Grain refiners additives

This hazard can be completely eliminated by the introduction of nucleants in the melt. These additives (grain refiners) crystallize in the melt at a temperature above the temperatures of lead dendrite formation. Thus, the growth of lead solid—solution crystals proceeds at a faster rate onto the already formed nuclei. As the latter are abundant in the melt, a fine-grain structure is obtained. 

Based on these facts, there are several possibilities to control the crystallite size the crystallite size can be decreased by increasing the current density for a constant the crystallite size can be increased for long due to Ostwald ripening the use of organic additives (grain refiners) enables the control of the crystallization process during the time because these molecules are adsorbed on the electrode surface in a reversible way and hinder the surface diffusion of the adatoms and changing the temperature influences all diffusion processes (ion diffusion in the electrolyte, surface diffusion of the nuclei) - if small crystallite sizes are desired, the deposition should be performed at lower temperatures. 

Fig. 3. Grain size as a function of antimony content A, without grain refiners B, with addition of selenium (4).

Fibrous stmctures represent a grain refinement of columnar stmcture. Stress-reHeving additives, eg, saccharin or coumarin, promote such refinement, as do high deposition rates. These may be considered intermediate in properties between columnar and fine-grained stmctures. 

Selenium acts as a grain refiner in lead antimony alloys (114,115). The addition of 0.02% Se to a 2.5% antimonial lead alloy yields a sound casting having a fine-grain stmcture. Battery grids produced from this alloy permit the manufacture of low maintenance and maintenance-free lead—acid batteries with an insignificant loss of electrolyte and good performance stability. 

Of the elements commonly found in lead alloys, zinc and bismuth aggravate corrosion in most circumstances, while additions of copper, tellurium, antimony, nickel, silver, tin, arsenic and calcium may reduce corrosion resistance only slightly, or even improve it depending on the service conditions. Alloying elements that are of increasing importance are calcium especially in maintenance-free battery alloys and selenium, or sulphur combined with copper as nucleants in low antimony battery alloys. Other elements of interest are indium in anodesaluminium in batteries and selenium in chemical lead as a grain refiner ". 

T orr. Reaction (1) is a simplified version of a reaction that involves the formation of subfluorides such as ReF4. Grain refinement is obtained by the addition of a small amount of H2O in the gas stream.The hydrogen reduction of the chloride is not normally used since it produces gas phase precipitation which is almost impossible to avoid. 

Du and co-workers [48, 66] studied the effect of carbon on the grain refinement of Mg-3A1 alloy. High grain refining efficiency was obtained when these alloys were refined by carbon. A further increase in efficiency was obtained by the combination of 0.2 wt% C and less than 0.2 wt% of a solute element (Ca, Sr) [48, 66], Addition of a higher Ca amount would increase the brittleness of the alloy [60, 66]. Similar results were demonstrated when 0.2 wt% Sr was added in the alloys instead of Ca [48]. 

The metal has very little commercial use. In elemental form it is a laser source, a portable x-ray source, and as a dopant in garnets. When added to stainless steel, it improves grain refinement, strength, and other properties. Some other applications, particularly in oxides mixed with other rare earths, are as carbon rods for industrial hghting, in titanate insulated capacitors, and as additives to glass. The radioactive isotope ytterbium-169 is used in portable devices to examine defects in thin steel and aluminum. The metal and its compounds are used in fundamental research. 

Mechanism of zinc electrodeposition on steel in acidic solution of zinc chloride was investigated [408] as a function of pH, grain-refining additives, and current density. 

Various boron compounds have been used as rocket fuels, diamond substitutes, and additives to aluminum alloys to improve electrical and thermal conductivity, as well as for grain refining. Boron hydrides are sensitive to shock and can detonate easily. Boron halides ate corrosive and toxic. 

GRAIN REFINER. An additive agent used lo obtain liner grains in a casling. The material is added to the molten metal prior to casting. 

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