Periodic Reporting for period 1 - CoMeTES (Performance study of innovative Corrosion and Mechanically resistant coated materials against molten salts for next-generation concentrated solar power plants and Thermal Energy Storage systems)
Periodo di rendicontazione: 2023-10-01 al 2025-09-30
In 1990, the first GIEC report was published, assessing the influence of human activities on the climate change and its harmful consequences for future generations. More than 30 years later, almost half of the global population is considered as highly vulnerable: drought, heatwaves, forest fires, storms, floods, water shortages…natural disasters have been multiplying at an alarming rate and reducing the emissions of greenhouse gases appears as the main today’s stake. Committed to the Paris Agreement, the European energy system increasingly needs to adapt while electricity cost is undergoing a record-breaking surge influenced by the geopolitical situation. In this framework, COMETES, financed by the European Commission (Marie-Sklodowska-Curie-Action postdoctoral fellowship 2023), was designed to meet the REPowerEU plan objectives: produce affordable, secure and sustainable energy for the European Union. Concentrated Solar Power (CSP) technology emerged these years as one of the most promising solutions. It uses an unlimited energy source that can be combined with thermal energy storage (TES) systems to manage the solar energy intermittency. However, this technology remains technically and economically uncompetitive compared to other energies. This is mainly due to a currently low solar-to-electric conversion and the use of expensive materials required to resist the operating conditions. In particular, these materials are in direct contact with high-temperature molten salts used as heat transfer fluid (HTF) which can lead to dramatic corrosion. One increasingly considered solution is to improve the thermodynamic efficiency of the plant using a Brayton cycle operating with supercritical CO2. However, these new systems would require higher operating temperatures (>700 °C) and thus the use of new molten salt systems (among which carbonates) which constitute a corrosive environment for the plant’s structural materials.
COMETES aims at managing this issue by using slurry aluminide coatings applied on low chromium steels. These coatings, especially developed at INTA, have already demonstrated their potential in nitrate and carbonate salts, and they present the advantage to be 25% less expensive than Ni-based alloys currently used as plant constituent materials. However, limitations remained to allow the industrial use of this technology and COMETES focused on overcoming the current barriers through 6 objectives:
- Develop and improve different coating deposition processes to allow large-scale exploitation and on-site O&M.
- Develop low-cost coated material solutions able to operate on CSP in new HTF/TES fluids at 700 °C.
- Investigate the mechanical and the Stress Corrosion Cracking (SCC) resistance of the coated materials developed.
- Investigate the suitability of the developed coatings on industrial-representative components.
- Investigate the repairability of the developed coating.
- Broaden the existing literature through significant dissemination, communication & exploitation productions.
These objectives were expected to allow the achievement, in the long term, of 3 general objectives (GO): 1/ Increase the solar-to-electric conversion of CSP plants by 20% and the storage capacity by >25%, 2/ Decrease the Capital Expenditures (CAPEX) by >2% and 3/ to decrease the Operational expenditures (OPEX) by 25% as well as the O&M. These expected progresses were supposed to reduce shutdowns, mitigate the risk of leaks, and allow more efficient integration of CSP with other renewable energy sources, contributing to Europe’s strategic goals for energy independence and sustainable power generation. In the framework of CoMeTES, the objective is to increase the Technology Readiness Level (TRL) of the coated technology from 3 to 5 in order to progress through a future industrial exploitation.
Beyond technical results and research analysis, CoMeTES was built to actively promote scientific dissemination, communication, and exploitation of knowledge and coating technology. Through publications, conference presentations, and public outreach activities, the project enhances the visibility of CSP-TES research and fosters collaboration across academic and industrial communities. This integrated approach ensures that the results not only advance scientific understanding but also lay the groundwork for future industrial adoption and societal impact, by supporting more sustainable, reliable, and cost-effective renewable energy technologies.
By achieving these objectives and increasing the TRL, COMETES paves the way for pilot-scale deployment at very short-term, industrial adoption, and measurable reductions in costs and emissions at mid-term, thus bridging the gap between lab-scale research and societal impact.
COMETES aims at managing this issue by using slurry aluminide coatings applied on low chromium steels. These coatings, especially developed at INTA, have already demonstrated their potential in nitrate and carbonate salts, and they present the advantage to be 25% less expensive than Ni-based alloys currently used as plant constituent materials. However, limitations remained to allow the industrial use of this technology and COMETES focused on overcoming the current barriers through 6 objectives:
- Develop and improve different coating deposition processes to allow large-scale exploitation and on-site O&M.
- Develop low-cost coated material solutions able to operate on CSP in new HTF/TES fluids at 700 °C.
- Investigate the mechanical and the Stress Corrosion Cracking (SCC) resistance of the coated materials developed.
- Investigate the suitability of the developed coatings on industrial-representative components.
- Investigate the repairability of the developed coating.
