Abstract
Corrosion behaviour of stainless steel 347 was investigated in a molten nitrate salt (60 wt% NaNO3 + 40 wt% KNO3) immersion at 565 °C for up to 3000 h. A growth of stratified oxide layers consisting of NaFeO2, Fe2O3 and Fe3O4 was observed on the stainless steel surface with a constant gravimetric corrosion rate of ~ 0.4 µm/year. The feasibility of using Ni3Al coatings deposited by means of air plasma spray for suppression of corrosion was investigated. Ni3Al coatings were observed to undergo a fast oxidation with a corrosion rate of ~ 2.7 µm/year in the first 500 h, and subsequently stabilise between 500 and 3000 h with no observable changes in microstructure, composition and weight at a corrosion rate of ~ 0.02 µm/year. The results presented in this study strongly suggest that Ni3Al coating suppresses the formation of oxide layers on the surface of stainless steel substrates and can be used as protection against corrosion in the presence of molten nitrate salts, which is of relevance to thermal energy storage applications.
Highlights
Thermal energy storage (TES) is arguably the key to further lower the levelised cost of energy (LCOE) of concentrated solar power (CSP) into a comparable level with other electricity generation technologies [1,2,3,4,5,6]
Assuming the Ni3Al coatings remain at a constant density of 6.67 × 1 03 mg/cm3, this is equivalent to an effective corrosion rate of ~ 2.7 μm/ year within the first 500 h and a practically negligible rate of ~ 0.02 μm/year from 500 to 3000 h
Assuming the SS347 substrates remain at a constant density of 8.03 × 1 03 mg/cm3, this is equivalent to an overall corrosion rate of ~ 0.4 μm/year in molten salts
Summary
Thermal energy storage (TES) is arguably the key to further lower the levelised cost of energy (LCOE) of concentrated solar power (CSP) into a comparable level with other electricity generation technologies [1,2,3,4,5,6]. A feasible storage medium for TES is molten salt with high temperature stability, low melting point, low viscosity and high thermal conductivity [8,9,10]. The commonly used solution to accommodate corrosion is to use thicker walls, as it would extend the lifetime operation of the material. It does not solve the corrosion issue and may not be applicable for components with fixed dimensions or tight tolerance [22]. Ni-based superalloys have been proposed to suppress corrosion, as they provide high temperature stability and resistance for molten salt corrosion compared to iron-based alloys [22, 24, 25].
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