings taken in each discrete region of implanted Cr (RI) and unaltered alloy (RU) both showed the formation of CrNiO4 with Ni(OH)2 found only in the non-implanted region. Also post-corrosion XPS analysis also confirmed the oxidation of Ni and Cr, which was to be expected based on the corrosion reactions reported in literature. This study demonstrates the advantages of in-situ TEM to study molten salt corrosion. Reduction of water and air contamination, as well as being able to simultaneously compare the influence of the alloy's composition on corrosion products will help in the understanding of the relationship between factors that cause severe corrosion. The possible contributions of this technique can lead to advancements which improve the lifetime and utility of CSPs.