High temperature corrosion in molten solar salt: The role of chloride impurities

Abstract

The role of chloride impurities in molten NaNO3‐KNO3 (solar salt) mixture on corrosion behavior of low‐chromium ferritic‐martensitic X20CrMoV11‐1 steel (X20) and stainless steel 316 (SS316) was studied at 600 °C. Gravimetric and metallographic methods were employed to characterize the kinetics of oxidation and the resulting corrosion products. Steel X20 showed non‐protective character in both low‐ (up to 0.02 wt% Cl−) and high‐ (up to 0.25 wt% Cl−) chloride salts by forming a thick and non‐compact oxide scale. A significant increase in weight gain was observed when X20 steel was immersed in the high‐chloride‐containing salt. Furthermore, the scale underwent severe deformation. SS316 showed superior corrosion resistance in both low‐ and high‐chloride salts. Oxide scales formed on both steels included two zones: an outer Na‐rich oxide and an inner mixed oxide based on Fe2O3 and Fe3O4 structures. The morphology and composition of these zones were significantly different on X20 and SS316 steels. A passive Cr‐rich oxide layer at the metal/oxide interface was characterized as a protective layer. In the case of stainless steel 316 this layer showed even higher continuity when tested in the high‐chloride salt resulting in better protection during the isothermal test.

Scale Formation and Degradation of Diffusion Coatings Deposited on 9% Cr Steel in Molten Solar Salt

“…This is in good agreement with the study by Audigié et al showing the formation of NaFeO 2 in the multi-layered oxide scale grown on P91 during exposure in molten nitrates at 580 • C [25]. The incorporation of Na into the oxide scales and the consequential formation of sodium ferrite is attributed to the transfer of Na cations from the salt melt to the scale surface and their inward penetration through oxide scale defects such as pores and cracks [18]. Moreover, N was enriched underneath the porous oxide scale (see Figure 7b and N map in Figure 7c), suggesting the nitridation of the sub-scale region, which was also observed in other studies investigating the molten nitrate corrosion of this alloy at 580 • C [6,25].…”

“…Furthermore, commercial solar salt contains a wide variety of impurities, such as chlorides and sulfates, which can increase the corrosion rate, as shown in [17,18]. Due to the above stated reasons, the corrosion resistance of structural materials employed in the receiver tubing system and the hot storage tank is a decisive factor for plant design and lifetime [4,6,[9][10][11]19].…”

“…Employment of cheaper alloys as structural materials can be a very efficient method to reduce costs in CSP plants. The corrosion behavior of low Cr-steels, austenitic steels and Ni-based alloys in molten nitrate salts at a wide temperature range has been investigated in different studies [2,10,13,14,17,18,[20][21][22], which showed higher corrosion resistance for austenitic steels and Ni-based alloys compared to ferritic-martensitic steels. Notwithstanding their high corrosion resistance, employment of austenitic steels or Ni-based alloys significantly increases the cost of structural materials in CSP plants.…”

Influence of Impurity SO42− on the Thermal Performance of Molten Nitrates Used for Thermal Energy Storage

“…They also found that the corrosion loss of mild steel, immersed for several hours in melts at 600 °C, was significantly increased by adding 7.5 wt.% Na 2 SO 4. What's more, Dorcheh et al . also studied the corrosion behavior of X20 steel and SS316 in molten solar salt at 600 °C.…”

Improving optical properties of diffusion-based absorber coatings for CSP tower systems

“…Material selection and lifetime estimation are challenging tasks due to the variety of degradation mechanisms occurring at the external and internal surfaces of the absorber tube. With regards to the inside of the tube, the nitrite and nitrate anions in combination with O 2 and salt impurities such as chlorides form a highly corrosive and challenging environment [2][3][4]. With respect to the outside of the absorber tube, the materials are concurrently exposed to dust erosion as well as thermocyclic oxidation due to fluctuating temperature.…”