Molten salts corrosion process

Electrochemical cells can also be used to attempt to obtain data on the mechanisms of the salt-induced corrosion processes. Cyclic voltammetry has been used [78-22] to obtain information on the oxidation and reduction reactions that may occur during molten salt corrosion. Chronopotentio-metric investigations with platinum as the working electrode in cells can also be used to determine and control the compositions of molten salts, as well as to measure the solubilities of various oxidation products in melts [25-28,30]. 

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 ... 

The most direct effect of defects on tire properties of a material usually derive from altered ionic conductivity and diffusion properties. So-called superionic conductors materials which have an ionic conductivity comparable to that of molten salts. This h conductivity is due to the presence of defects, which can be introduced thermally or the presence of impurities. Diffusion affects important processes such as corrosion z catalysis. The specific heat capacity is also affected near the melting temperature the h capacity of a defective material is higher than for the equivalent ideal crystal. This refle the fact that the creation of defects is enthalpically unfavourable but is more than comp sated for by the increase in entropy, so leading to an overall decrease in the free energy... 

Boron nitride is one of the most outstanding corrosion-resistant materials. It is inert to gasoline, benzene, alcohol, acetone, chlorinated hydrocarbons and other organic solvents. It is not wetted by molten aluminum, copper, cadmium, iron, antimony, bismuth, silicon, germanium, nor by many molten salts and glasses. It is used extensively as crucible material, particularly for molten metals, glasses and ceramic processing. 

Electrochemical processes in melts are often attended by side reactions and phenomena complicating the primary process. This is true, in particular, for the technically very important class of reactions in which a number of metals (calcium, barium, and others) are obtained electrometallurgically from molten salts. In many of these processes the metal that is deposited (sometimes in a highly disperse state) is found to interact with the corrosive melt for example, in a reaction such as... 

A dense and electronically insulating layer of LiA102 is not suitable for providing corrosion resistance to the cell current collectors because these components must remain electrically conductive. The typical materials used for this application are 316 stainless steel and chromium plated stainless steels. However, materials with better corrosion resistance are required for longterm operation of MCFCs. Research is continuing to understand the corrosion processes of chromium in molten carbonate salts under both fuel gas and oxidizing gas environments (23,25) and to identify improved alloys (29) for MCFCs. Stainless steels such as Type 310 and 446 have demonstrated better corrosion resistance than Type 316 in corrosion tests (29). 

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]. 

Among the pentavalent elements, the most important are niobium and tantalum. Niobium is an excellent material for surface treatment of steel materials for chemical industry due to its high hardness and corrosion-resistance in wet acidic conditions. Nowadays, niobium is also used for the preparation of superconductor tapes and it is used in other branches of industry, for instance in nuclear technology and metallurgy. Tantalum is also of similar importance. For these applications, it is necessary to prepare high purity metal. Molten salt electrolysis, as an alternative process to classical thermal reduction, provides niobium and tantalum with required quality. In order to optimize these processes, it is necessary to know details of both complex formation and redox chemistry of the species present in the melts. 

Aluminum alloys with niobium and tantalum Nb and Ta can be obtained in elemental form from high-temperature molten salts. Nb and Ta are widely used as coatings for corrosion protection as they form - like Al - thin oxide layers that protect the underlying material from attack. In technical processes several high-temperature molten salts are employed for electrocoating and the morphology of the deposit is strongly influenced by the composition of the baths. Some attempts have... 

Hot corrosion is designated as the accelerated attack of metals and ceramics in oxidizing environments by the presence of a thin molten salt film, for example, a fused sulfate, carbonate, chloride, or nitrate. In many high-temperature processes, molten salts are present either in partially molten ashes, as deposits on boiler tubes from conventionally fired plants such as waste fired boilers (chlorides, sulfates), as a single salt deposits on gas turbines (Na2S04), or as the electrolytes in molten carbonate fuel cells [(Li,K)2C03]. 

As the molten salt is electrolytic. Hot Corrosion processes involve electrochemical reactions like oxidation of the metal and reduction of melt components and dissolved gases. Hence, many of investigations of Hot Corrosion have been done by electrochemical techniques, mostly combined with conventional corrosion... 

In general, the solubility of oxides is a function of melt basicity and depends on the chemical composition of the passive layer. Hence, the corrosion process of alloys in molten salts is quite complicated as a result of the heterogeneous composition of the oxide scales formed on alloys. 

In the presence of molten salts, hot corrosion is unavoidable. Alloy components only slowdown the corrosion process. The naturally forming oxide layers are not sufficient to completely protect against hot corrosion. Composites have been used to resist salt corrosive attack at high temperatures. These composites consist of a metal or alloy core with a coating specially designed to resist hot corrosion. The metal core provides strength and the coating provides protection. 

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