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51318-11182-Effects of Pre-Oxidiation and Carburization on Corrosion in Molten Fluoride Salts

Static exposure tests of pre-oxidized stainless steel and carburized chromium were conducted in the molten fluoride salt, LiF-NaF-KF (46.5-11.5-42 mol %), FLiNaK to simulate the corrosion environment typical of some advanced nuclear reaction concepts.

Product Number: 51318-11182-SG
Author: Kevin J. Chan / Rebecca Ambrecht / Won Tae Choi / Preet M. Singh
Publication Date: 2018
Industry: Energy Generation
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Molten fluoride salt coolants are used in several advanced nuclear reactor concepts. Corrosion of structural alloys in molten fluorides is distinguished from other instances of high temperature corrosion by the lack of protection by oxide scales. Protective oxide scales do not form on surfaces exposed to molten fluoride due to the solubility of oxides and low oxygen activity. However, pre-oxidation of alloys can be done to create a continuous high quality oxide scale before exposure to the molten fluoride environment. Pre-oxidation has been shown to improve hot corrosion resistance of some alloys. Carbides are known to be resistant to attack by molten fluorides and could potentially be used to form a diffusion barrier on alloy surfaces. Alloy S31603 and alumina forming austenitic stainless steel OC5 were pre-oxidized in air or steam atmosphere to form surface oxide film. Chromium carbide layers were formed by carburization treatment of pure chromium in a hydrocarbon/hydrogen environment. Static exposure tests of these materials were conducted in the molten fluoride salt, LiF-NaF-KF (46.5-11.5-42 mol %), FLiNaK to simulate the corrosion environment. Pre-oxidation was not found to be a viable means of corrosion mitigation, while the carbide layers were found to improve corrosion resistance in molten fluoride environments.

Key words: MSR, FHR, molten salt, pre-oxidation, chromium carbide, molten fluoride, FLiNaK

Molten fluoride salt coolants are used in several advanced nuclear reactor concepts. Corrosion of structural alloys in molten fluorides is distinguished from other instances of high temperature corrosion by the lack of protection by oxide scales. Protective oxide scales do not form on surfaces exposed to molten fluoride due to the solubility of oxides and low oxygen activity. However, pre-oxidation of alloys can be done to create a continuous high quality oxide scale before exposure to the molten fluoride environment. Pre-oxidation has been shown to improve hot corrosion resistance of some alloys. Carbides are known to be resistant to attack by molten fluorides and could potentially be used to form a diffusion barrier on alloy surfaces. Alloy S31603 and alumina forming austenitic stainless steel OC5 were pre-oxidized in air or steam atmosphere to form surface oxide film. Chromium carbide layers were formed by carburization treatment of pure chromium in a hydrocarbon/hydrogen environment. Static exposure tests of these materials were conducted in the molten fluoride salt, LiF-NaF-KF (46.5-11.5-42 mol %), FLiNaK to simulate the corrosion environment. Pre-oxidation was not found to be a viable means of corrosion mitigation, while the carbide layers were found to improve corrosion resistance in molten fluoride environments.

Key words: MSR, FHR, molten salt, pre-oxidation, chromium carbide, molten fluoride, FLiNaK

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