Molten salts thin deposits

It was quite recently reported that La can be electrodeposited from chloroaluminate ionic liquids [25]. Whereas only AlLa alloys can be obtained from the pure liquid, the addition of excess LiCl and small quantities of thionyl chloride (SOCI2) to a LaCl3-sat-urated melt allows the deposition of elemental La, but the electrodissolution seems to be somewhat Idnetically hindered. This result could perhaps be interesting for coating purposes, as elemental La can normally only be deposited in high-temperature molten salts, which require much more difficult experimental or technical conditions. Furthermore, La and Ce electrodeposition would be important, as their oxides have interesting catalytic activity as, for instance, oxidation catalysts. A controlled deposition of thin metal layers followed by selective oxidation could perhaps produce cat-alytically active thin layers interesting for fuel cells or waste gas treatment. 

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]. 

Molybdenum, with respect to its physico-chemical properties and high corrosion resistance, is inevitably used as a construction material for the equipment used in chemical industry. Often only thin layer on the exposed surfaces is preferred. In such cases, electrodeposition from molten salts appears to be a very important method for the formation of compact molybdenum deposits on the electrically conductive substrates. This method may be also considered for the primary production of molybdenum metal. 

In discussing molten salt tests, it is useful to consider first thin deposits where diffusion through the deposit of reactants from the gas plays a significant role in the corrosion process, and thick deposits where such diffusion is less important and sometimes negligible. 

Matsuda T, Nakamura S, Ide K-i, Nyudo K, Yae S et al (1996) Oscillatory behavior in electrochemical deposition reaction of polycrystalline silicon thin films through reduction of silicon tetrachloride in a molten salt electrolyte. Chem Lett 569-570... 

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...

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