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Other Types of ReleasesPermeation leaksPermeation leaks involve diffusive transport of hydrogen molecules through the surface material. This is significantly more pronounced in storage tanks that do not have metallic containment, high storage pressure, have a high surface area and long residence times and occurs over an extended period of time. According to (ScheferRW:2006), permeation of hydrogen through a metal involves adsorption and dissociation of molecular hydrogen to atomic hydrogen on the inner surface, followed by diffusion of atomic hydrogen through the metal and finally recombination to molecular hydrogen and desorption at the outer surface. Equations are provided for the permeation rate of hydrogen through several common metals. The results show the sensitivity of hydrogen flux to type of material, temperature and pressure. In automotive applications for tanks without metallic containment (commonly referred to as Types 3 and 4, draft regulations and standards include limits on the acceptable permeation rates, For non-metallic liner materials, the draft ECE regulations permit a maximum hydrogen permeation rate of 1.0Ncm3/hr per litre internal volume of the container for the settled pressure at 150C for a full container at start of life (GRPE, 2003), while an SAE standard, J2578 adopts 75Ncm3/min (75NmL/min) at 850C/ end of life and at nominal working pressure for a standard passenger vehicle (the SAE rate is independent of the size of the storage system). ISODIS15869.3 permits up 2.8Ncm3/hr per litre internal volume of the container based on similar conditions to the ECE draft. ReferencesInvalid BibTex Entry! |