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In the course of energy transition the development of sustainable technologies for power generation providing base load supply is of particular importance. In comparison to photovoltaics concentrated solar power (CSP) Systems have great potential to fulfil this requirement by the use of thermal storage systems utilizing molten salt mixtures as heat transfer fluids. For this purpose, molten nitrates are frequently discussed due to their beneficial thermal and physical properties as well as high operation temperatures. In order to protect the piping system from degradation, coatings can be applied on the surface of the employed materials, which are commonly steels or Ni-based alloys. The goal is to achieve cost reduction to ensure an even more competitive position of the CSP technology with respect to other renewable sources on the market. In this study, the corrosion behavior of coated and uncoated ferritic-martensitic steels of type T91 and VM12 in molten salt (mixture of NaNO3 and KNO3) has been investigated under isothermal conditions. The diffusion coatings are based on potentially protective elements such as Al, Si or Cr and were applied on the steels either by pack cementation or slurry deposition. Characterization of the samples was conducted by means of optical microscope and EPMA in order to gain a deeper understanding of the occurring corrosion mechanisms and for the purpose of lifetime analysis.

The efficiency of concentrated solar power technology is based on the functionality of exterior and interior absorber tube surfaces. While a high solar absorptivity and thermocyclic oxidation resistance are targeted for the exterior surfaces, a high resistance against molten nitrate corrosion is essential for the functionality of the interior surfaces. In this study, a bi-layer coating concept combining a Cr-diffusion coating with a top-coat ensured a high thermocyclic oxidation resistance together with high absorptivity for the exterior surfaces. For the interior surfaces, the molten nitrate corrosion resistance of ferritic-martensitic steels was significantly increased via the application of diffusion aluminide coatings.

The efficiency of concentrated solar power technology is based on the functionality of exterior and interior absorber tube surfaces. While a high solar absorptivity and thermocyclic oxidation resistance are targeted for the exterior surfaces, a high resistance against molten nitrate corrosion is essential for the functionality of the interior surfaces. In this study, a bi-layer coating concept combining a Cr-diffusion coating with a top-coat ensured a high thermocyclic oxidation resistance together with high absorptivity for the exterior surfaces. For the interior surfaces, the molten nitrate corrosion resistance of ferritic-martensitic steels was significantly increased via the application of diffusion aluminide coatings.

Since the energy conversion efficiency of CSP systems is directly linked to the operating temperature of the molten salt, the next-generation of CSP plants is targeted to operate at temperatures over 700°C. Due to their higher thermal stability compared to nitrate salts, molten chloride salts are considered as a promising HTF candidate for the next-gen CSP plants. Despite the competitiveness of molten chlorides as HTF, structural materials (i.e. iron and nickel-based alloys) experience severe corrosion in chloride mixtures. Therefore, the second part of this study investigates the corrosion mechanisms of P91 and Inconel 600 in flowing Ar with partial embedding in molten NaCl-KCl and in molten NaCl-KCl-MgCl2 at 700°C. Improvement of scaling behavior of low-Cr steels in molten nitrates allows their application in modern CSP plants thereby increasing the cost-efficiency. Understanding the prevailing corrosion mechanisms in molten chlorides will further develop material selection and corrosion mitigation strategies.

In the course of energy transition the development of sustainable, efficient and renewable technologies for power generation providing base load supply is of crucial importance. CSP plants have great potential to fulfil all these requirements. However, mitigation of molten salt corrosion inside the absorber tube as well as improvement of optical properties on its outside are key challenges and part of ongoing developments. For this purpose, a Cr-Mn-diffusion absorber coating has been developed and is presented together with its improved version, an organic-based bilayer system. This study focusses on the continuous improvement of optical properties in terms of thermal efficiency (solar absorption and emittance) as well as qualification of both coatings with regards to long term durability (isothermal and thermocyclic stability).

In the course of energy transition the development of sustainable, efficient and renewable technologies for power generation providing base load supply is of crucial importance. CSP plants have great potential to fulfil all these requirements. However, mitigation of molten salt corrosion inside the absorber tube as well as improvement of optical properties on its outside are key challenges and part of ongoing developments. For this purpose, a Cr-Mn-diffusion absorber coating has been developed and is presented together with its improved version, an organic-based bilayer system. This study focusses on the continuous improvement of optical properties in terms of thermal efficiency (solar absorption and emittance) as well as qualification of both coatings with regards to long term durability (isothermal and thermocyclic stability).

Since the energy conversion efficiency of CSP systems is directly linked to the operating temperature of the molten salt, the next-generation of CSP plants is targeted to operate at temperatures over 700°C. Due to their higher thermal stability compared to nitrate salts, molten chloride salts are considered as a promising HTF candidate for the next-gen CSP plants. Despite the competitiveness of molten chlorides as HTF, structural materials (i.e. iron and nickel-based alloys) experience severe corrosion in chloride mixtures. Therefore, the second part of this study investigates the corrosion mechanisms of P91 and Inconel 600 in flowing Ar with partial embedding in molten NaCl-KCl and in molten NaCl-KCl-MgCl2 at 700°C. Improvement of scaling behavior of low-Cr steels in molten nitrates allows their application in modern CSP plants thereby increasing the cost-efficiency. Understanding the prevailing corrosion mechanisms in molten chlorides will further develop material selection and corrosion mitigation strategies.