Corrosion of High-Temperature Alloys in Molten Salts

Concentrated solar power (CSP) is an interesting technology that involves storing solar energy in the form of heat and subsequently converting it into electricity. The third-generation (Gen3) CSP plants aim to operate at temperatures >700\ub0C, necessitating the deployment of new heat storage materials that can withstand such high operating temperatures. Molten carbonate and chloride salt mixtures are promising candidates for Gen3-CSP plants. Nevertheless, the use of such melts poses a serious corrosion challenge for the metallic materials that contain them.The work of this thesis focuses on the behaviours of selected, high-temperature alloys that are in direct or indirect contact with salt melts. A special experimental set-up was established to mimic the conditions in hot thermal energy storage (TES) tanks. Salt melts containing NaK-nitrate, which is a state-of-the-art TES material, LiNaK-carbonate, and KMg-chloride were employed to study their effects on commercial and experimental alloys. The experiments were conducted at temperatures that exceeded 50–100\ub0C than required in the power plants. A comparative study of the corrosion resistance profiles of the chromia-forming and alumina-forming alloys in the three above-mentioned salt melts was conducted. Alloys exposed to nitrate melts were found to have the most predictable and highest levels of corrosion resistance compared to those exposed to the other melts. In stark contrast, the chromia-forming alloys in contact with carbonate melts showed catastrophic corrosion behaviours, characterised by a severe internal attack, i.e., carburisation, which progressed throughout the sample. On the other hand, the ferritic alumina-forming alloys showed an interesting and highly beneficial phase transformation of two LiAlO2 polymorphs upon exposure to (Li,Na,K)2CO3 at 800\ub0C. A dense, protective α-LiAlO2 scale was formed and slowly grew over time despite being thermodynamically unfavourable; moreover, an outer, less-protective γ-LiAlO2 phase formed. A comprehensive approach is adopted to study the microstructure and crystallographic evolution of these α/γ-lithium aluminate polymorphs. In addition, the consequences of pre-oxidation of the tested alloys are studied.Alloys exposed to the chloride melt underwent rapid degradation. The degradation was caused by selective element leaching. A transient Laves phase barrier formed in Kanthal\uae APMT to delay the selective chromium leaching. However, the aluminium was depleted and with high velocity instead. One section of this thesis is dedicated to studying and understanding better the corrosion of evaporated salt species on the metallic materials of hot storage tanks. Interestingly, it was found that evaporated salt species caused more-severe corrosion than direct contact between the alloy and molten\ua0 salt. Thus, metallic materials immersed in salt melts have not experienced the most-corrosive conditions in terms of salt/impurity mixtures. This conclusion is valid for the vessel set-up configuration used in this thesis, which includes the cover gas, salt melt and cover gas impurities

Alloys in Contact with Molten Salts for Thermal Storage Applications

The combination of a concentrated solar power (CSP) plant and a thermal energy storage (TES) system is a promising technology for power generation, in that it overcomes the challenges commonly faced by renewable energy systems, such as intermittency, dispatchability, and the gap between the energy supply and energy demand. The third generation (Gen3) CSP plants are designed to increase plant efficiency by using supercritical carbon dioxide (sCO2) instead of steam for the Brayton cycle gas turbines, requiring a minimum operating temperature of 750\ub0C for the TES materials. Operating the TES tanks at higher temperatures poses a serious challenge in terms of corrosion for the metallic tank components and risks catastrophic failure of the plant. This study aims to provide useful insights into the corrosion behaviour of metallic materials that come in contact with different salt melts.The first part of this licentiate thesis is a comparative study of the corrosion resistance of chromia forming (316H or 304L) vs alumina forming (Kanthal\uae\ua0APMT) alloys that were exposed to three salt melts chosen for the current and next-generation CSP plants. The following salt melts were selected: Solar Salt, which is a commercial binary nitrate salt mixture that is utilised in currently operating CSP plants; a ternary carbonate; and a binary chloride salt mixture, which are candidate TES media for the Gen3 CSP technology. Corrosion exposures were conducted at 650\ua0\ub0C for the nitrate experiments and 800\ua0\ub0C for the carbonate and chloride experiments. The corrosion assessments of the tested alloys focused on oxidation, dissolution, and internal attack. The main findings revealed that: \ua01) for nitrate exposures: alloy 316H and Kanthal\uae\ua0APMT showed good corrosion resistance in contact with the nitrate melts, even though an internal attack was detected on the chromia forming 316H alloy; this attack was relatively slow and predictable. \ua0The relatively good corrosion behaviour of the alloys in nitrate melt is partly due to the lower operating temperature compared to the carbonate and chloride exposures. 2) for chloride exposures: both 304L and Kanthal\uae\ua0APMT underwent rapid degradation upon exposure to chloride melt. The degradation of these materials is caused by leaching of elements, such as chromium and aluminium. Nonetheless, molybdenum in Kanthal\uae\ua0APMT affected the corrosion process by forming a Laves phase barrier to chromium leaching; however, this did not prevent the rapid leaching of aluminium from the alloy. 3) for carbonate exposures: alloys in contact with the carbonate melt behaved differently based on the type of oxide scale formed. The stainless steel 304L showed the poorest corrosion resistance among all tested alloys in the three melts, wherein severe carburisation was detected. In strong contrast to 304L, Kanthal\uae\ua0APMT showed good corrosion resistance because it formed a thin protective layer of α-LiAlO2, which makes it a promising candidate for Gen3 CSP plants. After 168 h of exposure, a phase transition from α→γ-LiAlO2 oxide scale was observed.The second part of this thesis is dedicated to ranking five alumina-forming alloys in contact with alkali carbonate melt at 800\ua0\ub0C up to 1000\ua0h. Four ferritic FeCrAl alloys: Kanthal\uae\ua0APMT, Kanthal\uae\ua0AF, Kanthal\uae\ua0EF 100, and Kanthal\uae\ua0EF 101, and one austenitic FeNiCrAl alloy, Nikrothal\uae\ua0PM58 were investigated in this study. All four ferritic alumina-forming alloys developed a thin, protective α-LiAlO2 scale. The thermodynamically stable γ-LiAlO2 nucleates on top of the α-LiAlO2 scale and forms non-protective crystals. However, no severe aluminium depletion was detected for at least 1000\ua0h. The austenitic Nikrothal\uae PM58 did not form a α-LiAlO2 scale at 800\ua0\ub0C due to the relatively slow diffusion of aluminium from the alloy towards the alloy/melt interface

