Hydrogen is made available at the surface of an aluminium melt through the reaction between aluminium and water:

In addition to atmospheric moisture, water comes into contact with the melt in a foundry environment from sources such as hygroscopic flux materials, moist foundry tools, and water formed from the combustion of natural gas in gas fired furnaces. Hydrogen produced through the reduction of water dissociates into its atomic form at the melt surface, dissolves into the melt boundary layer, and is finally transported to the bulk of the molten metal by diffusion. Problems are encountered in the aluminium industry due to the dramatic difference in hydrogen solubility between the solid and liquid states as shown. Consequently, as a casting containing a high level of hydrogen solidifies, hydrogen gas is released creating porosity. In the majority of cases dissolved hydrogen levels are minimised, although elevated gas levels may be desirable in order to oppose shrinkage when manufacturing near net shape castings. Dissolved hydrogen must be accurately measured and controlled in order to ensure high yields and high product quality.




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