Molten salts high-temperature corrosion

Molten salts promote rapid corrosion of many constmctional materials at relatively low temperatures. Low-melting-point salts include sodium salts from saline atmospheres, fireside ash, silicate insulation, contaminants in the feed, etc. Corrosion rates of several mm/year can be observed at temperatures as low as 520°C. High chromium- and nickel-containing alloys up to 50% Cr/50% Ni are employed. 

Data on high-temperature melts are still limited. Conventional methods are difficult to apply because of the high values of thermal conductivity. Other difficulties in measuring molten salts are their corrosiveness, high electrical conductivities, and the necessity of careful preparation. Special care should be taken to exclude convection errors, which are usually the most serious source of errors, even at room temperature. 

Ionic liquids are, quite simply, liquids that are composed entirely of ions. Thus, molten sodium chloride is an ionic liquid a solution of sodium chloride in water (a molecular solvent) is an ionic solution. The term ionic liquids was selected with care, as it is our belief that the more commonly used phrase molten salts (or simply melts) is referential, and invokes a flawed image of these solvents as being high-temperature, corrosive, viscous media (cf. molten cryolite). The reality is that room-temperature ionic liquids can be liquid at temperatures as low as — 96°C, and are typically colorless, fluid, and easily handled. To use the term molten salts to describe these novel systems is as archaic as describing a car as a horseless carriage. Moreover, in the patent and recent academic literature, ionic... 

Hot corrosion refers to corrosion between a metal-oxide and a molten salt deposit. It occurs at the solid-gas interface. Molten salts are extremely corrosive and their presence increases the rate of corrosion by two orders of magnitude when compared to high-temperature corrosion at similar temperatures and conditions [27—29]. They act as solvents, preventing the formation of a stable oxide, or they chemically react with the oxide layers. By transporting through, the salts may damage the protective oxide layers. Two different types of hot corrosion exist, namely. Type I and II. 

M. Fukumoto, C. Tachikawame, Y. Matsuzaka, M. Hara, Formation of Si diffusion layer on stainless steels and their high temperature corrosion resistance in molten salt, Corros. Sci. 56 (2012) 105—113. 

Structural material selection. The AHTR requires high-temperature corrosion resistant materials. Materials are the greatest challenge for all high-temperature reactors, including the AHTR. Materials have been identified that allow operation with molten salts to 750°C (Hastelloy-N). 

Vanadium in a fuel forms various metal compounds with low melting points, and causes molten-salt corrosion of steel called vanadium attack. Another example of high temperature corrosion is sulfidation. Carbon monoxide, carbon dioxide, and hydrocarbons form metal carbides at high temperatures and this is called carburization. Nitriding involves chemical reaction of nitrogen with metal. 

Although there are no aqueous electrolytes, high temperature corrosion is an electrochemical process, involving anodic and cathodic partial reactions. The metal oxides generated at the corroding surface or molten salts present at the surface form the electrolyte. 

Hot corrosion refers to high-temperature corrosion that takes place in presence of molten salt on the metal surface. Sodium sulfate deposits are most common. They are the result of the reaction equilibria (9.78) (9.82) or of a reaction between sodium chloride and SO3 ... 

The presence of a molten salts favors the dissolution of the surface oxides that otherwise limit the rate of high temperature corrosion. The dissolution of nickel oxide in molten sodium sulfate provides a good example. The solubility of NiO in Na2S04, at a given temperature, depends on the activity of Na20, which is established by the equilibrium ... 

Section V on Testing in Environments (H. Hack, Section Editor) includes chapters on outdoor and indoor atmospheres, seawater, fresh water, soils, concrete, industrial waters, industrial chemical, petroleum, high-temperature gases, organic liquids, molten salts, liquid metals, corrosion inhibitors, in-vivo, and microbiological effects. Each chapter provides a descriptive overview of the environment and factors and variables affecting corrosion rates and mechanisms. 

Haynes alloy 556 exhibits useful resistance to a wide variety of high-temperature corrosive atmospheres as well as molten salts. The presence of approximately 18% cobalt results in more resistance to sulfidation than... 

Finally, zirconium is an excellent material for laboratory equipment, such as crucibles and autoclaves. Zirconium crucibles have replaced platinum crucibles for handling certain molten salts. Properly oxidized zirconium and zircaloys can be used as insulating washers and measuring devices exposed to high-temperature corrosives. 

Roger JW. Fundamentals of High-temperature corrosion in molten salts. In Metals Handbook Corrosion. Metals Park, Ohio ASM International, 1987 50-5. 

Nickel sulfide, NiS, can be prepared by the fusion of nickel powder with molten sulfur or by precipitation usiag hydrogen sulfide treatment of a buffered solution of a nickel(II) salt. The behavior of nickel sulfides ia the pure state and ia mixtures with other sulfides is of iaterest ia the recovery of nickel from ores, ia the high temperature sulfide corrosion of nickel alloys, and ia the behavior of nickel-containing catalysts. 

Nickel and Ni-Cr alloys are among the most resistant metallic materials to corrosion and oxidation at high temperatures and are widely used to resist corrosion by gases and molten salts at elevated temperatures (see Sections 7.1 and 7.5). 

It has been established that salts can deposit or form on metals during gas-metal reactions. Molten layers could then develop at high operating temperatures. Consequently, the laboratory testing of corrosion resistance in molten salts could yield valuable results for evaluating resistance to some high-temperature gaseous environments. 

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