- Broaden the existing literature through significant dissemination, communication & exploitation productions.
These objectives were expected to allow the achievement, in the long term, of 3 general objectives (GO): 1/ Increase the solar-to-electric conversion of CSP plants by 20% and the storage capacity by >25%, 2/ Decrease the Capital Expenditures (CAPEX) by >2% and 3/ to decrease the Operational expenditures (OPEX) by 25% as well as the O&M. These expected progresses were supposed to reduce shutdowns, mitigate the risk of leaks, and allow more efficient integration of CSP with other renewable energy sources, contributing to Europe’s strategic goals for energy independence and sustainable power generation. In the framework of CoMeTES, the objective is to increase the Technology Readiness Level (TRL) of the coated technology from 3 to 5 in order to progress through a future industrial exploitation.
Beyond technical results and research analysis, CoMeTES was built to actively promote scientific dissemination, communication, and exploitation of knowledge and coating technology. Through publications, conference presentations, and public outreach activities, the project enhances the visibility of CSP-TES research and fosters collaboration across academic and industrial communities. This integrated approach ensures that the results not only advance scientific understanding but also lay the groundwork for future industrial adoption and societal impact, by supporting more sustainable, reliable, and cost-effective renewable energy technologies.
By achieving these objectives and increasing the TRL, COMETES paves the way for pilot-scale deployment at very short-term, industrial adoption, and measurable reductions in costs and emissions at mid-term, thus bridging the gap between lab-scale research and societal impact.
During CoMeTES, the technical work focused on three main work packages (WP2, WP3, and WP4) aimed at developing and validating slurry aluminide coatings for CSP-TES applications. Overall, nearly 90% of the planned tasks were completed, fully supporting the achievement of the project’s major scientific objectives. The remaining tasks were affected by anticipated and well-managed risks (e.g. delays in material delivery and welded sample fabrication). In parallel, additional unplanned but valuable work was incorporated, including the development of a numerical model that emerged as a relevant contribution to the project.
WP2 – Development & optimization of coating deposition processes
The coating deposition method was finely optimized (surface preparation, deposition method, post-treatments). The efficiency of the developed coated materials was evaluated through short-term molten salt corrosion tests and mechanical tests.
Key outcomes:
- formation of defect-free coatings on low-chromium steels, scalable deposition processes, and short-term corrosion resistance in both nitrate and carbonate molten salts at high temperatures (up to 700 °C).
- coatings were successfully produced on 347H austenitic steel.
- Mechanical properties of the coated materials were also evaluated, confirming that coating deposition did not compromise substrate strength.
- Coated P91 were also produced and proposed as low-cost alternative for current CSP-TES systems.
WP3 – High-temperature corrosion, mechanical resistance and coupling leading to stress corrosion cracking (SCC)
Test samples were coated and then tested through long-term HT-corrosion (up to 10,000 h at 700ºC) and 3 types of SCC tests: 1/ tensile tests on specimens pre-corroded during 500 and 1000 h of exposure to examine the corrosion effect on the mechanical resistance; 2/ HT-corrosion tests (1000 h) of coated specimens pre-stressed by 3-point bending to inspect the occurrence of potential cracks or decohesion due to mechanical loading and the extent of corrosion in these areas; 3/ U-bend immersion tests in molten salts to investigate the SCC synergetic effect.
Key outcomes:
- Long-term molten salt corrosion tests were performed, with exposure up to 10,000 h at 700ºC on 347H in Li-Na-K carbonate salts.
- These tests demonstrated the effectiveness of the coatings, including a self-healing phenomenon observed in deliberately cracked samples, indicating long-term durability even with the presence of cracks ting. This holds significant promise for industrial application.
- From a scientific point of view, these tests allowed to deeply investigate long-term high-temperature molten salt corrosion mechanism of austenitic alloys, allowing the development of new degradation theories.
- the influence of corrosion degradation on the mechanical behaviour of coated and uncoated samples was investigated, leading to a scientific publication.
- SCC tests using U-bends were successfully conducted, opening new paths for SCC investigation in molten salts.
WP4 – Suitability of coatings for industrial-representative components
Coatings were applied on components representative of real CSP and TES systems, including U-bends and welded samples. This WP was completed with the development of a valuable numerical model to simulate crack propagation in coated samples (not initially scheduled) and anticipate the risk of failure and crack propagation within the substrate.
Key outcomes:
- For the first time, the slurry coating was successfully deposited on a welded part made of 347H, and high-temperature molten salt corrosion tests confirmed significant increased lifetime of these components (today limited to 3 years in current CSP-TES systems).
- Dip coating method allowed optimal coating of complex-shape samples opening up to deposition processes adapted to industrial issues.
- a numerical model was developed to anticipate crack propagation in coated components, further supporting industrial applicability, predictive actions and improving the understanding of degradation mechanisms experimentally observed on coated materials.
