Received: 5 October 2016 | Revised: 3 May 2017 | Accepted: 10 May 2017 DOI: 10.1002/maco.201609300 ARTICLE High temperature corrosion in molten solar salt: The role of chloride impurities A. S. Dorcheh | R. N. Durham | M. C. Galetz DECHEMA-Forschungsinstitut, Theodor- Heuss-Allee 25, 60486 Frankfurt am Main, Germany Correspondence A. S. Dorcheh, DECHEMA- Forschungsinstitut, Theodor-Heuss-Allee 25, 60486 Frankfurt am Main, Germany. Email: soleimani@dechema.de Funding information Ferrostaal GmbH The role of chloride impurities in molten NaNO 3 -KNO 3 (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 Fe 2 O 3 and Fe 3 O 4 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. KEYWORDS chlorides, concentrated solar power, KNO 3 -NaNO 3 , molten nitrates, stainless steel 316, X20CrMoV11-1 1 | INTRODUCTION To date, molten salts based on alkali nitrates with a composition of 60 wt% NaNO 3 -40 wt% KNO 3 (also known as solar salts) are the main heat transfer fluids (HTF) used in concentrated solar power plants. They offer a combination of high density, high specific heat (C p ), low melting point, high thermal stability, and low vapor pressures (see Table 1). The circulation of salt in the power cycle relies on the molten state of the nitrate salts and this can only be granted by keeping the temperature in all transport and storage parts above the freezing point of the salt (∼225 °C). Currently, thermal efficiency of concentrated solar power (CSP) plants is between 30 and 40%. [1] Increasing the thermal efficiency requires lower salt melting points and elevated hot storage temperatures. Efforts to develop novel salt mixtures with lower melting points and higher heat capacities are increasing. [2–4] However, the marked degradation of nitrate salts which starts above 600 °C does not allow to increase the hot storage temperature further. [2] The combination of high operating temperatures and the corrosive chemical nature of molten salts induce hot corrosion in salt containments of the CSP system. Therefore, the appropriate material selection and life time estimation requires intensive hot corrosion studies Materials and Corrosion. 2017;1–9. www.matcorr.com © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim | 1