Corrosion of High-Temperature Alloys in Molten Salts

Doctoral thesis, 2023

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°C, 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°C 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°C. 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® 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  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.