Overall, the project achieved significant technical progress by developing cost-effective coatings capable of resisting high-temperature molten salt corrosion and mechanical stresses, increasing component lifetime, and demonstrating industrial applicability. The coating technology has reached TRL5 at the end of the project, providing confidence for future pilot-scale implementation at very short-term.
WP2 – Development & optimization of coating deposition processes
The coating deposition method was finely optimized (surface preparation, deposition method, post-treatments). The efficiency of the developed coated materials was evaluated through short-term molten salt corrosion tests and mechanical tests.
Key outcomes:
- formation of defect-free coatings on low-chromium steels, scalable deposition processes, and short-term corrosion resistance in both nitrate and carbonate molten salts at high temperatures (up to 700 °C).
- coatings were successfully produced on 347H austenitic steel.
- Mechanical properties of the coated materials were also evaluated, confirming that coating deposition did not compromise substrate strength.
- Coated P91 were also produced and proposed as low-cost alternative for current CSP-TES systems.
WP3 – High-temperature corrosion, mechanical resistance and coupling leading to stress corrosion cracking (SCC)
Test samples were coated and then tested through long-term HT-corrosion (up to 10,000 h at 700ºC) and 3 types of SCC tests: 1/ tensile tests on specimens pre-corroded during 500 and 1000 h of exposure to examine the corrosion effect on the mechanical resistance; 2/ HT-corrosion tests (1000 h) of coated specimens pre-stressed by 3-point bending to inspect the occurrence of potential cracks or decohesion due to mechanical loading and the extent of corrosion in these areas; 3/ U-bend immersion tests in molten salts to investigate the SCC synergetic effect.
Key outcomes:
- Long-term molten salt corrosion tests were performed, with exposure up to 10,000 h at 700ºC on 347H in Li-Na-K carbonate salts.
- These tests demonstrated the effectiveness of the coatings, including a self-healing phenomenon observed in deliberately cracked samples, indicating long-term durability even with the presence of cracks ting. This holds significant promise for industrial application.
- From a scientific point of view, these tests allowed to deeply investigate long-term high-temperature molten salt corrosion mechanism of austenitic alloys, allowing the development of new degradation theories.
- the influence of corrosion degradation on the mechanical behaviour of coated and uncoated samples was investigated, leading to a scientific publication.
- SCC tests using U-bends were successfully conducted, opening new paths for SCC investigation in molten salts.
WP4 – Suitability of coatings for industrial-representative components
Coatings were applied on components representative of real CSP and TES systems, including U-bends and welded samples. This WP was completed with the development of a valuable numerical model to simulate crack propagation in coated samples (not initially scheduled) and anticipate the risk of failure and crack propagation within the substrate.
Key outcomes:
- For the first time, the slurry coating was successfully deposited on a welded part made of 347H, and high-temperature molten salt corrosion tests confirmed significant increased lifetime of these components (today limited to 3 years in current CSP-TES systems).
- Dip coating method allowed optimal coating of complex-shape samples opening up to deposition processes adapted to industrial issues.
- a numerical model was developed to anticipate crack propagation in coated components, further supporting industrial applicability, predictive actions and improving the understanding of degradation mechanisms experimentally observed on coated materials.
Overall, the project achieved significant technical progress by developing cost-effective coatings capable of resisting high-temperature molten salt corrosion and mechanical stresses, increasing component lifetime, and demonstrating industrial applicability. The coating technology has reached TRL5 at the end of the project, providing confidence for future pilot-scale implementation at very short-term.
CoMeTES has significantly advanced the state of the art in CSP-TES materials and coatings technology by addressing key limitations previously identified in the literature:
1. Enhanced deposition methods for industrial scalability: While slurry aluminide coatings were previously limited to small or medium-sized components using spraying techniques requiring post-treatments, CoMeTES optimized both spraying and dipping coating deposition methods. Innovations included improved surface preparation, deposition on complex-shaped samples, optimization of the diffusion heat treatments. These developments allowed, for the first time, the deposition of defect-free coatings on 347H austenitic steel and on welded parts, demonstrating feasibility for large-scale, complex, and industrial components. Especially on welds, CoMeTES demonstrated uniform coverage, high corrosion resistance in Solar Salt and in Molten Carbonates, and significantly extended lifetime compared to uncoated welds. This overcomes a key limitation of TES systems, where weld degradation had previously restricted operational lifetimes to about three years. Mechanical tests are required to ensure the complete efficiency of the coated welds.
2. Superior high-temperature molten salt corrosion resistance: Previous studies rarely achieved corrosion rates below the target of 15 µm/year across multiple molten salts, limiting plant lifetimes. CoMeTES slurry coated 347H achieved long-term resistance in carbonate salts up to 10,000 h, demonstrating mass variations <5 mg/cm², effectively lowering corrosion rates below 10 µm/year. This extends component lifetimes significantly beyond the prior state of the art.
