Fluxes composed of chlorine and fluorine containing salts are used for degassing molten aluminum alloys. Degassing fluxes are commonly shaped in form of tablets. Degassing operation starts when a flux tablet is plunged by a clean preheated perforated bell to the furnace bottom. The flux components react with aluminum forming gaseous compounds (aluminum chloride, aluminum fluoride). The gas is bubbling and rising through the melt. Partial pressure of hydrogen in the formed bubbles is very low therefore it diffuses from the molten aluminum into the bubbles. The bubbles escape from the melt and the gas are then removed by the exhausting system. The process continues until bubbling ceases. Degassing flux may also be introduced by an injection method. In this case the inert gas serves as carrier for granulated flux. Besides the degassing effect the degassing treatment allows removing non-metallic inclusions suspended in the melt (cleaning effect).
Hydrogen, found in the foundry primarily due to the decomposition of water vapor, is very soluble in liquid aluminum alloys, but much less so in the solid phase. When aluminum solidifies, the excess dissolved hydrogen comes out of solution and forms bubbles, from pinhole size to much larger sizes. Metal which has not been degassed before casting is vulnerable to this type of defect. Degassing fluxes remove hydrogen, and also oxides and other impurities.