Superior protection by α-Al2O3/α-LiAlO2 double oxide scales against alkali carbonate corrosion

Ferritic alumina-forming alloys showed high corrosion resistance to molten carbonates by forming a protective α-LiAlO2 scale at their surface. In this work, four ferritic alumina-forming alloys were pre-oxidised before exposure to (Li,Na,K)2CO3-melt at 800 \ub0C for 1000 h. Only outer transient alumina, such as γ-alumina, reacts with the melt, forming LiAlO2. At the same time, the innermost α-Al2O3 scale remains intact. α-Al2O3, as well as α-LiAlO2, are slow-growing compressive stress-stabilised oxide scales. Thus, the combined presence of α-Al2O3/α-LiAlO2 improves the corrosion protection of materials investigated in this study, compared to non-pre-oxidised ones, by forming a superior double oxide scale

Perspectives on selected alloys in contact with eutectic melts for thermal storage: Nitrates, carbonates and chlorides

Increasing the operating temperature of molten salt-based concentrated solar power plants is of paramount importance to enable next-generation gas turbines and an overall increase in power conversion efficiency. The issue is how to mitigate the degradation of necessary metallic components in highly corrosive salt environments. In this study, three eutectic salt mixture candidates, nitrates, carbonates, and chlorides, are brought into contact with stainless steel (316H or 304L) and the FeCrAl alloy Kanthal\uae APMT. The post-exposure analysis is discussed in terms of the overall performance of each alloy as concerns mass change, scale growth, internal attack, and leaching. Significant reduction of corrosion is realised through the ability of Kanthal\uae APMT to form aluminium oxide species at the surface in contact with alkali nitrates and carbonates. On the other hand, aluminium is leached most efficiently in contact with chlorides, which causes a deeper attack on Kanthal\uae APMT than alloy 304L. The overall conclusion is that only by employing a holistic perspective on all individual measurements can a long-term performance estimation be formulated

Additional data and experimental setups, for a comparative study of alloys in contact to eutectic melts for thermal storage

Three different eutectic salt mixtures have been brought into contact with three different high temperature alloys to assess corrosion damages for next-generation CSPs. This article contains additional material to support findings and assessments reported on our main article in the Solar Energy Journal [https://doi.org/10.1016/j.solener.2021.06.069]. Five sections, A-E, provide data to ensure reproducibility and confidence in our claims in the main article. A newly designed experimental setup for high temperature exposures is described as well as impurities within used chemicals. Material thickness measurements document alloy consumption by eutectic salts. Reaction enthalpies are listed illustrating individual metal species in contact with salt species at relevant temperatures. Thermodynamic single point equilibrium calculations have extended environmentally induced Laves phase precipitation found for alloy Kanthal APMT in contact with molten chlorides

Evaporated Alkali Carbonate Effect on an Aluminum Diffusion Coated 253MA Vessel after 4000 h Discontinuous Operation— Lessons Learned

