Alloys in Contact with Molten Salts for Thermal Storage Applications

Licentiate thesis, 2021

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°C 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® APMT) 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 °C for the nitrate experiments and 800 °C 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:

 1) for nitrate exposures: alloy 316H and Kanthal® APMT 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.  The 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® APMT 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® APMT 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® APMT 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 °C up to 1000 h. Four ferritic FeCrAl alloys: Kanthal® APMT, Kanthal® AF, Kanthal® EF 100, and Kanthal® EF 101, and one austenitic FeNiCrAl alloy, Nikrothal® PM58 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 h. The austenitic Nikrothal® PM58 did not form a α-LiAlO2 scale at 800 °C due to the relatively slow diffusion of aluminium from the alloy towards the alloy/melt interface.