High-temperature oxidation corrosion selective

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In order to select the alloy best suited to this application, an extensive program of corrosion tests was carried out by ORNL (USA) on the available commercial nickel-base alloys and austenitic stainless steels [22,30—34]. These tests were performed in a temperature gradient system with various fluoride media and different temperatures (maximum temperature and temperature gradient). Chromium, which is added to most alloys for high-temperature oxidation resistance, is quite soluble in molten fluoride salts. Metallurgical examination of the surveillance specimens showed the corrosion to be associated with outward diffusion of Cr through the alloy. It was concluded that the chromium content shall be maintained as low as reasonably possible to keep appropriate air oxidation properties. The corrosion rate is marked by initial rapid attack associated with dissolution of Cr and is largely driven by the impurities in the salt [22,30—34]. This is followed by a period of slower linear corrosion rate... 

The fifth component is the stmcture, a material selected for weak absorption for neutrons, and having adequate strength and resistance to corrosion. In thermal reactors, uranium oxide pellets are held and supported by metal tubes, called the cladding. The cladding is composed of zirconium, in the form of an alloy called Zircaloy. Some early reactors used aluminum fast reactors use stainless steel. Additional hardware is required to hold the bundles of fuel rods within a fuel assembly and to support the assembhes that are inserted and removed from the reactor core. Stainless steel is commonly used for such hardware. If the reactor is operated at high temperature and pressure, a thick-walled steel reactor vessel is needed. 

Another type of membrane is the dynamic membrane, formed by dynamically coating a selective membrane layer on a finely porous support. Advantages for these membranes are high water flux, generation and regeneration in situ abiUty to withstand elevated temperatures and corrosive feeds, and relatively low capital and operating costs. Several membrane materials are available, but most of the work has been done with composites of hydrous zirconium oxide and poly(acryhc acid) on porous stainless steel or ceramic tubes. 

The selection of materials must also consider oxidation/reduction processes that occur in the absence of an aqueous electrolyte. Examples include sulfidation, destructive oxidation of alloys in air or steam at high temperatures, carburization, nitriding, fuel ash corrosion, and high-temperature hydrogen attack. 

RE enhancing the selective oxidation of Cr to form stronger Cr203 scales Perez et al. [64] studied the effect of Si, Mo, and Ce implantation in AISI 304 stainless steel to test their corrosion resistance. At high temperatures. Si was found to behave similarly to a RE metal. Mo has beneficial properties in halide environments. 

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