A vessel of a laboratory setup for hosting molten alkali carbonate immersion experiments was recently decommissioned after about 4000 h discontinuous operation at 800 \ub0C. In this article, we want to describe the long‐term damage of the vessel caused by evaporated alkali carbonate spe-cies in a carbon dioxide gas environment. The vessel is made of alloy 253MA and coated by an aluminum powder pack diffusion process. The degree of material loss and internal degradation did not correlate with the temperature profile across the vessel but rather with the vicinity to the gas and salt evaporation source. One millimeter of the vessel’s initial 5 mm wall thickness was com-pletely consumed at the strongest attacked location plus another 800 μm of internal attack beneath the metal–oxide interface

Managing Soft-errors in Transactional Systems

Abstract-Multicore architectures are becoming increasingly prone to soft-errors -i.e., transient faults caused by external physical phenomena such as electric noise and cosmic particle strikes. With increasing core counts, the soft-error rate is growing due to the accelerating transistor density on chips. The impact of these errors on business-critical applications that are being deployed on multicore hardware can be significant. We present an active replication-based approach that fully masks such errors for transactional applications. We partition computational cores, fully replicate objects across partitions, and concurrently execute transactional requests on all partitions, thereby enabling completely local object accesses. Transactional requests are globally ordered and delivered across partitions using optimistic atomic broadcast. Hardware message passing -an important emerging trend in multicore architectures -is exploited to mitigate communication costs. We report preliminary results obtained with an implementation of our approach on a 36-core Tilera TILE-Gx hardware, with an onchip scalable mesh network

Differentiation in corrosion performance of alumina forming alloys in alkali carbonate melts

Alkali carbonate melts are promising high temperature thermal storage media. In this work five alumina forming alloys have been exposed to a ternary LiNaK carbonate melt and CO2 at 800 \ub0C. The corrosion propagation was found to depend on the formation of a slow-growing LiAlO2 scale. Furthermore, the two polymorphs contributing to the LiAlO2 phase were monitored for up to 1000 h: a dense α-LiAlO2 scale and γ-LiAlO2 crystallites. We suggest a growth stress assisted formation of α-LiAlO2 relaxing into the outwards growing γ-LiAlO2 phase. This implies a deceleration of the α-LiAlO2 scale growth towards a steady state-thickness

Extracellular Biofabrication, Characterization, and Antimicrobial Efficacy of Silver Nanoparticles Loaded on Cotton Fabrics Using Newly Isolated Streptomyces

Biological method for silver nanoparticles synthesis has been developed to obtain cost effective, clean, nontoxic, and ecofriendly size-controlled nanoparticles. The objective of this study is extracellular biosynthesis of antimicrobial AgNPs using cell-free supernatant of a local Streptomyces sp. strain SSHH-1E. Different medium composition and fermentation conditions were screened for maximal AgNPs biosynthesis using Plackett-Burman experimental design and the variables with statistically significant effects were selected to study their combined effects and to find out the optimum values using a Box-Behnken design. The synthesized AgNPs were characterized using UV-visible spectroscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and energy dispersive X-ray spectroscopy. Rapid biosynthesis of AgNPs was achieved by addition of 1 mM AgNO3 solution to the cell-free supernatant. The produced particles showed a single surface plasmon resonance peak at 400 nm by UV-Vis spectroscopy which confirmed the presence of AgNPs. Streptomyces sp. SSHH-1E was identified as Streptomyces narbonensis SSHH-1E. Transmission electron microscopy study indicated that the shape of AgNPs is spherical and the size is ranging from 20 to 40 nm. Fourier transform infrared spectroscopy analysis provides evidence for proteins as possible reducing and capping agents. Furthermore, the biosynthesized AgNPs significantly inhibited the growth of medically important pathogenic Gram-positive and Gram-negative bacteria and yeast. The maximum biosynthesis of AgNPs was achieved at initial pH of 8, peptone of 0.5 g, and inoculum age of 48 h. The statistical optimization resulted in a 4.5-fold increase in the production of AgNPs by Streptomyces narbonensis SSHH-1E

Brief Announcement: Managing Resource Limitation of Best-Effort HTM

ABSTRACT The first release of hardware transactional memory (HTM) as commodity processor posed the question of how to efficiently handle its best-effort nature. In this paper we present Part-HTM, the first hybrid transactional memory protocol that solves the problem of transactions aborted due to the resource limitations (space/time) of current best-effort HTM. The basic idea of Part-HTM is to partition those transactions into multiple sub-transactions, which can likely be committed in hardware. Due to the eager nature of HTM, we designed a low-overhead software framework to preserve transaction's correctness (with and without opacity)