Corrosion by Molten Salts

Corrosion damage from molten salts can occur in a wide variety of materials and by different mechanisms. It has been pointed out that although many studies have been performed, quantitative data for materials selection and performance prediction are rarely available [15]. Molten salt corrosion is usually applicable to materials retaining the molten salt, as used in heat treating, solar and nuclear energy systems, batteries, fuel cells, and extractive metallurgical processes. Some factors that can make molten salts extremely corrosive include the following  

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The corrosion by molten salts was intensively investigated in connection with impurities of combustion gases [431, 433, 434, 436, 464]. The corrosion effect of NaCl in combustion environments is less pronounced, when the sulphur concentration in the fuel is higher [431]. The reason for this behaviour is that at high sulphur concentration Na2S04 is stable and thus the Na20 activity and the formation of sodium silicates is reduced [431]. These processes are analysed in [431, 433, 436]. 

Metals and ceramics come into contact with molten salts, either by design or by accident, in many high-temperature industrial processes, as in oil-fired boilers used for generating electricity, for example. The mechanisms of corrosion at elevated temperatures have received far less attention than those pertaining to aqueous solutions. The important differences between corrosion by molten salts and by aqueous media therefore need to be considered, together with the special features which distinguish the former phenomena. ... 

The treatment here is restricted to an aqueous environment which includes atmospheric condensates, fresh water, sea water, soils, etc. It should be realized, however, that other types of corrosion are of great practical importance including gaseous oxidation and corrosion by molten salts. 

The feasibility of the operation of a molten-sult-fueled reactor at a truly high temperature was demonstrated in 1954 in experiments with a reactor constructed at ORNL. The temperature of the fuel exiting from the core of this reactor was about 1500°E, and the temperature of the fuel at the inlet to the core was about 1200°E. The reactor was constructed before the mechanism and control of corrosion by molten salts had been fully explored, and therefore the experimental operation of the reactor was of short duration. Since the work was supported by the Aircraft Reactors Branch of the Atomic Energy Commission, the reactor was called the Aircraft Reactor Experiment (ARE). ... 

MSO is unsuited for treating materials with high inert content, such as asbestos, concrete, soils, and rubble. There is concern over emissions from MSO relating to particulate mercury content and radioactivity. MSO is inappropriate for wastes with high tritium levels. MSO pilot programs have encountered problems with carbon monoxide (CO) emissions. The corrosion of reactor materials by molten salt has remained a concern for the long-term operability of the system. The viscosity and volatility of the melt have to be controlled. There have been problems with material from the melt plugging air exhaust and feeder systems. 

R. A. Rapp and K. S. Goto, The hot corrosion of metals by molten salts . In Molten Salts, eds. J. Braunstein and J. R. Selman, Pennington, New Jersey, Electrochemical... 

Zirconium resists attack in some molten salts. It is very resistant to corrosion by molten sodium hydroxide to temperatures above 1000°C. It is also fairly resistant to potassium hydroxide. The oxidation properties of zirconium in nitrate salts are similar to those in air. 

Iridium — (L. iris, rainbow), Ir at. wt. 192.217(3) at. no. 77 m.p. 2446°C b.p. 4428°C sp. gr. 22.42 (17°C) valence 3 or 4. Discovered in 1803 by Tennant in the residue left when crude platinum is dissolved by aqua regia. The name iridium is appropriate, for its salts are highly colored. Iridium, a metal of the platinum family, is white, similar to platinum, but with a slight yellowish cast. It is very hard and brittle, making it very hard to machine, form, or work. It is the most corrosion-resistant metal known, and was used in making the standard meter bar of Paris, which is a 90% platinum-10% iridium alloy. This meter bar was replaced in 1960 as a fundamental unit of length (see under Krypton). Iridium is not attacked by any of the acids nor by aqua regia, but is attacked by molten salts, such as NaCl and NaCN. Iridium occurs uncombined in nature with... 

Ozeryanaya, I.N. (1985) Corrosion of metals by molten salts in heat-treatment processes. Metal Sci. Heat Treat, 3, 184-188. 

Corrosion phenomena induced by molten salts in Generation IV nuclear reactors... 

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