3. Mechanical and stress-corrosion cracking (SCC) improvements: Earlier work on slurry coatings focused mainly on corrosion resistance without systematic evaluation of mechanical and SCC behavior. CoMeTES conducted comprehensive tests showing the reciprocal influence of mechanical solicitation and molten salt exposure on bare steels, and the coatings maintained mechanical integrity. A self-healing phenomenon was observed in deliberately cracked coatings, further supporting protective behaviour in the coating defects and long-term durability.
4. Industrial applicability and TRL advancement: By successfully coating complex-shaped and welded industrial-representative components, CoMeTES demonstrated TRL5 readiness for CSP-TES systems. The ability to coat welded parts and maintain performance under high-temperature molten salt exposure is a unique result that enables future pilot-scale and industrial demonstrations at short-term and supports potential access to market and commercialisation at mid-term. IPR support is required by now to guide and optimize the commercial adoption of the coated system.
5. Key enablers for further uptake: To ensure further success and industrial deployment, additional actions are needed:
o Pilot-scale demonstration in operational CSP-TES plants
o Engagement with industrial partners to scale up deposition processes
o Protection of intellectual property for coating formulations and deposition methods
o Development of standards and regulatory frameworks for coating certification
o Optimization for other high-temperature energy or industrial applications beyond CSP
In summary, CoMeTES has overcome critical limitations of prior slurry aluminide coatings, including limited component size, insufficient mechanical and SCC data, and short TES lifetimes due to weld degradation. The project has produced cost-effective, high-performance coatings capable of extending component life, reducing operational risks, and supporting industrial adoption, thus moving CSP-TES technology closer to economically viable (expected CAPEX and OPEX reductions around 2 and 70%, respectively), large-scale deployment.
1. Enhanced deposition methods for industrial scalability: While slurry aluminide coatings were previously limited to small or medium-sized components using spraying techniques requiring post-treatments, CoMeTES optimized both spraying and dipping coating deposition methods. Innovations included improved surface preparation, deposition on complex-shaped samples, optimization of the diffusion heat treatments. These developments allowed, for the first time, the deposition of defect-free coatings on 347H austenitic steel and on welded parts, demonstrating feasibility for large-scale, complex, and industrial components. Especially on welds, CoMeTES demonstrated uniform coverage, high corrosion resistance in Solar Salt and in Molten Carbonates, and significantly extended lifetime compared to uncoated welds. This overcomes a key limitation of TES systems, where weld degradation had previously restricted operational lifetimes to about three years. Mechanical tests are required to ensure the complete efficiency of the coated welds.
2. Superior high-temperature molten salt corrosion resistance: Previous studies rarely achieved corrosion rates below the target of 15 µm/year across multiple molten salts, limiting plant lifetimes. CoMeTES slurry coated 347H achieved long-term resistance in carbonate salts up to 10,000 h, demonstrating mass variations <5 mg/cm², effectively lowering corrosion rates below 10 µm/year. This extends component lifetimes significantly beyond the prior state of the art.
3. Mechanical and stress-corrosion cracking (SCC) improvements: Earlier work on slurry coatings focused mainly on corrosion resistance without systematic evaluation of mechanical and SCC behavior. CoMeTES conducted comprehensive tests showing the reciprocal influence of mechanical solicitation and molten salt exposure on bare steels, and the coatings maintained mechanical integrity. A self-healing phenomenon was observed in deliberately cracked coatings, further supporting protective behaviour in the coating defects and long-term durability.
4. Industrial applicability and TRL advancement: By successfully coating complex-shaped and welded industrial-representative components, CoMeTES demonstrated TRL5 readiness for CSP-TES systems. The ability to coat welded parts and maintain performance under high-temperature molten salt exposure is a unique result that enables future pilot-scale and industrial demonstrations at short-term and supports potential access to market and commercialisation at mid-term. IPR support is required by now to guide and optimize the commercial adoption of the coated system.
5. Key enablers for further uptake: To ensure further success and industrial deployment, additional actions are needed:
o Pilot-scale demonstration in operational CSP-TES plants
o Engagement with industrial partners to scale up deposition processes
o Protection of intellectual property for coating formulations and deposition methods
o Development of standards and regulatory frameworks for coating certification
o Optimization for other high-temperature energy or industrial applications beyond CSP
In summary, CoMeTES has overcome critical limitations of prior slurry aluminide coatings, including limited component size, insufficient mechanical and SCC data, and short TES lifetimes due to weld degradation. The project has produced cost-effective, high-performance coatings capable of extending component life, reducing operational risks, and supporting industrial adoption, thus moving CSP-TES technology closer to economically viable (expected CAPEX and OPEX reductions around 2 and 70%, respectively), large-scale deployment.