Hot corrosion of metals by molten salts

An important indusuial interest is in the conosion of metals and ceramics by molten sodium sulphate/vanadate solutions. This is because turbines, which are usually nickel-based alloys, operating in a marine anuosphere, containing 

interact with the sulphur and vanadium oxides emitted from the combustion of technical grade hydrocarbons and die salt spray to form Na2S04 and NaV03- These conosive agents function in two modes, either the acidic mode in which for example, the sulphate has a high SO3 thermodynamic activity, of in the basic mode when the SO3 partial pressure is low in the combustion products. The mechanism of coiTosion is similar to the hot coiTosion of materials by gases widr the added effects due to the penetration of tire oxide coating by tire molten salt. 

It follows from the latter equation that the equilibrium constant, which has a value of lO at 1245K, where 

The composition of turbine blades is a complex mixture of alloying elements in solid solution in nickel. A typical alloy would contain Al, Cr, Mo, Co or Ta in amounts up to 10 atom per cent of the particular elements which are chosen. Of drese all except cobalt form substantially more stable oxides than 

Richardson. Physical Chemistry of Melts in Metallurgy, vols I II. Academic Press, London (1974). 

Waseda and J.M. Toguri The Structure and Properties of Oxide Melts. World Scientific, Singapore (1998) ISBN 981-02-3317-5. 

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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... 

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]. 

Alloys, used at high temperatures, obtain their protection from a dense and adherent oxide layer formed on the metal surface. The corrosive attack of metals and alloys in molten salts is due to the solubility of oxide scales by basic and acidic dissolution. This breakdown of the passive film gives rise to accelerated metal consumption by enhanced oxidation (Hot Corrosion). The phenomenon is closely related to pitting corrosion of metals and alloys in aqueous solutions. 

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... 

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. 

Figure 3.7 shows the perforated and pitted areas of the boiler tubes, respectively. The region near the pit, which is thinned due to corrosion, had shown the layers of deposits over the surface. The deposits, when analyzed by energy-dispersive X-ray spectroscopy (EDX), showed the presence of V, S, Al, Si, and O (Figure 3.8). Electron probe microanalysis (EPMA) showed that the deposits were rich in V compounds near the pit (Figure 3.9). Although floor-area tube temperature is normally between 300°C and 450°C, the deposit would increase the temperature due to poor heat transfer effect, which is sufficient to cause melting of the salt compounds in the deposit. Once these salts are in molten state, they would undergo fluxing reaction, destroying the protective layer, and consequently, the metal undergoes hot corrosion. The presence of V and S as noticed on pits...

The operation of waste incinerators is affected by the formation of corrosive gases and aerosols during the combustion process. This leads to severe corrosion of the metal compounds, e.g. heat exchanger tubes and water walls. As especially water walls have surface temperatures around 350°C, salt deposits are formed by condensation of aerosols within the flue gas on the metal surface. Heavy metal compounds like Pb and Zn cause the formation of eutectic salt mixtures with low melting points [1]. If salt deposits are molten, corrosion is accelerated in comparison to solid deposits. Such type of corrosion is known as hot corrosion . 

A model for hot corrosion beneath molten chloride is shown in Fig. 30.15 [8, 13]. In that model corrosion starts with the dissolution of Fe at the metal/salt melt interface and the formation of iron chloride. If the gas phase contains no chlorine species at the beginning, the chlorine source must be the molten chlorides. In this case chlorine can be released by the oxidation of the chloride. 

Direct resistance heating in which an electric current is passed directly through a section of the molten-salt piping has also been used successfully. Operating temperatures of this type of heater are limited only by the corrosion and strength limitations of the metal as the temperature is increased. Experience has indicated that heating of pipe bends by this method is usually not uniform and (jan be accompanied by hot spots caused by nonuniformity of liquid flow in the bend. 

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