Corrosion of High-Temperature Alloys in Molten Salts
Doktorsavhandling, 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.

Thermal energy storage



Molten salts

Phase transition

High-temperature corrosion

Alumina-forming alloys

Alkali transition metalates, Alkali aluminate

Chromia-forming alloys


Concentrated solar power

KA-salen, Kemigården 4, Chalmers.
Opponent: Prof. Sannakaisa Virtanen, Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany


Esraa Hamdy Mohamedin

Chalmers, Kemi och kemiteknik, Energi och material

E. Hamdy, A. Pochi, C. Geers Direct and indirect contact of an austenitic high-temperature alloy to eutectic chloride melts with possible consequences for inhibitor strategies

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

Corrosion Science,; Vol. 192(2021)

Artikel i vetenskaplig tidskrift

E. Hamdy, F. Liu, C. Geers Superior protection by α-Al2O3/α-LiAlO2 double oxide scales against alkali carbonate corrosion

Global warming has become a familiar term to the general public which can be described as the increase in the Earth’s surface heat due to heat-trapping by greenhouse gas emissions (GHS), mainly CO2. Since the industrial revolution, human activities, such as burning fossil fuels, have been mainly responsible for global warming. Sea ice loss, floods, wildfires, hurricanes, droughts and intense heat waves are consequences of global warming that scientists predicted a long time ago, and we have already been witnessing these events. Since GHG emissions are mainly generated from burning fossil fuels for power generation, switching to renewable and clean energy sources has become a necessity.

Solar energy is an abundant, clean, renewable source with no CO2 emissions. Concentrated solar power (CSP) is a promising technology that collects, stores and converts the energy from sunlight to thermal energy to drive a turbine or power an engine. Combining CSP plants with thermal energy storage systems allows these plants to store energy in the form of heat. Although the CSP technology provides a clean, reliable, efficient energy source, even outside of daylight hours, CSP still cannot compete with other conventional energy sources.

Third-generation (Gen3) CSP plants aim to increase plant efficiency and lower the kWh cost by operating at higher temperatures. Achieving such a target requires the employment of heat storage media that can withstand the required high temperature of the process, in this study, molten salts. Carbonate and chloride melts are considered promising candidates for heat storage materials. However, operating at temperatures >700°C in such melts creates serious corrosion challenges for metallic materials containing the salt. Corrosion is defined as the degradation of metals due to their interaction with the surrounding environment. However, when high-temperature alloys form a stable oxide scale at their surface, undesired reactions with corrosive species in the environment can be hindered from progressing. My research has focused on understanding the corrosion mechanisms of different commercial and experimental alloys in contact with molten salts proposed for the Gen3 CSP plant. A special set-up was constructed to mimic the conditions in hot storage tanks. My studies have generated data that improved our understanding of how alloys behave in direct and indirect contact with the salt melts and why alloy degradation occurs on different timescales.

Korrosion i Termisk Lagringsanläggningar

Vinnova, Formas, Energimyndigheten (via HTC), 2018-12-01 -- 2021-04-30.

VINNOVA (Termisk SolEl), 2018-12-01 -- 2020-06-30.

Kemi och kemiteknik, 2021-05-01 -- 2023-05-01.









Chalmers materialanalyslaboratorium



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5293



KA-salen, Kemigården 4, Chalmers.


Opponent: Prof. Sannakaisa Virtanen, Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany

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